Gamma ISP: Reference Manual

radcal_SLC and radcal_MLI

ANSI-C programs: radcal_SLC.c, radcal_MLI.c

NAME
radcal_SLC - Radiometric calibration of single-look complex (SLC) images
radcal_MLI - Radiometric calibration of multi-look intensity (MLI) images

These programs support the radiometric calibration of single-look complex (SLC) and multi-look intensity (MLI) images. The supported radiometric calibration steps include:
- range spreading loss correction
- antenna gain correction
- normalization reference area correction
- correction for calibration constant
In addition the output can be scaled with an additional scaling factor, as required, for example, for output in short integer format.

Not supported radiometric calibration steps include:
- correction for analog digital converter (ADC) saturation

SYNOPSIS
radcal_SLC <SLC> <SLC_par> <CSLC> <CSLC_par> [fcase] [antenna] [rloss_flag] [ant_flag] [refarea_flag] [sc_dB] [K_dB] [pix_area]

<SLC>	(input) SLC (fcomplex or scomplex)
<SLC_par>	(input) SLC parameter file of input SLC
<CSLC>	(output) radiometrically calibrated SLC (fcomplex or scomplex)
<CSLC_par>	(output) SLC parameter file of output calibrated SLC
[fcase]	format case (default = 1) 1: fcomplex --> fcomplex (pairs of float) 2: fcomplex --> scomplex (pairs of short integer) 3: scomplex --> fcomplex 4: scomplex --> scomplex
[antenna]	1-way antenna gain pattern file or - (if not provided)
[rloss_flag]	range spreading loss correction (default=0: no correction, 1: r^3 correction, -1: undo r^3 correction, 2: r^4 correction, -2: undo r^4 correction) (r^3 correction is used except for ASAR APS mode data where r^4 correction is used)
[ant_flag]	antenna pattern flag correction (default=0: no correction, 1: correction, -1: undo correction)
[refarea_flag]	normalization reference area correction (default=0: no correction, 1: correction to sigma0, 2: correction to gamma0, -1: undo 1, -2: undo 2)
[sc_dB]	scale factor in dB (default=0.0, ESA published ERS1 K_dB update=-0.39, recommended for the upscaling scomplex output format: 60.0 dB)
[K_dB]	Calibration factor in dB (default: -(value from SLC_PAR) )
[pix_area]	ellipsoid-based ground range sigma/gamma nought pixel reference area depending on refarea_flag. (1 or -1: sigma nought ref. area, 2 or -2: gamma nought ref. area; float format)

and

radcal_MLI <MLI> <MLI_PAR> <OFF_PAR> <CMLI> [antenna] [rloss_flag] [ant_flag] [refarea_flag] [sc_dB] [K_dB] [pix_area]

<MLI>	(input) MLI (float)
<MLI_PAR>	(input) SLC parameter file of input MLI image
<OFF_PAR>	(input) ISP offset/interferogram processing parameter file (or - for image in SLC/MLI geometry)
<CMLI>	(output) radiometrically calibrated output MLI (float format)
[antenna]	1-way antenna gain pattern file or - (if not provided)
[rloss_flag]	range spreading loss correction (default=0: no correction, 1: r^3 correction, -1: undo r^3 correction, 2: r^4 correction, -2: undo r^4 correction) (r^3 correction is used except for ASAR APS mode data where r^4 correction is used)
[ant_flag]	antenna pattern flag correction (default=0: no correction, 1: correction, -1: undo correction)
[refarea_flag]	normalization reference area correction (default=0: no correction, 1: correction to sigma0, 2: correction to gamma0, -1: undo 1, -2: undo 2)
[sc_dB]	scale factor in dB (default=0.0, ESA published ERS1 K_dB update=-0.39, recommended for the upscaling scomplex output format: 60.0 dB)
[K_dB]	Calibration factor in dB (default: -(value from SLC_PAR) )
[pix_area]	ellipsoid-based ground range sigma/gamma nought pixel reference area depending on refarea_flag. (1 or -1: sigma nought ref. area, 2 or -2: gamma nought ref. area; float format)

