Technique for assessing the bearing accuracy of distant h.f. transmitters

Travelling irregularities in the ionosphere can produce large errors in the measured bearings of distant h.f. transmitters. By utilising the small Doppler frequency shifts produced in the received signal during these events, periods containing errors can be recognised. A `difference Doppler¿ technique is described that can assess the accuracy of the bearings measured for any transmitter of unknown location. The performance of the system for a wide range of ionospheric conditions, including multimode, is described. Examples of the improvements obtained in bearing accuracy and variance under operational conditions are presented.