Pseudorandom noise ranging receiver which compensates for multipath distortion by dynamically adjusting the time delay spacing between e

What is claimed is:

1. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. a correlation means for synchronizing with the received version of the code the output of the code generator, the correlation means operating in an acquisition mode to synchronize the code generator to within one code chip and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode making correlation measurements that correspond to a correlator spacing that is substantially narrower than one code chip; and

ii. when operating in the acquisition mode making correlation measurements that correspond to stepping through the code in steps that are substantially wider than the narrow correlator spacing used in the tracking mode.

2. The receiver of claim 1, wherein the correlation means includes an early/late correlator that in the tracking mode makes correlation measurements to determine the correlation between the received version of the code and an early-minus-late version of the pseudo-random code produced by the code generator, the early/late correlator operating on signal samples that occur only when the early-minus-late version of the code is non-zero.

3. The receiver of claim 2, wherein the correlation means further includes a punctual correlator, which, in the tracking mode, makes correlation measurements associated with a punctual version of the code produced by the code generator.

4. The receiver of claim 3, wherein the early/late correlator operates as an early correlator and the punctual correlator operates as a late correlator when the correlation means is operating in the acquisition mode.

5. The receiver of claim 3, wherein the early/late correlator operates as a late correlator and the punctual correlator operates as an early correlator when the correlation means is operating in the tracking mode.

6. The receiver of claim 1 further including a plurality of code generators and an associated plurality of correlation means, each of the correlation means measuring the correlation between the output of associated code generator and the received version of the same code.

7. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. a correlation means for synchronizing with the received version of the code the output of the code generator, the correlation means operating in an acquisition mode to synchronize the code generator to within one code chip and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode making correlation measurements that correspond to a correlator spacing that is substantially narrower than one code chip, to reduce the adverse affects of multipath distortion on the code tracking operations; and

ii. when operating in the acquisition mode making correlation measurements that correspond to stepping through the code in steps that are substantially wider than the narrow correlator spacing used in the tracking mode.

8. The receiver of claim 7 further including a plurality of code generators and an associated plurality of correlation means, each of the correlation means measuring the correlation between the output of the associated code generator and the received version of the same code.

9. The receiver of claim 8, wherein each of the correlation means includes an early/late correlator that in the tracking mode makes correlation measurements to determine the correlation between the received version of the associated code and an early-minus-late version of the pseudo-random code produced by the code generator, the early/late correlator operating on signal samples that occur only when the early-minus-late version of the code is non-zero.

10. The receiver of claim 9, wherein each of the correlation means further includes a punctual correlator, which, in the tracking mode, makes correlation measurements associated with a punctual version of the code produced by the code generator.

11. The receiver of claim 10, wherein the early/late correlator operates as an early correlator and the punctual correlator operates as a late correlator when the correlation means is operating in the acquisition mode.

12. The receiver of claim 10, wherein the early/late correlator operates as a late correlator and the punctual correlator operates as an early correlator when the correlation means is operating in the acquisition mode.

13. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. correlation means for synchronizing the output of the code generator with a version of the code received at the receiver, the correlation means operating in an acquisition mode to synchronize the code generator to the received version of the code and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode simultaneously making early and late correlation measurements at a spacing of less than one code chip in which noise in the measured signals correlates; and

ii. when operating in the acquisition mode making correlation measurements in which the noise in the measured signals does not correlate.

14. The receiver of claim 13, wherein the early and late correlation measurements in the tracking mode are taken at a plurality of spacings, each of which is less than one chip.

15. The receiver of claim 13, wherein early and late correlation measurements are taken in the acquisition mode, at a spacing of at least one chip.

16. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. a correlation means for synchronizing the output of the code generator with a version of the code received at the receiver, the correlation means operating in an acquisition mode to synchronize the code generator to the received version of the code and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode making early minus late correlation measurements at a spacing of less than one code chip in which noise in the measured signals correlates; and

ii. when operating in the acquisition mode making early and late correlation measurements in which noise in the measured signals does not correlate.

17. The receiver of claim 16, wherein the early and late correlation measurements in the tracking mode are taken at a plurality of spacings, each of which is less than one chip.

18. The receiver of claim 16 wherein the early and late correlation measurements are taken at a spacing of at least one chip in the acquisition mode.

19. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. correlation means for synchronizing the output of the code generator with a version of the code received at the receiver, the correlation means operating in an acquisition mode to synchronize the code generator to the received version of the code and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode making early and late correlation measurements at a spacing of less than one code chip in which noise in the measured signals correlates and adverse effects of multipath distortion are minimized; and

ii. when operating in the acquisition mode making correlation measurements in which the noise in the measured signals does not correlate.

20. The receiver of claim 16, wherein the early and late correlation measurements in the tracking mode are taken at a plurality of spacings, each of which is less than one chip.

21. The receiver of claim 20 wherein early and late correlation measurements are taken in the acquisition mode, at a spacing of at least one chip.

22. A receiver for demodulating and decoding a composite radio-frequency ranging signal, consisting of a plurality of transmitted signals, one of which is modulated with a predetermined pseudo-random code, the receiver including:

A. a code generator for generating the pseudo-random code;

B. a correlation means for synchronizing the output of the code generator with a version of the code received at the receiver, the correlation means operating in an acquisition mode to synchronize the code generator to the received signal and operating in a subsequent tracking mode to track the received version of the code, the correlation means

i. when operating in the tracking mode making early minus late correlation measurements at a spacing of less than one code chip in which noise in the measured signals correlates and adverse effects of multipath distortion are minimized; and

ii. when operating in the acquisition mode making early and late correlation measurements in which noise in the measured signals does not correlate.

23. The receiver of claim 22, wherein the early and late correlation measurements in the tracking mode are taken at a plurality of spacings, each of which is less than one chip.

24. The receiver of claim 22, wherein the early and late correlation measurements in the acquisition mode are taken at a spacing of at least one chip.

Description Submit all comments and votes

FIELD OF THE INVENTION

This invention relates generally to digital radios which receive pseudorandom noise (PRN) encoded signals such as those used in navigation systems, and particularly to such a receiver adapted for use in signalling environments susceptible to multipath fading.

BACKGROUND OF THE INVENTION

Passive pseudorandom noise (PRN) ranging systems such as the United States' Global Positioning System (GPS) and the Russian Global Navigation System (GLONASS) allow a user to precisely determine his latitude, longitude, elevation and time of day. PRN ranging system receivers typically accomplish this by using time difference of arrival and Doppler measurement techniques on precisely-timed signals transmitted by orbiting satellites. Because only the satellites transmit, the need for two-way communications is avoided, and an infinite number of receivers may thus be served simultaneously.

In order for the receivers to extract the requisite information, the signals transmitted by the satellites must contain certain information. For example, within the GPS system, each carrier signal is modulated with low frequency (typically 50 Hz) digital data which indicates the satellite's ephemeris (i.e. position), current time of day (typically a standardized time, such as Greenwich Mean Time), and system status information.

Each carrier is further modulated with one or more unique, high frequency pseudorandom noise (PRN) codes, which provide a mechanism to precisely determine the signal transmission time from each satellite. Different types of PRN codes are used for different system applications. For example, within the GPS system, a so-called low-frequency "C/A code" is used for low cost, less accurate commercial applications, and a higher-frequency "P-code" is used for higher accuracy military applications.

Thus, a typical PRN receiver receives a composite signal consisting of one or more of the signals transmitted by the satellites within view, that is within a direct line-of-sight, as well as noise and any interfering signals. The composite signal is first fed to a downconverter which amplifies and filters the incoming composite signal, mixes it with a locally generated carrier reference signal, and thus produces a composite intermediate frequency (IF) signal. A decoder or channel circuit then correlates the composite signal by multiplying it by a locally generated version of the PRN code signal assigned to a particular satellite of interest. If the locally generated PRN code signal is properly timed, the digital data from that particular satellite is then properly detected.

Because the signals transmitted by different satellites use unique PRN codes and/or unique carrier frequencies, the receiver signals from different satellites are automatically separated by the multiplying process, as long as the locally generated PRN code has the proper timing. A delay lock loop (DLL) tracking system which correlates early, punctual, and late versions of the locally generated PRN code signal against the received composite signal is also typically used to maintain PRN code lock in each channel. The receiver's three dimensional position, velocity and precise time of day is then calculated by using the PRN code phase information to precisely determine the transmission time from at least four satellites, and by detecting each satellite's ephemeris and time of day data.

For more information on the format of the GPS CDMA system signals, see "Interface Control Document ICD-GPS-200, Sep. 26, 1984", published by Rockwell International Corporation, Satellite Systems Division, Downey, Calif. 90241.

