AVHRRAD

Description

Performs either a radiometric correction (also known as a solar zenith angle correction), visible channel calibration, or thermal channel calibration of AVHRR data. It can execute a single function or perform all three functions consecutively. AVHRRAD can also generate satellite zenith angles, solar zenith angles, and relative azimuth angles (instead of performing corrections or calibrations) when the input image is either 2,048 or 409 pixels wide.

Parameters

Name	Type	Caption	Length	Value range
FILI *	String	Input file name	1 - 192
FILO *	String	Output file name	1 - 192
DBIC	Integer	Input raster channel(s)	0 -
DBOC *	Integer	Output raster channel(s)	1 -
AVHRRSEG	Integer	AVHRR calibration/orbit text segment	0 - 1
TIMEMULT	Integer	Search time interval multiplier	0 - 1	Default: 1
CTYPE *	String	Correction/calibration type	3 - 3	SOL | VIS | THE | ALL | ANG Default: SOL
REPORT	String	Report mode	0 - 192	Quick links
MONITOR	String	Monitor mode	0 - 3	ON, OFF Default: ON

* Required parameter

Parameter descriptions

FILI

Specifies the name of the input file that contains the raw AVHRR image data and the AVHRRSEG text segment. The input file must be a PCIDSK file (created with FIMPORT) or an actual AVHRR disk file (for solar zenith angle correction, FILI can be a Level 1b AVHRR disk file but not any other AVHRR disk file format).

If FILI and FILO are the same, FILI must be a PCIDSK file; it cannot be an AVHRR disk file.

FILO

Specifies the name of the PCIDSK file to receive the corrected (or calibrated) image data. FILO must exist and be specified: AVHRRAD does not create a new output file.

FILO can be the same as FILI as long as the file type is PCIDSK.

DBIC

Specifies the image channels in the input file (FILI) to be corrected or calibrated.

If the correlation and calibration type (CTYPE) option is "ALL", this parameter must specify five input channels. The first two input channels must be the visible channels; the other three channels must be the thermal channels.

If the correlation and calibration type (CTYPE) option is "ANG", this parameter is ignored.

Ranges of channels or segments can be specified with negative values. For example, {1,-4,10} is internally expanded to {1,2,3,4,10}. When you are not specifying a range in this way, only 48 numbers can be specified explicitly.

DBOC

Specifies the channels in the output file (FILO) to receive the corrected or calibrated image data.

For each input channel specified (DBIC), an output channel must be specified in this parameter (unless the "ANG" CTYPE option is selected). Duplicate output channels are not allowed.

Ranges of channels or segments can be specified with negative values. For example, {1,-4,10} is internally expanded to {1,2,3,4,10}. When you are not specifying a range in this way, only 48 numbers can be specified explicitly.

AVHRRSEG

Specifies the text segment that contains AVHRR calibration and orbital data. This segment is automatically created when FIMPORT reads an AVHRR image.

If this parameter is not set, AVHRRAD searches for a valid AVHRR text segment in the input file.

Alternatively, you may create a new text segment or edit an existing one to use with AVHRRAD; see the Details section for more information.

TIMEMULT

This parameter applies to radiometric correction only. It specifies a multiplication factor that can increase the search-time interval in which AVHRRAD looks for the exact starting scan date of the input image. The starting scan date refers to the precise instant just prior to when the top-left pixel of the input image was scanned. By default, the multiplier is set to 1, indicating that a default search interval of 60 minutes should be used. If set to 2, a default search interval of 120 minutes would be used.

AVHRRAD uses date and time information in the text segment of the input PCIDSK file to approximate the exact starting scan date. The format of the date information in the text segment is described in the Details section.

CTYPE

Specifies the correction/calibration type.

REPORT

Specifies where to direct the generated report.

Available options are:

• TERM: generates a report on the terminal (default)
• DISK: generates a report on file "IMPRPT.LST"
• OFF: turns off report generation
• <filename>: appends a report to the specified file

MONITOR

The program progress can be monitored by printing the percentage of processing completed. A system parameter, MONITOR, controls this activity.

