| Environments | EASI |
| Sections | Description Parameters Parameter Details Details Example References Acknowledgement |
Generates a calibrated radar backscatter image from a RADARSAT or ENVISAT ASAR image, with correction for radiometric terrain effects. Also creates radar shadow and layover masks in the input image geometry.
Progress of this program can be monitored. See MONITOR section.
| Name | Prompt | Type | Length |
|---|---|---|---|
| FILE | Database Input File Name | Char | 1 - 192 |
| DBIC | Database Input Channel List | Int | 1 - 4 |
| DBIW | Database Input Window | Int | 0 - 4 |
| ORBIT | Orbit Segment Number | Int | 0 - 1 |
| DBGC | Database Ground Control Segment | Int | 0 - 1 |
| FILO | Database Output File Name | Char | 0 - 192 |
| DBOC | Database Output Channel List | Int | 0 - 2 |
| FILEDEM | Database DEM File Name | Char | 1 - 192 |
| DBEC | Database Elevation Channel | Int | 0 - 1 |
| BACKELEV | Background Elevation Value | Real | 0 - 1 |
| LCOVCOR | Land Cover Correction Exponent | Real | 0 - 1 |
| AMPORDB | Amplitude, Power or Decibels | Char | 0 - 3 |
| REPORT | Report Mode: TERM/OFF/filename | Char | 0 - 192 |
Specifies the name of the PCIDSK file containing the SAR channels and segments to use as input. The file must contain the SAR Orbital Ephemeris segment and SAR-specific array segments (Offset and Gain).
Specifies the image channel(s) to process. One, two or four values must be specified.
EASI>dbic=m | For non-SLC image, process one channel
EASI>dbic=m,n | For SLC image, process I and Q channels
EASI>dbic=k,l | For ASAR APP image, process both polarization
| channels
EASI>dbic=k,l,m,n | For ASAR APS image, process four SLC channels
| of both polarizations
Input channel pixel values should be digital numbers representing amplitude of radar signal scattered from each pixel. This is the format created by SAR distribution format import modules, such as CDASAR.
Specifies the image window to process from the input channel(s).
EASI>DBIW=Xoffset,Yoffset,Xsize,YSize EASI>DBIW= | Defaults to entire image
If DBIW is specified to a window size smaller than that of FILE, the orbit and GCP segments contained in FILE will not be output to FILO.
Specifies the number of the ORBIT segment on FILE. If ORBIT is undefined, the first orbit segment on file will be used.
EASI>ORBIT=2 | Use orbit information in segment 2 EASI>ORBIT= | Find and use the first orbit segment on FILE
Optionally specifies the number of the segment containing the collected Ground Control Points (GCPs, segment type 214). GCPs refine the geometric model of the SAR image geometry.
EASI>DBGC=k | Use GCP segment number k EASI>DBGC= | Do not use GCPs
Specifies the name of the PCIDSK image database to receive the generated backscatter image. If FILO is defaulted, FILE is used to store the output image.
EASI>FILO="filespec" EASI>FILO= | Use FILE for input and output
If FILO is specified but does not exist, a new file is created with the required number of "32R" channels to receive the output image data.
Specifies the image channel(s) on FILO to receive the generated backscatter channel(s). All output channels must be 32-bit real, or an error is issued.
EASI>DBOC=n | Write single output channel n
EASI>DBGC=n,m | Write two output channels n and m
| (valid only for ASAR APS or APP)
If FILO is to be created, DBOC is ignored.
If DBOC is the same as DBIC on the same image file, the output image is processed correctly and overwrites the input image.
Specifies the filename of the input PCIDSK image database containing elevation values.
EASI>FILEDEM="filespec"
Optionally specifies the channel in the file FILEDEM which contains the terrain elevation values.
EASI>DBEC=n | Use channel n in FILEDEM EASI>DBEC= | Use channel 1 in FILEDEM
Elevation values in DBEC are required to be in metres.
Optionally specifies a value which is used as a background (undefined or unknown) elevation value in FILEDEM. If specified then pixels in the elevation channel with this value are ignored.
