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Traverse Gravimeter Experiment (TGE)

Copyright © 2006 by Karl Dodenhoff.
All rights reserved.
Adapted by the author from
My Little Space Museum.
Last revised 29 March 2013.

Detail from NASA photo AS17-142-21730
TGE deployed off the rover by Gene Cernan, during EVA 3 at Station 8
at the base of the Sculptured Hills, Taurus-Littrow Valley.


Above Left - TGE preflight photo.  Note the odd blue thermal blankets.
(NASA Photo ID S72-53953)

Center Left - Internal photo of the TGE.  The sphere is the gimbal which housed the VSA sensor.
(NASA Photo ID S72-53952)

Center Right - Gene Cernan practicing with the TGE during EVA training.
(NASA Photo ID AP17-S72-44420)

Right - TGE as it appeared mounted at the rear of LRV 3.
(Detail from NASA Photo ID AS17-137-20979)

Above left - An excellent TGE diagram.
Center - The Vibrating String Accelerometer (VSA) sensor,
which was the heart of the system.

Right - Artist's concept of crewman operating TGE off the LRV.
(In reality, leaning forward like this was NOT a good idea!)

Apollo Experiment Number: S 199

Apollo Missions: 17

Wt: 14.6 kg

Dim: 50.8 x 27.9 x 24.8 cm

The Traverse Gravimeter Experiment (TGE) was a unique experiment carried on Apollo 17, mounted on the geopallet of the Lunar Roving Vehicle (LRV).  It was built by Draper Labs of MIT.  According to the Apollo 17 Preliminary Science Report "The primary goal of the traverse gravimeter experiment (TGE) was to make relative gravity measurements at a number of sites in the Apollo 17 landing area and to use these measurements to obtain information about the geological substructure. A secondary goal was to obtain the value of the gravity at the landing site relative to an accurately known value on Earth. Both these goals were successfully achieved by the experiment. A gravity tie has been obtained between the Taurus-Littrow landing site and the Earth with an estimated accuracy of approximately 5 mgal. Relative gravity measurements that can be used to infer the substructure of the area have been obtained at stations visited during each period of extravehicular activity (EVA)."

The primary sensor which formed the heart of the system was a Bosch Arma D4E Vibrating String Accelerometer (VSA), which consisted of a pair of strings mounted inside an apparatus which very precisely measured their vibrations when an electric current was passed through them.  The VSA assembly was in turn mounted inside a two axis gimbal system which served to level the system inside the TGE housing before the measurements were taken.

The TGE assembly consisted of the instrument assembly, a battery pack, a thermal blanket assembly, and an inner isoframe assembly.  The outer structure included a carrying handle and LRV mounting equipment, a small radiator and readout assembly, a power toggle switch, three operation pushbuttons, and three small legs at the base for operations off the LRV.

The system was simple to use and operate by the crew.  It could be operated both on and off the back of the LRV.  First, the crewman ensured that the system was level within 15 degrees of vertical.  He then pushed the "GRAV" button on the front of the housing.  After approximately three minutes, during which time the system was required to be undisturbed, a nine digit readout would appear on the display panel.  These numbers were then read down to the ground by the crewmen.  Another pushbutton, labeled "READ" was on the front face of the housing.  It allowed the crewmen to read the display after it went out, as it was designed to remain illuminated for only 20 seconds, to conserve battery power.  The third button was labeled "BIAS" which was used to obtain calibration readings when the system was "inverted".

The TGE was operated a total of 26 times during the three EVAs on Apollo 17, with excellent results.  The readings are catalogued in Table 13-1 from the Apollo 17 Preliminary Science Report.  As discussed in chapter 13 of the Preliminary Science Report, the results suggest that the basalt layer filling the valley is about a kilometer thick. A similar instrument, the Lunar Surface Gravimeter, experiment S 207, was deployed as part of the Apollo 17 ALSEP array, but it failed to function properly.

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