avcal [ parameter=value ... ] [ inputfile ... ]
Parameters are radiance_only, temp_units, vis_coefs, line_interval, solar_corr, full_report.
avcal converts AVHRR sensor count values to values with engineering units. For channels 1 and 2 the units are percent albedo. For channels 3, 4, and 5 the units can be either percent albedo or brightness temperature. This process is often referred to as radiometrically calibrating the data. New calibration coefficients are computed for each 100 input lines.
The calibration algorithm is based on information contained in Techniques for Data Extraction and Calibration of TIROS-N/NOAA Series Satellite Radiometers for Direct Readout Users by Levin Lauritson and Gary Nelson (NOAA-NESS Publication #107, July 1979). Briefly, the visible and near infrared data (channels 1 and 2) are converted from raw counts to percent albedo using a linear relationship determined prior to launch. The thermal infrared data (channels 3, 4, and 5) are converted from raw counts to radiances with a linear relationship that is based on the raw count value associated with cold space (roughly 3 degrees Kelvin) and the raw count value associated with the temperature of an onboard target (approximately 300 degrees Kelvin). A slight nonlinearity in channels 4 and 5 is corrected using a quadratic function of radiance. Lastly, the infrared radiances are converted to temperature using the inverse Planck function.
To calibrate visible channels, avcal uses the ASCII data file avcal.coef the location of which is defined by the SATDATA environment variable. This is a time-dependent file, each record of which contains the calibration information on per-satellite basis together with the date label, defining the starting date when this information became valid. See this file for more information.
The output report provides the intercept (intcpt) and slope (slope) for each channel's linear calibration equation. It also provides the space (space) and target (target) counts. Lastly, it provides the temperatures (average) of the four platinum resistance thermistors (prt), which are embedded in the onboard target.
The output data are stored as scaled 16 bit integers to reduce mass storage requirements. Subsequent processing functions convert these compressed values to their true values as needed.
One or more TeraScan datasets are required as inputs. There are no output TeraScan datasets; i.e. calibration is done in place with the results overwriting the input data.
OPTIONAL. This parameter determines whether avcal converts raw AVHRR channel 3, 4, or 5 data to radiance or to temperature.
Valid responses are yes and no. The default is no, which means convert to temperature.
avcal converts the raw AVHRR data to brightness temperatures with units of degrees Celsius, Kelvin, or Fahrenheit. Degrees kelvin or celsius should be used if the data will be used to estimate sea surface temperature using nitpix because nitpix does not currently support input data with units of fahrenheit.
Valid responses are [celsius, kelvin, fahrenheit]. The default is celsius.
OPTIONAL. avcal can overwrite gain and offset for visible channels with the user-supplied four real values of vis_coefs : slope (ch1), intercept (ch1), slope (ch2) and intercept (ch2).
Valid responses are any four real values. Accepting the default values (i.e. pressing RETURN instead of entering the actual values), no matter what they are, means that no overwriting will be done.
OPTIONAL. Defines the number of HRPT frames, after which the calibration coefficients will be updated.
Valid response is an integer value between 10 and 100. The default value is 100.
OPTIONAL. Determines whether or not to perform the solar zenith angle correction.
Valid responses are [yes, no]. The default is no.
If full_report=no, only the calibration report for the first 100 lines is printed. If full_report=yes, a calibration report is printed for every 100 lines of input data.
Valid responses are [yes, no]. The default is no.
Calibrate and report the contents of the TeraScan dataset named big. Notice that units, bad_value and real_scaling values all change after calibration. See bigdataset for information on the original contents of big.
[1] % avcal big
temp_units : char(10) ? [celsius]
full_report : char( 3) ? [no]
line 1 ch slope intcpt space target prt average
1 0.10634 -3.85 38.3 1 419.00
2 0.10749 -3.88 39.6 2 427.00
3 -0.00144 1.43 994.1 602.5 3 414.00
4 -0.17249 167.04 988.0 334.2 4 423.00
5 -0.19988 196.32 993.8 360.8
[2] % contents big
printout : char( 3) ? [no]
Dimensions:
1 1199 line
2 1410 sample
3 103
Global Attributes:
history byte
channels long 1 2 3 4 5
projection long std_projection sensor_scan
et_affine double 1 0 0 1 -67 -367
satellite byte noaa-9
sensor long std_sensor avhrr
pass_date long std_date 88/09/13
start_time double std_time 21:18:22.6
time_adjust double std_time 00:00:00
attitude double degrees 0 0 0
sensor_tilt double degrees 0
sensorient double degrees 0 45 0 55.3 -55.3
scan_samples long 2048
scan_rates double 6 40000 37.6991
orb_elem_date long std_date 88/09/13
orb_elements double 0.476962 0.828252 4e-05 0.001585
0.0363905 0.669068 0.275306
center_lat double std_latitude
center_lon double std_longitude
map_angle double degrees
Variable Attributes:
bad_value double
real_scaling double
Variables:
hrpt_header short
1 1199 line
2 103
avhrr_ch1 short albedo*100%
1 1199 line
2 1410 sample
bad_value : -32768
real_scaling : 0 100
avhrr_ch2 short albedo*100%
1 1199 line
2 1410 sample
bad_value : -32768
real_scaling : 0 100
avhrr_ch3 short temp_deg_c
1 1199 line
2 1410 sample
bad_value : -32768
real_scaling : 0 100
avhrr_ch4 short temp_deg_c
1 1199 line
2 1410 sample
bad_value : -32768
real_scaling : 0 100
avhrr_ch5 short temp_deg_c
1 1199 line
2 1410 sample
bad_value : -32768
real_scaling : 0 100
avhrr, nitpix, avin, avfix, fixline, avfilt, nitpix.
The calibration algorithm is based on information contained in Techniques for Data Extraction and Calibration of TIROS-N/NOAA Series Satellite Radiometers for Direct Readout Users by Levin Lauritson and Gary Nelson (NOAA-NESS Publication #107, July 1979).