Radiometric Calibration of the Cryogenic Limb Array Etalon Spectrometer (CLAES) on UARS

Abstract

The Cryogenic Limb Array Etalon Spectrometer (CLAES) will derive stratospheric temperatures and constituent number densities from the measurement of infrared spectral emissions, during its 18 month mission on board the NASA Upper Atmosphere Research Satellite (UARS). An experiment overview of the Cryogenic Limb Array Etalon Spectrometer (CLAES) is given by Roche and Kumer (ref. 1); an instrument overview is given by Burriesci, Naes, Springer, and Steakley (ref.2). This paper addresses the radiometric calibration of the CLAES. An extended area full aperture blackbody simulator (graybody) mounted on the instrument aperture door has been used for laboratory calibration, and will be used for repeated calibrations in orbit. The advantages of full aperture calibration sources are well understood; complete end-to-end calibration is provided, and the calibration accuracy is not limited by uncertainty in knowledge of the source position. The CLAES calibration source (fig. 1) is a 22.15 cm diameter anodized aluminum plate with concentric circular "γ" grooves to provide enhanced emissivity. The source is mounted on the CLAES aperture door with 4 fiberglass insulating supports. The perimeter and back of the calibration source are enclosed in an aluminum shield containing multilayer insulation (MLI). For laboratory calibration the source is heated with strips of heating "tape" on the back face. In orbit the source will be heated "passively" while the door is open, by exposure to radiation from the earth. Calibration source temperature is measured with an array of platinum resistance thermometers on the rear face of the source. During calibration intervals, the door will be closed and calibration data will be obtained over a range of temperature as the source cools to the internal radiative ambient of the cryogenic telescope (baffle temperature ≈ 130K). CLAES calibration goals for accuracy and precision are ≈3%; of particular concern is stability (repeatability) of the calibration source over a nominal 1.5 year mission. The primary sources of uncertainty in the instrument calibration are the knowledge of the source apparent emissivity, Δϵ, the thermometry error, ΔT, the uncertainty in wavelength to which the instrument is tuned, Δλ, and the uncertainty of the instruments spectral bandpass, Δδe. These items will be discussed below.