The Radiometer Atmospheric Cubesat Experiment Post-Launch Results

Abstract:

The Jet Propulsion Laboratory (JPL) developed the Radiometer Atmospheric CubeSat Experiment (RACE) that was lost during the Orbital 3 (Orb-3) launch anomaly on October 28, 2014. The 3U CubeSat mission would have measured 2 channels of the 183 GHz water vapor line and raised the technology readiness level (TRL) of various subsystems to 6. Despite the launch failure, several hundreds of hours of instrument operation data was taken, including measurements in thermal vacuum of the complete spacecraft system. These data is used to evaluate the 35 nm Indium Phosphide (InP) receivers, and the low noise amplifier (LNA) based internal calibration system.

The thermal vacuum measurements included frequent observations of a ‘cold’ and ‘hot’ target allowing for various receiver parameters to be calculated. The payload thermal vacuum data show that the receiver front ends performed as expected in terms of the gain (>35 dB) and drift (0.06 dB/K). The data also shows that integration could be performed with decreasing noise up to ~30 seconds, allowing for the system to be calibrated within that time period. The expected spacecraft calibration period would have been every 12 seconds.

The injected noise from the load terminated LNA show magnitudes from 50 – 150 K that can be tuned which would meet most requirements. However the temperature coefficient is large at ~3 K/K which is over an order of magnitude larger than typical noise diodes. For nanosatellite class spacecraft, the power required to properly maintain the physical temperature range (±0.1K) would be challenging. On larger spacecraft, this methodology may still be viable, depending on the availability of suitable noise diodes at 183 GHz.

While the CubeSat did not take measurements in space, the ground data in the relevant environment and extensive testing allows us to raise the following subsystems to TRL 6: 1) 183 GHz 35 nm InP receiver, 2) 183 GHz direct detect receiver and 3) 183 GHz LNA based calibration system.