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 = " Instrument Overview =================== The Galileo Photopolarimeter Radiometer (PPR) is a multi-purpose and multi-function instrument designed for both solar wavelength photopolarimetry and thermal infrared sounding and radiation budget measurements. Mounted on the Galileo Orbiter scan platform along with the SSI, NIMS, and UVS instruments, the PPR scene telescope has a single circular field of view of 2.5 mrad and uses scan platform slews and mosaic sequences to map out larger regions. The wavelength observed is determined by the position of a rotating filter wheel with 32 equally spaced positions (numbered 0 through 31). Positions 0 - 17 are used for three polarimetry wavelengths, 18 - 24 for seven thermal radiometry bands, and 25 - 31 for seven photometry bands. For the positions corresponding to polarimetry and photometry bands, the scene flux is directed by a relay lens through a Wollaston prism that splits the input into two orthogonally-polarized output beams, which are then focused onto a pair of silicon photodiode detectors. At each of the thermal radiometry positions, a chopper mirror alternately directs the scene flux and the space-view reference flux through the filter and then from an elliptical mirror on the back of the filter wheel to a pyroelectric detector. The PPR is a 5-kg instrument with overall dimensions of 45 x 39 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 ====================== Various preflight and inflight tests of the PPR are performed to characterize the instrument performance and calibration. Principal preflight tests included radiometric calibration of solar wavelength channels with standard lamps, polarization accuracy and instrumental polarization determination using various sources with known polarization, and radiometric calibration of thermal channels using blackbody calibration units with controllable temperatures in a thermal vacuum chamber environment. Inflight radiometric calibration is performed with a radiometric calibration target (RCT-PPR) that is separately mounted on the spacecraft and viewed by the PPR at zero cone angle. The RCT is passively cooled to a nominal temperature near 140 K, with the actual temperature monitored by two platinum resistance thermometers connected to the instrument so that their readouts can be included in the PPR science data stream. A small tungsten-filament lamp is mounted in the RCT interior with an elliptically shaped, sapphire cover plate mounted such that its outer surface approximately conforms to the inner conical surface of the RCT. When this lamp is commanded on, it serves as a radiometric and polarimetric reference standard for the shortwave channels. There is a photometric calibration target on the spacecraft that can be viewed by all the remote sensing instruments, thus providing a visible and near-IR radiometric reference by solar illumination of its well- characterized diffusely reflecting surface. Along with the other scan platform instruments, the PPR can use observations of relatively brighter stars as flux standards in the visible and near infrared. In addition, such observations are employed to characterize the off- axis light rejection of the PPR optical system and to determine the precise boresighting offsets between the scan platform instruments. Operational Considerations ========================== The PPR is designed so that it may be commanded to operate in several different modes and with selectable configurations within each of those modes. The selected mode and configuration parameters control the specific cycling of the filter wheel and thus the set of bands at which measurements are acquired during a single cycle. With such flexibility in determining the operational mode, it is possible to select a measurement cycle that is optimized for various spacecraft scan platform slew rates as well as providing measurement sets other than the default of cycling through all the PPR bands when science objectives so dictate for particular targets or circumstances. Basic instrument operational functions follow two very different processes depending on whether the measurements are photopolarimetry or thermal radiometry. 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. During polarimetry and photometry measurements, the chopper is kept stationary and is positioned so as not to block the scene flux. The amplified detector outputs are converted to a pair of 12-bit samples with dark-reading offsets of 190 DN. 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 a 3 K 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 preflight calibration, temperature monitoring of key optical elements, and an accurately-monitored, high emittance inflight calibration target (the RCT-PPR), are key to performing radiometrically useful measurements on a cool scene such as Jupiter with the warm PPR instrument. The amplified detector output is converted to a 12-bit sample with a zero radiance level offset of 1400 DN. In radiometry, the second sample of the pair is the 12-bit converted readout from one of ten thermistors monitoring the temperatures of internal optical elements or one of the two platinum resistance thermometers monitoring the RCT. As radiometry measurements are acquired, the instrument cycles through the twelve temperature elements for the temperature data that are placed in the second sample of the pair, with the instrument housekeeping status identifying the specific element number. 