CA2127737A1 - Satellite search methods for improving time to first fix in a gps receiver - Google Patents
Satellite search methods for improving time to first fix in a gps receiverInfo
- Publication number
- CA2127737A1 CA2127737A1 CA002127737A CA2127737A CA2127737A1 CA 2127737 A1 CA2127737 A1 CA 2127737A1 CA 002127737 A CA002127737 A CA 002127737A CA 2127737 A CA2127737 A CA 2127737A CA 2127737 A1 CA2127737 A1 CA 2127737A1
- Authority
- CA
- Canada
- Prior art keywords
- gps
- search
- gps receiver
- search range
- gps satellite
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- 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
- G01S19/23—Testing, monitoring, correcting or calibrating of receiver elements
- G01S19/235—Calibration of receiver components
-
- 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
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/28—Satellite selection
-
- 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
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
- G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
Abstract
ABSTRACT OF THE DISCLOSURE
An embodiment of the present invention includes a GPS receiver capable of concurrently tracking up to eight GPS satellites. The GPS
receiver comprises computer-implemented methods for parallel search, split search and precomputation. The parallel search method causes the apparent Doppler frequency spectrum to be sectioned into several segments and a first GPS satellite signal is searched for in parallel in each of the segments. When the first GPS fix is not found after a predetermined time has expired, a split search is begun.
An embodiment of the present invention includes a GPS receiver capable of concurrently tracking up to eight GPS satellites. The GPS
receiver comprises computer-implemented methods for parallel search, split search and precomputation. The parallel search method causes the apparent Doppler frequency spectrum to be sectioned into several segments and a first GPS satellite signal is searched for in parallel in each of the segments. When the first GPS fix is not found after a predetermined time has expired, a split search is begun.
Description
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SATELI.ITE SE~RCII METIIODS FOR IMPROVING TIME TO
FIRST FIX IN A GPS RECEIVER
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1. Field_Q~_~he Inven~iQn The invention relates generally to navigation systems and more specifically to global positioning system (GPS) devlces with rapid time-to-first-fix and re-acquisition ',~.'! performance.
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::` .. i 2. Description of ~he Prior Art Global positioning system receivers use signals received from typically three or more overhead satellites to determine navigational data such as position and velocity, and such 'b,-,`j 20 systems may also provide altitude and time. GPS
signals are available worldwide at no cost and ~i can be used to determine the location of a ::!
~`~i$, vehicle, such as an automobile, to within one city block, or better. Dual-frequency carrier `~
GPS receivers typically track a pair of radio . carriers, L1 and L2, associated with the GPS
satellites to generate accumulated delta-range , . .~
:.;q measurements (~DR) from P-code modulation on ~ those carrier frequencies and at the same time ,;
track L1 C/A-code to generate code phase measurements. Carrier frequency L1 is allocated ~i1 to 1575.42 MHz and carrier frequency L2 is .. ; positioned at 1227.78 Ml~z. Less expensive receivers tune only one carrier frequency, and therefore do not have adequate information to ~:, compute the local ionospheric delays that will ~i appear as position errors. At such frequencies, radio carrier signals travel by line-of-sight.
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Thus buildings, mountains and 'i - `
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; The constellation of GPS satellites in orbit about the earth presently comprises approximately seventeen individual satellites.
Each transmits one of thirty-two unique identifying codes in a code multiple access "!, arrangement. This allows all of the many GPS
satellites to transmit in spread speetrum mode ~;,J 10 at the same frequency (plus or minus a Doppler frequency shift of that frequency as results from the satellite's relatlve velocity).
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.I.i Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 "chip" code to one of the thirty-two pseudo random number (PRN) sequence codes that are preassigned to individual GPS
satellites. These eodes are not neeessarily being transmitted in phase with one another.
Therefore, "finding" a GPS satellite initially involves searehing various earrier frequeneies, to aeeount for Doppler frequeney shift and oseillator inaeeuraeies, and searching for a eode mateh, using 1023 different eode phases and twenty or more possible eorrelation eode ,.,, ., j .
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In large eities with many tall buildings, one or more of the GPS satellites being tracked by a particular receiver, may be temporarily bloeked. In some situations, sueh blockage can prevent all the overhead GPS satellites from being tracked and such outages can last for several minutes. GPS signals also beeome , unavailable to vehieles moving through underground or underwater tunnels.
At least one background art GPS five-'!""';';`' channel receiver directs all of its ehannels to :~ ....
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SATELI.ITE SE~RCII METIIODS FOR IMPROVING TIME TO
FIRST FIX IN A GPS RECEIVER
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` ~ACKGROUND OF THE INV~N~Q~
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1. Field_Q~_~he Inven~iQn The invention relates generally to navigation systems and more specifically to global positioning system (GPS) devlces with rapid time-to-first-fix and re-acquisition ',~.'! performance.
!~.
::` .. i 2. Description of ~he Prior Art Global positioning system receivers use signals received from typically three or more overhead satellites to determine navigational data such as position and velocity, and such 'b,-,`j 20 systems may also provide altitude and time. GPS
signals are available worldwide at no cost and ~i can be used to determine the location of a ::!
~`~i$, vehicle, such as an automobile, to within one city block, or better. Dual-frequency carrier `~
GPS receivers typically track a pair of radio . carriers, L1 and L2, associated with the GPS
satellites to generate accumulated delta-range , . .~
:.;q measurements (~DR) from P-code modulation on ~ those carrier frequencies and at the same time ,;
track L1 C/A-code to generate code phase measurements. Carrier frequency L1 is allocated ~i1 to 1575.42 MHz and carrier frequency L2 is .. ; positioned at 1227.78 Ml~z. Less expensive receivers tune only one carrier frequency, and therefore do not have adequate information to ~:, compute the local ionospheric delays that will ~i appear as position errors. At such frequencies, radio carrier signals travel by line-of-sight.
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Thus buildings, mountains and 'i - `
block reeeption.
; The constellation of GPS satellites in orbit about the earth presently comprises approximately seventeen individual satellites.
Each transmits one of thirty-two unique identifying codes in a code multiple access "!, arrangement. This allows all of the many GPS
satellites to transmit in spread speetrum mode ~;,J 10 at the same frequency (plus or minus a Doppler frequency shift of that frequency as results from the satellite's relatlve velocity).
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.I.i Particular satellites are sorted out of a resulting jumble of signals and noise by correlating a 1023 "chip" code to one of the thirty-two pseudo random number (PRN) sequence codes that are preassigned to individual GPS
satellites. These eodes are not neeessarily being transmitted in phase with one another.
Therefore, "finding" a GPS satellite initially involves searehing various earrier frequeneies, to aeeount for Doppler frequeney shift and oseillator inaeeuraeies, and searching for a eode mateh, using 1023 different eode phases and twenty or more possible eorrelation eode ,.,, ., j .
~ templates.
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In large eities with many tall buildings, one or more of the GPS satellites being tracked by a particular receiver, may be temporarily bloeked. In some situations, sueh blockage can prevent all the overhead GPS satellites from being tracked and such outages can last for several minutes. GPS signals also beeome , unavailable to vehieles moving through underground or underwater tunnels.
At least one background art GPS five-'!""';';`' channel receiver directs all of its ehannels to :~ ....
