Single-Point Calibration for Microwave Sounders: Application to TEMPEST-D

Passive microwave sounders are critical for accurate forecasts from numerical weather prediction models. These sensors are calibrated using a traditional two-point approach, with one source typically a free-space blackbody target and the second a clear view to the cosmic microwave background, commonly referred to as “cold space.” Occasionally, one or both of these calibration sources can become corrupted, either by solar/lunar intrusion in the cold space view or by thermal instability of the blackbody calibration source. A Temporal Experiment for Storms and Tropical Systems (TEMPEST) microwave sounder instrument is currently deployed on the International Space Station (ISS) for a 3-year mission. TEMPEST is also calibrated using a blackbody target and cold space view; however, the cold space view will be routinely obstructed by objects present on the ISS. Here we test an alternative single point calibration methodology that uses only the blackbody calibration target. We find the brightness temperature difference between this new approach and the traditional two-point calibration approach to be < 0.1 K when applied to 3 years of the TEMPEST CubeSat Demonstration (TEMPEST-D) mission data from 2018-2020. This approach is applicable to other microwave radiometers that experience occasional degradation of calibration sources, such as thermal effects, intrusions or instability of noise diodes.