A 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 the paper describes an experiment to collect 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 is presented as a result of 14 months of observation. The modifications made to the radar antenna and display in order to provide sufficient system sensitivity, are described. 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 radiosonde station, Ocean Weather Ship "L', 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 with reasonable confidence to ray bending in passage through elevated layers.