Comment lines in the text segment are indicated by a "!" character at the beginning of the line. The first line in the text segment must be as follows:
! AVHRR Calibration/Orbital DataWhen AVHRRSEG is not set, AVHRRAD chooses the first text segment in the input file which contains this line.
AVHRRAD does not utilize all of the information in the text segment generated by FIMPORT or MIAVHRR. The information which it does use is dependent upon the type of correction or calibration being performed. Note that the "ANG" angle generation option requires the same text segment information as the "SOL" radiometric correction option.
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 ...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 AVHRRAD. Note that the units 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 indicates the datum code that the GCP is referenced against. This line is optional. If not present, AVHRRAD 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 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 solar zenith angle 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.
SLOPES: 0.1020000 0.1030000 -0.0016797 -0.1581315 -0.1790746 INTERCEPTS: -4.1300001 -4.2100000 1.6692461 157.2673558 178.9532614The SLOPES line contains the slope coefficients for AVHRR channels 1 through 5. All five values must be specified if the SLOPES line is present. These values are typically satellite dependent. The units for these values are:
Slope (channel 1) percent reflectance/count Slope (channel 2) percent reflectance/count Slope (channel 3) milliWatts/m^2 sterad cm^-1/count Slope (channel 4) milliWatts/m^2 sterad cm^-1/count Slope (channel 5) milliWatts/m^2 sterad cm^-1/countThe INTERCEPTS line contains the intercept coefficients for AVHRR channels 1 through 5. All five values must be specified if the INTERCEPTS line is present. These values are typically satellite dependent. The units for these values are:
Intercept (channel 1) percent reflectance Intercept (channel 2) percent reflectance Intercept (channel 3) milliWatts/m^2 sterad cm^-1 Intercept (channel 4) milliWatts/m^2 sterad cm^-1 Intercept (channel 5) milliWatts/m^2 sterad cm^-1The text segment generated when FIMPORT or MIAVHRR is used to read in an AVHRR image may or may not contain a SLOPES and INTERCEPTS line. Some AVHRR formats (such as Level 1b) contain this information while others do not.
If the SLOPES and INTERCEPTS lines are not present in the text segment, then AVHRRAD will use default, pre-launch slope and intercept values for AVHRR visible channels 1 and 2. These pre-launch values are satellite dependent and can be found in the NOAA Polar Orbiter Data Users Guide. AVHRRAD will look for a SATID line in the text segment in order to acquire the satellite name. Please see the section on radiometric correction for a description of the format of the SATID line in the text segment.
AVHRRAD also looks for the SLOPES and INTERCEPTS lines in the text segment as in visible channel calibration. If these lines are not present in the text segment, then AVHRRAD will try to extract additional information from the text segment in order to compute the slope and intercept values for AVHRR thermal channels 3, 4, and 5. The format of this additional information in the text segment is as follows:
PRT(1): 403.000000 PRT(2): 410.000000 PRT(3): 401.000000 PRT(4): 404.000000 BLACKBODY(3): 613.000000 BLACKBODY(4): 320.000000 BLACKBODY(5): 317.000000 SPACE(3): 993.000000 SPACE(4): 993.000000 SPACE(5): 999.000000 AVALUES(1): 277.018 0.05128 0.0 0.0 0.0 AVALUES(2): 276.750 0.05128 0.0 0.0 0.0 AVALUES(3): 276.862 0.05128 0.0 0.0 0.0 AVALUES(4): 276.546 0.05128 0.0 0.0 0.0These lines can appear in any order in the text segment. The PRT, BLACKBODY, and SPACE lines are required. An error will result if they are not found. The AVALUES lines are optional if the satellite is one of: NOAA-9, NOAA-10, NOAA-12, NOAA-14. The reason for this is that AVHRRAD knows the A values for these satellites. For all other satellites, the A values are not known. They must be acquired and then entered into the text segment as AVALUES lines before performing a thermal channel calibration.
The four PRT lines are the 4 platinum resistance temperature counts. Each is a mean count obtained when a platinum resistance thermometer is used to measure the temperature of the internal blackbody target. These counts can be converted directly to temperature using a formula described in NESS 107 (See the AVALUES description below). Four thermometers are used because of the need for redundancy.
The three BLACKBODY lines are the mean counts obtained when the AVHRR sensor is viewing the internal blackbody target. BLACKBODY(3) is the count associated with AVHRR channel 3, BLACKBODY(4) is associated with AVHRR channel 4, and BLACKBODY(5) is associated with AVHRR channel 5.
The three SPACE lines are the mean counts obtained when the AVHRR sensor is viewing space. SPACE(3) is the count associated with AVHRR channel 3. SPACE(4) is associated with AVHRR channel 4, and SPACE(5) is associated with AVHRR channel 5. The presence of SPACE(1) or SPACE(2) lines in the text segment is ignored by AVHRRAD.
The 4 AVALUES lines are used to convert PRT counts to temperature. For example, the first line (AVALUES(1)) contains 5 coefficients which are used to convert the PRT1 count value into a temperature value in units of degrees Kelvin (K).
PRT, BLACKBODY, and SPACE lines are automatically generated by MIAVHRR or FIMPORT into the text segment (for all AVHRR formats). However, the AVALUES lines must be supplied by the user.