GAMMA Interferometric Point Target Analysis Software (IPTA): Reference Manual

pwr_stat

ANSI-C program: pwr_stat.c

NAME
pwr_stat - generate point list based on temporal variability of SLC intensity

SYNOPSIS
pwr_stat <SLC_tab> <SLC_par> <MSR> <plist> [MSR_min] [PWR_min] [roff] [loff] [nr] [nl] [norm_mode] [type]

EXAMPLE
pwr_stat testsite.SLC_tab - testsite.plist 1.5 1.0 - - - - 1 1

Generates a point list of coordinates of points with a low temporal variability of the SLC intensity and an intensity above the spatial average.

INSTALLATION
Source code pwr_stat.c in ./IPTA/src, executable version pwr_stat in ./IPTA/bin

DESCRIPTION
In the IPTA point lists contain the coordinates (range and azimuth pixel number relative to the reference SLC geometry) of the selected points. Data at these points (SLC values, interferogram values, ...) are stored in point data stacks with each record corresponding to a different SLC or interferogram. The point data stack of mask values contains for each point a flag (0 or 1) which indicates if the data of a point are considered (1) or not considered (0).

pwr_stat supports the generation of point lists of "point target candidates". pwr_stat identifies "point target candidates" based on a low temporal variability of the SLC intensity (i.e. a mean to temporal standard deviation ration above the indicated threshold) and an SLC intensity above an incidated threshold relative to the spatial average.

The physical basis for the methodology used is that point targets (i.e. SAR image resolution cells for which the scattering is dominated by a target which is small in size as compared to the size of the resolution cell) do not exhibit the speckle observed for extended targets. For a point target almost the same backscattering intensity is found when observing from slightly different directions. For an extended target the backscattering will strongly fluctuate if the change in direction corresponds to an offset of the order of the critical baseline known in interferometry. For point targets which remain unchanged over time it is expected that the temporal variability is small relative to the temporal average. For extended targets with a fully developped speckle behavior it is expected that the temporal variability is of the order of the temporal average.

pwr_stat uses the Mean to Standard deviation Ratio (MSR) as the (inverse) measure of the temporal variability. For fully developed speckle the MSR should be 1.0. Lower temporal variability corresponds to higher MSR values, so that 1.5 reasonable initial threshold to use.

As a second (or alternative) criteria an SLC intensity above minimum value can be indicated. As a simplification not an absolute value, but one relative to the spatial average is used. The idea behind this second criteria is to limit the points to dominating points (which need to be above the background level). For very small sets of SLC the intensity criteria may still be useful while the MSR criteria may get more unreliable due to poor statistics.

To get reliable statistics on the temporal variability it is a suficient number of scenes is required. With fewer than 20 scenes this criteria tends to become less reliable. In any case a 100% reliability cannot be expected, so that a subsequent control of the usefullness of the identified points remains necessary. On the other hand this methodology may also fail to detect useful points.

Important conditions for a successful use of pwr_stat are the accurate co-registration of the SLC and a good relative radiometric calibration. There are 3 options for relative radiometric calibration by the [mode] parameter.  Mode 0 applies no relative calibration scaling between scenes and assumes that the relative calibration of the sensor
is adequate. Mode 1 uses the average scene intensity to derive normalization factors, and can be used for areas with small temporal variation in backscatter (desert or urban areas). Finally Mode 2 (default)  identifies a number of point targets given the initial relative calibration. Because most of these points are point targets, they are assumed to have constant temporal backscatter and can be used for relative calibration between scenes.

To generate a point list which shall be used for an initial analysis it is recommended to set the MSR and intensity thresholds to relatively high values which will result in a relatively short list, but with a high fraction of "good points", i.e. points for which interferometric interpretation of the phases is possible.

To generate a candidate point list which shall be used to determine the final "long list" it is recommended to set the MSR threshold to a relatively low value and the intensity threshold to a value smaller than 1. (or even 0.0 to ignor this second criteria entireley). This will result in a long short list, but with a much lower fraction of "good points". But more important at this stage is that only very few "good points" are lost.

Instead of the full area of the SLC a sub-section can be selected using the roff, loff, nr, and nl command line parameters.

The file names of the co-registered SLC are provided in a text file (ascii) with each line containing one SLC file name. A possibility to generate this text file is to use a command such as: ls -1 *.rslc > testsite.SLC_tab.

Other programs to generate new point lists are image2pt, mkgrid, merge_pt, prox_pt, and sp_stat.

Programs to validate existing point lists are qc_pt and expand_pt.

OPTIONS
none.

SEE ALSO
expand_pt, image2pt,merge_pt, mkgrid, prox_pt, qc_pt, sp_stat, SLC_par, typedef_ISP.h, ipta.h.

© Copyrights for Documentation, Users Guide and Reference Manual by Gamma Remote Sensing, 2003.
UW, CW,TS, last change 18-Jun-2003.