The idea is simple. Put the reference receiver on
a point that's been very accurately surveyed and keep it there.
This reference station receives the same GPS signals as the roving receiver but instead
of working like a normal GPS receiver it attacks the equations backwards.
Instead of using timing signals to calculate its position, it uses its known position
to calculate timing. It figures out what the travel time of the GPS signals should be, and
compares it with what they actually are. The difference is an "error correction"
The receiver then transmits this error information to the roving receiver so it can use
it to correct its measurements.