Long-range propagation experiment to investigate the incidence of anomalous propagation in the North Atlantic

The significance of enhanced range on air-to-ground paths is reviewed and an experiment described which collected maximum range data on an oversea path. A secondary surveillance radar sited in the Hebrides was used to observe civil air traffic crossing the North Atlantic, and a cumulative distribution of range extending to 0.1% of the year obtained based on 14 months of observation. About 99% of the observations are shown to be satisfactorily modelled by employing an exponential atmosphere with linear N profiles as measured by a suitably sited radio-sonde station to model the first 1 km of height. The remaining 1% of observations, the extreme range cases, are examined in some detail and are attributed to ray bending in passage through elevated layers