CA2162926C - Pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing - Google Patents
Pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing Download PDFInfo
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- CA2162926C CA2162926C CA002162926A CA2162926A CA2162926C CA 2162926 C CA2162926 C CA 2162926C CA 002162926 A CA002162926 A CA 002162926A CA 2162926 A CA2162926 A CA 2162926A CA 2162926 C CA2162926 C CA 2162926C
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
- H04B1/7085—Synchronisation aspects using a code tracking loop, e.g. a delay-locked loop
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/22—Multipath-related issues
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/709—Correlator structure
Abstract
A receiver for pseudorandom noise (PRN) encoded signals consisting of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple digital correlators. The sampling circuit provides digital samples of a received composite signal to each of the several receiver channel circuits. The synchronizing circuits are preferably non-coherent, in the sense that they track any phase shifts in the received signal and adjust the frequency and phase of a locally generated carrier reference signal accordingly, even in the presence of Doppler or ionospheric distortion. The multiple correlators in each channel correlate the digital samples with locally generated PRN codes having multiple offsets, to produce a plurality of correlation signals. The plurality of correlation signals are fed to a parameter estimator, from which the delay and phase parameters of the direct path signal, as well as any multipath signals, may be estimated, and from which a range measurement may be corrected.
Description
2t62926 wo 95/14937 l Pcr/cAs4loo64o PSEUDORANDOM NOISE RANGING RECEIVER
WHlCH COMPENSATES FOR MULTIPATH DISTORTION BY MAKING
USE OF MIJLTIPLE CORRELATOR TIME DELAY SPACING
- Field of the lnvention This invention relates generally to digital receivers for pseudorandom noise (PRN) encoded signals such as those used in passive ranging systems, and in particular to such a receiver that has been adapted for use in signaling env;,unlll~n~s susceptible to n-llltipath fading.
Background of ~lle Inve~ on Passive ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigalion System (GLONASS) allo~ a user to precisely determine latitude, longitude, elevation, and time of day.
Ranging system receivers typically accG,-")lish this by decoding several precisely-timed signals transmitted by a group of special satelliles.
For eA~",ple, within the GPS system, each signal t,~"s",;~led by a satelli~e is modulated with low frequency (typically 50 ~Iz) digital data which indicates the satellite's position and time of day, no""al; ~ed to Greenwich Mean Time. Each satellite signal is further modulated with a unique, high frequency pseudorandomnoise (PRN) code, which provide a ..~C~ .., to plccisely deterrnine the lhle of sight signal trans,.l;ss;on time from each satellite.
The GPS system satellite constell~tion has been placed in geostationary orbit such that at least four satellites are within a direct line of sight at any given position on the earth. A typical PRN receiver thus receives a c~"")osite signal cons;~li.,g of several signals t,; n~ ;lled by the s~tellite~, as well as any noise and interfering signals. A decoder or channel circuit may then recover one of the I,;...~...;tled signals by c~"Jati"g (mul~ ,g) the co.."~Gsile signal with a locally generated reference version of the PRN code signal ~c~igned to the particular satellite of interest. If the locally generated PRN I c~re.~ce signal is p~ up~. Iy timed, the digital data from that pa, l;~ lar satellite may then be prc?erly detected SUBSTITUTE SHEET
WO 9S/14937 - PCI`/CA94100640 216292~ -2-The signals r~ce;~,d from .li~ t ~t~ tes are also automatically separated by the ~ lLi~ ?, process, because the signals lm~ d by .liffaent 5~tPtlitps use unique PRN codes having low or zero cross-co,lclalion power. The three .1~ nF onal position of the receiver and its ~,loci~y may then be resolved by using the PRN code phase il.rull~.dt;on to ~ ;3el~ d~,t.,.,.une the l~r~ 3~;0n tirne from at least four Qot~ t~Q~, and by det~ e each satellite's ephP . is andtime of day data.
In order to co"~lly d.,t~.,.u"e the offset ofthe PRN 1~ . ce signal, its rdative time delay is typically varied relative to the i~co... -~g signal until a m~xim~lm power level in the les~lling cG.,~lalion signal is determined. At the time offset co-l~ ,onding to this point of ..~ t,cc;~d power, the local re~clcnce signal is conQ;dered to be in s~ ,hlol~ .. with the inco.~ e signal, and the range ...eas,~- c.ue.lL may then be made. A so-called delay lock loop (DLL) l, ~.,kil.g system which correlates early, punctu~l, and late ~ ;on5 of the locally generated PRN code signal against the received co .posi~e signal thus pwrulllls these op~alions to ...~ in PRN code lock in each channel.
Because of this need to pl c~,;sely determine the exact p. ~,paga~ion time, a number of p. oblc.ns face the deJ;g..e. s of PRN receivers. One pl oble.., concc. "s accurate phase and frequency llackillg ofthe received signals; another p,oble."
concerns the correction of relative divergence between the received signals and the local PRN code signal generators in the presence of ionospheric distortion.
In addition, because GPS systems depend upon direct line of sight for communication propagation, any multipath fading can further distort received signal timing es-im~tes In the ideal system, only one signal, the signal taking the direct or shortest path, is present. However, since the ~ .c . ~ler uses an o~-u~idhcclional (wide angle) antenna for m~xin--lm coverage, and since it is so far away from the receiver, the prese.-ce of surrounding reflecting objects such as buildings and natural surface formations means that there are typically multiplepaths for the signal to take. Such a multipath signal takes a slightly di~. c.~l and longer router and thus arrives at the receiver at a different time.
SUBSTITUTE SH-EET
~162~2~
The exact number of .~ h signals present at any given ...u... .l is a r.~ ;on ofthe satdlite and LqntPnnq p~s;tiorlQ relstive to any and all .~ne~ g ûbjects. Th.,. erul c, in the typical ~;lualiûn, there may be none, or there may be many mllltipqth signals. Since the r....ll;p~h signals travel a longer ~ Cl~-)ce they will always be r~ce;~d at some time after the direct path signal and will inevitably suffer a loss in power due to the ~necl;on(s). This time delay equals the di~ ce in length b~ ,e.~ the direct path and the reflected path divided by the prcpa~qtion ~,lo~,;l~.
The effect of the p~sel)ce of .. 1l ;palh on the process of acquiring code lock is that there will always be some coll~laliol~ with the ,...~ Ih signals as well as with the desired, direct path signal.
The typical way of dealing with this is to design the PRN autocol I clalion -,liûns such that even a small offset from zero in time will yield a near zero value in the estimqte of the autocGI~ I,lalion function. In reality, however, the autoco,l~lalion power de~ ases linearly as the time offset h~cleases, in either the position or negative directions The multip~th co" ~lalion power only reaches zero when the PRN code offset is greater than plus or minus one chip. Since the carrier salll?ling rate is usually much higher than the PRN code chipp;"g rate, partial coll~làlions will occur at sub-chip offsets.
Thus, in the presence of rr ~Ikip~th distortion, most GPS receivers suffer a degradation in accuracy and an increase in processing time. This is especially true in high accuracy dil~,~.,L;al GPS appli.,alions, where pseudorange multipath will result in errors c, eep;,lg into the dirrc~ e.llial corrections, causing large position blases.
Unlike other error sources, .~ull;?a~h is typically u~co~lclated b~ en antenna locations. Thus, the base and remote receivers e"~,e, ;ence different multipath interference and as a result, simple di~ nc;ng bel.. eel- them will not cancel the errors due to IuullilJàlll distortion. Also, modellinp multipath for each antenna location is difficult and impractical.
SU~S~ITUTE SHEET
W O 95/14937 ~ 1 ~ 2 9 2 3 _ 4 _ PC~r/CA94/00640 A c~.,u..on method of ~ )"hiS to cale~ully choose the design of the antenna and careful site s~1e~ ~;on U~ t~,ly, it ,s often not possible to cnange either of these p~ ~..et~ . ~.. For ~ .k., if the antenna is to be mounted on an airplane fuselage, it will not be easily moved or te~lAeeA, and its shape is rece....;~ restricted due to r - c.J~.,~.Iic CQI -:d~ ~ions.
What is needed is a way to reduce the tracking errors present in PRN
rangtng l~cei~e.~, especi ~lly those ofthe lower-L~u.,l.~ CtA code type, in the 3ence of n...~ fading, without de~aJ;l~g the signal ac.3: ;~;on caF ' lity of the receiver, or increasing errors due to Doppler shift, sudden receiver motion, or other noise sources. The desired method of I cduc;ng mllltipath d;~u. lion wouldbe l~r.sl,a.ent to user, and operate within the GPS receiver itself, as opposed to u;l;ng a special antenna or receiver siting.
S~ l~r of the Invention Briefly, the invention is an improved receiver for pseudorandom noise (PRN) encoded signals consisting of a s-...r.!ine circuit, multiple carrier and code s~ l ol~h~g circuits, and multiple correlators, with each correlator having a selectable code delay spacing. The time delay spacing of the multiple correlators is distributed around an expected correlation peak to produce an estimate of the correlation function parameters which vary with respect to mnltip~th distortion.The parameters of interest discernable from an estim~te of the shape of the autocorrelation peak include the direct signal path time and phase offsets.
This information may be used in turn used to determine the offset estimates for locally generated PRN refel ence code and carrier phase tracking signals, or may be used to adjust a range measu,e....,At.
If mllltipath correlators are not available full-time for each ch~nnel, then the code delay for a fewer number of correlators, such as two for each channel, can be sequenced from epoch to epoch, so that over time, measu,~."c."s from several points on the col, ~lalion function can be taken.
Wo 95/14937 2, 1 ~; 2 9 2 ~ PCT/CA94/00640 In another c..-bod~ nl, the ul~jGlily ofthe cl.~nl-rlc in a ~e~,eivel can be left to operate normal!y, with one or more of the c~ nrlc being dedic~ted to co..1;...-uu~1y sequPn~ing from channel to channel to determine the mllhipath parameters for a partial PRN code being tracked.
There are several adva.lt~es to this ~~ g.~ In em~uu~ such as cG....,.~.c;al GPS coarse/~ icitioll (C/A) code appl~ Q~c~ where the mllltipath d.~tol l;on in the tcce;~_d co...pG;,i~e signal is of the same order of l~in.de as a PRN code chip time, the PRN receiver is capable of acquiring carrier and code lock over a wid~. range of OpC.al;llg cond;l;ons. Once the receiver is locked, it will ~vtom~tic-~lly remain locked, even in the p, ese"ce of multipath distortion.
An improvement in range ",ea~u~ ~.nen~ accuracy is actively accomplished -without special antenna design, and without specifying the particular location of the ~nt~-ln~, even in d;fI;,~nlial ranging applications.
SUBSTITU~E SHEET
W O 95/14937 ~ 1 ~ 2 9 2 6 6 - PC~r/CA94/00640 BriefDescription of ~e Dl~w-~-gs The above and further adv~ul~ges ofthe invention may be better understood by l~,f~Ulg to the following de~c~ on in conjunction with the acco...r~ u~ dl a-. ulg5, in which:
Fig. 1 is a high level block d;agl ~,. of a PRN receiver which i"co~ ~ol ales the invention, ;..chl.l;..Y its duw~co~ .ler, sampler, channel, and processor clrcults;
Fig. 2 is a block diagram of one of the channel circuits, showing multiple correlators being used in each ~h~nncl;
Fig. 3 is a block diag,a.ll of a carrier/code S~/IIClUcJlU~.;llg circuit used ineach channel circuit;
Fig. 4 is a block diagram of a correlator circuit used in each channel circuit;
Fig. 5 is a timing diagram showing the relative duration of various portions of a received PRN signal;
Fig. 6 is a plot of a direct path signal COI I JaliOn, a multirath signal correlation, and the resulting direct with multipath col l~,lation;
Fig. 7 is a plot of another multipath correlation with di~. enl phase offset;
Fig. 8 is a plot showing the n~llimiting effect of the channel 22;
Fig. 9 shows the resulting tracking error; and Fig. 10 is another plot of the band limited correlation function showing the di~l~ ibLIlion of the mnltiple correlators.
SUBE;TITUTE SHEET
216232i W O 95/14937 PC~r/CA94/00640 Detailed Desc~ ion of a ~ d Embodiment Now turning ~ ntion to the d. ~. .ngs, Fig. I is an overall block diagram of a pseudo.~do,ll noise (PRN) ranging,~cei~. 10 con..LI u~,lcd in ~c l.lance - with the i,.~.,Lion. It ;.. ~ des an antenna 11, a dow,lco"~ . 12, an in-phase (I) and quad,~lult; (Q) sampler 14, a ~locessor 16, a control bus 18, a channel bus 20 and ,..~ .le Clr~ C 22a, 22b, . . ., 22n, (collectively, the cl~ lC 22). TheillusL,~I~,d receiver 10 will be des_lil,ed herein as Op~.aliul~, within the United States' Global PGC;I;U~! ~g System (GPS) using the CGIlullC..(,;al, coarse acquisition (CtA) pseudorandom codes, ho..~ ., ada~t~lions to other ranging systems are also possible.
