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| Quick links | Description :: Parameters :: Parameter descriptions :: Details :: Example :: Algorithm |
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| Name | Type | Caption | Length | Value range |
|---|---|---|---|---|
| FILI * | String | Input file name | 1 - 192 | |
| FILO * | String | Output file name | 1 - 192 | |
| DBIC * | Integer | Input SAR slant range channel | 1 - 1 | |
| INPXSZ * | Float | Input spacing (range, azimuth) | 2 - 2 | 0 - |
| DELAY | Float | Time delay to first input pixel | 0 - 1 | 0.0000 - |
| HEIGHT * | Float | Height above ground (m) | 1 - 1 | 0.000000000 - |
| RESAMPLE | String | Resampling method | 0 - 8 | NEAR | BILIN | CUBIC Default: NEAR |
| MONITOR | String | Monitor mode | 0 - 3 | ON, OFF Default: ON |
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FILI
Specifies the name of the PCIDSK image file containing the SAR slant range image to convert to ground range.
FILO
Specifies the name of a new PCIDSK file to receive the converted ground range image. Although the output file is automatically created, you must specify a file name.
DBIC
Specifies the input image channel that contains the slant range SAR imagery to convert to ground range.
INPXSZ
Specifies, in meters, the pixel spacing for the input slant range SAR image, for both range and azimuth directions.
The pixel spacing for the output ground range image is always square, and equals the input pixel spacing in the azimuth direction (Ysize x Ysize).
DELAY
Specifies, in microseconds, the radar time delay to the first input pixel. If this parameter is not specified, the input image is assumed to be in nadir mode.
DELAY is used to determine the slant range distance, in meters, to the first input pixel in the SAR image. This is calculated using the following formula:
SLANT RANGE DISTANCE = DELAY * SPEED OF LIGHT / 2
The speed of light is 299.793 meters/microsecond. DELAY * the speed of light is the distance to the ground and back. This value is divided by 2 to give the distance one way. If not specified, the default DELAY value is:
DELAY = (2 * HEIGHT) / SPEED OF LIGHT.
Any user-specified DELAY value must be greater than or equal to the default value.
HEIGHT
Specifies, in meters, the assumed aircraft height above ground.
RESAMPLE
Specifies the resampling method to use for the conversion.
MONITOR
The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.
Available options are:
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SRTOGR performs a slant range to ground range conversion on the input SAR image, without requiring an elevation image or ground control points. SRTOGR can be used instead of the FLIGHT and STG functions when an elevation model does not exist, or where ground control points are difficult to collect (such as over water).
SRTOGR assumes a flat terrain and a constant aircraft or satellite height (Height) above ground.
The input pixel spacing for the slant range image (INPXSZ) determines the output pixel spacing for the ground range image. The output pixel spacing is always square, and equals the input pixel spacing in the azimuth direction.
The maximum number of input and output pixels per line that can be processed is 131072 8-bit pixels, or 32768 16/32-bit pixels.
SRTOGR assumes that the X direction is the range and the Y direction is the azimuth. The near range must be positioned on the left side of the image. If required, the ROT function can be used to properly orient the image.
Resampling is the process of sampling one or more input pixels to create one output pixel (this is distinct from the transform phase, which simply determines where the output pixel lies on the input image). In general, resampling is accomplished by taking a weighted average of a small window of input pixels around a point determined by the transform phase. The quality of the corrected output image and the time required for calculation is highly dependent on the chosen resampling method.
Nearest neighbor: This method takes the single input pixel nearest to the transformed point as the resampled output pixel.
Bilinear interpolation: This method takes a weighted average of the four input pixels around the transformed point.
Cubic convolution: this function uses the 4-point classic method. This variation of cubic convolution uses a 4x4 window of input pixels. This method is closer to the perfect sin(x)/x resampler than Nearest Neighbor or Bilinear Interpolation.
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Convert the slant range SAR image of Gatineau Park (gatineau.pix) to a ground range image.
The HEIGHT parameter contains the assumed height of the aircraft above the ground. The report from the example run of the FLIGHT function shows that the aircraft altitude is estimated at 6898.73 meters above sea level. The ground elevation on the DEM (on gatdtm.pix) ranges between 0 and 430 meters, with an average elevation of 236 meters above sea level, therefore, the height of the aircraft above the ground is between 6469 meters (at the highest point in the DEM) and 6899 meters (at sea level).
Because the DELAY is 43.1 microseconds, the slant range distance to the first ground pixel is 6460 meters, which is less than the height of the aircraft everywhere on the slant range image; therefore, the first few slant range pixels are not on the ground. On the original slant range image, one can see that about the first 70 slant range pixels are zeroes, meaning that there are about 70 times 4 = 280 meters from the first slant range pixel to the ground; therefore, HEIGHT was chosen to be the sum of 6460 plus 280, which is 6740 meters.
Because the elevation data is not considered during the slant to ground range conversion, the output ground range image is not as accurate as it would be if the FLIGHT and STGBIG functions were used to correct the image instead.
EASI>fili = 'gatineau.pix' ! input file EASI>filo = 'ground.pix' ! output file EASI>dbic = 1 ! input slant range SAR image EASI>inpxsz = 4.0,3.89 ! input spacing (range, azimuth) EASI>delay = 43.1 ! time delay to first input pixel (us) EASI>height = 6740 ! height above ground (m) EASI>resample = 'NEAR' ! nearest-neighbor resampling EASI>run SRTOGR
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The algorithm used by SRTOGR is derived from the FLTIMA function, which is part of LDIAS (Landsat Digital Image Analysis System) developed by CCRS (Canada Centre for Remote Sensing) in Ottawa, Canada. The equations were derived with the help of Dr. Bert Guindon of CCRS, who wrote the original FLTIMA function.
The algorithm is output driven. For each output pixel location in the output window on the master (output) file, the corresponding input pixel location on the uncorrected (input) file is determined. The pixel value at the input pixel location is copied to the output pixel location.
If S0 <= HEIGHT, then G0 = 0
Compute slant range pixel N given ground range pixel M:
N = (SQRT((G0 + M*INPXSZ(2))**2+HEIGHT**2) - S0) / INPXSZ(1)
Compute ground range pixel M given slant range pixel N:
M = (SQRT((S0 + N*INPXSZ(1))**2-HEIGHT**2) - G0) / INPXSZ(2)
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