The pseudorange is an approximation of the distance between a satellite and a GNSS receiver.
A GNSS receiver will attempt to measure the ranges of (at least) four satellites as well as their positions when their positional data were transmitted. With the satellites' orbital parameters supplied in the almanac within the message, each position can be calculated for any point in time.
The pseudoranges of each satellite are obtained by multiplying the time taken for each signal to reach the receiver by the speed of light. But because there will inevitably be accuracy errors in the time measured, the term pseudorange is used.
The problem is that the quartz oscillators used for timing in most GNSS receivers are only accurate to 1ppm at best. This means that if the clock hasn't been corrected for a week the distance error will be massive. Even when the clock is corrected, a second later the error can be hundreds of metres.
Fortunately, in a GNSS receiver the clock's time is used to measure the ranges to several different satellites simultaneously, and so all the measured ranges have the same error. Ranges with the same error are called pseudoranges. By finding the pseudorange of an additional fourth satellite, the time error can also be estimated.
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