WO2003016937A1

WO2003016937A1 - Improvements to tracking systems

Info

Publication number: WO2003016937A1
Application number: PCT/AU2002/001094
Authority: WO
Inventors: Gregory Charles Hurst; Wayne Norman Agutter; Robert Schenk; William Mark Carney; Charles Leigh Homes
Original assignee: Quiktrak Networks Ltd
Priority date: 2001-08-13
Filing date: 2002-08-13
Publication date: 2003-02-27
Also published as: CN1541338A; KR20040019407A; EP1428037A1; AUPR697101A0

Abstract

A spread-spectrum location and tracking system, is provided in which the vehicle (10) or object to be located emits a continual direct sequence spread spectrum radio signal (11). This transmission is received at a number of wellspaced receiving stations (12) in the coverage area and the differences in the times of arrival of the signals at these receivers are measured by determining the epoch corresponding to the correlogram peak. The carrier frequencies ƒ1, ƒ2, ƒ3 of transmitter transmissions received at a receiver site (12), are estimated and frequency errors apart from those due to relative motion between the transmitter and a particular receive site, will be seen as a common offset by all receive sites, and may thus be eliminated by suitable processing, allowing estimate of the velocity vector of a moving transmitter from a single set of measurements at (3) or more remote sites.

Description

Improvements to tracking systems

Introduction

The present invention relates generally to improvements in communications systems and in a preferred embodiment the invention is applied to a vehicle location and tracking system employing direct-sequence spread-spectrum (DSSS) transmission techniques.

Throughout this specification the word "comprise", or variations such as

"comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.

Summary of the Invention According to a first aspect, the present invention comprises a method of determining a speed and direction of motion of a mobile transmitter, the method comprising: transmitting a signal from the mobile transmitter; receiving the transmitted signal at three or more receivers positioned at spaced locations on a plane having known positions relative to one another; measuring time of arrival information for the signal received at each receiver; calculating transmission path length differences from the time arrival measurements from the transmitter to each of the receivers and using the path length differences to calculate a location of the transmitter relative to the receivers using multi-lateration; for the signal received at each receiver, determining a frequency offset from a nominal transmission frequency of the transmitter; using the frequency offsets measured at the receivers and the location of the transmitter relative to the receivers to calculate the instantaneous speed and direction of motion of the transmitter relative to the receivers in the plane of the receivers.

Preferably the receivers are located at known fixed positions relative to the earth's surface and spaced such that a plane defined by the receivers approximates the earth's surface, whereby the speed and direction of travel of the transmitter are calculated relative to the earth's surface.

Preferably in the 2 dimensional case with 3 receivers, the following calculations are performed to determine the speed and direction of travel of the transmitteπ- Where: i) the three receivers are located at base stations Bi, B₂, B₃; ii) the transmitter T is located such that the angle subtended between the line drawn from the first base station Bi and the transmitter T and the second base station B₂ and the transmitter T is θ, and the angle subtended between the line drawn from the first base station Bi and the transmitter T and the third base station B₃ and the transmitter T is δ; iii) the carrier frequency of the transmitted signal is f_c; iv) the maximum Doppler shift due to motion of the transmitter is;

f_d = f_c.v/c (1 )

where v is the speed of the transmitter in the plane of the base stations and c is the speed of propagation of radio signals; v) the measured carrier frequencies fi, f₂, f₃ of the signals received at the base stations Bi, B₂, B₃ respectively are related to the maximum Doppler shift by:

f₂ = f_c + fdCθs(θ - α) (3) f₃ = f₀ + f_dcos(α + δ) (4)

where α is the angle between the direction of travel of the transmitter T and the line joining the first base station Bi and the transmitter T such that α and f_d can be determined by solving:

f 1 - f ₂ = fd(cos α - cos(α - θ)) (5) and f ι - f 3 = fd(cos α - cos(α + δ)) (6)

where fi, f₂, f₃, θ, and δ are known.

The speed of the transmitter T can then be determined by substituting fd in equation (1 ), while α gives direction of travel relative to the line Bi - T. Measurements at more than three receivers can be used to provide an over-determined solution to reduce or eliminate the effects of ambiguity and/or random measurement errors.

This technique of velocity determination can be employed with any multiple receiver communication system where position can be unambiguously determined. However in the preferred embodiment the transmission system used is a direct- sequence spread-spectrum system in which the modulating signal is a filtered (band-limited) maximal length pseudo-random binary sequence (PRBS). The sequence length N (bits) and the chip rate f_n (code clock rate) are chosen such that N/f_n is greater than the signal propagation delay from the transmitter to the receivers.

