satvis [ parameter=value ... ] [ inputfile outputfile ] satvis [ parameter=value ... ] [ inputfile ... directory ]
Parameters are: max_sat_zenith, min_sun_reflect, min_ch4_temp, max_ch2_rad, min_ch1/ch2, max_ch2_delta, phase_function, box_lines, box_samples.
satvis computes the following variables for a given input dataset and stores them in a separate output dataset.
(1) cloud classification
CLEAR 0
SUN_GLINT 1
LOW_CLOUD 2
HIGH_CLOUD 4
PART_CLOUD 8
(2) Radiance ratio ch1/ch2 after Rayleigh effects have been
removed
(3) optical depth using marine, rural, continental or 2-term
Henyey- Greenstein scattering phase functions
All output is byte-scaled; real values are stored as 8-bit byte data as follows:
stored ch1/ch2 ratio = true ratio * 50
stored optical thick = true thick * 50
255 is used as the missing or bad value.
Pixels are classfied by the following scheme:
if ch2 radiance (from avcal) is negative, then BAD, else
if satellite zenith angle is too great, then BAD, else
if solar reflection angle is too small, then SUN GLINT, else
if ch4 brightness temp is too small, then HIGH CLOUD, else
if ch2 radiance is too great, then LOW CLOUD, else
if ch1/ch2 radiance ratio is big enough, then CLEAR, else
if ch2 range about pixel is small enough, then CLEAR,
else PART CLOUD
Thresholds for the above tests are provided by the user via function parameters. Radiance ratio, and optical thickness are computed only for clear pixels.
Input and output to this function can be 1) a list of input datasets followed by a directory or 2) one input dataset, followed by one output dataset. In the first case, output datasets are named the same as the inputs and are placed in the output directory. For example, if input dataset is INDIR/zzz, and the output directory is OUTDIR, the output dataset is OUTDIR/zzz.
This parameter specifies the number of lines (or height) in the cloud screening box.
Valid range is [1,2,3,4,5]. The default is 3.
This parameter specifies the number of samples (or width) in the cloud screening box.
Valid range is [1,2,3,4,5]. The default is 3.
Maximum allowable satellite zenith angle in degrees. Pixels with greater than the maximum satellite zenith angle will not be used in any computations; they will be assigned bad_value (see angles).
pixel satellite normal \ | \ | \ zen | \ | \ | \ | \ | \The default value is 50 degrees.
Minimum allowable solar reflection angle in degrees. Pixels with less than the minimum solar reflection angle will be classified as contaminated by sun glint and will not be used in further computations.
reflected pixel satellite sun normal sun . \ | / . \ | / . ref \ | / . \ | / . \ | / . \ | / . \ | / \/The default value is 50 degrees.
Minimum allowable channel 4 brightness temperature in degrees Celsius. Pixels with ch4 brightness temperature less than this minimum will be classified as covered by high cloud, and will not be used in further computations.
The default value is 0 degrees Celsius.
Maximum allowable channel 2 radiance in % albedo. Pixels with ch2 radiance greater than this maximum will be classified as covered by low cloud, and wil not be used in further computations.
The default value is 3 % albedo.
Minimum ratio of ch1 radiance to ch2 radiance. Pixels with ch1/ch2 ratio greater than this minimum will be classified as clear. Pixels with ch1/ch2 ratio less than this minimum may be classified as clear or partly cloudy, depending on a channel 2 radiance neighborhood test.
The default value is 1.5.
Maximum acceptable difference in channel 2 radiance (% albedo) computed as follows: If X[i], 1 = 1, ..., n is list of channel 2 radiances in a given test box, and X[m] is the ch2 radiance at the box's center, then the maximum difference is given by
n max | X[i] - X[m] | i=1The default value is .25 % albedo.
Scattering phase function to be used in computing optical thickness. There are four choices: marine, rural, continental, and hg2. The first three functions are evaluated from pre-computed lookup tables; the last function is a two-term Henyey-Greenstein function, adjusted for current pixel geometry and radiance values.
The default value is marine.
The following is an example of calculating cloud parameters from an Avhrr image dataset. Note: The dataset must be a calibrated dataset. For more on how to calibrate a dataset, see avcal.
in/out files : char(255) ? n11.93203.2226 n11.satvis max_sat_zenith : real ? [50] min_sun_reflect: real ? [50] min_ch4_temp : real ? [0] max_ch2_rad : real ? [3] max_ch2_delta : real ? [0.25] min_ch1/ch2 : real ? [1.5] phase_function : char( 9) ? [marine] cabo%
avhrr, nitpix, avin, avcal, avfilt3, avfix.
Last Update: $Date: 1998/05/29 20:42:39 $