TABLE OF CONTENTS For a radar onboard a flying platform (aircraft, drone, satellite) the direction dependent relative velocities of the ground scatterers cause corresponding Doppler frequency shifts of the clutter echoes. Against this broadened clutter spectrum one can apply no common filter, because one would thereby suppress too broad a Doppler frequency range of possible targets and would thus limit target detection inadmissibly and unnecessarily. With an active receiving antenna array, however, a signal field with samples in the space and time dimension can be offered for optimal signal processing for the detection of moving targets [1 7], even if those targets are slowly moving. The antenna array elements may form a linear or a planar array. The antenna can be looking with its broadside orientation forward or sideways relative to the flight direction. For an onboard multifunction phased-array radar, the forward-looking case is particularly of interest [5 8].
The clutter Doppler frequency shift depends on the platform velocity v, wavelength ? and the angle ? between the direction to the clutter scatterer and the flight velocity vector [8]:
On a cone with its axis given by the flight velocity vector the Doppler frequency shift is constant, its value depends only on the angle ?. The intersection of the cone with the flat ground plane is a hyperbola and on this curve we have a constant Doppler frequency. All curves with constant Doppler shifts are given by the set of hyperbolas as illustrated in Figure 13.1.
