| Environments | PYTHON :: EASI :: MODELER |
| Batch Mode | Yes |
| Quick links | Description :: Parameters :: Parameter descriptions :: Details :: Algorithm |
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| Name | Type | Length | Value range |
|---|---|---|---|
| Input: Input SAR slant range channel * | SAR Slant Range Layer port | 1 - 1 | |
| Output: Output ground range channel * | Output Ground Range Layer port | 1 - 1 | |
| Spacing (m) * | Float | 2 - 2 | 0 - |
| Delay | Float | 0 - 1 | 0.0000 - |
| Height Above Ground (m) * | Float | 1 - 1 | 0.000000000 - |
| Resampling Method | String | 0 - 1 | NEAR | BILIN | CUBIC Default: NEAR |
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Input: Input SAR slant range channel
Specifies the input image channel that contains the slant range SAR imagery to convert to ground range.
Output: Output ground range channel
Specifies the output image channel to receive the converted ground range.
Spacing (m)
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 Above Ground (m)
Specifies, in meters, the assumed aircraft height above ground.
Resampling Method
Specifies the resampling method to use for the conversion.
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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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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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