Available options are:

• ON: Turns the monitor ON (default)
• OFF: Turns the monitor OFF (recommended if running in batch or background mode)

Details

AVHRRAD performs either a radiometric correction (also known as a solar zenith angle correction), visible channel calibration, or thermal channel calibration of AVHRR data. It can execute a single function or perform all three functions consecutively.

For radiometric correction, one GCP, an approximate starting scan date and time, and a set of orbital element values are required for input. Radiometric calibration requires only the values of the AVHRR sensor calibration for input.

For radiometric correction, the input image should be either 2,048 pixels (HRPT and LAC data) or 409 pixels (GAC data) wide.

Alternatively, AVHRRAD can generate satellite zenith angles, solar zenith angles, and relative azimuth angles (instead of performing any corrections or calibrations) when the input image is either 2,048 or 409 pixels wide.

Radiometric correction (solar zenith angle correction)

The solar zenith angle along a scanline can vary significantly for AVHRR data due to the large AVHRR sensor scan angle. This difference in solar angle value along a scanline means that the corresponding ground objects represented by the AVHRR image are receiving unequal amounts of solar radiation. To account for this effect, a radiometric or solar zenith angle correction can be applied to the visible channels (1 and 2).

To perform a solar zenith angle correction, AVHRRAD must set transformation equations between the input image pixel coordinates and a latitude-longitude coordinate system. This requires information in the text segment, such as the satellite name (which is used to select the proper TLE orbital element data file), GCP values, and an approximate starting scan date and time of the input image. For a more complete description of the process for setting up these transformation equations, see the AVHRCOR documentation.

AVHRRAD performs a solar zenith angle correction by computing the value of each output pixel as follows:

PixelOut = PixelIn / cos(z)

where z is the solar zenith angle corresponding to the center of the input pixel. The solar zenith angle, z, is a function of (Pixel,Line) coordinates in the input image. The output pixel is still a digital count value (like the input pixel); hence, the output of a solar zenith angle correction can be supplied as input for a subsequent visible channel calibration.

Pixels in the input image that have a solar zenith angle greater than 85 degrees are considered invalid and are not corrected. A report, given at the end of the correction process, indicates how many pixels are invalid and the bounding box that encompasses the invalid pixels.

Visible channel calibration

AVHRR visible channels 1 and 2 are calibrated using the procedure described in the NOAA Polar Orbiter Data User's Guide. Each generated or calibrated pixel value is computed as follows:

PO = S(c) * PI + I(c)

AVHRRAD looks for the channel slope and intercept coefficients in the AVHRRSEG text segment. If no such coefficients are found, the default pre-launch values are used.

Any solar zenith angle correction should be performed before running a visible channel calibration, because the output of a solar zenith angle correction is still a digital count value and can be supplied as input for subsequent visible channel calibrations.

Thermal channel calibration

AVHRR thermal channels 3, 4, and 5 are calibrated using the procedure described in the NOAA Polar Orbiter Data User's Guide. Each generated or calibrated pixel value is computed as follows:

PO = PlanckEqn(E)

E  = S(c) * PI + I(c)

The PlanckEqn looks like this:

T = C2 * v / ln(1 + C1 * v^3 / E)

The constants C1 and C2 are defined as follows:

C1 = 1.1910659e-5  milliWatts/(m^2 sterad cm^-4)
C2 = 1.438833 cm K

After applying Planck's Equation, AVHRRAD does not perform any non-linearity corrections of the computed surface brightness temperatures. Any non-linearity corrections can be manually applied later.

AVHRRAD searches for channel slope and intercept coefficients from the AVHRRSEG text segment. If these values are absent, AVHRRAD extracts further information from the text segment to compute the slope and intercept coefficients. This extra information includes PRT counts, BLACKBODY counts, SPACE counts, and optional "A" coefficients for converting PRT counts to temperatures. The procedure for computing the slope and intercept coefficients is described in NESS 107, section 5.1.1.

