Voyager IRIS Data Set Description


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Data Set Description

    DATA_SET_NAME                  = "VG1/VG2 JUPITER IRIS 3 RDR V1.0"
    START_TIME                     = 1979-061T18:57:33Z
    STOP_TIME                      = 1979-194T02:54:32Z
    NATIVE_START_TIME              = "1630800"
    NATIVE_STOP_TIME               = "2075500"
    DATA_OBJECT_TYPE               = SPECTRUM
    DATA_SET_RELEASE_DATE          = 1994-02-14
    PROCESSING_LEVEL_ID            = 3
    PRODUCER_FULL_NAME             = "Barney J. Conrath"
    SOFTWARE_FLAG                  = Y
    PROCESSING_START_TIME          = 'N/A'
    PROCESSING_STOP_TIME           = 'N/A'

The data set contains measurements from both the infrared interferometer spectrometer and the broadband reflected solar radiometer and ancillary data. The data set is ordered by time as measured by the Flight Data System Count (FDSC). This represents the data frame number, modulo 60. Also included is pointing and other information on the geometry associated with a given data record.

Each record of the data set contains a header, radiometer observations, and interferometer observations.

The interferometer data consists of calibrated thermal emission spectra expressed as spectral radiances in Watt/cm2/sr/cm-1. The wavenumber corresponding to each spectral radiance value is not included in the data set; the beginning wavenumber and the constant wave number increment are given, permitting easy calculation of the appropriate wavenumber for each radiance. The calibrated radiances have been obtained from the directly measured interferograms of the planetary body, along with deep space calibration observations. The interferograms are first symmetrized to correct for the fact that they are not sampled at zero path difference and to also remove the asymmetry due to residual dispersion in the beamsplitter and compensator. The symmetrized interferograms are then apodized using a Hamming function and are cosine-transformed. The responsivity obtained from the deep space measurements and knowledge of the instrument temperature are then used to obtain calibrated radiances.

The radiometer data include a measurement integrated over the 45.6 sec required to take one interferogram and measurements sampled 8 times during the data frame. The latter include both high and low gain measurements. A steady target yields identical signals for the 8 samples each of high and normal gain data, and for the integrated data, within the resolution of each measurement. The integrating radiometer provides the best resolution (by a factor of ~3); however, for an accurate integrated measurement the target must remain stable in the field of view for an entire frame, which seldom happens.

Radiometer data are presented as Watts at the detector (wad), which is the integral across the instrument passband of the wavenumber-dependent power received at the detector (wad(nu)). For a target filling the instrument field of view:

        wad(nu) = Signal at instrument * instrument factor
                = Target object illumination
                  * target object reflectance
                  * instrument grasp
                  * instrument filter function

For the IRIS measurements, the target is illuminated by the sun; the target object reflectance is described by the bidirectional reflectance function; the instrument grasp is the telescope area-solid angle product, A * omega, multiplied by an obscuration factor, g; and the instrument filter function, t, is the wavelength dependent instrument passband.

Integrating over the passband gives: wad = I * (A * omega * g) * t

where I is the spectrally integrated radiance from the target, and t is the spectrally integrated radiance as attenuated by the instrument, normalized by the entering flux; as such, it is independent of the normalization of the target BDRF, and can be calculated using a normalized reflectance spectrum. The spectral geometric albedo, as determined from groundbased measurements was used [HANELETAL1981A].

The flux measured by the IRIS instrument is therefore:

I = WAD / (A * omega * g * t)

          A *  omega * g   = 0.02535
          t (Jupiter)      = 0.173 +/- 0.003
          t (Io)           = 0.171 +/- 0.003
          t (Europa)       = 0.173 +/- 0.003
          t (Ganymede)     = 0.168 +/- 0.003
          t (Callisto)     = 0.163 +/- 0.003
          t (target plate) = 0.156 +/- 0.002

For a detailed description of the data set contents, see [HANELETAL1980B]. Scientific results of IRIS observations of the Jovian system are contained in [HANELETAL1979A], [HANELETAL1979B], [PEARLETAL1979], [CONRATHETAL1981], [FLASARETAL1981], [GAUTIERETAL1981], [HANELETAL1981A], and [KUNDEETAL1982].

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Confidence Level Note

In evaluating the confidence level of a given data record, several factors should be taken into account. These include the interferometer data quality, instrument temperature drift, field of view smear, and pointing accuracy.

The interferometer data quality is indicated in the data sets by the parameter REJECT_CODE. The various values of this parameter have the following meaning:

          0 = good
          1 = too many spikes in interferogram
          2 = missing data in interferogram
          3 = zero peak in interferogram
          4 = no interferogram data, but
              radiometer data are available
          5 = interferogram symmetrization problem

Only code 0 indicates the interferogram is usable; however, the radiometer data may be available in the other cases.

When spectral data are selected for analysis, care must be taken to insure that excessive motion of the field of view on the target body has not occurred during the time the interferogram is taken. Smearing can be checked for by noting the differences in latitude/longitude of the Q5 points (center of field of view) at line counts 350 and 750. An additional check can sometimes be made by noting variations in the sampled radiometer data during a frame. Since the total infrared energy and the very low resolution components of the spectral data are recorded in a brief time surrounding the interferogram peak, even when smear occurs this information can be associated with the pointing information at the 24 second point, as can radiometer sample #3.

