Kenneth Overoye

Prelaunch and in-flight radiometric calibration of the Atmospheric Infrared Sounder (AIRS)

With 2378 infrared spectral channels ranging in wavelength from 3.7-15.4 /spl mu/m, the Atmospheric Infrared Sounder (AIRS) represents a quantum leap in spaceborne sounding instruments. Each channel of the AIRS instrument has a well-defined spectral bandshape and must be radiometrically calibrated to standards developed by the National Institute of Standards and Technology. This paper defines the algorithms, methods, and test results of the prelaunch radiometric calibration of the AIRS infrared channels and the in-flight calibration approach. Derivation of the radiometric transfer equations is presented with prelaunch measurements of the radiometric accuracy achieved on measurements of independent datasets.

On-board calibration techniques and test results for the Atmospheric Infrared Sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS) is a space based instrument developed for measurement of global atmospheric properties; primarily water vapor and temperature. AIRS is one of several instruments on board NASAs Earth Observing System Aqua spacecraft. AIRS operates in the 3.7 - 15.4 micron region and has 2378 infrared channels and 4 Vis/NIR channels. AIRS spatial resolution is 13.5 km from the orbit of 705 km and it scans ±49.5 degrees. AIRS has a set of on-board calibrators including a single infrared blackbody source, a parylene spectral calibration source, a space view and a Vis/NIR photometric calibrator. The on-board calibration subsystems are described along with a description of special test procedures for using them and results from several tests performed to date. Results are exceptional indicating that the instrument is performing better than expected.

Test and calibration of the AIRS instrument

A test system and set of procedures have been developed to fully test and calibrate the Atmospheric Infrared Sounder (AIRS), a facility instrument on NASAs EOS PM platform. The system has been used to test and calibrate, under simulated space conditions, the spatial, spectral, radiometric and polarization characteristics of AIRS for each of its 2378 spectral bands covering the IR range from 3.7 and 15.4 micrometer. Unique challenges included spectral line shape and out of band response characterizations over 3 decades of the response function, accurate radiometric response calibration for IR source brightness temperatures from 195 to 357 K, spectral channel center and width determinations to 3 ppm of wavelength, spatial co-registration determination of the 2378 bands to 0.002 degrees and polarization response determination over all bands. Measured sensitivity, spectral response and polarization response for the AIRS instrument met or bettered requirements. The test systems developed to meet these objectives are described and the procedures and summary results of testing are presented.

Operational readiness for the atmospheric infrared sounder (AIRS) on the Earth Observing System Aqua spacecraft

The Atmospheric Infrared Sounder project will measure global atmospheric water vapor and temperature with unprecedented resolution and accuracy. AIRS is an infrared instrument covering 3.7-15.4 microns in 2378 IR channels. This paper describes the AIRS mission and science objectives, the instrument design and operation, the calibration plan, the in-flight operational scenario and the Science Processing System. All aspects of the program are addressed here to demonstrate that the AIRS program is ready to transition to the flight segment of the program. The AIRS instrument meets the majority of instrument design requirements established in order to meet the scientific objectives. A well-defined operational approach has been established, and a sound calibration plan has been developed to ensure optimal performance throughout the life of the mission.

Absolute Radiometric Calibration Accuracy of the Atmospheric Infrared Sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002. AIRS has demonstrated in-flight NIST traceability and high radiometric accuracy. This accuracy is achieved in orbit by transferring the calibration from a Large Area Blackbody (LABB) to the On-Board Calibrator (OBC) blackbody during preflight testing. The LABB theoretical emissivity is in excess of 0.9999 and temperature uncertainty is less than 30 mK. The LABB emitted radiance is NIST traceable through precision Platinum Resistance Thermometers (PRTs) located on the internal surfaces. The radiometric accuracy predictions for AIRS based on the OBC, LABB, and pre-flight measurements give an accuracy of 0.2K, 3 sigma. AIRS pre-flight calibration coefficients have not changed in flight, preserving the link between observations and pre-flight calibration and characterization. An update is being considered that will improve accuracy and preserve traceability.

Performance of the atmospheric infrared sounder (AIRS) in the radiation environment of low-earth orbit

The Atmospheric Infrared Sounder (AIRS), a hyperspectral infrared sounder, was launched onboard NASAs Aqua spacecraft on May 4, 2002 into sun-synchronous polar Earth orbit for a mission expected to last 7 years. By monitoring calibration data from views of deep space and two on-board calibrators, we have identified a number of effects attributed to in-orbit radiation. Transient effects include 1. steps in the output level of individual channels, attributed to injection of charge into a large capacitor in the read-out electronics integrated circuit (ROIC); and 2. spikes in the calibration data and, by inference, in the scene data, attributed to the passage of ionizing radiation through the active region of the HgCdTe detectors. On-board signal processing corrects for most of the spike effects, and ground processing smoothes the hot and cold calibration data and provides a system of flags to alert the user in cases where the calculated radiances are still suspect. Persistent effects include 1. extremely rare degradations of channels due to large charge injection events; and 2. slow increases in noise levels for a small number of channels, attributed to bias shifts due to the slow accumulation of radiation dose in the ROIC input cells for some channels. In addition to these detector effects, two operational anomalies have been attributed to the high radiation levels in the South Atlantic Anomaly (SAA), one an unplanned cooler shut-down, the second an unplanned stopping of the scan mirror. This paper presents statistics on the frequency and location of these radiation events, and provides a description of the mechanisms by which such events are identified and accounted for. It should be emphasized that the vast majority of the 2378 AIRS infrared channels, and the instrument as a whole, have shown excellent stability and operability throughout the mission.

