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.
AIRS Mechanical Subsystem
The AIRS Instrument Support Structure (ISS) is designed to maintain instrument alignment stability and boresight relative to the spacecraft. It provides a 7 point interface to the spacecraft including a 3 point kinematic mount to the platform and maintains the alignment after exposure to the environmental loads ang gravity release.
Alignment is maintained evan after the instrument achieves operational temperature and provides thermal isolation between the three thermal zones of 155K, 190K and 290K. It provides high thermal conductivity between the HRS (Heat Rejection System) and the electronics. The ISS provides vibration isolation between the cryocoolers and the spectrometer.
Finally the ISS serves as electrical ground and insulation for sensitive electronic subsystems. The ISS baseplate is made of aluminum, while the support Trusses are made of beryllium, aluminum and titanium.
AIRS Optics and Focal Planes
Infrared Spectrometer Assembly
The multi-aperture, array grating spectrometer is an advanced pupil-imaging design providing high spectral resolution, wide spectral coverage and full spectral multiplex operation essential to meeting the AIRS science objectives. The design approach uses a coarse echelle grating in combination with high definition bandpass filters to create a two-dimensional color map compatible with state of the art IR FPA technology.
Upwelling radiance enters the system (see figure at right) via the cross-track scan mirror, where it is directed into a 4-mirror off-axis telescope assembly with a common 1.1 field stop to ensure high spatial registration for all spectral samples. The collimated energy exiting the telescope is incident on the spectrometer entrance slit plane containing eleven individual apertures arranged in two staggered columns. For precise radiometry, these slits are conjugate with the system entrance pupil and each is covered with an order-sorting bandpass filter, which forms the first stage of spectral separation. Ultimately, these eleven slits are imaged onto the focal plane, where each slit image contains the energy from one selected grating order. The entire wavelength range is mapped onto the FPA using orders 3-11. A second stage of filtering over each FPA array further defines the selected color band, rejects overlapping orders, and serves to reduce background photon levels.
The relationship between the entrance slits, grating orders, and FPA layout is illustrated in the lower right portion of the optics diagram. Energy passing through the entrance slits is re-imaged within the system where a tuning fork chopper (357 Hz) is incorporated for reduction of 1/f noise in the PC channels. The energy is then relayed onto a coarse (13 l/mm) grating surface (photo at right) where high spectral resolution separation occurs. Energy from each entrance slit is dispersed by the grating and re-imaged onto the focal plane by a wide field, off-axis F1.7/F2.0 Schmidt camera which provides a nominal 100 m x 200 m spectral resolution element format. Schmidt aberrations are corrected by an aspheric surface on the grating and a field flattener incorporated within the FPA assembly. A provision for commandable, micron level adjustment of alignment and focus is built into the Schmidt mirror assembly via three precision actuators and can be used in flight if necessary. So far in the mission, no alignment adjustments have been made.
Focal Plane Assembly
The AIRS IR Focal Plane Assembly (FPA) represents a major advance in infrared technology, incorporating a number of state of the art features in a space qualified, high reliability configuration. Key to its operation is an advanced hybrid PV/PC HgCdTe focal plane consisting of 10 PV modules and 2 PC modules each individually optimized for a particular grating order/wavelength range (see diagram).
The PV modules consist of 1, 2 or 4 bi-linear arrays of back-side-illuminated HgCdTe detectors, each bump mounted to a low power, 1.2 m CMOS read-out integrated circuit (ROIC) that provides the first stage of signal integration and multiplexing. The 10 PV modules contain a total of 4208 detectors multiplexed down to 26 outputs, with each PV module having a unique set of requirements and constraints in terms of wavelength coverage, signal and background flux, and sensitivity (D*). Technologically, the most stressing are the long wavelength modules, particularly M10 which covers the band from 12.7 m to 13.7 m and required a detector material cutoff wavelength of 15.6 m. Considerable development in the areas of LWIR detector material growth, radiation tolerant ROIC design and fabrication, and detector/ROIC interconnect was required to satisfy system requirements. To mitigate risk in the longest wavelength region, 13.7 m to 15.4 m, PC HgCdTe detector technology was used. This spectral region is covered in two modules with individual leads for each of the 274 low impedance PC detectors, a distinct disadvantage attendant with this technology.
The set of 12 modules is mounted to a common ceramic motherboard. The motherboard contains a complex arrangement of power, command and control, and signal interconnects to all PV modules as well as individual lead-outs for the PC detectors. The AIRS FPA is unique in its hybrid PV/PC approach and required special care in the routing, shielding and grounding of very low noise (nV) PC signals in the presence of high level (V) PV signals. A total of 526 leads interconnect to the motherboard assembly using a series of 10 high-density, thin-film flex cables specifically designed for cryogenic operation. Modules are individually assembled, tested and positioned onto the motherboard using computer controlled stages to maintain the requisite optical alignment tolerances of +/-15 m. Overlaying the focal plane module is an IR bandpass filter assembly containing 17 individual filters used for grating order selection as well as background suppression. The filter set is precisely registered to the focal plane using a dark mirror coated frame for stray light control. The assembly also includes a cold shield and a field flattener lens, which is part of the Schmidt exit optics.
AIRS Electronics Subsystem
Sensor Power Supply
Instrument power conditioning is provided by the Sensor Electronics Power Supply (SEPS), which operates at 29 Vdc input to produce 15 isolated, highly regulated outputs for system operation. Redundancy is provided at the supply level via a separate power module within the SEPS assembly. Cooler power conditioning and control is provided in separate Cooler Control Electronics module (CCE), one CCE for each Cooler assembly. Communication to either cryocooler is provided through a serial bus interface to the AIRS controller.