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-focus on one satelllte at init.ial turn-on, as is ;';!, indicated by the user display on such receivers.
This addresses the problem of satellite signal . frequency uncertainty that exists due to Doppler :....................................................................... .
~: 5 efEects and local oscillator inaccuracies in the :!i receiver. A search for a particular satellite in the apparent Doppler frequency spectrum is ~i conducted in parallel by segmenting the possible Doppler frequency spectrum into as many segments as there are rece;ver channels and appointing ~;.' each of the several receiver channels to attend to a search within a respective segment. The ~A~
~;~ single largest uncertainty stems from the random frequency possible from typical local ~ 15 oscillators at start-up. Therefore, the -.~$j ~,.',.7~ apparent Doppler frequency will be totally unknown, regardless of whether the actual Doppler frequency is known, as might be available if the present position is known.
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MMARY OF T~IE PRESENT INVENTION
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~i It is therefore an object of the present invention to provide a system and method for ~ reducing the time necessary for a first fix ,~,,,,~,,"j?,~. after turning on a GPS receiver. ~ -Briefly, a preferred embodiment of the present invention includes a GPS receiver capable of concurrently tracking up to eight GPS
~''Ct satellites. Firmware included in the GPS
i; receiver comprises computer-implemented methods for parallel search, split search and precomputation. The parallel search method causes the apparent Doppler frequency spectrum - to be sectioned into several segments and a ~i ".
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~^ -4-i first GPS satellite signal is searched for in parallel in each of the segments. If the first '5`A~. GPS satellite is not fo~nd after a predetermined time has expired, then other satellites are ~n 5 attempted until a first satellite is tracked.
Information from the parallel search is used to speed up acquisition of subsequent satellites. If enough satellites are not acquired for obtaining a first position fix, , 10 after a predetermined duration, then a split ,,., . ;-, ~,-,i'!,'';`i search is conducted. 'I'he search method allows a i~ lower cost local oscillator to be employed that ~,! lacks the temperature stability of prior art oscillators. The split search method causes as many as half of the available channels to be appointed to search for a first set of GPS
satellites while a second set of GPS satellites is sought in a search with the remaining .; channels. The precomputation method determines whi.ch GPS satellites are expected to be at the highest elevations over the next succeeding several hours and prepares a search list of those candidate GPS satellites for use when the GPS is next turned-on. If the time between turning-off and turning the GPS receiver back on . is only a few hours, the precornputed candidate . search list is indexed to supply a GPS
satellite's identity that should result in a quick find after a relative short search.
An advantage of the present invention is tha-t it provides a method for improving the time-to-first-fix for a GPS receiver.
Another advantage of the present invention is that a system is provided that reduces search .;` 35 delays encountered when a GPS receiver has been denied signal reception for several minutes at `~ ` the time the GPS receiver is first turned-on.
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~~ A further advantage of the present invention ".
`,~ is that a system is provided that searches two lists of possible GPS satellite signal sources, one list related to the last determined position and the second list interleaved with the first ,i) ,.
. that uses the remaining positions in the world : ., ~;~ for its position assumptions.
Another advantage of the present invention is that a system is provided that precomputes i~ 10 future tracking opportunities and therefore saves time in searching for GPS satellites to .~i, track when restarted after a short period of non-operatioIl.
A further advantage of the present invention ~ .
is that a system is provided that includes a parallel search that puts the most search power ,~ around a nominal frequency and then moves the '~ search outward after quickly searchiny the interior range.
Another advantage of the present invention is that a system is provided that includes a split search to minimi~e time-to~first-fix when the receiver antenna is covered or blocked at power up, while still allowing the receiver to be moved great distances while powered off by ~ not entirely trusting in the last position.
d~ ,7 These and other objects and advantages of the present invention will no doubt become ~ obvious to those of ordinary skill in the art '?':''''. ' 30 after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
IN THE DRAWINGS
.i. Fig. 1 is a block diagram of a GPS receiver ~: .
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: ., ~ :, ~, embodiment of the present invention that includes a set of computer-implemented methods ~i, in firmware;
Fig. 2A i.s a diagram of the radio spectrum and the apparent Doppler frequency spectrum divided into search ranges;
~,i Fig. 2B is a diagram of the radio spectrum ;, and the apparent Doppler frequency spectrum showing a ping-pong sequence within a typical 0 search range of E'ig. 2A;
'~, Fig. 3A is a flowchart of the parallel search methocl included as a computer subroutine in the firmware of the GPS receiver of Fig. l;
Fig. 3B is a flowchart of a satellite selection process included in that of Fig. 3A;
Fig. 9 is a flowchart of the split search method included as a computer subroutine in the firmware of the GPS receiver of Fig. l; and Fig. 5 is a flowchart of the precomputation of future tracking opportunities method included ~i as a computer subroutine in the firmware of the GPS receiver of Fig. 1.
DETAILED DES~RIPTION OF THE PREE'ERRED EMBODIMENT
Fig. 1 illustrates a global position system (GPS) receiver embodiment of the present ~, 30 invention, referred to herein by the general , ;.
'.~ reference numeral 10. GPS receiver 10 is a six-`~ channel type and includes a patch antenna 12, a low-noise amplifier 19, bandpass filter (BPF) ~ 16, a mixer 18, a local oscillator (LO) 20, an M 35 intermediate frequency (IF) stage 22, a phase demodulator 24, a multi-channel GPS code correlator 26 and a GPS navigation computer 28 .
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l with a firmware memory 30. Except Eor some -. parts of firmware 30, GPS receiver 10 may comprise a commercially available unit, such as .y a portable three-channel GPS receiver marketed by Trimble Naviqation (Sunnyvale, CA), under the trademark "ENSIGN".
Firmware 30 comprises computer instructions (programs) necessary for the deterrnination of geographic position and velocity of GPS receiver 10, together with the time. Such programs are conventional and include functions to vary the frequency output of LO 20 to sweep through the ~^$ apparent Doppler frequency speetrum to find signals captured by antenna 12 from GPS
satellites. Correlator 26 is fed a variety of pseudo-random number (PRN) eorrelation codes and phases to demodulate information carried by spread spectrum communication. GPS navigation computer 28 calculates the pseudo-ranges to a number of visible GPS satellites and uses orbit ephemeris and satellite almanac periodieally reeeived from the GPS satelli-tes to determine a position fix by triangulation with the predetermined positions of the GPS satellites.
When GPS receiver 10 is first powered up, only a guess ean be made as to whieh GPS
satellites will be in the vieinity, based on the last position and the almanae. A seareh through a list of possible PRN correlation codes must be conducted to identify and track those GPS
satellites that are actually available. After -~ enough GPS satellites are acquired for a fix, the GPS satellites to then seareh for can be accurately predicted, leaving no uncertainty about the respective horizon angle of each. At least three GPS satellites must typically be .~ tracked for a position determination output from , :
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A Doppler frequency shift of the transmitted carrier frequency of each GPS
satellite will occur that is dependent on the velocity (vector plus speed) relative to GPS
receiver 10. This actual Doppler frequency , .
shift will create a few kilohertz uncertainty about the assigned I,1 carrier frequency that must be overcome by sweeping through the radio spectrum -to find the desired signal. Almanac information can be used to predict what the ,~,;.
