Gamma ISP: Reference Manual

offset_pwr_tracking

ANSI-C program: offset_pwr_tracking.c

NAME
offset_pwr_tracking estimates range and azimuth offset fields for SLC images using intensity tracking

SYNOPSIS
offset_pwr_tracking <SLC1> <SLC2> <par1> <par2> <OFF_par> <offs> <snr> [rwin] [azwin] [offsets] [n_ovr] [thres] [rstep] [azstep] [rstart] [rstop] [azstart] [azstop] [pflag]

<SLC1>	(input) single-look complex image 1 (reference, fcomplex or scomplex)
<SLC2>	(input) single-look complex image 2 (fcomplex or scomplex)
<SLC1_par>	(input) SLC-1 ISP image parameter file
<SLC2_par>	(input) SLC-2 ISP image parameter file
<OFF_par>	(input) ISP offset/interferogram parameter file
<offs>	(output) offset estimates (fcomplex)
<snr>	(output) offset estimation SNR (float)
[rwin]	search region size (range pixels, enter - for default from offset parameter file)
[azwin]	search region size (azimuth pixels, enter - for default from offset parameter file)
[offsets]	(output) range and azimuth offsets and SNR data in text format, enter - for no output
[n_ovr]	SLC over-sampling factor (integer 2**N (1,2,4) default: 2)
[thres]	offset estimation quality threshold (enter - for default from offset parameter file)
[rstep]	step in range pixels (enter - for default: rwin/2)
[azstep]	step in azimuth pixels (enter - for default: azwin/2)
[rstart]	starting range pixel (enter - for default: 0)
[rstop]	ending range pixel (enter - for default: nr - 1)
[azstart]	starting azimuth line (enter - for default: 0)
[azstop]	ending azimuth line (enter - for default: nlines - 1)
[c_ovr]	correlation function oversampling factor (integer 2**N (1,2,4,8,16) default: 4)
[pflag]	print flag: 0: print offset summary, 1: print all offset data
[pltflg]	plotting flag: 0: none (default) 1: output plots in PNG format 2: screen output and PNG format plots 3: output plots in PDF format

EXAMPLE
offset_pwr_tracking 19990915.slc 19991020.slc 19990915.slc.par 19991020.slc.par 19990915_19991020.off 19990915_19991020.offs 19990915_19991020.snr 32 64 19990915_19991020.offsets 2 7.0 10 20 400 1600 1200 7200

Estimates range and azimuth offset fields (offsets of 19991020.slc relative to 19990915.slc) using the intensity tracking algorithm using a 32 x 64 pixels search chip size. An SLC over-sampling factor of 2 is applied to the SLC data. The search is repeated every 10 range pixels and every 20 azimuth lines, for the specified region of the image.

DESCRIPTION

offset_pwr_tracking estimates range and azimuth offsets over a grid for a pair of SLC images. These offset measurements can be used either for image co-registration or offset tracking of features such as glaciers.

The intensity correlation tracking algorithm is used. Notice that this algorithm requires features in the detected images. The algorithm is not well suited for small search chip sizes (< 16 x 16 pixels). Offsets are calculated for the indicated section of the reference SLC (SLC-1) at the indicated range and azimuth step. The related information is documented in the offset parameter file.

Throughout the image small SLC data patches are selected and used to determine the local range and azimuth offset fields. For each such offset estimate, the intensity cross-correlation function is calculated by convolution implemented using FFTs. The SLC data can be over-sampled by a factor of 2 (default) or 4 prior to cross-correlation. The location of the correlation maximum is estimated using a two-dimensional quadratic least squares fit. The correlation function can be interpolated by a specified factor (nominally 4) prior to fitting.

The range and azimuth size of the rectangular chips used in the calculation of the correlation function corresponds to the search window sizes indicated. The search is centered at the offsets indicated in the offset polynomials of the provided offset parameter file.

The size of the patches needs to be in-line with the features that shall be used in the matching and with the spatial dimension of expected variations in the offsets. This method of measuring offsets between SLC images is complementary to the fringe visibility method used in offset_SLC and offset_SLC_tracking.

For most cases a SNR threshold of 7.0 for the acceptance of an offset gives a reliable result. In cases of too low correlation, the use of larger search chip and window sizes is suggested.

The estimated offset fields and related matching SNR values are written to binary output files of fcomplex (*.offs) and float (*.snr) format. The offsets can be converted to offsets in the original (reference) geometry using the program offset_tracking. The program offset_fit can be used to obtain estimates of the coefficients of range and azimuth offset polynomials.

No search is conducted for chips with a 0 value in the center of the search chips. Setting the SLC values of areas which are not of interest to the 0 value may permit to strongly reduce the required processing time, while still covering the area of interest at high spatial resolution (small range and azimuth step).

The binary offsets file (*.offs) contains offsets in SAR image pixels (of the reference image) in fcomplex format. The real parts correspond to range offsets and the imaginary parts to azimuth offsets. The positions of the offset estimates are defined by the starting range and azimuth positions and the range and azimuth steps, as documented in the offset parameter file.

OPTIONS

Range and azimuth chip sizes, range and azimuth steps, and range and azimuth search regions can all be specified on the command line, permitting for example to use search chips, search windows, and steps which are larger in azimuth directions as may be appropriate considering the different ground-range and azimuth sampling of most SLC data.