As dish size increases the on-axis gain of the antenna increases but its beamwidth narrows. This increases the need for the antenna to achieve good pointing accuracy. If the satellite is slightly off station or it moves in orbit the antenna will not produce the signal level that would be expected from simple gain calculations.
If one studies orbital mechanics it will be found that a number of second order factors operate on geostationary satellites that will cause them not to remain at a fixed location on the geostationary orbital arc. Amongst other things the effects of the Moon and the Sun cause geostationary satellites to describe a pattern in the sky about their nominal position in the sky. If uncorrected the patterns gradually increase in amplitude until the satellite will move out of a design station keeping window. At this time the satellite operator fires station keeping fuel to put the satellite back within a required station keeping tolerance. With the Optus system antennas larger than about 4.5 m need to be able to track this movement of the satellite or they will suffer some loss of performance. For this reason large antennas are usually equipped with a motor or motors and an automatic control system to allow them to track the movement of a satellite around its nominal position.
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