Method and device for rapidly extracting time and frequency parameters from high dynamic direct sequence spread spectrum radio signals under interference

• US 6,407,699 B1
• Filed: 04/14/2000
• Issued: 06/18/2002
• Est. Priority Date: 04/14/2000
• Status: Active Grant

• Current Assignee: Yang, Chun
• Original Assignee: Yang, Chun
• Inventors: Yang, Chun
• Primary Examiner(s): Zanelli, Michael J.
• Assistant Examiner(s): GIBSON, ERIC M
• Application Number:
  US09/550,064
• Time in Patent Office: 795 Days
• Field of Search: 342/357.12, 342/357.01, 342/357.06, 342/418, 342/378, 342/357.15, 375/148, 375/343, 375/346
• US Class Current: 342/357.59
• CPC Class Codes: G01S 19/21 Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
• CPC Class Codes: G01S 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073
• CPC Class Codes: G01S 19/243 Demodulation of navigation message
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence

Abstract

A GPS receiver and associated method that embodies high-dynamic, fast acquisition, and interference-suppressing capabilities for the reception and processing of GPS signals from a plurality of GPS satellites to produce GPS signal time and frequency parameters and navigation fixes. The GPS receiver includes an antenna and an analog front-end to intercept the incoming radio-frequency signal, band-limiting amplify the signal, and to convert it to an appropriate intermediate frequency so that it may be converted to digital form. One or more high-speed digital signal processors (DSP) constitute an all digital software baseband processor that process the sampled and quantified signals to form a two-dimensional delay-Doppler map of correlation power and to extract the signal time and frequency parameters and navigation data. The baseband processor is organized into functionally identical channels, each dynamically assigned to a different satellite visible. The baseband processor performs the incoming signal time-tagging, transformation, replica generation, interference-suppressing, delay-Doppler mapping and frequency uncertainty planning. The baseband processor also performs integration for larger processing gain, parameter extraction, and operational management. A relatively slow-speed microprocessor, coupled to the baseband processor, integrates the signal time and frequency parameters and navigation data from a plurality of GPS satellites to produce a navigation solution by a Kalman filter or a least-squares estimator.

Claims

1. In a navigational system utilizing a plurality of satellites that transmit radio-frequency signals embedded with time and frequency parameters, a method of obtaining navigational fixes in the presence of interference, comprising the steps of:

receiving the radio-frequency signals from a plurality of the satellites;

converting the radio-frequency signals received to a predetermined intermediate frequency with a band-limiting amplifying gain, thereby producing converted signals;

generating a local replica code;

processing said converted signals to form a delay-Doppler map of correlation power between the converted signals and said local replica code;

utilizing said delay-doppler map to extract navigation data and both time and frequency parameters; and

combining said navigation data and said time and frequency parameters from said plurality of satellites to produce a navigation solution.

2. The method according to claim 1, wherein said step of processing said converted signals to form a delay-Doppler map of correlation power further includes the substeps of:

generating a code replica sequence spectrum;

obtaining the complex conjugates of the code replica sequence spectrum;

sampling said converted signals and tagging samples of said converted signals with a local time code;

forming a two-dimensional map of correlation power calculated from said code replica sequence and the tagged samples.

3. The method according to claim 2, further including the substep of extracting time-frequency parameters from each said two-dimensional map that have been cumulated over a predetermined time period.

4. The method according to claim 2, wherein said substep of obtaining the complex conjugates of the code replica sequence spectrum, includes;

generating a complete code sequence for each available satellite;

resampling the code sequence according to the given Doppler frequency estimate;

converting the code replica sequence to a frequency domain; and

taking the complex conjugates of the converted code replica sequence spectrum.

5. The method according to claim 2, wherein said substep of sampling said converted signals and tagging samples of said converted signals with a local time code, includes:

tagging each incoming sample with the receiver local time and storing the tagged samples in an extended samples buffer;

repositioning the start sample within the extended samples buffer according to the estimated location of a correlation peak.

6. The method according to claim 5, further including the substeps of:

suppressing any narrowband interference if present from the incoming spectrum by spectral filtering methods; and

using a pseudo quadrature sampling scheme to form in-phase and quadrature samples.

7. The method according to claim 2, wherein said step of sampling said converted signals and tagging samples of said converted signals with a local time code further includes:

establishing the local time base starting point by reading from a local real-time clock; and

maintaining the local time base by adding up each interval used in sampling.