The antenna 11 receives a c~ Gs;le signal Cs COfi~ P of the signals llim~ ed from all pall;.,;~,al;~-g s~rltiles within view, that is, within a direct line of sight of the antenna 1 1. When the GPS system is fully operational, signals from at least four and as many as eleven s~tpllit~c may be received simlllt~ncously at each location on the earth.
The co..~po~.;le signal Cs is fol ~aJ ded to the downconverter 12 to provide an intc.ll.ediale frequency signal, IF. The IF signal is a dowl-col--~erted and filtered version ofthe composite signal Cs. The do~..co.-~e.Ler 12 should have abandp~cs filter which is s--fficiently vide to permit several chips of the PRN coded signals to pass through. For the C/A code embodiment descl ibed here, this bandwidth is typically 8 MHz.
The downconverter 12 also generates a sample clock signal, Fs, which is four times the frequency of the IF signal, which indicates the points in time atwhich samples of the IF signal are to be taken by the sampler 14.
The sampler 14 receives the IF and Fs signals and provides digital samples of the IF signal to the c~ rlc 22 via the channel bus 20. The samples consist ofin-phase (I) and quadrature (Q) s~mple-c of the IF signal taken at the times indicated by the Fs signal, typically by an analog-to-digital converter which samples at pre~,;sely 90 phase, utatiolls of the IF signal's carrier frequency. With W O 95/14937 ~16 2 9 2 ~ 8 - PC~r/CA94/00640 the digital sample clock signal, Fs~ chosen acco,~;.,g to these ~ lr~ f c, that is, with four samples taken in every IF carrier cycle, the output sarnples from the sampler 14 are thus in in-phase and quadrature order as I,Q, -I, -Q, I,Q . . . and so on. The I and Q samples are then se~,a.dled and routed to the ch~ nPlc 22 on separate Is and Qs con~uctors of the channel bus 20, along with the Fs signal. For more details of one embodiment of the dow~cc~ erter 12 and sample 14, please refer to U.S. Patent No. 5,101,416 entitled ''~~ ;ch~ rl Digital Receiver for Global pocitioning System" issued March 31, 1992, and Z~ cd to NovAtel Col.l.llun.cations Ltd.
Each channel 22 is normally ~igned to process the signal ll; .c~ ed by one of the c~tellitçs which is p~ .,Lly within view of the antenna 11. A given channel 22 thus processes the Is and Q5 signals and tracks the carrier and code of the signal ~ h~ ..;Lled by its ~igned s~teliite As e,~ ;ned below, each channel 22 uses a carrier and code syncl,lon;~;ng circuit to frequency and phase-track the PRN encoded carrier signal by mai."a;.. ~ an eYrected Doppler offset unique tothe desired satellite Furthe. .no, e, each channel 22 co..lains multiple correlators to m~int~in phase lock with a locally generated PRN code l~f~.~nce signal as well, to remove the effects of any multipath distortion on the position measurement The locally generated PRN code reference signal is then used to decode the data from the assigned satellite The resulting decoded data, including the satellite's ephemeris, time of day, and status info~ ation, as well as the locally generated PRN code ph~se and carrier phase measurements, are provided to the p-ucessor 16 via the control bus 18 The ch~nnPls 22 are described in detail in co~e~,lion with Fig 2 The sampler 14 and ch~nnels 22 are controlled by the processor 16 via the control bus 18. The processor 16 includes a central processing unit (CPU) 162 which typically supports both synchronous-type input/output (I/0) via a multiple-bit data bus DATA, address bus ADDR, and control signals CTRL and syllcL~nous controller circuit 164, and an interrupt-type I/0 via the interrupt signals, INT and an interrup~ controller circuit 166. A timer 168 provides certain timing signals such as a measurement trigger MEAS ir-~ic~ting a request for a ~UBSTITUTE SHEET
WHlCH COMPENSATES FOR MULTIPATH DISTORTION BY MAKING
USE OF MIJLTIPLE CORRELATOR TIME DELAY SPACING
- Field of the lnvention This invention relates generally to digital receivers for pseudorandom noise (PRN) encoded signals such as those used in passive ranging systems, and in particular to such a receiver that has been adapted for use in signaling env;,unlll~n~s susceptible to n-llltipath fading.
Background of ~lle Inve~ on Passive ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigalion System (GLONASS) allo~ a user to precisely determine latitude, longitude, elevation, and time of day.
Ranging system receivers typically accG,-")lish this by decoding several precisely-timed signals transmitted by a group of special satelliles.
For eA~",ple, within the GPS system, each signal t,~"s",;~led by a satelli~e is modulated with low frequency (typically 50 ~Iz) digital data which indicates the satellite's position and time of day, no""al; ~ed to Greenwich Mean Time. Each satellite signal is further modulated with a unique, high frequency pseudorandomnoise (PRN) code, which provide a ..~C~ .., to plccisely deterrnine the lhle of sight signal trans,.l;ss;on time from each satellite.
The GPS system satellite constell~tion has been placed in geostationary orbit such that at least four satellites are within a direct line of sight at any given position on the earth. A typical PRN receiver thus receives a c~"")osite signal cons;~li.,g of several signals t,; n~ ;lled by the s~tellite~, as well as any noise and interfering signals. A decoder or channel circuit may then recover one of the I,;...~...;tled signals by c~"Jati"g (mul~ ,g) the co.."~Gsile signal with a locally generated reference version of the PRN code signal ~c~igned to the particular satellite of interest. If the locally generated PRN I c~re.~ce signal is p~ up~. Iy timed, the digital data from that pa, l;~ lar satellite may then be prc?erly detected SUBSTITUTE SHEET
WO 9S/14937 - PCI`/CA94100640 216292~ -2-The signals r~ce;~,d from .li~ t ~t~ tes are also automatically separated by the ~ lLi~ ?, process, because the signals lm~ d by .liffaent 5~tPtlitps use unique PRN codes having low or zero cross-co,lclalion power. The three .1~ nF onal position of the receiver and its ~,loci~y may then be resolved by using the PRN code phase il.rull~.dt;on to ~ ;3el~ d~,t.,.,.une the l~r~ 3~;0n tirne from at least four Qot~ t~Q~, and by det~ e each satellite's ephP . is andtime of day data.
In order to co"~lly d.,t~.,.u"e the offset ofthe PRN 1~ . ce signal, its rdative time delay is typically varied relative to the i~co... -~g signal until a m~xim~lm power level in the les~lling cG.,~lalion signal is determined. At the time offset co-l~ ,onding to this point of ..~ t,cc;~d power, the local re~clcnce signal is conQ;dered to be in s~ ,hlol~ .. with the inco.~ e signal, and the range ...eas,~- c.ue.lL may then be made. A so-called delay lock loop (DLL) l, ~.,kil.g system which correlates early, punctu~l, and late ~ ;on5 of the locally generated PRN code signal against the received co .posi~e signal thus pwrulllls these op~alions to ...~ in PRN code lock in each channel.
Because of this need to pl c~,;sely determine the exact p. ~,paga~ion time, a number of p. oblc.ns face the deJ;g..e. s of PRN receivers. One pl oble.., concc. "s accurate phase and frequency llackillg ofthe received signals; another p,oble."
concerns the correction of relative divergence between the received signals and the local PRN code signal generators in the presence of ionospheric distortion.
In addition, because GPS systems depend upon direct line of sight for communication propagation, any multipath fading can further distort received signal timing es-im~tes In the ideal system, only one signal, the signal taking the direct or shortest path, is present. However, since the ~ .c . ~ler uses an o~-u~idhcclional (wide angle) antenna for m~xin--lm coverage, and since it is so far away from the receiver, the prese.-ce of surrounding reflecting objects such as buildings and natural surface formations means that there are typically multiplepaths for the signal to take. Such a multipath signal takes a slightly di~. c.~l and longer router and thus arrives at the receiver at a different time.
SUBSTITUTE SH-EET
~162~2~
The exact number of .~ h signals present at any given ...u... .l is a r.~ ;on ofthe satdlite and LqntPnnq p~s;tiorlQ relstive to any and all .~ne~ g ûbjects. Th.,. erul c, in the typical ~;lualiûn, there may be none, or there may be many mllltipqth signals. Since the r....ll;p~h signals travel a longer ~ Cl~-)ce they will always be r~ce;~d at some time after the direct path signal and will inevitably suffer a loss in power due to the ~necl;on(s). This time delay equals the di~ ce in length b~ ,e.~ the direct path and the reflected path divided by the prcpa~qtion ~,lo~,;l~.
The effect of the p~sel)ce of .. 1l ;palh on the process of acquiring code lock is that there will always be some coll~laliol~ with the ,...~ Ih signals as well as with the desired, direct path signal.
The typical way of dealing with this is to design the PRN autocol I clalion -,liûns such that even a small offset from zero in time will yield a near zero value in the estimqte of the autocGI~ I,lalion function. In reality, however, the autoco,l~lalion power de~ ases linearly as the time offset h~cleases, in either the position or negative directions The multip~th co" ~lalion power only reaches zero when the PRN code offset is greater than plus or minus one chip. Since the carrier salll?ling rate is usually much higher than the PRN code chipp;"g rate, partial coll~làlions will occur at sub-chip offsets.
Thus, in the presence of rr ~Ikip~th distortion, most GPS receivers suffer a degradation in accuracy and an increase in processing time. This is especially true in high accuracy dil~,~.,L;al GPS appli.,alions, where pseudorange multipath will result in errors c, eep;,lg into the dirrc~ e.llial corrections, causing large position blases.
Unlike other error sources, .~ull;?a~h is typically u~co~lclated b~ en antenna locations. Thus, the base and remote receivers e"~,e, ;ence different multipath interference and as a result, simple di~ nc;ng bel.. eel- them will not cancel the errors due to IuullilJàlll distortion. Also, modellinp multipath for each antenna location is difficult and impractical.
SU~S~ITUTE SHEET
W O 95/14937 ~ 1 ~ 2 9 2 3 _ 4 _ PC~r/CA94/00640 A c~.,u..on method of ~ )"hiS to cale~ully choose the design of the antenna and careful site s~1e~ ~;on U~ t~,ly, it ,s often not possible to cnange either of these p~ ~..et~ . ~.. For ~ .k., if the antenna is to be mounted on an airplane fuselage, it will not be easily moved or te~lAeeA, and its shape is rece....;~ restricted due to r - c.J~.,~.Iic CQI -:d~ ~ions.
What is needed is a way to reduce the tracking errors present in PRN
rangtng l~cei~e.~, especi ~lly those ofthe lower-L~u.,l.~ CtA code type, in the 3ence of n...~ fading, without de~aJ;l~g the signal ac.3: ;~;on caF ' lity of the receiver, or increasing errors due to Doppler shift, sudden receiver motion, or other noise sources. The desired method of I cduc;ng mllltipath d;~u. lion wouldbe l~r.sl,a.ent to user, and operate within the GPS receiver itself, as opposed to u;l;ng a special antenna or receiver siting.
S~ l~r of the Invention Briefly, the invention is an improved receiver for pseudorandom noise (PRN) encoded signals consisting of a s-...r.!ine circuit, multiple carrier and code s~ l ol~h~g circuits, and multiple correlators, with each correlator having a selectable code delay spacing. The time delay spacing of the multiple correlators is distributed around an expected correlation peak to produce an estimate of the correlation function parameters which vary with respect to mnltip~th distortion.The parameters of interest discernable from an estim~te of the shape of the autocorrelation peak include the direct signal path time and phase offsets.
This information may be used in turn used to determine the offset estimates for locally generated PRN refel ence code and carrier phase tracking signals, or may be used to adjust a range measu,e....,At.
If mllltipath correlators are not available full-time for each ch~nnel, then the code delay for a fewer number of correlators, such as two for each channel, can be sequenced from epoch to epoch, so that over time, measu,~."c."s from several points on the col, ~lalion function can be taken.
Wo 95/14937 2, 1 ~; 2 9 2 ~ PCT/CA94/00640 In another c..-bod~ nl, the ul~jGlily ofthe cl.~nl-rlc in a ~e~,eivel can be left to operate normal!y, with one or more of the c~ nrlc being dedic~ted to co..1;...-uu~1y sequPn~ing from channel to channel to determine the mllhipath parameters for a partial PRN code being tracked.
There are several adva.lt~es to this ~~ g.~ In em~uu~ such as cG....,.~.c;al GPS coarse/~ icitioll (C/A) code appl~ Q~c~ where the mllltipath d.~tol l;on in the tcce;~_d co...pG;,i~e signal is of the same order of l~in.de as a PRN code chip time, the PRN receiver is capable of acquiring carrier and code lock over a wid~. range of OpC.al;llg cond;l;ons. Once the receiver is locked, it will ~vtom~tic-~lly remain locked, even in the p, ese"ce of multipath distortion.
An improvement in range ",ea~u~ ~.nen~ accuracy is actively accomplished -without special antenna design, and without specifying the particular location of the ~nt~-ln~, even in d;fI;,~nlial ranging applications.
