INTERCALIBRATION OF PASSIVE MICROWAVE SENSORS USING THE DYNAMIC MICROWAVE RADIOMETER ON THE NASA INCUS MISSION BASED ON HERITAGE FROM THE TEMPEST MISSIONS

S. Reising¹, V. Chandrasekar¹, C. Radhakrishnan¹, S. T. Brown², S. van den Heever¹

¹Colorado State University, ²NASA/Caltech Jet Propulsion Laboratory

The INvestigation of Convective UpdraftS (INCUS) is a NASA Earth Venture mission (EVM-3) that will provide the first global systematic investigation into convective mass flux, the vertical transport of air and water, and its evolution within deep tropical convection. The overarching goal of the INCUS mission is to understand why, when and where tropical convective storms form, and why only some storms produce severe weather. INCUS is led by PI Susan van den Heever of Colorado State University (CSU), in collaboration with NASA/Jet Propulsion Laboratory (JPL), Blue Canyon Technologies, and Tendeg Systems. INCUS consists of a series of three small satellites, to be launched from a single Firefly Alpha and fly in formation, each carrying a Ka-band radar based on RainCube and one cross-track scanning radiometer based on TEMPEST. A novel time-differencing approach among the three satellites flown in close succession (30, and 90, and 120 seconds apart) will provide the first estimates of convective mass flux across the tropics.

The selection of the INCUS as a NASA Earth Venture Mission (EVM-3) is based in part on the prior success of two pathfinder CubeSat missions: RainCube, the first weather radar on a CubeSat, led by NASA/JPL, and the Temporal Experiment for Storms and Tropical Systems – Demonstration (TEMPEST-D) mission, led by CSU, producing the first global (up to 58 degrees latitude) observations from a multi-frequency microwave radiometer on a CubeSat, operating for nearly three years in LEO.

In 2007, the Global Precipitation Mission (GPM) Intersatellite Radiometer Calibration Working Group (XCAL) was formed to provide a quantitative basis for consistent calibration of passive microwave sensors operating in Low-Earth Orbit (LEO) in the GPM constellation. In general, measurements from various passive microwave sensors can be compared over homogeneous Earth scenes with spatial and temporal collocation. However, the passive microwave sensors in the GPM constellation are heterogeneous not only in sensor design and manufacturing but also in their center frequencies, polarizations, bandwidths, spectral responses, scanning patterns, beam sizes, etc. To compensate for these sensor differences, the double-difference method was developed. First, a radiative transfer model is used to simulate theoretical brightness temperatures that would be measured by each of two specific sensors, the difference between which would be equal to the inter-sensor bias if there were no radiometric calibration errors. Then the two observed brightness temperatures from each of these sensors are subtracted from their corresponding theoretical brightness temperatures, forming two single differences. Finally, the two single differences are subtracted from each other, removing any common bias due to radiative transfer model errors and yielding the double difference. This double difference provides a quantitative basis to determine the radiometric calibration error between any two passive microwave sensors.

The INCUS Dynamic Microwave Radiometer (DMR) is a build-to-print cross-track scanning passive millimeter-wave radiometer based on the TEMPEST-D and TEMPEST-H8 sensors. As such, the double-difference technique is being applied to TEMPEST-D and TEMPEST-H8 using the GPM Global Microwave Imager (GMI) and the Microwave Humidity Sensors (MHS) on NOAA and EUMETSAT satellites. The calibration coefficients resulting from the double-difference technique for DMR will be used to include observations from the other sensors in the GPM constellation in INCUS mission science and provide much greater spatial and temporal coverage beyond that available from the DMR sensor alone.

TEMPEST-D, a NASA Earth Venture Technology mission, produced global atmospheric science data, a well-calibrated, highly stable radiometer over three years of operations. TEMPEST-D brightness temperatures were validated using scientific and operational microwave sensors, including GPM/GMI and four MHS sensors, operating at similar frequencies to TEMPEST-D channels at 87, 164, 174, 178 and 181 GHz. Using the double-difference approach, TEMPEST-D performance was shown to be comparable to or better than much larger scientific and operational sensors, in calibration accuracy, precision, stability and instrument noise, during its nearly 3-year mission.

A duplicate TEMPEST sensor produced alongside TEMPEST-D was integrated with the Compact Ocean Wind Vector Radiometer (COWVR) from NASA/JPL and launched by the U.S. Space Force to demonstrate low-cost space technologies to improve global weather forecasting. COWVR/TEMPEST were launched on the STP-H8 mission on December 21, 2021, and have performed coordinated observations of Earth’s oceans and atmosphere from the ISS since January 7, 2022. Retrievals of water vapor profiles, clouds, and precipitation from COWVR/TEMPEST-H8 are being performed in collaboration between JPL and CSU.

Previous studies have validated the accuracy and precision of TEMPEST-D brightness temperatures using clear-sky oceanic observations. Recent advances extended the validation of TEMPEST-D and TEMPEST-H8 brightness temperature observations over tropical cyclones using GPM/GMI brightness temperatures and GPM/DPR vertical cumulative reflectivity.

Prior studies demonstrated accurate quantitative precipitation estimation using machine learning over CONUS. Recent advances expanded this capability to a global basis using GPM/GMI and AMSR-2 datasets for training and validation and IMERG rain rates for cross comparison. The heritage of TEMPEST-D and TEMPEST-H8 will be used to demonstrate the potential for remote sensing of precipitation from the Dynamic Microwave Radiometer on the INCUS mission.