NOAA KLM User's Guide

Section 7.0

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Introduction Page, NOAA KLM TOC, Acronyms
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7.0 Calibration of NOAA KLM Instruments

The calibration of a satellite radiometer involves finding the transfer function between digital counts from the instrument and the scene input spectral radiance for all the instrument's spectral channels.

In the infrared region of the spectrum, calibration is accomplished by using accurate blackbodies in a thermal/vacuum (T/V) environment. A T/V chamber is used to avoid atmospheric effects and to better simulate the space environment in which the instrument must ultimately operate. The T/V chamber contains accurate extended area blackbody cavities which are used as infrared sources. Calibration traceability to the National Institute for Standards and Technology (NIST) is via the temperature of the blackbody.

In the visible region, calibration is performed on a test bench. No significant errors are introduced by performing the calibration in this manner. The laboratory source most used to calibrate visible channels is an integrating sphere. The integrating sphere is the device used to create a visible radiance which is traceable to the NIST. Calibration of the AVHRR and HIRS visible channels uses one 102 cm (40-in) sphere exclusively in order to aid inter-instrument calibration consistency.

In the ultraviolet region, calibration is performed using several calibrated standard lamps; a quartz-halogen lamp for use in the 250-400 nm range, an argon mini-arc for the 160-280 nm range and a deuterium (D2) lamp covering the total spectral range. In the microwave region, calibration is performed in a T/V environment using calibrated blackbody radiometric sources.

Calibration updates to characterize the in-orbit performance of the visible and near-infrared channels of the AVHRR/3 will be introduced in the Level 1b data as and when deemed necessary by NESDIS; in addition, they will be accessible on NESDIS and ORA Home pages on the World Wide Web and different electronic bulletin boards. They will also be distributed in hard copy. The same multimedia approach will be adopted to obtain user feedback so that the calibration information given to the user community can be optimized.

The SEM-2 TED and MEPED responses are calibrated with particle (proton and electron) beams, with the results summarized in a Calibration Report for each instrument. The TED calibrated responses vary slightly from unit to unit, since the Continuous Dynode Electron Multiplier (CDEM) detectors have slightly varying detection efficiencies. The MEPED particle responses are essentially identical for all units. The TED and MEPED also have In-Flight Calibration cycles which verify the stability of the electronic gains, thresholds, channel logic, and detector noise.

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