Home Engineering



Doppler CentroidThe crosstrack velocity is the most interesting parameter of the moving target in GMTI applications. Therefore, in most existing algorithms, the moving target is classified according to the values of their crosstrack velocities [13]. For example, if the moving target has a slow crosstrack velocity that its spectrum is submerged by the clutter, it is defined as a slow moving target. However, when the Doppler centroid of the moving target exceeds the limit of PRF, the Doppler ambiguity is induced. The Doppler ambiguity exists when the crosstrack velocity meets the condition
In that case, the “fast” or “slow” crosstrack velocity [4] cannot accurately reflect the spectrum character of the moving target. In the existence of the Doppler ambiguity, the moving targets can be classified according to the locations of their spectra. We classify the moving targets into three types, as shown in Fig. 2.4. The red and black triangles in Fig. 2.4 represent the Doppler spectra of the moving target and the clutter, respectively. The spectra of the moving targets of Type I are completely located out of the clutter. Hence, a highpass filter is able to separate the moving target from the clutter. In Type II, partial spectra of the moving Fig. 2.4 Illustration of the three types of moving targets in the rangeDoppler domain target are submerged by the clutter. Type III represents the moving targets that are completely submerged by the clutter. The targets with slow crosstrack velocities and the targets with fast crosstrack velocities that aliased into the baseband, belong to Type III. In addition, given that the alongtrack velocity has no relationship with the Doppler centroid, the alongtrack moving targets also belong to Type III. The velocity information of the moving targets for each type is illustrated in Table 2.1, where B_{a} and B_{m} denote the Doppler bandwidths of the clutter and the moving target, respectively, and k is the number of the Doppler ambiguity. In a whole synthetic aperture, the number of azimuth samples N_{a} = PRF • T_{a}, where T_{a} represents the synthetic aperture time. Thus the number of azimuth dislocation samples can be calculated as
The synthetic aperture time T_{a} = LR, where L_{a} is the azimuth aperture size. Substitute it into Eq. 2.3.8, it yields that Table 2.1 Classification of moving targets
Substitute azimuth sample interval d_{a} = L into Eq. 2.3.9, the azimuth dislocation is
According to Eq. 2.3.10, the azimuth dislocation of a moving target is determined by its crosstrack velocity. 
<<  CONTENTS  >> 

Related topics 