The content from this description has been adapted from the CAPS
  instrument description paper [YOUNGETAL2004].
 
 
  Instrument Overview
  ===================
 
    The CAPS instrument was comprised of three sensors: the Electron
    Spectrometer (ELS), the Ion Beam Spectrometer (IBS), and the Ion
    Mass Spectrometer (IMS). The ELS sensor measured the velocity
    distribution of electrons from 0.7 eV to 30 keV, a range that
    permited coverage of thermal electrons found at Titan and near the
    ring plane as well as more energetic trapped electrons and auroral
    particles. The IBS sensor measured ion velocity distributions with
    very high angular and energy resolution from 1 eV to 50 keV. It was
    specially designed to measure sharply defined ion beams expected in
    the solar wind at 9.5 AU, highly directional rammed ion fluxes
    encountered in Titan's ionosphere, and anticipated field-aligned
    auroral fluxes. The IMS sensor was designed to measure the
    composition of hot, diffuse magnetospheric plasmas and
    low-concentration ion species with an atomic resolution
    M/delta M ~70 and, for certain molecules, (such as N2+ and CO+),
    effective resolution as high as ~2500. The three sensors were
    mounted on a motor-driven actuator that rotates the entire
    instrument over approximately one-half of the sky every 3 minutes.
 
 
  Scientific Objectives
  =====================
 
    The Cassini Plasma Spectrometer (CAPS) had many complimentary
    science objectives during the Cassini mission.  The CAPS objectives
    were as follows:
 
      - Perform in-situ studies of dust-plasma interactions in Saturn's
        magnetosphere
      - Study Saturnian auroral phenomena
      - Observe Saturn Kilometric radiation (SKR) generation
      - Study magnetospheric external and internal boundaries,
        structures and plasma domains (including flux tubes)
      - Study microphysics of the bow shock, magnetosheath, magnetopause
        and magnetotail
      - Study magnetospheric dynamics (driven by solar wind, internal
        rotation, substorms, radial transport, and mass loading)
      - Study the composition of the ionosphere, exposphere and
        atmosphere at Titan
      - Study the interaction of Titan's upper atmosphere and ionosphere
        with Saturn's magnetosphere (precipitation, ion pickup)
      - Study the structure of upstream, bow shock, wake, and flux tube
        regions around Titan
      - Determine composition of ionized molecules originating from the
        ionosphere and the icy satellite's expospheres
      - Study the composition of the exposphere and surfaces around icy
        satellite
      - Look at the interaction of the magnetosphere (precipitation, ion
        pickup) with icy satellite exosphere
      - Study the structure of upstream, bow shock, wake, and flux tube
        regions around the icy satellites
      - Study the composition of ring exposphere and ring particle
        surfaces, characterize ring/magnetoshere interactions,
        dust-plasma interactions, ring particle dynamics and erosion,
        and study the interactions of ring plasma with Saturn's
        ionosphere
 
 
 
  Operational Considerations
  ==========================
 
    The ELS measured differential electron velocity distributions at
    densities as low as 10**(-3) cm**(-3). The measurement range
    extended from 1 eV to 28 keV. The lower end of this range, allowed
    detailed studies of secondary electron fluxes that contribute to
    ionization and chemical processes taking place at Titan and
    elsewhere. At tens of keV ELS contributed to studies of trapped
    electrons and those associated with saturnian aurora. Throughout its
    energy range ELS provided a global survey of plasma density,
    temperature and electron pitch angle distributions that were needed
    to derive a comprehensive view of plasma dynamics within the
    magnetosphere and, for roughly 50% of the mission, in the solar wind
    and magnetosheath.
 
    The large amount of time that Cassini spent in the solar wind during
    the Cassini tour presented the opportunity to study both the solar
    wind's intrinsic characteristics and its interactions with the
    magnetosphere of Saturn, and the comet-like magnetosphere of Titan.
    The IBS energy and angular resolution (few percent and ~2 degrees
    respectively) provided the capability of making solar wind
    measurements, and for observing ion ram fluxes at Titan and auroral
    ion beams. The IBS energy range extends from 1 eV up to 50 keV.
 
    The IMS was designed to provide comprehensive measurements in all
    regions of the magnetosphere. IMS relies on time-focused optics
    combined with carbon foil technology. It was designed to separate
    atomic species with high resolution, and to identify isobaric
    molecular species such as CH4+, NH2+, and O+ (all with M/Q = 16,
    where M/Q is the mass/charge ratio) or N2+ and CO+ (M/Q = 28) that
    would otherwise have required a very large conventional instrument
    to achieve. Because of its 1 eV to 50 keV energy range IMS was also
    used to study the composition of Titan's ionosphere at a few eV,
    complementing INMS [WAITEETAL2004], and to study energetic trapped
    ions, complementing the MIMI/CHEMS investigation [KRIMIGISETAL2004].
 
 
  Detectors
  =========
 
    ELS
    ---
 
      The ELS sensor was a hemispherical top-hat electrostatic analyzer
      (ESA) similar to that described by [CARLSONETAL1983].  The
      detector consisted of a chevron microchannel plate (MCP) pair with
      a gold-coated copper spacer 66 microns thick positioned between
      the two plates.  At operating voltage, the measured full width
      half max (FWHM) pulse height distribution was 130%. The
      resistivity of the glass in the MCP was low enough to allow the
      plate to respond to count-rates up to 1 x 104 mm**(-2) s**(-1) or
      approximately 106 electrons per anode per second, without
      saturation causing significant gain degradation. The MCP high
      voltage could be varied from 0 to 3.5 kV in steps of approximately
      60V. This allowed the MCP bias to be increased throughout the
      mission to recover possible gain loss.  The bias voltage at the
      input to the MCP was maintained at +150 V to ensure all electrons
      have sufficient energy to be detected.  During calibration, the
      operational voltage on the MCP was approximately +2.4 kV.
 