All corrections and calibrations

When the "ALL" correlation or calibration type option is selected, all corrections and calibrations are performed; specifically, a solar zenith angle correction is applied to the first two input channels, followed by a visible channel calibration of these same channels. Finally, the last three input channels receive a thermal channel calibration.

Angle generation

When the "ANG" correlation or calibration option is selected, the satellite zenith angle, solar zenith angle, and relative azimuth angle are computed for each input pixel and written to the output channels. The angle unit values are in degrees. Scaling is performed only if the output is a 16-bit channel. In this instance, the angle values are multiplied by 100; if you have 16-bit output channels, the pixel values must be divided by 100 to recover the true angle value in degrees.

The satellite zenith angle is the angle between the zenith line (pointing straight up) and the direction to the satellite. The satellite zenith angle is close to zero degrees for pixels near the center of the raw image. It increases to more than 68 degrees for pixels near the left or right ends of the image. The angle values range from zero to approximately 68 degrees.

The solar zenith angle is the angle between the zenith line and the direction to the sun. Values can range from zero to 180 degrees; however, for the purposes of solar zenith angle correction, values greater than 85 degrees are invalid.

The relative azimuth angle is defined as the absolute difference between the satellite azimuth angle and the solar azimuth angle and ranges between zero and 180 degrees. The relative azimuth angle is discontinuous at the center of the input image because the satellite azimuth angle is undefined exactly at the image center (the satellite zenith angle is zero degrees).

CALCULATIONS

The following sections describe the equations and calculations used to compute the three angle quantities.

Satellite Zenith Angle

Given:

x	=	image pixel coordinate (ranges from 0.0 to 2048.0)
SatAltitude	=	Satellite altitude (fixed at 833.3 km)
EarthRadius	=	Earth equatorial radius (fixed at 6378.135 km)

1. Compute the satellite scan or view angle for the given pixel coordinate

                   (1024.0 - x)
   SatScanAngle = ------------ * 55.3846 degrees
                   1024.0
   

2. Compute the satellite zenith angle using the sine law.

   	sin(SatZenAngle)			sin(SatScanAngle)
   -------------------------  =  -----------------
   SatAltitude + EarthRadius        EarthRadius
   

   Values of SatZenAngle range from 0 to +68 degrees, regardless of which side of nadir the pixel is on.

Greenwich Mean Sidereal Time

Preliminaries: Modified Julian Dates is used instead of Julian Dates because MJDs turn over at midnight instead of at noon, and are easier to work with. Refer to the Astronomical Almanac for details.

Given:

MJD	=	Modified Julian Date
MJD2000	=	51544.5	=	Modified Julian Date of epoch J2000.0, which is defined as January 1, 2000, 12h UT.
SecPerDay	=	1440.0
SolarSiderealDayRatio	=	1.00273790934

1. Compute the integral and fractional MJD

   IntMJD	=	floor(MJD)
   FracMJD	=	MJD - IntMJD
   

2. Compute Tu, which is the interval of time, measured in Julian centuries of 36525 days of universal time (mean solar days), elapsed since January 1, 2000, 12h UT.

   The Ast. Almanac gives the following equation for Tu:

   Tu = (JD - 2451545.0) / 36525
   

   We will modify this equation to use a Modified Julian Date instead. The value of Tu is not affected because there is a constant offset between JD and MJD values - and MJD is also referenced against the January 1, 2000, 12h UT epoch.

   Tu = (IntMJD - MJD2000) / 36525.0;
   

3. Compute the GMST value at 0h UT. This value is in seconds.

   GMST	=	24110.54841 + 8640184.812866 * Tu + 0.093104 * Tu * Tu +
   		(-6.2e-6) * Tu * Tu * Tu;
   

4. Add the appropriate mean sidereal time interval to GMST using the fractional day value.

   GMST	=	GMST + FracMJD * SecPerDay * SolarSiderealDayRatio
   

5. Reduce the GMST value to between 0 and 86400 seconds. Then convert it to an angular measure (degrees).

   GMST	=	GMST / SecPerDay * 360.0 degrees
   

Solar Hour Angle and Declination

Given:

MJD	=	Modified Julian Date
GMST	=	Greenwich Mean Sidereal Time that corresponds to MJD
Lon	=	Longitude of some point on Earth's surface (degrees East)
Lat	=	Latitude of some point on the Earth's surface (degrees North)
MJD2000	=	51544.5	=	Modified Julian Date of epoch J2000.0, which is defined as January 1, 2000, 12h UT.

