Performance and Results from the Juno Microwave Radiometer

Juno is a New Frontiers mission to study Jupiter and carries as one of its payloads a six-frequency microwave radiometer to perform atmospheric sounding of the Jovian atmosphere to pressures of approximately 250 bars [1]. Juno was launched from Kennedy Space Center on August 5, 2011 and reached Jupiter orbit on July 4, 2016. The Microwave Radiometer (MWR) operates from 600 MHz to 22 GHz and was designed and built at the Jet Propulsion Laboratory. Because the mission is operating in the harsh Jovian radiation environment, it required an ambitious radiometer design that pushed the limits for an internally calibrated radiometer. The MWR uses noise diodes and a PIN-diode Dicke switch located inside the receiver. Typically, internally calibrated radiometers are designed to minimize the loss and the temperature gradient between the antenna and the noise calibration sources. However, for the MWR, the receivers with the calibration sources needed to reside inside a centralized radiation vault and the large antennas were up to 1.5 m away on the outward facing side of the spacecraft. Producing calibrated antenna temperatures requires a correction for up to 3 dB of front-end loss with a 150 K temperature gradient. In other words, 50% of the received signal at the internal calibration plane originates in the front-end of the radiometer. Additionally, the MWR is operating on a spacecraft rotating through the strong Jovian magnetic field, requiring targeted magnetic shielding of the ferrite isolator inside the receiver.