This step can in general best be performed after the COARSE coregistration and before the FINE. (The coarse offset in pixel direction is used to evaluate the polynomial for the Doppler Centroid frequency.) The FINE steps can benefit a lot from this filtering (TODO add plots).
By processing the RAW data to SLC at the mean Doppler centroid frequency this step can be avoided in the InSAR processing chain. (For ESA SLC images this cannot be done obviously.)
Normally the step S_FILTAZI is performed at the same time. (requires a PROCESS S_FILTAZI card in the input file, see chapter 16.) However, we kept this two seperate steps to be able to only filter the slave images in a large stack (all slaves coregistered on the same master image). This has the advantage that for each interferogram of the stack not a large file is created for the master. The disadvantage of not filtering the master of course is that a small part of the spectrum of the master is not shared with the slave spectrum, yielding loss of coherence in the interferogram.