Closer spacing of geostationary satellites through adaptive nulling at the ground terminal

The results of a preliminary study suggest that interference to the downlink of one geostationary satellite from that of another satellite in the adjacent orbital slot can be substantially reduced by the addition of a simple adaptive nulling system to the ground terminal antenna. Alternatively, the nulling system allows the satellites to be spaced significantly closer together with no increase in interference. Significant improvement is obtained with simple open-loop antenna pointing. The improvement is greater, however, if the adaptive antenna automatically tracks the direction of the desired source. For a dish antenna with a single receiving feed horn, the extra hardware required to provide the adaptive capability consists of two more receiving feed horns, a variable weighting device for each of the feed lines (now three in number) and the adaptive control electronics. An example shows that the minimum spacing between one satellite and an interfering satellite, consistent with certain assumed operating requirements, could be reduced from the
2.25\deg
needed for a simple single beam antenna to
1.35\deg
, for the adaptive antenna using open loop pointing, to as little as
0.7\deg
, for the adaptive antenna using automatic pointing. The adaptive hulling also provides essentially the same protection against downlink interference from the two other satellites occupying the slots on both sides of the satellite serving the terminal. The three degrees of freedom of the postulated nulling system, however, cannot cope with three or more nearby interfering satellites.