Effects of sun transit and rainfall on satellite communication links

Satellite communication links are affected by sun transit and rainfall, among others. These affects must be taken into accounts in satellite link design and operation. This talk describes the effects and techniques to quantify and alleviate the effects. Sun transit causes satellite link outage and occurs when the pointing angles from the receive earth station to the satellite and to the sun are so nearly coincident that additional noise power presented by the sun causes the demodulator to operate below its Eb/No-threshold. In general a sun transit outage cannot be avoided and occurs around the spring and fall equinoxes for a few minutes a day and for several days; its occurrence, however, can be predicted in advance with an accuracy of less than a minute by the Vuong & Forsey technique. Rainfall attenuates and depolarizes the carrier; it also raises the antenna temperature of a receive earth station. Its affects are more pronounced at Ku-band and higher frequencies, at heavy rain zones such as Vietnam and at low elevation angles. Rain attenuation statistics can be predicted from various models with the most popular ones being ITU´s and Crane´s. The combined effects of uplink rainfall and downlink rainfall to a satellite communication link can be estimated accurately and simply by the Vuong & Vuong technique. Rainfall affects can be alleviated through design satellite links with high link margins, use of ALC (automatic level control) available with a satellite transponder, UPC (uplink power control) which can be equipped to a transmit earth station, adaptive data rate or adaptive FEC (forward error correction) coding rate, among others.