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Availability: Archived at NSSDC, accessible from elsewhere

Time span: 1991-10-29 to 1991-10-30


This description was generated automatically using input from the Planetary Data System.

This data set contains the RDR data for the Galileo Orbiter PPR instrument for the period corresponding to the Gaspra asteroid encounter observations in October 1991. Data Set Overview ================= As described in the Project Galileo Experiment Data Record Software Interface Specification (cf. ASCII and word processor text versions in DOCUMENT subdirectory), the Galileo Data Management System generates Low Rate Science (LRS) EDR files for eight Galileo Orbiter instruments including the PPR. EDR files are organized into blocks of data corresponding to a major frame, or one RIM cycle consisting of 91 minor frames. The PPR generates 18 bytes of instrument data for each 2/3-sec interval corresponding to one minor frame count. For each of the 18-byte PPR minor frame records, the first six bytes 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. Data files of the R_EDR archive are a reformatted version of the PPR EDR data and tabulate all of this data in an ASCII Table format. The present Reduced DR (RDR) archive is also an ASCII Table format with the same number of records (rows) as the R_EDR files for the respective data sets, but with science data number values converted to reduced thermal radiometry brightness temperatures and polarimetry-photometry radiances and linear polarization degree and direction as appropriate. The detached PDS label for each file specifies the record format and describes in detail each parameter of the record. NOTE = The following is from the PPR instrument paper as Published in Space Science Reviews, Vol. 60, by Kluwer Academic Publishers. The complete document is located at /DOCUMENT/PPR_INST/PPR.HTM. 4.2. INFLIGHT CALIBRATION Several means of inflight calibration will be utilized by the PPR to update the preflight calibration of thephotopolarimetry channels and to provide the prime radiometric calibration of the radiometry channel. These include: (1) an internal calibration lamp within the PPR aft optics; (2) the third orientation position for each of the halfwave retarders which interchanges the roles of the two silicon detectors; (3) the radiometric calibration target (RCT-PPR) that is separately mounted on the spacecraft and can function either as a blackbody source or to provide a slightly polarized lamp output signal for the visible/near-infrared region; (4) a spacecraft supplied photometric calibration target (PCT) that provides a standard of spectral radiance that can be viewed by all instruments located on the spacecraft scan platform; and (5) viewing stars and spatially unresolved planets. 4.2.1. Internal Cal Lamp The small tungsten filament lamp located within the aft optics of the PPR provides a means to track any changes with time of the silicon detector/amplifier channels. The spectral output of the lamp is modified with the use of a color glass filter (Schott BG-18). The stability of this long-life (50 000-hour) lamp is further enhanced by operating it only intermittently, at a derated power level, and with a controlled, slow turn-on characteristic. The lamp is energized intermittently only during cycle mode operation while the chopper is being driven to its rest position following radiometry mode sampling at the solar + thermal filter wheel position and if the cal lamp command bit is set to the ON state. (The estimated total on time is less than 200 hours during the 7-year pre-launch testing period plus the post-launch inflight period.) 4.2.2. Internal Polarimetric Calibration As previously discussed, one key feature for achieving accurate polarimetry with the PPR is the ability to cross-calibrate the detectors by measuring simultaneously the orthogonal polarization components of the scene radiance. The third halfwave retarder position (with fast axis oriented at 45- to the plane of deviation of the Wollaston prism) effectively interchanges the scene polarization components incident on the two detectors. This permits maintaining the polarimetric accuracy even in the presence of slow relative changes of the two detector channels with time. temperature, radiation, etc. 4.2.3. RCT-PPR Design The RCT-PPR will serve in a dual calibration role for the PPR. The primary role will be as a thermal calibration target which closely approximates a blackbody source when viewed along the RCT-PPR axisymmetric axis. Due to spacecraft space limitations, it was necessary to restrict the overall length of the target. To achieve the desired normal emittance (e > 0.998) the geometry of the interior portion of the target has a truncated conical form with a center cylindrical section. This provides on-axis performance approximately equivalent to a cone with half the apex angle and twice the overall length. The end of the central cylindrical portion is not viewed by the PPR since this area is within the central obscuration of the PPR telescope. The interior of the target is a smooth (specularly reflecting) black-painted surface to achieve a higher on-axis emittance than would be possible with a rough (diffuse) surface for the same geometry. The calculated on-axis emittance of the RCT-PPR is greater than 0.998 based on the reflectance versus incident angle for the interior surfaces. The RCT-PPR is designed and mounted such that it will be passively cooled at Jupiter to a temperature of 145 ~15 K. The wall thickness is chosen to assure worst case temperature gradients of less than 0.5 K. The temperature of the RCT-PPR is monitored by two platinum resistance thermometers (PRTs) that are calibrated by the manufacturer (resistance versus temperature) to an accuracy 0.2 K. These PRTs are read out directly by the PPR along with a low temperature coefficient resistor also mounted on the RCT-PPR to allow a first-order correction for spacecraft cabling resistance. The annular aperture of the target is designed to accommodate the 3-a worst case relative misalignments resulting from possible spacecraft environmental and mounting factors specified by the Galileo Project