Fico estimates the sub-pixel offset between two given complex images. It runs from a control file, usually generated by resolve. This control file contains the gross single-pixel offset between the complex images, as well as some housekeeping information.
Fico creates a grid on top of both given complex images. A small chip of each image is extracted at each grid point. Fico then generates small interferograms by shifting and interfering these image chips. For each small interferogram, Fico measures the phase variance, which is similar to the interferogram coherence. Better interferograms have smaller phase variance (less phase noise), and hence higher coherence.
Fico then interpolates to find the best possible offset between the two images at that grid location. Fico then writes this offset to its output file, and moves on to the next grid location.
At each grid location, if no offset between the images results in a reasonable interferogram, Fico skips that point. This generally happens over water, heavy vegetation, or other decorrelated areas.
If the two SAR images are of totally different locations, or have too large a spatial or temporal baseline, and hence have no coherent phase between them, Fico will detect this bad interferogram and end in error.
Sadly, Fico has no way to tell the difference between a bad interferogram and a bad pair of image offsets. To tell the difference, you can measure the image offset by hand between the two amplitude images of your interferogram. If this hand offset matches (within a few pixels) the offsets in the Fico control file, you have a bad interferogram-- try to order better data.
But if your hand offset does not match the offsets in the Fico control file, you might be able to continue by overwriting the values in the Fico control file, and re-running Fico.
Remember, amplitude images have usually been multilooked (by a factor of 5 for ERS), so to get the offsets in the single- look complex file you must multiply the Y coordinate by the amount of multilooking (5 for ERS).
Fico is usually called by register_ccsd or register_sic, but can be run alone.
Fico determines the sub-pixel offset between two SAR complex files.
Back to main interferometry page.
Last Updated: September 1, 1998
If you have any questions, please feel free to email olawlor@acm.org