According to a second aspect, the present invention comprises a method of determining a speed and direction of motion of a mobile transmitter, the method comprising: transmitting a signal from the mobile transmitter; receiving the transmitted signal at four or more receivers positioned at spaced locations having known positions relative to one another in space, and which are not contained in a single plane; measuring time of arrival information for the signal received at each receiver; calculating relative transmission path length differences from the time arrival measurements from the transmitter to each of the receivers and using the path length differences to calculate a location of the transmitter relative to the receivers using multi-lateration; for the signal received at each receiver, determining a frequency offset from a nominal transmission carrier frequency of the transmitted signal; using the frequency offsets at the receivers and the location of the transmitter relative to the receivers to calculate the instantaneous speed and direction of motion of the transmitter relative to the receivers.

Preferably in the three dimensional case with four receivers, the following calculations are performed to determine the speed and direction of travel of the transmitteπ-

Where: i) four receivers are located at base stations Bi, B₂, B_3|, B₄; ii) the transmitter T is located such that an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the second base station B₂ to the transmitter T is θ, an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the third base station B₃ to the transmitter T is δ, and an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the fourth base station B₄ to the transmitter T is φ; iii) the carrier frequency of the transmitted signal is f_c; iv) the maximum Doppler shift due to motion of the transmitter is;

f_d = f_c.v/c (1 )

where v is the speed of motion of the transmitter and c is the speed of propagation of radio signals; v) the measured carrier frequencies f-i, f₂, f₃, of the signals received at the base stations Bi, B₂, B₃, B₄ respectively are related to the maximum Doppler shift by:

fι = fo + fdFι(α, β, θ, δ, φ) (7) f₂ = f_c + fdF₂(α, β, θ, δ, φ) (8) f₃ = fc + fdF₃(α, β, θ, δ, φ) (9) f₄ = f_c + f_dF₄(α, β, θ, δ, φ) (10)

where α is the angle between the direction of travel of the transmitter T and the line joining the first base station Bi and the transmitter T, and β is the angle between the direction of travel of the transmitter T and the line joining the first base station B₂ and the transmitter T; vi) , β and fd are determined by solving equations 7, 8, 9 and 10, where fi, f₂, f₃, , θ, δ and φ are known; and vii) the speed of the transmitter T is determined by substituting f_d in equation (1 ), and α and β give direction of travel relative to the lines Bi - T and B₂ - T.

In the three dimensional case, measurements at more than four receivers can be used to provide an over-determined solution to reduce or eliminate the effects of ambiguity and/or random measurement errors.

According to a third aspect, the present invention comprises a method of determining a frequency error in a signal received by a direct sequence spread- spectrum communications receiver, the method comprising: mixing a received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signal over predetermined measurement periods of time to produce a plurality of sample points for each measurement period; processing the sample points from each measurement period to produce a contiguous set of complex correlograms; from the set of complex correlograms, determining a set of target points amongst the complex points in the correlogram which best represent the received signal, (typically the position of peaks or the leading edges of those peaks (target points) in a composite correlogram); monitoring the phase angle of the corresponding target points in successive complex correlograms to determine a sequence of phase changes (rotations) in the received signal; determining the mean rate of change of phase of the target points, and

(directly) translating the mean rate of change of phase to a frequency representing the frequency error of the received signal relative to the nominal or expected frequency of the received signal.

The frequency error is preferably used to compensate the mixing frequency to match the received frequency and thereby achieve an intermediate frequency signal closely matched to the intended intermediate frequency of the receiver.

Expressed generally, for typical embodiments, the intermediate frequency signal is sampled at a frequency fs equal to the chip rate fc times number of samples n_s per predetermined measurement period t_s divided by the product of the spreading code length Nc and the number of spreading code cycles Cs per predetermined measurement period ts

fs = fc . n_s /(Nc . Cs)

such that the number of samples n_s per predetermined measurement period t_s is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length ts which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

In a preferred embodiment, the intermediate frequency signal is sampled at approximately 4MHz over successive approximately 4 msec periods to produce sets of 16384 complex samples. These sample sets undergo processing to produce the correlograms for each respective period. The generated correlograms are complex, but in general only the magnitude is considered to determine the index (time offset) of the set of points which best represent the received signal (usually the peaks in the correlograms). This time offset (determined as an average or other suitable function over the complete set of sample correlograms) is used as a measure of time of arrival of the signal for position determination.