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. The user of such data should be aware of these effects and remove them either by inspection from the artificial variation of data caused by boom obscuration or by use of a map of boom locations in spacecraft clock angle space. The PPR can be commanded to use a 'boom operation mode' in which the filter wheel is fixed at a single position for an entire spacecraft spin period. With this type of operation, the booms affect the same ranges of sample numbers within the set of samples acquired during each spin period. Detectors ========= Visible and Near-Infrared ------------------------- DETECTOR_ID = VISIBLE DETECTOR_TYPE = Si MINIMUM_WAVELENGTH = 0.36 MAXIMUM_WAVELENGTH = 0.97 The PPR uses two silicon photodiodes for the visible and near- infrared region (out to 0.97 micrometer). For typical Jovian radiances, the signal to noise ratio for the polarimetry channels exceeds 2000 and for the photometry channels is greater than 1000 except for the 0.892 methane absorption band, which is approximately 400. The Jovian albedo of Woodman et al. (1979, Icarus, v.37, 73) was used to set polarimetry and photometry gain levels. Thermal Infrared ---------------- DETECTOR_ID = IR DETECTOR_TYPE = LiTaO3 MINIMUM_WAVELENGTH = 0.3 MAXIMUM_WAVELENGTH = 110 The PPR uses a lithium tantalate pyroelectric detector for the thermal infrared sounding bands and for the broad-band bolometric channels. Sensitivity for the radiometry channels was affected by low long-wavelength responsivity of the detector and low filter transmission at several bands. The signal to noise ratio for a 130 K target was measured to be 30 at 17 micrometers; 20 at 21 micrometers; 41 at 27.5 micrometers; 12 at 37 micrometers; 33 at >42 micrometers; and 460 for the solar plus 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 and 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. Optics ====== The PPR employs a 10-cm aperture scene-view telescope of Cassegrainian Dall-Kirkham design, which gives excellent image quality for the 2.5 mrad (0.143 degree) instantaneous field of view, with the image quality being dominated by diffraction at the longer wavelengths. The alternating view of space for thermal radiometry observations 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 (on the back side of the wheel) used to direct scene flux towards the radiometry detector. Filters ======= Filter Wheel Measurement Function Center Bandwidth Position Wavelength (micrometers) (micrometers) --------------------------------------------------------------------- 1, 3, 5 Polarimetry 0.9446 0.0108 7, 9, 11 Polarimetry 0.6785 0.0087 13, 15, 17 Polarimetry 0.410 0.060 18 Radiometry, thermal 16.8 4.2 sounding - channel A 19 Radiometry, thermal 21.0 3.0 sounding - channel B 20 Radiometry, thermal 35.5 6.9 sounding - channel D 21 Radiometry, thermal 27.5 7.2 sounding - channel C 22 Radiometry, thermal -- 45-110 sounding - channel E 23 Radiometry, bolometric - -- 0.3-4.0 solar 24 Radiometry, bolometric - -- 0.3-110 solar plus thermal 25 Photometry, medium strength 0.6187 0.0070 methane absorption band 26 Photometry, continuum 0.6333 0.0086 27 Photometry, weak ammonia 0.6480 0.0074 absorption band 28 Photometry, medium strength 0.7887 0.0119 ammonia absorption band 29 Photometry, continuum 0.8293 0.0119 30 Photometry, weak methane 0.8403 0.0071 absorption band 31 Photometry, strong methane 0.8918 0.0111 absorption band Mounting Offset =============== The PPR is mounted on the Galileo Orbiter scan platform, with its scene telescope optical axis nominally coaligned with the view optical axes of the SSI, NIMS, and UVS instruments. Field of View ============= The PPR instrument has a circular field of view, with a diameter of 2.5 mrad, or 0.143 degrees. Parameters ========== When the PPR is operating, it delivers an 18-byte science and housekeeping data record to the spacecraft command and data handling system every two-thirds of a second, i.e., once each minor frame of the spacecraft clock. The first six bytes of this record are housekeeping data that completely specify the instrument status, both commanded parameters and position within operational measurement mode cycles. The remaining twelve bytes are three sets of science data sample pairs and their associated identifying parameters and parity check bit. Operational Modes ================= The commanded state of the PPR includes the selection of one of five basic operational modes: (1) cycle, (2) photopolarimetry plus photometry, (3) photometry, (4) radiometry, and (5) position select. In cycle mode, the PPR collects data at all of the photopolarimetry (polarimetry), radiometry, and photometry filter positions. Filter wheel stepping is unidirectional from low to high filter position number. In photopolarimetry plus photometry mode, the filter wheel stepping is unidirectional in the same manner as for cycle mode, but data is collected only at the polarimetry and photometry filter positions, with the instrument simply stepping through the radiometry positions. In the photometry mode, the PPR collects data only at the photometry positions, using bidirectional stepping of the filter wheel. Note that this involves stepping not only through the photometry positions, 25 - 31, but also positions 24 and 0 at which the dark current level restoration (DC-restore) function is performed. In the radiometry mode, data is collected only at the radiometry positions (18 - 24), using bidirectional stepping of the filter wheel. In the position select mode, the PPR collects data at a single filter or a contiguous series of filter positions, with the specific sequence determined by additional command parameters that specify the starting filter position and the number of additional positions (with choices of 0, 1, 2, and 5 additional positions). If more than one filter position is