      Electrons leaving the rear of the MCP traversed a gap of 500
      microns before striking the anode.  A voltage of +82 V applied
      between the anode surface and the back surface of the MCP
      optimized spreading of the charge cloud leaving the MCP. The anode
      had eight discrete 20 degree-wide electrodes separated by 150
      microns. The active anode area was formed by 10 micron thick gold
      on a Deranox 975 Alumina substrate. The area of the separator
      contacting the MCP was coated with 10 microns of gold. A signal
      ground plane incorporated into the bottom layer of the multilayer
      ceramic provided electromagnetic screening of the anode from the
      analyzer structure.
 
 
    IBS
    ---
 
      The IBS sensor was similar to ELS, as it was also based on the
      principles of a curved-electrode electrostatic analyzer.  The
      primary difference, aside from its larger radius, was that the
      spherical IBS electrodes extend 178 degrees from the entrance
      aperture to channel-electron multiplier detectors located at the
      exit.
 
      A unique aspect of IBS was the method used to obtain high
      angular resolution 3-D velocity space measurements. Based on the
      crossed-fan field-of-view concept employed in an earlier solar
      wind ion instrument [BAMEETAL1978], it was possible to obtain the
      required angular resolution by tilting the acceptance fans of each
      aperture 30 degrees relative to the others.  With that in mind,
      there were three curved 2.5 x 15 mm apertures in the IBS faceplate,
      each with a nominal acceptance fan of +/- 1.5 degrees FWFM in
      azimuth (set by the ESA characteristics) and +/- 75 degrees FWFM
      in elevation angle (set by the apertures) from the normal to the
      plane of the aperture.  If we define the middle aperture as being
      along the 0 degree radius from the center of the instrument
      faceplate, the other two apertures were located at +/- 30 degrees
      relative to it. There were three CEM detectors located 180 degrees
      around the faceplate from each of the apertures, i.e. in the
      position where ions entering the apertures from any transmitted
      direction come to a focus. The FOV of the middle aperture was
      oriented such that its long (polar) dimension was parallel to the
      azimuthal (Z) axis of the CAPS actuator. The FOV of the other two
      apertures were therefore 'crossed' with inclinations of +/- 30
      degrees with respect to that of the middle aperture.
 
      The three matched detectors used in IBS were custom-built CEMs
      chosen for their rugged construction out of solid ceramic, their
      high gain characteristics (typical plateau ~10^8), and their
      availability in nearly any desired form factor (the manufacturer
      was Dr. Sjuts Optotechnik GmBH of Germany). The input funnel
      interior dimensions are 5 x 20 mm, which was more than adequate to
      intercept all of the ions converging at each of the sensor's three
      focal points. Ninety percent transmission grids were stood off
      from the front of the CEM funnels and biased negatively with
      respect to the funnels in order to minimize secondary electron
      loss.  The input assembly (grid and funnel) was biased at negative
      high voltage so that all transmitted ions received
      post-acceleration equivalent to the CEM bias. A CEM responded to
      an incoming ion by emitting an electron charge pulse at its exit,
      which was biased at about -100 V.
 
 
    IMS
    ---
 
      The IMS sensor approach was to choose a novel technique based on
      isochronous (time-focusing) mass spectrometry based on carbon
      foils.  In order to achieve IMS measurement goals the properties
      of a toroidal ESA were matched to those of the TOF analyzer. The
      combination of an ESA with carbon-foil based TOF measurements
      had been previously developed and flown in space by Gloeckler and
      colleagues [GLOECKLER&HSIEH1976, GLOECKLERETAL1995,
      GLOECKLERETAL1998, HAMILTONETAL1990] and Young [YOUNGETAL1992,
      YOUNGETAL1998B]. Early development of the LEF concept
      [GLOECKLERETAL1995], resulted in a high-resolution 1-D (in energy)
      isochronous instrument used to measure the collimated flow of the
      solar wind. However, [MCCOMASETAL1990] and [MCCOMAS&NORDHOLT1990]
      were the first to describe the principles and operation of a
      cylindrically symmetric LEF analyzer capable of making 2-D
      energy-angle measurements. [MCCOMASETAL1998] and
      [NORDHOLTETAL1998] further describe the LEF application to IMS.
 
      Ions initially enter the top-hat portion of the ESA through a
      grounded collimator (the top-hat was similar to the geometry used
      in the ELS). A flat circular plate truncates the inner ESA toroid
      at the point where it joins the top-hat section. There are two
      advantages to toroidal geometry: (1) Toroids have two radii of
      curvature that independently control ion focusing within the TOF
      section (in spheres the two radii are degenerate and equal, in
      cylinders one radius is infinite). (2) The top-hat entrance
      aperture of a toroid like the one used in IMS was larger per unit
      of ESA plate surface area than for a comparably sized spherical
      top-hat, giving higher sensitivity per unit of sensor weight
      [WOLLNIK1971, YOUNGETAL1988]. In the elevation plane, ions are
      formed into a beam by a combination of vanes in the entrance
      collimator and field-correcting slits (one per 20 degree pixel) at
      the exit of the ESA.
 