However, the effect of any frequency error in the received signal is to induce a rotation in the phase of the peak values of the correlograms. By monitoring the difference between the phases of peak values in successive correlograms, the rate of rotation and hence the frequency error in the original signal can be estimated.

According to a fourth aspect, the present invention comprises a method of improving the signal to noise ratio of a spread-spectrum receiver by reducing the effects of random noise and noise due to multi-path reception, the method comprising: mixing a received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signal over predetermined periods of time to produce a plurality of sample points for each period; processing the sample points from each period to produce a succession of complex correlograms; monitoring the phase of the peak values in successive correlograms to determine a rate of phase rotation of the received signal; determining from the rate of rotation of the received signal the frequency error of the received signal relative to the nominal or expected frequency of the signal; reprocessing the signal with compensation for the determined frequency error to modify the correlograms such that the phase of the signal peaks in successive correlograms is aligned; and performing a complex accumulation of the correlograms to provide some amount of cancellation of noise due to its randomly distributed phase.

Again expressed generally, for typical embodiments, the intermediate frequency signal is sampled at a frequency fs equal to the chip rate fc times number of samples ns per predetermined measurement period ts divided by the product of the spreading code length Nc and the number of spreading code cycles Cs per predetermined measurement period ts

fs = fc ■ n_s /(Nc . Cs)

such that the number of samples ns per predetermined measurement period ts is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length t_s which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

In a preferred embodiment, the intermediate frequency signal is sampled at approximately 4MHz over successive approximately 4 msec periods to produce sets of 16384 sample points. These sample sets undergo processing to produce the complex correlograms for each respective period. The effect of any frequency error in the received signal is to induce a rotation in the phase of the peak value(s) of the correlograms. By monitoring the difference between the phases of successive correlogram peak values the rate of rotation and hence the frequency error in the original signal can be estimated.

Usual signal processing utilises accumulation (or averaging) of successive correlograms in order to improve the signal to noise ratio. Because of the inherent phase rotation of correlogram data, this processing is done using magnitude information only (incoherent accumulation). The effect of taking the correlogram magnitude is to rotate all data (noise plus signal) into a common plane and thus both noise and signal magnitude become linearly additive in the accumulation process. A net SNR improvement is achieved because the noise magnitude at any point in the correlograms varies randomly in amplitude with successive correlograms whereas the signal amplitude is generally large and constant.

However, if the rotation in successive correlograms is removed by adjusting the phase of the successive correlograms so that the signal peaks are phase aligned, then it is possible to perform a complex accumulation in which case the randomly distributed noise phase would produce some cancellation. In effect, the noise increases rms-wise per accumulation whereas the signal would increase linearly per accumulation.

A potential benefit of the de-rotation technique is in the reduction of multi-path effects. The normal signal processed in the receive site, comprises the direct path signal plus a number of reflected signals. As reflected signals must, by definition, have taken a different path they will exhibit different Doppler frequency shifts if the transmitter and/or the reflecting body is in motion. When the received correlogram is de-rotated to align the phase of the primary (direct path) signal, the multipath signals may be rotated out of the primary signal plane, resulting in a decrease in observed multi-path amplitude when correlograms are accumulated.

According to a fifth aspect, the present invention comprises a method of determining the angle of arrival of a signal, the method comprising: mixing the received signals from two spaced antennas at a single receiver site to intermediate frequency signals; successively making a plurality of samples of the intermediate frequency signals over predetermined periods of time to produce a plurality of complex sample points for each period for each of the signals; processing the complex sample points from each period to produce a succession of correlograms for each of the two signals; comparing the phases of the peak values of the correlograms of the two signals; determining from the difference in phase between the two signals the difference in time of arrival of the two signals and hence, from the geometry of the location of the two antennas, the angle of arrival of the signal from the transmitter. In previous applications, the rate of phase rotation between successive correlogram measurements was determined in order to estimate the transmission frequency error. In the case of a measurement of simultaneous signals received at spatially separated antennas, the phase difference between correlograms calculated for each measurement represents the difference in path length of the signal from transmitter to antenna. Differential path length measurement allows calculation of the angle of arrival of the transmission relative to the antenna baseline.