~`~ Doppler frequency should be, and so the search can be constrained.
However, inaccuracies in the absolute frequency output of LO 20 can cause the spectrum ~.:.. .
of apparent Doppler frequencies to be broader than would otherwise be the case, e.g., plus or ~i minus ten kilohertz. The prior art obtains quicker searches through the apparent Doppler frequency spectrum by using very high quality temperature-compensated crystal oscillators (TCXO). Such high quality local oscillators are expensive and add significant cost to GPS
receiver 10. These oscillators have stabilities on the order of one to two parts per million (ppm). Low cost oscillators can vary as much as twenty ppm. Therefore, a compromise is traditionally made between the expense of the local oscillator and the time-to-first-fix required to search through the apparent Doppler frequency spectrum.
~;. GPS receiver 10 has only to sweep within approximately three hundred hertz of a GPS
satellite signal to establish a lock on its carrier frequency. Such frequency sweeping is ~ implemented by causing navigation computer 28 to j,~,~',!,,, load a variety of divider values for numerically ,, i,., 1 ~' .
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. ;;. g ~; controlled oscillators (MCO) that are a part of LO 20. Phase lock loop (PLL) circuitry, ~` conventional to such de~ices within LO 20, provides the abil;ty to capture and lock within r~l 5 a limited range. Therefore, when searching for ;`~ a signal from a GPS satellite, a strategy, as ~- illustrated in Figs. 2A and 2B is used.
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In Fig. 2A, a three-channel GPS receiver is directed to align its three channels (CH0, Clll and CH2) shoulder-to-shoulder in a first se-t (S1) of bins centered on a nominal center .l frequency. Fig. 2B shows a "ping-pong" search strategy that is used within each bin to locate the GPS sate:llite's signal. Initially, the search begins close in on one side (+ is shown) to the nominal center frequency of the respective bin, and bounces close in to the . .
opposlte side (- 1n the example). The bounce :~ back and forth takes the search within each bin wider and wider until the whole bin has been ~`~ searched. In Fig. 2A, once the search of the three bins of S1 have been completed, a second set (S2) of bins is tried next. In Fig. 2A, CH0 : , `
and CH1 are appointed to the next two higher bins above S1, and CH2 is appointed to the next bin below S1. The selection of which channels to use and which side gets the odd remainder ~-; channel is arbitrary. To balance the odd channel being used above the nominal frequency . 30 in S2, a set three (S3) has two bins below and . one bin above the nominal center frequency.
Many conventional search strategies for finding a signal within a pair of range boundaries exist, and many are compatible with the macro search strategy embodiment of the present invention described herein. The present invention is therefore not limited to the high , ~ . "
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,~ and low p.ing-pong search just described.
A parallel search computer-implemented di process 40, illustrated by the flowcharts of Flgs. 3A and 3B, causes GPS navigation computer ~ 5 28 to segment the apparent Doppler frequency i.~ spectrum into several sections (e.g., first through thi.rd search ranges in Fig. 2A) and conducts parallel searches within each section using the multiple GPS receiver channels ~; 10 available. In the case of a portable three-channel GPS receiver being used for GPS receiver 10, a part of the apparent Doppler frequency spectrum is divided into a lower, a nominal and an upper frequency segment. Enough time is allowed for parallel searches within each frequency segment to search plus or minus one kilohertz. If a GPS satellite signal is not found, the upper and lower channels are stepped out further so that each may search another segment of plus or minus one kilohertz.
Process 40 comprises a power up step 41, a ~'! step 42 that selects a satellite to search for, ;~ a step 43 predicts a nominal center frequency ~;l for the selected satellite, a step 44 partitions -the radio frequency spectrum and appoints the ~ available GPS receiver channels such that they i clus-ter about the nominal center frequency. A
~-i`1 step 45 causes each of the GPS receiver channels . ~, . .
`'~ to search a first set of respective frequency :., ;~
,i 30 bands in parallel (S1 in Fig. 2A). However, the `~i' apparent Doppler frequency spectrum that may -~ need to be searched may be wider than S1. A
decision is made in a step 46 whether a signal . has been found. If not, a step 47 decides ;~1 35 whether a search limit has been exceeded. For ~`~ example, the search limit could be a limit of time and/or of frequency. If the search limit . ,;, ,-`'.''''.~
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has been exceeded, then control passes back to .~ step 42 to select another satellite. Otherwise, a step 48 appoints the available channels to a ~ .
~ next set of frequency bands. A step 49 , .....
. 5 estimates the oscillator error based on both :~ predicted and measured Doppler frequency. Then a split search is conducted. All subsequent ~ searches before the first fix use this estimate ;~' to refine, bound, or minimize the search range for other satellites. For example, if the actual Doppler frequency for a satellite is one ` kilohertz (Kllz) and the oscillator error is ten kilohertz, each satellite would take the limit to search, e.g., ten Kllz. Thus the ten K~lz . 15 range is only searched once, quickly.
~~ In Fig. 3B, step 92 is shown to comprise a ,,~""î' step 50 in which the highest GPS satellite is tried. If a signal from that GPS satellite was found, a step 51 directs the program control to exit. Otherwise, a step 52 selects the next highest GPS satellite not already tried. If a signal from that GPS satellite was found, a step ;i 53 directs the program control to exit.
Otherwise, a step 54 again tries the highest GPS
~;~ 25 satellite. If a signal from that GPS satellite f~- was found, a step 55 directs the program control ~~ to exit. Otherwise, a step 56 selects the next highest GPS satellite which appears to be below the horizon limit. A step 57 looks to see if , 30 all the available GPS satellites have been - -tried. If not, control returns to step 50.
Otherwise, a step 58 does a reset. For example, if there are six GPS satellites SV1 through SV6 with respective elevation angles of 70, 40, . 35 10, -5, -15 and -90~, then the order of selecting just described will select SV1, SV2, SV1, SV5, SV1, SV3, SV1, SV4, SV1 and SV6.
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~ l2-!`'',. After a reset as a result of step 58, the order will again be, SV1, SV2, SV1, SV5, SV1, SV3, ~ SVl, SV9, SV1 and SV6. It will eventually try !:':`',` all the satellites, regardless of elevation.
In the present invention, if the Doppler -~ frequency error is close in, as is usually the ;~i case, it may be found faster than can be done with the prior art. lhis results from placing more search capability near the predicted frequency, whlch ls aften very close to the actual frequency. For exampler if the actual ~4:' frequency is at -4 KHz and nominal at -l KHz, the present invention can Eind the signa] in the first pass. It would take a maximum search time to find the signal for a non-precomputed or a non-centered alternate technique.
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With greater numbers of GPS receiver i channels available in GPS receiver lO, more ~;., search ranges can be searched in parallel at any ;~ 20 one time. With an even number of channels, e.g., six channels, the search ranges are i~ preferably positioned in frequency such that the - nominal frequency is near the center of one of , ~ the two middle search ranges, rather than along -~ 25 a border between them. Although this makes the `~ search range a little lopsided around the -.
;i nominal frequency, the probability of finding a GPS satellite signal is a bit better at the ` nominal frequency and a search will conclude sooner because searches typically start at the .,, .j ~ center of each search range, not the edges.
-`i Fig. 2A is typical of the sequence of such ~.~1 searches from S1 through S3 and beyond.
