GPS operates at a negative SNR. That also uses code division multiple access to allow multiple transmitters to operate on the same frequency and the signals do not interfere.
Imagine you're watching a trail of animal footprints in the snow, but the snow has covered parts of the trail (noise), so some prints are unclear. You're trying to figure out exactly which path the animal took. The Viterbi algorithm is like a detective method for doing just that, but instead of animal tracks, it's used for decoding messages or signals that got partially scrambled during transmission.
I use it with ham radio, where the software I use sends signals well under the noise floor (-23db) that can still get across call signs, signal reports, and maybe a thank you. The naked ear would not hear a thing at the low end of received signal.