Scan-angle-dependent radiometric modulation due to polarization for the Atmospheric Infrared Sounder (AIRS)

Space based remote sensing instruments employing scanning mirrors to acquire data on the earth can experience a radiometric modulation with scan angle (striping) due to polarization effects. Mirrors inherently introduce polarization that depends on the angle of incidence and orientation of the mirror. In the case of the Atmospheric Infrared Sounder (AIRS) the angle of incidence is constant, however the orientation of the mirror changes with scan angle. The polarization of the scan mirror couples with that of the aft optics for spectral separation to produce a radiometric modulation of the signal with scan angle. Data acquired during instrument testing on the polarization of the spectrometer were combined with data obtained for the scan mirror to model the expected radiometric modulation. Results were compared with direct measurements of the modulation obtained during radiometric testing while viewing a large area blackbody. Agreement is very good and shows that the modulation is very small and can be modeled to an accuracy consistent with the radiometric calibration budgets. Both modeled and measured results are presented for representative bands in the instrument as well as a discussion of the modeling techniques and equations used.

Spectral resolution modeling with partial coherence for the Atmospheric Infrared Sounder instrument

We generalize the spectral resolution model that was used for the Atmospheric Infrared Sounder (AIRS) instrument to account for the observed disparity between the predicted resolution and its measurement. The prelaunch-measured spectral resolution of the AIRS instrument was shown to be narrower or better than the prediction by 3% to roughly 14% with the narrowing increasing with wavelength across the AIRS infrared spectral band of 3.7 to 15.4 µm. The prediction was based on the common practice of using the slit as a secondary source, but with a tacit assumption of spatial incoherence for the slit illumination. We show that the narrowing of the spectral resolution is caused by partial coherence at the slit illumination, and is accounted for naturally by using the primary source and a system point spread function P, which includes the optical contributions of the preslit source optics or telescope in addition to that of the spectrometer. We model the disparity versus wavelength, and show a good comparison to that measured and reported in the literature.

Spectral test and calibration of the atmospheric infrared sounder

Spectral characterization of the Atmospheric Infrared Sounder (AIRS) instrument during ground Thermal/Vacuum tests posed a number of difficult challenges due to the high spectral resolution and accurate knowledge requirements. A Fourier transform spectrometer was used in external step-scan mode to characterize the spectral response functions (SRFs) of the 2378 infrared detectors in the focal plane array which is part of the AIRS grating spectrometer. This paper summarizes the test development and characterization results. Special post-test data analysis was needed separately to determine the effects of interference in the order-separating entrance filters, which have a different temperature dependence from that of the otherwise unperturbed SRFs. This separation, which was successfully accomplished, provides calibration of the AIRS SRF shape over the full expected range of instrument temperatures.

Atmospheric Infrared Sounder (AIRS) thermal test program

The Atmospheric Infrared Sounder (AIRS) has been developed for the NASA Earth Observing System (EOS) program with a scheduled launch on the first post meridian (PM-1) platform in December 2000. AIRS is designed to provide both new and more accurate data about the atmosphere, land and oceans for application to climate studies and weather predictions. Among the important parameters to be derived from AIRS observations are atmospheric temperature profiles with an average accuracy of 1 K in 1 kilometer (km) layers in the troposphere and surface temperatures with an average accuracy of 0.5 K. The AIRS measurement technique is based on passive infrared remote sensing using a precisely calibrated, high spectral resolution grating spectrometer providing high sensitivity operation over the 3.7 micrometer - 15.4 micrometer region. To meet the challenge of high performance over this broad wavelength range, the spectrometer is cooled to 155 K using a passive two-stage radiative cooler and the HgCdTe focal plane is cooled to 58 K using a state-of-the-art long life, low vibration Stirling/pulse tube cryocooler. Electronics waste heat is removed through a spacecraft provided heat rejection system based on heat pipe technology. All of these functions combine to make AIRS thermal management a key aspect of the overall instrument design. Additionally, the thermal operating constraints place challenging requirements on the test program in terms of proper simulation of the space environment and the logistic issues attendant with testing cryogenic instruments. The AIRS instrument has been fully integrated and thermal vacuum performance testing is underway. This paper provides an overview of the AIRS thermal system design, the test methodologies and the key results from the thermal vacuum tests, which have been completed at the time of this publication.