As wireless communication networks continue to advance, new applications for wireless technology continue to be developed. The Global Positioning System (GPS), which generally enables the determination of location information, had been limited by Selective Availability (SA), which is the intentional degradation of the standard positioning service (SPS) signals by a time varying bias. SA is controlled by the United States Department of Defense and was used to limit accuracy for non-U.S. military and government users. Although there were ways to overcome SA and provide accurate location information, such GPS receivers capable of providing accurate location information were often expensive. However, on May 1, 2000, SA was turned off, enabling highly accurate GPS receivers at a significantly lower cost.
Further, recent regulations enacted by the Federal Communications Commission (FCC) have created a new market for GPS receivers. For example, recent requirements by the FCC have required that cellular telephones provide location information to a degree of accuracy which could be provided by GPS receivers.
Another application for GPS receivers can be found in the area of telematics. Telematics is a term generally related to the provisioning of data and/or services to vehicles. One particularly beneficial aspect of a telematics system is the transmission of location information related to a vehicle in the event of an emergency condition. For example, if a vehicle is in an accident and an air bag is deployed, the telematics unit in the vehicle will automatically contact a public safety answering point (PSAP) and transfer information such as the location of the device or information related to the status of vehicle systems.
GPS receivers can be generally very sensitive to reference oscillator frequency jumps. An oscillator can have its frequency deviate if the oscillator is subjected to mechanical vibrations, which is common in portable devices which incorporate a GPS receiver. Although a very accurate oscillator can be used in a GPS receiver, such oscillators can be expensive. Prior art GPS receivers have used a single numerically controlled oscillator (NCO) per satellite signal being tracked. The DSP processor which performs the searching, acquisition, and tracking of the GPS signals controls the frequencies of the NCOs. Unfortunately, the tracking bandwidth is very narrow. As a result, if a channel is being tracked, a deviation of NCO frequency beyond the bandwidth of the tracking loop can cause the receiver to drop out of track. Once the receiver is out of track, it has to reacquire the signal which can take a few seconds of time.
Another prior art receiver uses a parallel correlator architecture in order to increase the number of correlations done per unit time, and therefore decreases the time taken to acquire a signal from a satellite. In order to acquire a signal, it is necessary to know the frequency of the of the intermediate frequency signal provided to the correlators and the code phase of the replica pseudonoise (PN) code. Often neither are known. As a result, the search process involves correlating through the entire range of code phases, one NCO frequency at a time. The NCO frequency is adjusted after the receiver has exhausted all possible code phases and has not found a large correlation value. By increasing the number correlators used during a search, the receiver will spend less time per NCO frequency. The receiver will thus be able to search through more frequency bins per unit time, thereby decreasing the time needed for the receiver to acquire a GPS signal. Although such a receiver allows for fast recovery of acquisition from a jump in oscillator frequency, it will not prevent the receiver from being knocked out of track when an oscillator jump occurs.
Accordingly, there is a need for an improved receiver and method for determining an approximate intermediate frequency in a wireless communication device which provides a high degree of protection from tracking dropout due to fluctuations in the reference oscillator frequency.