Azisar: Along-track interferometry
Azimuth Interferometry (azisar) | Along-track InSAR
The phase difference between two radar images forms an interferogram (InSAR). The difference in phase between the two SAR images is a measure of delays in propagation of the emitted waves. The phase delay is due to difference in wave velocity or to a change of position of the scatterers between the two acquisitions. Phase delay is usually referred to as apparent line-of-sight (LOS) displacements. Using information contained in the individual focused SAR images, a similar phase image of along-track displacements can be obtained. The procedure consists in filtering both focused images to forward-looking and backward-looking SAR images. The phase difference between the forward- and backward-looking interferograms is a measure of azimuthal displacements between the two acquisitions (Bechor and Zebker, 2006; Barbot et al., 2008a). Formation of the azimuthal InSAR requires a double difference that cancels topography and atmospheric signals in the resulting azisar phase. Azimuthal displacements can also be obtained by cross-correlation of radar amplitude images. The advantage of azisar compared to offset tracking is a much higher resolution and a faster procedure. Azisar phase does not require phase unwrapping.
The software is written by Yuri Fialko and myself implements the formation of the azisar interferogram using the filtering approach of (Barbot et al., 2008a). It runs as an add-on to ROI_PAC. Before creating your own along-track InSAR images, you will need to get the original version of ROIPAC. Next, download the azisar software.
The additions consist in the following C codes contained in the tar ball which need to be compiled and copied to $INT_BIN
In addition, the scripts resamp_slc.pl and process_2pass.pl have been modified to incorporate the azisar processor. Read a few explicative slides to get an overview of the azimuthal interferometry method. The filtering approach is explained in an appendix of (Barbot et al., 2008a).
Frequently Asked Questions
Some investigators have reported difficulties installing or using the azisar software. Here are simple ways to ammend them.
- The software stops if the SLC images become larger than X (Gb).
The filtering of SAR data in the azimuth direction requires Fourier-transforming columns of data. Ideally, this would done on the whole image in one pass, but considering memory and I/O limitations, it is done on a buffer of columns. The length of the buffer is defined in azisar.h with the pre-processor variable INTMAX. One may solve the size limit by 1) increasing the available memory on the machine, and 2) decreasing the variable INTMAX from say, 32768 to 16384.
- The makefile to compile resamp_slc_roi.F does not work with my compiler.
This file is essentially identical to the original resamp_roi.F of Werner, Rosen and Hensley, except for the additional output of both resampled SAR images. So if you have a successful makefile for this file, it should work for resamp_slc_roi.F as well. A common source of error is the pre-processor option "-Dsecondo=secondo_", which needs to be changed to "-Dsecondo=secondo" for some compilers.