HF ADAPTIVE MODEM AIR-TO-GROUND TEST.

This report presents the accumulation of experimental data, computational processing, and engineering analysis of one-way HF digital data transmitted form an aircraft to a ground station. The aircraft traveled from Riverside, California to Honolulu, Hawaii to Anchorage, Alaska to Rome, New York with the receive site located in Newport Beach, California. In this test data was transmitted at rates of 1200, 1800, and 2400 bps. The tested modes include one full rate mode and three distinct half rate models all utilizing phase-shift keying modulation with non-diversity operation. These modes are as follows 1 full rate - multitone quaternary encoding 2 half rate - multitone binary encoding 3 half rate - double bit length of full rate mode 4 half rate - half the number of tones of full rate mode. The channel conditions are identified by the processing of digitized analog sensors within the modem standard circuitry. The sampled circuits include the quadrature and in-phase phase detectors, keyed filter envelope amplitudes both on a per tone basis. In addition to the normal data signals, CW and ON-OFF tone modes were transmitted and detected to correlate with the continuously monitored sensors. The identifying channel parameters are fade rate, phase offset, phase standard deviation, pulse spreading, coherent bandwidth, minimum crosscorrelation value and signal-to-noise ratio. These are used to allow modem mode performance to be related to the channel character. The results of this test indicate that the modern sensors do identify the channel character in sufficient detail to select an optimum mode and that half-rate mode offers superior performance a majority of the time. Author