Just to be clear about this, "Less accurate" and "deliberately degraded" are not the same thing. L1 C/A is at this point is running at it's full potential accuracy. It's true to say it provides less accuracy than the L1 and L2 P(Y) codes (and the newer SPS and PPS signals), but that is by design, not because it's being "degraded".
Oh and to the original question, attenuation and scattering due to (falling) atmospheric precipitation should have a negligible effect on the GPS link budget. However significant deposits on the antenna radome could indeed have a debilitating effect.
Last edited by ion_berkley; 27th Jan 2013 at 07:33.