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Analysis of Altitude Errors Arising from Area Target Returns in FM Altimeters.

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A theoretical analysis is pressented for the errors, in a conventional and servoed slope FM altimeter, caused by the use of finite beamwidth antennas over an area target. In this analysis, an area target is considered to be a level surface as far as general physical characteristics are concerned, but is rough as far as acting as a backscatterer of energy. Errors in altitude measurements are determined analytically for different terrain models. Error curves are shown as a function of antenna beamwidth. The results show that a positive error exists for both types of altimeters and the error increases with the increase in antenna beamwidth. For a given antenna beamwidth, the servoed slope altimeter has smaller errors compared to those of a conventional FM altimeter, especially when the terrain reflectivity is independent of the incidence angle. The difference between the two altimeters becomes small as the antenna beamwidth decreases. For the case of sea surface as a target, the differences between the two altimeters is negligible and the increase in errors with the increase in antenna beamwidth is small. For this reason, it may be advantageous to use a wider antenna beamwidth small antenna especially when antenna stability is a problem, provided the antenna beamwidth is not so wide as to violate the assumption of level surface due to sea surface wave structure. For a situation where the altimeter antenna is directed at an angle alpha from the vertical, the error in general increases with alpha more so for a conventional altimeter. However, the increase is quite small for broadbeam antennas. This result also points to the advantage of using broadbeam antennas. Author

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  • Flight Control and Instrumentation

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