The AIRS Instrument
The Atmospheric Infrared Sounder (AIRS) instrument incorporates numerous advances in infrared sensing technology to achieve a high level of measurement sensitivity, precision, and accuracy. Two items stand out because they were considered extremely high risk at the time the AIRS instrument development was approved; yet they both were extremely successful. The first of these is the development of low-noise detectors and read-out electronics for the AIRS wavelength range of 3.7 to 15.4 microns. The second is the low-vibration and long lifetime pulse tube cryocooler used to maintain the IR detectors near 58 K.
AIRS was built under system contract to NASA's Jet Proplusion Laboratory (JPL) by Lockheed Martin Infrared Imaging Systems (LMIRIS) of Lexington, Massachusetts. LMIRIS has since become part of BAE Systems. The detector development was done in-house by LMIRIS. The pulse tube coolers were developed by TRW in Redondo Beach, California under subcontract to LMIRIS. TRW has since become Northrop-Grumman Space Technologies (NGST). The AIRS coolers are the first of the TRW pulse tube coolers to fly. Others have since been used on a variety of missions. Another division of TRW/NGST built the Aqua spacecraft.
The heart of the instrument is a cooled (155 K) array grating spectrometer operating over the entire AIRS infrared (IR) spectral range at a spectral resolution (lamdba / delta lambda) of 1200. A grating disperses infrared energy across arrays of high-sensitivity HgCdTe detectors. The concept requires no moving parts for spectral encoding and provides 2378 spectral samples, all measured simultaneously in time and space. Simultaneity of measurement is an essential requirement for accurate temperature retrievals under partly cloudy conditions.
AIRS looks toward the ground through a cross-track rotary scan mirror which provides +/- 49.5 degrees (from nadir) ground coverage along with views to cold space and to on-board spectral and radiometric calibration sources every scan cycle. The scan cycle repeats every 8/3 seconds. Ninety ground footprints are observed each scan. One spectrum with all 2378 spectral samples is obtained for each footprint. A ground footprint every 22.4 ms. The AIRS IR spatial resolution is 13.5 km at nadir from the 705.3 km orbit.
There is also a set of visible and near infrared detectors (Vis/NIR) divided into four intermediate and broadband spectral channels. The Vis/NIR spatial resolution is approximately 2.3 km. The Vis/NIR channels provide a diagnostic imaging capability for observing low-level clouds. The Vis/NIR light goes through a four-color imaging photometer at ambient temperatures to the Vis/NIR detectors. The stability of the Vis/NIR photometer is monitored using on-board lamps.
At the same time, the IR light passes through a multi-aperture spectrometer whose elements are passively cooled to about 155 K by a two-stage radiator assembly. Using space views and a view of a hot on-board blackbody every scan line, the science software (running on the ground) calibrates the IR radiances to absolute accuracy 3% or better. The wavelengths of each channel are calibrated using observations from scene data of well-understood atmospheric spectral lines, with help from an on-board spectral source, a sheet of thin Parylene film.
The IR spectrometer has eleven separate apertures, each of which is eventually imaged onto the focal plane. In the optical path is a coarse grating (13 lines/mm) which disperses the light into its component wavelengths. A combination of filters at the 11 apertures, the use of multiple orders of the grating (3-11), and order-separation filters on the focal plane, ensures that each IR detector is exposed to light from just one well-known narrow band of wavelengths.
After dispersal by the grating, the IR light is passed through a specially designed window into a dewar that contains the focal plane. The temperature of the focal plane is tightly controlled at a value near 58 K, using two 1.5 W capacity Split Stirling pulse tube cryocoolers.
The IR detectors are divided into 12 modules containing 17 linear arrays distributed in a two-dimensional pattern on the cold focal plane. The detectors are of two types\photovoltaic (PV) and photoconductive (PC). It was originally hoped that the entire focal plane could be PV, a more advanced and compact technology. But PV detectors sensitive to the longest AIRS wavelengths were not available by the time the instrument design had to be frozen. So two of the twelve detector modules (for 13.7 to 15.4 micron wavelengths) are PC.
The overall electronics design emphasized radiation tolerance and redundancy to increase lifetime. The latest space qualified SMT and FPGA technology was used. The system is microprocessor-controlled and has a high degree of configuration flexibility via ground commands. Critical low noise PC signal processing functions are performed on-board but off the focal plane, in parallel with high-level PV signal processing operations. The electronics are complex by virtue of the detector count and the redundancy configuration, and considerable effort was placed on high-density packaging of over 30,000 parts.