Self-contained, single-frequency GPS instruments fitted
on lightweight stations suitable for helicopter-sling payloads
became a critical part of volcano monitoring during the
September 2004 unrest and subsequent eruption of Mount St.
Helens. Known as “spiders” because of their spindly frames,
the stations were slung into the crater 29 times from September 2004 to December 2005 when conditions at the volcano
were too dangerous for crews to install conventional equipment. Data were transmitted in near-real time to the Cascades
Volcano Observatory in Vancouver, Washington. Each fully
equipped unit cost about $2,500 in materials and, if not
destroyed by natural events, was retrieved and redeployed as
needed. The GPS spiders have been used to track the growth
and decay of extruding dacite lava (meters per day), thickening
and accelerated flow of Crater Glacier (meters per month), and
movement of the 1980-86 dome from pressure and relaxation
of the newly extruding lava dome (centimeters per day).