HF transmitter SSL (single site location) with combined ionospheric update

HF-SSL is a useful technique for identifying the location of HF transmitters when information is only available from a single site. The main source of error in the technique has generally been found to be an inability to model in sufficiently accurate detail the ionospheric effects on the signal propagation which require accurate characterisation of the ionosphere at the time of observation. The method presented reduces errors from this source by using a Nelder-Mead (1965) function minimisation algorithm to produce an optimised solution for a real-time updated ionosphere mode as well as the HF transmitter location. The method can be applied to the situation where two simultaneous paths exist from the transmitter to receiver, e.g. 1E and 1F2 or 1F2 and 2F2. This is a condition which is often satisfied and it has been found that super-resolution methods can be used to successfully decompose the received HF wavefield, received on a suitable antenna array, into its constituent component giving the DOA of each. Thus such a scheme compliments more traditional SSL methods using interferometry which are often only performed for single mode propagation. Further, in the extended method, measured angles of inclination to known transmitters and differential time delays between modes can also be employed in the optimisation, producing a more accurate estimate of both the ionosphere model and propagation distance.