The semiautomatic device uses a human-assisted placement and triggering mechanism. The instrument launches the test mass at a maximum rate of once each minute. The FG5 dropping chamber mechanism produces a 20-cm vertical free fall that the test object traverses in 0.2 s. One of our primary design goals was to produce a chamber competitive with this number. For portability, it was also desirable to create a chamber substantially smaller than the FG5.
Interest exists in using absolute gravimeters for dynamic airborne and marine applications.
Relative spring-type gravimeters used on moving platforms typically store a filtered sample at 1 Hz. Thus, it was desirable that the new chamber throw at or better than this standard 1-Hz rate. To achieve these design goals, the new chamber uses some of the FG5 features, including a motor-driven mechanism for throwing, and a cofalling, dragfree chamber around the test mass to shield it from air resistance and electromagnetic forces. To obtain a smooth launch, we increased the symmetry of the throwing mechanism and kept the design simple and robust. Fig. 2 .