| Abstract: |
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. 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 needed to reside inside a centralized radiation vault and the large antennas were up to 1.5m away on the outward facing side of the spacecraft. Producing calibrated antenna temperatures requires a correction for up to 3dB of front-end loss with a 150K temperature gradient. In other words, 50% of the received signal at the internal calibration plane originates in the front-end of the radiometer and a parameterized hardware radiative transfer model is needed to transfer the reference plane to the input of the antenna. 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.
In this talk, we will review the radiometer design and the unique calibration approach that was required due to the size, distributed nature and complex thermal environment of the system. We will discuss in-flight performance through the 5-year cruise phase and the first 11 science orbits. We will show that better than 2% absolute calibration and 0.1% relative calibration was achieved. We will show comparisons of in-flight derived antenna patterns using the planet as a point source and pre-launch models that predicted the pattern at the antenna’s 150K operating temperature. We will compare the predicted/measured magnetic susceptibility of the design to the derived in-flight estimates. These results are relevant to internally calibrated radiometers in general and in particular to the notional P-band and L-band radiometers envisioned for future Earth science missions. Finally, we will share some of the initial science results from the MWR. These include pole-to-pole atmospheric soundings revealing the depth of the belts and zones on the planet, microwave images of the Great Red Spot showing the 3-D structure of the storm and the inferred global distribution of moist convection.
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