In a sixth aspect, the present invention provides a method of determining a frequency error between two signals received by a spread-spectrum communications receiver from two transmitters separated in distance from each other, the method comprising: mixing each received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signals over predetermined measurement periods of time to produce a plurality of sample points for each measurement period; processing the sample points from each measurement period to produce a contiguous set of complex correlograms; from the set of complex correlograms, determining a set of target points amongst the complex points in each correlogram which best represent the received signals; monitoring the phase angle of the corresponding target points in successive complex correlograms of each received signal to determine a sequence of phase changes in the received signals relative to each other; determining the mean rate of change of phase of the target points, and translating the mean rate of change of phase to a frequency representing the frequency error of one of the received signals relative to the other received signal; and using the frequency error information to synchronise to the received signals.

Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 generally illustrates a vehicle tracking system in which the improvements to a communication system according to the present invention might be used; and Figure 2 is a diagram showing the trigonometry used to calculate a speed and direction of a moving transmitter from frequency shifts in received signals.

Figure 3 is a block diagram illustrating the receiver processing used in the preferred embodiment.

Detailed Description of the Preferred Embodiments

The present invention will be described by way of example with reference to a multi-vehicle location and tracking system generally illustrated in Figure 1 , which employs a direct-sequence spread-spectrum communication system as the means of determining vehicle position.

In direct-sequence spread-spectrum transmission systems, the receiver extracts information transmitted by cross-correlating the incoming composite stream with the spreading code associated with the transmission. When the clock rate and the epochs of the in-coming and locally-generated spreading codes match, the spread-spectrum signal is folded (condensed) into a small number of adjacent points in the correlogram whilst all other non-matching signals (generally noise) remain spread across the whole correlogram. The value of the cross-correlation between the incoming signal and the noise-free version of the spreading code as a function of relative time delay (epoch) over one complete cycle of the spreading code is termed the correlogram.

Referring to Figure 1 it may be seen that in a spread-spectrum location and tracking system, the transmitter on the vehicle 10 or object to be located emits a continual direct sequence spread-spectrum radio signal 11. This transmission is received at a number of well-spaced receiving stations 12 in the coverage area and the differences in the times of arrival of the signals at these receivers are measured by determining the epoch corresponding to the correlogram peak. Inverse hyperbolic navigation techniques then may be used to compute the position of the transmitter at the central computer 13 which then sends this information to an operator terminal. In the preferred embodiment, the spreading code is a 511 bit, maximal length pseudo-random binary sequence (PRBS) and is clocked at a rate of 1 Mbit sec. In this case there is one complete cycle of the spreading code every 511 microsecs.

Embodiments of the present invention use a novel technique to estimate the carrier frequency f , /₂ , f₃ on transmitter transmissions received at a receiver site 12. Generally this frequency will be in error compared to its nominal value due to:

Frequency offset in the transmitter due to measurement and quantisation error in the carrier frequency synthesis control loop; • Frequency offset in the transmitter due to errors in the frequency- reference oscillator on the transmitter;

Frequency offset in the received signal at the receive site due to Doppler change due to relative motion between the transmitter and the receive site. All frequency errors apart from that due to relative motion between the transmitter and a particular receive site will be seen as a common offset by all receive sites and may thus be eliminated by suitable processing, allowing estimate of the velocity vector of a moving transmitter from a single set of measurements at 3 or more remote sites. The technique employed to estimate frequency error involves estimating the phase angle rotation in the peaks of the successive complex correlograms used to generate time-of-arrival data at a site. As a side benefit, once this phase rotation is known, the rotation may be removed from the original data, leading to improvements in the correlogram signal to noise ratio and hence improved tracking accuracy.

The same technique may be used to measure the carrier phase difference between a signal received at diverse receive antennas, allowing for the calculation of the angle of arrival of the direct-path signal and hence the direction vector of the transmitter relative to the antenna baseline.

Estimation of Received Frequency Error

The receive site signal processing assumes that the transmitter is transmitting on the correct frequency and mixes the received signal to an intermediate frequency (IF) signal appropriately. The IF is sampled at approximately 4MHz over approximately 4 msec periods to produce sample sets of 16384 complex points. This sample set undergoes processing to produce a correlogram. This processing comprises taking the Fourier Transform of the sample sets, multiplying the resulting complex frequency spectrum by a template comprising the complex conjugate of the Fourier Transform of the expected signal (with zero frequency error and known reference epoch), and then taking the inverse Fourier Transform of the result. (This process is illustrated in figure 3.) In the preferred embodiment, all operations following analog to digital conversion may be performed in a general purpose computer processing apparatus. The generated correlogram contains complex correlation values, but in general only the magnitude of these values is considered to determine the epoch (peak position) of the signal, which is used for position fitting.

