On the improvement of low elevation angle satellite communications impaired by tropospheric fading effects

Performance impairments occur on C- and Ku-band satellite communications links because of severe tropospheric fading effects when the elevation angle of the ground station is very low (<50). Two cases of satellite communications link impairment are discussed; the first is for data transmission from a high latitude ground station as found in NASA's Earth Orbiting Satellite (EOS) program. The second is for data transmission from LEO/MEO satellites at all latitudes. Experimental data shows that deep fading of satellite communications signals because of severe tropospheric absorption is as much as 27 dB during the worst summer month between a high latitude station and a geosynchronous satellite. With this amount of deep fading, if would be extremely difficult to transmit low-BER data during the summer months. The mitigation techniques proposed consist of: (a) adaptive coding by using a combination of the Reed-Solomon code, convolutional code, and interleaving, and (b), for lower orbit LEO satellites a shaped (depressed cardioid) antenna pattern. Using the same adaptive coding technique, the amount of contact time by ground stations at any latitude with LEO/MEO satellites could be increased substantially by enabling operation to very low elevation angles generally considered unusable. Using the shaped (depressed cardioid) antenna pattern, the received signal level could be maximized at very low elevation angles.