SUBSTITU~E SHEET
W O 95/14937 ~ 1 ~ 2 9 2 6 6 - PC~r/CA94/00640 BriefDescription of ~e Dl~w-~-gs The above and further adv~ul~ges ofthe invention may be better understood by l~,f~Ulg to the following de~c~ on in conjunction with the acco...r~ u~ dl a-. ulg5, in which:
Fig. 1 is a high level block d;agl ~,. of a PRN receiver which i"co~ ~ol ales the invention, ;..chl.l;..Y its duw~co~ .ler, sampler, channel, and processor clrcults;
Fig. 2 is a block diagram of one of the channel circuits, showing multiple correlators being used in each ~h~nncl;
Fig. 3 is a block diag,a.ll of a carrier/code S~/IIClUcJlU~.;llg circuit used ineach channel circuit;
Fig. 4 is a block diagram of a correlator circuit used in each channel circuit;
Fig. 5 is a timing diagram showing the relative duration of various portions of a received PRN signal;
Fig. 6 is a plot of a direct path signal COI I JaliOn, a multirath signal correlation, and the resulting direct with multipath col l~,lation;
Fig. 7 is a plot of another multipath correlation with di~. enl phase offset;
Fig. 8 is a plot showing the n~llimiting effect of the channel 22;
Fig. 9 shows the resulting tracking error; and Fig. 10 is another plot of the band limited correlation function showing the di~l~ ibLIlion of the mnltiple correlators.
SUBE;TITUTE SHEET
216232i W O 95/14937 PC~r/CA94/00640 Detailed Desc~ ion of a ~ d Embodiment Now turning ~ ntion to the d. ~. .ngs, Fig. I is an overall block diagram of a pseudo.~do,ll noise (PRN) ranging,~cei~. 10 con..LI u~,lcd in ~c l.lance - with the i,.~.,Lion. It ;.. ~ des an antenna 11, a dow,lco"~ . 12, an in-phase (I) and quad,~lult; (Q) sampler 14, a ~locessor 16, a control bus 18, a channel bus 20 and ,..~ .le Clr~ C 22a, 22b, . . ., 22n, (collectively, the cl~ lC 22). TheillusL,~I~,d receiver 10 will be des_lil,ed herein as Op~.aliul~, within the United States' Global PGC;I;U~! ~g System (GPS) using the CGIlullC..(,;al, coarse acquisition (CtA) pseudorandom codes, ho..~ ., ada~t~lions to other ranging systems are also possible.
The antenna 11 receives a c~ Gs;le signal Cs COfi~ P of the signals llim~ ed from all pall;.,;~,al;~-g s~rltiles within view, that is, within a direct line of sight of the antenna 1 1. When the GPS system is fully operational, signals from at least four and as many as eleven s~tpllit~c may be received simlllt~ncously at each location on the earth.
The co..~po~.;le signal Cs is fol ~aJ ded to the downconverter 12 to provide an intc.ll.ediale frequency signal, IF. The IF signal is a dowl-col--~erted and filtered version ofthe composite signal Cs. The do~..co.-~e.Ler 12 should have abandp~cs filter which is s--fficiently vide to permit several chips of the PRN coded signals to pass through. For the C/A code embodiment descl ibed here, this bandwidth is typically 8 MHz.
The downconverter 12 also generates a sample clock signal, Fs, which is four times the frequency of the IF signal, which indicates the points in time atwhich samples of the IF signal are to be taken by the sampler 14.
The sampler 14 receives the IF and Fs signals and provides digital samples of the IF signal to the c~ rlc 22 via the channel bus 20. The samples consist ofin-phase (I) and quadrature (Q) s~mple-c of the IF signal taken at the times indicated by the Fs signal, typically by an analog-to-digital converter which samples at pre~,;sely 90 phase, utatiolls of the IF signal's carrier frequency. With W O 95/14937 ~16 2 9 2 ~ 8 - PC~r/CA94/00640 the digital sample clock signal, Fs~ chosen acco,~;.,g to these ~ lr~ f c, that is, with four samples taken in every IF carrier cycle, the output sarnples from the sampler 14 are thus in in-phase and quadrature order as I,Q, -I, -Q, I,Q . . . and so on. The I and Q samples are then se~,a.dled and routed to the ch~ nPlc 22 on separate Is and Qs con~uctors of the channel bus 20, along with the Fs signal. For more details of one embodiment of the dow~cc~ erter 12 and sample 14, please refer to U.S. Patent No. 5,101,416 entitled ''~~ ;ch~ rl Digital Receiver for Global pocitioning System" issued March 31, 1992, and Z~ cd to NovAtel Col.l.llun.cations Ltd.
Each channel 22 is normally ~igned to process the signal ll; .c~ ed by one of the c~tellitçs which is p~ .,Lly within view of the antenna 11. A given channel 22 thus processes the Is and Q5 signals and tracks the carrier and code of the signal ~ h~ ..;Lled by its ~igned s~teliite As e,~ ;ned below, each channel 22 uses a carrier and code syncl,lon;~;ng circuit to frequency and phase-track the PRN encoded carrier signal by mai."a;.. ~ an eYrected Doppler offset unique tothe desired satellite Furthe. .no, e, each channel 22 co..lains multiple correlators to m~int~in phase lock with a locally generated PRN code l~f~.~nce signal as well, to remove the effects of any multipath distortion on the position measurement The locally generated PRN code reference signal is then used to decode the data from the assigned satellite The resulting decoded data, including the satellite's ephemeris, time of day, and status info~ ation, as well as the locally generated PRN code ph~se and carrier phase measurements, are provided to the p-ucessor 16 via the control bus 18 The ch~nnPls 22 are described in detail in co~e~,lion with Fig 2 The sampler 14 and ch~nnels 22 are controlled by the processor 16 via the control bus 18. The processor 16 includes a central processing unit (CPU) 162 which typically supports both synchronous-type input/output (I/0) via a multiple-bit data bus DATA, address bus ADDR, and control signals CTRL and syllcL~nous controller circuit 164, and an interrupt-type I/0 via the interrupt signals, INT and an interrup~ controller circuit 166. A timer 168 provides certain timing signals such as a measurement trigger MEAS ir-~ic~ting a request for a ~UBSTITUTE SHEET
2~ 6292~
range ...e~ n~ to be taken. The op~ ion ofthe ~)roc~;,ol 16 and its various r~ nc irnplern~nted in software will be better u~du.~loou from the following on The cGl"pG~;le signal Cs rc~i~cd from the ~nt~nn~ 1 I typically consists of signals ~ c..~ d by all s~ lI;t~ S within view (that is, within a direct line-of-sight of the l~,cei~. 10), any i.~ .ing signals, such as ,~ . signals and noise.
The carrier signal used by the GPS C/A ranging system is an ~band carrier at 1.57542 GigaHertz (GHz) with a PRN code rate of 1.023 I-ffIz and a nominal ed power of-160 dBW. Natural bacl~.u"d.noise at about -204 dBWlHz is typically mixed in with the L-band signals. In aA~ition, one or more ,a~h signals are present in the composite signal Cs, as will be desc.i~ed below. For more detailed i,~. ..,alion on the forrnat of the GPS system signals,see "Interface Control Doc,l...en~ ICD-GPS-200, September 26, 1984", p~1b!ished by Rockwell Intemational Col~olation, Satellite Systems Division, Downey, California 90241.
Fig. 5 shows, on a ~ Io~ led time scale, the relative durations of various co."l)on~"ls of a typical PRN ranging signal Ir~c~ ed by a GPS satellite and certain of the signals in a pi efe" ed embodiment of a channel 22n. A single carrier cycle has a particular duration, C. A single cycle of the digital sample signal clock Fs, consists of K carrier cycles.
A PRN code chip incl~ldes N cycles of the Fs signal, and a PRN code epoch consists of Z PRN code chips, where Z is also Icnown as the sequerlcc length of the PRN code. One data bit typically conslsts of T PRN code epochs.
For the p~ e~ d embodiment of the invention adapted to receive the GPS Ll ranging signal, the carrier frequency is 1575.42 MHz, and K is 77, so that Fs equals 20.46 MH~. In addition, a con~ N is 20, so that the PRN code chip rate is 1.023 MHz, and Z is 1023, so that the PRN code epoch rate is 1 kHz. T, another con~ , is also 20, so that the data bit rate is 50 Hz.
A channel circuit 22n is shown in detail in Fig. 2. It include5 a carrier and code s~"cl-ro,u,e- circuit 220, a PRN code generator 230, a carrier phase shi~er23~, ll~l.ll;~Jlc correlators 240-1, 240-2, 240-3, ..., 240-m (collectively, correlators SUBSTITUTE SHEET
WO 95/14937 ~16 2 9 2 6 - PCT/CA94/00640 240), a code shift register 250, an early minus late (or early, late) J;sc. i,.u..ator 260, a dot prodl;ct (or p~ uol, early minus late) discriminator 265, and a mllhipath p~u~ ler ectimotor 270.
The PRN code ~..e.ator 230 uses signals output by the ~....I..onlLer 220 to ~cn~,.dte a local PRN ~ nce signal, PRN CODE. The p~ ,uL PRN
CODE signal generated at any given time depends upon which satellité it is desired for the channel to be tuned to, as sPIected by the SAT II) input. PRN
code gcnc.dlol~. such as codé generator 230 are well known in the art.
The s~ncl-- oni~er 220 is a single numerically controlled osrillotor (NCO) which uses the sample clock Fs and a~.~Jrop. iale in ,l. uc~ions from the processor 16 to provide the control signals re~u;,~,d by PRN code gene.~10r 230 and correlators 240 in the carrier or code carrier phase to non-coherently track the errors caused by residual Doppler and multirath distortion.
Before contin..in~ with a detailed ~I;c~,..c~:~n ofthe chaMel 22n, refer to Fig. 3, which is a detailed block diagram ofthe carrier and code s~...,l..o..i~;r 220.
This element jnrl1ld~s an expected Doppler rate register 221, an accum~loted delta range (ADR) register 222, and a fine chip counter 224. A code phase generator circuit 226 also inrludes a subchip counter 226a, chip counter 226b, epoch counter 226d. Buffer circuits 227, 228, and 229 allow the processor 16 to load, read, and add to or subtract from the contents of the various counters andregisters in the synchroni~ r 220.
The s~". h.or.;~er 220 accepts the sarnple clock signal Fs from the channel bus 20, an expected Doppler value EDOPP and col ~ ~cled values for the registersand counters 222, 224 and 226 from the control bus 18. In response to these inputs, it provides a clock signal CLK and reset signal RST to the PRN code generator 230, as well as in~..ul,t signals INT1, INT4, and INT20 to the controlbus 18. An in~A~qneQus carrier phase angle Pctimqte is also provided via bits ~o, J~ 1. .7~p to the correlators 240.
The contents of the ADR register 222 and code phase generator 226 provide an i..cl~..l;.neous estimqte of the l,dn ,.,.;l time of the particular satellite SUE~STITUTE SHEET
- W 095/14937 ~ 1 6 2 9 2 6 PCT/CA~ ^6 signal ~C,;~rd to channel 22n. The di~ ,..ce b_h.'~,CIl this e~timqte of the ll~.~.~ul time and the ~ ~,cei-e. time of day (as estimated by the timer 168 in Fig. 1) is then taken as the propa~ iol~ tirne of that signai plus any I cce;~e- clock offset.
By .~ lyulg the p~ u~a~,alion time by the speed of light, a precise ~ bul enlenlofthe range from the r~cei~,~. 10 to the ~c~ign~d satellite may then be made by the pr~cessor 16. These ll~ ule.ll~ occur at selected time i...~ d by the ~ul~,l,.,nl strobe MEAS from the timer 168, and are t,ypicaiiy taken .eoul~ across ail the cl~ ~n~lc 22. The t~S~Illu~E~ range to each satellite is then used by the p.ocessor 16 to compute the position and or velocity of the receiver 10.
In operation, the expected Doppler rate register 221 is first loaded via the proces~or bus 18 with an ~stimqtPd Doppler offset EDOPP for the particular sateliite tracked by channel 22n. In most ;l.~l~n~CS, such as when the receiver 10 has been operating for some time, the EDOPP Pstimqte may be taken from ~lmqnqc data already received from satellites to which the receiver 10 has been a~ ,luolli~d, since the alm~n~c data from each satellite jn~ludes an estimated position of all other operating s~tlollites However, if this ~lmqn~c data is notavailable, such as when the receiver 10 is first turned on, this estimate may bedeterrnined by succ~ccive applo~i...alion t~ s, as desclibcd in the afole.l.e,.lioned U.S. Patent No. 5,101,416.
The Doppler value is s~,e~ ed in carrier Doppler cycles per Fs pulse. For example, if the expected Doppler frequency is +4.45 kiloHertz (kHz), which is a possible Doppler frequency for a stationary receiver and an approaching satellite, dividing by a typical Fs frequency of 20.46 MHz for the GPS L I embodiment results in an expected Doppler shift of a~ o~i-,-alely 0.00044 carrier cycles per Fs pulse. Specified in this way, the Doppler value will always be less than one.
The ADR register 222, which provides an estim7te of the range to the satellite being tracked, is divided into a whole cycle portion 222w and a partial cycle portion 222p. As shown, an adder 223isa. ~ngcd to add the contents of the Doppler register 221 to the partial cycle portion 222p of the ADR 222 upon the occu~,~nce of every Fs pulse. The most signific~rlt bits 0, 1, ..., p of the partial SUBSTITUTE SHEET
WO 95/14937 21~ 2 9 2 6 PCT/CA94/00640 cycle portion 222p thus provides an ;ncl~ ~aneous e~ected carrier phase angle in cydes.