For more information on the format of the GLONASS system signals, see "The GLONASS System Technical Characteristics and Performance", Working Paper, Special Committee on Future Air Navigation Systems (FANS), International Civil Aviation Organization (ICAO), Fourth Meeting, Montreal, Quebec, Canada, 2-20 May 1988.

A number of problems face the designer of PRN receivers. One problem concerns accurate phase and frequency tracking of the received signals; another problem concerns the correction of relative divergence between the received signals and the local PRN code signal generators in the presence of ionospheric distortion. In addition, because GPS systems depend upon direct line of sight for communication propagation, any multipath fading can further distort received signal timing estimates.

Certain GPS system designers have realized that the tracking error caused by multipath distortion in the out-of-phase condition can be reduced by narrowing the delay spacing between the early and late correlators in the DLL. However, this has heretofore not been thought to be advantageous under a wide range of operating conditions, since the DLL is then more susceptible to loss of lock due to sudden dynamic motions of the receiver. See, for example, Hagerman, L. L., "Effects of Multipath on Coherent and Non-coherent PRN Ranging Receiver", Aerospace Corporation Report No. TOR-0073(3020-03)-3, 15 May 1973.

As a result, most present-day PRN receivers use a DLL time-delay spacing of one PRN code bit (or chip) time. Historically, there have been several reasons for this adherence to one chip-time spacing.

For example, early PRN receivers were invariably of the P-code, or high frequency variety. Since P-code chip time is relatively narrow as compared with the correlator DLL spacing, it was feared that Doppler and random noise considerations would cause loss of PRN code lock if the correlator spacing was made any narrower.

Furthermore, narrower correlator spacing is not particularly desirable, as it increases the time required to lock onto a given PRN signal. This is of particular concern in PRN ranging systems, where often times many codes and code delays must be tried.

Finally, it has been thought that because a narrowed correlator spacing requires a higher precorrelation bandwidth, the resulting higher sampling rates and higher digital signal processing rates were not justified.

What is needed is a way to reduce the tracking errors present in PRN ranging receivers, especially those of the lower-frequency C/A code type, in the presence of multipath fading, without degrading the signal acquisition capability of the receiver, or increasing errors due to Doppler shift, sudden receiver motion, or other noise sources.

SUMMARY OF THE INVENTION

Briefly, the invention is an improved receiver for pseudorandom noise (PRN) encoded signals consisting of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple digital autocorrelators which form a delay locked loop (DLL) having dynamically adjustable code delay spacing.

The sampling circuit provides high-rate digital samples of a received composite signal to each of the several receiver channels. Each receiver channel includes a synchronizing circuit and a least two autocorrelators. The synchronizing circuits are non-coherent, in the sense that they track any phase shifts in the received signal and adjust the frequency and phase of a locally generated carrier reference signal accordingly, even in the presence of Doppler or ionospheric distortion. The autocorrelators in each channel form a delay lock loop (DLL) which correlates the digital samples of the composite signal with locally generated PRN code values to produce a plurality of (early, late), or (punctual, early-minus-late) correlation signals. The time delay spacing between the (early, late), and (punctual, early-minus-late) correlation signals is dynamically adjustable. Thus, during an initial acquisition mode, the delay spacing is relatively wide, on the order of approximately one PRN code chip time. However, once PRN code synchronizm has been achieved, the code delay spacing is narrowed, to a fraction of a PRN code chip time.

There are several advantages to this arrangement, especially in environments such as GPS C/A code applications wherein the multipath distortion in the received composite signal is of the same order of magnitude as a PRN code chip time. The PRN receiver is capable of acquiring carrier and code lock over a wide range of operating conditions, and once it is locked, will remain locked, even in the presence of multipath distortion.

Noise reduction is achieved with the narrower DLL spacing because the non-coherent synchronizer provides noise components of the (early, late) or (punctual, early-minus-late) signals which are correlated, and thus tend to cancel one another.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a PRN receiver which operates according to the invention, including its downconverter, sampler, channel, and processor circuits;

FIG. 2 is a block diagram of the downconverter circuit;

FIG. 3 is a block diagram of the channel circuit;

FIG. 4 is a block diagram of a carrier/code synchronizing circuit used in each channel circuit;

FIG. 5 is a timing diagram showing the relative duration of various portions of a received PRN signal;

FIG. 6 is a block diagram of a correlator circuit used in each channel circuit;

