Instrument Overview
  ===================
    The Galileo Probe Net Flux Radiometer (NFR) measured net and
    upward radiation fluxes in Jupiter's atmosphere between about
    0.44 bars and 14 bars, using five spectral channels to separate
    solar and thermal components.  The instrument used an optical
    head extending through the probe wall to obtain views of the
    Jovian atmosphere.  It sampled upward and downward radiation
    fluxes with a single 40 degree (full angle) conical field of view
    chopped between directions +/- 45 degrees from horizontal.
 
    The NFR consists of two major sub-assemblies: the electronics
    module (EM), and optical head (OH).  The electronics module is
    about 13 cm x 19.5 cm x 16 cm high, while the optical head is
    about 8.5 cm x 8 cm high x 10.5 cm long.  The total weight of the
    NFR is 3.134 kg, of which the optical head is 0.672 kg.  The
    electronics module has four feet which are bolted to the Probe
    instrument shelf.  The optical head, supported only by its
    attachment to the electronics module, extends out of the Probe
    through the Probe thermal blanket and aeroshell to allow the NFR
    sensors to view atmospheric radiation.
 
    The instrument was built by Martin Marietta Astronautics Group.
    The principal investigator is Dr.  Lawrence Sromovsky of the
    University of Wisconsin Space Science and Engineering Center.
 
 
  Scientific Objectives
  =====================
    On December 7, 1995 the Galileo Probe Net Flux Radiometer made
    the first in-situ measurements of radiative fluxes within
    Jupiter's atmosphere.  The instrument's first targets were the
    primary drives for atmospheric motions: absorbed solar radiation
    and the flux of energy from the planet's interior.  Because solar
    radiation absorption and planetary emission occur at different
    places and altitudes, net radiative heating and cooling result in
    buoyancy differences that force atmospheric motions.  An
    understanding of Jovian circulation thus requires knowledge of
    the vertical profile of radiative heating and cooling and its
    horizontal distribution as well.
 
    The NFR contributed to this understanding by measuring the
    difference between upward and downward radiation fluxes, the net
    flux, as a function of altitude during probe descent.  Because
    the radiative power per unit area absorbed by an atmospheric
    layer is equal to the difference in net fluxes at the boundaries
    of the layer, the vertical derivative of the net flux defines the
    radiative energy absorbed per unit volume, and thus defines the
    radiative heating (or cooling) of the atmosphere.
 
    However, because the Galileo probe provided only one sample
    profile of Jovian atmospheric conditions at one location and
    time, it is especially important to understand why the measured
    radiative energy deposition occurs: we might then have some idea
    of how to apply the results to other atmospheric regions which
    were not samples.  The NFR experiment contributes to
    understanding horizontal variations by making spectral
    measurements which illuminate the mechanisms by which radiation
    interacts with the atmosphere.  Five broad spectral bands were
    used to separate the vertical distribution of radiative heating
    by sunlight and radiative cooling and heating by exchanges of
    thermal infrared radiation.  The profiles of radiation flux also
    contain signatures of the substances that absorb and emit
    radiation - gases and particulates, and thus provide independent
    constraints on models of atmospheric composition and cloud
    structure.  when these are interpreted with other probe
    measurements and linked with orbiter observations they provide a
    basis for using orbiter observations to extend radiative heating
    determinations to other locations on the planet.
 
    A full treatment of scientific objectives and results can be
    found in [SROMOVSKYETAL1998].  Preliminary scientific results
    were documented in [SROMOVSKYETAL1996].
 
 
  Calibration
  ===========
    Calibration of the instrument was carried out at the University
    of Wisconsin.  Details of calibration have been documented in
    [SROMOVSKYETAL1992] and [SROMOVSKY&FRY1994].  The calibration
    constants and algorithms used for the flight instrument can be
    found in [SROMOVSKYETAL1998].
 
 
  Operational Considerations
  ==========================
    Several special factors affected the quality of data acquired
    during descent into Jupiter's atmosphere.  Among them were
    temperatures outside the range of instrument calibration, and
    thermal perturbations which we were unable to duplicate in the
    laboratory.  Details of our treatment of these factors can be
    found in [SROMOVSKYETAL1998].
 
 
  Optical Head
  ============
    The optical head contains a rotating optics assembly, support
    structure, apertures, reference blackbody sources and a position
    control system.  The rotating assembly includes detectors,
    field-of-view shaping optics, and heated diamond window, all of
    which rotate as a unit between three different pairs of four
    distinct angular orientations.  Because the rotating optics could
    not structurally support a high pressure differential, the
    interior of the rotating optics is designed to admit ambient
    external gas during descent.
 
    To inhibit possible condensation on interior optical surfaces, as
    flow into the rotor is controlled by vents within the electronics
    module and at the base of the rotor.  These vents take advantage
    of the dynamic pressure distribution around the Galileo descent
    probe to constrain gas flow.  Gas enters the probe through a
    large vent at the back (aft) of the probe.  some of this gas
    enters the electronics module through a molecular sieve filter at
    the top of the electronics.  The filtered gas, also warmed (in
    upper descent) or cooled (in lower descent) by the electronics
    module, enters the rear of the optical head through a vent at the
    base of the electronics module.  From this point the gas flows
    partly into the rotor through the rotor base vent (near the rear
    bearing) and partly through the motor and rotor bearings into the
    forward part of the optical head and then through the apertures
    into the external atmosphere.  The differential pressure driving
    this flow is approximately 1 mb and at 1 atmosphere the total
    flow rate through the optical head will be approximately 100
    cc/sec.
 
    When the optical system is not viewing external radiation, it
    views one of two internal radiation sources: an ambient blackbody
    source which is thermally coupled to the wall of the front
    housing, and a heated blackbody source which is servo-controlled
    to a temperature of approximately 107 deg C (the servo point is
    attained in air or vacuum, but generally is not attained in He or
    H2 atmospheres where high gas conductivity limits the blackbody
    temperature to a maximum differential above the ambient
    atmospheric temperature).
 
 
  Detector Package
  ================
    All NFR spectral channels use LiTaO3 pyroelectric thermal
    detectors to convert absorbed radiation power to electrical
    signals.  The NFR detector package contains an array of six
    detectors mounted in close proximity on a single circuit board.
    Spectral filters are mounted in a filter frame which is
    hermetically sealed to the detector circuit board, trapping xenon
    gas in the volume between the detectors and the filter frame.
    (All channels experience additional spectral filtering by the 0.2
    mm thick diamond window at the entrance to the rotating optics,
    the spectral reflectivity of the mirrors, and the spectral
    response of the detectors.) The backfill of the very heavy xenon
    gas is used to buffer the small amount of hydrogen gas which will
    diffuse into the detector package during descent.  The buffering
    effect maintains low thermal conductivity inside the detector
    package and thereby eliminates significant thermal crosstalk
    which otherwise would occur via gas conduction between detector
    elements.  There are two filter frames: an upper frame containing
    only a CaF2 long-wave blocker for channel C, and a lower filter
    frame containing five spectral filters and one opaque blocker for
    the blind channel.  Each pyroelectric detector element consists
    of a crystal approximately 1 mm x 2 mm x 25 microns thick mounted
    on 0.015 inch high mesas made of a vibration dampening material
    called Visilox.  Black paint (3M velvet) is applied to the top
    surface so as to cover the active (electroded) area but not the
    entire detector.  The typical paint thickness is 20-25 microns.
 
    Detectors have a primary thermal time constant of approximately
    110 ms, an electrical capacitance of about 40 pF, and a
    responsivity of approximately 1400 V/W.  In the laboratory the
    dominant noise source is Johnson noise associated with the
    detector load resistor.
 
 
  Spectral Response
  =================
    The NFR made measurements in five parallel spectral channels.
    Two solar channels provided complete integration of all solar
    wavelengths from 0.3-3.5 microns (B) and a red-weighted subset
    from 0.6-3.5 microns (E) in which methane absorption is most
    significant.  A broadband thermal channel (A) from 3-200 microns
    measured sources and sinks of Jupiter's thermal radiation as a
    whole.  Channel C (3.5-5.8 microns) sampled the narrow band
    5-micron window in Jupiter's atmosphere where gaseous absorption
    is relatively low.  Channel D (14-150 microns) sampled the
    hydrogen-dominated longwave region of the thermal spectrum.
    Channel F is a blind channel that measured non-radiative detector
    perturbations, needed to correct for similar perturbations in the
    other channels.  None of the spectral channels has a flat
    responsivity, and thus energy deposition profiles and heating and
    cooling rates are somewhat model dependent.  To compute a heating
    rate due to thermal radiation exchange requires a uniform
    weighting of the entire thermal spectrum.  But since the spectrum
    isn't measured, we must integrate the spectral flux density of a
    model spectrum that leads to the same simulated NFR measurement.
 
 
  Electronics
  ===========
    The NFR electronics consist of: digital circuits, analog circuits
    (detector pre-amplifiers, post-amplifiers, demodulators and
    integrators), gain select amplifier, analog to digital converter,
    housekeeping monitors, motor driver, and optics position sensors.
 
 
  Digital Circuits
  ================
    The microprocessor system consists of the 1802 CPU (Central
    Processing Unit), 256 words of RAM (Random Access Memory), 6144
    8-bit words of PROM (Programmable Read-Only Memory), nine I/O
    (Input/Output) ports and a power-up reset circuit.  The RAM is
    used to provide 256 bytes of temporary storage for data values
    during data accumulation and manipulation.  The six 1-Kbyte PROMs
    contain the program necessary to operate the instrument.  At any
    given time only one of the PROMs is turned on and for only 1
    microsecond of the 8 microsecond machine cycle, providing a
    factor of 48 reduction of power consumption by the PROMs.  Six
    8-bit wide output ports are used to control the non-digital NFR
    subsystem.  Three input ports are used to read data from the NFR
    subsystems.  The 2048 Hz spacecraft clock is divided down to a
    4-Hz signal which is used to interrupt the microprocessor.  The
    Minor Frame signal from the spacecraft is used to synchronize the
    4-Hz timer and also to synchronize the software with spacecraft
    timing.  The software does not begin cycling in its normal mode
    until the microprocessor detects a Minor Frame interrupt.
 
    The PROM software controls the sequence, timing, and duration of
    motor control pulses.  After each cycle of the optical rotor,
    position sensor phototransistors are read to determine if the
    optics are at the correct position.  If any one of the 22 half
    cycles of one instrument cycle results in an incorrect optics
    position, the microprocessor notes this in the data stream by
    setting the motor position error bit to one for that IC.
 
 
  Analog Circuits
  ===============
    There are six channels of analog processors, one for each
    detector.  Each channel includes a detector signal pre-amplifier,
    a post-amplifier, a demodulator and an integrator.  This six
    pre-amplifiers are housed in a hybrid package placed adjacent to
    the detectors on the rotating optics.  The rest of the analog
    circuits reside on two circuits boards within the electronics
    module.
 
 
  Pre-Amplifiers
  ==============
    Each pre-amplifier is a DC differential amplifier with a gain of
    6.67, using U423 dual JFET inputs.  The effective input load
    resistance of 0.909E+10 ohm (1E+10 ohm in parallel with 1E+11
    ohm) in combination with the typical detector capacitance of 40
    pF leads to a detector electrical droop time constant of 0.36 s.
    with this droop a typical detector will generate an electrical
    offset of 0.055 V per deg C per minute of thermal ramp.  Because
    the load resistance is so much less than 1E+13 ohm detector
    resistance, detector noise is dominated by the Johnson noise of
    the load resistors (modified by the detector shunt capacitance,
    of course).
 
 
  Post-Amplifiers
  ===============
    The six parallel post-amplifiers each consists of three
    non-inverting amplifiers in series (except for channels B and E
    which have one inversion to compensate for an inversion built
    into the detector package).  Single pole RC filters are used to
    block the DC component from the hybrid and to tailor the
    frequency response of the circuit.  The filter components are
    chosen to give a maximum response at 16 Hz.  This may seem
    strange in view of our fundamental 2-Hz detector signal.
    However, this filter function acts somewhat like a
    differentiator, which, in combination with the following
    integrator, results in a very small sensitivity to the details of
    signal transitions and a high sensitivity only to the final
    values attained after each flip of the rotor.  This effect
    minimizes asymmetry errors.
 
    FET (Field Effect Transistor) switches at the input of each post-
    amplifier allow the inputs to be grounded through a 100 ohm
    resistor, providing a zero reading to be integrated as the Analog
    Zero (AZ) data.  The AZ value is intended to be a measure of
    offset in the integration circuitry.
 
    The gain of the post amplifiers is tailored to the dynamic range
    expected from each channel.  To extend the dynamic range of
    channels A, C, and D, which receive much stronger signals from
    the internal heated blackbody than they do from Jupiter's
    atmosphere (at least in laboratory situations), the third
    amplifiers of the circuits for those channels (and also for
    channel F) have two possible gains selectable with a FET switch.
    The gain is switched to 8 for analog zero, up flux, and net flux
    measurements, and switched to unity for the blackbody calibrate
    measurement.  The solar channels, B and E, receive relatively
    weak signals from the on-board calibration source and thus do not
    need gain reduction capabilities.
 
 
  Demodulator and Integrator
  ==========================
    Each demodulator is a gain unity, reversible polarity amplifier,
    the polarity of which is controlled by two FET switches,
    synchronized to the 2-Hz NFR decommutation signal.
 
    The integrator consists of an inverting amplifier with a 0.82
    microfarad capacitor in the feedback loop and a 1 megohm resistor
    connected between the output of the demodulator and the input to
    the integrating amplifier.  A FET switch is placed in parallel
    with the capacitor to short out the charge after a measurement
    has been taken.  Two other switches control input to the
    integrator.  In one configuration the output of the demodulator
    is connected to the integrator (enabling integration); in the
    other configuration the demodulator output is disconnected and
    the integrator input is grounded (holding the integrated value
    for readout by the A/D converter).
 
 
  Gain Select Amplifier (GSA)
  ===========================
    All six integrator outputs and all housekeeping monitor outputs
    are routed by a 22-channel multiplexer to the gain selection
    circuits which properly scale those analog signals for input to
    the Analog to Digital Converter (ADC)described below.  A
    3-channel gain select multiplexer selects either the output of
    the 22-channel multiplexer or the output of one of two cascaded
    amplifiers, each with a gain of eight.  The three multiplexed
    channels view the output of the 22-channel multiplexer at gains
    of 1, 8, and 64.
 
 
  Analog to Digital Converter (ADC)
  =================================
    The ADC is a 12-bit, +10 to -10 V, successive approximation type
    converter, used over a +5 to -5 V 11-bit range only.  The most
    significant bit (bit 11) indicates polarity of the signal, and
    the second most significant bit indicates a positive or negative
    overrange.  Data reported to the Probe telemetry include the
    sign, two bits indicating the gain setting of the GSA, and nine
    bits of data from the ADC (bits 1 through 9 - bit zero is
    unused).
 
    The microprocessor changes the gain of the GSA as required to
    obtain an on-scale reading for the ADC.  The GSA is first set to
    its maximum gain of 64.  If the processor detects that the ADC is
    in an overrange condition (input greater than +5 or less than -5
    V), it sets the GSA to a gain of 8.  If this also leads to an
    overrange condition, the GSA is set to a gain of one.
 
 
  Housekeeping Monitors
  =====================
    The following is a list of housekeeping data that the NFR reports
    in the probe telemetry stream:
 
          HB - Hot Blackbody Temp
          A1 - Ambient Wall Warm Temp
          A2 - Ambient Wall Cold Temp
          DT - Detector Temperature
          WT - Window Temperature
          ET - Electronics Temperature
          V1 - +10 Volt ADC Reference
          V2 - +7 Volt Supply
          BI - Hot BB Current
          WI - Window Heater Current
          G1 - GSA Cal for gain=1
          G2 - GSA Zero for gain=1
          G3 - GSA Cal for gain=8
          G4 - GSA Zero for gain=8
          G5 - GSA Cal for gain=64
          G6 - GSA Zero for gain=64
 
    The diode voltage drop change with temperature of a 1N4148 serves
    as the temperature sensor for the DT and ET temperature monitors.
    The HB, A1, A2 and WT temperature sensors are Fenwal GB38SM43
    thermistors.  The HB sensor is located on the back of the hot
    blackbody printed circuit resistor.  The A1 and A2 thermistors
    are located in an aluminum mount on the ambient wall.  The WT
    sensor is located in the window housing structure on the rotating
    optics.  The DT sensing diode is mounted directly to the detector
    board on the rotating optics.
 
 
  Motor Driver
  ============
    The motor driver is basically a pair of 28-V H-bridge circuits
    capable of driving up to 250 mA of reversible current through
    each of the two motor coils.  The microprocessor controls the
    motor driver by writing a one into the appropriate latch bit to
    turn on one of two coils in one of two polarities.  In addition
    to these four latch bits, there is one additional bit reserved
    for controlling eddy current damping by shorting one of the two
    coils.  The exact time and duration of each motor coil pulse is
    controlled by the PROM software, and is tuned, prior to burning
    PROMs, to obtain stable symmetric optical head rotation
    characteristics.
 
    To obtain stable rotor motion characteristics under varying
    temperature conditions, the motor drive currents are stabilized
    by a current regulator circuit.
 
 
  Optics Position Sensors
  =======================
    The rotor gear on the rotating optics has four slots cut into it
    so that four LED-photo transistor pairs mounted around the gear
    can determine if the rotor is at 0, 90, 180, or 270 degrees (+/-
    5 deg).  Only one photo- transistor will be turned on indicating
    the position of the rotor.  If the correct transistor is not
    illuminated, this condition is reported in the data stream by
    setting the position error flag.
 
 
  Operating Modes
  ===============
    The detectors, field-of-view shaping optics, and heated diamond
    window all rotate as a unit between three different pairs of four
    distinct angular orientations.  Besides the upward and downward
    viewing positions, there are two horizontal positions, one
    providing a view of an ambient blackbody, and the other providing
    a view of a heated blackbody reference, both blackbodies being
    located in opposite instrument walls.  In the Net Flux (NF) Mode,
    the rotating optics chops between upward and downward views.  In
    the Blackbody Calibrate (BC) Mode the FOV is chopped between the
    ambient and heated blackbody references.  In Upflux Mode (UF),
    the FOV is chopped between the downward viewing direction and the
    ambient blackbody reference.  Time-integrated measurements are
    returned every six seconds for all channels in parallel.  During
    each 2-minute Data Cycle (DC) there are 20 Integration Cycles
    (IC) during which decommutated, filtered, and integrated signals
    are computed.  Among these there are 17 cycles of net flux
    measurements, one upflux measurement, one blackbody calibration
    measurement, and one analog zero measurement.  All chopping is at
    a 2-Hz rate, and the decommutated signal integration generally
    extends for 5.5 seconds (the first 11 of 12 cycles for each six
    seconds).  The two net flux measurements following AZ and UF mode
    measurements (as well as AZ mode measurements themselves) are
    'short-cycled', meaning that only the last five cycles are
    integrated, because of large DC level shifts after changing
    operating modes.
 
 
  Measured Parameters
  ===================
    The Net Flux Radiometer actually measures net radiance rather
    that net flux.  This net radiance measurement is convoluted by
    the instrument field-of-view and the non-flat spectral response.
    A discussion of the relationship between measured net radiance
    and true net flux can be found in [SROMOVSKYETAL1998] and
    [SROMOVSKYETAL1992].
 
 
  Timing
  ======
    NFR data can be related to data from other probe instruments via
    the time after Minor Frame Zero.  This parameter is included in
    all descent measurement files, for each data point.