Instrument Overview
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    (excerpted from [WITTEETAL1992])
 
    Abstract.  -- The properties (density, bulk velocity relative
    to the solar system, and temperature) of the local interstellar
    gas, represented by neutral helium penetrating the heliosphere,
    will be measured in-situ for the first time by the ULYSSES GAS
    instrument.  By employing the solar gravitational field as a
    natural velocity analyser, the bulk velocity relative to the
    solar system and the temperature of the E as can be derived
    from the angular distributions of the particles measured in at
    least two widely separated points in the heliosphere.  The gas
    density can be determined if a composition corresponding to
    cosmic abundances is assumed.  The neutral particles are
    detected via the secondary electrons or ions which are emitted
    upon particle impact from a freshly deposited lithium-fluoride
    (LiF) layer.  The physical principles and assumptions on which
    the experiment is based, the main technical features of the
    instrument, and first measurements in space are briefly
    described.