However, the effect of any frequency error in the received signal is to induce a rotation, or progressive variation in the phase, of the complex correlogram samples. By monitoring the difference between the phases of successive correlogram peak values the mean rate of phase rotation and hence the frequency error in the received signal may be estimated.

Application to Velocity Estimation

The transmission frequency error measured at each receive site comprises a fixed error due to frequency offsets in the transmitter, plus a random measurement error plus the error due to Doppler shift of the frequency due to the component of the transmitter velocity vector in the direction of the receiving site. The common frequency error may be estimated given the transmitter and receive site locations and the observed frequency error at 3 or more receive sites. Once the common component is removed, the remaining residual errors may be used to calculate the relative motion between the transmitter and the receive sites. Measurements at more than 3 sites can be used to provide an over-determined solution to reduce or eliminate the effect of ambiguities and/or random measurement error.

Application to SNR Improvement

The accumulation of 4-msec samples of data to produce successive correlograms results in coherent accumulation of 8 complete code cycles for each correlogram. Such coherent accumulation provides maximum signal-to- noise ratio (SNR) improvement but can only be applied over a time during which the signal remains coherent. This time is determined by the frequency uncertainty of the carrier signal due to carrier frequency errors and errors due to Doppler shift and is practically limited to around 4 msec in the preferred embodiment. Usual signal processing utilises incoherent accumulation (or averaging) of successive correlograms in order to improve the signal to noise ratio further. Because of the inherent phase rotation of correlogram data, this processing can only be done using magnitude information only (i.e. incoherent accumulation).

The effect of taking the correlogram magnitude is to rotate all data (noise plus signal) into a common plane and thus both noise and signal become additive in the accumulation process. A net signal-to-noise ratio improvement is achieved because the noise magnitude is random in amplitude whereas the signal amplitude is generally large and constant.

However, if the rotation in successive correlograms is removed so that the signal peaks (representing the epoch of the signals) are phase aligned, then it is possible to perform a complex accumulation in which case the randomly distributed noise phase would produce some cancellation. In effect, the noise increases rms-wise per accumulation whereas the signal would increase linearly per accumulation. In effect, the coherence time has been extended indefinitely to provide all the benefits of coherent accumulation.

A specific application of the de-rotation technique is in the reduction of multi-path effects. The normal signal processed in the remote site comprises the direct path signal plus a number of reflected signals. As reflected signals must, by definition, have taken a different path they will exhibit different Doppler frequency shifts if the transmitter and/or the reflecting body is in motion. When the received correlogram is de-rotated to align the phase of the primary (direct path) signal, the phase of the multipath signals will typically exhibit phase rotation. The coherent accumulation of such phase rotating multipath signals will typically exhibit constructive and destructive accumulation and therefore not increase linearly with each accumulation step. This results in a decrease in observed multi-path amplitude when complex correlograms are accumulated.

Application to Angle of Arrival Measurements

In previous applications, the rate of phase rotation between successive correlogram measurements was determined in order to estimate the transmission frequency error. In the case of a measurement of a common signal simultaneously received at spatially separated antennas, the (carrier) phase difference between signal peaks in corresponding correlograms represents the difference(s) in path length of the common signal from its transmitter to each antenna. Differential path length measurement allows calculation of the angle of arrival of the transmission relative to the antenna baseline.

Application to General Frequency Measurement The techniques previously discussed for measurement of frequency error in the present embodiment may be more generally applied to the measurement of the error between any two frequency sources that are separated in distance and may thus be used as a remote frequency synchronisation method.