FIG. 7 is signal flow graph representation of the delay lock loop (DLL) operations performed by the correlator circuit and processor circuits to acquire PRN code lock;

FIG. 8 is a plot of calculated tracking error envelope versus multipath delay for various correlator code delay spacings and pre-correlation filter bandwidths;

FIG. 9 is a plot of calculated tracking error envelope versus multipath delay for various correlator code delay spacings at a pre-correlation filter bandwidth of 20 MegaHertz (MHz);

FIG. 10 is a plot of the difference between pseudo-range (PR) and accumulated delta range (ADR) measurements versus time for various PRN ranging receivers in a multipath environment, showing the improvement afforded by the invention; and

FIG. 11 is a plot of the differential measurement of FIG. 10 having the P-code data subtracted from the two C/A code data traces, which further shows the reduction in variance of the range measurements possible with the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Now turning attention to the drawings, FIG. 1 is an overall block diagram of a pseudorandom noise (PRN) ranging receiver 10 constructed in accordance with the invention. It includes an antenna 11, a downconverter 12, an in-phase and quadrature sampler 14, a processor 16, a control bus 18, a channel bus 20 and multiple channels 22a, 22b, . . . , 22n, (collectively, the channels 22). The illustrated receiver 10 will be primarily described as operating within the United States' Global Positioning System (GPS) using the so-called C/A codes, however, adaptations to other PRN ranging systems are also possible.

The antenna 11 receives a composite signal C.sub.s consisting of the signals transmitted from all participating satellites within view, that is, within a direct line of sight of the antenna 11. When the GPS system is fully operational world-wide, twenty-four satellites will be in operation, with as many as eleven GPS satellites being received simultaneously at some locations.

The composite signal C.sub.s is forwarded to the downconverter 12 to provide an intermediate frequency signal, IF, which is a downconverted and filtered version of the composite signal C.sub.s. The downconverter 12 also generates a sample dock signal, F.sub.s, which indicates the points in time at which samples of the IF signal are to be taken by the sampler 14. The downconverter 12 is discussed in greater detail in connection with FIG. 2.

The sampler 14 receives the IF and F.sub.s signals and provides digital samples of the IF signal to the channels 22 via the channel bus 20. The samples consist of in-phase (I.sub.s and quadrature (Q.sub.s) amplitude samples of the IF signal taken at the times indicated by the F.sub.s signal, typically by an analog-to-digital converter which samples at precisely 90.degree. phase rotations of the IF signal's carrier frequency. The Nyquist sampling theorem dictates that the sampling rate be at least twice the bandwidth of the IF signal. With the digital sample clock signal, F.sub.s, chosen according to these guidelines, the output samples from the sampler 14 are thus in in-phase and quadrature order as I,Q,--I, --Q, I,Q . . . and so on. The I and Q samples are then routed on separate signal buses, I.sub.s and Q.sub.s, along with the F.sub.s signal, to the channels 22.

Each channel 22 is assigned to process the signal transmitted by one of the satellites which is presently within view of the antenna 11. A given channel 22 thus processes the I.sub.s and Q.sub.s signals and tracks the carrier and code of the signal transmitted by its assigned satellite.

In particular, each channel 22 uses a carrier/code synchronizing circuit to frequency and phase-track the PRN encoded carder signal by maintaining an expected Doppler offset unique to the desired satellite. Each channel 22 also maintains a phase lock with a locally generated PRN code reference signal, by using two correlators connected as a delay lock loop (DLL).

The locally generated PRN code reference signal is then used to decode the data from the assigned satellite. The resulting decoded data, including the satellite's ephemeris, time of day, and status information, as well as the locally generated PRN code phase and carrier phase measurements, are provided to the processor 16 via the control bus 18. The channels 22 are described in detail in connection with FIG. 4.

The sampler 14 and channels 22 are controlled by the processor 16 via the control bus 18. The processor 16 includes a central processing unit (CPU) 162 which typically supports both synchronous-type input/output (I/O) via a multiple-bit data bus DATA, address bus ADDR, and control signals CTRL and synchronous controller circuit 164, and an interrupt-type I/O via the interrupt signals, INT and an interrupt controller circuit 166. A timer 168 provides certain timing signals such as the measurement trigger MEAS. The operation of the processor 16 and its various functions implemented in software will be better understood from the following discussion.

Referring now to FIG. 2, the downconverter 12 includes a bandpass filter 120, low noise amplifier 121, mixer 122, intermediate-frequency filter 123, and final amplifier 124.

