The Apollo 17 Traverse Gravimeter was a self-contained, self-leveling lunar gravimeter powered by an internal battery and capable of resolving gravitational acceleration differences with an effective accuracy of 0.5 to 1 milligal on the Moon.  The primary objective of the Traverse Gravimeter (TG) Experiment (Apollo Experiment S-199) was to make a high-accuracy relative survey of the lunar gravitational field in the Apollo 17 landing area and to use these measurements to obtain information about the geological substructure.  A secondary goal was to make an Earth-Moon gravity tie. Specific objectives were to measure the value of gravity, relative to the value at a lunar base station, at selected known locations along the lunar traverse and to measure the value of gravity at a known point on the lunar surface (base station), relative to the gravity at a known point on Earth.

Instrument Overview
  ===================
    The Apollo 17 Traverse Gravimeter was a self-contained, self-leveling
    lunar gravimeter powered by an internal battery and capable of
    resolving gravitational acceleration differences with an effective
    accuracy of 0.5 to 1 milligal (1 gal = 0.01 meter/second^2) on the
    Moon.  The primary objective of the Traverse Gravimeter (TG)
    Experiment (Apollo Experiment S-199) was to make a high-accuracy
    relative survey of the lunar gravitational field in the Apollo 17
    landing area and to use these measurements to obtain information about
    the geological substructure.  A secondary goal was to make an
    Earth-Moon gravity tie. Specific objectives were to measure the value
    of gravity, relative to the value at a lunar base station, at selected
    known locations along the lunar traverse and to measure the value of
    gravity at a known point on the lunar surface (base station), relative
    to the gravity at a known point on Earth.
 
    The Traverse Gravimeter instrument package was rectangular in shape
    with a cylindrical surface at the front. It was approximately 51-cm
    high, 28-cm wide, and 25-cm deep, with a mass of 12.7 kg.  The package
    was insulated by a multilayer blanket for thermal protection.  A
    folding handle at the top of the instrument was used for hand carrying
    and for securing the instrument to the Lunar Roving Vehicle (LRV).  It
    had three footpads at the base for surface operations. On top of the
    case was a nine digit display.  A radiator for thermal control was
    also located at the top of the case.  Inside the case was a 7.5-volt,
    375- watt-hour battery pack and a two-axis gimbal system which
    contained the gravity sensor housed in a thermally protected and
    evacuated two-stage oven assembly, comprising an intermediate and a
    precision oven.  The oven assembly was enclosed in an electronic frame
    and ceramic insulating ball supported by the gimbal assembly, which
    was attached through bearings to the housing.  Pendulums acted as
    level sensors, supplying signals to stepper motors and a gear train
    which position the gimbal assembly so that the gravity sensor is
    within 3 minutes of arc of vertical.  The ovens maintained a constant
    temperature (within 0.01 K) near 322 K using electric heaters and a
    resistance thermometer.
 
    The gravity sensor was a Bosch Arma D4E vibrating string accelerometer.
    Each of the two strings is energized and generates continuous vibrations
    with a frequency dependent on the local gravity.  The difference between
    the frequencies is a simple function of gravity.  The apparatus is a
    cylinder with ring magnets at either end.  At the center of the cylinder
    is a soft spring attached to cylindrical masses above and below, the
    masses are attached to the insides of the cylinder by cross supports.
    At the center of each mass the electrically conducting vibrating strings
    are attached, passing through the ring magnets before extending out each
    end of the cylinder.  When a voltage is applied across the string, the
    resulting current, running through the ring magnetic field, caused
    motion in the string and induced a voltage.  The voltage was regenerated
    through a stable high-gain amplifier and fed back to the string.  The
    signal was fed to a phase-lock loop module to determine the difference
    frequency.  The difference frequency was measured using a precision
    125-kHz clock.
 
    The TG could be operated mounted on a pallet on the back of the LVR
    or placed directly on the surface.  The vertical axis of the package
    had to be within 15 degrees of vertical to make a measurement and
    required about 3 minutes of undisturbed operation. For a normal
    measurement, the instrument would level itself and then measure the
    difference frequency of the strings.  The number of counts of the
    precision clock, from which the frequency can be determined, would be
    given as the first seven digits on the display.  The eighth and ninth
    digit were for thermal monitoring.  For calibration, the gravimeter
    could be inverted and a 'bias' measurement would be made.
 
    In all, 26 measurements were made over the course of three extra
    vehicular activities (EVAs).  The first measurement on each EVA was a
    thermal monitor reading at the Lunar Module (LM) site to check the
    thermal condition of the gravimeter.  Two inverted bias readings were
    made, one near the beginning of the campaign and one as the last
    measurement.  The instrument was disturbed during reading 8, this
    reading is considered to be unreliable.  Following the reading at
    Station 2 on the second EVA, which was particularly high, the
    principal investigator requested a reading be made between Stations 2
    and 3, this is referred to as reading 2a.  During the traverse from
    Station 9 to the LRV on the third EVA, the pallet on which the
    traverse gravimeter was mounted apparently swung loose and banged
    against the LRV, potentially shocking the instrument.  Therefore, the
    last two measurements, 25 and 26, made at the LM site after this
    incident, are considered to be of suspect reliability.  A total of
    eight gravity readings were made at the LM site and 13 were made at
    other stations along the traverses, along with the three thermal
    readings and two bias measurements.
 
    Three pairs of readings were made at the same position, the first on
    the rover and the second on the surface (readings 2 and 3, 21 and 22,
    23 and 24).  These showed a decrease of roughly 6 milligal on the
    ground compared to on the rover.  One possibility, suggested by
    post-mission engineering tests on the flight spare and engineering
    prototype, was that a one way bias shift could have been induced by
    moving the gravimeter.  If the shift was unrecoverable it would result
    in a 'staircase' effect, with the bias (i.e. the effective baseline
    reading) increasing each time the gravimeter was transferred from the
    rover to the ground.
 
    For more information, see the Apollo 17 Preliminary Science Report
    [APOLLO17A1973], the Apollo 17 Traverse Gravimeter Motion Sensitivity
    Test Report [APOLLO17B1973], the Apollo Scientific Experiments Data
    Handbook [APOLLOSEDH1974], Buck, et al. (1973) [BUCKETAL1973], Mamon
    (1971) [MAMON1971], and Talwani, et al. (1973) [TALWANIETAL1973].
 
  This instrument description was provided by the NASA National Space
  Science Data Center.

extracted metadata from PDS3 catalog and
                    modified to comply with PDS4 Information Model

Apollo 17 LM Traverse Gravimeter

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