EXAMPLE

radcal_SLC 1352.slc_esa 1352.slc_esa.par 1352.cslc_esa 1352.cslc_esa.par 4 ERS1_antenna.gain 1 1 1 59.61

Calibrates the input SLC (provided by an official ESA Processing and Archiving Facility (PAF) in scomplex format, i.e. pairs of short integer) for the range spreading loss, the antenna diagram, the normalization reference area correction, the calibration constant (provided by the PAF and found in the SLC_PAR), the update of the calibration constant (-0.39 dB), and an additional scaling factor or +60.0 dB to scale the values into an appropriate value range to be stored again in scomplex format. The output format is scomplex. SQR(re) + SQR(im) corresponds to 1.e06 times the backscattering coefficient sigma0.

radcal_SLC 1352.slc_esa 1352.slc_esa.par 1352.cslc_esa 1352.cslc_esa.par 3 ERS1_antenna.gain 1 1 2 -0.39

Calibrates the input SLC (provided by an official ESA Processing and Archiving Facility (PAF) in scomplex format, i.e. pairs of short integer) for the range spreading loss, the antenna diagram, the normalization reference area correction, the calibration constant (provided by the PAF and found in the SLC_PAR), and the update of the calibration constant (-0.39 dB). The output format is fcomplex. SQR(re)+SQR(im) corresponds to the backscattering coefficient gamma0 (defined as sigma0/cos(incidence angle).

radcal_MLI 1352.mli 1352.mli.par - 1352.cmli - 0 0 1 -0.39 -48.13

Calibrates the input MLI (generated by multi-looking of an ESA PAF provided SLC using the program multi_look) for the normalization reference area correction, the calibration constant -48.13 dB, and the update of the calibration constant (-0.39 dB). The output mli values correspond to the backscattering coefficient sigma0.

radcal_MLI 1352_2435.pwr1 1352.slc.par 1352_2435.off 1352_2435.pwr1.tmp - 0 0 -1 0.0

followed by

radcal_MLI 1352_2435.pwr1.tmp 1352.slc.par 1352_2435.off 1352_2435.pwr1.gamma0 - 0 0 2 0.0

Transforms 1352_2435.pwr1 from sigma0 backscattering coefficients to gamma0 values.

radcal_MLI 1352_2435.pwr1.tmp 1352.slc.par 1352_2435.off 1352_2435.pwr1.gamma0 - 0 0 2 0.0 - pix_area

in addition, writes the ellipsoid-based gamma nought normalization area (since refarea_flag = 2) to the output file pix_area.

DESCRIPTION
radcal_SLC performs radiometric corrections for SAR data including the following terms:

- range spreading loss correction
- antenna gain correction
- normalization reference area correction
- correction for calibration constant

according to to the calibration procedure described in the document: "ERS-1 SAR Calibration", H. Laur, ESA-ESRIN, 17. October 1992, and "ERS-1 SAR Radiometric Calibration", H. Laur, P. Meadows, J.I. Sanchez, and E. Dwyer, CEOS SAR Calibration Workshop, ESA WPP-48. This approach can be applied to other SAR sensors. Different sensors produce products that have already corrected for this factors. Ideally, all the spatially varying corrections have already been performed on the delivered image products and only a constant scale factor is required to scale the data to obtain relative radar cross-section. Early systems such ERS-1, ERS-2, and JERS-1 required spatially dependent corrections such as for the elevation antenna pattern while current systems usually do not since the correction has already been performed by the SAR processor.

In order to provide flexibility scomplex (short complex) and fcomplex (float complex) formats are supported for the input and output SLC. When selecting scomplex as output format it has to be taken into account that the output values have to be in the range of the short integer values. With the expected SAR backscatter levels of -10 dB we recommend to scale the calibrated intensities with a factor of 1'000'000 (+60dB). This scales the real and imaginary parts with a factor 1'000 (30dB). With this scaling a backscattering value of -10 dB transfers to real and imaginary parts of the order of 316. To achieve maximum accuracy it is recommended to use fcomplex format for the calibrated SLC data (especially for calibration experiments with strong point targets). In general processing data with the Gamma software is best done using float complex format for complex data to avoid radiometric and phase errors.

