(b) RDM algorithm
RDM algorithm is a representative algorithm that detects moving targets with the Doppler modulation rate change. It is firstly invented as a motion compensation algorithm that corrects the motion error of the platform during the flight. J.R. Moreira first applied this algorithm in GMTI, the principle of RDM algorithm is: The change of Doppler modulation rate can be detected by the relationship between neighboring Doppler power spectra. The moving target can be indicated by its Doppler centroid, and the along-track velocity and cross-track acceleration can be estimated.
There are mainly three limitations of RDM algorithm: first, only targets with the along-track velocity and cross-track acceleration can be detected, but the target with only the cross-track velocity cannot be detected; second, RDM algorithm is not effective for the detection of multiple targets which are located nearby; third, the Doppler parameter estimation precision is not high, and two motion parameters cannot be derived from one Doppler parameter.
Shear average algorithm also uses the Doppler modulation rate to detect moving targets. This algorithm uses the contrast ratio of sub-images focused with different Doppler modulated rates to judge whether there is a moving target in the sub-image. Moreover, shear average algorithm has also the disadvantages same as RDM algorithm.