SATID: NOAA-12 YEAR: 1994 DAY: 203.599031 GCP: LONG = -96.117188 LAT = 49.062500 GCP: X = 1024.5 Y = 0.5 GCP: D = 0 TLELINE: ... TLE Line 1 ... TLELINE: ... TLE Line 2 ...Except for the TLE lines, the spacing of the blanks is not important. But, if any of the required information is missing in the text segment, AVHRCOR will not be able to perform the geometric correction.
It is possible to create your own text segment with this format for use with AVHRCOR. Use a system text editor to create a text file with this format. Then use TEXREAD to read the text file into a text segment.
The SATID specifies the name of the satellite which captured the image. This information is used to select the proper TLE file (Two Line Element File) for satellite orbit predictions.
The YEAR specifies the year in which the image was captured.
The DAY specifies the approximate starting scan date (or time) of the image in GMT or UTC. This value does not have to be the exact starting scan date. But it does have to be within 60 minutes of the exact date. 60 minutes is equivalent to 0.042 days. The DAY value indicates the day of the year. It consists of an integral portion which ranges from 1 to 365 (or 366 for leap years) and a fractional portion which indicates the time of day. DAY 1 refers to January 1 whereas DAY 203 refers to July 22 in a non-leap year or July 21 in a leap year. Note that the search interval (60 minutes) can be increased through the use of the TIMEMULT parameter.
The first two GCP lines specify a single GCP to be used with AVHRCOR. Note that the units for the LAT/LONG component of the GCP are decimal longitude and latitude. For example, LONG = 80.0 is the same as 80 degrees East longitude. Similarly, LAT = -30.0 is the same as 30 degrees South latitude. The GCP X value must be a number between 0.0 and the number of pixels in the input image. The GCP Y value must be a number between 0.0 and the number of lines in the input image. The use of non-integer GCP X/Y values allows the user to specify a GCP for any part of a pixel. As an example, suppose we have a 200 pixel by 100 line image. The exact center of the top-left pixel in this image would have coordinates of X = 0.5 and Y = 0.5. The exact center of the bottom-right pixel would have coordinates of X = 199.5 and Y = 99.5. The exact center of the whole image would have coordinates of X = 100.0 and Y = 50.0.
The third GCP line specifies the datum code that the GCP is referenced against. This line is optional. If not present, AVHRCOR will use a default value of D = 1 (indicating datum code D001 or WGS 72). WGS 72 is used as the default because Level 1b AVHRR data from NOAA/SAA contains GCPs which are in the WGS 72 system. See the DATUM CODES heading for a partial listing of valid datum code numbers.
The GCP lines are optional. If they are not present in the text segment, no refinement of the starting scan date of the image will be performed. The (approximate) image date specified by the YEAR and DAY lines will be taken to be the precise image date. In this case, the TIMEMULT parameter will have no effect. The results of the geometric correction may be less accurate in this situation, depending on the accuracy of the approximate image date.
The two TLE lines are optional. If present, they will override the default behaviour which is to look up TLE entries in a TLE file. Please see the section, TLE DATA FILE, for further information.