The Natural Measurements of a GPS Receiver

Contrary to popular belief, the natural measurements of a GPS receiver are not pseudorange or delta pseudorange. The natural measurements are replica code phase which can be converted into satellite transmit time and replica carrier Doppler phase (if the GPS receiver is in phase lock with the satellite carrier signal) or replica carrier Doppler frequency (if the receiver is in frequency lock with the satellite carrier signal). This paper describes the natural measurements of a GPS receiver and how they may be converted into pseudorange and delta pseudorange or, in the case of the replica carrier Doppler phase measurements, how they can be converted into the so-called integrated carrier Doppler phase measurements used for ultra-precise static and kinematic surveying or positioning. The most important concept presented in this paper is the measurement relationship between the replica code phase in the GPS receiver and the satellite transmit time. It will be seen that this relationship is totally unambiguous for P(Y) code, but can be ambiguous for C/A code. Every C/A code GPS receiver is vulnerable to this ambiguity problem and, under weak signal acquisition conditions, the ambiguity will occur. When the ambiguity does occur in a C/A code GPS receiver, it causes serious range measurement errors, which, in turn, result in severe navigation position errors. Some commercial C/A code GPS receivers do not have built-in safeguards to detect or mitigate this potential problem.