Gamma ISP: Reference Manual

ANSI-C program: init_offset.c

NAME
init_offset - This program computes the initial range and azimuth offset (range and azimuth offset of SLC-2 relative to SLC-1). SLC data can be stored either as pairs of 4-byte (real) (fcomplex) or 2-byte short (scomplex) values Data processed by ESA are provided in scomplex format.

SYNOPSIS
init_offset <SLC-1> <SLC-2> <SLC1_par> <SLC2_par> <OFF_par> [rlks][azlks] [rpos] [azpos] [offr] [offaz] [thres] [cflag]

<SLC-1>	(input) single-look complex image 1 (reference)
<SLC-2>	(input) single-look complex image 2
<SLC1.par>	(input) SLC-1 ISP image parameter file
<SLC2.par>	(input) SLC-2 ISP image parameter file
<OFF_par>	ISP offset/processing parameter file
[rlks]	number of range looks (default=1)
[azlks]	number of azimuth looks (default=1)
[rpos]	center of patch in range (samples) (enter - for default: image center file)
[azpos]	center of patch in azimuth (lines) (enter - for default: image center) file)
[offr]	initial range offset (default=0)
[offaz]	initial azimuth offset (default=0)
[thres]	correlation SNR threshold (default = OFF_par)
[rwin]	range window size (default: 512)
[azwin]	azimuth window size (default: 512)
[cflag]	copy offsets to the range and azimuth offset polynomials in the OFF_par 0: no 1: yes (default)

init_offset 1352.slc 1610.slc 1352.par 1610.par 1352_1610.off 1 1

DESCRIPTION
init_offset reads the single look complex SAR images SLC-1 and SLC-2, the corresponding SLC parameter files SLC-1.par and SLC-2.par and the ISP offset/processing parameter file. For a multi-look intensity image at a user defined location, the two-dimensional cross correlation function is calculated and the peak of the correlation function yields the initial offset of SLC-2 relative to SLC-1.

Confidence in the offset estimate is measured by comparing the height of the correlation peak relative to the average level of the correlation function to yield an correlation SNR. SNR values greater than 6.0 indicate coregistration better than 1 pixel. The default threshold is read from the OFF_par.

OPTIONS
In order to avoid ambiguity problems and still to achieve an accurate estimates init_offset is first run with multi-looking, followed by a second run at single look resolution:

init_offset 8059.slc 8560.slc 8059.par 8560.par 8059_8560.off 1 1

The offset estimate is written to 8059_8560.off. In the second run the first estimate (as stored in 8059_8560.off) is used as initial guess.

An alternative approach for the initial offset estimation based on the orbit data provided in the SLC paramter files is included in the ISP program init_offset_orbit. Especially in the case of large registration offsets or any kind of problems with the registration the (additional) use of init_offset_orbit is recommended.