Gamma ISP: Reference Manual

offset_fit

offset_fit computes registration offset polynomials from offsets file generated by one of the programs offset_pwr, offset_SLC, offset_SLC_tracking, offset_pwr_tracking,

SYNOPSIS
offset_fit <offs> <snr> <OFF_par> [coffs] [coffsets] [thres] [npoly] [interact_flag]
 

EXAMPLE
offset_fit 1352_1610.offs 1352_1610.snr 1352_1610.off 1352_1610.coffs 1352_1610.coffsets 3.0

Derives the 4-parameter regression polynomials with a minimum SNR threshold of 3.0

DESCRIPTION

offset_fit computes range and azimuth registration offset polynomials from offsets estimated by one of the programs offset_pwr, offset_SLC,offset_pwr_tracking, or offset_SLC_tracking. The input range and azimuth offsets and the corresponding SNR estimates (used a measure for the quality of the individual estimates) are provided in binary files.

This program generates 6-term polynomial models of range and azimuth offsets using linear least-squares. It has the capability to determine if particular offset measurements are in error and removes these from the input data set based upon two criterea: SNR of the offset measurement, and deviation of the measurement with respect to the current model values. Offset estimates with an SNR below the indicated threshold are rejected for the final fit. Different SNR thresholds are required depending on the offset estimation method used. Recommended values for the supported methods are listed below.

The second criterea for rejection of values is based upon the deviation of the offset measurement away from an initial fit of the data. This process can be repeated interatively using the initally culled data to get a new estimate that is used for comparison with the culled data. Tighter thresholds are applied as the interation proceeds. Termination occurs when there is no further improvement in the standard deviation of the offsets relative to the model and is a matter of judgement.

Offset polynomial estimation can be done either automatically or interactively. The program queries the user in the interactive mode for the SNR and maximum range and azimuth errors. Offset data meeting the threshold criterea are then used to determine a new global fit to the estimates. Typically, in the first round of culling, the points 3 or more standard deviations away from the initial least-squares fit are rejected.

For interferometric repeat pass data a co-registration accuracy of better than 0.2 pixels (re-sampling of SLC-2 is required) allows limiting  the loss of correlation by registration errors to below 10%.

The formats and definitions of the individual parameters in the are ISP offset/processing parameter file are found in the ISP type definition file typedef_ISP.h and in the description of the ISP offset/processing parameter file and in the description of the create_offset  program. The coordinates of the image section selected to be processed are relative to SLC-1.

The 4-parameter polynomials used to model the range and azimuth offsets are of the type:
range_offset = A0 + A1*range + A2*azimuth + A3*range*azimuth + A4*range*range + A5*azimuth*azimuth
azimuth_offset = B0 + B1*range + B2*azimuth + B3*range*azimuth + B4*range*range + B5*azimuth*azimuth

The resulting polynomials are written out to the ISP offset/processing parameter file. The polynomial order can be indicated on the command line. Valid orders are 1,3,4 or 6. The corresponding polynomials are (r: range pixel number, az: azimuth pixel number):

OPTIONS
The use of a 3-parameter polynomial may be preferred over a 4-parameter polynomial if the SLC data are very noisy.

SEE ALSO
typedef_ISP.h, OFF_par,offset_pwr, offset_SLC,offset_SLC_tracking, offset_pwr_tracking, init_offset_orbit, init_offset, create_offset