Avionics Navigation Systems, 2nd Edition

10 Doppler and Altimeter Radars

10.1 DOPPLER RADARS

10.1.1 Functions and Applications

The primary function of a Doppler radar is to continuously determine the velocity vector of an aircraft with respect to the ground. If the measurement is made in, or has been converted to, an Earth-referenced coordinate frame and resolved about north and east, the velocity components can be integrated into distance traveled from a known point of departure and the aircraft's geodetic present position and course and distance to destination can be calculated. Thus, a Doppler radar can be the primary sensor of a dead reckoning navigation system or one of the sensors in a multisensor system. The velocity is determined by measuring the Doppler shift of microwave signals transmitted from the aircraft in several narrow beams pointed toward the surface at relatively steep angles, backscattered by the surface and received by the Doppler radar receiver.

A Doppler radar has the following advantages over other methods of velocity measurement or dead reckoning navigation:

1. Velocity is measured with respect to the Earth's surface. This is in contrast to air data systems which measure velocity with respect to the air mass and to most terrestrial radio navigation systems in which velocity measurement is based on differencing of successive position measurements.
2. It is self-contained, that is, it requires no ground-based stations or satellite transmitters.
3. The airborne transmitter power requirements are extremely ...