Calibration and validation of small satellite passive microwave radiometers: MicroMAS-2A and TROPICS

Abstract

Miniaturized microwave radiometers deployed on nanosatellites in Low Earth Orbit are now demonstrating cost-effective weather monitoring capability, with increased temporal and spatial resolution compared to larger weather satellites. MicroMAS-2A is a 3U CubeSat that launched on January 11, 2018 with a 1U 10-channel passive microwave radiometer with channels near 90, 118, 183, and 206 GHz for moisture and temperature profiling and precipitation imaging. 1 The Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission is projected to launch in 2020, and its 1U 12-channel passive microwave radiometer is based on the current CubeSat mission MicroMAS-2A. TROPICS will provide rapid-refresh measurements over the tropics and measure environmental and inner-core conditions for tropical cyclones. 2 In order to effectively use small satellites such as MicroMAS-2A and TROPICS as a weather monitoring platform, calibration must ensure consistency with state of the art measurements, such as the Advanced Technology Microwave Sounder (ATMS), which has a noise equivalent delta temperature (NEDT) at 300 K of 0.5 -3.0 K. 3 In this work, we present initial analysis from the MicroMAS-2A radiometric bias validation, which compares MicroMAS-2A measured brightness temperatures to simulated brightness temperatures calculated by the Community Radiative Transfer Model (CRTM) using input from GPS radio occultation (GPSRO), radiosonde, and numerical weather prediction (NWP) atmospheric profiles. We also model solar and lunar intrusions for TROPICS, and show that the frequency of intrusions with a scanning payload allows for the novel opportunity of using the solar and lunar intrusions as a calibration source.

Correcting Calibration Drifts Using Solar and Lunar Intrusions for Miniaturized Microwave Radiometers

“…Over a year period, the TROPICS satellites have lunar and solar intrusion events fifteen times a day (~5745 total over the course of a year), with intrusions occurring once every orbit. In comparison, ATMS only has 44 total lunar intrusions over the course of a sample year [17,19].…”

“…A satellite in a 550 km, 30°inclination orbit was modeled in Systems Tool Kit (STK) with a dual spinner configuration, with the instrument modeled to scan at 30 rpm. Solar and lunar intrusions occurred in a sector of ± 57° from zenith over the course of the 2019 modeled year [17,19]. Cold calibration for the TROPICS SVs should thus not be performed during this ± 57° from zenith sector.…”

“…We then examined intrusions over the course of a day. The solar and lunar intrusions were shown to occur periodically every orbit [17,19]. As the sensor scans around the spacecraft, intrusions occur every two seconds during the intrusion event.…”

“…As the sensor scans around the spacecraft, intrusions occur every two seconds during the intrusion event. For a sample intrusion event with a 3° FOV, there are 23 intrusions at elevation angles ranging from 82.3° to 82.5 that occur over 46 seconds [17,19].…”

A preliminary assessment of the value and impact of multiple configurations of constellations of EON-MW, a proposed 12U microwave sounder CubeSat for global NWP

“…Various 'vicarious' methods will be employed post launch to evaluate pointing and calibration accuracy and stability, for example, by monitoring the position of coastlines as evident in surface sensitive channels, and by comparing observed BTs to BTs simulated from collocated profiles of temperature and water vapor from radiosondes, GPSRO retrievals, and NWP analyses (e.g. Crews et al, 2018). The reader should note the distinction usually made between the sensor noise values and the BT observation errors usually used to simulate and assimilate these data.…”