Data Set Overview
  =================
     This data set consists of all of the spacecraft potential data generated
     from the Cassini Plasma Spectrometer (CAPS) electron spectrometer
     uncalibrated data.

     The data set currently contains the spacecraft potential data from the
     Electron Spectrometer (ELS_SCPOT).  The data is also contained in the
     electron moments file.  Data was generated using uncalibrated data from
     the electron spectrometer on Cassini CAPS.

     The uncalibrated data were acquired in a mix of CAPS operating modes
     beginning with the first instrument checkout in January 1999 and
     containing throughout the Cassini Tour and through the end of prime
     mission.  The data set covers the time period from 1999-004T03:07:47
     UT until end of prime mission (July 2008).  In addition, it will cover
     data received during extended missions.  Sampling rates were variable
     and depended upon the downlink capabilities and other activities
     on-board.

     For times when CAPS is not producing data due to being turned off or
     due to communication issues, the data set will not contain data.

     Additionally, there will be no data during times when the calculations
     are not possible.

  Electron Moment File Discussion
  ==========

     First, some notes on the code itself - it assumes a zero bulk flow
     velocity, due to the limited view of the ELS.  Zero velocity in assumed
     in the spacecraft frame.

     In plasmas, the zeroth moment gives the density, the first moment gives
     the bulk flow velocity vector, and the second moment gives a pressure
     tensor or a temperature tensor.  Each moment in turn requires the
     previous moments, i.e. the velocity moment requires density and the
     pressure moment requires density and velocity.  But as we don't have a 3d
     sensor (like Cluster), we have to assume the plasma is isotropic, i.e.
     what you see in one direction you also see in the opposite direction.
     So, any velocity vector that you measure in one direction will be exactly
     cancelled by the velocity vector in the opposite direction.  So in our
     case the bulk flow velocity will be zero.  In this case the
     pressure/temperature tensor will become a scalar.

     Secondly, the calibrated numbers.  We have to convert the data into phase
     space density so we can subtract off the photoelectrons properly.  This
     can be summarized as follows:

     The ELS accumulation time is 0.0234375s.  So:

          data in counts/sec = data recorded / accumulation time

     after taking into account telemetry periods where several accumulation
     periods were summed or averaged.

     To get the higher calibrated units, which are anode and energy dependent:

          data in DEF = data in counts/sec / geometric factor

          data in DNF = data in DEF / (energy * electron charge)

          data in PSD = (data in DEF * electron mass squared) /
                        (2 * (energy * electron charge) squared)


     The geometric factor (with efficiency rolled in) can be found in the
     CALIB directory.  The units on geometric factor are str m^2 eV/eV per
     anode. The updated file is always on the ELS managed web site:
     http://www.mssl.ucl.ac.uk/~lkg/ELS_calibrations/geometric_factor.html
     For more information, the ELS calibration paper talks in depth about the
     geometric factor. [LEWISETAL2010]

     Background subtraction comes from analysis of data from 15th Sep 06,
     14th Jan 07 and 17th Jun 07, which assigns background levels per anode
     based on actuator position.  The analysis was published in 2009 in a
     paper by Arridge et al [ARRIDGEETAL2009].

     Here's a summary of what the code does.

     Read in data, average by A-cycle (and A-cycle is a 32 second instrument
     cycle).
     Per A-cycle:
       Subtract background, taking the actuator position from the mid-point
           of the A-cycle.
       Convert data to PSD (phase space density), remove photoelectrons.
       Remove spacecraft potential (also taken from mid-point of the
           A-cycle) from instrument energy array
       Get the radial cell limits for each energy bin in m/s from the
           spacecraft-potential-subtracted energy array
       For each bin, density = 4pi/3 * data * (top limit velocity ^3 -
           bottom limit velocity ^3)
       Sum all bin densities to get total density.
       Calculate temperature constant t_const =
           (4 * mass_electron * pi) / (15 * density * Boltzmann's constant)
       For each bin, temperature = t_const * data * (top limit velocity ^5 -
           bottom limit velocity ^5)
       Sum all bin temperatures to get total temperature.
       Convert temperature from Kelvin to eV with
           temperature = temperature * 8.61752d-5