Determination of Speed and Direction

Referring to Figure 2, the determination of speed and direction of travel of a vehicle operating within the tracking system of the preferred embodiment is performed as follows:

Where: i) three receivers are located at base stations Bi, B₂, B₃; ii) the transmitter T is located such that the angle subtended between the line drawn from the first base station Bi and the transmitter T and the second base station B₂ and the transmitter T is θ, and the angle subtended between the line drawn from the first base station Bi and the transmitter T and the third base station B₃ and the transmitter T is δ; iii) the carrier frequency of the transmitted signal is f_c; iv) the maximum Doppler shift due to motion of the transmitter is;

f d = f c V/C (1 )

where v is the speed of the vehicle and c is the speed of propagation of radio signals; v) the measured carrier frequencies f-i, f₂, f₃ of the signals received at the base stations B-i, B₂, B₃ respectively are related to the maximum Doppler shift by:

fi = f_c + fdcos(α) (2) f₂ = f_c + fdθos(θ - α) (3) h = fc + fdθos(α + δ) (4) where α is the angle between the direction of travel of the transmitter T and the line joining the first base station Bi and the transmitter T;

such that α and fd can be determined by solving:

f 1 - f 2 = fd(cos α - cos(α - θ)) (5) and f 1 - f 3 = fd(cos α - cos(α + δ)) (6)

where i, f₂, f3, θ, and δ are known. The speed of the transmitter T can then be determined by substituting fd in equation (1), while α gives direction of travel relative to the line Bi - T.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A method of determining a speed and direction of motion of a mobile transmitter T, the method comprising: transmitting a signal from the mobile transmitter; receiving the transmitted signal at three or more receivers positioned at spaced locations on a plane having known positions relative to one another; determining time of arrival information for the signal received at each receiver; calculating transmission path length differences from the transmitter to each of the receivers and using the path length differences to calculate a location of the transmitter relative to the receivers using multi-lateration; for the signal received at each receiver, determining a frequency offset from a nominal transmission carrier frequency of the transmitted signal; using the frequency offsets at the receivers and the location of the transmitter relative to the receivers to calculate the instantaneous speed and direction of motion of the transmitter relative to the receivers in the plane of the receivers.

2. The method of claim 1 , wherein the receivers are located at known fixed positions relative to the earth's surface and spaced such that a plane defined by the receivers approximates the earth's surface, whereby the speed and direction of travel of the transmitter are calculated relative to the earth's surface.

3. The method of claim 1 or 2, wherein: i) three receivers are located at base stations Bi, B₂, B₃; ii) the transmitter T is located such that an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the second base station B₂ to the transmitter T is θ, and an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the third base station B₃ to the transmitter T is δ; iii) the carrier frequency of the transmitted signal is f_c; iv) the maximum Doppler shift due to motion of the transmitter is;

fd = fcV/C (1 )

where v is the speed of motion of the transmitter in the plane of the base stations and c is the speed of propagation of radio signals; v) the measured carrier frequencies fi, f₂, f3 of the signals received at the base stations Bi, B₂, B₃ respectively are related to the maximum Doppler shift by:

f ι = fc + fdcos(α) (2) f2 = fc + fdCθs(θ - α) (3) fa = fc + fdθθs(α + δ) (4)

where α is the angle between the direction of travel of the transmitter T and the line joining the first base station Bi and the transmitter T; vi) and fd are determined by solving:

f 1 - f 2 = fd(cos α - cos(α - θ)) (5) and f ι - f ₃ = f_d(cos α - cos(α + δ)) (6)

where fi, f₂, f3, θ, and δ are known; and

vii) the speed of the transmitter T is determined by substituting f_d in equation (1 ), and α gives direction of travel relative to the line Bi - T.

4. The method of claim 1 , 2 or 3, wherein measurements are made at more than three receivers to provide an over-determined solution to reduce the effects of ambiguities and/or random measurement errors.

5. The method of claim 1 , 2, 3or 4, wherein the transmission system is a direct- sequence spread-spectrum system in which the modulating signal is a filtered (band-limited) maximal length pseudo-random binary sequence (PRBS) with a sequence length N (bits)and a chip rate f_n (code clock rate) chosen, such that N/f_n is greater than a signal propagation delay from the transmitter to the receivers.

6. A method of determining a speed and direction of motion of a mobile transmitter T, the method comprising: transmitting a signal from the mobile transmitter; receiving the transmitted signal at four or more receivers positioned at spaced locations having known positions relative to one another in space, and which are not contained in a single plane; determining time of arrival information for the signal received at each receiver; calculating relative transmission path length differences from the transmitter to each of the receivers and using the path length differences to calculate a location of the transmitter relative to the receivers using multi- lateration; for the signal received at each receiver, determining a frequency offset from a nominal transmission carrier frequency of the transmitted signal; using the frequency offsets at the receivers and the location of the transmitter relative to the receivers to calculate the instantaneous speed and direction of motion of the transmitter relative to the receivers.

