ANSI-C program: ptarg_cal_SLC.c
NAME
ptarg_cal_SLC: Point target analysis and
radiometric calibration of SLC images
SYNOPSIS
ptarg_cal_SLC
<SLC_par> <SLC> <r_samp> <az_samp>
<psigma> <c_r_samp> <c_az_samp>
<ptr_image> <r_plot> <az_plot> <pcal>
[osf] [win] [pltflg] [psz] [csz]
| <SLC_par> | (input) SLC image parameter file |
| <SLC> | (input) SLC image in FCOMPLEX or SCOMPLEX
format |
| <r_samp> | point target range sample number |
| <az_samp> | point target azimuth line number |
| <psigma> |
radar cross-section of the
calibration target in m**2 |
| <c_r_samp> |
clutter region center
range sample number |
| <c_az_samp> |
clutter region center
azimuth line number |
| <ptr_image> | (output) oversampled point
target image, with and without phase gradient, width is
osf*psz |
| <r_plot> | (output) range point target response plot data (text format) |
| <az_plot> | (output) azimuth point target response plot data (text format) |
| [pcal] |
(output) measured point
target parameters and radiometric calibration factor (text
format) |
| [osf] | image over-sampling factor, 2, 4, 8, 16, 32, 64 (enter - for default: 16) |
| [win] |
(input) maximum search
window offset (samples) (enter - for default: 1) |
| [pltflg] |
plotting mode flag: 0: none 1: output plots in PNG format (default) 2: screen output and PNG format plots 3: output plots in PDF format |
| [psz] | point target region size (samples) (enter - for default: 16) |
| [csz] | clutter region size (samples) (enter - for default: 16) |
| [c_image] | (output) clutter region image (FCOMPLEX format) |
EXAMPLE
ptarg_cal_SLC 19990421.slc
19990421.slc.par 203 48 10.0 203 70 cr2.slc
cr2.r_plot cr2.az_plot cr2.pcal
Integrates the target backscatter over an 16x16 window about
the coordinates specified on the command line. Determines the
exact point target location of a calibration target, oversamples
the SLC about the target, and determines the range and azimuth
response. The average clutter level is determine from a region
close to the target. This information is used to determine
the absolute radiometric calibration factor. A text file
pcal is generated that
contains the results of the radiometric analysis.
DESCRIPTION
ptarg_cal_SLC is a
point target analysis tool used to determine the radiometric
calibration factor for SLC images given that the image contains a
calibration target with known radar cross section (RCS). The
input SLC (in FCOMPLEX or SCOMLEX format) is read and the
response of the point target at or near the indicated location is
analyzed. To estimate the approximate point target location the
SLC can be displayed using the display program disSLC.
The user must also provide the center coordinates of a clutter
region chosen close to the point target and that is
representative of the area where the point target is
located. The radiometric calibration factor is determined
using the integral algorithm described by Gray et al. 1990
[1].
We start from an SLC that is corrected for antenna pattern,
slant range spreading loss, ellipsoidal corrected reference area,
except for the calibration constant. This means that the
SLC image is radiometrically corrected except for the calibration
constant and terrain topography effects. If this is not the case
then the SLC has to be corrected using radcal_SLC. Sigma0 values have units
of m**2/m**2 (dimensionless) and Radar Cross Section is in units
of m**2.
Then RCS (radar cross section) of a calibrated (sigma0) SLC pixel
is K*A*(Re**2 + Im**2),
where K is the
radiometric calibration factor. The defined reference area
A is given by
ground_range_pixel_spacing * azimuth_pixel_spacing. The
RCS of an area is given by the sum of RCS of the pixels
within the area. This includes the calibration target RCS
and clutter RCS.
RCS_CR_with_clutter = K * SUM(A
* P_i)
over the 16x16 region where the sum is of the individual pixel
intensities, P_i,
defined as (Re**2
+Im**2).
The clutter cross-section is determined using an area without the
target but has the same characteristics (area with the same
characteristics close to the calibration target).
RCS_clutter_per_pixel = K*
SUM(A*P_i)/N_c
over the clutter estimation region(32*32), where N_c is the number of pixels in the
clutter region(32*32).
RCS_CR = RCS_CR_with_clutter
- (N * RCS_clutter_per_pixel)
where N is number of samples in the 8x8 point target
region.
