An ionospheric mode detection system for HF communications application

The injection of sounding pulses into a communications channel was used as a technique for mode detection. This pulse sounding procedure was shown to reliability recognize propagation modes within the following limits. The maximum time resolution of the system is approximately 0.5ms between arriving pulses. Thus modes with nearly equal times of flight are not easily resolved. The majority of errors recorded were due to random noise. The pulse sounding system is a low cost aid to HF communications which enables the structure of signals received via multiple propagation paths to be evaluated. From this information an estimate of the expected fading induced error rate is obtained by comparing the relative amplitudes of the active modes. The measured relative time delays between modes provide an indication of the maximum data transmission rate possible before bit errors occur due to multipath propagation. Moreover, the pulse sounding signal can provide the (S/N) ratio for a given channel from which error rate estimates can be based.