7. The method of claim 1 or 2 wherein: i) four receivers are located at base stations Bi, B₂, B3,, B₄; ii) the transmitter T is located such that an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the second base station B₂ to the transmitter T is θ, an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the third base station B₃ to the transmitter T is δ, and an angle subtended between lines drawn respectively from the first base station Bi to the transmitter T and from the fourth base station B₄ to the transmitter T is φ; iii) the carrier frequency of the transmitted signal is f_c; iv) the maximum Doppler shift due to motion of the transmitter is;

f_d = fc.v/c (1 )

where v is the speed of motion of the transmitter and c is the speed of propagation of radio signals; v) the measured carrier frequencies f-i, f₂, f₃, U of the signals received at the base stations Bi, B₂, B3, B₄ respectively are related to the maximum Doppler shift by:

fι = f_c + fdFι(α, β, θ, δ, φ) (7) f₂ = fc + f_dF₂(α, β, θ, δ, φ) (8) f₃ = f_c + f_dF₃(α, β, θ, δ, φ) (9) f₄ = f_c + f_dF₄(α, β, θ, δ, φ) (10)

where α is the angle between the direction of travel of the transmitter T and the line joining the first base station Bi and the transmitter T, and β is the angle between the direction of travel of the transmitter T and the line joining the first base station B₂ and the transmitter T; vi) α, β and f_d are determined by solving equations 7, 8, 9 and10, where fi, f₂, f3, , θ, δ and φ are known; and vii) the speed of the transmitter T is determined by substituting fd in equation (1), and α and β give direction of travel relative to the lines Bi - T and B₂ - T.

8. The method of claim 6 or 7, wherein measurements are made at more than four receivers to provide an over-determined solution to reduce the effects of ambiguities and/or random measurement errors.

9. The method of claim 6, 7 or 8, wherein the transmission system is a spread-spectrum system with a PN code of length Nbits and a chip rate f_n (code clock rate) for the PN code chosen, such that N/f_n is greater than a signal propagation delay from the transmitter to the receivers.

10. A method of determining a frequency error in a signal received by a direct sequence spread-spectrum communications receiver, the method comprising: mixing a received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signal over predetermined measurement periods of time to produce a plurality of sample points for each measurement period; processing the sample points from each measurement period to produce a contiguous set of complex correlograms; from the set of complex correlograms, determining a set of target points amongst the complex points in the correlogram which best represent the received signal; monitoring the phase angle of the corresponding target points in successive complex correlograms to determine a sequence of phase changes in the received signal; determining the mean rate of change of phase of the target points, and translating the mean rate of change of phase to a frequency representing the frequency error of the received signal relative to the nominal or expected frequency of the received signal.

11. The method of claim 10, wherein the position of target points in the complex correlograms are located by identifying the peak in the magnitude of each complex correlogram.

12. The method of claim 11 , wherein the sample sets undergo processing to produce the correlograms for each respective period, the generated correlograms are accumulated and the time of arrival of the signal for position determination is determined by determining the position of a set of points with a peak magnitude of the accumulated correlograms.

13. The method of claim 11 or 12, wherein the location of the target point in each complex correlogram is chosen by locating the first point in a set of points determined to represent a peak of the respective correlogram.

14. The method of claim 12 or 13, wherein the effect of a frequency error in the received signal is measured by monitoring the difference between the phases of successive correlogram peak values and estimating the frequency error by measuring the rate of phase variation.

15. The method of claim 10, 11 , 12, 13 or 14, wherein the measured frequency error is used to compensate the mixing frequency to match the received frequency and thereby achieve an intermediate frequency signal closely matched to an intended intermediate frequency of the receiver.

16. The method of claim 10, 11 , 12, 13, 14 or 15, wherein the intermediate frequency signal is sampled at a frequency fs equal to the chip rate fc times number of samples ns per predetermined measurement period ts divided by the product of the spreading code length Nc and the number of spreading code cycles Cs per predetermined measurement period ts

fs = fc • n_s /(Nc . Cs)

such that the number of samples ns per predetermined measurement period ts is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length ts which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

17. A method of improving the signal to noise ratio of a spread-spectrum receiver by reducing the effects of random noise and noise due to multi-path reception, the method comprising: mixing a received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signal over predetermined periods of time to produce a plurality of sample points for each period; processing the sample points from each period to produce a succession of complex correlograms; monitoring the phase of a peak value in successive correlograms to determine a rate of phase rotation of the received signal; determining from the rate of rotation of the received signal the frequency error of the received signal relative to the nominal or expected frequency of the signal; reprocessing with compensation for the estimated frequency error to modify the correlograms such that the phase of the signal peaks in successive correlograms are aligned; and performing an accumulation of the complex correlograms whereby noise cancellation occurs due to its randomly distributed phase.

