Requirements on channel probes for automatic channel selection

Whilst strategies can be developed to maintain high data rate communications under benign conditions, the same waveforms are inappropriate when the channel quality is low. It seems likely, therefore, that many future HF point to point communicators will have a number of waveforms at their disposal. These will range from the high capacity variety which operate over the benign channel to relatively low capacity schemes which operate well over the disturbed channel. It is one of the purposes of ARCS (automatic radio control system) to make that selection between waveforms; the aim is clearly to maintain a high data rate whilst also keeping the bit error rate to an acceptably low value. In order to fulfil the above role an ARCS probe must be designed to measure the full range of conditions which may occur. Furthermore, the probe must be able to use those measurements appropriately to select from amongst the available waveforms. Strategies for accomplishing this are the subject of active research and include considerations such as probe duration, S/(N+I) evaluation, determination of multipath, Doppler shifts and spreads, the impact of bimodal Doppler spectra and the time rate of change of the aforementioned parameters. This paper does not attempt to cover all of these factors but restricts itself to illustrating the range of Doppler shifts and spreads which can be experienced on difficult high latitude HF channels. These channels represent extremes in Doppler related distortion.