Improvement of the SSR mode S data link using a distributed ground system

In air traffic control extensive use is planned of data links. One of the major candidates is the data link capability of SSR mode S. The data link capability of SSR mode S however is rather limited when using conventional rotating antennas with a short and fixed time-on-target, and a long time interval between two successive beam scans. This results in a high access delay and too long transaction times for long messages consisting of several ELM frames. The paper presents a method for the analysis of the cause of reply losses, and then discusses a novel approach to the data link´s shortcomings, while maintaining the surveillance capability: the substitution of a number of interrogators with a rotating antenna by a distributed network of (listen-only) SSR mode S ground system receivers and a transmitter. The results show that the mode S transponders suffer from a high reply loss, by occupancy due to mode A/C interrogations, and shows that the novel approach offers adequate data link performance while maintaining the reply loss numbers at an acceptable low probability