We can now solve for the calibration constant K:
K = RCS_CR/(SUM(A * P_i)-
(N * SUM(A * P_i)/N_c))
Note: the two sums are for the region with the target, and the
clutter region without the target respectively. This
expression can be simplied by extracting A from within the
sums:
K = RCS_CR/(A *
(SUM(P_i)- (N * SUM(P_i)/N_c))
The output pcal file has the following format:
#r_pix, az_pix,
r_pos, az_pos,
pk_mag, pk_phase_deg, r_3dB_width,
az_3dB_width, r_10dB_width, az_10dB_width, r_pslr_dB,
az_pslr_dB
202.871 47.350 681125.223
81.996 1.48090e+03
-123.904
1.771
2.966
3.118
5.277
-21.669 -23.996
#cl_r_pix, cl_az_pix, cl_ave_pwr,
ptr_range, inc_angle, ptr/clutter_dB, beta0_cl_dB,
sigma0_cl_dB, K_beta0_dB, K_sigma0_dB
203
70
1.2616e+04 681125.399
0.74661 26.7047
-20.4380
-22.1183 -61.4471
-63.1274
The first line describes the point target parameters including
the following parameters:
rpix:
range pixel position of the target peak (samples)
az_pix:
azimuth pixel position of the target peak (lines)
rpos:
slant range of the pixel peak
azpos:
meters along-track position of the pixel peak
pk_mag:
magnitude of the peak
pk_phase_deg: phase in degrees of the pixel
peak
r_3dB_width: 3 dB range width of the peak
in meters
az_3dB_width: 3 dB azimuth width of the peak in
meters
r_10dB_width: 10 dB range width of the peak in
meters
az_10dB_width 10 dB azimuth width of the peak
in meters
r_pslr_dB range peak
sidelobe ratio in dB
az_pslr_dB azimuth peak
sidelobe ratio in dB
The second line describes the clutter statistics and
radiometric calibration parameters:
cl_r_pix
clutter region center range pixel
cl_az_pix clutter
region center azimuth pixel (line)
cl_ave_pwr clutter average
power level in
ptr_range point target
range
inc_angle incidence
angle at the point target
ptr/clutter_dB ratio of the point target energy to
clutter in dB
beta0_cl_dB beta0 of the clutter
region dB
sigma0_cl_dB sigma0 of the clutter region
dB
K_beta0_dB calibration
constant for beta0 dB
k_sigma0_dB calibration constant
for sigma0 dB
For the point target response analysis, the SLC is oversampled
with an oversampling factor that can be set on the command
line using the osf
parameter. The location of the intensity maximum is determined.
Centered at the intensity maximum the range and azimuth response
is determined and stored as text files and optionally plotted
using Gnuplot. Parameters estimated from the data include the
sub-pixel peak location, expressed in input SLC samples, the
signal intensity in dB, the signal intensity in linear scale, and
the signal phase. The over-sampled SLC around the point target is
saved as a 2-D image in FCOMPLEX format.
The required parameters in the SLC parameter file are:
date: YYYY MM DD (2012 10
15)
image_format: (SCOMPLEX, FCOMPLEX)
line_header_size: integer (should be nominally 0)
range_samples : integer
azimuth_lines: integer
image_geometry: SLANT_RANGE
range_looks:
1
azimuth_looks: 1
range_scale_factor: 1.0000
azimuth_scale_factor: 1.0000
azimuth_deskew: ON
azimuth_angle:
90.0 deg. (for right-looking)
radar_frequency: Hz
range_pixel_spacing: meters
azimuth pixel_spacing: meters
near_range_slc:
meters
sar_to_earth_center: meters
earth_semi_major_axis: 6378137.0000 m
earth_semi_minor_axis: 6356752.3141 m
number_of_state_vectors: 0
Other parameters should be set to
0.0
ptarg_cal_SLC estimates the the 3 dB and 10 dB peak widths in range and azimuth, and the Peak to Side-Lobe Ratio (PSLR). The response measured over a larger region including integrated sidelobe ratio can be performed using the program ptarg_SLC. The search region for the point target peak is located in the center of the data segment extracted from the entire image as specified by the r_samp and az_samp parameters command-line parameters. The size of the search region defaults to +/- 1 pixel of the center coordinates but can be increase to +/- 4 samples by entering a value for the win command-line parameter. The interpolated image has dimensions determined by the over-sampling factor. The oversampled point target image is 256x256 for an 16x16 image oversampled with osf=16. Two images are written to the output ptr_image. The first image has any linear phase trend removed, and the second image is an interpolated version of the original SLC segment.
The user can specify on the command line the size of the point-target window and the size of the clutter window using the command line parameters psz and csz The clutter window should be at least as large as the window used for extracting the point target. Increasing the size of the window for the point target beyond where there is any return will only increase errors since the contribution of the clutter has increased relative to the point target energy. It should usually not be necessary to change the size of the windows to differ from the default.
The user has the option to write out the data values in the
clutter region in FCOMPLEX format to a data file by specifying
the c_image parameter.
This parameter is the name of the file created to contain the
data values from the clutter region. The clutter data are
not oversampled so that the number of data values per line and
the number of lines are equal to the value of the csz command line parameter.
[1] Gray, L.A., P.
Vachon, C. Livingstone, T. Lukowski, "Synthetic Aperture Radar
Calibration using Reference Reflectors," IEEE Transactions on
Geoscience and Remote Sensing, Vol. 28, No. 3, pp. 374-383, May
1990.
SEE ALSO
Users Guide, ptarg_cal_MLI, radcal_SLC, ptarg_SLC