ISCCP Calibration Coefficients

Calibration Coefficient Tables

The calibration adjustment coefficients found in these tables are meant to be applied either to scaled visible radiances (values from zero to one) or to brightness temperatures (IR in Kelvins):

for visible scaled radiances,
	L =  S * Ln + I
for infrared brightness temperatures,
	T =  S * Tn + I

where L and Ln are the corrected and Nominal scaled visible radiances, T and Tn are the corrected and Nominal brightness temperatures, and S and I are the respective slope and intercept values given in the calibration tables.

The values of Ln and Tn are determined from the original telemetry count values using the Nominal calibration supplied by the operator for each satellite radiometer. The ISCCP version of these data (the B3 dataset) represents the values of Ln and Tn as standard 8-bit count representing integers from 0 to 254 with the value 255 reserved to indicate "no data". Although the Nominal calibrations used by ISCCP are obtained from the satellite operators, there may have been changes made later. The ABSOLUTE calibrations given here are valid ONLY when used with the assumed Nominal calibration and spectral response functions, both of which are given in the Radiance Calibration Report. If a different Nominal calibration is assumed, then these coefficients need to be modified to account for this difference.

Several sets of S and I values are given for each satellite. For the geostationary satellites (GMS, GOES, METEOSAT, INSAT), the first set of coefficients gives the normalization to the current afternoon polar orbiter (see list of dates below indicating the reference polar orbiter) and the second set gives the final absolute calibration that accounts for the normalization to the reference polar orbiter and for any adjustments to that polar orbiter. For the polar orbiters (NOAA), the Total Correction is composed of two parts: a normalization to the original reference standard and an absolute correction including trend correction.

 Date Range       Reference Polar Orbiter
 ---------------  -----------------------------------------
 Aug 81 - Jan 85  NOAA-7
 Feb 85 - Oct 88  NOAA-9
 Nov 88 - Sep 94  NOAA-11
 Oct 94 - Jan 95  Interpolation between NOAA-11 and NOAA-14
 Feb 95 - Sep 01  NOAA-14
 Oct 01 - Dec 05  NOAA-16

It is important to obtain the FINAL CALIBRATION posted here since small adjustments are implemented during D data production. The coefficients reported here represent final calibration through June 2005. Coefficients after that period are still under review. All available calibration is posted here, including months where the B3 dataset has not yet been produced.

Note: The ISCCP BT data covering the period 1983 through June 2005 is now available by individual satellite in our download site.
Additionally, the Absolute IR coefficients have been updated with the latest calibrations and made effective on April 20, 2007.


Calibration Overview
This figure shows the overview of the ISCCP calibration procedure.
NOAA Afternoon
This figure shows the drift in orbit time of the NOAA afternoon polar orbiters carrying the AVHRR instrument. There is a significant drift in the equator crossing time over the life of a given satellite. This has implications for both monitoring the calibration of a satellite over time and also the bridging of the gap between satellites.
NOAA Morning
Similar to the previous figure but includes the morning satellites and in the lower panel shows the change in cosine of the solar zenith angle due to the drift in orbit. This complicates the monitoring of surface targets over time. Note also the difference in behavior between the morning (left) and afternoon (right) polar orbiters.
Visible Reflectance
This shows the increase in visible reflectance associated with the Pinatubo eruption. The plot is an anomaly plot showing monthly means minus climatology. The time period labeled "Pinatubo Effect" was not used in the calibration calculations.
Surface Reflectance 
Anomaly
This plot shows the surface reflectance anomaly of the AVHRR Channel 1 observations after calibration.
Surface Reflectance
This plot shows the surface reflectance (global, clear-sky) of AVHRR Channel 1 observations (monthly means) before and after calibration.
IR Normalizations
This plot shows the effect of the ISCCP infrared normalizations. NOAA-9 was chosen as the standard for the IR channel and the other satellites' data are normalized to NOAA-9. The statistics plotted are the 10th (lower) and 90th (upper) population percentiles of water pixels. These are fairly stable statistics which show the behavior of the instrument at both the warm (10th percentile) and cold (90th percentile) ends of the data.
Calibration Plots
This plot shows the before and after calibration plots at the cold end of the data. The magnitude of the calibration corrections is usually less than a degree although it is more for NOAA-7 and even larger for NOAA-16. The jump observed for NOAA-16 data is being investigated.

Documentation

Brest, C.L., W.B. Rossow, and M. Roiter, 1997: Update of Radiance Calibrations for ISCCP. J. Atmos. Ocean Tech., 14, 1091-1109. Read abstract/document.

Rossow, W.B., C.L. Brest, and M. Roiter, 1996: International Satellite Cloud Climatology Project (ISCCP) Update of Radiance Calibrations. WMO/TD.-No. 736. World Meteorological Organization. Read abstract/document (79 Pages).

Desormeaux, Y., W.B. Rossow, C.L. Brest, and C.G. Campbell, 1993: Normalization and calibration of geostationary satellite radiances for ISCCP. J. Atmos. Ocean Tech., 10, 304-325. Read abstract/document.

Rossow, W.B., Y. Desormeaux, C.L. Brest, and A.W. Walker, 1992: International Satellite Cloud Climatology Project (ISCCP) Radiance Calibration Report. WMO/TD-No. 520, WCRP-77, World Meteorological Organization. Read abstract/document. Read addendum to document (PS). NOTE: Mistakes in the document discovered post-publication are corrected by errata pages which are available as PostScript replacement pages.

Brest, C.L., and W.B. Rossow, 1992: Radiometric calibration and monitoring of NOAA AVHRR data for ISCCP. Int. J. Remote Sens., 13, 235-273. Read abstract and/or download PDF document.


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