      A voltage applied to the inner ESA electrode created an electric
      field in the top-hat that deflects ions into the toroid. As with
      ELS and IBS, only particles within a particular range of E/Q and
      direction of arrival were transmitted through the ESA to the TOF
      analyzer. Ions that successfully exited the ESA were accelerated
      by +/- 14.56 kV into one of eight ultra-thin (~100 angstrom)
      carbon foils distributed around the entrance to the TOF analyzer
      (one foil per angular 'pixel').  Ions falling through the
      accelerating field, gained sufficient velocity perpendicular to
      the foil so that they would penetrate the foils even at the lowest
      energies (~ 1eV). Higher post-acceleration voltages would
      naturally have been desirable, but would have driven up instrument
      size, mass, and power in addition to risk.
 
 
  Electronics
  ===========
 
    ELS sensor electronics
    ----------------------
 
      The electronics were accommodated on four circuit boards
      integrated to a single motherboard consisting of flexible and
      rigid sections.  This design eliminated the need for an internal
      cable harness, and at the same time coupled ELS to the CAPS/DPU
      interface connector.
 
      MCP pulses collected on eight anodes were passed to an equal
      number of Amptek(R) A111F charge amplifier/discriminators that
      converted raw signals above a predetermined threshold into 5 V,
      300 ns logic pulses. Thresholds were set in hardware to 3.4 x 105
      electrons, which yielded an equivalent level of 25 mV (into 2.3
      pF), giving good rejection of electronic noise. A decrease in the
      threshold level by 2.5 mV increased spurious electronic noise
      counts by a factor of ten. (This relationship held over a wide
      range of thresholds.  MCP dark counts and penetrating radiation
      were the main remaining contributors to background.).
 
      Front and rear MCP bias voltages were provided by Zener diodes,
      which required filtering at these low currents (around 10
      microAmperes). The MCP anodes were biased at high voltage so
      signal pulses had to be decoupled by high voltage capacitors
      before the signal goes to the amplifiers which shared the same
      circuit board with the HV bias/anode coupling circuitry. The HV
      section was carefully designed and laid out to support a maximum
      field of 800 V/mm.
 
      The SMU (sensor management unit) received and interpreted sensor
      commands sent by the CAPS DPU and accumulated and transmited ELS
      data back to the DPU. It stored the sequence of high voltage steps
      to be applied to the analyzer, the grid voltage setting, and the
      MCP voltage table. SMU circuitry supplied stimulation test pulses
      of variable amplitude and frequency to the amplifier/discriminator
      channels. Under control of the CAPS DPU, the SMU clock speed could
      be successively halved to lengthen the data acquisition period
      from 31.25 to 1000 ms/step, creating progressively longer energy
      sweeps. Furthermore, the sample deadtime could be varied between
      25% and 12.5% of the sample period to increase counting rate
      capability at high rates.
 
 
    IBS sensor electronics
    ----------------------
 
      For IBS, all of the electronics were mounted on two circular
      circuit boards housed in the ESA hemispherical cavity.  The upper
      board contained both the ESA and the CEM high voltage power
      supplies, while the low-voltage amplifier/discriminators, test
      pulser, and digital-to-analog converters (DACs) were contained on
      the lower board. The electrical interface from the IBS to the CAPS
      DPU was a single 62-pin connector.
 
      The CEM HV supply, which provided a negative bias voltage to the
      three IBS detectors in parallel, was commanded by an 8-bit word
      over a range of 0 to -4.0 kV. The ESA supplied biases to the inner
      ESA hemisphere with a stepped negative potential that determined
      the instantaneous energy of transmitted ions. For IBS to be able
      to function as desired, the ESA HVPS had to cover the voltage
      range -0.05 to -2600 V in 1.284% increments with a time between
      steps of 7.8125 ms. In order to have sufficient resolution over
      the entire voltage range, the ESA supply was designed with three
      ranges (-0.05 to -1.85 V, -1.85 to -69.6 V, and -69.6 to -2600 V)
      each of which was 12-bit controllable. This results in a total of
      12,288 possible voltage steps although only 852 logarithmically
      spaced steps were required for full coverage of the voltage range
      with 1.284% spacing. During operation a 14-bit data word
      controlling the ESA stepper supply was written to a latch in the
      IBS electronics every 7.8125 ms (one-eighth of the IMS stepping
      interval of 62.5 ms). The basic operational mode consisted of a
      255-step voltage scan (corresponding to an energy scan) lasting
      2 s.
 
 
    IMS sensor electronics
    ----------------------
      This section describes the IMS electronics subsystems by tracing
      the order in which signals are acquired and processed, namely FEE
      (front-end-electronics) to TDC (time-to-digital converter) to SAM
      (spectrum analyzer module), and then to CPU2.
 
      Front-End Electronics
      ----------------------
        Once charge from an MCP detector was collected on an anode, it
        enters a series of amplifiers, discriminators, and logic that
        identifies the event and begins processing. In this discussion
        the signals are identified as 'START' and 'STOP'. Both event
        types are handled in essentially identical ways. When a START
        event is detected the signal is split into two parts. One is
        used to identify the location of the event in one of eight
        elevation angles or one of the two stop channels (LEF or ST).
        This part of the signal (termed the identification or ID signal)
        is processed through fast (0.2 microsecond deadtime) preamp and
        level-discriminators and can be used to correct for deadtime in
        the slower (2.2 microsecond) circuits. The other 'half' of the
        START signal is sent to the TDC where it triggers the timing
        circuitry. Similar processes apply to the STOP signals.
 
