Calibration and Validation of Microwave Atmospheric Sounders on CubeSats and Small Satellites for Applications in Weather Prediction and Climate Monitoring
Authors: Reising Steven C. , Berg Wesley , Brown Shannon T. , Gaier Todd C. , Kummerow Christian D. , Chandrasekar Venkatchalam , Padmanabhan Sharmila , Lim Boon H. , Schulte Richard , Goncharenko Yuriy , Radhakrishnan Chandrasekar 
Published: March 2020

&lt;p&gt;Passive microwave radiometer systems have provided both temperature and water vapor sounding of the Earth&amp;#8217;s atmosphere for several decades, including MSU, AMSU, MHS, ATMS, etc.&amp;#160; Due to its ability to penetrate clouds, dust, and aerosols, among global datasets, microwave atmospheric sounding provides the most valuable quantitative contribution to weather prediction.&amp;#160; Long-term, well-calibrated sounding records can be indispensable for climate measurement and model initialization/validation.&amp;#160; Hence, passive microwave sounders are deployed on large, operational satellites and operated by NOAA, EUMETSAT and other similar national/international organizations.&lt;/p&gt;&lt;p&gt;In the past five years or so, advances in CubeSats and other small satellites have enabled highly affordable space technology, providing access to space to private industries, universities and smaller nations.&amp;#160; This provides a valuable opportunity for organizations such as NOAA and EUMETSAT to explore the added value of acquiring data from passive microwave sounders on small, low-cost spacecraft for relatively small investments, both for sensor and spacecraft acquisition and launch.&amp;#160; This provides the potential for deployment of constellations of low-Earth orbiting microwave sounders to provide much more frequent revisit times than are currently available.&lt;/p&gt;&lt;p&gt;For passive microwave sounding data to be valuable for weather prediction and climate monitoring, each sensor needs to be calibrated and validated to acceptable accuracy and stability.&amp;#160; In this context, the first CubeSat-based multi-frequency microwave sounder to provide global data over a substantial period is the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) mission.&amp;#160; This mission was designed to demonstrate on-orbit capabilities of a new, five-frequency millimeter-wave radiometer to enable a complete TEMPEST mission using a closely-spaced train of eight 6U CubeSats with identical low-mass, low-power millimeter-wave sensors to sample rapid changes in convection and surrounding water vapor every 3-4 minutes for up to 30 minutes.&amp;#160; TEMPEST millimeter-wave radiometers scan across track and observe at five frequencies from 87 to 181 GHz, with spatial resolution ranging from 25 km to 13 km, respectively.&lt;/p&gt;&lt;p&gt;The TEMPEST-D satellite was launched on May 21, 2018 from NASA Wallops to the ISS and was successfully deployed on July 13, 2018, into a 400-km orbit at 51.6&amp;#176; inclination.&amp;#160; The TEMPEST-D sensor has been operating nearly continuously since its first light data on September 5, 2018.&amp;#160; With more than 16 months of operations to date, TEMPEST-D met all of its Level-1 mission objectives within the first 90 days of operations and has successfully achieved TRL 9 for both instrument and spacecraft systems.&amp;#160;&lt;/p&gt;&lt;p&gt;Validation of observed TEMPEST-D brightness temperatures is performed by comparing to coincident observations by well-calibrated on-orbit instruments, including GPM/GMI and MHS on NOAA-19, MetOp-A and MetOp-B satellites. Absolute calibration accuracy is within 0.9 K for all except the 164 GHz channel, well within the required 4 K for all channels. Calibration stability is within 0.5 K for all channels, also well within the 2 K requirement. TEMPEST-D has NEDTs similar to or lower than MHS. Therefore, although the TEMPEST-D radiometer is substantially smaller, lower power, and lower cost than operational radiometers, it has comparable performance, i.e. instrument noise, calibration accuracy and calibration stability.&lt;/p&gt;