When the partial cycle register 222p has a carry out, the whole number portion 222w is il~W~ ed and the fine chip counter 224 is also i,~cr~ led If the partial cycle register 222p requires a borrow, then the whole number portion222w and fine chip counter 224 ue de~ n .led The subchip counter 226a is clocked by the Fs signal and controlled by the fine chip counter 224. Subchip counter 226a is nomin~lly a 0 to N-l counter controlled directly by the Fs signal, but may be adjusted to count one extra cycle or one fewer cycle depe..d;i,g upon the state of the fine chip counter 224. In particular, when the fine chip counter carries out, i.e., h,cl~.,lenls from K-l to 0, a cycle is stolen from the sub chip counter 226a to keep it sylll,hl oni~id with the ADR 222. In other words, this event causes the subchip counter 226a to count only to N-2 ~or one iteration.
When the fine chip counter 224 bollOwa, i.e., decl.,ll,e.,ls from 0 to K-l, a cycle is added to the subchip counter 226 so that it counts from 0 to N for one iteration.
By periodically removing or adding one cycle of the sample clock Fs, the locally generated PRN code (as controlled by the output signals RST and CLK of code phase generator 226) remains synchronized with the locally generated carrier phase (as i.,~ ed by the state of the ADR 222). With this a~ ~ ,.ng~ en~, as long as the carrier phase indic~ted by the ADR 222 remains locked to the inco,n;"~, carrier, the code phase generator 226 will remain locked to the incoming PRN
code. This is accomplished non-coherently, in the sense that the local left;re.,ce signal, Fs, need not remain phase locked to the carrier of the inte""ediate frequency signal, IF, in order for the PRN code generator 230 to remain phase-locked.
The most cignifir~nt bit of the subchip counter 226a is used as a clock signal, CLK, to indicate a PRN code chip edge. In the pref~ d embodiment for SU~STITUTE SHEET
the GPS Ll carrier, the subchip counter 226a counts from zero to ~ineteen since N equals twenty, i.e., there are twenty Fs cycles per PRN code chip (Fig. 5).
The chip counter 226b is used to det~ "c the duration of a cornrle~e PRN code s~ .P .fe. For the GPS e ~ there are 1,023 C/A code chips in - a PRN code epoch, and thus the chip counter 226b counts from zero to 1022.
The most c;gn;lir~ bit, INT1 h..l;. ~.~c the end of a co~plele PRN code epoch tothe p-ucessor 16; it is also used to reset the local PRN code generator 230.
Another clock signal, INT4, which is four times the rate of INT1 (i.e., the third most cignifi~ ~nt bit of the chip counter 226b) is also generated. Both INTI andrNT4 may be used to ;.~ u~l the p, ucei~or 16 to service the correlators 240 during an initial locking se~u....c~ as will be de cl ibcd shortly Finally, the epoch counter 226d is used to indicate the end of a data bit, after T PRN code epochs. This i...i;caliol- is given by the most significant bit of the epoch counter 226d, which is output as the INT20 signal.
The carrier tracking loop is inherently much more sensitive than the code DLL and is able to l--easu,~; small changes ~ ,~I,e.,.ely accurately. Assuming ~he carrier loop is tracking pl ope.ly, the fine chip counter 224 in conjunction with the subchip counter 226a, enables the channel 22n to accurately track any relative motion of the receiver 10 with respect to the satellite.
By returning now to Fig. 2, the operation of a typical receiver channel 22n will be understood in greater detail. A carrier phase shifler 235 accepts the lS and Qs samples, along with the ;..~ cO~Ic carrier phase bits ~ 2, ., ~p, from the sync}lon;~er 220. The carrier phase shi~er 235 then phase rotates the Is andQs samples by an amount inriic~ted by the i~ rlcous carrier phase angle estim~te generated by the s),llcl,i ûni~el 2Z0, and prûvides outputs ID and QD
acco~ding to the following e,.l"e~;ons:
ID=IS cos(II) + Qs sin(II) QD=Qs cos(II) - Is sin(rI) SUBSTITUTE SHEET
6~,16 2 ~ ~ ~ 14 -where PRN is the current vaiue of the PRN CODE input and II is the ;..c~ tQuS carrier phase ~ e ,~ese~lled by the bits 7~ 2, ..."~p Since the Is and Qs sampie~ are in digital fonn, this phase shift opc.atiol- is ~e.ru.~,ed by appropriate digitai circuits. By removing the i~ ni .~us carrier shift in thesame op.,. alion at every Fs clock pulse, signais with very high Doppler offsets may be pl~,cesscd before any ~;~.;r.~ power loss is e nr,o~ t~
The shift register 250 I-,cei~es the PRN code signal from the PRN code generator 230 and generates a pluraiity, m, of time-shifted PRN code replica signals PRNt I, PRNt2, ..., PRNtm. The time shifts i...~ ed to the PRN code replica signals are typically d;~Llib-lLed about an expected code time delay, in a manner which will be desc, il,ed below in co~.ne.~Lion with Figs. 6 through 10.
The time shifted PRN code replica signals PRNtl, PRNt2, ... PRNtm are then fed to ~yecLi~re ones ofthe correlators 240-1, 240-2, ... 240-m. Each correlator 240-i in turn feeds the i..co...;..g phase-shifted samples ID, QD and the rei~yec~ e PRN code replica PRNti ~ciened to that correlator, to a co..,~,leA
multiplier circuit. As shown in Fig 4, the rnl~ltirlier circuit inrlude5 a pair of multipliers 245I-i, 245Q-i A pair of accum~ tors 247I-i and 247Q-i perform a low fre~uency filtering function on the correlated ID and QD data, by simple accumulation of succ~ssive samples, to produce averaged correlator outputs Ij and Qi The disc.h..;..ators 260 and 265, typically ;.l,ple.l.ented by prog,~l,...~mg the processor 18 (Fig I ) operate on the outputs of the m correlators I 1, Q I, I2, Q2 , Im, Qm to deterrnine the carrier and PRN code drift When a difference in the locally generated PRN code and the received code is detecterl, the a~...,l.,u.li~cr 220 is co.l~ cd by adjusting the internal values in its counters, 222, 224 or 226~ or Doppler register 22l. The dis~;l;lll;llators 260 to 265 and a~ ,h~ o,fi2er 220 thus co.n~,'ete a delay lock loop (DLL).
In an initial synchlon;~lion mode, the (early, late) dis~.i,..;..ator 260 is used. this dia~ l ;...inator 260 makes use of the fact that when the s~ ,l.ron;~;l 220 8U4SrITUTE SHEET
W O 95/14937 ~16 2 9 ~ b PC~rtCA94/00640 is exactly in phase, one of the COIl ~,lalc. ~ 240 which is early by a certain fraction of a PRN code chip time is ~rected to have the same output power as another co~ lalor 240 which is late by the same fraction of a PRN code chp time.
A collJàlor output selector 261 thus selec,ts one of the m pairs of - co. I ~lalor outputs as the early co., ~,lalor outputs, IE, QE and another pair as the late co,l~lalor outputs IL, QL. The early co,.~làlor output lE, QE is pref~,.ubly taken from one of the co..~lalo.~ 240 having a PRN code replica signal PRNt;
which was e I-e~lcd to be early by one-half a PRN code chip time. The late correlator outputs IL, QL are taken from a correlator 240 which is eA~ecLed to be late by one half a chip time. Thus, the time delay spacing of the early and latecorrelators 240 in this initial ~lcq..;~;~;on mode is one chip time.
Next, the PRN code for the desired satellite is loaded into the PRN code generator 230 via the SEL lines. ~11 pGSaiblC r, e~"~e~.~ if S and code phase delays are then succecsively tried in an attempt to obtain frequency and code lock withthe satellite signal received from the ~ ned to the channel 22. In particular, the carrier delays are swept by trying ~Lrre.~.lt EDOPP values. Different code delays are swept by ~djustin~ the code counters 224, 226a, and 226b via the buffers 227, 228, and 299. At each code and frequency offset, the outputs IE, QE and IL, Q~, are read and a power level is c~lc..l~ted by the disc.i...;nalor 260, such as bydetern~ining the square of the dia~l ~"ce in their powers. This di~e~ el~ce is con.yal ed to a predetermined threshold to determine whether the satellite has been locked ont or not. If lock is not indic~te~, the next carrier and code phase aretried.
The dis-,-;---;nator 260 must be allowed to dwell for an approp,iale time at each code and carrier delay. When sea. cl.i..g for strong satellites, where the signal to noise ratio is above 45dBHz, a dwell time as short as 1/4 of a PRN code epochiâ used. For weaker s?tPllitec~ a dwell time app.oxi..,ately equal to the PRN code epoch time is used.
Alte~llali~ely~ a Ji~.e.~t, so-called dot-product disc.i...;..~lor 265 may be used. in this mode, another selector 264 selects three of the correlators output - CUBSl I I UTE SHEET
W O 95tl4937 ~ ~ 6 2 9 ~ ~ 16 - PC~r/CA94/00640 pairs as y~ l (P), early (E), and late (L). The dis.";...:u~or function of interest in this c~ lator is IE L Ip + QE-L- QP
which is why it is referred to as a dot-product disel;.,ui~a~or. In general, carrier phase tracking is better with respect to noise in the dot-product mode because of the availability of a pllnchl~l power estimqte~ which has a greater signal strength in the p.~3~..,ce of noise.
After initial code locking, the delay be~ .. the wly (E) and late (L) correlators may be slowly dec, ~sed by cl-~r.g;.~g which of the correlators 240 are selected as the early and late correlators. By na" uw;ng the DLL delay in this manner, the noise level of the disc,;..~-n-~;orl function p~ . ro"--ed by the power es1im~for 265 is de~ ased, and its accuracy is increased.
Both of these discl i,,unalors 260 and 265 operate under the assumption that the co" ~lalion function is ideal, that is, that it is symrnetric and will peak at or near the position of the punctual correlator. As indicated by the curve 300 shown in Fig. 6, this is indeed true for the direct path signal, which will have a peak power at a time offset equal to zero code chips. In this ideal situation, the co-,~lalion power decreases linearly as the offset increases, in both the negative and positive directions, until the magnitude of the offset equals zero at a timedifference of plus or minus one code chip time. The amplitude scale in Fig. 6 isnormalized to a peak power equal to one ( 1).
However, this ideal situation will only occur if there are no multipath signals present. Since the number of rm~ltiratll signals present at any given location is a function of the satellite and antenna position rela~ive to any reflectin objects, anywhere from zero to several individual multipath signals may be present at any given time.
In some ;~ nces~ the CO-I. Ialion between a multipath signal and the local , efc. ~nce PRN signals will have a similar triangular shape. The shape is the same because the mllitipath signal is identic~l to the direct path signal but takes aslightly di~.~..l, longer path to the receiver. The curve 302 ,ep-~sel-ts the SUBSTITUTE SHET
WO 95/14937 216 2 9 ~ ~ PCTICA94/00640 cu,l~,lalion b~l..~.l the same ~,fc,~,nce PRN signal and a ""J~ .alh signal having aD ~ditiongl p, Opo~1;QI~ delay, r~,p.~ 3- ~t;~g the e ~lition~l time required for the lh signal to reach the receiver. In the CAal~ C shown, the ~ lition~
p,o~a~,~l;on delay is 0.2 chips.
Since the .. II;p~lh signal also arrives at the receiver, there will always be some co,r~,lalion with both the direct path signal and the "...1l;~, 1h signal. The ~,sul~ , coll.,ldtioD r~....l;n~ is thus the sum ofthe direct path c~ hlion 300 and .alh coll.,làlion 302, yeildng an acutally obs_.ved cc~ lalion ru -~l;on 304 As is event from the curve, in the pre~ense of such a mllltipath signal, the observed coll~,lation function 304 deviates from that of the direct path signal, which is the signal which the receiver channel 22n is actually attc..")th,g to lock onto.
In the e,.aln~lc shown in Fig. 6, the mllltir~th signal was ~c~ ~. ed to have a di~rentiàl phase of zero degrees, that is, the phase as that of the direct path signal. In this ;..Cl~ ~ce the peak of the correlation plot 304 still occurs near the zero offset, and the arorc.l,e..tioned power estim~tors 260 and 265 may still tend to find the co, I ~lalion peak at or near the correct offset. However, in other h.c~ ces, this is not the case.
In particular, it is also quite possible that in the act of being reflected by an object, the carrier of the multir~th signal will have had a relative phase difference imparted upon it which is di~le,.l from the phase of the carrier of the direct path signal. The amount of this carier phase offset depends upon the exact position of the reflectin~ object with respect to the satellite and the receiver. The effect on the non-coherent receiver channel 22n which has been des~libed herein is that the mllltip~th co"~lation signal will in effect be multiplied by a const~nt eaqual to the cosine of this phase di~I~l ence.
