Modulation index application for satellite adjacent downlink interference identification

Adjacent downlink satellite interference usually occurs because of the presence of side lobes in addition to the desired main lobe in the radiation pattern of the satellite ground station antenna. If the angular separation between two adjacent satellites is not sufficiently large, it is quiet possible that the power radiated through the antenna´s radiation pattern, whose main lobe is directed towards the intended satellite, interferes with the received signal from the adjacent satellite. This problem becomes more difficult if both satellites transmit at the same frequency. The SARSAT (Search and Rescue Satellite Aided Tracking) system is designed to provide distress alert and location data in order to assist in search and rescue operation. SARSAT uplink detects and locates distress beacons (406 MHz) activated at distress location. The system calculates the location of distress event using Doppler processing techniques. Processed data is repeatedly retransmitted through SARSAT downlink to Local User Terminals (LUT) when satellites are in view and also stored in on-board memory. The downlink of all SARSAT LEO (Low Earth Orbit) satellites uses the same 1544.5 MHz frequency. In case those satellites are within the main lobe of the local user terminal antenna, transmissions from adjacent satellites act as interference to each other. This can result in missed bursts or no solutions received at LUT, consequently no data provided about the distress location*. Modulation index application for adjacent satellite downlink interference identification, for periods where the interference duration may be significant, is presented in this paper. Measurement results recorded at SARSAT terminals are provided.