Redundancy requirements coupled with the large number of focal plane outputs were key factors driving the overall electronics architecture. To minimize size, mass and power high-density component and packaging techniques were used throughout the design, including SMT components, custom hybrids, and a great reliance on FPGA technology. The development was made more difficult by the reduction in the industry's space qualified component product lines, which required the program to shoulder the burden of parts qualification in many instances.
In total, over 30,000 electronic components are used in AIRS, all of which meet MIL STD 975 Grade 2 or better requirements.
The AIRS electronics architecture is a redundant, fully synchronized, radiation tolerant design, providing a highly flexible microprocessor-controlled configuration commandable from the ground. As shown in the photo, the electronics are partitioned into functionally distinct modules, each of which uses the latest space qualified, high-density SMT and FPGA component technologies. On-board signal processing functions are contained in the Sensor Electronics Module (SEM) which processes 26 multiplexed, high level PV outputs along with 274 low level PC signals via a pipeline data processing scheme operating at 6 Mwords/s.
Prior to the pipeline processing, the PV signals are multiplexed and digitized at 12-bit resolution. Each PC signal is hardwired to an individual low noise preamplifier, which has been hybridized to minimize circuit area, band limited, digitized at 12 bits, and then combined with the PV data. The pipeline process includes charged particle mitigation and 2-pixel spectral summation for PV detectors, PC signal demodulation at 357 Hz, along with digital integration of 16 FPA subsamples to match the full footprint dwell time of 22.4 ms.
Vis/NIR sensor data along with engineering data are inserted into the data stream, which is then output to the Aqua spacecraft at an average rate of 1.27 Mb/s using a CCSDS packet protocol and a TAXI interface.
Command and control, redundancy management, engineering data collection, and on board servo control functions are contained in the Actuator Drive Module (ADM) which interfaces with the spacecraft via a MIL STD 1553 bus. A radiation tolerant imbedded processor (Harris RTX 2010) is used for command and control functions with program code primarily written in C and operated out of RAM. Instrument redundancy management to the circuit card level is via ground command of a series of 96 relays, the drivers for which are packaged in hybrid form to minimize circuit area. Scan mirror control uses a digital servo to provide a programmable 2-speed, 2.67 s rotary scan cycle with less than 0.8 mrad error over the 2 sec, 100 ground scan segment.
The cryocooler control electronics (see photo) operate off a 28 Vdc spacecraft bus and provide high efficiency, synchronized drive to the compressor pistons. Cold head temperature (+/-10 mK) and compressor vibration output (<0.5 Newton) are controlled by software feedback loops based on a cold head temperature sensor, a capacitive piston position sensor, and an accelerometer mounted on the compressor. The overall mass of the AIRS cryocooler system is 37 kg including both redundant assemblies along with the support structure.
The development of the space qualified pulse tube cryocooler was a key accomplishment for the program and represents a major advance in cryogenic technology. The AIRS pulse tube cryocooler has demonstrated excellent performance in terms of temperature control, operating efficiency and vibration output. Both cryocoolers achieved a thermodynamic efficiency (system input power/cooling capacity) of approximately 62 W/W at 55 K operation with a net cooling capacity of 1.5 Watts. Cold head temperature control has been measured to be better than +/-10 mK at 55 K. Vibration output as measured during cryocooler acceptance testing was well within specification for nearly all cryocooler harmonics and subsequent system level tests have shown no evidence of cryocooler vibration interference.
AIRS Cryogenic and Cooling Systems
The focal plane assembly operates at 58 K for high sensitivity and is packaged in a permanent vacuum dewar which mates directly to the 155 K grating spectrometer. This complex thermal mechanical arrangement in combination with the large number of leads placed great demands on the dewar design. The dewar outer shell is constructed primarily of stainless steel and uses a glass inner bore to accommodate the 58 K/155 K thermal transition. Low loss heat transfer from the focal plane to the pulse tube cryocooler is provided through a single crystal sapphire rod connected to the dewar end well. Attachment of the rod to the cryocooler uses a flexible coupling.
Low vibration, long life focal plane operation near 58 K is critical to the success of AIRS, and the rapid advance of pulse tube cryocoolers has proven to be a key enabling technology in this area. The AIRS focal plane cryocooler, developed under contract with TRW (now NGST), is a fully redundant pulse tube refrigerator with each redundant assembly consisting of an actively balanced compressor, separate pulse tube coldhead, and independent control electronics.
The pulse tube cryocooler is a variation of the Stirling cycle refrigerator, where the moving displacer in the traditional coldhead assembly is replaced by a passive combination orifice and gas reservoir to bring about the proper phase relationship between pressure and mass flow rates. The result is an accessible cold spot for FPA cooling without the penalties of coldhead vibration, electromagnetic interference, and displacer reliability inherent in the standard approach. AIRS was the first NASA instrument to use pulse tube coolers.
Radiators and Earth Shield Assembly
The AIRS radiator assembly (not shown) is a two-stage flat panel design. The first stage is an open-back beryllium structure with a painted black aluminum hexel radiating surface. It extends around the spectrometer and operates in the 171-190 K temperature range. The second stage radiator provides extended cooling to 145-160K. The second stage radiator is an aluminum honeycomb and face sheet structure with a black painted aluminum hexel radiating surface.
The AIRS Earth Shield Assembly provides shielding of the cold radiator surfaces to earth radiation. The basic design is a honeycomb base with aluminum face sheets and core. The side panels are beryllium and fold inward toward the middle in the closed position. The door was activated once after launch and will remain open throughout the mission.
Decontamination heaters on the optical bench provide enough energy to the spectrometer to outgas condensed water in the event icing occurs after opening the Earth Shield.