There exists a fundamental level of electrical noise at all frequencies typically controlled by the background temperature that sets the minimum level of signal you can resolve before it gets washed out in noise. GPS uses multiple coherent (noise being random instead of coherent does not combine at the same rate) copies of the signal that can be recombined to increase their effective signal, so even though they are transmitted below the universal noise floor, they are above it once recombined.

*edited for spelling