PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = GO INSTRUMENT_ID = PPR OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "PHOTOPOLARIMETER RADIOMETER" INSTRUMENT_TYPE = "PHOTOPOLARIMETER RADIOMETER" INSTRUMENT_DESC = "The Galileo photopolarimeter radiometer (PPR) is a hybrid instrument consisting of a visual/near infrared photometer and polarimeter, with additional capability to measure thermal infrared radiation. A single circular field of view is used. The wavelength observed is determined by the position of a rotating filter wheel. A silicon photodiode serves as detector for the visible and near-IR bands, while a pyroelectric detector is employed for the thermal IR and bolometric bands. The PPR is a 5 kg instrument with a 10 cm cassegrainian Dall-Kirkham telescope. The overall dimensions of the PPR are 45 X 19 X 33 cm. Scientific Objectives ===================== The primary science objectives and anticipated results of the PPR experiment are to: (1) determine the vertical and horizontal distribution of cloud and haze particles in the atmosphere of Jupiter, including their size, shape and refractive index; (2) Determine the energy budget of Jupiter and the variations in amount and spatial distribution of reflected solar radiation and emitted thermal radiation for Jupiter and its satellites, including the thermal structure of the atmosphere and the vertical distribution of absorbed solar radiation in the atmosphere of Jupiter; (3) Measure and map the photometric, polarimetric and radiometric properties of the major satellites of Jupiter. Instrument Calibration ====================== As part of the instrument calibration, preliminary measurements were made to characterize the off-axis light rejection of the PPR optical system. A more complete determination of stray light rejection as a function of angle from the PPR optical axis will be performed during inflight calibration opportunities (including also the boresighting of the PPR with respect to other Galileo instruments) by viewing such astronomical objects as stars and spatially unresolved planets. Inflight calibration of flux is performed with a black radiometric cal target on the spacecraft. It also carries a visible light source that serves as a reference standard for the shortwave channels. There is a photometric cal target on the spacecraft that can be viewed by all the remote sensing instruments. It is illuminated by the sun. The PPR can also observe stars as flux standards in the visible and near IR. Operational Considerations ========================== It should be noted that for certain scan platform pointing directions (below about 90 degrees cone angle), spacecraft booms on the spinning part of the spacecraft sweep periodically through the fields of view of all remote sensing experiments. It is necessary to be aware of these effects and remove them either by examination or by use of a map of their locations. Detectors ========= Visual and Near-Infrared ------------------------ DETECTOR_ID = VISUAL DETECTOR_TYPE = Si MINIMUM_WAVELENGTH = 0.41 MAXIMUM_WAVELENGTH = 0.945 The PPR uses two silicon photodiodes for the visual and near-infrared region (out to 945 nm). The measured signal to noise ratio for the polarimetry channels exceeds the science-dictated minimum of 1000. For the photometry channels, the requirement of 200 is exceeded. The Jovian albedo of Woodman et al (1979) was used to set gain levels. Thermal Infrared ---------------- DETECTOR_ID = IR DETECTOR_TYPE = LiTaO3 MINIMUM_WAVELENGTH = 14 MAXIMUM_WAVELENGTH = 110 The PPR uses for the thermal infrared region a lithium tantalate pyroelectric detector. Sensitivity for the radiometry channels was affected by low detector long-wavelength responsivity and low filter transmission. The signal to noise ratio for a 130K target was measured to be 30 at 17 microns; 20 at 21 microns; 41 at 27.5 microns; 12 at 37 microns; 33 at >42 microns; and 460 for the solar + thermal band with thermal input only. Electronics =========== The low power electronics for the PPR instrument retains much of the radiation-proven circuitry from the predecessor instruments built for the Pioneer missions, but most of the discrete control logic has been replaced by a radiation-hard CMOS microprocessor system. The analog circuitry consists of two silicon photodiode polarimetry/photometry channels, one pyroelectric radiometry channel, and an analog multiplexer which presents the three signal channels and the temperature telemetry channels to the 12-bit analog to digital converter. The digital circuitry decodes the serial spacecraft commands and formats the instrument signal and telemetry data for transfer to the spacecraft via the command and data bus. The digital system also provides timing signals for analog channel and mechanism controls. The power subsystem conditions the 30 vdc spacecraft input power and