GPS Receiver

Details GPS Receiver GPS Receiver

2002-11-18

2004-08-17

Issing, Gregory C. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Directive

Including a satellite

Reexamination Certificate

active

06778135

ABSTRACT:

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
This U.S. patent application is related to the following concurrently filed U.S. patent applications:
i) USING FFT ENGINES TO PROCESS DECORRELATED GPS SIGNALS TO ESTABLISH FREQUENCIES OF RECEIVED SIGNALS by Warloe et al, Ser. No. 10/298,447, filed Nov. 18, 2002;
ii) ADDRESS TRANSLATION LOGIC FOR USE IN A GPS RECEIVER by Warloe et al, Ser. No. 10/298,983, filed Nov. 18, 2002;
iii) SAVING POWER IN A GPS RECEIVER BY CONTROLLING DOMAIN CLOCKING by Warloe et al, Ser. No. 10/298,415, filed Nov. 18, 2002; and
iv) AVOIDING INTERFERENCE TO A GPS RECEIVER FROM WIRELESS TRANSMISSIONS BY TIME MULTIPLEXING GPS RECEPTION by Warloe et al, Ser. No. 10/298,414, filed Nov. 18, 2002, wherein these related U.S. patent applications are incorporated herein by reference in their entireties.
FIELD OF THE INVENTION
The present invention relates to a global positioning system (GPS) receiver, and in particular to an improved GPS receiver capable of efficiently performing a two-dimensional search for a received GPS signal and avoiding interference due to a transmission by an associated wireless communications device.
BACKGROUND OF THE INVENTION
The global positioning system (GPS) is based on an earth-orbiting constellation of twenty-four satellite vehicles each broadcasting its precise location and ranging information. From any location on or near the earth, a GPS receiver with an unobstructed view of the sky should be able to track at least four satellite vehicles, thereby being able to calculate the receiver's precise latitude, longitude, and elevation. Each satellite vehicle constantly transmits two signals, generally referred to as L
1
and L
2
. The L
1
signal from a satellite vehicle contains a unique pseudo-random noise code ranging signal (C/A code) with a chipping frequency of 1.023 MHz, system data with a bitrate frequency of 50 Hz, and an encrypted precise-code (y-code) with a chipping frequency of 10.23 MHz all being modulated onto a carrier frequency of 1575.42 MHz. The L
2
signal consists of the system data and y-code being modulated onto a carrier frequency of 1227.60 MHz.
In order to calculate a three-dimensional location, a receiver must determine the distance from itself to at least four satellite vehicles. This is accomplished by first determining the location of at least four satellite vehicles using ephemeris data received from the satellites. Once the locations of the satellites have been determined, the distance from the receiver to each of the satellites is calculated based upon the current estimate of receiver position. The measurement of the distance from the receiver to a satellite is based on the amount of time that elapsed between the transmission of a ranging signal from each satellite vehicle and the reception of that chip symbol by the receiver. In particular, the estimated position of the receiver is then corrected based upon a time epoch associated with the received ranging signal.
In order to acquire the L
1
or L
2
signal, the receiver must match the C/A code or y-code carried in the L
1
signal, or the y-code carried in the L
2
signal, with an internally generated code. For the C/A code, this is typically done by correlating the two signals by shifting the generated code through the 1023 possible time offsets of the C/A code until the generated code matches the C/A code carried in the L
1
signal. To improve the performance of the search, the generated code may be shifted at shorter intervals than a whole chip. For example, 2046 one-half chip positions may be searched. At the time offset when the generated code matches the C/A code carried in the L
1
signal, the two signals will cancel out, leaving only the carrier frequency and system data.
In addition to finding the time offset of the C/A code or y-code carried in the L
1
signal or the y-code carried in the L
2
signal, the frequency of the received L
1
or L
2
signal is typically determined. This may be done by generating a local L
1
or L
2
signal, and correlating this, together with the generated C/A or Y code with the received signal. Because of the movement of the satellite vehicles relative to the earth, the received frequency will experience a Doppler shift of +/−4,500 Hz from the transmitted frequency of the L
1
or L
2
signal. Another source of frequency uncertainty is the imperfection of the local oscillator, which typically can add a frequency offset of +/−20 ppm, or +/−30 kHz. However, a good part of this offset is due to variations in temperature, and may be modeled by a GPS receiver with a temperature sensor. With this modeling, the remaining temperature uncertainty could be around 10 kHz. Receiver movement may also cause a Doppler effect, however, this effect is usually insignificant when compared to the movement of the satellite vehicles in a commercial application. Due to the conventional method of the GPS signal detection, the receiver generated L
1
or L
2
signal needs to be within less that 500 Hz of the received signal for a successful search. Typically the frequency of the generated signal is incremented in 750 Hz intervals as the receiver searches for the correct code/carrier combination.
Therefore, a two-dimensional search of an approximately 30,000 Hz frequency range and the possible time offsets of the C/A code or the y-code must be made in order to acquire the L
1
or L
2
signal. GPS receivers have been designed to concurrently search all possible time offsets for the C/A code in the L
1
signal at a single frequency, thereby requiring an enormous number of correlators. After searching all 2046 time offsets, the frequency is then changed, and all time offsets are search for the new frequency. This process is repeated until the approximately 30,000 Hz frequency range has been searched.
In many applications, the GPS receiver is incorporated into a mobile device. Typically, the mobile device is powered by a battery, where battery life is a major concern of both the consumer and designers of the mobile device. Therefore, it is desirable for the GPS receiver to minimize power consumption. In addition, it is desirable to avoid degradation of the GPS receiver due to a strong signal being transmitted from the mobile phone.
Thus, there remains a need for a GPS receiver capable of concurrently searching the approximately 30,000 Hz range of frequencies to determine the precise frequency of the L
1
or L
2
signal and that reduces the number of correlators used to determine the time offset of the C/A code or the y-code carried in the L
1
or the y-code in carried in the L
2
signal. Further, there remains a need for the GPS receiver to minimize power consumption and avoid performance degradation caused by the transmission for an associated wireless communications device.
SUMMARY OF THE INVENTION
The global positioning system (GPS) receiver of the present invention operates to search a range of frequencies by correlating a baseband signal corresponding to a received GPS signal with a generated frequency and a generated code at a plurality of time offsets. Address translation logic operates to group results of the correlation according to the plurality of time offsets by translating addresses received from a direct memory access (DMA) controller in order to improve the efficiency of performing a fast Fourier transform (FFT) on the results of the correlation. The transformed data produced by the FFT process are used by the GPS receiver to determine a frequency of the received GPS signal and a time offset associated with a ranging code carried in the received GPS signal.
In addition, the GPS receiver includes jammer response circuitry that operates to control the correlation of the baseband signal with the generated frequency and the generated code at the plurality of time offsets based on detecting a transmission from an associated wireless transmitter, thereby avoiding performance degradation in the GPS receiver due to interference caused by the transmission. In general, the jammer response circuitry activat

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