Thus, m~yimllm values for a mllltipath co.l~,lalion signal will occur when - the relative carrier phase di~. .,nce is 0 or l 80, and a minirmlm col I .,lation values will occur when this phase .I;~.~,nce is 90 or 270. In Fig. 6, this carrier phase difl~ ce was ~sl.med to be zero, and thus a positive co~l~lation existed n the mllltip~th and direct path signals. However, as shown in Fig. 7, when the mllltip~th signal has a carrier phase .liÇ~ ce of 180, a negative co"~lalion 6U~ I ITUTE SHEET
wo 95/14937 2 ~ ~ 2 9 2 18 - PCT/CA94/00640 with the le~.hlcc signal will result, since the cosine of 180 is minus one. The ;y~ correlation r~ on 306 thus has a nc~ mp~itude peak, and the sum of the direct path and multipath COIl~laliOllS 308 no longer peaks near zerotime offset, and has 5i~;fir~ shape distortion.
This in turn means that the early, pu~ udl~ and late CO" ~làlo. ~., as p,ucessed by the ul;;,e,;...;..~tc"s 260 and 265 alone, may not - le ~ t~ly track the PRN code of the desired direct path signal. The early minus late d;s~"..,.."alor260, in particular, ac5,~."~s that the power at the early and late collJalor loc-l;onc is the sarne, and that the dot product disc,;.";.,alor 265 assumes that the power at the output ofthe puncutal co,relàlor is the pGS;Iiol~ of mq~rimum co"lalion power.
Thus, ~lthough the DLL llack;l)g loop formed by the dis.,lhllàlors 260 and/or 265 may be adequate for intial code phase locking, they may in some ces tend to urge the receiver 22 to lock to a position which is offset from the actual position. This in turn leads to errors in the measurement of the propagation time from the C~trllite~ and ultimately, errors in the c~lcul~ted position of the receiver l O.
In the e,~ .pl~.y PRN ranging system being dic~s,ed the inco..~ii.g signal is band limted by the sampling rate of the channel 22n, which further e~ects theshape of the correlation function. That is ,na~ .led by a rounding offof the peaks in the correlation function, and a shift in the positive dir,ection. Taking into consideration the limited bandwidth, a~)proAh"ately 8 MHz in this embodiment, the actual correlator I es~,onse will thus be somewhat smoothed out and shifted in time, such as that shown in Fig. 8.
As better seen in the ...~g~ ed view of Fig. 9, the early, punctual, and late correlators will tend to lock onto a postion which is offset in the positive time direction by appro~;ll,alely 0.05 chips in this eAa,.,ple. This, in turn, leads to a range mea~llre.,.~nt error of the same m~gnit~ld~, which may mean a clict~nce error of more than 10 meters in the eAalllple shown.
~UBSTITUTE SHEET
~l6~9~3 WO 95/14937 PCr/CA94/00640 tionql ones ofthe multiple col,~laloJ~ 240-1, 240-2, ..., 240-m can ~us be used to generate a more accurate e;.~ e of the sl.ape of the t~ posite ;pa~h and direct path co.l~,lalion signal, which in turn can be used to develop better update cs~ al~s for the PRN code DLL and carrier phase tlacLllg loops.
To ul~dc~ d this further, CQI~ der that signals generated in the channel 22n, even in the pl~s~,nce of ,,..~II;p~ can be pred;.,led to follow a msthe ..-~ir~l model. ~AC~.. ;.~g that the channel 22n makes use of a non-coherent carrier and code ~y~chlo~ c;t 230 to adjust for the carrier and Doppler error as de~,l;l,ed above, then the colll~on~.lls of the inputs to the corre!ators ID and QD due to the direct path signal can be e,~,.eised, rcs~,ecli~ely, as I = A Rf(~k) CS(~k) Q = A Rf(~k) sin (~k) where A is an amplitude factor depe.ld;l,g upon the signal to noise ratio at theinput to the receiver, Rf~t) is the unfiltered PRN code autocol,elalion function, ~k is the code tracking error at time tk, and ~k is the residual phase tracking error also at time tk. The contribution of a given multipath signal can thus be e~y~ essed as:
Im = Rf~k-~) cos(~m + ~k) Qm = Rf~k-o) sin(~m + ~k) where ~ is the relative time delay of the mllltip~th signal, and ~m is the relative phase of the m~kirath c~"l~,on~ carrier.
SOBSTITUTE SHEET
wo 95/14937 '~16 ~ 9 2 3 PCT/CA94/00640 By a~pljil~g these equal;ol~s to the e~presa;ol~ for the dot-product disc~ or 2t'5, its output can be ...odclled as:
D~k = {Rf(~k - d/2) - Rf~k + d/2)} Rf(~k) +
a2 ~Rf(~k - d/2 - ~) - Rf(lk + d/2 - ~)} Rf(~k - ~) +
a {Rf~k - d/2) - Rf(~k + d/2)} Rf(~k - ~) cos(~m) +
a ~Rf(Tk - d/2 - o) - Rf(~k + d/2 - ~)} Rf(~k) cos(~m) with d being the spacing bel~ n the early and late co" clatola in PRN code chips A similar e"~ sion can thus be developed in the same manner for the I;
and Qj signals generated by an exemplary col,~latur 240-i in le~yonse to each one of the plurality, j, of multipath signals:
Ii =A [R,(~ +tj) cos(~) + ~j R~(rk +tj +~j) cos(~k +~mi)]
Qj A [R,(rk +tj) sin(~k) + ~aj-Rt(Tk +tj +~j) sin(~k +~7j)]
~ - o where A is the relative amp!itude of the direct path co.,.ponent, Rf is the cross col,~lation between the incoming filtered signal PRN code and the unfiltered reÇercnce code, tj is the re~,tnce code offset of correlator 240-i, ~k is the code tlackillg error at time tk (also referred to as epoch k)-~k is the carrier phase error at epoch k, aj is the relative amplitude of mllltipath component j, ~j is the delay of muhipath col"~one.l~ mj is the phase of .,.~ h compolle~l~ j, and n is the number of multipath co.npon~,.-ls. A measured orrelator power output, Pj, can bedete,..ulled for each correlator 240-i as SU8STITUTE SHEE~
WO 9S/14937 2 16 2 9 2 ~ PCT/CA94/00640 Pj = ~II2 + Q2, from which the actual coll~laLiion curve shown in Fig. 8 can be de~ ul~ed.
Fig. 10 is an e r ~ Y plot ofthe same COI I~hliOn signal shown in Fig. 8, ~LUWillg how in the case of m equal to 19, the t; for each of the nineteen correlators may be evenly di~ ulcd across the col l ~lation time scale. Thus, bydistributing the spacing of the COll eld~ul ~ 240 with di~el e.ll code offsets t;, and then s~-,p!i~ the outputs of the multirle correlators 240-1, 240-2, ..., 240-n, the above system of equations can be formed to solve for the aj, âj, and ~mj of each~nullipath Colllpollc.lL, as well as the code tlaChillg error ~k carrier phase error ~k of the direct path signal.
The system of equqtionc cn be solved in any suitable fashion by the p~ aul~ler estim~or 270 (which is also typically embodied in so~ware for the plûcessor 18). For tAa ..~le, least squares estimation, m~imllm likelihood e~lhllaLion, and other signal parameter estimation techniques which are known inthe art may be used.
Once the set of simlllt~neous equations is solved for code tracking error k and carrier phase error ~k of the direct path signal, the estimates of the values of these parameters can be used by the system in several different ways.
In one approach, the tlachillg loop r'~m~nted by the disc~ ors 260 and/or 265 can be left in place, and the code tracking error ~k can be used as afinal correction to the range estim~e~ as ..~ ;..ed by the carrier and code ,cl,ror~ l 220.
2162~2~
wo 95/14937 - 22 - PCr/CAg4/00640 Alternatively, ~c,~,.n;.~g the paral,..;ter estim~tor 270 can perform the n~ceiS,..y ç~lcnl~tions quickly enough, the code tracking error ~k and-carrier phase error ~k may be fed back to control the carrier and code s~--,lu ~,.u~r 220.
In particular, the carrier phase error, ~k, can be added by the p.ocessor 18 back into the range register 222, and the code tracking error Ik can be added into the PRN code register 226.
Also, de~e-..t;--g upon the system ~e.~uh~ c.lts~ the multiple correlators 240 may be implen-~nted in difI~.e.lt ~ays. One approach would be to have the physical number of correlators in each channel equal to m. This approach might be neceS~A.y in applications where the receiver is subjected to env;,-~l""e.,ls where the mnltipath changes rapidly.
However, in most hic~ cc ~, once intial code lock is aquired, the correlation power estimqtPs can be ~cumed to be stable for at least several seconds at a time. Thus, there may be as few as two physical correlators per channel, as long as the co..~lalo-~ have adjustable code delay spacing. The codedelay spacing may be sequenced from epoch to epoch, so that over time, measurements of the correlation power at each one of the m time offsets (tj's) of interest are taken.
In another approach, all of the correlators in all of the ch~nn~lc 22a, 22b, ..., 22n can be periodically set to track the same PRN code for a certain period of time. All channels 22 can then be sequenced through the PR~ codes for each satellite of interest, so that over time, all desired Ij and Qj signals will be determined.
Finally, the majority of the ch~nnels 22 in a receiver may be left to operate in the dis.,. ;",inators mode, while one or more of the cl,annels 22 is dedic~ted to continually sequencing from channel to channel to SU~ IenI the correlation parameter estim~tes for each satellite being tracked.
8UBSTITUTE S~IEET
range ...e~ n~ to be taken. The op~ ion ofthe ~)roc~;,ol 16 and its various r~ nc irnplern~nted in software will be better u~du.~loou from the following on The cGl"pG~;le signal Cs rc~i~cd from the ~nt~nn~ 1 I typically consists of signals ~ c..~ d by all s~ lI;t~ S within view (that is, within a direct line-of-sight of the l~,cei~. 10), any i.~ .ing signals, such as ,~ . signals and noise.
The carrier signal used by the GPS C/A ranging system is an ~band carrier at 1.57542 GigaHertz (GHz) with a PRN code rate of 1.023 I-ffIz and a nominal ed power of-160 dBW. Natural bacl~.u"d.noise at about -204 dBWlHz is typically mixed in with the L-band signals. In aA~ition, one or more ,a~h signals are present in the composite signal Cs, as will be desc.i~ed below. For more detailed i,~. ..,alion on the forrnat of the GPS system signals,see "Interface Control Doc,l...en~ ICD-GPS-200, September 26, 1984", p~1b!ished by Rockwell Intemational Col~olation, Satellite Systems Division, Downey, California 90241.
Fig. 5 shows, on a ~ Io~ led time scale, the relative durations of various co."l)on~"ls of a typical PRN ranging signal Ir~c~ ed by a GPS satellite and certain of the signals in a pi efe" ed embodiment of a channel 22n. A single carrier cycle has a particular duration, C. A single cycle of the digital sample signal clock Fs, consists of K carrier cycles.
A PRN code chip incl~ldes N cycles of the Fs signal, and a PRN code epoch consists of Z PRN code chips, where Z is also Icnown as the sequerlcc length of the PRN code. One data bit typically conslsts of T PRN code epochs.
For the p~ e~ d embodiment of the invention adapted to receive the GPS Ll ranging signal, the carrier frequency is 1575.42 MHz, and K is 77, so that Fs equals 20.46 MH~. In addition, a con~ N is 20, so that the PRN code chip rate is 1.023 MHz, and Z is 1023, so that the PRN code epoch rate is 1 kHz. T, another con~ , is also 20, so that the data bit rate is 50 Hz.
A channel circuit 22n is shown in detail in Fig. 2. It include5 a carrier and code s~"cl-ro,u,e- circuit 220, a PRN code generator 230, a carrier phase shi~er23~, ll~l.ll;~Jlc correlators 240-1, 240-2, 240-3, ..., 240-m (collectively, correlators SUBSTITUTE SHEET
WO 95/14937 ~16 2 9 2 6 - PCT/CA94/00640 240), a code shift register 250, an early minus late (or early, late) J;sc. i,.u..ator 260, a dot prodl;ct (or p~ uol, early minus late) discriminator 265, and a mllhipath p~u~ ler ectimotor 270.
The PRN code ~..e.ator 230 uses signals output by the ~....I..onlLer 220 to ~cn~,.dte a local PRN ~ nce signal, PRN CODE. The p~ ,uL PRN
CODE signal generated at any given time depends upon which satellité it is desired for the channel to be tuned to, as sPIected by the SAT II) input. PRN
code gcnc.dlol~. such as codé generator 230 are well known in the art.
The s~ncl-- oni~er 220 is a single numerically controlled osrillotor (NCO) which uses the sample clock Fs and a~.~Jrop. iale in ,l. uc~ions from the processor 16 to provide the control signals re~u;,~,d by PRN code gene.~10r 230 and correlators 240 in the carrier or code carrier phase to non-coherently track the errors caused by residual Doppler and multirath distortion.
Before contin..in~ with a detailed ~I;c~,..c~:~n ofthe chaMel 22n, refer to Fig. 3, which is a detailed block diagram ofthe carrier and code s~...,l..o..i~;r 220.