1. Compute the number of days from J2000.0.

   Days = MJD - MJD2000
   

2. Compute the mean longitude and the mean anomaly of the sun. The formulae can be found on page C24 of the 1988 Ast Almanac.

   MeanLongitude	=	280.460 + 0.9856474 * days
   MeanAnomaly	=	357.528 + 0.9856003 * days
   

3. Compute the ecliptic longitude of the sun. See page C24 of the Ast. Almanac.

   EclipticLongitude	=	MeanLongitude + 1.915 * sin(MeanAnomaly) + 0.020 * sin(2.0 * MeanAnomaly);
   

4. Compute the obliquity of the ecliptic in radians. See page C24 of the Ast. Almanac.

   ObliquityOfEcliptic	=	23.439 - 0.0000004 * days
   

5. Compute the right ascension of the sun. See page C24 of the Ast. Almanac. Ensure that the right ascension and ecliptic longitude values are in the same hemisphere.

   tan(SolarRightAscension)	=	cos(ObliquityOfEcliptic) * tan(EclipticLongitude)
   

6. Compute the declination of the sun.

   sin(SolarDeclination)	=	sin(ObliquityOfEcliptic) * sin(EclipticLongitude)
   

7. Compute the local (apparent) sidereal time (as an angle). Use GMST as a close approximation to GST (Greenwich Apparent Sidereal Time).

   LST	=	GST + Lon
   

8. Compute the hour angle of the sun. A negative hour angle indicates the sun is east of the observer's location.

   SolarHourAngle	=	LST - SolarRightAscension
   

Solar Zenith Angle

Given:

Lat	=	North latitude of a point on the Earth's surface
SolarHourAngle already computed
SolarDeclination already computed

1. Compute the solar zenith angle using this formula from [R1].

   cos(SolarZenithAngle)	=	sin(SolarDeclination) * sin(Lat) + cos(SolarDeclination) * cos(Lat) * cos(SolarHourAngle)
   

Relative Azimuth Angle

Given:

Lon	=	East longitude of a point on the Earth's surface
Lat	=	North latitude of a point on the Earth's surface
SatLon	=	East longitude of the satellite nadir.
SatLat	=	North latitude of the satellite nadir.

SatScanAngle	already computed
SolarHourAngle	already computed
SolarDeclination	already computed
SolarZenithAngle	already computed
SatelliteZenithAngle	already computed

1. Compute the solar azimuth angle using equation 2.7 from [R1].

   sin(SolarDeclination)	=	cos(SolarZenithAngle) * sin(Lat) + sin(SolarZenithAngle) * cos(Lat) * cos(SolarAzimuthAngle)
   If SolarHourAngle < 0.0 then
       SolarAzimuthAngle	=	-1 * SolarAzimuthAngle
   EndIf
   

2. Compute the Earth angle, which is the angle from the point on the Earth's surface - to the center of the Earth - to the satellite.

   EarthAngle	=	SatelliteZenithAngle - SatScanAngle
   

3. Compute the satellite hour angle and declination.

   SatelliteHourAngle	=	Lon - SatLon
   SatelliteDeclination	=	SatLat
   

4. Compute the satellite azimuth angle using equation 2.7 of [R1].

   sin(SatelliteDeclination)	=	cos(EarthAngle) * sin(Lat) + sin(EarthAngle) * cos(Lat) * cos(SatelliteAzimuthAngle)
   If SatelliteHourAngle < 0.0 then
       SatelliteAzimuthAngle	=	-1 * SatelliteAzimuthAngle
   EndIf
   

5. Compute the relative azimuth angle.

   RelativeAzimuthAngle	= fabs(SolarAzimuthAngle - SatelliteAzimuthAngle)
   If ( RelativeAzimuthAngle > 180 degrees ) then
       RelativeAzimuthAngle = 360 degrees - RelativeAzimuthAngle
   EndIf
   

References for Angle Computations

[R1] Green, Robin M., "Spherical Astronomy". Cambridge University Press, 1985.

