GNSS works in the L band (1–2 GHz). That choice is a compromise: low enough to pass relatively well through the atmosphere and light vegetation, high enough for usable antenna dimensions. All GPS frequencies are derived from a single base clock of 10.23 MHz.
| Band | Frequency | Signals | Use |
|---|---|---|---|
| L1 | 1575.42 MHz | C/A, P(Y), M, L1C | civil + military; most widely used |
| L2 | 1227.60 MHz | P(Y), M, L2C | military + (modern) civil; ionospheric correction |
| L5 | 1176.45 MHz | L5 (Safety-of-Life) | civil aviation; more powerful, more robust |
The other systems partly sit on top of these: Galileo E1 overlaps L1, and E5a coincides with L5. That is deliberate — it makes multi-constellation receivers possible (Protection and anti-jam), but it also means that a single jammer on the L1 frequency hits GPS and Galileo at once.
The ionosphere delays the signal by an amount that depends on frequency. A receiver that measures on two bands at once (e.g. L1 and L2/L5) can calculate that delay and remove it — which is why accurate and military receivers are always dual-frequency. This matters for EW too: dual-band receivers are harder to jam completely than single-band ones.