System and method for fast code phase and carrier frequency acquisition in GPS receiver

• US 20020064210A1
• Filed: 09/10/2001
• Published: 05/30/2002
• Est. Priority Date: 09/18/2000
• Status: Active Grant

• Current Assignee: SkyBitz, Inc. (Ametek Incorporated)
• Original Assignee: SkyBitz, Inc. (Ametek Incorporated)
• Inventors: Sullivan, Mark C.
• Granted Patent: US 7,027,486 B2
• Time in Patent Office: Days
• US Class Current: 375/145
• CPC Class Codes: G01S 19/235 Calibration of receiver components
• CPC Class Codes: G01S 19/258 relating to the satellite constellation, e.g. almanac, ephemeris data, lists of satellites in view
• 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
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

A GPS receiver acquires carrier frequency and Gold code phase using short segments of a received GPS signal. In one embodiment, a 1-ms segment of the GPS signal is transformed to the frequency domain. This is multiplied by a frequency representation of the Gold code. The resulting product is converted to the time domain, and a peak is detected. The location of the peak corresponds to the code phase. If no peak is located, the carrier frequency is changed. Full- and half-bin steps in carrier frequency are considered. Processing gain is achieved by using longer segments of the input signal, for example 4 or 16 ms and integrating 1-ms segments. Considerations are provided for compensating for the effects of a transition, should it occur in the short segment of the GPS signal being processed. Integrations can be performed using non-coherent and coherent techniques. Adjustments are made for non-integral millisecond segment lengths.

Claims

1. A GPS receiver, comprising:

a receiver for receiving a composite GPS signal comprising a received spreading code having a received spreading code frequency;

a digitizer for digitizing the received composite GPS signal at a sample rate;

a spreading code generator to generate a spreading code;

a convolver to convolve received composite GPS signal with the generated spreading code to generate a convolved output;

means for estimating a clock error due to the sample rate not being equal to an integral multiple of the received spreading code frequency;

a metric derived from the convolved output and the estimated clock error; and

means for maximizing the metric to determine a total time offset and a frequency offset.

2. The GPS receiver recited in claim 1, wherein the convolver performs a linear convolution.

3. The GPS receiver recited in claim 1, wherein the convolver performs a plurality of circular convolutions.

4. The GPS received recited in claim 1, further comprising:

means for estimating a data sequence modulating the spreading code;

means for generating an ideal convolver output corresponding to a convolution of an ideal composite GPS signal with the generated spreading code;

a multiplier for multiplying the convolver output with a complex conjugate of the estimated data sequence and a complex conjugate of the ideal convolver output; and

wherein the metric comprises an integration of application of the multiplier to consecutive segments of the collected GPS signal.

5. The GPS receiver recited in claim 1, wherein the means for maximizing the metric searches across different pre-determined time offsets to maximize the metric.

6. The GPS receiver recited in claim 1, wherein the convolver performs the convolution in the frequency domain.

7. A method for deriving a code phase and carrier frequency offset for use in a GPS receiver, comprising the steps of:

receiving a composite GPS signal comprising a received spreading code sequence having a receiver spreading code sequence frequency;

sampling the received composite GPS signal at a sample rate

generating a generated spreading code sequence;

convolving the received GPS signal with the generated spreading code;

estimating a clock error due to the sample rate not being equal to an integral multiple of the received spreading code frequency;

deriving a metric from the convolved output and the estimated clock error; and

maximizing the metric to determine a total time offset and a frequency offset.

8. The method recited in claim 7, wherein the convolving step comprises the step of performing a linear convolution.

9. The method recited in claim 7, wherein the convolving step comprises the step of performing a plurality of circular convolutions.

10. The method recited in claim 7, further comprising the steps of:

estimating a data sequence modulating the spreading code;

generating an ideal convolver output corresponding to a convolution of an ideal composite GPS signal with the generated spreading code; and

multiplying the convolver output with a complex conjugate of the estimated data sequence and a complex conjugate of the ideal convolver output; and

integrating an output of the multiplier over consecutive segments of the collected GPS signal.

11. The method recited in claim 7, wherein the maximizing step comprises the step of searching across different pre-determined time offsets to maximize the metric.

12. The method recited in claim 7, wherein the convolving step further comprises the steps of:

converting the received GPS signal to a frequency domain;

converting the generated spreading code sequence to the frequency domain;

multiplying the converted received GPS signal and spreading code sequences to generate a product; and

converting the product to a time domain.