'':.':! If antenna 12 (Fig. 1) is covered when GPS
~::! 35 receiver lO is first turned on, navigation computer 28 would typically discard any 1~ assumptions or precomputed information it may :-.:
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~ 13-;; have had about which GPS satellites are probably i~ in the vicinity. ~fter a search had failed to track such GPS satellltes, other GPS satellites would be tried. In the prior art, the original ~1 5 set of GPS satellites would not be retried until all the others in orbit about the world had been tried first. The round-robin time of such a search can be exceedingly long, and can cause a user to assume GPS receiver 10 is inadequate or . ` 10 not functional. Prior art GPS receivers avoided ."
such problems by requiring the user to initialize the Ullit by asking for the current position. This is unpopular with users, because a GPS receiver is supposed to provide one with a position, not ask for it. Therefore, such queries have been eliminated, but the uncertainty still remains.
A split search computer-implemented process 60 is diagrammed in Fig. 4. In a step 62, the eight GPS satellites with the highest elevation ~` based on an estimate of position are selected.
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-.- It may be the case, however, that the assumed ~- horizon may be wrong. In a step 64, a decision is made whether a sufficient number of GPS
satellites are being tracked to obtain a current , ~
~ position fix. If not, a step 66 tests to see `.~ ':' ` if, since power up, a time-out has been exceeded and there are still an insufficient number of GPS satellites to compute a fix. For example, if five minutes has elapsed, a step 68 is engaged. Otherwise, control returns to step 64 to allow more time. Step 68 re-computes ~; elevations of GPS satellites every thirty ~",!.~ seconds and bases a search using one half of the GPS satellites based on position and the others 4~; ,`;,,.
regardless of position. For example, GPS
satellites below the apparent horizon will be ~. ' , .....
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~!: . As mentioned in reference to Figs. 2A and ;~ 2~, lnitial]y, only one set of search ranges or bi.ns is computed, based on the last position, -time and almanac, until a five minute time-out occurs. Then the "split" aspect is begun. A
fi.rst search l.ist preferably includes the highest elevation satellites that have been predicted by navigation computer 28 using the GPS almanac and the last position fix to be the !,~ most likely to be available for tracking.
` second list may include those satellites that - have been predicted by navigation computer 28 by . sequentially searching for satellites ;~:. 15 independent of position, thus their eleva-tion .~ angles may be negative based on -the present '~' ! posi-tion could be erroneous. This strategy is based on the assumption that if GPS receiver 10 has been moved since the last operational . 20 period, the travel i.s more probably to have been ., ~j a shorter trip rather than a longer trip, and ~-` therefore the horizon of the previous fix is ~:` more likely than the opposite side of the world, .' for example.
It can require as much as three seconds ,,:;;
after powering-on GPS receiver 10 to allow .,j navigation cornputer 28 to compute which PRNs -to ~ use, based on current time, a current almanac ;~.;.'9 and a previous position fix stored in memory. A
,;~; 30 precomputation process 80, illustrated in Fig.
This addresses the problem of satellite signal . frequency uncertainty that exists due to Doppler :....................................................................... .
~: 5 efEects and local oscillator inaccuracies in the :!i receiver. A search for a particular satellite in the apparent Doppler frequency spectrum is ~i conducted in parallel by segmenting the possible Doppler frequency spectrum into as many segments as there are rece;ver channels and appointing ~;.' each of the several receiver channels to attend to a search within a respective segment. The ~A~
~;~ single largest uncertainty stems from the random frequency possible from typical local ~ 15 oscillators at start-up. Therefore, the -.~$j ~,.',.7~ apparent Doppler frequency will be totally unknown, regardless of whether the actual Doppler frequency is known, as might be available if the present position is known.
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MMARY OF T~IE PRESENT INVENTION
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~i It is therefore an object of the present invention to provide a system and method for ~ reducing the time necessary for a first fix ,~,,,,~,,"j?,~. after turning on a GPS receiver. ~ -Briefly, a preferred embodiment of the present invention includes a GPS receiver capable of concurrently tracking up to eight GPS
~''Ct satellites. Firmware included in the GPS
i; receiver comprises computer-implemented methods for parallel search, split search and precomputation. The parallel search method causes the apparent Doppler frequency spectrum - to be sectioned into several segments and a ~i ".
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~^ -4-i first GPS satellite signal is searched for in parallel in each of the segments. If the first '5`A~. GPS satellite is not fo~nd after a predetermined time has expired, then other satellites are ~n 5 attempted until a first satellite is tracked.
Information from the parallel search is used to speed up acquisition of subsequent satellites. If enough satellites are not acquired for obtaining a first position fix, , 10 after a predetermined duration, then a split ,,., . ;-, ~,-,i'!,'';`i search is conducted. 'I'he search method allows a i~ lower cost local oscillator to be employed that ~,! lacks the temperature stability of prior art oscillators. The split search method causes as many as half of the available channels to be appointed to search for a first set of GPS
satellites while a second set of GPS satellites is sought in a search with the remaining .; channels. The precomputation method determines whi.ch GPS satellites are expected to be at the highest elevations over the next succeeding several hours and prepares a search list of those candidate GPS satellites for use when the GPS is next turned-on. If the time between turning-off and turning the GPS receiver back on . is only a few hours, the precornputed candidate . search list is indexed to supply a GPS
satellite's identity that should result in a quick find after a relative short search.
An advantage of the present invention is tha-t it provides a method for improving the time-to-first-fix for a GPS receiver.
Another advantage of the present invention is that a system is provided that reduces search .;` 35 delays encountered when a GPS receiver has been denied signal reception for several minutes at `~ ` the time the GPS receiver is first turned-on.
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~~ A further advantage of the present invention ".
`,~ is that a system is provided that searches two lists of possible GPS satellite signal sources, one list related to the last determined position and the second list interleaved with the first ,i) ,.
. that uses the remaining positions in the world : ., ~;~ for its position assumptions.
Another advantage of the present invention is that a system is provided that precomputes i~ 10 future tracking opportunities and therefore saves time in searching for GPS satellites to .~i, track when restarted after a short period of non-operatioIl.
A further advantage of the present invention ~ .
is that a system is provided that includes a parallel search that puts the most search power ,~ around a nominal frequency and then moves the '~ search outward after quickly searchiny the interior range.
Another advantage of the present invention is that a system is provided that includes a split search to minimi~e time-to~first-fix when the receiver antenna is covered or blocked at power up, while still allowing the receiver to be moved great distances while powered off by ~ not entirely trusting in the last position.
d~ ,7 These and other objects and advantages of the present invention will no doubt become ~ obvious to those of ordinary skill in the art '?':''''. ' 30 after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
IN THE DRAWINGS
.i. Fig. 1 is a block diagram of a GPS receiver ~: .
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Fig. 2A i.s a diagram of the radio spectrum and the apparent Doppler frequency spectrum divided into search ranges;
~,i Fig. 2B is a diagram of the radio spectrum ;, and the apparent Doppler frequency spectrum showing a ping-pong sequence within a typical 0 search range of E'ig. 2A;
'~, Fig. 3A is a flowchart of the parallel search methocl included as a computer subroutine in the firmware of the GPS receiver of Fig. l;
Fig. 3B is a flowchart of a satellite selection process included in that of Fig. 3A;
Fig. 9 is a flowchart of the split search method included as a computer subroutine in the firmware of the GPS receiver of Fig. l; and Fig. 5 is a flowchart of the precomputation of future tracking opportunities method included ~i as a computer subroutine in the firmware of the GPS receiver of Fig. 1.
