Method and apparatus for processing satellite positioning system signals

This application is a divisional patent application of and claims priority to co-pending U.S. patent application Ser. No. 10/838,517 filed May 4, 2004. U.S. patent application Ser. No. 10/838,517 is a continuation-in-part of the following U.S. patent applications:

(1) U.S. patent application Ser. No. 10/295,332, filed Nov. 15, 2002, which is a continuation of U.S. patent application Ser. No. 09/990,479, filed Nov. 21, 2001 (now U.S. Pat. No. 6,487,499), which is a continuation of U.S. patent application Ser. No. 09/553,930, filed Apr. 21, 2000 (now U.S. Pat. No. 6,453,237), which claims priority to U.S. provisional patent application No. 60/130,882, filed Apr. 23, 1999;
(2) U.S. patent application Ser. No. 10/359,468, filed Feb. 5, 2003, which is a continuation of U.S. patent application Ser. No. 09/989,625, filed Nov. 20, 2001 (now U.S. Pat. No. 6,587,789), which is a divisional of U.S. patent application Ser. No. 09/615,105, filed Jul. 13, 2000 (now U.S. Pat. No. 6,411,892);
(3) U.S. patent application Ser. No. 10/665,703, filed Sep. 19, 2003, which is a divisional of U.S. patent application Ser. No. 09/900,499, filed Jul. 6, 2001 (now U.S. Pat. No. 6,704,348), which is a continuation-in-part of U.S. patent application Ser. No. 09/861,086, filed May 18, 2001 (now U.S. Pat. No. 6,606,346);
(4) U.S. patent application Ser. No. 09/993,335, filed Nov. 6, 2001.
Each of the aforementioned patent applications, patents, and provisional application are incorporated by reference herein in their entireties.

1. Field of the Invention

The present invention relates to signal correlators for digital signal receivers and, more particularly, to a method and apparatus processing satellite positioning system signals in a mobile receiver.

2. Description of the Background Art

Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver.

More specifically, each GPS signal available for commercial use utilizes a direct sequence spreading signal defined by a unique pseudo-random noise (PN) code (referred to as the coarse acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carrier signal (referred to as the L1 carrier) and uniquely identifies a particular satellite. The PN code sequence length is 1023 chips, corresponding to a one millisecond time period. One cycle of 1023 chips is called a PN frame or epoch.

GPS receivers determine the time delays between transmission and reception of the signals by performing a series of correlations between the incoming signals and internally generated PN signal sequences (also referred to as reference codes or PN reference codes). For each incoming signal, the correlation process can be lengthy, as both the time delay and the exact frequency of the signal are unknown. To test for the presence of a signal, the receiver is tuned to a particular frequency, and the incoming signal is correlated with the PN reference code having a particular code shift corresponding to a time delay. If no signal is detected, the PN reference code is shifted and the correlation is repeated for the next possible time delay. To find a given signal, receivers traditionally conduct a two dimensional search, checking each delay possibility over a range of frequencies. In addition, each individual correlation is typically performed over one or more milliseconds in order to allow sufficient signal averaging to distinguish the signal from the noise. Because many thousands frequency and time delay possibilities are checked, the overall acquisition process can take tens of seconds.

The measured time delays produced by the correlation process are referred to as “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each measured time delay, then one has true, unambiguous pseudoranges. A set of four pseudoranges together with knowledge of absolute times of transmission of the GPS signals and satellite positions in relation to these absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission (or reception) are needed in order to determine the positions of the GPS satellites at the times of transmission and hence to compute the position of the GPS receiver.

Accordingly, each of the GPS satellites broadcasts a model of satellite orbit and clock data known as the satellite navigation message. The satellite navigation message is a 50 bit-per-second (bps) data stream that is modulo-2 added to the PN code with bit boundaries aligned with the beginning of a PN frame. There are exactly 20 PN frames per data bit period (20 milliseconds). The satellite navigation message includes satellite-positioning data, known as “ephemeris” data, which identifies the satellites and their orbits, as well as absolute time information (also referred to herein as “GPS time” or “time-of-day”) associated with the satellite signal. The absolute time information is in the form of a second of the week signal, referred to as time-of-week (TOW). This absolute time signal allows the receiver to unambiguously determine a time tag for when each received signal was transmitted by each satellite.