EASI> BACKELEV= | All elevation pixels are valid EASI> BACKELEV=b | Ignore elevation pixels with value b
Optionally specifies exponent for the land cover correction. The exponent is a real value between 0.0 and 2.0. The value of 0.0 results in no correction applied. Values between 0.5 and 1.0 are recommended for general use, and work best in forested areas.
EASI>LCOVCOR=0.7 | Apply the recommended correction EASI>LCOVCOR=0.0 | Do not apply the land cover correction EASI>LCOVCOR= | Use the default, no correction
Please refer to the DETAILS section for more information on this parameter.
Specifies whether to output amplitude, power or decibel (dB) values of the derived radar backscatter.
EASI>AMPORDB= | Default is "DB" EASI>AMPORDB="DB" | Output decibels EASI>AMPORDB="AMP" | Output amplitude values EASI>AMPORDB="POW" | Output power values
Specifies the file to which the report should be appended.
EASI>REPORT="filename" EASI>REPORT= | Defaults to terminal output
Note: The following names have special meaning:
EASI>REPORT="TERM" | generates reports on your terminal EASI>REPORT="DISK" | generates reports on file "IMPRPT.LST" EASI>REPORT="OFF" | cancels report generation
The report provides total number of processed pixels, number of pixels with the elevation value available, and the number of detected shadow and layover pixels.
RTCSIGM generates a calibrated radar backscatter image from the input scaled radar image. It also corrects the derived values for radiometric terrain effects, derived from the supplied digital elevation model (DEM). All processing is performed in the raw SAR image geometry, and there is no spatial resampling of image pixels. For a zero-elevation DEM the results of RTCSIGM should be very similar to the output of SARSIGM for the same input image.
The input image must not be processed by the SARBETA or SARSIGM programs before running RTCSIGM.
FILE specifies the database which contains the uncorrected Radarsat-1 or ASAR image. This file should be created using the SAR image reading program, CDASAR. It should contain an ORBIT segment, the image channels specified by DBIC, and SAR Offset and SAR Gain array segments. The array segments are identified by RTCSIGM automatically, but an error is issued if they are not present, are corrupted, or are inconsistent with the image parameters.
DBIC specifies the channels on FILE that are to be processed. One, two or four channels should be specified. Two channels must be specified for Radarsat-1 SLC images, and ASAR single polarization images (ASA_IMS). Two channels may be also specified for the detected ASAR Alternating Polarization (ASA_APP or ASA_APM) images. Four input channels can be specified only for ASAR Alternating Polarization SLC images (ASA_APS). Each pair of SLC channels is combined (detected) on reading and processed to result in a single calibrated and terrain corrected backscatter channel.
For Single Look Complex (SLC) images, the first channel of each channel pair is taken as the "I" (in-phase or real) channel, and the second as the "Q" (quadrature or imaginary) channel.
DBIW specifies a rectangular window in the input image FILE to process. If this parameter is defaulted, the whole FILE image will be processed.
ORBIT specifies the segment on FILE containing the satellite orbital and ephemeris information. This segment should be created by the SAR image reading program. If the parameter is defaulted, the program will search FILE for an orbit segment, and will use the first segment found. If the ORBIT segment does not exist, program will abort with an error.
DBGC optionally specifies the number of a segment on FILE that contains Ground Control Points (GCPs). If provided, the GCPs are used to refine the model of the viewing geometry.
FILO specifies the database which will contain one or two image channels, and two bitmaps. The image channel(s) will contain the calibrated and radiometrically corrected input image channel(s). The first bitmap will have set to 1 all pixels where the layover occurs. The second bitmap will have set to 1 all pixels where the shadow occurs.
FILO can be defaulted, in which case output channels are assumed to exist on FILE, and the whole image must be processed. If FILO is specified and the file does not exist, it will be created. If the DBIW parameter is defaulted to the entire window, all Orbit (type 160) and GCP (type 214) segments are copied from FILE. If FILO exists it will be used for output, provided its dimensions agree with the size of the window specified by DBIW, the channels specified by DBOC exist, and are all 32-bit real.