DETAILED DES~RIPTION OF THE PREE'ERRED EMBODIMENT
Fig. 1 illustrates a global position system (GPS) receiver embodiment of the present ~, 30 invention, referred to herein by the general , ;.
'.~ reference numeral 10. GPS receiver 10 is a six-`~ channel type and includes a patch antenna 12, a low-noise amplifier 19, bandpass filter (BPF) ~ 16, a mixer 18, a local oscillator (LO) 20, an M 35 intermediate frequency (IF) stage 22, a phase demodulator 24, a multi-channel GPS code correlator 26 and a GPS navigation computer 28 .
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l with a firmware memory 30. Except Eor some -. parts of firmware 30, GPS receiver 10 may comprise a commercially available unit, such as .y a portable three-channel GPS receiver marketed by Trimble Naviqation (Sunnyvale, CA), under the trademark "ENSIGN".
Firmware 30 comprises computer instructions (programs) necessary for the deterrnination of geographic position and velocity of GPS receiver 10, together with the time. Such programs are conventional and include functions to vary the frequency output of LO 20 to sweep through the ~^$ apparent Doppler frequency speetrum to find signals captured by antenna 12 from GPS
satellites. Correlator 26 is fed a variety of pseudo-random number (PRN) eorrelation codes and phases to demodulate information carried by spread spectrum communication. GPS navigation computer 28 calculates the pseudo-ranges to a number of visible GPS satellites and uses orbit ephemeris and satellite almanac periodieally reeeived from the GPS satelli-tes to determine a position fix by triangulation with the predetermined positions of the GPS satellites.
When GPS receiver 10 is first powered up, only a guess ean be made as to whieh GPS
satellites will be in the vieinity, based on the last position and the almanae. A seareh through a list of possible PRN correlation codes must be conducted to identify and track those GPS
satellites that are actually available. After -~ enough GPS satellites are acquired for a fix, the GPS satellites to then seareh for can be accurately predicted, leaving no uncertainty about the respective horizon angle of each. At least three GPS satellites must typically be .~ tracked for a position determination output from , :
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A Doppler frequency shift of the transmitted carrier frequency of each GPS
satellite will occur that is dependent on the velocity (vector plus speed) relative to GPS
receiver 10. This actual Doppler frequency , .
shift will create a few kilohertz uncertainty about the assigned I,1 carrier frequency that must be overcome by sweeping through the radio spectrum -to find the desired signal. Almanac information can be used to predict what the ,~,;.
~`~ Doppler frequency should be, and so the search can be constrained.
However, inaccuracies in the absolute frequency output of LO 20 can cause the spectrum ~.:.. .
of apparent Doppler frequencies to be broader than would otherwise be the case, e.g., plus or ~i minus ten kilohertz. The prior art obtains quicker searches through the apparent Doppler frequency spectrum by using very high quality temperature-compensated crystal oscillators (TCXO). Such high quality local oscillators are expensive and add significant cost to GPS
receiver 10. These oscillators have stabilities on the order of one to two parts per million (ppm). Low cost oscillators can vary as much as twenty ppm. Therefore, a compromise is traditionally made between the expense of the local oscillator and the time-to-first-fix required to search through the apparent Doppler frequency spectrum.
~;. GPS receiver 10 has only to sweep within approximately three hundred hertz of a GPS
satellite signal to establish a lock on its carrier frequency. Such frequency sweeping is ~ implemented by causing navigation computer 28 to j,~,~',!,,, load a variety of divider values for numerically ,, i,., 1 ~' .
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. ;;. g ~; controlled oscillators (MCO) that are a part of LO 20. Phase lock loop (PLL) circuitry, ~` conventional to such de~ices within LO 20, provides the abil;ty to capture and lock within r~l 5 a limited range. Therefore, when searching for ;`~ a signal from a GPS satellite, a strategy, as ~- illustrated in Figs. 2A and 2B is used.
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In Fig. 2A, a three-channel GPS receiver is directed to align its three channels (CH0, Clll and CH2) shoulder-to-shoulder in a first se-t (S1) of bins centered on a nominal center .l frequency. Fig. 2B shows a "ping-pong" search strategy that is used within each bin to locate the GPS sate:llite's signal. Initially, the search begins close in on one side (+ is shown) to the nominal center frequency of the respective bin, and bounces close in to the . .
opposlte side (- 1n the example). The bounce :~ back and forth takes the search within each bin wider and wider until the whole bin has been ~`~ searched. In Fig. 2A, once the search of the three bins of S1 have been completed, a second set (S2) of bins is tried next. In Fig. 2A, CH0 : , `
and CH1 are appointed to the next two higher bins above S1, and CH2 is appointed to the next bin below S1. The selection of which channels to use and which side gets the odd remainder ~-; channel is arbitrary. To balance the odd channel being used above the nominal frequency . 30 in S2, a set three (S3) has two bins below and . one bin above the nominal center frequency.
Many conventional search strategies for finding a signal within a pair of range boundaries exist, and many are compatible with the macro search strategy embodiment of the present invention described herein. The present invention is therefore not limited to the high , ~ . "
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,~ and low p.ing-pong search just described.
A parallel search computer-implemented di process 40, illustrated by the flowcharts of Flgs. 3A and 3B, causes GPS navigation computer ~ 5 28 to segment the apparent Doppler frequency i.~ spectrum into several sections (e.g., first through thi.rd search ranges in Fig. 2A) and conducts parallel searches within each section using the multiple GPS receiver channels ~; 10 available. In the case of a portable three-channel GPS receiver being used for GPS receiver 10, a part of the apparent Doppler frequency spectrum is divided into a lower, a nominal and an upper frequency segment. Enough time is allowed for parallel searches within each frequency segment to search plus or minus one kilohertz. If a GPS satellite signal is not found, the upper and lower channels are stepped out further so that each may search another segment of plus or minus one kilohertz.
Process 40 comprises a power up step 41, a ~'! step 42 that selects a satellite to search for, ;~ a step 43 predicts a nominal center frequency ~;l for the selected satellite, a step 44 partitions -the radio frequency spectrum and appoints the ~ available GPS receiver channels such that they i clus-ter about the nominal center frequency. A
~-i`1 step 45 causes each of the GPS receiver channels . ~, . .
`'~ to search a first set of respective frequency :., ;~
,i 30 bands in parallel (S1 in Fig. 2A). However, the `~i' apparent Doppler frequency spectrum that may -~ need to be searched may be wider than S1. A
decision is made in a step 46 whether a signal . has been found. If not, a step 47 decides ;~1 35 whether a search limit has been exceeded. For ~`~ example, the search limit could be a limit of time and/or of frequency. If the search limit . ,;, ,-`'.''''.~
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has been exceeded, then control passes back to .~ step 42 to select another satellite. Otherwise, a step 48 appoints the available channels to a ~ .