specified, then the filter wheel steps through those positions in a bidirectional sequence. Timing of the various PPR operations is based upon the same 30 Hz clock used for the chopper mirror, i.e., a clock cycle of 33.3 msec. The data sample integration period for polarimetry and photometry filter positions is 7 clock cycles and for radiometry positions is 14 clock cycles. Stepping the filter wheel between adjacent positions requires 6 clock cycles. In cycle mode, the start-up of the chopper at filter wheel position 18 to begin radiometry sampling and its stopping sequence at position 24 require 49 clock cycles. Various command parameters control the specific nature of the sequence within each of the PPR modes. In addition to the programmed filter position and the number of additional filter positions that control the range of positions at which data are collected in the position select mode, these parameters are as follows. GAIN STEP PP/PH: Selects gain setting for photopolarimetry and photometry channels from among 16 levels, with each being separated from adjacent levels by a factor of approximately 1.4. GAIN STEP RAD: Selects gain setting for radiometry channel from among four levels, with each being a factor of 2 greater than the next lower level. NUMBER OF SAMPLES: Selects the number of samples of data to be accumulated at each active filter wheel position before stepping to the next filter, with the choices being 1, 4, 16, and 256 samples. In position select mode, the commanded number of samples is used as the number of cycles of position select operation occurring between DC-restore sequences (if the DC-restore parameter is commanded on). NUMBER OF SAMPLES MULTIPLIER: Selects the multiplier for number of samples at radiometry positions as either 1 or 4. So if the number of samples were specified as 16 and the number of samples multiplier were 4, then 16 samples would be accumulated at each polarimetry and photometry position and 64 samples at each radiometry position. The maximum number of samples is limited to 256. A multiplexed function of this parameter is that when the external cal lamp is activated, the multiplier selects the lamp to be on for just six samples if it is set at 1 and keeps the lamp on for all the samples acquired during the spin period if it is set at 4. CAL LAMP: Enables or inhibits the preprogrammed calibrator lamp operation depending on whether the command bit is 1 or 0, respectively. Enables the external cal lamp if it is set at 1 and if the boom sequence operation parameter is enabled as well. DC-RESTORE: Initiates special DC-restore cycle in position select mode or under boom sequence operation wherein the filter wheel is stepped to the appropriate DC-restore filter wheel position, the DC- restoration function is performed followed by readout of a single dark level sample, and the filter wheel is returned to the next position of the sequence of the selected mode. BOOM SEQUENCE OPERATION: Stepping of the filter wheel between adjacent functional positions is initiated once per spacecraft spin period based upon PPR receipt of spin period data and spin reference signal from the spacecraft. The external cal lamp in the RCT is enabled only in boom sequence operation and if the cal lamp parameter is also set to enabled. TEMPERATURE RANGE SELECT: Selects the initial high or low range for thermistor temperature sensors. If the thermistor output is out-of- range for the selected case, instrument software automatically switches the range for that particular thermistor readout. Because of the differences in time required for specific steps of the instrument operation, the various operational modes of the PPR result in the generation of the 18-byte science data records at variable rates. Those rates range from just slightly slower than that at which the spacecraft performs the readout of those records (viz., once each minor frame, or two-thirds of a second) to a rate that is approximately three times slower. Accordingly, the PPR design uses two internal 18-byte buffers that are alternately filled, with one buffer being active, or in the state of being filled, and the other containing the previous 18 bytes of science data for the sequence. At the time of the each spacecraft readout of PPR data, it is only the non-active buffer that is presented and placed into the telemetry stream, and whenever that buffer has been previously transferred, the PPR sets a flag in the housekeeping data of that record to indicate that it is a 'repeat' record. Thus, the PPR data stream in general contains redundant data in the form of these repeat records. As a consequence, the spacecraft event times and scan platform pointing angles that are 'attached' to each minor frame in the spacecraft telemetry correspond to a variably later time than that at which the actual data samples in the PPR science data record for that minor frame were acquired. The basic PPR data reduction software accounts for the specific operational modes and adjusts these times and scan platform pointing angles to provide accurate values for each of the three sample pairs within the 18-byte science records. 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. Hunten, D.M., L. Colin and J.E. Hansen, Atmospheric Science on the Galileo Mission, Space Sci. Rev., 44, 191-240, 1986 Johnson, T.V., C.M. Yeates and R. Young, Space Science Reviews Volume on Galileo Mission Overview, Space Sci. Rev., 60, 3-21, 1992" END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "RUSSELLETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HUNTENETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "JOHNSONETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END