On-orbit Calibration of the Tomorrow.io Microwave Sounders

Abstract

The Tomorrow.io Microwave Sounder (TMS) constellation represents a significant advancement in rapid-revisit passive microwave sensing from space. Building upon the NASA TROPICS (Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats) mission, each TMS satellite hosts a 12-channel microwave radiometer on a 6U CubeSat, utilizing a novel dual-calibration approach with both noise diodes and an internal calibration target (ICT). The system is optimized for atmospheric temperature and humidity sounding, as well as precipitation retrieval, with measurements centered on key absorption lines at 118 and 183 GHz, with window channels at 91 and 204 GHz. This manuscript details the Level 1B algorithm for converting raw instrument counts into calibrated brightness temperatures. Calibration accounts for instrument nonlinearity, antenna pattern effects, and temperature-dependent noise diode output, and is validated against multiple sources including ERA5 reanalysis, ATMS and GMI coincident observations, and simulations from radiosonde and GNSS-RO-derived profiles. Performance assessments for the first seven TMS satellites demonstrate radiometric noise (NEDT) generally within 0.3–1.0 K, calibration accuracy within 1 K across most channels and space vehicles, and high calibration stability over time. Comparisons to TROPICS show marked improvements, especially in the W- and F-band channels. Some residual biases remain, particularly in space vehicles that experience a widely varying thermal environment, but are being addressed in future algorithm updates. Overall, TMS provides reliable, rapid-revisit global observations that can complement larger, more expensive systems with less frequent coverage, supporting applications in tropical cyclone monitoring, precipitation nowcasting, and medium-range weather forecasting.