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. 5 estimates the oscillator error based on both :~ predicted and measured Doppler frequency. Then a split search is conducted. All subsequent ~ searches before the first fix use this estimate ;~' to refine, bound, or minimize the search range for other satellites. For example, if the actual Doppler frequency for a satellite is one ` kilohertz (Kllz) and the oscillator error is ten kilohertz, each satellite would take the limit to search, e.g., ten Kllz. Thus the ten K~lz . 15 range is only searched once, quickly.
~~ In Fig. 3B, step 92 is shown to comprise a ,,~""î' step 50 in which the highest GPS satellite is tried. If a signal from that GPS satellite was found, a step 51 directs the program control to exit. Otherwise, a step 52 selects the next highest GPS satellite not already tried. If a signal from that GPS satellite was found, a step ;i 53 directs the program control to exit.
Otherwise, a step 54 again tries the highest GPS
~;~ 25 satellite. If a signal from that GPS satellite f~- was found, a step 55 directs the program control ~~ to exit. Otherwise, a step 56 selects the next highest GPS satellite which appears to be below the horizon limit. A step 57 looks to see if , 30 all the available GPS satellites have been - -tried. If not, control returns to step 50.
Otherwise, a step 58 does a reset. For example, if there are six GPS satellites SV1 through SV6 with respective elevation angles of 70, 40, . 35 10, -5, -15 and -90~, then the order of selecting just described will select SV1, SV2, SV1, SV5, SV1, SV3, SV1, SV4, SV1 and SV6.
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~ l2-!`'',. After a reset as a result of step 58, the order will again be, SV1, SV2, SV1, SV5, SV1, SV3, ~ SVl, SV9, SV1 and SV6. It will eventually try !:':`',` all the satellites, regardless of elevation.
In the present invention, if the Doppler -~ frequency error is close in, as is usually the ;~i case, it may be found faster than can be done with the prior art. lhis results from placing more search capability near the predicted frequency, whlch ls aften very close to the actual frequency. For exampler if the actual ~4:' frequency is at -4 KHz and nominal at -l KHz, the present invention can Eind the signa] in the first pass. It would take a maximum search time to find the signal for a non-precomputed or a non-centered alternate technique.
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With greater numbers of GPS receiver i channels available in GPS receiver lO, more ~;., search ranges can be searched in parallel at any ;~ 20 one time. With an even number of channels, e.g., six channels, the search ranges are i~ preferably positioned in frequency such that the - nominal frequency is near the center of one of , ~ the two middle search ranges, rather than along -~ 25 a border between them. Although this makes the `~ search range a little lopsided around the -.
;i nominal frequency, the probability of finding a GPS satellite signal is a bit better at the ` nominal frequency and a search will conclude sooner because searches typically start at the .,, .j ~ center of each search range, not the edges.
-`i Fig. 2A is typical of the sequence of such ~.~1 searches from S1 through S3 and beyond.
'':.':! If antenna 12 (Fig. 1) is covered when GPS
~::! 35 receiver lO is first turned on, navigation computer 28 would typically discard any 1~ assumptions or precomputed information it may :-.:
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~ 13-;; have had about which GPS satellites are probably i~ in the vicinity. ~fter a search had failed to track such GPS satellltes, other GPS satellites would be tried. In the prior art, the original ~1 5 set of GPS satellites would not be retried until all the others in orbit about the world had been tried first. The round-robin time of such a search can be exceedingly long, and can cause a user to assume GPS receiver 10 is inadequate or . ` 10 not functional. Prior art GPS receivers avoided ."
such problems by requiring the user to initialize the Ullit by asking for the current position. This is unpopular with users, because a GPS receiver is supposed to provide one with a position, not ask for it. Therefore, such queries have been eliminated, but the uncertainty still remains.
A split search computer-implemented process 60 is diagrammed in Fig. 4. In a step 62, the eight GPS satellites with the highest elevation ~` based on an estimate of position are selected.
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-.- It may be the case, however, that the assumed ~- horizon may be wrong. In a step 64, a decision is made whether a sufficient number of GPS
satellites are being tracked to obtain a current , ~
~ position fix. If not, a step 66 tests to see `.~ ':' ` if, since power up, a time-out has been exceeded and there are still an insufficient number of GPS satellites to compute a fix. For example, if five minutes has elapsed, a step 68 is engaged. Otherwise, control returns to step 64 to allow more time. Step 68 re-computes ~; elevations of GPS satellites every thirty ~",!.~ seconds and bases a search using one half of the GPS satellites based on position and the others 4~; ,`;,,.
regardless of position. For example, GPS
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~!: . As mentioned in reference to Figs. 2A and ;~ 2~, lnitial]y, only one set of search ranges or bi.ns is computed, based on the last position, -time and almanac, until a five minute time-out occurs. Then the "split" aspect is begun. A
fi.rst search l.ist preferably includes the highest elevation satellites that have been predicted by navigation computer 28 using the GPS almanac and the last position fix to be the !,~ most likely to be available for tracking.
` second list may include those satellites that - have been predicted by navigation computer 28 by . sequentially searching for satellites ;~:. 15 independent of position, thus their eleva-tion .~ angles may be negative based on -the present '~' ! posi-tion could be erroneous. This strategy is based on the assumption that if GPS receiver 10 has been moved since the last operational . 20 period, the travel i.s more probably to have been ., ~j a shorter trip rather than a longer trip, and ~-` therefore the horizon of the previous fix is ~:` more likely than the opposite side of the world, .' for example.
It can require as much as three seconds ,,:;;
after powering-on GPS receiver 10 to allow .,j navigation cornputer 28 to compute which PRNs -to ~ use, based on current time, a current almanac ;~.;.'9 and a previous position fix stored in memory. A
,;~; 30 precomputation process 80, illustrated in Fig.
5, can be run periodically after the receiver ~: has begun computing positions. Process 80 . builds a table, in a step 82, of the particular .:~ PRNs that should be used for various times in :~ 35 the near future. Therefore, if GPS receiver 10 is powered down and then powered back up a few hours later, GPS navigation computer can index ....
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and use the ]ist of P~Ns previously computed, in . a step 84, and thus save time to a first fix.
Ephemeris information obtained during previous periods of operation will be fresh enough and useful for only five or six hours after collection. ~lmanac information typically will -~.~ . .
have a useful life of approximately four weeks.
~' However, if the period of non-operation exceeds ~` several hours, the new ephemeris data collected must be processed and the advantage of precomputation will be proportionately diminished. Therefore, precomputation process -~ ao includes the on-going background routine 82 that builds a table of which PRNs should be used for various times in the near future and indexes each by time. The initialization subroutine 84, executed at power-up of GPS receiver 10, fetches ~:~ the PRNs from the table according to the current , . .
time.
Once a first GPS satellite signal has been --~ found, the apparent Doppler frequency can be -~ further constrained in searches for additional , - .
:; satellites, thus speeding up the search for ~` those additional satellites and the time it wi]l take to compute a first position fix. This occurs when the oscillator uncertainty is much greater than the Doppler frequency caused by the relative motion between the satellite and the a~ -.
receiver.