      Time-To-Digital Converter
      ----------------------
 
        Once a START pulse triggers the TDC, the state of the ID
        discriminators is captured giving the identity of the event
        origin. Subsequent START events are ignored for timing purposes
        but are counted as ID events. Once initiated, the TDC converts
        time directly into a digital word using a clock and vernier
        technique in the following manner. A valid START enables a gated
        80 MHz clock that increments a counter. A subsequent STOP that
        occurs later than 40 ns (the pulse pair resolution of the TDC)
        after the START event, inhibits that counter, thus providing a
        coarse time measurement with a resolution of 12.5 ns. If no STOP
        event was recorded the counter times out at 1600 ns,
        corresponding to the longest time required for the heaviest,
        lowest energy ions to cross the TOF optics as discussed above.
        To obtain finer time resolution, the phase of the clock relative
        to the clock edge is measured at the instant the STOP was
        received by means of the delay line vernier implemented using a
        12.5 ns delay line with 16 taps. As the STOP pulse propagates
        down the delay line, a changing pattern of 1's and 0's appears
        on the delay line taps. This pattern is latched at the end of
        every clock period. When a STOP occurs the resulting digital
        pattern represents the time of the STOP signal relative to the
        clock edge. The FEE logic then encodes it into a binary word
        that becomes the fine time measurement. Thus the TDC resolution
        is effectively 12.5 ns/16 = 0.781 ns, corresponding to
        1600 ns/0.781 ns = 2048 channels. If the TDC is instructed to
        identify molecular events then the gated clock is fed to two
        independent counters.  The first STOP causes one counter to be
        inhibited and the vernier pattern to be read out and stored. The
        second STOP inhibits the second counter and causes a second
        vernier pattern to be read out.
 
        There are several possible outcomes of events detected by the
        TDC logic in addition to the ideal outcome (false events).
        The rate of accidental coincidence events is dependent in a
        complicated non-linear way on the total event rates.  False
        events and background must be removed from IMS data before they
        are usable.  After a TOF measurement is completed, the TDC
        encodes an 11-bit TOF value, a 3-bit elevation sector value, a
        resolution bit signaling ST or LEF data, and a 'continuation'
        bit indicating a molecular event. Each 16-bit TOF word is then
        sent to a first-in-first-out (FIFO) buffer read out by the
        spectrum analyzer module (SAM).  The TDC is then reset in
        preparation for the next measurement.
 
        During a single IMS sampling period (62.5 ms) the TDC also
        accumulates the number of ID events corresponding to the eight
        start anodes, two stop anodes, two timing discriminator
        channels, number of times when no coincident stop TOF event was
        detected, total number of TOF measurements completed, and the
        number of events recorded in two other configurable channels.
	These configurable channels can be set to be read out into the
        data stream.  To achieve the best possible performance at high-
        event rates, the TDC operates as a non-paralyzeable counter with
        a fixed deadtime of 2.187 microseconds for TOF measurements. ID
        measurements (collected in 'SINGLES' counters), on the other
        hand, have a 0.2 microsecond deadtime making them suitable for
        correcting the slower timing measurement rates. Finally, the
        FEE/TDC system has built-in test pulsers that can stimulate any
        START or STOP pair with one of 24 selectable time delays at
        periodic event rates up to 1 MHz.
 
      Spectrum Analyzer Module
      ----------------------
        The SAM processing method allows a high level of TOF data
        compression (~1000:1) to be achieved with little sacrifice in
        accuracy. The key to carrying out this process at high-event
        rates and within the short IMS sampling interval (62.5ms) is a
        high-speed, deterministic deconvolution technique that we will
        refer to as the SAM algorithm. The hardware that supports this
        algorithm has a distributed, pipelined architecture based on
        four loosely coupled, reconfigurable modules.  Once SAM
        processes a single data sample (i.e., data gathered in 62.5 ms)
        they are transferred to the CPU2 processor and the cycle
        repeated. Although data transfers from FEE to TDC and TDC to SAM
        occur at random rates, control of all other processing, data
        transfers and command and control functions are synchronized,
        taking place during the final 12.5% (7.81 ms) of the sampling
        interval.
 
        Dual memory banks allow raw data acquisition to proceed while
        CPU2 and SAM processors both access data from the previous 4.0s
        acquisition interval. At the end of each interval, spectra that
        were processed during the previous interval are read from SAM by
        CPU2. In addition, new, selectively binned TOF spectra, are read
        into the SAM processor. There, during the next measurement
        interval, the SAM algorithm deconvolves TOF spectra, extracting
        selected ion M/Q values. Once analysis is complete, SAM
        interrupts CPU2, which then reads the M/Q-sorted data. SAM is
        then ready for the next acquisition and processing cycle. SAM
        operates as a CPU2 slave. The analysis algorithm is coded in the
        Ada language and executed in the 1750A microprocessor module.
        Its address space is accessible by CPU2, and program and local
        data are loaded into the module by CPU2. However, SAM itself
        cannot read or write to CPU2 memory. The three remaining modules
        (TOF, TOFACC, and EVENT) are each autonomously controlled by
        separate gate arrays. In order to speed up TOF data processing,
        dedicated paths support concurrent data flow from the input FIFO
        to the appropriate processing modules. In addition, each module
        manages its own local processing and external access to data.
        SAM acquires data from the TDC via a FIFO buffer at a maximum
        rate of 5 x 105 periodic events/s. The FIFO can be configured by
        command to enable dual-stop operation (for molecules) and data
        acquisition from selected elevation ID channels.
 
