The essence of this chapter:
- GNSS (GPS, GLONASS, Galileo, BeiDou) provides position and time through trilateration: four pseudoranges solve x, y, z and the clock error.
- The signals lie in the L band (L1 1575.42 / L2 1227.60 / L5 1176.45 MHz) and are told apart by unique PRN codes (CDMA).
- The three GPS codes increase in robustness: C/A (open, narrow, vulnerable), P(Y) (encrypted, faster, more robust), M code (modern, BOC, directly acquirable, the strongest).
- Processing gain explains everything: it lifts the signal out from below the noise (like pulse compression in radar), but it is finite — and because the signal is so weak, a jammer of a few watts can deny GNSS over a large distance.
- Jamming drowns the signal (denial); spoofing/meaconing falsifies position or time (deception) and is often the greater threat.
- Protection is layered: FRPA (fixed pattern, limited) vs CRPA (array with null/beam steering, powerful and also anti-spoofing), supplemented by M code, INS coupling, multi-constellation and alternative PNT.
- All of this falls under NAVWAR — EW applied to the navigation spectrum: protecting your own PNT and denying the opponent's.
With that, the learning path connects to current events: after the classic EW of radar, IR and missiles, the fight over position and time has become one of the most decisive arenas of modern combat.