Photopolarimeter Radiometer PPR

The Galileo PPR used a 10 cm diameter Cassegrain telescope to observe a 2.5mrad diameter circular FOV in a number of photometric, polarimetric, and radiometric channels. Polarimetry was done in three spectral channels at 410 nm, 678 nm, and 945 nm, while photometry was done in seven narrowband channels at 619 nm, 633 nm, 648 nm, 789 nm, 829 nm, 840 nm, and 892 nm, which covered various important methane and ammonia absorption features. PPR also had seven radio-metry bands. One of these used no filters and observed all the radiation, both solar and thermal, while another let only solar radiation through (i.e., wavelengths less than 4 ^m). The difference between the solar-plus-thermal and the solar-only channels gave the total thermal radiation emitted. Five further broadband channels were included at 17 ^m, 21 ^m, 27 ^m, 36 ^m, and >42 ^m. Unfortunately, for a substantial part of the mission the PPR filter wheel became stuck, which somewhat limited the data return from this instrument.

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