This element jnrl1ld~s an expected Doppler rate register 221, an accum~loted delta range (ADR) register 222, and a fine chip counter 224. A code phase generator circuit 226 also inrludes a subchip counter 226a, chip counter 226b, epoch counter 226d. Buffer circuits 227, 228, and 229 allow the processor 16 to load, read, and add to or subtract from the contents of the various counters andregisters in the synchroni~ r 220.
The s~". h.or.;~er 220 accepts the sarnple clock signal Fs from the channel bus 20, an expected Doppler value EDOPP and col ~ ~cled values for the registersand counters 222, 224 and 226 from the control bus 18. In response to these inputs, it provides a clock signal CLK and reset signal RST to the PRN code generator 230, as well as in~..ul,t signals INT1, INT4, and INT20 to the controlbus 18. An in~A~qneQus carrier phase angle Pctimqte is also provided via bits ~o, J~ 1. .7~p to the correlators 240.
The contents of the ADR register 222 and code phase generator 226 provide an i..cl~..l;.neous estimqte of the l,dn ,.,.;l time of the particular satellite SUE~STITUTE SHEET
- W 095/14937 ~ 1 6 2 9 2 6 PCT/CA~ ^6 signal ~C,;~rd to channel 22n. The di~ ,..ce b_h.'~,CIl this e~timqte of the ll~.~.~ul time and the ~ ~,cei-e. time of day (as estimated by the timer 168 in Fig. 1) is then taken as the propa~ iol~ tirne of that signai plus any I cce;~e- clock offset.
By .~ lyulg the p~ u~a~,alion time by the speed of light, a precise ~ bul enlenlofthe range from the r~cei~,~. 10 to the ~c~ign~d satellite may then be made by the pr~cessor 16. These ll~ ule.ll~ occur at selected time i...~ d by the ~ul~,l,.,nl strobe MEAS from the timer 168, and are t,ypicaiiy taken .eoul~ across ail the cl~ ~n~lc 22. The t~S~Illu~E~ range to each satellite is then used by the p.ocessor 16 to compute the position and or velocity of the receiver 10.
In operation, the expected Doppler rate register 221 is first loaded via the proces~or bus 18 with an ~stimqtPd Doppler offset EDOPP for the particular sateliite tracked by channel 22n. In most ;l.~l~n~CS, such as when the receiver 10 has been operating for some time, the EDOPP Pstimqte may be taken from ~lmqnqc data already received from satellites to which the receiver 10 has been a~ ,luolli~d, since the alm~n~c data from each satellite jn~ludes an estimated position of all other operating s~tlollites However, if this ~lmqn~c data is notavailable, such as when the receiver 10 is first turned on, this estimate may bedeterrnined by succ~ccive applo~i...alion t~ s, as desclibcd in the afole.l.e,.lioned U.S. Patent No. 5,101,416.
The Doppler value is s~,e~ ed in carrier Doppler cycles per Fs pulse. For example, if the expected Doppler frequency is +4.45 kiloHertz (kHz), which is a possible Doppler frequency for a stationary receiver and an approaching satellite, dividing by a typical Fs frequency of 20.46 MHz for the GPS L I embodiment results in an expected Doppler shift of a~ o~i-,-alely 0.00044 carrier cycles per Fs pulse. Specified in this way, the Doppler value will always be less than one.
The ADR register 222, which provides an estim7te of the range to the satellite being tracked, is divided into a whole cycle portion 222w and a partial cycle portion 222p. As shown, an adder 223isa. ~ngcd to add the contents of the Doppler register 221 to the partial cycle portion 222p of the ADR 222 upon the occu~,~nce of every Fs pulse. The most signific~rlt bits 0, 1, ..., p of the partial SUBSTITUTE SHEET
WO 95/14937 21~ 2 9 2 6 PCT/CA94/00640 cycle portion 222p thus provides an ;ncl~ ~aneous e~ected carrier phase angle in cydes.
When the partial cycle register 222p has a carry out, the whole number portion 222w is il~W~ ed and the fine chip counter 224 is also i,~cr~ led If the partial cycle register 222p requires a borrow, then the whole number portion222w and fine chip counter 224 ue de~ n .led The subchip counter 226a is clocked by the Fs signal and controlled by the fine chip counter 224. Subchip counter 226a is nomin~lly a 0 to N-l counter controlled directly by the Fs signal, but may be adjusted to count one extra cycle or one fewer cycle depe..d;i,g upon the state of the fine chip counter 224. In particular, when the fine chip counter carries out, i.e., h,cl~.,lenls from K-l to 0, a cycle is stolen from the sub chip counter 226a to keep it sylll,hl oni~id with the ADR 222. In other words, this event causes the subchip counter 226a to count only to N-2 ~or one iteration.
When the fine chip counter 224 bollOwa, i.e., decl.,ll,e.,ls from 0 to K-l, a cycle is added to the subchip counter 226 so that it counts from 0 to N for one iteration.
By periodically removing or adding one cycle of the sample clock Fs, the locally generated PRN code (as controlled by the output signals RST and CLK of code phase generator 226) remains synchronized with the locally generated carrier phase (as i.,~ ed by the state of the ADR 222). With this a~ ~ ,.ng~ en~, as long as the carrier phase indic~ted by the ADR 222 remains locked to the inco,n;"~, carrier, the code phase generator 226 will remain locked to the incoming PRN
code. This is accomplished non-coherently, in the sense that the local left;re.,ce signal, Fs, need not remain phase locked to the carrier of the inte""ediate frequency signal, IF, in order for the PRN code generator 230 to remain phase-locked.
The most cignifir~nt bit of the subchip counter 226a is used as a clock signal, CLK, to indicate a PRN code chip edge. In the pref~ d embodiment for SU~STITUTE SHEET
the GPS Ll carrier, the subchip counter 226a counts from zero to ~ineteen since N equals twenty, i.e., there are twenty Fs cycles per PRN code chip (Fig. 5).
The chip counter 226b is used to det~ "c the duration of a cornrle~e PRN code s~ .P .fe. For the GPS e ~ there are 1,023 C/A code chips in - a PRN code epoch, and thus the chip counter 226b counts from zero to 1022.
The most c;gn;lir~ bit, INT1 h..l;. ~.~c the end of a co~plele PRN code epoch tothe p-ucessor 16; it is also used to reset the local PRN code generator 230.
Another clock signal, INT4, which is four times the rate of INT1 (i.e., the third most cignifi~ ~nt bit of the chip counter 226b) is also generated. Both INTI andrNT4 may be used to ;.~ u~l the p, ucei~or 16 to service the correlators 240 during an initial locking se~u....c~ as will be de cl ibcd shortly Finally, the epoch counter 226d is used to indicate the end of a data bit, after T PRN code epochs. This i...i;caliol- is given by the most significant bit of the epoch counter 226d, which is output as the INT20 signal.
The carrier tracking loop is inherently much more sensitive than the code DLL and is able to l--easu,~; small changes ~ ,~I,e.,.ely accurately. Assuming ~he carrier loop is tracking pl ope.ly, the fine chip counter 224 in conjunction with the subchip counter 226a, enables the channel 22n to accurately track any relative motion of the receiver 10 with respect to the satellite.
By returning now to Fig. 2, the operation of a typical receiver channel 22n will be understood in greater detail. A carrier phase shifler 235 accepts the lS and Qs samples, along with the ;..~ cO~Ic carrier phase bits ~ 2, ., ~p, from the sync}lon;~er 220. The carrier phase shi~er 235 then phase rotates the Is andQs samples by an amount inriic~ted by the i~ rlcous carrier phase angle estim~te generated by the s),llcl,i ûni~el 2Z0, and prûvides outputs ID and QD
acco~ding to the following e,.l"e~;ons:
ID=IS cos(II) + Qs sin(II) QD=Qs cos(II) - Is sin(rI) SUBSTITUTE SHEET
6~,16 2 ~ ~ ~ 14 -where PRN is the current vaiue of the PRN CODE input and II is the ;..c~ tQuS carrier phase ~ e ,~ese~lled by the bits 7~ 2, ..."~p Since the Is and Qs sampie~ are in digital fonn, this phase shift opc.atiol- is ~e.ru.~,ed by appropriate digitai circuits. By removing the i~ ni .~us carrier shift in thesame op.,. alion at every Fs clock pulse, signais with very high Doppler offsets may be pl~,cesscd before any ~;~.;r.~ power loss is e nr,o~ t~
The shift register 250 I-,cei~es the PRN code signal from the PRN code generator 230 and generates a pluraiity, m, of time-shifted PRN code replica signals PRNt I, PRNt2, ..., PRNtm. The time shifts i...~ ed to the PRN code replica signals are typically d;~Llib-lLed about an expected code time delay, in a manner which will be desc, il,ed below in co~.ne.~Lion with Figs. 6 through 10.
The time shifted PRN code replica signals PRNtl, PRNt2, ... PRNtm are then fed to ~yecLi~re ones ofthe correlators 240-1, 240-2, ... 240-m. Each correlator 240-i in turn feeds the i..co...;..g phase-shifted samples ID, QD and the rei~yec~ e PRN code replica PRNti ~ciened to that correlator, to a co..,~,leA
multiplier circuit. As shown in Fig 4, the rnl~ltirlier circuit inrlude5 a pair of multipliers 245I-i, 245Q-i A pair of accum~ tors 247I-i and 247Q-i perform a low fre~uency filtering function on the correlated ID and QD data, by simple accumulation of succ~ssive samples, to produce averaged correlator outputs Ij and Qi The disc.h..;..ators 260 and 265, typically ;.l,ple.l.ented by prog,~l,...~mg the processor 18 (Fig I ) operate on the outputs of the m correlators I 1, Q I, I2, Q2 , Im, Qm to deterrnine the carrier and PRN code drift When a difference in the locally generated PRN code and the received code is detecterl, the a~...,l.,u.li~cr 220 is co.l~ cd by adjusting the internal values in its counters, 222, 224 or 226~ or Doppler register 22l. The dis~;l;lll;llators 260 to 265 and a~ ,h~ o,fi2er 220 thus co.n~,'ete a delay lock loop (DLL).
In an initial synchlon;~lion mode, the (early, late) dis~.i,..;..ator 260 is used. this dia~ l ;...inator 260 makes use of the fact that when the s~ ,l.ron;~;l 220 8U4SrITUTE SHEET
W O 95/14937 ~16 2 9 ~ b PC~rtCA94/00640 is exactly in phase, one of the COIl ~,lalc. ~ 240 which is early by a certain fraction of a PRN code chip time is ~rected to have the same output power as another co~ lalor 240 which is late by the same fraction of a PRN code chp time.
A collJàlor output selector 261 thus selec,ts one of the m pairs of - co. I ~lalor outputs as the early co., ~,lalor outputs, IE, QE and another pair as the late co,l~lalor outputs IL, QL. The early co,.~làlor output lE, QE is pref~,.ubly taken from one of the co..~lalo.~ 240 having a PRN code replica signal PRNt;
which was e I-e~lcd to be early by one-half a PRN code chip time. The late correlator outputs IL, QL are taken from a correlator 240 which is eA~ecLed to be late by one half a chip time. Thus, the time delay spacing of the early and latecorrelators 240 in this initial ~lcq..;~;~;on mode is one chip time.
Next, the PRN code for the desired satellite is loaded into the PRN code generator 230 via the SEL lines. ~11 pGSaiblC r, e~"~e~.~ if S and code phase delays are then succecsively tried in an attempt to obtain frequency and code lock withthe satellite signal received from the ~ ned to the channel 22. In particular, the carrier delays are swept by trying ~Lrre.~.lt EDOPP values. Different code delays are swept by ~djustin~ the code counters 224, 226a, and 226b via the buffers 227, 228, and 299. At each code and frequency offset, the outputs IE, QE and IL, Q~, are read and a power level is c~lc..l~ted by the disc.i...;nalor 260, such as bydetern~ining the square of the dia~l ~"ce in their powers. This di~e~ el~ce is con.yal ed to a predetermined threshold to determine whether the satellite has been locked ont or not. If lock is not indic~te~, the next carrier and code phase aretried.
The dis-,-;---;nator 260 must be allowed to dwell for an approp,iale time at each code and carrier delay. When sea. cl.i..g for strong satellites, where the signal to noise ratio is above 45dBHz, a dwell time as short as 1/4 of a PRN code epochiâ used. For weaker s?tPllitec~ a dwell time app.oxi..,ately equal to the PRN code epoch time is used.
Alte~llali~ely~ a Ji~.e.~t, so-called dot-product disc.i...;..~lor 265 may be used. in this mode, another selector 264 selects three of the correlators output - CUBSl I I UTE SHEET
W O 95tl4937 ~ ~ 6 2 9 ~ ~ 16 - PC~r/CA94/00640 pairs as y~ l (P), early (E), and late (L). The dis.";...:u~or function of interest in this c~ lator is IE L Ip + QE-L- QP
which is why it is referred to as a dot-product disel;.,ui~a~or. In general, carrier phase tracking is better with respect to noise in the dot-product mode because of the availability of a pllnchl~l power estimqte~ which has a greater signal strength in the p.~3~..,ce of noise.