18. The method of claim 17, wherein the time of arrival of the signal for position determination is determined by determining a position of a set of points with a peak magnitude of the correlogram.

19. The method of claim 17 or 18, wherein the location of the target point in each complex correlogram is chosen by locating the first point in a set of points determined to represent a peak of the respective correlogram.

20. The method of claim 17, 18 or 19, wherein the effect of a frequency error in the received signal is measured by monitoring the difference between the phases of successive correlogram peak values and estimating the frequency error by measuring the rate of phase variation.

21. The method of claim 17, 18, 19 or 20, wherein the measured frequency error is used to compensate the mixing frequency to match the received frequency and thereby achieve an intermediate frequency signal closely matched to an intended intermediate frequency of the receiver.

22. The method of claim 17, 18, 19, 20 or 21.wherein the intermediate frequency signal is sampled at a frequency f_s equal to the chip rate f_c times number of samples n≤ per predetermined measurement period ts divided by the product of the spreading code length N_c and the number of spreading code cycles Cs per predetermined measurement period ts

fs = fc • n_s /(N_c . Cs)

such that the number of samples n≤ per predetermined measurement period ts is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length ts which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

23. The method as claimed in any one of claims 17 to 22, wherein the estimated frequency error is used to adjust the phase of successive correlograms whereby the signal peaks are phase aligned, and the accumulation step is performed using a complex accumulation of the correlograms.

24. The method of claim 23, wherein the accumulation of complex correlograms is used to reduce multi-path effects.

25. A method of determining the angle of arrival of a signal, the method comprising: mixing the received signals from two spaced antennas at a single receiver site to intermediate frequency signals; successively making a plurality of samples of the intermediate frequency signals over predetermined periods of time to produce a plurality of complex sample points for each period for each of the signals; processing the complex sample points from each period to produce a succession of correlograms for each of the two signals; comparing the phases of the peak values of the correlograms of the two signals; determining from the difference in phase between the two signals the difference in time of arrival of the two signals and hence, from the geometry of the location of the two antennas, the angle of arrival of the signal from the transmitter.

26. The method of claim 25, wherein the intermediate frequency signal is sampled at a frequency fs equal to the chip rate fc times number of samples ns per predetermined measurement period ts divided by the product of the spreading code length N_c and the number of spreading code cycles Cs per predetermined measurement period t_s:-

fs = fc • n_s /(Nc . Cs)

such that the number of samples n_s per predetermined measurement period t_s is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length t_s which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

27. A method of determining a frequency error between two signals received by a spread-spectrum communications receiver from two transmitters separated in distance from each other, the method comprising: mixing each received signal to an intermediate frequency signal; successively making a plurality of samples of the intermediate frequency signals over predetermined measurement periods of time to produce a plurality of sample points for each measurement period; processing the sample points from each measurement period to produce a contiguous set of complex correlograms; from the set of complex correlograms, determining a set of target points amongst the complex points in each correlogram which best represent the received signals; monitoring the phase angle of the corresponding target points in successive complex correlograms of each received signal to determine a sequence of phase changes in one received signal relative to the other received signal; determining the mean rate of change of phase of the target points, and translating the mean rate of change of phase to a frequency representing the frequency error of the received signal relative to the other received signal; and using the frequency error information to synchronise the received signals.

28. The method of claim 27, wherein the position of target points in the complex correlograms are located by identifying peaks in the magnitude of each complex correlogram.

29. The method of claim 27 or 28, wherein the intermediate frequency signals are sampled at a frequency fs equal to the chip rate fc times number of samples ns per predetermined measurement period ts divided by the product of the spreading code length N_c and the number of spreading code cycles Cs per predetermined measurement period ts

fs = fc • n_s /(Nc . Cs)

such that the number of samples n_s per predetermined measurement period ts is a power of 2 selected to give a sampling frequency greater than or equal to the Nyquist Rate, and where the predetermined measurement periods have a period length ts which is a period over which coherence is generally assured and ts is an integer number of spreading code cycle periods.

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