      Spectrum Analyzer Module
      ----------------------
        The IMS high voltage (HV) system was made up of five supplies
        controlled independently by CPU1.  The supplies are contained in
        two separate units:
           1. nominally +/- 15 kV in HVU1
           2. the two MCP supplies and ESA stepping supply in HVU2
        All supplies feature current-limiting and breakdown protection,
        as well as analog monitoring (digitized in the data stream).
        HVU1 is powered by filtered +30 V from spacecraft primary power
        whereas the HVU2 supplies use +15 V generated by CAPS.
        High-voltage cables rated at >= 2.5 times the respective supply
        voltages connect HVU1 and HVU2 to IMS sensor electrodes (40 kV
        cables are used for HVU1 connections). All HV connectors were
        custom designed and fabricated from low-void ceramic to prevent
        high-electric field concentrations that might lead to HV
        breakdown over the course of the mission. Metal covers placed
        over the HV terminals reduce electric stresses and provide
        mechanical as well as contamination protection.
 
        The HVU1 supply is composed of independent positive and negative
        voltage converters whose output is maintained at +/- 1% over all
        line and temperature variations with very low ripple. The
        primary purpose of HVU1 is to provide the negative and positive
        high voltages that together create the LEF by correctly biasing
        a string of 30-high ohmic resistors running the length of the
        TOF ring stack. HVU1 also generates regulated 1200 V referenced
        to -15 kV that is used to bias the first plate in the LEF MCP
        stack. Because power for the LEF MCP regulator is derived from
        the -15 kV supply the latter must reach a minimum of approx.
        -12 kV in order for the LEF MCP to reach ~950 V and operate
        correctly. A voltage drop of -100 V is placed on a grid in front
        of the MCP to return secondary electrons to the MCP surface,
        thus maintaining high efficiency.  To make a compact but
        reliable unit, electric fields in the HVU1 supplies were kept
        below 1 kV/mm relative to the grounded housing. Transformer and
        high voltage assemblies were not encapsulated in order to avoid
        multi-material composite structures that could introduce
        mechanical stresses that might in turn lead to dielectric faults
        and eventual breakdown. Instead, after fabrication and cleaning,
        the interior surfaces and components were coated with
        Parylene-C, a tough, low-outgasing polymer with high-dielectric
        strength.
 
        Both of the MCP supplies are located in the HVU2 as is the ESA
        supply. The MCP supplies are virtually identical in construction
        and operation, relying on a pulse-width modulator control
        circuit and a resonant converter operating at a switching
        frequency of 100 kHz. The capacitor networks provide rf
        filtering. A resistor divider biases the field-correcting
        aperture located at the ESA exit to approximately one half the
        ESA plate potential. Another function of the MCP supplies, in
        addition to powering the two detectors, is supplying a
        suppression bias voltage to high transmission grids to return
        secondary electrons to the MCP (the same function provided by
        HVU1 for the LEF MCP). Similar potentials at the back of the
        third stages of both detectors accelerate electron charge clouds
        toward their respective anodes.
 
  Calibration
  ===========
 
    ELS calibration
    ---------------
 
      ELS was calibrated in the Mullard Space Science Laboratory (MSSL)
      electron calibration facility developed for Cluster
      [JOHNSTONEETAL1997]. A mercury lamp generated UV that struck a
      gold layer deposited on a quartz disk. From this, photoelectrons
      were extracted by applying a bias potential to the gold surface,
      creating an electron beam 15 cm in diameter with divergence less
      than 1 degree (at 1 keV) and good uniformity over the ELS
      aperture. During calibration ELS was mounted on a two-axis rotary
      table and turned to allow electrons from defined directions to
      enter (additional information regarding the calibration theory can
      be found in [YOUNGETAL2004]). A micron-metal shield inside the
      vacuum chamber shielded the calibration volume by reducing the
      residual magnetic field to less than 10% that of the Earth.
      Electron beams with energies above ~30 eV showed minimal
      directional deviation.
 
      Beam current measurements that provide absolute calibration were
      made with a faraday cup and picoammeter.  During calibration
      sequences beam stability was monitored with a CEM.  A tritium
      source provided a cross check after each sensor re-configuration
      to maintain consistency during calibration.
 
      Calibration of the flight model was made at ten electron energies
      between 2.3 and 16260 eV (see [YOUNGETAL2004] for details). Two
      basic types of data were taken: First a finely stepped elevation
      angular scan was made at constant energy and beam azimuth angle.
      Second, a full three-dimensional calibration (energy, elevation,
      azimuth) was obtained at defined resolutions in the three
      dimensions. The most detailed calibrations were made at 125 and
      960 eV.  In each case some 150,000 data points, corrected for dead
      time and beam monitor readings, were summed to produce a plot.
      The 8 anodes showed a nearly uniform response with some loss of
      transmission at the two end anodes. This was to be expected because
      the grid holder cuts off incident trajectories at +/- 80 degree
      elevation. A summary of 125 and 960 eV calibration data can be
      found in [YOUNGETAL2004].  Energy-angle scans with a 125eV
      electron beam were made at the azimuthal center of each of the
      eight anodes.  The plots generated showed the analyzer peformance
      in 3-dimensions was consistent from one anode to the next and
      deviated little from instrument simulations.
 
