4.10 Microwave Altimeter

4.10 Microwave Altimeter

Microwave altimeters or radar altimeters are used to measure the distance between the platform (usually satellite or aircraft) and the ground surface. Some applications of microwave altimetry are in ocean dynamics of the sea current, geoid surveys, and sea ice surveys. Therefore, precise measurement of the satellite orbit and the geoid should be carried out.

The principle of satellite altimetry is shown in Figure 4.10.1. The ground height or the sea surface height is measured from the reference ellipsoid. If the altitude of the satellite Hs is given as the height from the reference ellipsoid, the sea surface height HSSH is calculated as follows.

HSSH = Hs - Ha where Ha : measured distance between satellite and the sea surface.

The sea surface height is also represented by the geoid height Hg that is measured between the geoid surface and the reference ellipsoid and the sea surface topography DH is given as follows.

HSSH = Hg + H

The sea surface topography results from ocean dynamics such as sea current, wave height, tidal flow etc., which can be determined if the geoid height Hg is given. The distance between a satellite and the ground surface or sea surface; Ha, is measured on the basis of the travel time of the transmitted microwave pulses. From the time (t=0), when the first edge of pulse arrives at the surface, to the time (t= ) when the end edge of a pulse with a width of arrives at the surface, the received power increases linearly as shown in Figure 4.10.2. The received pulses are composed of echoes from various parts of the sea surface. Therefore the travel time from a satellite to the sea surface can be calculated by averaging the received pulses. Pulse compression techniques will be also applied (see 4.2) in order to obtain a high frequency pulse for improvement of the resolution.

Ocean wave height can be estimated if the relation between the average scattering coefficient and elapsed time as seen in the different gradients in Figure 4.10.3 is taken into account.