Modernization of Blackbody Temperature Control for the enhanced MODIS Airborne Simulator (eMAS)

This paper describes a modernization effort to a critical subsystem onboard NASA's Enhanced MODIS Airborne Simulator (eMAS). A new Blackbody Temperature Controller subsystem greatly improves eMAS' infrared-imaging calibration and offers substantial gains in measurement and control performance. Reductions in Size, Weight and Power (SWaP) over prior instrument hardware are also achieved. Real-time control from the ground via the NASA Airborne Science Sensor Net is enabled by the ARM-based, embedded controller. A linear-drive stage delivers 140W peak power to a Thermoelectric Cooler (TEC) array for the blackbody. The paper details the rationale for selection of a linear power stage and corresponding analyses and prototyping that traded PWM versus linear methods. Linear drive is often considered grossly inefficient; a deeper analysis and novel packaging techniques indicate otherwise for this specific application. A survey of high-performance, temperature-measurement techniques reveals that new System-on-Chip (SoC) offerings substantially reduce complexity over prior state-of-the-art with little compromise in performance. Packaging volume of the new technology also radically shrinks. Lab results indicate measurement readout-noise (15mK, 3σ) improvements over legacy eMAS hardware. Selection of the temperature sensor and NIST-traceable calibration yields an overall systematic error of 60mK typical, 141mK worst-case. A full measurement system error budget is presented. The measurement stability aids performance and tuning of the control algorithm, also discussed. Two controllers have been built and integrated into the instrument in preparation for the upcoming deployment season. The new development is best understood in the context of the eMAS whiskbroom imager, its infrared capability, and the implementation of its blackbody infrared references. The blackbodies are built using circular plates of copper, coated with a special, high-emissivity paint that appears diffuse at infrared wavelengths. A pair of references on either side of the spectrometer's field of view provides an in-system, two-point calibration every scan line. An array of TEC modules is sandwiched between each plate and a forced-air cooled heatsink. A single sensor is read out at high precision with room for a coarse, auxiliary channel. Historically, temperature readout and control of these references were performed with a system mounted far from the spectrometer optical bench. Low-level analog signals were sent over long (10') cables in close proximity to the TEC array's high-current drive. This rendered the temperature measurement highly susceptible to systematic noise. The legacy system also utilized an analog control loop that could not be tuned or modified over time. The new controller leverages modern embedded system techniques to radically shrink the packaging volume and enable real-time control. Low-noise methods and clever thermal management allow the controller to be placed in close proximity to the reference plate. The overall result is an improvement in performance, reduction in system volume, and a drop in weight. Aircraft managers always welcome a weight reduction for airborne sensors; however, the eMAS technicians tasked with repeatedly installing the system are the real beneficiaries. Overall, the modernization effort represents a major step forward for this unique NASA Airborne Science asset.