AIRS Scanning and Calibration Subsystem
Scan Head Assembly
The AIRS Scan Head Assembly consists of the primary scan mirror, the rotating baffle which surrounds the scan mirror and protects it from contamination and stray light, the scan Motor Encoder Assembly (MEA), the and the scan head housing. The Scan Head Assembly also contains the sunshields, the VIS/NIR Sensor Assembly and the On-Board Calibrators.
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:
- During the first two seconds, the mirror rotates at 49.5 degrees/second, generating a scan line with 90 footprints of the Earth Scene, each with a 1.1-degree diameter Instantaneous Field of View (IFOV).
- During the remaining 0.667 seconds, the scan mirror completes one complete revolution, with four independent views of cold space, one view into a 310 K radiometric calibrator (the On-Board Calibrator [OBC] blackbody), one view into a spectral reference source (Parylene), and one view into a photometric calibrator.
The VIS/NIR photometer, which contains four spectral bands, each with nine pixels along track, with a 0.185-degree IFOV, is bore sighted to the IR spectrometer to allow simultaneous visible and infrared scene measurements.
The scan head support system consists of a three point interface to the base ring of the instrument support truss. Its primary function is to support scan head and maintain boresight. It is comprised of beryllium.
The MEA consists of a two phase 24 pole brushless DC torquer motor with redundant windings. It was developed by BEI Motion System Co. It contains an optical encoder with redundant read heads and motor commutation using a master code disks. The Bearings are an angular contact duplex pair mounted back-to-back.
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.
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.