Gamma MSP: Reference Manual

af

ANSI-C program: af.c

NAME autof - Estimation of effective along-track velocity for SAR processing using cross-correlation of azimuth looks

SYNOPSIS af <SAR_par> <PROC_par> <SLC> [rwin] [azwin] [dr] [daz] [thres] [update_flg] [a1_flg] [b0_flg] [offsets] [dac_flg] [n_ovr] [roff] [azoff]

<SAR_par>	(input) MSP SAR sensor parameter file
<PROC_par>	(input) MSP processing parameter file
<SLC>	(input) (input) single-look complex image
[rwin]	range window size (enter - for default: 1024)
[azwin]	azimuth window size (enter - for default: 4096)
[dr]	range sample increment (enter - for default: 1024, enter 0 for single patch)
[daz]	azimuth line increment (enter - for default: 4096, enter 0 for single patch)
[thres]	offset estimation SNR threshold (enter - for default: 10.000)
[update_flg]	update effective velocity flag (enter - for default: 1) 0: no update of effective velocity in PROC_par file 1: update effective velocity in PROC_par (default)
[a1_flg]	fit a1 for first derivative of the effective velocity w.r.t.range 0: no (default) 1: yes
[b0_flg]	fit b0 for first derivative of the effective velocity w.r.t. along-track time 0: no (default) 1: yes
[offsets]	(output) range and azimuth offsets and SNR data in text format, enter - for no output
[dac_flg]	Doppler ambiguity correction flag 0: Doppler centroid remains unchanged: exit if Doppler ambiguity error detected 1: Doppler centroid ambiguity corrected (default)
[n_ovr]	SLC oversampling factor (1,2,4: enter - for default: 1)
[roff]	range offset for single patch center
[azoff]	azimuth offset for single patch center

EXAMPLE af ERS2_CCRS.par p19991020.slc.par 19991020.slc 1024 4096 - - 10

states that the af program should use the SAR sensor parameter file ERS2_CCRS.par and SAR parameter file p19991020.slc.par to analyze the SLC 10001020.slc. The autofocus analysis is performed on patches that are 1024 pixels in range and 4096 azimuth lines. The default increment between patches is selected using the - - values for parameters dr and daz. The SNR threshold of 10 is used to exclude estimates with possibly large errors in the range and azimuth offsets.

DESCRIPTION af calculates an estimate of the along-track velocity of the radar platform using cross correlation of images derived from different segments ("looks") of the azimuth doppler spectrum. An azimuth shift between these two images is proportional to a platform velocity error. af selects a set of patches covering the entire SLC mage, generates the two looks, and estimates a velocity error for each patch.

To use af, first process the range-compressed data using az_proc with the effective velocity derived from the state vectors. After this, apply af and determine an update for the effective velocity and reprocess the full frame using az_proc. Then check the image focus by rerunning af. Do not run af multiple times before reprocessing the image!

Each of the sub-aperture images is formed from a bandwidth equal to 0.35 times the radar PRF. Typically the processing bandwidth for an SLC is 0.8*PRF and hence the frequency separation between looks is .45*PRF. The two looks are obtained by filtering the image using an FIR bandpass filter. After filtering the images are detected and cross-correlated using a 2D FFT to measure the range and azimuth offsets. A biquadratic interpolator determines the offset correlation peak with sub-pixel accuracy.

Range offsets occur only if there is uncompensated range migration in the processor. This can occur when the doppler centroid is incorrect by some multiple of the PRF and termed a doppler ambiguity errror. Azimuth offsets between the looks indicate an error in the azimuth chirp rate. This error in chirp rate is directly proportional to an error in the along-track velocity. See the references: Synthetic Aperture Radar: Systems and Signal Processing, J. Curlander and R. MacDonough, Wiley Interscience, 1991, Chapter 5 and Digital Processing of Synthetic Aperture Data, Algorithms and Implemenation, Ian Cumming and Frank Wong, Artech House, 2005, Chapter 13.

Low correlation and inaccurate offset measurement can occur if the scene is devoid of contrast, such as over the open ocean or on an ice sheet. A threshold for the SNR of the correlation signal can be specified on the command line as the parameter thres and has a default value of 10. The size of the SLC patches is specified by the parameters rwin and azwin, with the spacing between patches by the parameters dr and daz. A recommended patch size is 1024 range samples by 4096 azimuth lines. The dr and daz parameters are set by default to 2*rwin and 2*azwin respectively resulting in approximately 6 estimates in a single ERS frame.

The program has the option to turn on or off the update of the effective velocity parameter in the MSP PROC_par. As a final check, it may in some cases be useful to turn off the update. Similarly, the user can select if the doppler centroid should be updated if there is a large range shift between looks indicating a possible ambiguity error in the doppler centroid.

Since multiple measurements are made over the scene, af is able to find areas with good correlation. A fit of the autofocus derived velocity with respect to azimuth and slant range is possible if the a1_flg and b0_flg command line parameters are set to 1. This may be useful when processing very long data stips. By default, the effective velocity estimate is the only estimated parameter. Note that the the azimuth processor az_proc takes into account earth curvature in using the effective velocity parameter from the PROC_par. The polynomial parameters a1 and b0 specify across-track and along-track corrections to the effective velocity value calculated and used by az_proc.