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s~ 30 In the present invention, if the first . searched satellite is not found after lapse of a ;.~
period of time sufficient to search the entire Doppler frequency spectrum, then a first assumption that such first satellite should have been visible given a probable position, is `i discarded. Then a second assumption is made that causes the next lowest elevation satellite ~,~
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~;~ elevations tend to have the strongest signals and a mlnlmum of Doppler Erequency shlEt.) If the second assumption fails, the first assumption is retried, followed again by the second assumption and then a third assumption .- that causes the still next lowest elevation satellite to be sought. Each assumptLon will `~ typically dwell for twenty seconds.
Every fourth time, a satellite below the horizon is tried, in case the receiver has moved a large distance. The assumptions about which satellites are visible can become invalid when a GPS receiver has been shipped a great distance ;. 15 since the last time it was turned-on and allowed to acquire a signal. However, such assumptions , cannot be discarded too quickly or too -~r~ permanently because temporary problems can take a while to clear and the initial assumptions could well be correct. For example, a GPS
~;i receiver in a car parked in a tall parking -~ ` garage could be den:ied GPS signal availability after turn-on for several minutes while the car :~ makes it way out through the structure.
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i i 25 The present invention further provides for ;: ,~,.
a precomputation of which GPS satellites will be visible at various times in the near future.
This allows a GPS receiver to be turned-off and ~;- then turned back on up to several hours later , . . .
and to use the identity of the GPS satellite ~` ~ost likely to be at the highest elevation, and thus the most visible, in an initial search.
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not found after lapse of a period of time ~; sufficient to search the entire Doppler frequency spectrum, based on the current . ., ^ .
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Although the present invention has been described in terms of the presently preferred ;.l 15 ernbodiments, it is to be understood that the ~' disclosure is not to be interpreted as limiting.
. Various alterations and modifications will no ,., . ,~
doubt become apparent to those skilled in the ar-t after having read the above disclosure.
~ccordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.
What is claimed is:
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and use the ]ist of P~Ns previously computed, in . a step 84, and thus save time to a first fix.
Ephemeris information obtained during previous periods of operation will be fresh enough and useful for only five or six hours after collection. ~lmanac information typically will -~.~ . .
have a useful life of approximately four weeks.
~' However, if the period of non-operation exceeds ~` several hours, the new ephemeris data collected must be processed and the advantage of precomputation will be proportionately diminished. Therefore, precomputation process -~ ao includes the on-going background routine 82 that builds a table of which PRNs should be used for various times in the near future and indexes each by time. The initialization subroutine 84, executed at power-up of GPS receiver 10, fetches ~:~ the PRNs from the table according to the current , . .
time.
Once a first GPS satellite signal has been --~ found, the apparent Doppler frequency can be -~ further constrained in searches for additional , - .
:; satellites, thus speeding up the search for ~` those additional satellites and the time it wi]l take to compute a first position fix. This occurs when the oscillator uncertainty is much greater than the Doppler frequency caused by the relative motion between the satellite and the a~ -.
receiver.
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s~ 30 In the present invention, if the first . searched satellite is not found after lapse of a ;.~
period of time sufficient to search the entire Doppler frequency spectrum, then a first assumption that such first satellite should have been visible given a probable position, is `i discarded. Then a second assumption is made that causes the next lowest elevation satellite ~,~
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~;~ elevations tend to have the strongest signals and a mlnlmum of Doppler Erequency shlEt.) If the second assumption fails, the first assumption is retried, followed again by the second assumption and then a third assumption .- that causes the still next lowest elevation satellite to be sought. Each assumptLon will `~ typically dwell for twenty seconds.
Every fourth time, a satellite below the horizon is tried, in case the receiver has moved a large distance. The assumptions about which satellites are visible can become invalid when a GPS receiver has been shipped a great distance ;. 15 since the last time it was turned-on and allowed to acquire a signal. However, such assumptions , cannot be discarded too quickly or too -~r~ permanently because temporary problems can take a while to clear and the initial assumptions could well be correct. For example, a GPS
~;i receiver in a car parked in a tall parking -~ ` garage could be den:ied GPS signal availability after turn-on for several minutes while the car :~ makes it way out through the structure.
.....
i i 25 The present invention further provides for ;: ,~,.
a precomputation of which GPS satellites will be visible at various times in the near future.
This allows a GPS receiver to be turned-off and ~;- then turned back on up to several hours later , . . .
and to use the identity of the GPS satellite ~` ~ost likely to be at the highest elevation, and thus the most visible, in an initial search.
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:~ AEter acquiring the firs-t satellite, a Y~ split search is conducted if a position fix is Y'.~:, ;iX
not found after lapse of a period of time ~; sufficient to search the entire Doppler frequency spectrum, based on the current . ., ^ .
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Although the present invention has been described in terms of the presently preferred ;.l 15 ernbodiments, it is to be understood that the ~' disclosure is not to be interpreted as limiting.
. Various alterations and modifications will no ,., . ,~
doubt become apparent to those skilled in the ar-t after having read the above disclosure.
~ccordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.
What is claimed is:
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Claims (5)
1. A method for searching for acquisition of a signal from a GPS satellite with a multiple channel GPS receiver, the method comprising the steps of:
pre-computing a table of pseudo random number (PRN) codes associated with GPS
satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position;
fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table;
searching for a signal from a first GPS satellite associated with said fetched PRN
code in parallel using said multiple channels of said GPS receiver such that a first search range is positioned with a nominal frequency for said GPS satellite signal at its mid-point and a first channel is appointed to search said first search range and a second channel is appointed to search in parallel a second search range adjacent to said first search range; and splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS
satellite signal was not found.
pre-computing a table of pseudo random number (PRN) codes associated with GPS
satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position;
fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table;
searching for a signal from a first GPS satellite associated with said fetched PRN
code in parallel using said multiple channels of said GPS receiver such that a first search range is positioned with a nominal frequency for said GPS satellite signal at its mid-point and a first channel is appointed to search said first search range and a second channel is appointed to search in parallel a second search range adjacent to said first search range; and splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS
satellite signal was not found.
2. The method of claim 1, wherein:
the pre-computing of said table includes ephemeris information obtained previous to said period of non-operation and said table extends forward in time for period limited by the validity of said ephemeris information.
the pre-computing of said table includes ephemeris information obtained previous to said period of non-operation and said table extends forward in time for period limited by the validity of said ephemeris information.
3. A method for searching for acquisition of a signal from a predetermined candidate GPS
satellite with a multiple channel GPS receiver, the method comprising the steps of:
pre-computing a table of pseudo random number (PRN) codes associated with GPS
satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position; and fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table.
satellite with a multiple channel GPS receiver, the method comprising the steps of:
pre-computing a table of pseudo random number (PRN) codes associated with GPS
satellites predicted to be visible at a plurality of times in the future based on a current time and GPS satellite almanac and a predetermined geographic position; and fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table.