provides the necessary regulated and filtered voltages for instrument operation. This subsystem also contains the power driver circuitry for the actuator controls and calibration lamp. Filters ======= Wavelength Number Name Type Minimum Center Maximum ----------------------------------------------------------------------- 1 UNK MULTILAYER INTERFERENCE 0.9392 0.9446 0.950 7 POLARIMETRY MULTILAYER INTERFERENCE 0.6741 0.6785 0.6828 13 POLARIMETRY MULTILAYER INTERFERENCE 0.3800 0.4100 0.4400 25 PHOTOMETRY MULTILAYER INTERFERENCE 0.6152 0.6187 0.6222 26 PHOTOMETRY MULTILAYER INTERFERENCE 0.6290 0.6333 0.6376 27 PHOTOMETRY MULTILAYER INTERFERENCE 0.6443 0.6480 0.6517 28 PHOTOMETRY MULTILAYER INTERFERENCE 0.7827 0.7887 0.7946 29 PHOTOMETRY MULTILAYER INTERFERENCE 0.8233 0.8293 0.8352 30 PHOTOMETRY MULTILAYER INTERFERENCE 0.8367 0.8403 0.8438 31 PHOTOMETRY MULTILAYER INTERFERENCE 0.8862 0.8918 0.8973 24 SOLAR_THERMAL UNK 0.3 55 110 23 SOLAR UNK 0.3 2 4 18 A UNK 14.7 16.8 18.9 19 B UNK 19.5 21.0 22.5 21 C UNK 23.9 27.5 31.1 20 D UNK 32.1 35.5 38.9 22 E UNK 45 77 110 Optics ====== The PPR is a multi-purpose and multi-function instrument designed for both solar wavelength photopolarimetric and thermal infrared measurements. The 10-cm aperture scene-view telescope is a cassegrainian Dall-Kirkham design which gives excellent image quality for the 2.5 mrad (0.14 inch) instantaneous field of view, with the image quality being dominated by diffraction at the longer wavelengths. The view of space in radiometry mode is via reflection from the chopper mirror and the planar space-view telescope mirror. Located on the filter/retarder wheel are spectral filters used to define the required spectral bandpasses, the halfwave retarders used for the polarization analysis of the scene, and the ellipsoidal mirrors used to direct scene flux towards the radiometry detector. Mounting Offset =============== The PPR is mounted on the Galileo scan platform, nominally coaligned with the SSI, NIMS, and UVS instruments. Field of View ============= The PPR instrument has a circular field of view, with a diameter of 2.5 mrads. Operational Modes ================= Radiometry ---------- During radiometry measurements, the chopper is operated at 30 Hz and alternately directs the flux from the scene-view and space-view telescopes through the field stop. At each radiometry position the flux passed by a radiometry filter is reflected radially outward from the filter/retarder wheel by means of an ellipsoidal mirror (one mounted on the wheel at each of the seven radiometry positions). The flux reflected by the ellipsoidal mirror is collected by a condenser system consisting of a truncated conical reflector with a small diamond lens mounted onto the small end of the cone. The focused, modulated flux is detected by a lithium tantalate pyroelectric detector. The alternate views from the scene-view and space-view telescopes allow the scene radiance to be referenced to the space background (approximately to a 3K blackbody). There are four optical elements designated as 'radiometric stops' that serve to restrict the modulated flux reaching the detector to that received from the scene or space, or from the internal instrument elements which are radiometrically 'balanced' to first order. These radiometric stops, together with careful pre-flight calibration, temperature monitoring of key optical elements, and an accurately-monitored, high emittance inflight calibration target (the RCTPPR), are key to performing radiometrically useful measurements on a 'cool' scene, such as Jupiter, with the 'warm' PPR instrument. Polarimetry and Photometry -------------------------- For polarimetry and photometry measurements, the flux from the scene is collected by the scene-view telescope and focused onto a circular field stop subtending 2.5 mrad. Flux passed by the field stop then is modified by passage through optical elements located on the filter/retarder wheel. At the polarimetry positions, the flux passes through a halfwave retarder and a spectral filter, while at photometry positions it passes only through a filter. The relay lens directs the flux through the Wollaston prism which serves as a polarizing beam-splitter and produces two spatially-separated and orthogonally-polarized output beams. The detector lenses focus these beams onto the two silicon photodiodes. For polarimetry and photometry measurements the chopper is stationary and is positioned so as not to block the scene flux." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "RUSSELLETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT OBJECT = REFERENCE REFERENCE_KEY_ID = "RUSSELLETAL1992" REFERENCE_DESC = "Russell, E.E., F.G. Brown, R.A. Chandos, W.C. Fincher, L.F. Kubel, A.A. Lacis, and L.T. Travis, Galileo Photopolarimeter/Radiometer Experiment, Space Sci. Rev. 60 p. 531-563, 1992." END_OBJECT = REFERENCE END