Solar and Lunar Calibration for Miniaturized Microwave Radiometers

Miniaturized microwave radiometers deployed on nanosatellites in Low Earth Orbit (LEO) are now demonstrating the ability to provide science-quality weather measurements, such as the 3U Micro-sized Microwave Atmospheric Satellite-2A (MicroMAS-2A). The goal of having cost-effective miniature instruments distributed in LEO constellations is to improve temporal and geospatial coverage. The Time-Resolved Observations of Precipitations structure and storm Intensity with a Constellation of Smallsats (TROPICS) is a constellation of six 3U CubeSats, based on MicroMAS-2A, scheduled to launch no earlier than 2020. Each CubeSat hosts a scanning 12-channel passive microwave radiometer. TROPICS will improve temporal resolution to less than 60 minutes compared to larger satellites in polar orbit, such as NOAA-20 which hosts the Advanced Technology Microwave Sounder (ATMS) and has a revisit rate of 7.6 hours. [1] The improved refresh rate will provide high value observations of inner-core conditions for tropical cyclones [2]. In order to effectively use miniaturized microwave radiometers on small satellites such as MicroMAS-2A and TROPICS operationally, new approaches to calibration are needed to achieve state-of-the-art performance. Calibration on nanosatellite platforms present new challenges, as standard blackbody targets are too bulky to fit on 3U CubeSats. Instead, internal noise diodes are used for calibration, with long-term drifts measured on the order of 0.2% to 3.0% (approximately 0.4 K to 6.0 K) [4]. Blackbody calibration targets such as used on ATMS have 0.14 K error or better for warm calibration [3]. In order to provide state of the art calibration for CubeSats, methods must be developed to trend and correct noise diode drift. We develop a new way to continuously calibrate CubeSat constellations, such as TROPICS, by incorporating frequent and periodic solar and lunar intrusions as an additional source of information to counter noise diode drift. These solar and lunar intrusions also occur for existing satellites hosting microwave radiometers in polar orbits, but are much more infrequent than for the scanning payload on the TROPICS constellation, and are typically treated as an observational and calibration limiting constraint. The higher occurrence rate of intrusions for TROPICS motivates the novel idea of using the intrusions to support calibration. An algorithm is developed to compare expected effective brightness temperature from solar and lunar measurements to actual measured brightness temperatures. Future work includes developing a detailed error budget for the algorithm and testing the algorithm using actual sun and moon measurements taken by MicroMAS-2A and ATMS. In this paper we discuss background information, provide our approach, and show initial results.