If FILO exists, the ORBIT and GCP segments are transferred only if they do not already exist on FILO, FILO is the same size as FILE, and the DBIW parameter is defaulted to the entire image.
DBOC specifies the channels on FILO that are to be used for output. For SLC images the number of channels in DBOC must be exactly half of the number in DBIC; in all other cases the two numbers must be equal. If FILO is to be created DBOC is ignored, and can be defaulted. All channels specified in DBOC must be 32-bit real, or an error is detected. If the output is requested to FILE, the output channel(s) can be the same as the input channels, provided they are 32-bit real. Since this is not the case with the images imported by CDASAR, this practice is not recommended.
FILEDEM specifies the database which contains the DEM. The DEM can be in any georeferencing projection. It is assumed that the elevation values are in metres.
DBEC specifies the channels on FILEDEM that contains elevation values. If DBEC is defaulted, the first channel will be used.
BACKELEV optionally specifies the background elevation in the DEM. Typically BACKELEV is used when the data in the elevation channel are incomplete (e.g., elevation only in an area of interest, or the edges of the DEM are clipped). In this case the pixels with no known elevation are filled with an arbitrary value, such as -9999. If this is the case, BACKELEV should be set to this value. Pixels without elevation values are set to a no-data value (0) in FILO.
Calibration is the conversion of Digital Numbers stored in pixel values to radar brightness values. This is done according to Radarsat-1 and ASAR data format specifications, by applying offset and scale values provided in the distribution format, and converted to PCI SAR Offset and SAR Gain array segments by data importing programs.
The Radiometric Terrain Correction (RTC) corrects for the total ground area scattering into a single image pixel, accounting for terrain slope in the range (towards radar) and azimuth (along satellite path) directions. The two slope components are used to derive local incident angle, defined as the angle between the normal to terrain surface, and the direction towards the radar. The angle derivation is based on SAR antenna position, Earth shape, and the DEM values at and around the pixel. The RTC darkens pixels on slopes facing the radar antenna, and brightens pixels on slopes facing away from the antenna.
The radiometric correction algorithm, implemented with the assistance of Dr. Thierry Toutin of the Canada Centre for Remote Sensing, is based on Reference [1].
The radiometrically corrected value can be further adjusted for response of a uniform land cover. The correction is based on Reference [2].
The land cover correction compensates for anisotropic response of volume scatterers. It is inversely proportional to the power of the cosine of the total (3D) local incidence angle. The correction is enabled by selecting the exponent through the parameter LCOVCOR to a value between 0.0 and 2.0. Setting LCOVCOR to 0.0 disables the correction.
The correction is 1 (has no effect) for slopes exactly facing the sensor, and increases pixel values on slopes inclining away from the sensor. Different land covers might require different exponents, but at present the same correction is applied to the whole image. The values between 0.5 and 1.0 were found to perform well, although some experimentation might be required to find an optimal value for a particular image. A successful correction results in similar tone for areas known to have similar land cover, but located on differently-sloping terrain.
RTCSIGM creates layover and shadow masks, stored in two bitmaps. Layover areas correspond to slopes facing the sensor which are steeper than the angle between the local vertical and the direction towards the satellite antenna. In such areas the higher terrain points are imaged ahead of the lower terrain points. Layover pixels are adjusted, but the corrected values represent the average backscatter of the ground area projecting into a single pixel, and any spatial variability is lost.
Radar shadow areas correspond to back slopes facing more than 180 degrees away from the direction to the satellite (self-shadowing), or to areas screened from the sensor by a high peak between the satellite and the terrain point (cast shadows). Since no energy is backscattered from such areas, the corresponding pixel values represent thermal and electronic noise of the sensor. RTCSIGM converts shadow pixels to radar backscatter values (sigma nought), but does not correct them otherwise.
Radar backscatter values are naturally computed in power domain. Power values have a positive range of real values usually not exceeding tens or low hundreds, with the majority of values less than 1. Filtering and texture analysis operations should be performed in the power domain.