The composite signal C.sub.S received from the antenna 11 typically consists of PRN modulated signals from all satellites within view (that is, within a direct line-of-sight of the receiver 10), any interfering signals, and noise. The PRN modulated signals of interest typically use L-band carder frequencies--the carrier signals used by various PRN ranging systems are as follows:

______________________________________ PARAMETERS FOR CERTAIN PRN RANGING SYSTEMS PRN L-Band Carrier Frequency Code Rate Power ______________________________________ GPS L1 C/A 1.57542 GHz 1.023 MHz --160 dBW GPS L1 P 1.57542 GHz 10.23 MHz --163 dBW GPS L2 1.22760 GHz 10.23 MHz --166 dBW GLONASS C/A 1.602 . . . 1.616 GHz 511 KHz GLONASS P 1.606 . . . 1.616 GHz 5.11 MHz ______________________________________

Natural background noise at about -204 dBW/Hz is typically mixed in with the L-band signals as well.

The composite signal C.sub.s is first fed to the bandpass filter 120 which is a low insertion-loss filter having a bandpass at the desired carrier frequency. The bandpass filter 120 should be sufficiently wide to allow several harmonics of the PRN code chips to pass. In the preferred embodiment for GPS C/A code reception, this bandwidth is at least 10 MHz.

After the received signal passes through the low-noise pre-amplifier 121, the mixer 122 downconverts it from the carrier frequency to a desired intermediate frequency that is within the frequency range of the sampler 14. The intermediate frequency filter 123 is also a bandpass filter. It serves as a pre-correlation filter having a sufficiently narrow bandwidth to remove any undesired signals, but sufficiently wide to maintain the desired bandwidth for detection. As will be described later, the bandwidth selected for this precorrelation filter 123 significantly affects the performance of the receiver 10 in multipath fading environments, and again is typically at least 10 MHz.

The final amplifier 124 is used as a pre-amplification stage to provide the output IF signal with appropriate amplification. Although the illustrated downconverter 12 is a single-stage downconverter, there could, of course, be additional intermediate stages.

A local reference oscillator 125 provides a stable frequency, digital, signal as the sample clock signal, F.sub.s, to both a synthesizer 132 and the sampler 14 (FIG. 1). A voltage controlled oscillator (VCO) 131, also coupled to the reference oscillator 125, generates an analog local oscillator reference signal, LO, whose frequency is a predetermined harmonic of the fundamental frequency of the digital reference signal, F.sub.s. This is accomplished by the synthesizer 132, which frequency-divides the LO signal by a predetermined number, multiplies it with the sample clock signal F.sub.s, and then feeds this output to a low-pass filter 133 which, in turn, provides a control voltage to the VCO 131. The VCO provides the reference signal LO to the synthesizer 132 and mixer 122.

A typical channel 22n is shown in FIG. 3. It includes a carrier/code synchronizer circuit 220, PRN code generator 230, two correlators 240a and 240b (collectively, correlators 240), and a code delay line formed by the flip-flops 250 and 251, XOR gate 255, and a switch 256.

Briefly, the synchronizer 220 is a single numerically controlled oscillator (NCO) which uses the sample clock F.sub.s and appropriate instructions from the processor 16 to provide the/control signals required by PRN code generator 230 and correlators 240 to non-coherently track the frequency and any carrier phase error caused by residual Doppler, as well as to track the PRN code.

The code generator 230 uses signal pulses output by the synchronizer 220 to generate a local PRN reference signal, PRN CODE, corresponding to the PRN code associated with the satellite assigned to channel 22n. The PRN CODE signal is also forwarded to the delay line flip-flops 250 and 251 which provide the PRN CODE signal, with selected delays, through the XOR gate 255 and switch 256 to the correlators 240. PRN code generators such as code generator 230 are well known in the art.

The correlators 240 also receive the I.sub.s, Q.sub.s, and F.sub.s signals from the channel bus 20. They may be configured in two modes--the switch 256 is used to select between the modes. In the first, (early, late) mode, correlator B 240b is configured as an early correlator and correlator A 240a is configured as a late correlator. This first mode is preferably used for initial PRN code synchronization. In a second, (punctual, early-minuslate) mode, correlator B 240b is configured as "early minus late" and correlator A 240a as punctual. This second mode is used for carrier and PRN code tracking. Both correlators 240 correlate, rotate, and accumu