[R2] 1988 Astronomical Almanac.

Definitions

TLE data file

For radiometric correction, AVHRRAD requires Two-Line Element (TLE) data files. A TLE file is a text file that contains information about a specific satellite's orbital parameters. Each pair of lines in a TLE file describes a satellite's orbital position in space on a specific date. The $PCIHOME/etc folder contains TLE files for various NOAA satellites; there is one TLE file for each satellite. A TLE file has a .2le file name extension.

If you are working with very recent AVHRR data, you may need to obtain more up-to-date TLE files, which are publicly available and may be acquired from the Internet. When you download a TLE file, be sure to rename it properly for example, noaa-12.2le) and save it to the current working folder. Files obtained from various Internet sites can be edited; for example, some of the files contain TLE entries for more than one satellite and distinguish between satellites by preceding each TLE entry with the satellite name. To construct a proper TLE file that AVHRRAD can use, you must extract the relevant TLE entries from the file but omit the satellite name portion. TLE files saved to the current working directory override any in the $PCIHOME/etc folder.

For best results, ensure that the TLE file contains an entry with a date that is close to the scan date of the AVHRR image that is radiometrically corrected. The dates of TLE entries are typically separated by one or two days.

Text segment format

AVHRRAD requires a text segment input that is automatically generated when FIMPORT imports a raw AVHRR image. You can create your own text segment file for AVHRRAD using a text editor. The text segment can be read using TEXREAD.

You can also edit an existing text segment by using TEXWRIT to write out the text segment contents to a text file, editing the text file, and reading it back into the text segment with TEXREAD.

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 Data

When AVHRRSEG is not set, AVHRRAD uses the first text segment in the input file that contains this line.

AVHRRAD does not use all of the information in the text segment generated by FIMPORT; the information that it uses depends on the type of correction or calibration being performed. The "ANG" angle generation option uses the same text segment information as the "SOL" radiometric correction option.

Radiometric Correction (Solar Zenith Angle Correction)

For radiometric (solar zenith angle) correction, AVHRRAD searches for text with the following format in the text segment:

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 ...

Visible Channel Calibration

For visible channel calibration, AVHRRAD looks for text with the following format in the text segment:

SLOPES: 0.1020000 0.1030000 -0.0016797 -0.1581315 -0.1790746
INTERCEPTS: -4.1300001 -4.2100000 1.6692461 157.2673558 178.9532614

The 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 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 text segment generated when FIMPORT reads an AVHRR image might 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, AVHRRAD uses 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 User's Guide. AVHRRAD searches for a SATID line in the text segment for the satellite name. For a description of the format of the SATID line in the text segment, see the Radiometric Correction section.

Thermal Channel Calibration

To calibrate a thermal channel, AVHRRAD searches for a SATID line in the text segment and acquires the satellite name. For a description of the format of the SATID line in the text segment, see the section on Radiometric Correction.

AVHRRAD also looks for the SLOPES and INTERCEPTS lines in the text segment in the visible channel calibration. If these lines are absent in the text segment, AVHRRAD extracts any additional information from the text segment 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 0.0

These lines can appear in any order in the text segment. The PRT, BLACKBODY, and SPACE lines are required; an error results if they are not found. The AVALUES lines are optional if the satellite is one of the following: NOAA-9, NOAA-10, NOAA-12, NOAA-14. AVHRRAD knows the A values for these satellites. For all other satellites, the A values are unknown; they must be acquired and entered into the text segment as AVALUES lines before performing a thermal channel calibration.

