The stationary receiver is the key. It ties all the satellite measurements into a solid local reference.

Here's how it works:

The problem

Remember that GPS receivers use timing signals from at least four satellites to establish a position. Each of those timing signals is going to have some error or delay depending on what sort of perils have befallen it on its trip down to us.

(For a complete discussion of all the errors review the "Correcting Errors" section of the tutorial.)

Since each of the timing signals that go into a position calculation has some error, that calculation is going to be a compounding of those errors.