8. The method according to claim 6, wherein said step of suppressing any narrowband interference further includes:

monitoring the converted signals for any sudden power surge above an average noise level; and

filtering the converted signals if such a surge is detected.

9. The method according to claim 2, further including:

shifting said converted signals up and down circularly along a frequency axis, thereby creating a shifted incoming spectrum;

multiplying the shifted incoming spectrum with said code replica spectrum on a bin-to-bin basis to form a correlation function spectrum;

filtering the correlation function spectrum;

converting said correlation function spectrum from frequency-domain back to a time-domain to form the delay-Doppler map of correlation power;

cumulating the delay-Doppler maps of correlation power over a time period, made sufficient long by the given data bit sign, to narrow the equivalent noise bandwidth against wideband interference; and

detecting the presence of multipath signals from the correlation function spectrum to estimate multipath parameters for mitigation and processing.

10. The method according to claim 9, wherein said step of shifting said converted signals up and down circularly along a frequency axis further includes looping through a selected Doppler shift range.

11. The method according to claim 9, further including:

performing inverse discrete Fourier transforms for selected time lags to form a delay-Doppler sub-map of correlation power.

12. The method according to claim 2, wherein said substep of extracting time-frequency parameters from each said two-dimensional map that have been cumulated over a predetermined time period, further includes:

detecting the presence of said signal by threshold testing;

interpolating delay and Doppler estimates to a predetermined resolution;

generating a pseudo range measurement from an interpolated delay estimate and a local time tag;

generating a delta range measurement from the interpolated Doppler estimate and the selected Doppler shift; and

generating a carrier phase measurement from an interpolated complex correlation power and the estimated Doppler frequency.

13. The method according to claim 12, further including the steps of:

generating an estimate of carrier to noise density ratio;

achieving a navigation data bit sync based on a series of correlation values; and

formatting a time tag, pseudo range, delta range, carrier phase, carrier-to-noise ratio, and bit sync into a pre-specified set of observables.

14. The method according to claim 12, further including:

integrating the pseudo range measurements over at least a data bit interval;

taking an integer part of an interpolated location of the correlation peak to reposition a first-sample of the incoming sample segment; and

specifying a reduced range of delay lags for inverse transformation of correlation function from the frequency domain to the time domain.

15. The method according to claim 12, further including:

integrating the delta range measurements over at least a data bit interval;

converting a Doppler frequency estimate into a replica code resampling rate; and

specifying a reduced range of Doppler shifts for the frequency-domain Doppler removal from the converted signals.

17. The method according to claim 1, further including the steps of:

obtaining signal samples from a plurality of satellites;

repositioning said signal samples in a samples buffer;

time-tagging said signal samples with a local receiver time;

transforming said signals samples from a time domain to a frequency domain;

transforming said local code replica from the time domain to a frequency domain;

taking a complex conjugate of said local code replica;

multiplying said converted signals with said code replica code to produce a product spectrum;

transforming said product spectrum from the frequency domain back to the time domain and producing a delay-Doppler map of correlation power;

detecting a presence of a signal in the delay-Doppler map;

performing dynamic estimation of time and frequency parameters associated with any found signal;

obtaining data bit synchronization, and;

outputting navigation data bit and signal time and frequency parameters.

18. The method according to claim 17, further including:

resampling the local code replica according to an estimated Doppler frequency;

removing narrowband interference from incoming signals;

circularly shifting incoming signals along a frequency axis;

selecting the frequency of the circular shifting;

filtering said product spectrum;

selecting a range for the transformation to the time domain;

averaging the delay-Doppler map over a predetermined time interval;

interpolating the delay-Doppler map for finer time-frequency resolution;

integrating time-frequency parameter estimates over time to smooth out noise; and

estimating a carrier to noise density ratio as an indication of signal strength.

16. A receiver for receiving signals from satellites that are part of the global positioning system, said receiver including:

an antenna;

a radio frequency front-end that receives satellite signals from the satellites via the antenna, wherein said radio frequency front-end band limits the signals and converts the signals to an intermediate frequency;

a baseband processor containing a plurality of functionally identical channels, wherein a separate channel is dynamically assigned to each signal of a different satellite and each functionally identical channel determines navigation data and both time and frequency parameters embedded in the satellite signal processed by that channel; and

a navigation processor for receiving the navigation data from each baseband processor channel and calculation a positional fix.