An ionospheric mode detection sounder for H.F. data communications
A major problem experienced in HF data communications is that of multipath interference. Interference between the components, of a transmitted signal, arriving at the receiver over different propagation paths can produce serious errors in data transmission. A pulse sounding technique has been developed to resolve the interfering modes in the time domain. This signal consists of a O.7ms pulse of radio wave energy sent within a 2Oms break in transmission, repeated every 9Oms. The sounding signal is generated by a small computer which keys the radio transmitter. After reflection in the ionosphere the signal was received and digitised. A second computer then analyses the digital record to discover the amplitude and time of arrival of any sounding pulses present. The results are then compared with predictions to identify the active modes. Pulse sounding experiments were undertaken over four paths of lengths from 4 to 2156km. E,Es,F,2F,3F and auroral propagation paths were successfully recognised. This information could be used to advise on the likelihood of interference fading or more positively to control an elevation steerable antenna to adaptively cancel out this unwanted effect. To investigate the relationship between ionospheric conditions and the error rate in data communications a pseudo random test data message was transmitted with the pulse sounding. Measurements in the error rate in the message were conducted over the 578km path from Elgin to Leicester. It proved possible to establish an approximate correlation between the recorded data errors and the ionospheric conditions derived from the pulse sounding. This allows error rates to be estimated from the sounding results.