 
    IBS calibration
    ----------
 
      Initial modeling of the IBS was chiefly concerned with determining
      manufacturing tolerances required to achieve the desired
      energy-angle resolution and transmission efficiency of the ESA
      [VILPPOLAETAL1993]. Analysis of simulation results showed that it
      was necessary to align the two ESA hemispheres relative to one
      another with an accuracy of better than ~25 microns in order to
      obtain the desired energy resolution of delta E/E = 0.015.
      Furthermore, deviations of the ESA plate surfaces could not depart
      from perfect sphericity by more than 300 microns (0.3% of plate
      radius) if >90% transmission efficiency was to be maintained.
 
      Initial calibration results showed that IBS responded as expected
      except for the presence of a few small but unusual features. Of
      these, a double-bend in the energy-polar angle response was the
      most notable. To address these findings, a more accurate
      simulation of the sensor was developed and used to investigate
      IBS response in more detail [VILPPOLAETAL1996]. Improvements made
      to the model included addition of curved apertures at the correct
      standoff distance from the ESA plates, introduction of fringing
      fields, and a realistic description of the ion beam that matched
      that used in calibration. These upgrades to the model did not,
      however, account for the bend feature seen. A further refinement
      of the model allowed the introduction of slight asymmetries in ESA
      plate geometry [VILPPOLAETAL2001].  The resulting simulations with
      asymmetric hemispheres and a slight (few tens of microns)
      misalignment of the two hemispheres produced good agreement with
      laboratory results. While the flight sensor's ESA plates may
      indeed have been slightly misaligned as suggested by simulations,
      the response of the engineering model IBS was almost identical to
      the flight model, which would be surprising if a random
      misalignment occurred. Moreover, the responses of the three
      individual fan apertures in both IBS models were also very similar.
      This suggests that the unexpected calibration response was due to
      a small systematic error in alignment or was inherent in the
      overall electro-optic design and not a function of alignment
      accuracy. As of 2004, the latest simulations suggested that a
      small systematic manufacturing fault might be to blame.
 
      Calibration of IBS took place in ion beam facilities at Los Alamos
      and SwRI (the same facilities that were used to calibrate IMS).
      The work at Los Alamos concentrated on angle-angle and angle-
      energy responses while absolute energy and sensitivity
      calibrations took place at SwRI. At Los Alamos ions were produced
      in a radio-frequency discharge ion source and then accelerated
      down a 3 m flight tube into the calibration chamber where IBS was
      located. Both external supplies and the IBS internal power
      supplies were used in calibration. A nitrogen beam was typically
      used which was accelerated to between 0.3 keV and 60 keV. Beam
      location and uniformity were measured but not absolute ion
      current. Typical operating pressures were in the low 10-8 Torr
      range.
 
      The ion current extracted from the Los Alamos source was quite
      stable but not easily varied over a wide dynamic range. Therefore
      a series of slits were employed to adjust the current delivered to
      the target chamber. IBS was mounted on a stand whose orientation
      could be adjusted in one translational axis (across the beam) and
      two rotational axes. Thus the incident ion beam could be made to
      impinge on the IBS aperture at any desired combination of
      elevation and azimuthal angles. All of the diagnostic, motion
      control and data acquisition systems were computer-controlled.
      During a typical calibration run, three separate 1-D scans in
      energy, elevation and azimuthal angle would be taken across the
      center of the response function. Three central 2-D cuts through
      the response function would then be taken, followed by a series of
      energy-azimuth cuts along the elevation axis at 10 degree
      intervals. Finally, data were corrected for deadtime losses and
      variation in the beam current during the calibration run. Angular
      data were transformed from laboratory to spacecraft coordinates.
 
      After calibration at Los Alamos, the IBS was integrated with the
      CAPS flight instrument and underwent checks of calibration in the
      ion beam at SwRI. The latter was similar to that Los Alamos used
      for IMS as described below. Typical beam spread at SwRI had delta
      theta was approx. 0.2 degrees and delta E/E was approx. 0.005.
      One difference in the two calibration systems was important: the
      integrated CAPS unit was positioned to calibrate mainly IMS. Thus
      the rotation axes of the calibration goniometer were centered on
      the IMS FOV. Since the plane of the IBS apertures was offset 32.0
      cm from the central axis of the IMS and ELS sensors, the IBS
      aperture plane was neither coplanar nor co-aligned with the other
      two sensors: Any rotation of CAPS tended to move the IBS FOV out
      of the ion beam. The displacement of the aperture through the
      small IBS maximum angular acceptance of approximately +/- 2
      degrees was less than 1 cm, well within the diameter of the
      calibration beam as seen from IBS. Thus only calibrations of the
      energy response at azimuth = elevation = 0 degrees and of absolute
      sensitivity were possible.
 
 
    IMS calibration
    ----------
 
      Calibration was performed with a Von Ardenne plasma discharge
      source at SwRI and a radio frequency source at LANL [ALTON1993].
      Beam intensity was fairly stable over short time periods (~1 hour)
      but could be controlled only by pressure feedback in the source
      itself. The beam used for calibration at SwRI was monochromatic
      and plane parallel to a high degree. It was verified to have an
      angular spread of 0.2 degrees, about 2.5% of the width of IMS
      acceptance in angle. Beam width in energy was (delta E)/E = 0.005,
      about 3% of the IMS passband. Only the mass resolution of the beam
      separator was below expectations (M/(delta M))beam approximately
      equal to 40.
 
