Fade countermeasures experiment

It is important to determine the dynamic nature of fading before attempting fade countermeasures. The rate of change of fading is important, as is the frequency of occurrence. Long term propagation measurement programmes have been run, though there is at present a lack of data at 20/30 GHz. The ESA OPEX propagation measurement program will provide useful data for Olympus paths over the next few years. The findings of the various studies are summarised in the following section. The ATS-6 satellite provided many measurements during 1975/6 at 20/30 GHz at an elevation angle of 23° to the UK, and these have been compiled by ESA. Measurements were made of the duration of fades above a given amplitude, as well as of the statistical distribution of fading. The results interestingly show that shallow fades generally last for ten minutes or less with a majority lasting for less than 60 seconds. For greater attenuations, 10 dB or more, the percentage of fades lasting over one minute was greater, although half of all fades lasted for less than 60 seconds. It was also noted that high altitude ice crystals can cause depolarisation without significant attenuation, this must be considered when designing systems using frequency reuse by orthogonal polarisation. The ESA summarised results combine measurements made at Bradford, Eindhoven, Gometz-Ville, Langley, Leeheim, Martlesham and Winkfield, with radiometric data gathered at Martlesham, Langley and Winkfield to give a total observation time of 8116 hours at 20 GHz and 28829 hours at 30 GHz. The results are reproduced here