TABLE OF CONTENTS Radar altimeters were some of the first commercial applications of radar principles. They transmit energy downward from an aircraft, measure the time until a return is received, convert the time inter val to altitude, and display or record it. They differ from big radars in several ways. Because they are relatively short range ( ?30 km maximum), their power-aperture product can be small, their receivers noisy, and their signal processing off optimum. Nevertheless, they must have the ability to report altitude to a fine resolution (approximately <30 m) and follow faithfully (in a few milliseconds) rapid changes in altitude say, the combination of an aircraft in a dive passing over a cliff. Such performance is not too difficult to attain, so the emphasis is on reliability and economy. Most important is minimum encroachment on the carrier vehicle, which means the system must be small, lightweight, and nonintrusive, requiring no changes in vehicle mass properties or aerodynamics.
The radar altimeter needs an antenna beam sufficiently wide that altitude readings can be obtained over any specified flight attitude (perhaps a few tens of degrees for transport aircraft to close to 90 for highly maneuverable aircraft) or a beam that is steerable over those angles. The antenna needs to be flush mounted to provide smooth air flow. Horns, crossed dipoles, and slots potted in a dielectric are obvious approaches. Multiple-beam switched arrays are possible and could be made to meet all specifications, but they are generally too...
