SARSIM1

Description

Creates a range-azimuth simulated SAR image with layover and shadow masks, based on an elevation image.

Parameters

Name	Type	Length	Value range
InputElevation: Input elevation channel *	Raster port	1 - 1
Output: Output simulated radar channel	Raster port	0 - 1
OutBitLayover: Output layover bitmap channel	Bitmap port	0 - 1
OutBitShadow: Output shadow bitmap channel	Bitmap port	0 - 1
Elevation	Float	0 - 2	Default: 1,0
Background Elevation Value	Float	0 - 1
Spacing *	Float	2 - 2
Altitude Above Sea Level *	Float	1 - 1	0 -
Head Angle	Float	0 - 1	0 - 360 Default: 0
Minimum Look Angle *	Float	1 - 1	0 - 90
Near Range Point on DEM	Float	0 - 2
Oversampling Factor	Integer	0 - 1	3 - 15 Default: 5
FLIP	String	0 - 1	ON | OFF Default: OFF
Working Memory Size	Integer	0 - 1	1 -
OutModel: SAR modeling text segment	Text port	0 - 1

* Required parameter

Parameter descriptions

InputElevation: Input elevation channel

Specifies the image channel from the input file (FILI) that contains the elevation data from which the simulated image data is created.

The elevation data must be georeferenced in one of the conformal projections listed in the Details section.

Output: Output simulated radar channel

Specifies the 16-bit unsigned output image channel to receive the simulated radar image.

The specified output file must not already exist.

OutBitLayover: Output layover bitmap channel

Specifies the output channel to receive the overlay bitmap mask.

OutBitShadow: Output shadow bitmap channel

Specifies the output channel to receive the shadow bitmap mask.

Elevation

Optionally specifies the scale and offset to apply to the elevation values in the input elevation channel (DBEC), to convert values to metres. If this parameter is not specified, scale defaults to 1, and offset defaults to 0.

The equation for the conversion is:

elevation in metres = scale * (value + offset)

For example, to convert elevation values from feet to meters, where a value of 0 represents 1000 feet:

0.3048,1000        | metres = 0.3048 * (value + 1000)

Background Elevation Value

Optinally specifies the background elevation value, or NoData value, in the input elevation layer. Pixels with this value are ignored during processing. If this parameter is not specified, all elevation pixels are included in processing.

BACKELEV is typically used when the data in the elevation channel is incomplete. For example, if the elevation data is only in an area of interest, or if the edges of the DEM are clipped, the pixels with no known elevation value are filled with an arbitrary value (such as -9999). In such a case, set BACKELEV to that value to ensure that those pixels are excluded.

Spacing

Specifies the output pixel spacing, in meters, for simulated SAR imagery.

Specify pixel spacing for both the range (off track) and azimuth (along track) directions.

Altitude Above Sea Level

Specifies the platform altitude, in meters, above sea level for the simulated SAR image.

Head Angle

Specifies the heading angle, in degrees, clockwise from due north for the simulated SAR image.

The Heading Angle is between 0.0 and 360.0 degrees.

If the simulated SAR image will be used for geocoding an actual SAR image (using SARGEO), set this parameter to the APPROXIMATE heading angle for the actual SAR image. An exact value is not necessary, because SARGEO determines the actual heading using the ground control points that tie the simulated SAR image to the actual SAR image.

Minimum Look Angle

Specifies the minimum range look angle, in degrees.

Near Range Point on DEM

Specifies the georeference coordinates for the first pixel (near range point) of the simulated SAR image.

This parameter does not necessarily have to specify a point on the input elevation image. By default, this value points to the upper-left corner of the elevation image.

For descending images, this value should be the coordinate of the upper-right coordinate of the DEM. For ascending images, this value should be the coordinate of the lower-left coordinate of the DEM.

Oversampling Factor

Specifies the oversampling factor. Greater oversampling factors lead to a better visual effect, but may slow down the execution of SARSIM1.

FLIP

Optionally specifies whether or not to flip the simulated image.

If ON, the simulated image is flipped about the center horizontal axis. If OFF, the image is not flipped.

This option is used mainly for ascending images. Most ascending images are north-down and south-up. The simulated image using SARSIM1 will also be north-down and south-up. For all RADARSAT products except ScanSAR images, however, the images are flipped north-up and south-down. This may cause problems when collecting ground control points because of opposite orientations. Setting this option to ON will simulate an image with north-up and south-down orientation.

Working Memory Size

Specifies the amount of working memory to use to run this function.

SARSIM1 requires a substantial amount of memory to create the simulated image. In addition, it requires 2 bitmaps of sizes Xsize*Ysize/8 in which to store the output channel and shadow bitmap.

For the simulated image, the default value is 16 MB, but the user may change this value if more or less RAM memory is available on the system. Allow extra memory for the output channel and shadow bitmaps; a typical recommendation is half of the RAM available on the system.

OutModel: SAR modeling text segment

Specifies the name of the text segment created after running this function.

