Survivable HF communications

Robust HF uses a noncoherent FSK (frequency-shift-keyed) signal. It uses fast pseudo-random frequency hopping and a rugged modulation format which makes it impervious to multipath, atmospherics and man-made noise. It is optimized as an antijam system. Robust HFs operating frequency range is 2 to 30 MHz. The pseudorandom frequency hopping uses variable bandwidth and adapts to the best frequencies between the lowest usable frequency and the maximum usable frequency. The system operates skywave (long distance) or ground wave (up to 100 km, or even, more over water). Buried hard antennas have been tested and show almost no degradation in system performance. Robust HF uses powerful error coding with variable code rates and code combining, which automatically adapt according to link conditions. The result is exceptional processing gain. A powerful cyclic redundancy check (CRC) is included to prevent delivery of corrupt data. System bit rates run as high as 600 bit/s, averaging about 75 bit/s, depending on the code rate