Data Set Overview
    =================
      This data set contains the complete collection of the published
      Cassini radio occultation electron density profiles (EDP) of the
      Titan ionosphere as of September 2008. EDP files are stored as
      ASCII tables.
 
      The design of the Cassini spacecraft's tour of the Saturnian system
      permitted multiple radio occultations of Titan's ionosphere to be
      carried out. Of these, a group of four occultations was incorporated
      into the primary  Cassini mission and the resulting EDP files
      are included in this archive. These occultations occured on
      Titan flybys T12(19 March 2006), T14(20 May 2006), T27(26 March 2007),
      and T31(28 May 2007). Of these, the first two probed middle latitudes
      in the Southern Hemisphere (14.3 degrees South and 36.3 degrees South
      for T12 and 19.7 degrees South and 21.3 degrees South for T14). The
      other two occultations made observations in polar and near-polar
      locations (74.6 degrees South and 60.6 degrees North for T27 and
      75.4 degrees North and 74.0 degrees South for T31). All observations
      occurred near the terminator, with the entry measurements occurring
      on the dark side (solar zenith angle(SZA) 92.2 degrees to 95.8
      degrees), and the exits on the sunlit side (SZA 85.7 degrees to 90
      degrees). The exit measurements at midlatitudes were also near the
      dusk terminator. The following table provides a summary of the data:
 
    Obs  Date        File Name                       Ram (deg) SEP (deg)
    ---- ----------- ------------------------------- --------- ---------
    T12N 19 Mar 2006 S19TIIOC2006_078_0000_X_001.EDP   73.7     131.5
    T12X 19 Mar 2006 S19TIIOC2006_078_0000_X_001.EDP   87.5     131.5
    T14N 20 May 2006 S20TIIOC2006_140_1203_N_001.EDP  100.1      72.6
    T14X 20 May 2006 S20TIIOC2006_140_1203_X_001.EDP   66.4      72.6
    T27N 26 Mar 2007 S28TIIOC2007_085_0000_N_001.EDP   93.9     138.2
    T27X 26 Mar 2007 S28TIIOC2007_085_0000_X_001.EDP   60.7     138.2
    T31N 28 May 2007 S30TIIOC2007_148_1737_N_001.EDP   98.1      76.8
    T31X 28 May 2007 S30TIIOC2007_148_1737_X_001.EDP   78.4      76.8
 
      For more details see A.J. Kliore et al. (J. Geophys. Res., 113,
      A09317,doi:10.1029/2007JA012965, 2008).
 
 
    Parameters
    ==========
      Each EDP file contains a table  of electron number density versus
      altitude, where the altitude is determined by height of the closest
      approach point of the radio line-of-sight relative to the surface
      of Titan. The tables also include the planetocentric north latitude,
      solar zenith angle, solar time, and standard deviation of the
      measurements.
 
 
    Processing
    ==========
      During the occultation period, Cassini transmits three sinusoidal
      radio signals of 0.94, 3.6 and 13 cm wavelength (Ka-, X-, and S-band
      respectively). They are coherently generated on board Cassini from
      a common ultrastable crystal oscillator (USO).
 
      Digital samples of the in-phase and quadrature signal components
      were recorded at multiple sampling rates and bandwidths using
      the open-loop Radio Science Receivers (RSR) of the DSN. The data
      in these archive are based on processing signals recorded at 1 kHz
      bandwidth. The RSR as well as all subsystems of the DSN are driven
      by a highly stable frequency and timing system based on a hydrogen
      maser. Accurate antenna pointing is required especially at higher
      frequencies (e.g., Ka-band).
 
      The RSR data are processed by first detecting the signal carrier
      via software. A phase-locked loop or a series of Fast Fourier
      Transforms are typical detection methods depending on factors such
      as the signal-to-noise ratio as well as frequency and amplitude
      dynamics. Once detected, the signal is upconverted to 'sky frequency'
      and then frequency residuals are produced by removing a model of
      the apparent relative motion between the spacecraft and ground
      station. These residulas contain the science information on the
      atmosphere/ionosphere of the planet and form the basic data from
      which the atmospheric and ionospheric properties are extracted.
 
      The procedures for the analysis and inversion of these data are
      described in detail in A.J. Kliore et al. (Space Sci. Rev. 115, 1-70,
      doi:10.1007/s11214-004-1436-y, 2004). The residuals are detrended
      by fitting a least squares straight line to the baseline data and
      subtracting it from the raw residuals. The residuals are then used
      to compute the refractive bending angle at each data time point.
      This requires a precise ephemeris of Cassini relative to Titan,
      iterative light propagation time solutions linking the spacecraft,
      Titan, and the DSN station. The gravitational field of Titan was
      assumed to be spherical. Once the refractive bending angle and its
      corresponding ray asymptote distance with respect to the center of
      refraction are computed for each data point, the Abel integral
      transform, assuming spherical symmetry, is used to invert these
      data and to produce a vertical profile of refractivity.
 
      Since the radio refractivity is proportional to the electron density
      times the inverse square of the frequency, the electron density can
      be determined, as the frequencies are known very precisely. The
      altitude is determined by height of the closest approach point of
      the radio line-of-sight relative to the surface of Titan.
 
 
    Data
    ====
      The EDP directory contains all the data files (i.e., the electron
      density profiles), which are accompanied by detached PDS labels
      describing their contents. The naming convention of these data
      files is:
 
           sssttIOCyyyy_ddd_hhmm_[NX]_vvv.EDP
 
      where:
 
        sss - Cassini Sequence, during which the data were collected
              (for this data set, sss = S19, S20, S28 and S30)
 
        tt - target ID (for this data set, tt = TI, for Titan)
 
        IOC - Ionospheric Occultation
 
        yyyy_ddd_hhmm - year, day of year, hour, min, in Spacecraft Event Time
                       (SCET), for the start of the observation, during which
                       these data were collected.
 
        [NX] - N denotes entry occultation; X denotes exit occultation
 
        vvv - version number
 
 
    Software
    ========
     None.
 
 
    Media/Format
    ============
      The archival data set is written on DVD-R media. The DVD-R volumes
      conform to the 'UDF_ISO-9660_BRIDGE' structure as required by PDS.

Overview
    ========
      In general, this is a good data set.
 
 
    Review
    ======
      This archival data set was reviewed by the Cassini Radio Science
      Team prior to submission to the Planetary Data System (PDS).
 
 
    Data Coverage and Quality
    =========================
      The data are contained in one volumes (DVD), CORS_1001,
      and they cover the complete collection of the published Cassini
      radio occultation electron density profiles of the Titan ionosphere
      as of September 2008. The table, which was included in the section
      'Data Set Overview' of this document, provides a summary of the data.
 
      There are no known problems with this data set.
 
 
    Limitations
    ===========
      The limitations in this data set follow from the quality of
      the execution, which is described above under Data Coverage
      and Quality.