SARSIM1 creates a text segment to store the seven parameters DBIW, ESCALE, BACKELEV, OUTSIZE, ALTI, HEAD, MINLOOK, and NRPOINT. This text segment is stored in the DEM file, and may be used as input by SARGEO.

Details

SARSIM1 creates a range-azimuth simulated SAR radar image and two bitmaps showing layover and shadow areas from an input elevation image.

This function uses the elevation data from a specified input channel (DBEC) under a defined window area (DBIW), plus image geometry information to generate a simulated SAR image. A new output file is created (FILO) to store the simulated image and the shadow and overlay bitmap masks.

Georeferencing data is stored in the georeferencing segment (segment 1) of the input file (FILI).

The user must specify basic satellite parameters, such as the output pixel spacing (OUTPXSZ), the heading angle in degrees clockwise from due north (HEAD), the altitude above sea level in meters (ALTI), the SAR minimum look angle (MINLOOK), and an arbitrary point (NRPOINT) specified in georeference coordinates for the near range point (first pixel) for the simulated image. To achieve the best visual results, the output pixel spacing should be equal to the pixel size for the database.

The oversampling factor (OVERSAMP) is dependent on the relative sampling intervals of the DEM and the simulated image, the viewing geometry, and the steepest slopes in the region of interest. The default oversampling factor value is 5. Greater oversampling factors lead to a better visual effect, but may slow down the execution of the function.

SARSIM1 creates a text segment to store the seven parameters DBIW, ESCALE, BACKELEV, OUTSIZE, ALTI, HEAD, MINLOOK, and NRPOINT. This text segment is stored in the DEM file, and may be used as input by SARGEO.

Algorithm

The algorithm used by SARSIM1 is derived from the GENSIM function, which is part of SABOTS (SAR Analysis Based On Terrain Simulation) developed by Dr. Bert Guindon of CCRS (Canada Centre for Remote Sensing) in Ottawa, Canada.

The algorithm used by SARSIM1 can be found in the paper, "Analytic Formulation of Spaceborne SAR Image Geocoding and "Value-Added" Product Generation Procedure Using Digital Elevatoin Data", by B. Guidon and M. Adair. See the References section for details.

Simulated SAR geocoding consists of a DEM sampled in the desired output projection and a SAR image in a nominal ground range-azimuth projection (that is, projected to ground range from slant range without terrain correction). Along any range line, the relationship between true ground range (x) and image ground range (x') coordinates will be given using the following equation:

x' = x + (ez/ex)*h

This function uses an image model to derive the transformation from three-dimensional DEM coordinates (E, N, h) and two-dimensional SAR image space (Gr, Az), where E, N, and h are the DEM easting, northing, and vertical coordinates, and Gr and Az are the image range and azimuth coordinates.

For simulation, a spherical earth approximation and an orbit of constant heading, velocity, and altitude are assumed. The relative azimuth coordinate can be written as follows:

Az = E*sin(omega) - N*cos(omega)

For a SAR image processed to a nominal ground range-azimuth projection, the nominal ground range coordinate of point B relative to point A (excluding terrain parallax) is given by the following:

Gr = R3*(Alfa-Beta)

If the DEM coordinates of point B are not at the same azimuth as A, the true ground range will be equal to the perpendicular distance from B to the straight line passing through A, which is aligned with the satellite heading direction. The equation of such a line is as follows:

C1*E + C2*N + C3 = 0 

The generation of the simulation requires, in addition to the coordinate transformation, a radiometric response model to compute the backscatter contribution for each DEM sample, and a method to cope with the non-uniform mapping of DEM samples into the SAR image space.

SARSIM1 resolves this problem by oversampling DEM by factor 'f' via bilinear interpolation, and each new sub-sample is mapped to the nearest output coordinate grid point. By increasing the response level of an output grid point by a constant increment each time a DEM sub-sample is mapped to it, the resulting simulated image will automatically incorporate the desired range compression radiometric model.

The minimum required oversampling factor 'f' is dependent on the relative sampling intervals of the input DEM (dSd), the output SAR image, the viewing geometry, and the steepest slopes in the region of interest.

In order that at least one sub-sample is mapped to each output cell:

            dx f >= ( (---) (dSd**2)/SQRT(2*dSd**2) )**-1 dx' max

(dx/dx')max is the scale factor (true ground range to output ground range) for the steepest reverse slope. The presence of layover and shadow may be detected by computing, for each DEM sub-sample, the magnitude of the SAR look angle (shadow), the slant range (layover), and the simulated coordinates (Gr, Az). Monitoring the change in the slant range and the look angle along each range line is sufficient to detect the presence or absence of layover and shadow in each simulated pixel.

References

B. Guindon, M. Adair, 1992. Analytic Formulation of Spaceborne SAR Image Geocoding and "Value-Added" Product Generation Procedure Using Digital Elevation Data, Canadian Journal of Remote Sensing, Vol. 18, pp. 2-11.