A change in detector temperature can cause a shift in the absolute calibration of the measured infrared radiance. The detector temperature, IR_DET_TEMP, should be within 0.1 or 0.2K of 200K. If it is not, the spectrum can be corrected using I(corrected) = I(uncorrected) - B(IR_DET_TEMP) + B(200) where B(T) is the Planck radiance at temperature T.

The Noise Equivalent Spectral Radiance (NESR) provides a measure of the random errors of the spectra, expressed in radiometric units. It is defined as the radiance corresponding to a signal to noise ratio of unity and represents the one-sigma uncertainty in an individual spectrum. It is calculated from the standard deviation of measurements taken while the instrument is viewing deep space. Values of the NESR at selected wave numbers are given in the following table. A detailed listing of NESR versus wave number can be found in the NESR files included with each data set volume.

                IRIS NESR (1.E-8 W/cm**2/sr/cm**-1)

                              Wavenumber (cm**-1)
                    200   400   600   800   1000  1500  2000

        V1 Jupiter  3.05  0.43  0.56  0.75  0.65  1.03  1.89

        V1 Saturn   2.62  0.56  0.55  0.73  0.66  1.07  2.32

        V2 Jupiter  2.84  0.60  0.80  2.02  2.50  1.79  3.57

        V2 Saturn   2.93  0.49  1.07  3.54  1.84  2.94  (1)

        V2 Uranus   3.84  0.70  2.63  2.29   (2)

        V2 Neptune  3.34  0.88  3.48  1.79   (3)

        (1) Noisy
        (2) Spectra truncated at 799 cm**-1
        (3) Spectra truncated at 898 cm**-1

Radiometer calibration consists of a verification of instrument stability by repeated determinations of t(target plate), based on observations of a diffusely scattering target plate mounted on the spacecraft. The calibration conversion to Watts at the detector takes into account the detector response and electrical gains. Observations of the target plate were carried out before and after each encounter with the exception of after the Voyager 2 Saturn encounter when jamming of the instrument scan platform caused the maneuver to be aborted. The spacecraft was oriented so that an on- board, diffusely scattering target plate was illuminated by the sun at 30 degrees from the surface normal of the plate. The plate was then viewed by IRIS, to provide a check on the stability of the radiometer calibration, and by the other scan platform-mounted instruments. Each observational sequence proceeded as follows: view deep space, view target, view deep space (occasionally an interval is included when the IRIS field of view is only partially on the target; this results in data with an intermediate signal level in the data set). The difference in signals between target and space observations is then multiplied by the square of the spacecraft-sun distance to provide a normalized calibration signal (the calibration factor).

The principal sources of uncertainty are possible system nonlinearities, signal variability associated with instrument response to changing orientation relative to the sun, signal variability associated with sudden acquisition or loss of the bright target, and quantization. At Jupiter and Saturn, overall uncertainty is dominated by possible system nonlinearities (estimated as =< 0.5%). Otherwise, fluctuations in the deep space signal due to excursions in scan platform pointing immediately before and after viewing the target are significant; these arise because the instrument has a transient response to time- varying illumination by the sun. Transients due to abrupt acquisition of the target damp out in the first few frames of the target observation.

Files containing the target plate observations are included with each data set volume. The data format follows that of the standard Voyager IRIS records. However, all of the spectral data have been set to zero. Bad or missing integrated radiometer and normal gain radiometer data have also been set to zero; however, bad or missing normal gain radiometer data have been set to a constant negative value (corresponding to -1 DN, since the normal gain radiometer offset for Voyager 1 IRIS is equal to 0 DN). Tables summarizing the radiometer calibrations are contained in the calibration section of the Voyager 1 and 2 IRIS instrument catalog templates.

The pointing information provided in the SEDRs is derived from knowledge of the spacecraft position (determined from trajectory analysis), the spacecraft orientation (as indicated by sun and star sensor data and by displacements within the limit cycle), and the articulation of the scan platform on which the instruments are mounted. When adequate data are not available (due to downlink loss, for example), predicted values for pointing information are used. The quoted 3-sigma pointing uncertainty is 0.15 degrees (to be compared with the 0.25 degree diameter of the IRIS field of view). In addition, there are sometimes systematic errors in the SEDR pointing values for entire data sequences or links that take the form of approximately constant offsets in the given field of view locations on the picture body. When high accuracy in pointing knowledge is required, it is best to refer directly to images obtained simultaneously with the IRIS data, using the pointing changes between the line count 350 and 750 Q5 values to correct for spatial drift between the times of interferogram peaks and the shuttering of images. In correlating images and IRIS observations, note that images are not read out during the frames in which they are shuttered. When a single image is made during a frame, its assigned FDSC is one decimal count (modulo 60) greater than the FDSC of that frame. When both cameras are shuttered simultaneously (in the imaging modes BOTSIM and BSIMAN), the narrow angle image of the pair is read out first (with FDSC augmented by 0.01), and the wide angle image is read out second (with FDSC augmented by 0.02). IRIS FDSCs are never augmented.

