Data Set Overview
    =================
      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/cm**2/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)
 
           where: A * omega * g    = 0.02535
                  t (Uranus)       = 0.149 +/- 0.003
                  t (target plate) = 0.156 +/- 0.002
 
      For a detailed description of the data set contents, see
      [MAYOETAL1988].  Scientific results of IRIS observations of the
      Uranian system are contained in [HANELETAL1986],
      [CONRATHETAL1987], [FLASARETAL1987], [GIERASCHETAL1987], and
      [PEARLETAL1990].
 
      Processing Level Id            : 3
      Software Flag                  : Y
 
 
    Parameters
    ==========
 
      Integrated Visible Radiance
      ---------------------------
        The broadband, reflected solar radiometer signal integrated
        over the 45.6 seconds that IRIS data are taken within the
        48 second data frame.  The quantity given is power at the
        detector in Watts.
 
        Sampling Parameter Name        : TIME
        Data Set Parameter Name        : INTEGRATED_VISIBLE_RADIANCE
        Sampling Parameter Resolution  : 45.600000
        Minimum Sampling Parameter     : 48.000000
        Maximum Sampling Parameter     : 48.000000
        Sampling Parameter Interval    : 48.000000
        Minimum Available Sampling Int : 48.000000
        Data Set Parameter Unit        : WATT
        Sampling Parameter Unit        : SECOND
 
 
      Sampled Visible Radiance
      ------------------------
        Series of 8 radiometer samples taken during a 48 second data
        frame with the high gain channel.  The quantity given is
        power at the detector in Watts (-1.0 indicates data off the
        planet).
 
        Sampling Parameter Name        : TIME
        Data Set Parameter Name        : SAMPLED_VISIBLE_RADIANCE
        Sampling Parameter Resolution  : 2.400000
        Minimum Sampling Parameter     : 6.000000
        Maximum Sampling Parameter     : 6.000000
        Sampling Parameter Interval    : 6.000000
        Minimum Available Sampling Int : 6.000000
        Data Set Parameter Unit        : WATT
        Sampling Parameter Unit        : SECOND
 
 
      Thermal Radiance
      ----------------
        Radiance (W cm-2 ster-1) within a 4.3 cm-1 spectral interval.
 
        Sampling Parameter Name        : TIME
        Data Set Parameter Name        : SAMPLED_VISIBLE_RADIANCE
        Sampling Parameter Resolution  : 2.400000
        Minimum Sampling Parameter     : 6.000000
        Maximum Sampling Parameter     : 6.000000
        Sampling Parameter Interval    : 6.000000
        Minimum Available Sampling Int : 6.000000
        Data Set Parameter Unit        : WATT/CM**2/SR/CM**-1
        Sampling Parameter Unit        : SECOND
 
 
    Source Instrument Parameters
    ============================
 
      N/A
 
    Measurement Information
    =======================
 
      N/A
 
    Processing
    ==========
 
      Processing History
      ------------------
 
        Source Data Set ID          : NSSDC 77-076A-03C
        Software                    : UNK
        Product Data Set ID         : VG2-U-IRIS-3-RDR-V1.0

Overview
    ========
      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 sections 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.