urn:nasa:pds:context:instrument:tg.a17l 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.a17l::1.0 instrument_to_instrument_host Apollo 17 Preliminary Science Report, NASA SP-330, published by NASA, Washington, D.C., 1973. reference.APOLLO17A1973 Apollo 17 mission - lunar roving vehicle traverse gravimeter experiment motion sensitivity test, NASA Johnson Space Center, JSC-07948, Houston, Texas, Dec. 1973. reference.APOLLO17B1973 Apollo Scientific Experiments Data Handbook, NASA Technical Memorandum X-58131, JSC-09166, published by NASA Johnson Space Center, Houston, Texas, Aug. 1974. reference.APOLLOSEDH1974 Buck, S. W., J. S. Eterno, G. Mamon, R. T. Martorana, R. G. Scott, and W. A. Vachon, Traverse gravimeter experiment final report, R-739, Charles Stark Draper Laboratory, Massechussetts Institute of Technology, Cambridge, Massechussetts, August 1973. reference.BUCKETAL1973 Mamon, G., Traverse gravimeter for the lunar surface, E-2603, Charles Stark Draper Laboratory, Massechussetts Institute of Technology, Cambridge, Massechussetts, August 1971. reference.MAMON1971 Talwani, M., G. Thompson, B. Dent, H.-G. Kahle, and S. Buck, Traverse gravimeter experiment, In Apollo 17 Preliminary Science Report, NASA SP-330, published by NASA, Washington, D.C., 1973. reference.TALWANIETAL1973 TRAVERSE GRAVIMETER Atmospheric Sciences not applicable not applicable 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.