      During calibration a known ion species was selected, the energy of
      the beam set, and J0 measured. Because it was far simpler than
      varying the energy of the ion beam, the voltage on the ESA was
      micro-stepped at increments of ~0.3% of the passband. Angular
      measurements were made by placing the IMS on a three-axis
      goniometer (two angles plus translation) and rotating it about the
      center of the IMS FOV.
 
      Because of the considerable time required to obtain the full set
      of data needed to calibrate the geometric factor (G) for one
      energy and mass, only a few selected points were chosen for full
      calibration of G. On the other hand, calibration of the IMS
      response to mass, energy, and J0 could be achieved relatively
      quickly. A wide range of energies was calibrated using N+ as a
      standard. Similarly, a wide range of ion species was calibrated
      using a few specific ESA energies as standards. At every
      opportunity the value of J0 was measured to quantify absolute
      calibration. Correctly associating the center of TOF peaks with
      corresponding M/Q values for known gasses established TOF
      calibration, the correction factor sigma(E, M) for energy losses
      in the foil, and the 'effective spring constant'.
 
      The mass-analyzed ion beam at LANL was used initially to test IMS
      energy-angle response and TOF characteristics. From the latter,
      initial coefficients for the Spectrum Analyzer Module (SAM)
      algorithm could be calculated for the purpose of simulations.
      During these tests cross-talk between elevation-angle channels
      caused by internal particle scattering was discovered. Coating the
      inner plate of the ESA with copper sulfide black, an
      anti-scattering agent, eliminated most corss-talk, but some
      remained. Final calibration of the IMS in flight configurations
      was carried out at SwRI. Only about 30% of planned activities were
      completed. Fortunately that work covered nearly all aspects of IMS
      performance, although not to the desired depth, particularly in
      the area of energy-angular models of IMS (both refurbished to be
      as similar as possible to the flight model).
 
 
  Mounting
  ========
 
    One particular concern regarding location was obtaining good
    separation from the main Cassini engines and thrusters (potential
    sources of chemical contamination), separation from the radioisotope
    thermoelectricgenerators (potential source of penetrating background
    radiation), and separation from any sources of electrostatic
    charging. With all these considerations in mind, the best location
    for CAPS turned out to be on the underside of the
    fields-and-particles pallet adjacent to the MIMI/CHEMS instrument
    [KRIMIGISETAL2004] and just below the INMS [WAITEETAL2004]. Although
    meeting all of the above criteria for location, CAPS still did not
    have an acceptable field-of view because it was fixed to the
    spacecraft body and thus could only view in directions constrained
    by spacecraft orientation. In order to counteract this limitation,
    the CAPS sensors were mounted on a rotating platform driven by a
    motor actuator capable of sweeping the CAPS instrument by ~180
    degrees around an axis parallel to the spacecraft Z-axis. In this
    way nearly 2pi steradians of sky could be swept approximately every
    3 minutes regardless of spacecraft motion or lack thereof.  While
    not ideal for plasma measurements under all circumstances (e.g. when
    the spacecraft body blocked the direction looking into a plasma
    flow), careful design of observing periods permited effective
    performance under most conditions.
 
    The ELS and IMS axis of symmetry and the IBS field of view were all
    perpendicular to the sense of rotation for the CAPS instrument,
    which was about an axis parallel to the Z-axis.  When the actuator
    was at position 0, the field of view (FOV) was boresighted with the
    imaging subsystems out the -Y axis.  Further, azimuths were defined
    in the spacecraft X-Y plane and elevation was defined as being
    parallel to the spacecraft Z-axis.
 
    Even though adding a rotational platform provided a means to turn
    the instrument, the spacecraft still occluded parts of the CAPS
    field of view.  At ~ +80 degrees azimuth, parts of the fields and
    particles pallet (FPP), the neighboring MIMI/LEMMS instrument, and
    the RTG shielding obscured the CAPS field of view.  A visual
    description of this discussion can be found in the CAPS instrument
    paper [YOUNGETAL2004].
 
 
  Operational Modes
  =================
 
    Depending on mission phase and spacecraft power resources, CAPS
    transitioned to a lower-power state.  This sleep state was preferred
    to being powered-off, as it did not require power-cycling the high
    voltages and the time to recover to full science functionality was
    shorter.  In sleep mode, the ESA voltages were set to zero and the
    IBS, ELS, and IMS voltages were stepped down to safe levels.  In
    addition, the +/- 14.56 kV voltages were lowered to a safe value.
    The IBS and ELS sensors could be brought out of this state in ~15
    minutes. The IMS sensor required ~2 hours.  While in sleep the
    instrument did not produce any science packets, though it did
    continue to produce houskeeping data.  All spacecraft operating
    modes (OpModes) planned for tour allowed CAPS to remain on and
    operating.  Use of sleep mode was expected to be limited to
    activities associated with Saturn orbit insertion and the Huygens
    probe mission.
 
    Within the normal science state CAPS was capable of performing a
    wide range of table-driven programs devoted to science data
    gathering. As part of the science planning process, observational
    modes were specified that controlled the range of velocity space
    covered by the sensors, the rate at which velocity space was
    scanned, the conditions under which IMS data was taken, and the data
    products returned from the three sensors.  Voltage tables that
    controlled each sensor's ESA, as well as the range of actuator
    angles scanned, could be independently programmed thus controlling
    the ranges of velocity space covered by CAPS.
 
