From [RAGENTETAL1992]:
 
  Instrument Overview
  ===================
    The Nephelometer is designed to achieve the desired objectives by
    comparing simultaneous measurements of the light scattered at
    five angles from a well-defined volume of atmosphere in the
    vicinity of the Probe with theoretical models of light scattering
    from particulate matter.  A similar approach was successfully
    used by Marov et al.  (1980), for measurements made from the
    Venera Probes in the Venus atmosphere.
 
    A cloud or haze is characterized by the way in which it scatters
    light.  In particular, each unit of volume illuminated by a beam
    of light will scatter the light at a given angle, theta, in
    proportion to the product of the particle number density, n, and
    the probability of the particles in that volume to scatter light
    into a unit solid angle at that angle, the differential
    scattering cross section, [d sigma/d Omega]_theta.  The
    Nephelometer measured this quantity, at five angles.
    Measurements are then compared with calculations of the same
    quantities for model aerosols to obtain the best agreement with
    the experimental data.  Results of such comparisons yield mean
    particle sizes, particle number densities, and indications of
    non-sphericity of the particles and/or absorption in the
    particles.  The accuracy with which these quantities can be
    determined depends on the accuracy of the experimental data and,
    to a small extent, on the availability of subsidiary information,
    for example hints or particle composition from other experiments
    on the Probe.  A description of one method of performing such
    comparisons to obtain the best fit to the data is given by Marov
    et al. (1980).
 
    The instrument contains the following components: (1) pulsed
    solid state laser light sources, (2) solid state scattered light
    detectors, (3) collimating, defining, collecting optics,
    including a deployable axicon (axially-symmetric conical) mirror
    system, and spectral filters, (4) optical alignment, surface
    condensation and source output monitors, (5) other housekeeping
    measurement systems to monitor instrument operation and
    performance, and (6) analog and digital electronics circuitry and
    power supplies.  The mechanical structure, deployment system, and
    thermal design assure that the instrument will survive the severe
    launch, cruise phase.  atmospheric entry, and descent
    environments.
 
    A number of complicating factors must be considered in the design
    of the instrument.  For example, the required high sensitivity to
    small scattered light signals and the relatively large background
    light levels (up to 10^6 times as large as the minimum signal
    levels), as well as the large dynamic range of expected signals
    (of the order of 10^5 to 10^6), necessitate very careful signal
    processing.  An irradiating light beam collimated highly enough
    for the measurement of small angle scattering in the forward
    direction, yet powerful enough to provide sufficient scattered
    light for measurement of the relatively small scattering at wide
    angles is required.  This requirement is further complicated by
    the need to reduce instrumentally scattered light, the severely
    limited space, and the need for reliable source operation after
    an extended multiple-year cruise phase.  In addition, large
    zero-signal baseline effects may be caused by electrical signals
    induced by the operation of high power pulsed sources near very
    sensitive detector circuitry.  There is a need to survive not
    only the severe launch, cruise phase, and atmospheric entry
    environments, but also the intense high-energy radiation in
    passing through the Jovian radiation belts.  The effects of this
    radiation on the reliability and stability of electronic
    components and circuitry need to be carefully considered in the
    instrument design.  Finally, there are the requirements of
    relatively low allowable weight, space, power, and data rate.
 
    Physically, the instrument is constructed in three parts, a
    vented sensor head containing the forward scatter unit, a vented
    sensor head containing the backward scatter configuration, and a
    pressure-tight electronics unit containing the bulk of the
    electronics.  A photograph is shown in Figure 1.  The scaled unit
    is capable of withstanding pressures of greater than 20 bars with
    negligible leakage.  The vented sensor heads, containing
    components also capable of withstanding pressures greater than 20
    bars, are connected to the electronics unit with cables
    terminating in pressure-tight connectors sealed into the wall of
    the electronics unit.  Both units are mounted onto the aft side
    of the instrument shelf of the Probe.  The faces of the sensor
    units are flush with the Probe skin, and the
 
                                TABLE I
 
    Instrument characteristics.  The dynamic range for all channels
    is approximately 10^6, and the mean source wavelength for both
    forward and backscatter sources is approximately 904 nm.  The
    effective sampling volume decreases for strong signals as the
    number of sampled pulses is reduced.
 
