Global navigational satellite systems (GNSS) are known and include the global positioning system (GPS) and the Russian global orbiting navigational satellite system (GLONASS). GNSS-based navigational systems are used for navigation and positioning applications. In the GPS navigational system, GPS receivers receive satellite positioning signals from a set of up to 32 satellites deployed in 12-hour orbits about earth and dispersed in six orbital planes at an altitude of 10,900 nautical miles. Each GPS satellite continuously transmits two spread spectrum, L-band signals: an L1 signal having a frequency fL1 of 1575.42 MHz, and an L2 signal having a frequency fL2 of 1227.6 MHz. The L1 signal from each satellite is modulated by two pseudo-random codes, the coarse acquisition (C/A) code and the P-code. The P-code is normally encrypted, with the encrypted version of the P-code referred to as the Y-code. The L2 signal from each satellite is modulated by the Y-code. The C/A code is available for non-military uses, while the P-code (Y-code) is reserved for military uses.
GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite (e.g., the travel time). The receiver generates a set of codes identical to those codes (e.g., the Y-code or the C/A-code) transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. By receiving GPS signals from four or more satellites, a receiver unit can accurately determine its position in three dimensions (e.g., longitude, latitude, and altitude). Receivers typically utilize the fourth satellite to accommodate a timing offset between the clocks in the receiver and the clocks in the satellites. Additional satellite measurements can be used to improve the position solution.
Conventional GPS receivers integrate the baseband quadrature components (I and Q) of the satellite signals, implement a power detection function, and then follow this with post detection summation. If the frequency error is large, acquisition of the signal is more difficult because of the effective bandwidth of the coherent integration process. One solution to this problem has been to shorten the sampling interval and to implement a banked filter process such as a Fast Fourier transform (FFT). Multiple outputs from the FFT can then be detected and summed in post detection filters. However, the FFT method has several disadvantages. For example, if the signal frequency falls between FFT frequency bins, it will be significantly attenuated. Also, if the signal frequency is near the outer edges of the FFT response, it will be attenuated by the integration process prior to sampling. Further, the FFT requires multiplication if more than four points are used. Further still, utilization of an FFT typically produces frequency bins with frequencies up to plus and minus one-half the interval between samples, thereby causing the frequency bins on the extremes of the pattern to be non-useful because of the attenuation caused by integration over the segment.
Co-pending and commonly assigned patent application Ser. No. 08/963,930, filed on Nov. 4, 1998, entitled “MULTIPLE FREQUENCY BIN PROCESSING” teaches a method for increasing signal acquisition in GPS receivers which overcomes many of the disadvantages of FFT described above. As taught in this co-pending application, an algorithm for a banked filter provides low attenuation between frequency bins. Using the technique described in this application, all of the frequency bins produce useful results and no multiplications are required to implement the algorithms. A shortened sampling interval is utilized in the banked filter process, with the multiple outputs of the banked filter being detected and summed in post detection filters. The invention as taught in this co-pending application provides enhanced signal acquisition.
Once the signal is acquired by the GPS receiver, the signal can be tracked using local versions of the GPS signal code and carrier. However, as the signal begins to degrade, the receiver can lose the ability to track the carrier, and the receiver enters a mode of operation known as “State 3” operation. State 3 is a standard GPS mode of operation where the signal has degraded to the point where the receiver cannot track the carrier itself, but can track the code if an adequate frequency reference is available. Typically, the frequency reference comes from an inertial navigation system (INS), for example in the form of velocity information from the INS. So long as the receiver is close to being on frequency, the code portion of the GPS signals can still be tracked with the aid of the frequency reference.
Extended Range Correlation (ERC) is a mode of operation similar to the State 3 mode of operation. However, in the ERC mode of operation, the receiver will typically integrate the signal for multiple seconds, as opposed to integration for one second as is common in State 3 operation. The ERC mode of operation is an attempt by the receiver to filter out additional noise in order to track the signal for as long as possible. In the State 3 and ERC modes, as the INS output or other frequency reference information begins to degrade, the quadrature components I and Q of the satellite signals become more difficult to track. An improved method of tracking the GPS signal after acquisition and during these modes of operation would be a substantial improvement in the art.