When attempting to correlate IRIS data with that from other Voyager instruments, it may be necessary to take into account the relative offsets of the centers of the fields of view of the various instruments. Offsets relative to the center of the ISS Narrow Angle camera field of view are given in the tables below. Elevation is positive to the right within the imaging field of view and crosselevation is positive downward. The offsets are expressed both in degrees and in Narrow Angle pixels.

        Voyager 1:

          Instrument   Elevation    Cross-Elevation

          IRIS         +0.020 deg     +0.024 deg
                     (+37.7 pixels) (+45.3 pixels)

          ISS(WA)      +0.0315 deg    +0.0247 deg
                     (+59.4 pixels) (+46.6 pixels)

          UVS          +0.010 deg     -0.030 deg
                     (+18.9 pixels) (-56.6 pixels)

        Voyager 2:

          Instrument   Elevation    Cross-Elevation

          IRIS         +0.016 deg     -0.009 deg
                     (+30.2 pixels) (-17.0 pixels)

          ISS(WA)      -0.0308 deg    -0.0068 deg
                     (-58.1 pixels) (-12.8 pixels)

          UVS           0.0 deg       +0.08 deg
                       (0.0 pixels)(+150.9 pixels)

          PPS          -0.06 deg      +0.003 deg
                    (-113.2 pixels)  (+5.7 pixels)

One difference should be noted between the IRIS data sets described here and those previously deposited in the National Space Science Data Center (NSSDC data set I.D. Numbers: 77-084A-03A; 77-084A-03B; 77-076A-03A; 77-076A-03B; 77 -076A-03C; 77-076A-03D). The spectral radiances in the earlier data sets were listed with a wave number spacing of 1.39051 cm-1. This represents a significant over sampling of the data for which the apodized spectral resolution is 4.3 cm-1. In order to make the present data set more compact, the sampling interval for the spectral radiances was reduced to 2.15 cm**-1. This was accomplished by Fourier transforming the original data, resampling at larger intervals, and transforming back to the spectral space. It should be noted that the full intrinsic information content of the data has been preserved with this procedure, and the two forms of presentation of the data are entirely equivalent insofar as the information content is concerned.

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Parameter Information

    NOISE_LEVEL                    = 'N/A'

    NOISE_LEVEL                    = 'N/A'

    NOISE_LEVEL                    = 'N/A'

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Hanel, R., B. Conrath, M. Flasar, V. Kunde, P. Lowman, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, D. Gautier, P. Gierasch, S. Kumar, and C. Ponnamperuma, Infrared Observations of the Jovian System from Voyager 1, Science, 204, 972-976, 1979.

Hanel, R., B. Conrath, M. Flasar, L. Herath, V. Kunde, P. Lowman, W. Maguire, J. Pearl, J. Pirraglia, R. Samuelson, D. Gautier, P. Gierasch, L. Horn, S. Kumar, and C. Ponnamperuma, Infrared Observations of the Jovian System from Voyager 2, Science, 206, 952-956, 1979.

Gautier, D., B. Conrath, M. Flasar, R. Hanel, V. Kunde, A. Chedin, and N. Scott, The Helium Abundance of Jupiter from Voyager, J. Geophys. Res., 86, 8713-8720, 1981.

Hanel, R. A., B. J. Conrath, L. W. Herath, V. G. Kunde, and J.A. Pirraglia, Albedo, Internal Heat, and Energy Balance of Jupiter: Preliminary Results of the Voyager Infrared Investigation, J. Geophys. Res., 86, 8705-8712, 1981.

Flasar, F. M., B. J. Conrath, J. A. Pirraglia, P. C. Clark, R. G. French, and P. J. Gierasch, Thermal Structure and Dynamics of the Jovian Atmosphere 1. The Great Red Spot, J. Geophys. Res., 86, 8759-8767, 1981.

Conrath, B. J., F. M. Flasar, J. A. Pirraglia, P. J. Gierasch, and G. E. Hunt, Thermal Structure and Dynamics of the Jovian Atmosphere 2. Visible Cloud Features, J. Geophys. Res., 86, 8769-8775, 1981.

Kunde, V. G., R. Hanel, W. Maguire, J. P. Baluteau, A. Marten, N. Husson, and N. Scott, The Tropospheric Gas Composition of Jupiter’s North Equatorial Belt (NH3, PH3, CH3D, GeH4, H20) and the Jovian Isotopic D/H Ratio, Astrophys. J., 263, 443-467, 1982.

Pearl, J., R. Hanel, V. Kunde, W. Maguire, K. Fox, S. Gupta, C. Ponnamperuma, and F. Raulin, Identifications and Gaseous SO2 and New Upper Limits for Other Gases on Io, Nature, 280, 755-758, 1979.

Hanel, R. A., L. W. Herath, V. G. Kunde, and J. C. Pearl, Voyager Infrared Interferometer Spectrometer and Radiometer (IRIS) - Documentation for Reduced Data Records (RDR) for Jupiter, Doc. X-693-821-8, Goddard Space Flight Center, 1980.

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