PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "Lyle Huber, September 1998" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = GP INSTRUMENT_ID = NEP OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "GALILEO PROBE NEPHELOMETER" INSTRUMENT_TYPE = "NEPHELOMETER" INSTRUMENT_DESC = " 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. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "RAGENTETAL1992" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END