IceCube: 883-GHz Cloud Receiver and Calibration on a Spinning, Thermally-Stabilized CubeSat

Ice clouds play a key role in Earth's radiation and water budget in the upper troposphere. Accurate cloud ice measurements require good instrument sensitivity to volume scattering and microphysical properties of ice particles, and submillimeter-wave radiometry is a promising technique for this cloud remote sensing. IceCube will demonstrate an 883-GHz receiver technology on 3-U CubeSat to enable accurate cloud ice measurements in future science missions. The IceCube cloud radiometer requires accurate (<2 K) radiometric calibration from a low Earth orbit (LEO) environment similar to the International Space Station (ISS). However, large thermal variations and limited CubeSat resources create great engineering challenges in meeting this requirement. The IceCube team pioneers innovative solutions for calibrating a "free-running" radiometer, using a noise injection circuit to monitor the post-mixer gain, and a spinning CubeSat to acquire the space radiance periodically. In this approach, the mixer gain and its temperature dependence will be characterized from preflight laboratory measurements, while the large post-mixer gain variation will be calibrated from on-flight operation. During the nominal flight operation, to assure the receiver calibration quality, the mixer ambient temperature will be thermally stabilized to 20 ± 2°C with passive paraffin packs. Preliminary testing of the instrument engineering model (EM) is very encouraging, indicating that the proposed calibration approach should work well for a wide range of thermal conditions. The new calibration approach will greatly simplify the system design and radiometric calibration of spaceborne microwave radiometers. More results from instrument TVAC testing will be presented, along with the IceCube operation concept and verification plan of the 883-GHz radiance measurement. The IceCube project is supported by NASA ESTO and SMD/ATIP programs.