Constellations to Climate: Advances in Microwave Radiometer Technology Enabling Sustainable Observations using Small Satellites

Passive microwave radiometer systems, such as SSM/I, AMSR-E, AMSU, WindSat and GMI, have been providing important Earth observations for over 30 years, including by not limited to surface wind vector, atmospheric and surface temperature, water vapor, clouds, precipitation, snow and sea ice. These data are critical for weather forecasting and the longevity of the record, along with careful calibration, has also enabled the extraction of climate records. But the future of these systems, conically scanning systems in particular, is uncertain. The next generation conical sensor planned for NPOESS was canceled in part due to the sensor cost and complexity and a replacement has not yet been identified. A solution may lie in smaller, lower-cost but equally capable sensors manifested on free-flying small-satellites which can open the door to new possibilities and an avenue for sustainable passive microwave observation. Among the possibilities are deployment in constellations to shorten revisit time to improve weather forecasting or routine deployment of single sensors over time to ensure an unbroken long duration climate record.

In recent years, there has been significant technology development in microwave radiometers on CubeSats, but a fundamental limitation is the small size of the U-form factor (<10cm) constraining operation to higher frequencies (>90 GHz) where reasonable spatial resolutions can be achieved and limiting observations to atmospheric temperature, water vapor and precipitation. Lower frequency microwave radiometers (6-90 GHz), which are needed for measurements of sea surface temperature, wind vector, snow, sea ice, and rain, are more suited to ESPA-class spacecraft (EELV Secondary Payload Adapter) where larger apertures (>50cm) can be accommodated. In this paper, we will describe recent technology advances toward meter-class small-sat based conical passive microwave sensors. The paper will focus on the Compact Ocean Wind Vector Radiometer (COWVR), which was recently delivered by the Jet Propulsion Laboratory (JPL) to the US Air Force for a spaceborne technology demonstration mission.

COWVR is an 18-34 GHz fully polarimetric radiometer with a 75cm aperture designed to provide measurements of ocean vector winds with an accuracy that meets or exceeds that provided by WindSat in all non-precipitating conditions, but using a simpler design which has both performance and cost advantages. This paper will give an overview of the COWVR instrument and mission and its performance estimated from pre-launch calibration data. While the COWVR mission is a focused technology demonstration mission, the sensor design is scalable to a much broader frequency range while retaining its low-cost advantage. We will describe extensions of the COWVR design that have been developed and the capabilities of such systems when deployed in a constellation scenario or climate monitoring scenario. We will also describe deployable reflector technologies being developed at JPL to enable large apertures (>2-meter) to stow inside an ESPA volume (<80cm) and be suitable for operation from 6-200 GHz. This removes any limitations on the spatial resolution of the sensor, even when launched as a secondary payload in the constrained ESPA volume.