After initial code locking, the delay be~ .. the wly (E) and late (L) correlators may be slowly dec, ~sed by cl-~r.g;.~g which of the correlators 240 are selected as the early and late correlators. By na" uw;ng the DLL delay in this manner, the noise level of the disc,;..~-n-~;orl function p~ . ro"--ed by the power es1im~for 265 is de~ ased, and its accuracy is increased.
Both of these discl i,,unalors 260 and 265 operate under the assumption that the co" ~lalion function is ideal, that is, that it is symrnetric and will peak at or near the position of the punctual correlator. As indicated by the curve 300 shown in Fig. 6, this is indeed true for the direct path signal, which will have a peak power at a time offset equal to zero code chips. In this ideal situation, the co-,~lalion power decreases linearly as the offset increases, in both the negative and positive directions, until the magnitude of the offset equals zero at a timedifference of plus or minus one code chip time. The amplitude scale in Fig. 6 isnormalized to a peak power equal to one ( 1).
However, this ideal situation will only occur if there are no multipath signals present. Since the number of rm~ltiratll signals present at any given location is a function of the satellite and antenna position rela~ive to any reflectin objects, anywhere from zero to several individual multipath signals may be present at any given time.
In some ;~ nces~ the CO-I. Ialion between a multipath signal and the local , efc. ~nce PRN signals will have a similar triangular shape. The shape is the same because the mllitipath signal is identic~l to the direct path signal but takes aslightly di~.~..l, longer path to the receiver. The curve 302 ,ep-~sel-ts the SUBSTITUTE SHET
WO 95/14937 216 2 9 ~ ~ PCTICA94/00640 cu,l~,lalion b~l..~.l the same ~,fc,~,nce PRN signal and a ""J~ .alh signal having aD ~ditiongl p, Opo~1;QI~ delay, r~,p.~ 3- ~t;~g the e ~lition~l time required for the lh signal to reach the receiver. In the CAal~ C shown, the ~ lition~
p,o~a~,~l;on delay is 0.2 chips.
Since the .. II;p~lh signal also arrives at the receiver, there will always be some co,r~,lalion with both the direct path signal and the "...1l;~, 1h signal. The ~,sul~ , coll.,ldtioD r~....l;n~ is thus the sum ofthe direct path c~ hlion 300 and .alh coll.,làlion 302, yeildng an acutally obs_.ved cc~ lalion ru -~l;on 304 As is event from the curve, in the pre~ense of such a mllltipath signal, the observed coll~,lation function 304 deviates from that of the direct path signal, which is the signal which the receiver channel 22n is actually attc..")th,g to lock onto.
In the e,.aln~lc shown in Fig. 6, the mllltir~th signal was ~c~ ~. ed to have a di~rentiàl phase of zero degrees, that is, the phase as that of the direct path signal. In this ;..Cl~ ~ce the peak of the correlation plot 304 still occurs near the zero offset, and the arorc.l,e..tioned power estim~tors 260 and 265 may still tend to find the co, I ~lalion peak at or near the correct offset. However, in other h.c~ ces, this is not the case.
In particular, it is also quite possible that in the act of being reflected by an object, the carrier of the multir~th signal will have had a relative phase difference imparted upon it which is di~le,.l from the phase of the carrier of the direct path signal. The amount of this carier phase offset depends upon the exact position of the reflectin~ object with respect to the satellite and the receiver. The effect on the non-coherent receiver channel 22n which has been des~libed herein is that the mllltip~th co"~lation signal will in effect be multiplied by a const~nt eaqual to the cosine of this phase di~I~l ence.
Thus, m~yimllm values for a mllltipath co.l~,lalion signal will occur when - the relative carrier phase di~. .,nce is 0 or l 80, and a minirmlm col I .,lation values will occur when this phase .I;~.~,nce is 90 or 270. In Fig. 6, this carrier phase difl~ ce was ~sl.med to be zero, and thus a positive co~l~lation existed n the mllltip~th and direct path signals. However, as shown in Fig. 7, when the mllltip~th signal has a carrier phase .liÇ~ ce of 180, a negative co"~lalion 6U~ I ITUTE SHEET
wo 95/14937 2 ~ ~ 2 9 2 18 - PCT/CA94/00640 with the le~.hlcc signal will result, since the cosine of 180 is minus one. The ;y~ correlation r~ on 306 thus has a nc~ mp~itude peak, and the sum of the direct path and multipath COIl~laliOllS 308 no longer peaks near zerotime offset, and has 5i~;fir~ shape distortion.
This in turn means that the early, pu~ udl~ and late CO" ~làlo. ~., as p,ucessed by the ul;;,e,;...;..~tc"s 260 and 265 alone, may not - le ~ t~ly track the PRN code of the desired direct path signal. The early minus late d;s~"..,.."alor260, in particular, ac5,~."~s that the power at the early and late collJalor loc-l;onc is the sarne, and that the dot product disc,;.";.,alor 265 assumes that the power at the output ofthe puncutal co,relàlor is the pGS;Iiol~ of mq~rimum co"lalion power.
Thus, ~lthough the DLL llack;l)g loop formed by the dis.,lhllàlors 260 and/or 265 may be adequate for intial code phase locking, they may in some ces tend to urge the receiver 22 to lock to a position which is offset from the actual position. This in turn leads to errors in the measurement of the propagation time from the C~trllite~ and ultimately, errors in the c~lcul~ted position of the receiver l O.
In the e,~ .pl~.y PRN ranging system being dic~s,ed the inco..~ii.g signal is band limted by the sampling rate of the channel 22n, which further e~ects theshape of the correlation function. That is ,na~ .led by a rounding offof the peaks in the correlation function, and a shift in the positive dir,ection. Taking into consideration the limited bandwidth, a~)proAh"ately 8 MHz in this embodiment, the actual correlator I es~,onse will thus be somewhat smoothed out and shifted in time, such as that shown in Fig. 8.
As better seen in the ...~g~ ed view of Fig. 9, the early, punctual, and late correlators will tend to lock onto a postion which is offset in the positive time direction by appro~;ll,alely 0.05 chips in this eAa,.,ple. This, in turn, leads to a range mea~llre.,.~nt error of the same m~gnit~ld~, which may mean a clict~nce error of more than 10 meters in the eAalllple shown.
~UBSTITUTE SHEET
~l6~9~3 WO 95/14937 PCr/CA94/00640 tionql ones ofthe multiple col,~laloJ~ 240-1, 240-2, ..., 240-m can ~us be used to generate a more accurate e;.~ e of the sl.ape of the t~ posite ;pa~h and direct path co.l~,lalion signal, which in turn can be used to develop better update cs~ al~s for the PRN code DLL and carrier phase tlacLllg loops.
To ul~dc~ d this further, CQI~ der that signals generated in the channel 22n, even in the pl~s~,nce of ,,..~II;p~ can be pred;.,led to follow a msthe ..-~ir~l model. ~AC~.. ;.~g that the channel 22n makes use of a non-coherent carrier and code ~y~chlo~ c;t 230 to adjust for the carrier and Doppler error as de~,l;l,ed above, then the colll~on~.lls of the inputs to the corre!ators ID and QD due to the direct path signal can be e,~,.eised, rcs~,ecli~ely, as I = A Rf(~k) CS(~k) Q = A Rf(~k) sin (~k) where A is an amplitude factor depe.ld;l,g upon the signal to noise ratio at theinput to the receiver, Rf~t) is the unfiltered PRN code autocol,elalion function, ~k is the code tracking error at time tk, and ~k is the residual phase tracking error also at time tk. The contribution of a given multipath signal can thus be e~y~ essed as:
Im = Rf~k-~) cos(~m + ~k) Qm = Rf~k-o) sin(~m + ~k) where ~ is the relative time delay of the mllltip~th signal, and ~m is the relative phase of the m~kirath c~"l~,on~ carrier.
SOBSTITUTE SHEET
wo 95/14937 '~16 ~ 9 2 3 PCT/CA94/00640 By a~pljil~g these equal;ol~s to the e~presa;ol~ for the dot-product disc~ or 2t'5, its output can be ...odclled as:
D~k = {Rf(~k - d/2) - Rf~k + d/2)} Rf(~k) +
a2 ~Rf(~k - d/2 - ~) - Rf(lk + d/2 - ~)} Rf(~k - ~) +
a {Rf~k - d/2) - Rf(~k + d/2)} Rf(~k - ~) cos(~m) +
a ~Rf(Tk - d/2 - o) - Rf(~k + d/2 - ~)} Rf(~k) cos(~m) with d being the spacing bel~ n the early and late co" clatola in PRN code chips A similar e"~ sion can thus be developed in the same manner for the I;
and Qj signals generated by an exemplary col,~latur 240-i in le~yonse to each one of the plurality, j, of multipath signals:
Ii =A [R,(~ +tj) cos(~) + ~j R~(rk +tj +~j) cos(~k +~mi)]
Qj A [R,(rk +tj) sin(~k) + ~aj-Rt(Tk +tj +~j) sin(~k +~7j)]
~ - o where A is the relative amp!itude of the direct path co.,.ponent, Rf is the cross col,~lation between the incoming filtered signal PRN code and the unfiltered reÇercnce code, tj is the re~,tnce code offset of correlator 240-i, ~k is the code tlackillg error at time tk (also referred to as epoch k)-~k is the carrier phase error at epoch k, aj is the relative amplitude of mllltipath component j, ~j is the delay of muhipath col"~one.l~ mj is the phase of .,.~ h compolle~l~ j, and n is the number of multipath co.npon~,.-ls. A measured orrelator power output, Pj, can bedete,..ulled for each correlator 240-i as SU8STITUTE SHEE~
WO 9S/14937 2 16 2 9 2 ~ PCT/CA94/00640 Pj = ~II2 + Q2, from which the actual coll~laLiion curve shown in Fig. 8 can be de~ ul~ed.
Fig. 10 is an e r ~ Y plot ofthe same COI I~hliOn signal shown in Fig. 8, ~LUWillg how in the case of m equal to 19, the t; for each of the nineteen correlators may be evenly di~ ulcd across the col l ~lation time scale. Thus, bydistributing the spacing of the COll eld~ul ~ 240 with di~el e.ll code offsets t;, and then s~-,p!i~ the outputs of the multirle correlators 240-1, 240-2, ..., 240-n, the above system of equations can be formed to solve for the aj, âj, and ~mj of each~nullipath Colllpollc.lL, as well as the code tlaChillg error ~k carrier phase error ~k of the direct path signal.
The system of equqtionc cn be solved in any suitable fashion by the p~ aul~ler estim~or 270 (which is also typically embodied in so~ware for the plûcessor 18). For tAa ..~le, least squares estimation, m~imllm likelihood e~lhllaLion, and other signal parameter estimation techniques which are known inthe art may be used.
Once the set of simlllt~neous equations is solved for code tracking error k and carrier phase error ~k of the direct path signal, the estimates of the values of these parameters can be used by the system in several different ways.
In one approach, the tlachillg loop r'~m~nted by the disc~ ors 260 and/or 265 can be left in place, and the code tracking error ~k can be used as afinal correction to the range estim~e~ as ..~ ;..ed by the carrier and code ,cl,ror~ l 220.
2162~2~
wo 95/14937 - 22 - PCr/CAg4/00640 Alternatively, ~c,~,.n;.~g the paral,..;ter estim~tor 270 can perform the n~ceiS,..y ç~lcnl~tions quickly enough, the code tracking error ~k and-carrier phase error ~k may be fed back to control the carrier and code s~--,lu ~,.u~r 220.
In particular, the carrier phase error, ~k, can be added by the p.ocessor 18 back into the range register 222, and the code tracking error Ik can be added into the PRN code register 226.
Also, de~e-..t;--g upon the system ~e.~uh~ c.lts~ the multiple correlators 240 may be implen-~nted in difI~.e.lt ~ays. One approach would be to have the physical number of correlators in each channel equal to m. This approach might be neceS~A.y in applications where the receiver is subjected to env;,-~l""e.,ls where the mnltipath changes rapidly.
However, in most hic~ cc ~, once intial code lock is aquired, the correlation power estimqtPs can be ~cumed to be stable for at least several seconds at a time. Thus, there may be as few as two physical correlators per channel, as long as the co..~lalo-~ have adjustable code delay spacing. The codedelay spacing may be sequenced from epoch to epoch, so that over time, measurements of the correlation power at each one of the m time offsets (tj's) of interest are taken.
In another approach, all of the correlators in all of the ch~nn~lc 22a, 22b, ..., 22n can be periodically set to track the same PRN code for a certain period of time. All channels 22 can then be sequenced through the PR~ codes for each satellite of interest, so that over time, all desired Ij and Qj signals will be determined.
Finally, the majority of the ch~nnels 22 in a receiver may be left to operate in the dis.,. ;",inators mode, while one or more of the cl,annels 22 is dedic~ted to continually sequencing from channel to channel to SU~ IenI the correlation parameter estim~tes for each satellite being tracked.
