The Input SAR data to be registered can be either a slant range image or a ground range image (RANGETYP). If the input image is ground range, then the assumed aircraft height above the ground (HEIGHT) must be specified. This height must be the same value that was assumed when the input ground range image was produced.
If there are no elevation data (DBEC is defaulted), then OALTI, OHEAD, OPOINT, and OCOEFF output parameters are calculated based on the geometry of the flight path alone.
Prior to running FLIGHT, the user must collect at least ORDER+2 ground control points, which map points on the uncorrected SAR image to points on the master file containing the elevation data. The GCP segment must reside on the uncorrected file (FILE) and must be in either PIXEL or UTM coordinates. The best method of collecting the GCPs is to use the GCIT program in the Geometric Correction package, which maps points on the input image to user specified UTM coordinates. The user should obtain a topographical map of the area of interest and select at least 4 to 6 good ground control points.
The user must specify initial estimates of flight path parameters for aircraft altitude above sea level in metres (ALTI), aircraft heading in degrees clockwise from due north (HEAD), and an arbitrary point on the flight path (POINT) specified in UTM coordinates (easting and northing). Given these estimates, the program calculates more accurate values for these flight path parameters and saves them in output parameters (OALTI, OHEAD and OPOINT) which are the input to STG. Accurate initial estimates for ALTI, HEAD and POINT decrease the execution time of FLIGHT.
FLIGHT also calculates the polynomial coefficients (OCOEFF) of a specified order (ORDER), which are used to determine the line numbers on the input SAR image, given their distance along the flight path in metres from the coordinates calculated for OPOINT. OCOEFF is also required by STG.
FLIGHT assumes that elevation data has already been registered to an UTM grid (See PACE Geometric Correction Package), and that the UTM coordinates of the rectangle bounding the area covered by the elevation data have been defined with the GEOSET program. In addition, the scale and offset factors (ESCALE) for elevation data are also required in order to convert the elevation values to metres above sea level.
The input pixel spacing in metres (INPXSZ), and the time delay in microseconds (DELAY) for the input SAR image, are another requisite.
The user may specify an error tolerance (MAPTOL) in metres, which is used to determine when the estimates for the output flight path parameters have reached a desired level of accuracy. If MAPTOL is not specified, it defaults to the horizontal pixel size of elevation data as derived from FILM's georeferencing segment (i.e. ABS(UPLEFT(1) - LORIGHT(1)) / DBSZ(1)).
If the actual residual error is greater than the map tolerance error (MAPTOL), a warning message is printed in the report, and the user should run FLIGHT again, setting ALTI = OALTI, HEAD = OHEAD, and POINT = OPOINT. If, after several runs, the residual error does not change and is still greater than the map tolerance error, then try selecting more or better ground control points in the DBGC segment, and run FLIGHT again. However, you may decide that the residual error is good enough and proceed with running STG with the present output flight path parameters.
FLIGHT generates a report (REPORT) on the flight path parameters and ground control points used in estimating the output parameters. Report can be turned off (REPORT="OFF") if desired.
FLIGHT assumes that the radar image is right-looking (range increases from left to right), and that the look direction is parallel to the lines of the image. If the look direction is perpendicular to the lines of the image (from top to bottom), or at some arbitrary angle to the image coordinate system, then the image must be rotated prior to processing (using Geometric Correction Package). If the radar image is left-looking, then HEAD must be set to 180 degrees opposite of actual aircraft heading in order to simulate a right-looking image.