Conventional GPS receivers acquire signals from GPS satellites with a precise local oscillator (LO) signal (i.e., with little frequency offset) and with a precise sample clock. Because GPS receivers operate at extremely low signal levels, long coherent combining of received GPS symbols is performed. The coherent time is generally limited by the LO frequency drift and sample clock time errors with respect to the GPS signals. In a conventional GPS receiver, a crystal oscillator used to generate these precise signals is relatively expensive and is typically accurate to within about 0.5 parts-per-million (ppm). The use of less expensive crystal oscillators with a greater time/frequency drift error (e.g., up to accuracy levels of +/−20 ppm) may result in sampling the received signals at incorrect sample times and/or performing correlations with excessive frequency offsets. This makes it difficult, if not impossible, to acquire the GPS signals.
Thus, there are general needs for GPS receivers that can acquire GPS signals using less expensive and/or less accurate crystal oscillators. There are also needs for GPS receivers that compensate for the sample time error and frequency offset due to less expensive and/or less accurate crystal oscillators, particularly during signal acquisition.
USPTO Backgrounds
USPTO Backgrounds