How does spread spectrum technology provide anti-jam capability for military communications?

Spread spectrum technology provides anti-jam (AJ) capability for military communications by spreading the transmitted signal's energy across a bandwidth much wider than the minimum required for the data rate, making the signal resistant to narrowband and partial-band jamming. The processing gain of the spread spectrum system, defined as PG = BW_spread / BW_data (the ratio of the spread bandwidth to the data bandwidth), represents the advantage the communication system has over a jammer. Two primary techniques are used: direct sequence spread spectrum (DSSS, the data signal is multiplied by a high-rate pseudo-random noise (PN) code that spreads the signal bandwidth; at the receiver, the same PN code is used to despread the signal, concentrating the desired signal energy back into the data bandwidth while spreading the jammer energy across the full spread bandwidth; the processing gain is PG = chip_rate / data_rate; for a 10 Mcps chip rate and 10 kbps data: PG = 1000 = 30 dB) and frequency hopping spread spectrum (FHSS, the carrier frequency is rapidly changed according to a pseudo-random hopping pattern; a narrowband jammer can only jam the current hop frequency, which changes hundreds or thousands of times per second; the fraction of hops jammed is approximately BW_jammer / BW_hop_band; for 1000 hop frequencies and a jammer covering 10 frequencies: only 1% of hops are jammed, and error correction coding recovers the lost data). The anti-jam margin is: AJ_margin = PG + coding_gain - required_Eb/N0 - system_losses. For a DSSS system with PG = 30 dB, coding gain = 5 dB, required Eb/N0 = 10 dB, losses = 3 dB: AJ margin = 22 dB (the system can tolerate a jammer 22 dB stronger than the signal).