TABLE OF CONTENTS In this chapter some basic properties of airborne clutter are analysed. These include space-Doppler characteristics, the space-time covariance matrix and the associated azimuth-Doppler.
From (2.29), we can conclude that the ground is totally Doppler coloured since any pair of angles ( ?, ?) denotes an individual clutter Doppler frequency. Curves of constant Doppler frequency on the ground are called isodops.
The clutter Doppler frequency due to a certain stationary scatterer on the ground is proportional to the radial velocity [1] (see Figure 2.1 and (2.14)):
In accordance with Figure 2.1, and (2.29), (3.1) becomes in ground coordinates
Notice that we assumed that the radar platform flies parallel to the ground and that the ground is planar. Clutter arrivals may come from all possible azimuth angles ? = 0 , ...,360 . The Doppler frequency depends only on the platform velocity and the angle of arrival, however, not on the array geometry. For scatterers at a particular range (elevation and angle ?) the clutter Doppler bandwidth extends from
which means that the clutter bandwidth is larger at long range (small ?) than at close range.
For convenience we define a relative Doppler frequency
From Figure 2.1, we obtain the following relations:
so that (3.4) becomes
which is a set of hyperbolas
For H = 0 the hyperbolas turn into straight lines:
For f r = 1 or f r = ?1 the isodop is a straight line in the flight direction,...
