AIRS

On-board Calibrators

Within the AIRS scan cavity, the scan mirror several on-board calibration sources. First, AIRS acquires four individual footprints of space each scan. These are used as a baseline radiometric reference to compensate for drift of the detector signals. In addition to the space view, the IR channels view the On-Board Calibrator (OBC) Blackbody, a spectral reference source (parylene) and the Vis/NIR channels view a photometric calibrator.

A 360-degree rotation of the scan mirror generates a scan line of IR data every 2.667 seconds. The scan mirror motor has two speeds:

The OBC Blackbody provides a warm (nominally 308K) calibration reference for the infrared channels. It consists of a beryllium housing and cavity and weighs approximately 2.0 kg. The design is a wedge shape with a cavity depth/aperture ratio of 2:1. The aperture is sized for a 1.6 FOV plus timing motion and 0.5" oversize. This results in one clear footprint of the OBC Blackbody every scan line. The OBC Blackbody is located diametrically opposite to nadir (180 scan angle) and is temperature controlled to its nominal set point temperature to within 50 mK. The OBC blackbody has four temperature sensors. Sensors 1 and 2 are located on the first bounce plate, sensor 3 is located on the 2nd bounce plate, and sensor 4 is located near the aperture. The OBC Blackbody is radiometrically calibrated to an external reference prior to flight.

The AIRS spectral reference source consists of a mirror coated with Parylene, a thin film polymer, that has a unique spectral signature. The parylene retroreflects the optical image so that the instrument views itself, a cold target, modified by the spectrum of the Parylene. The resulting spectral features are broad, but useful for spectral stability trending.

The Vis/NIR photometric calibrator consists of three selectable lamps and an all reflective collimator. The optical system covers a 0.2 x 3.3 degree field of view and include a lamp monitor diode for lamp stability control. Source uniformity is achieved with a diffuser plate.