     Then get a quality factor by:
       Get peak energy of a Maxwellian from the actual density and
           temperature from the moments calculated. Calculate the
           theoretical peak energy that a Maxwellian would have with the
           temperature from the calculated moments (identically equal to
           TWICE the temperature in energy units):
           kbt = temperature * charge_e
       And work out the mid-energy of the bin that comes in (energy), and
           the geometric factor at that energy (geom)
       Get peak phase space density of same:
           peak_psd = density*SQRT((mass_e/(2*!PI*kbt))^3)*EXP(-1)
       Get the peak count rate from that lot:
           peak_cr=peak_psd*2(energy*charge_e)^2*geom/(mass_e^2)
       Get the Poissonian error on the peak count rate:
           std_dev_cr=SQRT(peak_cr/accutime)
       Roll them into a quality factor:
           quality_factor=(peak_cr-42.7)/std_dev_cr
     As a note, 42.7 Hz is the one count level.


  Data
  ====

    The data are stored in multiple data files and have been organized
    in folders, first by higher order data type and then by year.  Each file
    contains a maximum of 24 hours of data.  Note that data is included in
    the file based on the start time, and not the end time of the data.
    Format of the data files can be found in the CAPS instrument archive
    specification [FURMANETAL2013].  The format can also be found in the
    .LBL files in the FM/HIGHERORDER/SCPOT/YYYY specific directory,
    co-located with the data.


  Ancillary Data
  ==============

    Ancillary data can be found in the ANC data file provided with the
    CAPS UNCALIBRATED data set.

  References
  ========
    [FURMANETAL2005] CAPS standard data products and archive volume
      software interface specification, Version 1.9, JPL SIS ID:
      IO-AR-017, Southwest Research Institute, San Antonio, TX 78250,
      2005.

    [ARRIDGEETAL2009] The effect of spacecraft radiation sources on
      electron moments from the Cassini CAPS electron spectrometer,
      Planetary and Space Science, 57, 854-869,
      doi:10.1016/j.pss.2009.02.011, 2009.

    [GURNETTETAL2004] The Cassini Radio and Plasma Wave Investigation,
      Space Sci. Rev. 114, 395-463, 2004.

    [LEWISETAL2008] Derivation of density and temperature from the Cassini
      Huygens CAPS electron spectrometer, Planetary and Space Science, 56,
      901-912, doi: 10.1016/j.pss.2007.12.017, 2008.

    [LEWISETAL2010] The calibration of the Cassini-Huygens CAPS Electron
      Spectrometer, Plan. and Space Sci., 58, 427?436,
      doi:10.1016/j.pss.2009.11.008, 2010.

    [THOMSENETAL2005] Numerical moments computation for CAPS/IMS, Los
      Alamos National Laboratory Report LA-UR-05-1542, 2005.

    [THOMSENETAL2010] Survey of ion plasma parameters in Saturn's
      magnetosphere, J. Geophys. Res., 115, A10220,
      doi:10.1029/2010JA015267, 2010.

    [WILSONETAL2012] PDS User's Guide for Cassini Plasma Spectrometer
      (CAPS), 2012.

    [YOUNGETAL2004] Cassini Plasma Spectrometer Investigation,
      Space Sci. Rev. 114, 1-112, 2004.

Review
  ======
    These data have been reviewed by the instrument team and are of the
    highest quality that can be generated at this time. Science results
    based on these data have been published in several journals (Science,
    Nature, JGR, etc.). After submission to the PDS, these data will be
    approved through the peer review process.

  Data Coverage and Quality
  =========================

    Gaps
    ----
      There are many gaps in the CAPS data stream and there are many
      different sources for these gaps.  Sources of gaps are as follows:
          a. telemetry outages
          b. data policing violations (CAPS data volume higher than
             allocated)
          c. incorrect spacecraft data management commanding
          d. telemetry commanding during Cruise
          e. instrument anomalies
          f. instrument modes which don't return all data products
          g. planned instrument power-off and/or sleep periods
          h. instrument off due to bus imbalance/short issues
      When there is no data for a time period, one of the above sources
      is the reason behind the gap.  There is no indicator to which of
      the sources is responsible for the gap in data coverage.

    Poor Data
    ---------
      When the code generating electron moments and spacecraft potential
      generates data that is outside the expected minimum or maximum, the
      data is replaced with fill values.  Hence, some time periods may appear
      to have data, but many of records have been filled.  The assumption is
      that the code is working, but enviormental effects do not lend to
      correct data.

  Limitations
  ===========
      The main limitation for the Electron moments and spacecraft potential
      files is when the code generates data outside the minimum and maximum
      values.  This limitation will be addressed by the team with further use
      of the data.