----------------------------------------------------------------------
Performance
 
Scatter channels       5          16         40         70         180
                                                                  (Bkwd)
Sensitivity,
m^-1 sr^-1 cnt^-1  9.3x10^-7  5.1x10^-7  1.3x10^-7  1.5x10^-7  1.1x10^-8
 
  Mean scattering
   angle,  degrees     5.82       16.01      40.01      70.00      178.1
 
  Angular resolution,
   FWHM, degrees       0.64        1.08       1.72       1.76        4.0
 
  Effective sampling
   volume, 1           1.25        0.63       0.65       0.40       16.4
 
Physical description
 
Mechanical
  Weight, kg
 
    Sensor assembly                1.4
    Electronics                    3.0
    Total                          4.4
 
Dimensions, cm
  Sensor assembly                  50.8 x  8.9 x 12.7
  Electronics                      18.8 dia x 16.5
 
Electrical
  Power, W
    Instrument                     4.8
    Heater                         6.5
    Total                         11.33 average
  Data rate                       10 bps
  Data storage on Probe           800 bits
  Data output                     a digital, 2 bilevel
  Timing signals                  minor frame
  Commands                        3 stored, 4 real time
----------------------------------------------------------------------
 
    instrument is oriented on the Probe so that sampled volumes
    extend out of the Probe essentially radially.  A 'closeout'
    structure is used to seal the edges of the sensor faces to the
    Probe skin.  A deployable arm containing the axicon mirror
    segments, as well as the pyrotechnic pin puller that activates
    the deployment mechanism, extends from the upper corner of the
    top of the sensor unit out through the Probe skin.  This assembly
    allows forward scattering sample volumes to be situated in
    relatively undisturbed air, outboard of flow regimes near the
    skin of the Probe in which aerodynamic effects may severely
    modify the particle size distributions with respect to the true
    ambient free-stream distributions.  Calculation of these effects
    for the present case have been performed using modified methods
    similar to those described by Chow (1979).  The detector external
    windows and the axicon mirror assembly are electrically heated
    continuously during Probe descent to prevent condensation of
    atmospheric vapors.  During transit to Jupiter and the period of
    high heating on entry into the Jovian atmosphere, the Probe is
    immersed in the heat shield with the axicon mirror arm stowed in
    its undeployed position.  Targets are mounted on the inner
    surface of the heat shield, scattering fixed amounts of light
    from the forward and backward irradiating sources.  This
    scattered light is measured by the instrument, permitting checks
    of calibration stability during the long test and cruise phases
    of the mission, and shortly before entry into the Jovian
    atmosphere.  Initiation of the Nephelometer experiment begins
    after entry and deployment of the Probe parachute, removal of the
    Probe from the heat shield, and deployment of the axicon mirror
    arm.
 
 
  Scientific Objectives
  =====================
    The objective of the Nephelometer Experiment aboard the Probe of
    the Galileo mission is to explore the vertical structure and
    microphysical properties of the clouds and hazes in the
    atmosphere of Jupiter along the descent trajectory of the Probe
    (nominally from 0.1 to > 10 bars).  The measurements, to be
    obtained at least every kilometer of the Probe descent, will
    provide the bases for inferences of mean particle sizes, particle
    number densities (and hence, opacities, mass densities, and
    columnar mass loading) and, for non-highly absorbing particles,
    for distinguishing between solid and liquid particles.  These
    quantities, especially the location of the cloud bases, together
    with other quantities derived from this and other experiments
    aboard the Probe, will not only yield strong evidence for the
    composition of the particles, but, using thermochemical models,
    for species abundances as well.  The measurements in the upper
    troposphere will provide 'ground truth' data for correlation with
    remote sensing instruments aboard the Galileo Orbiter vehicle.
    The instrument is carefully designed and calibrated to measure
    the light scattering properties of the particulate clouds and
    hazes at scattering angles of 5.8, 16, 40, 70, and 178 degrees.
    The measurement sensitivity and accuracy is such that useful
    estimates of mean particle radii in the range from about 0.2 to
    20 microns can be inferred.  The instrument will detect the
    presence of typical cloud particles with radii of about 1.0
    microns, or larger, at concentrations of less than 1 cm^3.
 
 
  Calibration
  ===========
    Two methods were used to calibrate the Nephelometer.  The first
    is similar to the method described by Pritchard and Elliott
    (1960), as modified for application to the present case.  This
    technique involves recording the response of each of the
    scattering channels to the scattered light produced by a
    diffusely scattering target positioned perpendicular to the
    source beam optical axis, as the target is stepped along the
    source beam until the sensitive volume for each channel has been
    traversed.  For the forward-scattering channels a carefully
    documented diffusely transmitting screen mounted into the end of
    a set of telescoping tubes is used.  The transmitting screen
    transmittance is carefully measured using a standard integrating
    sphere and the screen's angular response and polarization
    characteristics are documented with a specially constructed
    goniometer.  Similar procedures are used to verify the
    characteristics of a large specially constructed Lambertian
    reflector that was used to calibrate the backward scattering
    channel.  Calibrated neutral density attenuating filters are used
    in front of the collecting optics for the detectors in each
    channel to maintain the signals within the dynamic range of the
    instrument.  The manner of relating the readings obtained using
    this scanning method to the calibration constants to be used in
    measuring actual aerosols is described below.
 