to assure that the PPR will view only the high emittance portion of the target during calibration. Through the use of the RCT-PPR and the preflight calibrations used to assess the influence of temperature changes of the PPR optical elements, it is expected that the overall radiometric calibration of the PPR thermal bands can be maintained within the desired ~1 K over the duration of the Galileo Mission. A small tungsten-filament lamp is mounted in one portion of the RCT-PPR interior surface. With the source commanded ON, flux from the source passes through an elliptically shaped, plane-parallel sapphire plate mounted such that the outer surface approximately conforms to the inner conical surface of the target. The flux transmitted to the PPR is partially polarized due to different S and P Fresnel reflectances of the inclined plate. Thus, this source will be useful in assessing possible photometric and polarimetric changes of the entire optical train of the PPR over the course of the mission. 4.2.4. Spacecraft PCT The PCT is intended to serve as a standard of spectral radiance for the scan platform mounted instruments by reflecting sunlight from a diffusely reflecting surface with well-characterized reflectance properties. Since this target can be viewed by all scan platform mounted instruments, the PCT is expected to be particularly useful in the role of calibration intercomparison among instruments. 4.2.5. Viewing Astronomical Objects Orienting the PPR to view such astronomical objects as stars or spatially unresolved planets will be used to provide both cross-check of the absolute photometric calibration of the PPR silicon photodiode channels and as an additional means to track any responsivity changes with time, temperature, radiation, etc. Sirius is a star which will provide an adequate signal-to-noise ratio for this purpose by aggregating a sufficient number of samples. Similarly, viewing unresolved (object subtending less than the PPR field of view) planets at phase angles accessible from Earth will allow intercomparisons to be made with ground-based photometric calibration. 4.3. SIGNAL-TO-NOISE PERFORMANCE 4.3.1. Photopolarimetry Channels Four separate band gains are used for the photopolarimetry channels, with the value applied (as described in the electronics section) being dependent on the filter/retarder wheel position as determined by the encoder. This will provide signal outputs of similar magnitude for the three polarimetry and seven photometry bands for typical scene spectral radiances. The channel and band gains were set to provide signal levels at Gain Step 8 of approximately 2000 DN for the three polarimetry bands (each with a separate band gain) and of approximately 1500 DN for the 648 nm photometry band (a single band gain is applied to all seven photometry bands). For setting these levels, the Jovian albedo values of Woodman et al. (1979) were used. The noise of the photopolarimetry channels is essentially independent of signal level, resulting primarily from the 100 megohm feedback resistors in the pre-amplifiers. As a result, the signal-to-noise ratio (SNR) varies only slowly with temperature over the PPR operating temperature range (since the Johnson noise varies as the square root of the absolute temperature). The measured SNR performance of the PPR photopolarimetry channels substantially exceeds the science-dictated, minimum SNR requirements of 1000 for the polarimetry bands and 200 for the photometry bands. 4.3. 2. Radiometry Channel Achieving the SNR performance desired for the PPR science investigations utilizing the radiometry channel produces far greater stress on instrument design that is the case for the photopolarimetry channels. The inevitable Galileo mission mass and size constraints on science instruments required substantial compromise on performance characteristics. To optimize the SNR performance of the pyroelectric detector required thinning the LiTaO4 detector element to the maximum extent possible. For the PPR application, ion-beam milling was used to provide thicknesses in the 5 to 6 Ym range. In order to provide good optical absorption with low mass, the detector was coated with an evaporated gold-black coating. The wide range of absorbing characteristics found in the literature for gold blacks increased the risk with this approach, but on balance offered the best overall choice for the PPR requirements. Optimization curves for the noise components of the PPR pyroelectric detector is illustrated in Figure 11. Measured noise data for the PPR detector were near the levels predicted. However, relative spectral response measurements indicated levels substantially below specification. The lower than expected long wavelength responsivity, combined with the lower than specified filter transmittance for some of the filters (much lower for the 37 Ym band) led to the inability to meet the instrument SNR performance specifications for four of the seven radiometry channel bands. The measured versus specified SNR performance is indicated in Table VI. The solar plus thermal band measurement tabulated includes only the thermal component; the solar band is not included in the table, but comfortably exceeds the specifications. Three of the four out-of-spec bands have SNR performances about 60% of specification, while the 37 Ym band (D filter) is about one-third of the desired level. Fortunately the mission profile and the flexibility designed into instrument operation allows for observational 'work-arounds' to achieve nearly all of the anticipated science goals. The obvious approach of increasing the number of samples to improve the SNR (by the square root of the increase factor) is the principal observation strategy to achieve the radiometry science goals.

These data are available on-line from the Planetary Data System (PDS) at:

Alternate Names



  • Planetary Science: Fields and Particles

Additional Information



Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II



NameRoleOriginal AffiliationE-mail
Dr. James E. HansenData ProviderNASA Goddard Institute for Space
Dr. Larry D. TravisGeneral ContactNASA Goddard Institute for Space
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