DME coherency measurement using a radio receiver implemented on FPGA device

In civilian air traffic regulations, specifically L10 in the Czech Republic, there is no regulation concerning coherency, respectively phase stability, of ground signal carrier. In case that a ground beacon of DME system uses a coherent signal, it could be used to help to improve accuracy of the system or to add other parameters of aircraft flight measured by this system. We can think of more precise measurement of radial speed of the aircraft towards the beacon using Doppler frequency shift measurement or adding carrier phase measurements for improvement of accuracy of distance measurement. Some of these prepositions may also require improving other parameters of the signal from current regulations description. But these improvements are by itself in the best interest of the DME system and concerning some parameters of signals used by some other types of radio location or radio navigation systems. This paper is focused on measurement of coherency of impulses transmitted by ground beacon of DME using specifically created radio receiver with digital signal processing based on FPGA. The receiver is designed to synchronize on first impulse of randomly selected impulse pair and then to compare phase shift of next impulses with phase shift of the first one. The results of this operation are then stored to a memory for evaluation. The measurement is realized on signal transmitted by ground beacon LKTB by receiving and processing of the signal with the assumption that for correct evaluation of this specific parameter it is not necessary to distinguish between answers for the questions from actual aircrafts and random squitters, which, concerning the fact, that DME beacon transmits relatively constant number of impulse pairs per second, make it possible to measure this parameter without the need of a source of interrogation for the beacon.