says that as light moves through a given medium, low-frequency signals get
"refracted" or slowed more than high-frequency signals.
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.