    The Nephelometer instrument produces counts, C, in proportion to
    the product of particle differential scattering cross section, at
    angle theta, [dsigma/dOmega]_theta (with units of m^2 sr^-1), and
    particle number density, n (with units of m^-3) with combined
    units for this product, n[dsigma/dOmega]_theta of m^-1 sr^-1.
    The proportionality constant is the product of source intensity
    I_s, effective sampling volume V_eff, and
    detector/electronics/optics gain constant K.  The instrument
    count output C can be written as follows:
 
     C = (KI_sV_eff)n[dsigma/dOmega]_theta = (1/E)n[dsigma/dOmega]_theta
 
    and, the desired measured value,
 
     n[dsigma/dOmega]_theta = CE = C/(KI_sV_eff) .
 
    In response to a diffuse calibration target normal to the source
    beam at position x, filling an effective area A_eff(x), and
    having reflectivity (or transmission) at angle of T cos theta,
    the instrument count output will be given by
 
     C(x) = t(x) = [KI_sA_eff(x)] (T cos theta)/pi .
 
    Because the normal calibration target is so bright, it is
    necessary to reduce the amount of scattered radiation reaching
    the detector with an attenuator of attenuation factor F.  By
    moving this calibration target along the beam over all x at which
    response is obtained, and integrating the response over all x, we
    obtain
 
    integral [C(x) dx] =
 
    integral [t(x) dx] =
 
    K(I_s/piF)(T cos theta) integral [A_eff(x) dx] =
 
    K(I_s/piF)(T cos theta)V_eff    .
 
    Thus, the proportionality constant E, in units of m^-1 sr^-1
    count^-1, can then be evaluated from
 
    E = (KI_sV_eff)^-1 =
 
    T cos theta{(piF)(integral[t(x) dx])}^-1  .
 
    In practice it is also necessary to make small corrections to
    account for the deviation of the reflection or transmission
    screens from true diffuse behavior, and polarization
    characteristics of the sources, screens, and detection system.
    The accuracy of this calibration procedure is a function of the
    accuracy of our knowledge of the reflection (or transmission) of
    the screen used to calibrate the Nephelometer and its simulation
    of diffuse reflection (or transmission), the accuracy of the
    measurement of the attenuation factor of the attenuator, the
    accuracy of the data taken at each target position, and the
    accuracy of the integration yielding the calibration factor.
    Estimates of the overall accuracy range from less than +- 5
    percent for the 5, 15, and 180 degree channels to less than +- 10
    percent for the 40 and 70 degree channels.
 
    The second type of calibration method involves obtaining the
    response of the instrument to a well-documented 'standard'
    aerosol environment.  These tests were performed in a large test
    chamber at Particle Measuring Systems, Inc.  (PMS) of Boulder,
    Colorado.  An aerosol with a very narrowly dispersed size
    distribution was produced by atomizing a suspension of spherical
    polystyrene or polyvinyl toluene particles into a large spherical
    chamber.  The particle sizes were measured using standard
    electron microscope sizing techniques developed for aerosol
    research at Ames Research Center.  The density of particles and
    the proportion of single particles to 'doublets','triplets',
    etc., in the actual aerosol was documented using standard
    particle sizing instrumentation manufactured and calibrated by
    PMS.  Nephelometer responses were recorded for a variety of
    particle sizes, particle densities and particle composition.  The
    calibration for each of the scatter channels was then determined,
    using Mie-scattering cross sections calculated for the
    PMS-documented aerosol distributions.  In general, the results
    obtained were within 30 to 50 percent (often within 10 percent)
    of those measured using the first method.  However, the
    variations in the results of repeated experiments in the particle
    chamber indicated that the results were less reliable than those
    of the target scanning technique.  Closer investigation indicated
    a number of variables in the test conditions that were apparently
    difficult to control.  For example, small persistent air currents
    in the test chamber were present, produced during aerosol
    injection, by thermal gradients, by the sampling of the PMS
    instrumentation, or by other causes.  These currents introduced
    inhomogeneities and differences in the particle densities as
    measured by the test instrumentation and the Nephelometer.  In
    addition, it proved to be difficult to produce an aerosol with a
    low enough content of aggregate particles, such that these larger
    particles did not appreciably affect the measured scattering
    cross sections.  It was suspected that some of the particles
    might also have been electrically charged and that electrical
    effects, for example, at the chamber walls, may have produced
    differences between the aerosol sampled by the PMS instruments
    and the Nephelometer.