Amplitude values (where amplitude equals the square root of the radar backscatter or power) would have a positive range of real values usually not exceeding low teens.
Decibels will produce a range of real numbers in the range from high negative teens to low positive teens. If quantitative measurements of radar backscatter over point and distributed targets is the objective, AMPORDB should be set to "DB". It is important to note that decibel values are on a logarithmic scale. As a result, certain mathematical operations applied to the dB output channel may not be valid. For example, the averaging of pixels will not yield a correct mathematical result. Any such operations should be done on power values and the result then converted to dB.
The data may be later converted from decibels to amplitude or power values (or from amplitude or power values to decibels) with theEASI + modelling functions or the MODEL program. This is documented in the ALGORITHM section of the program SARSIGM.
The correction algorithm, although approximate, should result in output image free from most of terrain-induced radiometric distortions, particularly in areas of gentle or moderate terrain relief. There may be some artifacts remaining in the high relief areas of the image, as the true information is severely distorted there (layover slopes) or not available at all (radar shadow).
Due to the extreme sensitivity of SAR sensor to terrain relief, errors in the DEM and in the geometric model may also create visible artifacts. Common DEM errors include over-smoothed mountain peaks and valley bottoms, as well as filtered out minor terrain features. SAR sensor responds strongly to even small changes of local slopes, and their effects will be visible in the input image. If a particular feature is absent from the DEM, the correction cannot be applied and an artifacts will be visible in the output image. Any artifacts in the DEM (spikes, dropped pixels, spurious ridges) will also distort the output image. To avoid errors, the best available DEM should be used, at a resolution identical or close to the resolution of the output image.
The geometric model errors manifest themselves as local shifts between the true and modelled pixel positions, which in turn causes some pixels to be over-corrected, and other under-corrected. This is visible as bright and dark lines following local elevation extrema, such as ridges and valley bottoms. These artifacts can be minimized by deriving the most accurate model possible with the available GCPs. Unfortunately, due to the accumulated errors of the total processing chain, such effect are unlikely to be removed completely.
To convert and correct a small area, enter the following:
EASI> FILO="radar.pix" EASI> DBIC=1 EASI> DBIW=1000,1000,512,512 EASI> ORBIT=2 EASI> DBGC= | No GCP segment EASI> FILO="rtcsigma.pix" | To be created automatically EASI> DBOC= | Defaulted for a new file EASI> FILEDEM="dem_1.pix" EASI> BACKELEV= | All elevations are valid EASI> LCOVCOR= | Do not apply land cover correction EASI> AMPORDB= | Default output units (DB) EASI> REPORT="rtcsigma.rpt" EASI> R RTCSIGM
An ASAR Alternating Polarization image is available, and it was read from the CD by CDASAR. To convert and correct the whole image with the optional Land Cover Correction, enter the following:
EASI> FILO="asa_app.pix" EASI> DBIC=1,2 EASI> DBIW= | Process whole image EASI> ORBIT=2 EASI> DBGC= | No GCP segment EASI> FILO="app_rtc.pix" | To be created automatically EASI> DBOC= | Defaulted for a new file EASI> FILEDEM="dem_2.pix" EASI> BACKELEV=-9999 EASI> LCOVCOR=0.7 EASI> AMPORDB= | Default output units (DB) EASI> REPORT="app_rtc.rpt" EASI> R RTCSIGM
[1]. Van Zyl, J., B.D. Chapman, P. Dubois, J. Shi, 1993.
The Effect of Topography on SAR Calibration, IEEE Transactions on
Geoscience and Remote Sensing, Vol. 31, No. 5, pp. 1036-1043.
[2]. Stussi, N., A Beaudoin, T. Castel and P. Gigord, 1995.
Radiometric correction of multi-configuration spaceborne SAR data
over hilly terrain, Proceedings of Int. Symposium on Retrieval
of Bio- and Geophysical Parameters from SAR Data for Land
Applications, Toulouse, France, 10-13 October 1995, pp. 469-478.
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