Data Set Overview
  =================
    The Cassini Radio and Plasma Wave Science (RPWS) Langmuir Probe continuous
    current data set comprises calibrated probe current measurements acquired
    at up to 20 Hz rate at fixed bias voltage during the entire Cassini
    mission.  This particular data set includes spacecraft event time, bias
    voltage, and measured current for each sampled data point.  Data are
    presented in tables organized so as to have fixed-length records for ease
    in data handling.  These can be used to infer high temporal resolution
    density fluctuations between standard Langmuir Probe sweeps.  If related
    to densities determined during a sweep prior to or following the 20 Hz
    data, or using other electron density estimates, the relative currents can
    be assigned absolute densities.


  Parameters
  ==========
    This data set comprises spacecraft event time, bias voltage, and measured
    current for each sampled data point that was acquired by the RPWS.


  Processing
  ==========
    Data in this data set were processed by the use of a number of software
    programs which assemble segmented mini-packets in the raw telemetry
    packets into complete sets then de-compress the data that were compressed
    by one of a number of compression algorithms by the RPWS flight software
    onboard. The conversion from telemetry data to physical measured electric
    current units is made through lookup tables created by two 3rd order
    polynomial fits to the ground tests calibration data. The ground test
    calibration data are presented in Chapter 9 of the  RPWSCAL document in
    the DOCUMENT directory.  These data are calibrated using the best
    calibration tables and algorithms available at the time the data were
    archived.

    The continuous current data themselves consist either of 21.5s long time
    series of currents at the indicated bias voltage consisting of 432
    samples each (20Hz), or of 624s long time series of currents consisting
    of 38 samples (survey, 61mHz).  The time series are usually made every 24s
    (20Hz) each during targeted moon flybys or during inner magnetosphere
    crossings.  In the outer magnetosphere the rate is lower with time series
    starting each 680s (survey).  Prior to and during SOI the bias voltage was
    set based on the most recent sweep to use an optimum setting.  Afterwards
    the bias was set to +11.5V, except during deep (<1200km from C/A) Titan
    flybys where a fixed bias of +4V was used.  Some of the time series data
    show data spikes of external interference origin.  These have not been
    corrected for, but  can easily be filtered away by the user.

    There are several literature references regarding how to utilize the
    continuous current data specific for the RPWS Langmuir probe analysis.

    The Langmuir probe spherical sensor is 5 cm in diameter, and coated with
    (baked) TiN.


  Data
  ====
    The RPWS Langmuir Probe continuous current data set is given as daily
    ASCII tables of three columns.  The first column is spacecraft event time
    (SCET), the second column is the set bias potential in volts, and the
    third column is the measured current in amps.


  Ancillary Data
  ==============
    Ancillary data archived separately with volume set id
    USA_NASA_PDS_CORPWS_0XXX include a series of files that describe the modes
    of the RPWS as a function of time and provide a time-ordered listing of
    Instrument Expanded Block (IEB) trigger commands (the mode by which the
    RPWS is reconfigured). Also a detailed description of each of the modes
    (or IEBs) is provided.

    Other data which are ancillary to this data set and which are archived
    separately from this collection are the Navigation and Ancillary
    Information Facility's SPICE kernels describing the position and attitude
    of Cassini and various solar system bodies as a function of time.


  Coordinate System
  =================
    This data set consists of calibrated current time series measured by the
    RPWS Langmuir probe.  These data are presented as detected by the sensor
    and are not rotated into any other coordinate system.  If desired, the
    SPICE kernels can be used with the SPICE toolkit to convert from the
    spacecraft frame to virtually any frame which may be of use in analyzing
    these data.  For some applications, the data may be considered point
    measurements at Cassini and may be entirely adequate with no coordinate
    transformations at all.  However, for analysis purposes using this data
    set, the photo-electron cloud around the spacecraft, the photo-electron
    emission from the Langmuir probe itself, as well as possible ram plasma
    wake effects will indeed depend on the orientation of the spacecraft
    versus the solar and plasma ram flow directions.


  Media/Format
  ============
    These data are supplied to the Planetary Data System as an online
    collection of files in a PDS volume directory structure or as a standard
    compressed archive, such as ZIP or TAR.GZ, of this collection.

Confidence Level Overview
  =========================
    This data set contains calibrated current measurements acquired by the
    Cassini RPWS Langmuir probe at 20 Hz rate for various moon flybys and
    inner magnetospheric crossings, and slower survey rate otherwise.  Every
    effort has been made to ensure that all data returned to JPL from the
    spacecraft are included and that the calibration is accurate.  The noise
    level of the Langmuir probe is estimated to be close to 100 pA derived
    from the sampled current data for negative bias voltage in thin plasma
    regions (<0.1 cm**-3).  Calibration and other systematic errors are
    smaller than the noise level.  In micro-meter sized dust regions the
    individual dust hits  can be seen as spikes in the voltage data.  These
    have not been corrected for since it contains potential science results.
    Furthermore, some of the current data show artificial data spikes of
    external interference origin. These have not been corrected either, but
    can easily be filtered away by the user. Wake effects may also affect the
    quality of the data if the probe is in the wake of the spacecraft.  The
    measurement characteristics of this data set has otherwise a very good
    confidence level.


  Review
  ======
    The RPWS Langmuir probe continuous current data will be reviewed
    internally by the Cassini RPWS team prior to release to the PDS. The data
    set will also be peer reviewed by the PDS.


  Data Coverage and Quality
  =========================
    All data in the stated interval are included, to the best of our knowledge
    and attempts to determine completeness.  In general, the  RPWS/LP
    instrument started to operate just after launch in 1997, thereafter with
    intermittent operation periods near Earth and Jupiter flybys until three
    months before Saturn Orbit Insertion (SOI).  The RPWS/LP was thereafter
    operated more or less continuously from SOI and forward, and gave higher
    resolution data during each moon flyby.  Data gaps are due to spacecraft
    anomaly resolution or simply to downlink gaps, some of which were imposed
    by limitations on DSN station availability.  The continuous current data
    are only acquired in particular modes of the Langmuir probe, dependent on
    available telemetry and are typically scheduled during icy satellite and
    Titan flybys.


  Limitations
  ===========
    Occasional RPWS Langmuir probe current measurements have interference in
    individual samples.  The RPWS HFR receiver produced these interference
    signals in the LPR in certain negative bias voltage ranges.  These samples
    can be filtered away before analysis and should not pose a problem for the
    user analysis that deliver densities.  Thermal electron output data will
    usually not be available, due to spacecraft photoelectron current
    contamination, for number densities below 1 cm**-3 and/or for temperatures
    above 10eV.