    The range of actuator motion was a compromise between time
    resolution and angular coverage. Sweeping through 180 degrees (close
    to the maximum range) required approximately 200 seconds, while
    sweeping through a 24 degree range took only 48 seconds. The
    actuator could also be held at a constant position, producing a
    two-dimensional cut of velocity space at the 2 or 4 second energy
    sweep period of the sensors. This was the planned mode of operation
    during icy satellite encounters, which were very rapid (e.g. during
    Enceladus encounters, the spacecraft crossed the satellite's
    geometric wake in under 50 seconds).
 
    Solid-state recorder (SSR) memory on the spacecraft was allocated on
    a downlink pass or obervation period basis based on the detailed
    Cassini science plan.  Depending on the CAPS allocation and
    particular measurement objectives during the day, one or more data
    rates and their duration were chosen to produce the required volume.
 
    In addition to the normal science mode, CAPS had an engineering mode
    in which multiplier gains were checked, pulser stimulation signals
    were used to test and calibrate detector signal chains, and detector
    background was measured for an extended period. Before reaching
    Saturn the engineering mode was operated as frequently as cruise
    data volume and pointing restrictions allowed. After reaching Saturn
    this mode was part of a periodic instrument calibration occurring
    approximately once every 50 days.
 
 
  Measured Parameters
  ===================
 
 
  ------------------------------------------------------------------------
  Table I.  CAPS sensor performance summary
  ------------------------------------------------------------------------
  Parameter                              IMS          ELS          IBS
  ------------------------------------------------------------------------
                              Med. Res.   High Res.
                              ---------------------
 
  Energy/charge response
    Range(eV/e)                     1-50,280       0.6-28,750     1-49,800
    Resolution(delta E/E)             0.17            0.17         0.014
 
  Angular response
    Elevation sectors (number)         8               8            3
    Instantaneous FOV              8.3x160deg      5.2x160deg  1.4x150 deg
    Angular resolution             8.3x20deg       5.2x20deg   1.4x1.5 deg
 
  Mass/charge response
    Range (amu/e)             1 ~ 400     1 ~ 100     ---          ---
    Resolution(M/delta M)        8           60       ---          ---
    Energy-geometric factor*
     (cm^2 sr eV/eV)           5x10^-3    5x10^-4   1.4x10^-2    4.7x10^-5
 
  Temporal response
    Per sample (s)                 6.25x10^-2     3.125x10^-2  7.813x10^-3
    Energy-elevation (s)              4.0             2.0          2.0
    Energy-elevation-azimuth (s)                180
  ------------------------------------------------------------------------
  *Applies to total field-of-view and includes efficiency factors
 
 
    References:
    ===========
 
    [YOUNGETAL2004]
 
      D.T. Young, J.J. Berthelier, M. Blanc, J.L. Burch, A.J. Coates,
      R. Goldstein, M. Grande, T.W. Hill, R.E. Johnson, V. Kelha, D.J.
      McComas, E.C. Sittler, K.R. Svenes, K. Szego, P. Tanskanen, K.
      Ahola, D. Anderson, S. Bakshi, R.A. Baragiola, B.L. Barraclough,
      R.K. Black, S. Bolton, T. Booker, R. Bowman, P. Casey, F.J.
      Crary, D. Delapp , G. Dirks, N. Eaker, H. Funsten, J.D. Furman,
      J.T. Gosling, H. Hannu la, C. Holmlund, H. Huomo, J.M. Illiano, P.
      Jensen, M.A. Johnson, D.R. Linder, T. Luntama, S. Maurice, K. P.
      McCabe, K. Mursula, B.T. Narheim, J.E. Nordholt, A. Preece, J.
      Rudzki, A. Ruitberg, K. Smith, S. Szalai, M.F. Thomsen, K.
      Viherkanto, J. Vilppola, T. V ollmer, T.E. Wahl, M. Wuest, T.
      Ylikorpi, C. Zinsmeyer, Cassini Plasma Spectrometer Investigation,
      Space Sci. Rev. 114, 1-112, 2004.
 
    [KRIMIGISETAL2004]
 
      Krimigis, S.M., D.G. Mitchell, D.C. Hamilton, S. Livi, J.
      Dandouras, S. Jaskulek, T.P. Armstrong, J.D. Boldt, A.F. Cheng, G.
      Gloeckler, J.R. Hayes, K.C. Hsieh, W.-H. Ip, E.P. Keath, E.
      Kirsch, N. Krupp, L.J. Lanzerotti, R. Lundgren, B.H. Mauk, R.W.
      McEntire, E.C. Roelof, C.E. Schlemm, B.E. Tossman, B. Wilken, and
      D.J. Williams, Magnetosphere Imaging Instrument (MIMI) on the
      Cassini Mission to Saturn/Titan, Space Sci Rev. 114, 233-329,
      2004.
 
    [WAITEETAL2004]
 
      Waite, J.H., W.S. Lewis, W.T. Kasprzak, V.G. Anicich, B.P. Block,
      T.E. Cravens, G.G. Fletcher, W.-H. Ip, J.G. Luhmann, R.L. McNutt,
      H.B. Niemann, J.K. Parejko, J.E. Richards, R.L. Thorpe, E.M.
      Walter, and R.V. Yelle, The Cassini Ion and Neutral Mass
      Spectrometer (INMS) Investigation, Space Sci. Rev. 114, 113-231,
      2004.