1. Field of the Invention
The present invention relates to satellite navigation systems, and, more particularly, to satellite navigation systems employing modulation schemes to enhance signal-detection sensitivity.
2. Description of the Related Art
A satellite navigation system, such as the Global Positioning System (GPS), comprises a constellation of satellites that transmit GPS signals that can be used by a wireless terminal to determine the wireless terminal's position.
FIG. 1 is a schematic diagram of GPS system 100 of the prior art. In prior art system 100, one or more satellites 101 of a satellite constellation transmit GPS signals 102 that are received by a wireless terminal 103. As is known in the field, the positioning operation is performed by receiving GPS signals 102 from three or more satellites. The basic method of determining position is based on knowing the time difference for each of the satellites. The time difference for each satellite is the time required for a GPS signal 102 initiated at the satellite to be received by wireless terminal 103. When GPS signals 102 from three satellites are simultaneously received, a "two-dimensional" position (latitude and longitude) can be determined. When GPS signals 102 are received from four or more satellites simultaneously, a "three-dimensional" position (latitude, longitude, and altitude) can be determined.
Each satellite 101 orbits earth at a known speed and is located at a known distance apart from the other satellites. Each satellite 101 transmits a unique GPS signal 102 which includes a carrier signal with a known frequency f modulated using a unique pseudo-random noise (PN) code and navigational data associated with the particular satellite 101, wherein the PN code includes a unique sequence of PN chips and navigation data includes a satellite identifier, timing information and orbital data, such as elevation angle .alpha..sub.j and azimuth angle .phi..sub.j.
Wireless terminal 103 generally comprises a GPS receiver 105 for receiving GPS signals 102, a plurality of correlators 107 for detecting GPS signals 102 and a processor 109 having software for determining a position using the navigation data. GPS receiver 105 detects GPS signals 102 via PN codes. Detecting GPS signals 102 involves a correlation process wherein correlators 107 are used to search for PN codes in a carrier frequency dimension and a code phase dimension. Such correlation process is implemented as a real-time multiplication of a phase shifted replicated PN codes modulated onto a replicated carrier signal with the received GPS signals 102, followed by an integration and dump process.
In the carrier frequency dimension, GPS receiver 105 replicates carrier signals to match the frequencies of the GPS signals 102 as they arrive at GPS receiver 105. However, due to the Doppler effect, the frequency f at which GPS signals 102 are transmitted changes an unknown amount .DELTA.f.sub.j before GPS signal 102 arrives at GPS receiver 105--that is, each GPS signal 102 should have a frequency f+.DELTA.f.sub.j when it arrives at GPS receiver 105. To account for the Doppler effect, GPS receiver 105 replicates the carrier signals across a frequency spectrum f.sub.spec ranging from f+.DELTA.f.sub.min to f+.DELTA.f.sub.max until the frequency of the replicated carrier signal matches the frequency of the received GPS signal 102 wherein .DELTA.f.sub.min and .DELTA.f.sub.max are a minimum and maximum change in frequency GPS signals 102 will undergo due to the Doppler effect as they travel from satellites 101 to GPS receiver 105, i.e., .DELTA.f.sub.min.English Pound..DELTA.f.sub.j.English Pound..DELTA.f.sub.max.
In the code phase dimension, GPS receiver 105 replicates the unique PN codes associated with each satellite 101. The phases of the replicated PN codes are shifted across code phase spectrums R.sub.j (spec) until replicated carrier signals modulated with the replicated PN codes correlate, if at all, with GPS signals 102 being received by GPS receiver 105, wherein each code phase spectrum R.sub.j (spec) includes every possible phase shift for the associated PN code. When GPS signals 102 are detected by correlators 107, GPS receiver 105 extracts the navigation data ND from the detected GPS signals 102 and uses the navigation data to determine a location for GPS receiver 105, as is well-known in the art.
Correlators 107 are configured to perform parallel searches for a plurality of PN codes across the frequency spectrum f.sub.spec and the code phase spectrums R.sub.j (spec). In other words, each of the plurality of correlators 107 are dedicated to searching for a particular PN code across each possible frequency between f+.DELTA.f.sub.min to f+.DELTA.f.sub.max and each possible for that PN code. When a correlator 107 completes its search for a PN code, correlator 107 searches for another PN code across each possible frequency between f+.DELTA.f.sub.min to f+.DELTA.f.sub.max and each possible phase shift for that PN code. This process continues until all PN codes are collectively searched for by the plurality of correlators 107. For example, suppose there are twelve satellites 101 thus there would be twelve unique PN codes. If GPS receiver 105 has six correlators 107, then GPS receiver 105 would use its correlators 107 to search for two sets of six different PN codes at a time. Specifically, correlators 107 search for the first six PN codes, i.e., a first correlator searches for PN-1, a second correlator searches for PN-2, etc. Upon completing the search for the first six PN codes, correlators 107 search for the next six PN codes, i.e., a first correlator searches for PN-7, a second correlator searches for PN-8, etc.
For each PN code being searched, correlator 107 performs an integration and dump process for each combination of frequency and phase shifts for that PN code. For example, suppose the frequency spectrum f.sub.spec includes 50 possible frequencies for the carrier signal and the code phase spectrum R.sub.j (spec) for a PN code includes 2,046 possible half-chip phase shifts. To search for every possible combination of frequency and half-chip phase shifts for the PN code, the correlator 107 would then need to perform 102,300 integrations. A typical integration interval for correlators 107 is 1 ms, which is generally sufficient for GPS receiver 105 to detect GPS signals 102 when the wireless terminal has a clear view of the sky or a direct line-of-sight to satellites 101. Thus, for the above example, 102.3 seconds would be required for one correlator 107 to search every possible combination of frequency and half-chip phase shifts for a PN code.
One disadvantage of the prior art is that, if GPS signal 102 is attenuated by a building or other obstacles, it may become impossible for a wireless terminal to receive sufficiently strong GPS signals from the minimum number of satellites needed to determine the position of the wireless terminal. This results in an interruption of the position determination. To compensate for weaker GPS signals and enhance detection of GPS signals 102, correlators 107 can be configured with longer integration intervals. In other words, detection is more accurate with longer integration intervals.
However, the presence of the navigation data limits the signal-detection capabilities of a wireless terminal by limiting the length of the integration interval to 20 ms.