8UBSTITUTE S~IEET
Claims (19)
1. A receiver for demodulating and decoding a composite radio frequency (RF) signal consisting of a plurality of transmitted pseudorandom noise (PRN) encoded signals comprising:
an RF downconverter, connected to receive the composite RF
signal and to provide a composite intermediate frequency (IF) signal;
means for generating a local sample clock signal;
a sampling circuit, connected to receive the composite IF signal and the local sample clock signal, and to provide digital in-phase (I) samples and quadrature (Q) samples of the composite IF signal;
a plurality of channel circuits, each channel circuit for demodulating and decoding one of the transmitted PRN encoded signals, and connected in parallel with the other channel circuits such that the each channelcircuit receives the I samples and Q samples at the same time as the other channel circuits, wherein each channel circuit further comprises:
a PRN code signal generator, connected to receive a synchronizing adjustment signal and to provide a local reference PRN code signal;
means for decoding the I and Q sample signals, connected to receive the I samples, the Q samples, and the local reference PRN code signal, the decoding means for multiplying the I and Q samples by the local reference PRN
code signal, and providing decoded I and Q samples;
autocorrelation means, connected to receive the decoded I and Q
samples, for determining the autocorrelation power level at multiple code phase delays; and parameter estimator means, connected to receive the autocorrelation power level at multiple code phase delays, for determining paramenters of a direct path signal to the receiver.
an RF downconverter, connected to receive the composite RF
signal and to provide a composite intermediate frequency (IF) signal;
means for generating a local sample clock signal;
a sampling circuit, connected to receive the composite IF signal and the local sample clock signal, and to provide digital in-phase (I) samples and quadrature (Q) samples of the composite IF signal;
a plurality of channel circuits, each channel circuit for demodulating and decoding one of the transmitted PRN encoded signals, and connected in parallel with the other channel circuits such that the each channelcircuit receives the I samples and Q samples at the same time as the other channel circuits, wherein each channel circuit further comprises:
a PRN code signal generator, connected to receive a synchronizing adjustment signal and to provide a local reference PRN code signal;
means for decoding the I and Q sample signals, connected to receive the I samples, the Q samples, and the local reference PRN code signal, the decoding means for multiplying the I and Q samples by the local reference PRN
code signal, and providing decoded I and Q samples;
autocorrelation means, connected to receive the decoded I and Q
samples, for determining the autocorrelation power level at multiple code phase delays; and parameter estimator means, connected to receive the autocorrelation power level at multiple code phase delays, for determining paramenters of a direct path signal to the receiver.
2. A receiver as in claim 1 wherein each channel circuit further comprises:
means for providing an expected carrier phase signal;
a synchronizing circuit, connected to receive the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal, and connected to provide an accumulated carrier phase signal and a PRN code phase control signal, the accumulated carrier phase signal and PRN code phase signals being synchronous with each other;
carrier rotation means, connected to receive the I samples, the Q
samples, and the accumulated carrier phase signal, for phase-rotating the I and Q
samples by an amount indicated by the accumulated carrier phase signal;
wherein the autocorrelation means provides the synchronizing adjustment signal to the synchronizing circuit, and wherein the PRN code phase signal is provided to the PRN code signal generator.
means for providing an expected carrier phase signal;
a synchronizing circuit, connected to receive the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal, and connected to provide an accumulated carrier phase signal and a PRN code phase control signal, the accumulated carrier phase signal and PRN code phase signals being synchronous with each other;
carrier rotation means, connected to receive the I samples, the Q
samples, and the accumulated carrier phase signal, for phase-rotating the I and Q
samples by an amount indicated by the accumulated carrier phase signal;
wherein the autocorrelation means provides the synchronizing adjustment signal to the synchronizing circuit, and wherein the PRN code phase signal is provided to the PRN code signal generator.
3. A receiver as in claim 1 wherein the autocorrelation power level is determined at more than three code phase delay points.
4. A receiver as in claim 1 wherein the direct path signal parameter being estimated is a direct path code tracking error.
5. A receiver as in claim 1 wherein the direct path signal parameter being estimated is a direct path carrier phase tracking error.
6. A receiver as in claim 1 wherein the parameter estimator means also estimates the phase tracking error of one or more multipath signals.
7. A receiver as in claim 1 wherein the parameter estimator or means also estimates the carrier tracking error of one or more multipath signals.
8. A receiver as in claim 1 wherein the autocorrelation power level at multiple code phase delays is determined by a like multiple number of physical correlator circuits.
9. A receiver as in claim 1 wherein the autocorrelation power level at multiple code phase delays is determined by a adjusting the code delay spacing of one or more physical correlator circuits.
10. A receiver for demodulating and decoding a composite radio frequency (RF) signal consisting of a plurality of transmitted pseudorandom noise (PRN) encoded signals comprising:
an RF downconverter, connected to receive the composite RF
signal and to provide a composite intermediate frequency (IF) signal;
means for generating a local sample clock signal;
a sampling circuit, connected to receive the composite IF signal and the local sample clock signal, and to provide digital in-phase (I) samples and quadrature (Q) samples of the composite IF signal;
a plurality of channel circuits, each channel circuit for demodulating and decoding one of the transmitted PRN encoded signals, and connected in parallel with the other channel circuits, such that each channel circuit receives the I samples and Q samples at the same time as the other channel circuits, wherein each channel circuit further comprises.
means for providing an expected carrier phase signal;
a synchronizing circuit, connected to receive the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal, and connected to provide an accumulated carrier phase signal and a PRN code phase control signal, the accumulated carrier phase signal and code phase signals being synchronous with each other;
a PRN code signal generator, connected to receive the PRN code phase control signal and to provide a local reference PRN code signal;
decoding and carrier rotation means, including means for simultaneously demodulating and decoding the I and Q sample signals, connected to receive the I samples, the Q samples, the local reference PRN code signal, and the accumulated carrier phase signal, the demodulating means for phase-rotating the I and Q samples by an amount indicated by the carrier phase signal, and the decoding means for multiplying the I and Q samples by the local reference PRN
code signal, and providing decoded I and Q samples;
correlation means, connected to receive the decoded I and Q samples, for determining multiple correlation power levels for at least three code phase delays;
parameter estimating means, connected to receive the multiple correlation power levels, and for providing and estimate of a direct path time delay and one or more multipath time delays to the receiver; and means for providing the synchronizing adjustment signal to the synchronizing circuit by comparing the estimates of the direct path time delay and the one or more multipath time delays, wherein a first and second code phase delay are dynamically narrowed by being set to a predetermined wide correlator delay equal to a PRN code chip timeduring an initial PRN code lock acquisition period, and by being set thereafter to a predetermined narrow correlator delay equal to a fraction of a PRN code chip time.
an RF downconverter, connected to receive the composite RF
signal and to provide a composite intermediate frequency (IF) signal;
means for generating a local sample clock signal;
a sampling circuit, connected to receive the composite IF signal and the local sample clock signal, and to provide digital in-phase (I) samples and quadrature (Q) samples of the composite IF signal;
a plurality of channel circuits, each channel circuit for demodulating and decoding one of the transmitted PRN encoded signals, and connected in parallel with the other channel circuits, such that each channel circuit receives the I samples and Q samples at the same time as the other channel circuits, wherein each channel circuit further comprises.
means for providing an expected carrier phase signal;
a synchronizing circuit, connected to receive the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal, and connected to provide an accumulated carrier phase signal and a PRN code phase control signal, the accumulated carrier phase signal and code phase signals being synchronous with each other;
a PRN code signal generator, connected to receive the PRN code phase control signal and to provide a local reference PRN code signal;
decoding and carrier rotation means, including means for simultaneously demodulating and decoding the I and Q sample signals, connected to receive the I samples, the Q samples, the local reference PRN code signal, and the accumulated carrier phase signal, the demodulating means for phase-rotating the I and Q samples by an amount indicated by the carrier phase signal, and the decoding means for multiplying the I and Q samples by the local reference PRN
code signal, and providing decoded I and Q samples;
correlation means, connected to receive the decoded I and Q samples, for determining multiple correlation power levels for at least three code phase delays;
parameter estimating means, connected to receive the multiple correlation power levels, and for providing and estimate of a direct path time delay and one or more multipath time delays to the receiver; and means for providing the synchronizing adjustment signal to the synchronizing circuit by comparing the estimates of the direct path time delay and the one or more multipath time delays, wherein a first and second code phase delay are dynamically narrowed by being set to a predetermined wide correlator delay equal to a PRN code chip timeduring an initial PRN code lock acquisition period, and by being set thereafter to a predetermined narrow correlator delay equal to a fraction of a PRN code chip time.
11. A method for demodulating and decoding a composite signal consisting of a plurality of transmitted pseudorandom noise (PRN) encoded signals comprising:
sampling the composite signal in synchronism with a local sample clock signal, and thereby providing digital in-phase (I) samples and quadrature (Q) samples of the composite signal;
feeding the I samples and Q samples to a plurality of channel circuits, each channel circuit connected in parallel with the other channel circuits such that each channel circuit receives the I samples and Q samples at the same time as the other channel circuits;
within each channel circuit:
generating a local reference PRN code signal in response to a synchronizing adjustment signal;
decoding the I and Q samples, by multiplying the I and Q samples by the local reference PRN code signal, and in response thereto, producing decoded I and Q samples;
determining the correlation power level of the I and Q samples at multiple code phase delays; and estimating the parameters of a direct signal path to the receiver from the correlation power levels at the multiple code phase delays
sampling the composite signal in synchronism with a local sample clock signal, and thereby providing digital in-phase (I) samples and quadrature (Q) samples of the composite signal;
feeding the I samples and Q samples to a plurality of channel circuits, each channel circuit connected in parallel with the other channel circuits such that each channel circuit receives the I samples and Q samples at the same time as the other channel circuits;
within each channel circuit:
generating a local reference PRN code signal in response to a synchronizing adjustment signal;
decoding the I and Q samples, by multiplying the I and Q samples by the local reference PRN code signal, and in response thereto, producing decoded I and Q samples;
determining the correlation power level of the I and Q samples at multiple code phase delays; and estimating the parameters of a direct signal path to the receiver from the correlation power levels at the multiple code phase delays
12. A method as in claim 11 further comprising:
providing an expected carrier phase signal;
producing an accumulated carrier phase signal and a PRN
code phase control signal which are synchronous with one another, by combining the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal; and phase-rotating the I and Q samples by an amount indicated by the accumulated carrier phase signal.
providing an expected carrier phase signal;
producing an accumulated carrier phase signal and a PRN
code phase control signal which are synchronous with one another, by combining the sample clock signal, the expected carrier phase signal, and a synchronizing adjustment signal; and phase-rotating the I and Q samples by an amount indicated by the accumulated carrier phase signal.
13. A method as in claim 11 wherein the correlation power level is determined at more than three code phase delays.
14. A method as in claim 11 wherein the direct path signal parameter being estimated is a direct path code tracking error.
15. A method as in claim 11 wherein the direct path signal parameter being estimated is a direct path carrier phase tracking error.
16. A method as in claim 11 wherein a phase tracking error of one or more multipath signals is also estimated.
17. A method as in claim 11 wherein a carrier tracking error of one or more multipath signals is also estimated.
18. A method as in claim 11 wherein the autocorrelation power level at multiple code phase delays is determined by a like multiple number of physical correlator circuits.
19. A receiver as in claim 11 wherein the autocorrelation power level at multiple code phase delays is determined by a adjusting the code delay spacing of one or more physical correlator circuits.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/158,511 US5414729A (en) | 1992-01-24 | 1993-11-29 | Pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing |
| US158,511 | 1993-11-29 | ||
| PCT/CA1994/000640 WO1995014937A1 (en) | 1993-11-29 | 1994-11-22 | Pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2162926A1 CA2162926A1 (en) | 1995-06-01 |
| CA2162926C true CA2162926C (en) | 2000-10-24 |
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ID=22568471
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002162926A Expired - Lifetime CA2162926C (en) | 1993-11-29 | 1994-11-22 | Pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing |
Country Status (6)
| Country | Link |
|---|---|
| US (1) | US5414729A (en) |
| JP (1) | JPH09505404A (en) |
| CN (1) | CN1128064A (en) |
| AU (1) | AU8137894A (en) |
| CA (1) | CA2162926C (en) |
| WO (1) | WO1995014937A1 (en) |
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| SG43672A1 (en) * | 1992-01-24 | 1997-11-14 | Novatel Communications Limited | A pseudorandum noise ranging receiver which compensates for multipath distortion by dynamically adjusting the time delay spacing between early and late correlators |
-
1993
- 1993-11-29 US US08/158,511 patent/US5414729A/en not_active Expired - Lifetime
-
1994
- 1994-11-22 JP JP7514715A patent/JPH09505404A/en active Pending
- 1994-11-22 AU AU81378/94A patent/AU8137894A/en not_active Abandoned
- 1994-11-22 CN CN94192920A patent/CN1128064A/en active Pending
- 1994-11-22 CA CA002162926A patent/CA2162926C/en not_active Expired - Lifetime
- 1994-11-22 WO PCT/CA1994/000640 patent/WO1995014937A1/en active Application Filing
Also Published As
| Publication number | Publication date |
|---|---|
| CA2162926A1 (en) | 1995-06-01 |
| WO1995014937A1 (en) | 1995-06-01 |
| AU8137894A (en) | 1995-06-13 |
| CN1128064A (en) | 1996-07-31 |
| JPH09505404A (en) | 1997-05-27 |
| US5414729A (en) | 1995-05-09 |
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