Unlike the static GPS units mentioned in my previous post, these units are designed for mobility. The survey involves taking readings at set benchmarks along a survey line in the caldera. Any small changes in the position of the benchmarks give indications of deformation which could preceed an eruptive phase.
A gravity survey was also conducted. These expensive and incredibly sensitive devices measure the torque on a metal spring attached to a weight to measure local gravity anomalies. These anomalies could indicate movement of magma beneath the surface.
images courtesy of USGS