The four PRT lines are the four 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 by 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 views the internal blackbody target. BLACKBODY(3) is the count that is 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 views 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 four AVALUES lines convert PRT counts to temperature. For example, the first line (AVALUES(1)) contains five coefficients that 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 FIMPORT into the text segment for all AVHRR formats; however, you must supply the AVALUES lines.

Datum codes

The following shows some examples of valid datum code numbers:

D = -1     NAD27 (USA, NADCON)
D = -2     NAD83 (USA, NADCON)
D = -3     NAD27 (Canada, NTv1)
D = -4     NAD83 (Canada, NTv1)
D = 800    Normal Sphere
D = 0      WGS 1984
D = 1      WGS 1972

For a complete list of datum codes, refer to 'Projections and earth models' in the Technical Reference section of the CATALYST Professional Online Help.

Examples

Perform a radiometric (solar zenith angle) correction of a two-channel input database, and generate the results to the same two channels in the same file. The AVHRR calibration and orbital data is stored in segment 3. A report is printed on the screen.

EASI>fili	=	'avhrr.pix'
EASI>filo	=	'avhrr.pix'
EASI>dbic	=	1,2	! input channels
EASI>dboc	=	1,2	! overwrite input channels
EASI>avhrrseg	=	3	! AVHRR text segment
EASI>timemult	=		! time intervals 60 minutes; default 1
EASI>ctype	=	'SOL'	! perform solar zenith correction

EASI>RUN AVHRRAD

Perform a visible channel calibration of channels 1 and 2. No report is generated. AVHRRAD searches for a valid AVHRR text segment.

EASI>fili	=	'avhrr.pix'
EASI>filo	=	'avhrr.pix'
EASI>dbic	=	1,2	! input channels
EASI>dboc	=	1,2	! overwrite input channels
EASI>avhrrseg	=		! search automatically
EASI>timemult	=		! time intervals 60 minutes; default 1
EASI>ctype	=	'VIS'	! perform visible channel calibration

EASI>RUN AVHRRAD

Perform a thermal channel calibration of channels 3, 4, and 5.

EASI>fili	=	'avhrr.pix'
EASI>filo	=	'avhrr.pix'
EASI>dbic	=	3,4,5	! input channels
EASI>dboc	=	3,4,5	! overwrite input channels
EASI>avhrrseg	=	3
EASI>timemult	=	
EASI>ctype	=	'THE'	! perform thermal channel calibration

EASI>RUN AVHRRAD

Perform all corrections and calibrations to the input PCIDSK file. The results are written back to the same file.

EASI>fili	=	'avhrr.pix'
EASI>filo	=	'avhrr.pix'
EASI>dbic	=	1,2,3,4,5	! input channels
EASI>dboc	=	1,2,3,4,5	! overwrite input channels
EASI>avhrrseg	=		! search automatically
EASI>timemult	=	
EASI>ctype	=	'ALL'	! perform all three corrections and calibrations

EASI>RUN AVHRRAD

Generate the satellite zenith angle, solar zenith angle, and relative azimuth angle values into the output PCIDSK file.

EASI>fili	=	'avhrr.pix'
EASI>filo	=	'angles.pix'
EASI>dbic	=		! does not need to be set
EASI>dboc	=	1,2,3	! output channels
EASI>avhrrseg	=		! search automatically
EASI>timemult	=	
EASI>ctype	=	'ANG'	! generate satellite zenith, solar zenith
			! and relative azimuth angle

EASI>RUN AVHRRAD

References

Kidwell, Katherine B., ed., NOAA Polar Orbiter Data User's Guide, June 1995, (also available online at http://www2.ncdc.noaa.gov/POD).

NOAA Technical Memorandum, 1979 (revised 1988), "Data Extraction and Calibration of Tiros-N/NOAA Radiometers", ed. Walter G. Planet, (NESS 107 - Rev. 1, U.S. Department of Commerce).

Di, Liping and Donald C. Rundquist. "A One-Step Algorithm for Correction and Calibration of AVHRR Level 1b Data", PE & RS, 60, no. 2 (February 1994): 165-171.

Green, Robin M., "Spherical Astronomy". Cambridge University Press, 1985.