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.