NASA TROPICS MISSION: L1 RADIANCE POST-LAUNCH CALIBRATION AND VALIDATION AFTER TWO YEARS ON ORBIT

V. Leslie¹, W. Blackwell¹, M. DiLiberto¹, G. Perras¹

¹MIT Lincoln Laboratory

The NASA Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission, selected by NASA as part of the Earth Venture–Instrument (EVI-3) program, hosts a high-performance microwave radiometer on a 3U CubeSat with a constellation of four CubeSats. The overarching goal for TROPICS is to provide nearly all-weather observations of 3-D temperature and humidity, as well as precipitation horizontal structure, at high temporal resolution to conduct high-value science investigations of tropical cyclones utilizing a constellation of CubeSats. Before Rocket Lab launched the four CubeSats in May 2023, NASA and Lincoln Laboratory launched the engineering qualification unit (TROPICS Pathfinder) into a sun-synchronous orbit in June 2021 and continued generating data products until Dec. 2023. Of the four CubeSats launched in May 2023, two continue to operate into Sept. 2025 (TROPICS-03 & TROPICS-05). TROPICS-07 stopped generating data in July 2023, and TROPICS-06 stopped in March 2025. This presentation covers the TROPICS constellation mission’s validation activities.

To provide temperature profiles using seven channels near the 118.75 GHz oxygen absorption line, water vapor profiles using three channels near the 183 GHz water vapor absorption line, imagery in a single channel near 90 GHz for precipitation measurements (when combined with higher resolution water vapor channels), and a single channel at 205 GHz that is more sensitive to precipitation-sized ice particles. Noise Diodes (ND) are an attractive option for calibration for small satellite sounders, especially in the face of volume constraints that preclude extensive radiative shielding of internal calibration targets that is utilized in operational missions.

Pre-launch characterization consisted of antenna pattern measurements, spectral response measurements, and TVac calibration using precision external calibration targets. Post-launch analysis indicated a significant benefit to using a new calibration approach relative to the original, more traditional calibration scheme. The new approach uses early-orbit observations minus simulations (O-S) to optimize the calibration parameters by minimizing the difference between the O-S under clear-sky conditions. A multilayer feed-forward neural network was used in some channels to infer and correct complicated calibration artifacts, such as relatively high channel mixing and non-linearity. The calibration parameters used in the minimization follow a generalized version of the periodic absolute calibration equation typically used for cross-track passive microwave radiometers. Calibration parameter regression and machine learning use various telemetry as predictors. Finally, the scan bias corrections of the sidelobe contamination are an affine function with each term a constant-coefficient polynomial regression with spot number as the predictor.

After the early-orbit optimization freezes the calibration parameters, a causal calibration sustainment system is in place that minimizes potential calibration drift due to changes in noise diode output power. The sustainment system uses GEOS-5 NRT forward processing NWP output for its timeliness.

Calibration and Validation activities consisted of simulations and radiance-to-radiance comparisons. The first validation compared observations minus simulations (O-S), over the mission lifetime, under clear-sky conditions (identified by GOES binary cloud mask) and cloudy non-precipitating conditions for channels with weighting functions peaking above most clouds. The simulations use the ERA-5 reanalysis and a Rosenkranz line-by-line absorption radiative transfer model. Both land and ocean surfaces were modelled (ocean emissivity used SURFEM-Ocean from RTTOV 13.2 and land emissivity was estimated using a surface channel radiance and RTM). This presentation will also present a second validation method of double differencing near coincidental observations between TROPICS and ATMS. The matchup used all spots under any weather conditions with criteria that the matchup had to be within an hour, less than 5 km apart, and similar scan angles within 15 degrees (i.e., to account for the large altitude difference). The two-part validation process of simulations and operational microwave sensor comparisons are a comprehensive validation methodology that accounts for different error sources (e.g., simulation errors) while providing a statistically significant data set over sufficiently fine temporal resolution.

The validation data is presented in a number of ways to adequately validate the radiances, that is, as histograms, scatterplots, statistical metrics (e.g., mean and std. dev.), and O-S maps. Furthermore, biases are plotted as a function of instrument temperature, scene temperature, latitude, and scan position to identify potential hidden issues. Performance is on par with present operational sounders.

DISTRIBUTION STATEMENT A. Approved for public release. Distribution is unlimited. This material is based upon work supported by the National Aeronautics and Space Administration under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration.