Whispering in a crowded canteen and still being understood: that works if the listener knows exactly which words to expect. In the same way, a GPS receiver uses its known code to pick out a signal that is weaker than the noise.
In practiceThat same margin is also the weak spot: a jammer only has to exceed it. That is why a small jammer on a lorry can deny GPS over a large area, while military codes with a bigger margin (Codes and signals) demand more jamming power.
Understand this one concept and you understand both why GNSS can be jammed and how it defends itself. The received GNSS signal is weaker than the thermal noise of the receiver itself — typically around −160 dBW, some 20–30 dB below the noise floor. So you cannot even "hear" the signal; yet GNSS works. How?
Through despreading: the receiver multiplies the incoming signal by a local copy of the PRN code (Codes and signals) and integrates. The real signal, which carries exactly that code, adds up coherently into a sharp peak; noise and signals without that code average out. The gain this provides is called processing gain — exactly the same principle as pulse compression and pulse integration in radar (Pulse compression and pulse integration).
Two consequences that carry the rest of the chapter:
- The weakness: processing gain is large, but finite. A jammer only has to raise the noise by more than those ~43 dB above the signal to swamp the despreading. Because the GNSS signal is already so weak, that takes astonishingly little power (The fundamental vulnerability to GNSS jamming).
- The strength: the faster the code (P/M), the greater the processing gain and the more jamming power is needed. That is why the military codes are more robust — not through magic, but through bandwidth.