eters. Larger reflectors can only be launched by using some form of unfurlable mesh or panel reflector. The trend toward smaller footprints on the Earth can be met either by using a larger reflector or by using a higher frequency, both of which involve higher costs. To date most spot beam communication satellites have used two prime-focus offset reflectors, one for transmit and one for receive, producing footprints on the Earth's surface that are elliptical because of the curvature of the Earth. Satellite Antennas for Telemetry, Tracking, and Command Fig. 9.17 Multiple spot beams generated by an array feed on a spacecraft. Multiple Beams It was early recognized that by using a single reflector and an array of feeds it was possible to produce multiple beams on the Earth (see Figure 9.17). This has the advantage that most of the antenna subsystem is reused, with the penalty of having to design and make the array of feed horns and the beam-forming network behind the array. The array feed elements must be compact SO that they occupy the minimum space in the focal plane of the offset reflector. At the same time the cross-polarization must be low. This tends to mean that corrugated horns cannot be used, and small-diameter dual- mode rectangular or circular horns are preferred. The maximum number of beams depends on the tolerable aberrations because array elements that are off-axis will have degraded performance. Shaped Beams It is desirable to optimize the shape of the satellite beam on the Earth's surface SO as to conserve power and to not waste energy by illuminating portions of the oceans. An example is shown in Figure 9.18. Shaped beams can be produced in two ways. Multiple, overlapping beams produced by an offset parabolic reflector and an array of feeds can be used. This approach is an extension of multiple beams and has the advantage that it is possible to design for reconfiguration by incorporating switching systems into the beam-forming network. The alternative approach is to use a single, high-performance fee