4. A method for searching in parallel for acquisition of a signal from a GPS satellite with a multiple channel GPS receiver, the method comprising the steps of:
pre-selecting a particular carrier frequency as a nominal frequency within an apparent Doppler frequency spectrum for GPS
satellite signals;
partitioning said apparent Doppler frequency spectrum into a plurality of adjacent search ranges such that a particular first search range near the center of said apparent Doppler frequency spectrum includes said nominal frequency;
appointing a first of said GPS
receiver channels to search said first search range;
appointing a second of said GPS
receiver channels to search a second search range above and adjacent to said first search range;
appointing any but said first of said GPS receiver channels to search a third search range below and adjacent to said first search range;
searching within said first and at least one of said adjacent search ranges for said signal from a GPS satellite and terminating the search if said signal is found;
appointing any but said first of said GPS receiver channels to search a fourth search range above and adjacent to said second search range;
appointing any but said first of said GPS receiver channels to search a fifth search range below and adjacent to said third search range;
searching within said first and at least one of said adjacent search ranges for said signal from a GPS satellite.
pre-selecting a particular carrier frequency as a nominal frequency within an apparent Doppler frequency spectrum for GPS
satellite signals;
partitioning said apparent Doppler frequency spectrum into a plurality of adjacent search ranges such that a particular first search range near the center of said apparent Doppler frequency spectrum includes said nominal frequency;
appointing a first of said GPS
receiver channels to search said first search range;
appointing a second of said GPS
receiver channels to search a second search range above and adjacent to said first search range;
appointing any but said first of said GPS receiver channels to search a third search range below and adjacent to said first search range;
searching within said first and at least one of said adjacent search ranges for said signal from a GPS satellite and terminating the search if said signal is found;
appointing any but said first of said GPS receiver channels to search a fourth search range above and adjacent to said second search range;
appointing any but said first of said GPS receiver channels to search a fifth search range below and adjacent to said third search range;
searching within said first and at least one of said adjacent search ranges for said signal from a GPS satellite.
5. A multiple channel GPS receiver, comprising:
GPS navigational computer means for pre-computing a table of pseudo random number (PRN) codes associated with GPS satellites predicted to be visible at a plurality of times in the future based on a current time and GPS
satellite almanac and a predetermined geographic position;
table indexing means for fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table;
radio receiver means for searching for a signal from a first GPS satellite associated with said fetched PRN code in parallel using said multiple channels of said GPS receiver such that a first search range is positioned with a nominal frequency for said GPS satellite signal at its mid-point and a first channel is appointed to search said first search range and a second channel is appointed to search in parallel a second search range adjacent to said first search range; and multi-channel GPS receiver means for splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS satellite signal was not found.
GPS navigational computer means for pre-computing a table of pseudo random number (PRN) codes associated with GPS satellites predicted to be visible at a plurality of times in the future based on a current time and GPS
satellite almanac and a predetermined geographic position;
table indexing means for fetching a PRN code from said table according to a time said GPS receiver is re-powered up after a period of non-operation that followed the step of pre-computing said table;
radio receiver means for searching for a signal from a first GPS satellite associated with said fetched PRN code in parallel using said multiple channels of said GPS receiver such that a first search range is positioned with a nominal frequency for said GPS satellite signal at its mid-point and a first channel is appointed to search said first search range and a second channel is appointed to search in parallel a second search range adjacent to said first search range; and multi-channel GPS receiver means for splitting said multiple channels between searching for said first GPS satellite and a second GPS satellite if said first GPS satellite signal was not found.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/095,953 US5402347A (en) | 1993-07-22 | 1993-07-22 | Satellite search methods for improving time to first fix in a GPS receiver |
| US08/095,953 | 1993-07-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2127737A1 true CA2127737A1 (en) | 1995-01-23 |
Family
ID=22254349
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002127737A Abandoned CA2127737A1 (en) | 1993-07-22 | 1994-07-11 | Satellite search methods for improving time to first fix in a gps receiver |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US5402347A (en) |
| EP (1) | EP0635728B1 (en) |
| JP (1) | JP2892941B2 (en) |
| CA (1) | CA2127737A1 (en) |
| DE (1) | DE69420641T2 (en) |
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| JP6118535B2 (en) * | 2012-10-31 | 2017-04-19 | 古野電気株式会社 | Positioning signal search device, positioning device, positioning signal search method, and positioning signal search program |
| US10845487B2 (en) | 2017-06-13 | 2020-11-24 | Microsoft Technology Licensing, Llc | Acquisition in global navigation satellite systems based on previously acquired satellites |
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| US4578678A (en) * | 1983-11-14 | 1986-03-25 | The United States Of America As Represented By The United States National Aeronautics And Space Administration | High dynamic global positioning system receiver |
| US4785463A (en) * | 1985-09-03 | 1988-11-15 | Motorola, Inc. | Digital global positioning system receiver |
| US4972431A (en) * | 1989-09-25 | 1990-11-20 | Magnavox Government And Industrial Electronics Company | P-code-aided global positioning system receiver |
| JPH03146890A (en) * | 1989-11-02 | 1991-06-21 | Pioneer Electron Corp | Satellite wave capturing system for gps receiver |
| JP2529419B2 (en) * | 1989-11-14 | 1996-08-28 | 松下電器産業株式会社 | GPS receiver satellite selection method |
| JPH03162619A (en) * | 1989-11-21 | 1991-07-12 | Pioneer Electron Corp | Satellite radio wave acquisition system for gps receiver |
| JPH0748076B2 (en) * | 1989-11-30 | 1995-05-24 | パイオニア株式会社 | Satellite radio wave capturing method of GPS receiver |
| JPH03295483A (en) * | 1990-04-13 | 1991-12-26 | Pioneer Electron Corp | Satellite radio wave pickup system of gps receiver |
| JPH03235079A (en) * | 1990-02-13 | 1991-10-21 | Pioneer Electron Corp | Gps receiver |
| JPH03293576A (en) * | 1990-04-11 | 1991-12-25 | Pioneer Electron Corp | Satellite radio wave catching apparatus of gps receiver |
| US5117232A (en) * | 1990-06-04 | 1992-05-26 | Raytheon Company | Global system positioning receiver |
| US5101416A (en) * | 1990-11-28 | 1992-03-31 | Novatel Comunications Ltd. | Multi-channel digital receiver for global positioning system |
| US5148452A (en) * | 1990-12-31 | 1992-09-15 | Motorola, Inc. | Global positioning system digital receiver |
| US5192957A (en) * | 1991-07-01 | 1993-03-09 | Motorola, Inc. | Sequencer for a shared channel global positioning system receiver |
| US5175557A (en) * | 1991-07-18 | 1992-12-29 | Motorola, Inc. | Two channel global positioning system receiver |
| JPH05157826A (en) * | 1991-12-09 | 1993-06-25 | Hitachi Ltd | Gps satellite signal receiving method and gps receiving device |
-
1993
- 1993-07-22 US US08/095,953 patent/US5402347A/en not_active Expired - Lifetime
-
1994
- 1994-07-07 DE DE69420641T patent/DE69420641T2/en not_active Expired - Fee Related
- 1994-07-07 EP EP94110580A patent/EP0635728B1/en not_active Expired - Lifetime
- 1994-07-11 CA CA002127737A patent/CA2127737A1/en not_active Abandoned
- 1994-07-21 JP JP6169655A patent/JP2892941B2/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| DE69420641T2 (en) | 2000-06-08 |
| DE69420641D1 (en) | 1999-10-21 |
| EP0635728B1 (en) | 1999-09-15 |
| JP2892941B2 (en) | 1999-05-17 |
| US5402347A (en) | 1995-03-28 |
| EP0635728A1 (en) | 1995-01-25 |
| JPH07191127A (en) | 1995-07-28 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |