Instrument Overview
  ===================
    From [SEIFF&KNIGHT1992]:
 
    The ASI experiment will operate over an altitude range of about
    870 km in two measurement modes, one to accommodate conditions of
    high-speed entry at low ambient density; the other, the very
    different conditions of parachute descent.  The entry mode begins
    at a nominal ambient density threshold of 10^-11 kg m^-3
    (presently believed to be about 750 km altitude above the 1 bar
    level) where the Probe deceleration is expected to be about 15
    micro g.  From measurements of Probe deceleration under the
    action of atmospheric drag, atmospheric densities will be
    derived.  The density profile is integrated above a given
    altitude to define the pressure at that level, and the
    temperature profile is then obtained through the equation of
    state, given the variation of atmospheric mean molecular weight
    with altitude.  This mode of operation is continued until the
    Probe deploys its parachute, nominally at the 100 mb level (47 km
    above the 1 bar level).
 
    Then, during the subsequent parachute descent of the Probe to the
    nominal end-of-mission depth at 16 bars, the thermal structure of
    the atmosphere is defined by measurements of temperature,
    pressure, and acceleration in descent mode.  Current estimates
    are that the mission could extend to below 20 bars, perhaps as
    deep as 25 bars.
 
    At entry, the Probe is effectively a 48 km s^-1 meteor, enveloped
    by a bow-shock wave and a thin shock layer of ionized,
    luminescent gases at extreme temperature (~15000 K at peak).
    Under these conditions, measurements of the ambient atmosphere by
    means of conventional low-density sensors are clearly infeasible;
    it is futile to extend sensors into the shock layer or through
    the bow-shock wave, because they quickly burn away and, outside
    the probe bow wave, develop shock layers of their own.
    Measurements of atmospheric density by way of Probe
    decelerations, however, provides a direct means of sensing the
    atmosphere.
 
    After the parachute is deployed, the heat shield is jettisoned
    and temperature and pressure sensors are exposed to the ambient
    atmosphere.  Acceleration measurements are continued, but at much
    less frequent intervals, and in the absence of large vertical
    winds, continue to define atmospheric density.  From the three
    measured state variables, temperature, pressure, and density, the
    atmospheric mean molecular weight may be determined.  Other
    instruments on the Probe, including the mass spectrometer and the
    helium abundance detector, will probably determine the molecular
    weight more accurately, however.  In these circumstances, the
    accelerometer data may be used to define the magnitude of large
    vertical winds in waves or gusts.
 
    Altitudes relative to any convenient reference level (e.g., the 1
    bar level) are defined by the temperature and pressure data
    integrated in the equation of hydrostatic equilibrium.  These
    data will be used to establish the altitudes of measurement for
    use by all Probe experiments.  They also define the rate of
    descent, which is necessary to the analysis of Doppler wind
    experiment.  In addition, atmospheric turbulence, radius to the
    center of the planet, and Probe angle of attack are sensed or
    derived.
 
    The nominal end of mission, at 16 bars, is at an altitude of
    about -120 km below the 1 bar level.  The vertical measurement
    range of about 870 km will be covered in a little over one hour -
    the initial 700 km in less than 4 min.  The time in descent on
    the parachute at velocities from the order of 100 to 30 m s^-1
    could be up to 75 min.
 
    The instrument electronics perform a number of essential
    functions.  They receive and execute commands from the Probe
    systems and thereby set the experiment mode (calibrate, entry, or
    descent), control the measurement sequences, select sensor
    ranges, collect data from the three sensor sets, amplify signals,
    perform A/D conversions, do some onboard data processing, and
    condition instrument power.  The microprocessor controlled
    electronics were designed and built by Martin Marietta Aerospace,
    in Denver, Colorado.
 
    This is a fourth generation entry probe atmosphere structure
    instrument.  Its predecessors were used on the four probes of the
    Pioneer Venus Mission, on the Viking Mission to Mars, and on the
    Planetary Atmosphere Experiments Test (PAET) entry probe into the
    Earth's atmosphere.  Best features of earlier instruments were
    retained and some new capabilities have been added.
 
 
  Scientific Objectives
  =====================
    (1)  To accurately define the state properties as a function of
         altitude below the 100 mb level to ~20 bars. (They have never
         been measured below the 1 bar level.)
 
    (2)  To define the currently highly uncertain state properties of
         the upper atmosphere.
 
    (3)  To measure the stability of the atmosphere, and identify
         convective layers and stable layers, where they exist.
 
    (4)  To detect cloud levels from changes in lapse rate at their
         boundaries.
 
    (5)  To define state properties within the clouds, and thus provide
         supplementary information on cloud composition.
 
    (6)  To search for and characterize wave structures in the
         atmosphere.
 
    (7)  To search for and measure intensity and scale of turbulence in
         the atmosphere.
 
    (8)  To measure vertical flow velocities above a threshold of about
         0.3 m s^-1.
 
    (9)  To establish an altitude scale for use in correlating all
         Probe experiment data.
 
    (10) To define the probe vertical velocity, necessary to the
         analysis of the Doppler wind experiment.