Summary One of the major problems confronting a telemetry receiving station is that of self calibration, particularly an end-to-end calibration, on a frequent and routine basis. For this purpose an external signal source is needed, preferably one in the far field of the antenna. The sun is such a source for L-and S-band systems--its usefulness depends on knowledge of its emission at the time it is used, since it is a variable source.Examined here are the characteristics of the sun as a source of electromagnetic energy in the 10 centimeter region, and the methods by which it could be used to determine receiving system noise temperature. Limitations of the methods are also described.
DiscussionThe most rigorous calibration of a telemetry system requires a far field source free of multipath transmission phenomena producing a known field intensity at the receiving antenna aperture. The source should be modulated to simulate an actual operation, and the receiving system should be in the operational configuration. Performance of the system should be measured at the output terminals of the system, usually a playback from a magnetic tape. (Ref 1. ) For a 24-ft diameter receiving antenna at 2250 MHz a source would have to be at least 2500 ft distant to be in the far field. At this distance, it would have to be at least 60 ft above the local horizon to place the first antenna minimum above the horizon. These are formidable and expensive constraints, particularly for mobile stations.An alternative to system calibration in one step is to calibrate in two steps. This is possible since the noise performance of the system is principally determined in the radio frequency circuits. The principal variables are antenna efficiency, transmission line losses, and preamplifier noise temperature, if one assumes that local electromagnetic interference has been controlled and reduced to a negligible value. The wide band characteristics of the circuit elements involved make them generally insensitive to signal characteristics. (One should question the intermodulation characteristics of the
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