Receivers in a global navigation satellite system (GNSS), such as the Global Positioning System (GPS), use range measurements that are based on line-of-sight navigation signals broadcast from satellites. A receiver measures a time-of-arrival of one or more broadcast signals. This time-of-arrival measurement includes a time measurement based upon a coarse acquisition (C/A) coded portion of a signal, called pseudo-range, and a phase measurement.
The navigation signals from satellites in a GNSS are broadcast on carrier signal frequencies and use one or more pseudo-random codes. Navigation information, such as the pseudo-range and/or the phase measurement, in the navigation signals may be recovered in a receiver using correlation in a code tracking loop. Correlation may de-spread an energy of the navigation signals and thereby may de-modulate signals encoded with one or more pseudo-random codes. The correlation operation in effect mixes a received navigation signal from a satellite with a replica of the signal generated in the receiver. Generation of the replica of the signal in the receiver includes generating a corresponding pseudo-random code. If a phase, carrier signal frequency and timing of the replica signal substantially duplicate the received navigation signal from the satellite, an output power is maximized. If there are timing errors in the replica signal, the output power is reduced if the timing of the pseudo-random code is in error by less than one pseudo-random code bit, or is zero if the timing error is greater than or equal to the pseudo-random code bit.
There are, however, many different pseudo-random codes corresponding to navigation signals from different satellites in a GNSS (for those GNSSs that utilize code diversity multiple access methods to mitigate inter-satellite interference) as well as different GNSSs. In order to recover a wide variety of navigation signals, the receiver often will have circuitry for generating many different pseudo-random codes. This circuitry adds complexity and cost to the receiver. As a consequence, there is a need for improved generation of pseudo-random codes in GNSS receivers.