The absolute calibration constant is stored in the SLC parameter file and is expressed in dB. For example the calibration constants for ERS-1 and ERS-2 are -48.52 dB and -50.09 dB respectively. The calibration scale factor can also be used for the scaling required for output SLC in scomplex format. The scaling factors are for the backscattering intensity are proportional to the square of the real and imaginary parts of the SLC.

If the radar data have not been corrected for the antenna elevation pattern, the programs can perform this correction. The antenna file (text format) contains relative gain values as a function of elevation angle relative to boresight. There are two columns in the file, where the first column is the angle relative to the antenna boresight in degrees, and the second column has values of the relative power gain as a decimal number. Note that the gain is not in decibels (dB). The gain used to radiometrically correct the data for each look angle is linearly interpolated from the the input antenna gain table. It is recommended that the spacing of the sample values of the antenna pattern should be spaced at most 1/50 the antenna beam width. In the case of ERS-1, with a 5 degree elevation beamwidth degrees the spacing is 0.1 degrees.

Here is a section of the table from ERS-1 radar for the range -1.0 to 1.0 degrees:.

-1.000   1.00612
-0.900   1.00520
-0.800   1.00404
-0.700   1.00265
-0.600   1.00127
-0.500   1.00011
-0.400   0.99896
-0.300   0.99850
-0.200   0.99850
-0.100   0.99896
0.000   1.00000
0.100   1.00173
0.200   1.00381
0.300   1.00647
0.400   1.00937
0.500   1.01239
0.600   1.01543
0.700   1.01918
0.800   1.02294
0.900   1.02695
1.000   1.03086

As output normalization type either sigma0 or gamma0 can be selected. For sigma0 the pixel reference area used in the normalization corresponds to the product of the azimuth pixel spacing and the projection of the slant range pixel spacing to a local horizontal surface (taking into account the selected reference ellipsoid). gamma0 is defined as sigma0/cos(inc_ang) with inc_ang the incidence angle defined as the angle between the look vector and the normal to the local reference ellipsoid. One reason for selecting gamma0 values instead of sigma0 values is the slightly reduced incidence angle, and therefore slant range, dependence. For a perfect Lambert scatterer the backscattering coefficient gamma0 is constant. Notice that SLC images processed with the GAMMA Modular SAR Processor (MSP) are already radiometrically calibrated. The image geometry is either specified with the ISP SLC/MLI parameter file and the ISP/offset parameter file (as it is the case for interferometric products) or by the ISP SLC/MLI parameter file only for MLI files generated by the program multi_look. In the later case - is indicated instead of a valid ISP/offset parameter file name.

For ENVISAT ASAR data a similar procedure as for ERS is used. Main differences are that for APS mode (alternative polarization) the r^4 range spreading loss correction has to be used instead of the r^3 correction which is used for ERS and all other ASAR modes. For ASAR the reference incidence angle is 90.0 degrees for all modes. The calibration constant has changed throughout the ENVISAT mission. The value for the image to be processed is stored in the ISP parameter file under calibration_gain. This value is automatically read in or can be specified on the command line.

By specifying the last optional parameter, pix_area, the ellipsoid-based ground-range sigma/gamma nought pixel normalization area is written to disk in float format. The choice between sigma nought or gamma nought normalization area is made according to the user-defined refarea_flag (1 or -1: sigma nought ref. area, 2 or -2: gamma nought ref. area). The resulting pixel normalization area can be used in combination with terrain-based normalization areas obtained from the program pixel_area for an optimized calibration of SAR data.

OPTIONS
Setting the option flags to negative values allows to specifically undo calibration corrections. This may be used, for example, to convert sigma0 values to gamma0 values, by first undoing the normalization reference area correction and then applying the alternate normalization reference area correction.

SEE ALSO
typedef_ISP.h, SLC_par, pixel_area