GPS RECEIVER OPERATION |
There are two range-type measurements that can be made on the GPS signals:
Pseudo-ranges, and
Carrier phase observations.
Both are a product of the operation of the GPS receiver (that is, the acquisition and maintenance of signal tracking), both are used for GPS navigation (position, velocity and time -- PVT -- determination), and both have a role in the specialised data processing that characterises GPS surveying. Before studying these measurements it is useful to consider the overall GPS hardware tracking operation (in a much abbreviated form!).
The received satellite signal level is actually less than the background noise level, hence correlation techniques are used to obtain the satellite signals. A typical satellite tracking sequence begins with the receiver determining which satellites are visible above the horizon. Satellite visibility is estimated from predictions of present PVT, and on the stored satellite almanac information residing within the receiver. (If no stored almanac information exists, or only a very poor estimate of PVT is available, the receiver will carry out a "sky search", attempting to randomly locate and lock onto a signal. The receiver will then decode the Navigation Message and read the almanac information about all the other satellites in the constellation.) A carrier-tracking loop is used to track the carrier frequency while a code-tracking loop is used to track the C/A and/or P code signals. The two tracking loops have to work together in an iterative manner, aiding each other in order to acquire and track the satellite signals.
The receiver's carrier-tracking loop will locally generate an L1 carrier frequency (or L2 if the receiver is capable of tracking this frequency) which differs from the received carrier signal due to a Doppler offset of the carrier frequency. This Doppler offset is proportional to the relative velocity along the line-of-sight to the satellite. In order to maintain lock on the carrier, the carrier-tracking loop must, in effect, adjust the frequency of the receiver-generated carrier until it matches the incoming carrier frequency. The amount of this offset is the "beat" frequency which can be processed to give a periodic carrier phase measurement. The derivative of this phase measurement is the "Doppler" measurement, which is used to determine the receiver's velocity.
What role does the code-tracking loop play in this process?
In order for the carrier-tracking loop to acquire
the incoming satellite signal in the first place the carrier signal
must be made visible above the background noise. This is generally done
by the code-tracking loop using the code-correlating technique
to "reconstruct" the carrier wave (see discussion below under
"Carrier Phase Measurements"). A by-product of code-tracking are
the pseudo-range measurements.
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© Chris Rizos, SNAP-UNSW, 1999