By comparing the delays of the two different carrier frequencies of the GPS signal, L1 and L2, we can deduce what the medium (i.e. atmosphere) is, and we can correct for it.

Unfortunately this requires a very sophisticated receiver since only the military has access to the signals on the L2 carrier.

Civilian companies have worked around this problem with some tricky strategies.

Unfortunately they're so secret if we told you how they work we'd have to kill you.