That the signals are not effected by atmospheric conditions is not true.
How does your weather radar work? By reflection of precipitation, right?
In the GPS we have two channels. One the L1 is used for civil use.
So why two channels?
The L1 contains the Precision code and the C/A (which is for civil use).
Both codes contains a model of the atmosphere. But the C/A time code is dithered to degrade the accuracy.
Anyways. The Military will use the L1 and the L2.
The L1 and L2 contains both the Precision code.
Since the channels have different frequencies the refraction in the atmosphere will be different. So the two P-codes will arrive at the receiver at different times. Because of the propagation in the atmosphere.
So the UHF band is affected by precipitation to some extent.
But that the receiver will stop working all together is something I have not heard of..