DESCRIPTION |
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.
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