Until recently, we relied exclusively on electronic distance measurements (EDM) to determine the horizontal movements that occur on active volcanoes and on tiltmeters and leveling surveys to measure vertical motions. The new Global Positioning System (GPS), however, allows us to measure horizontal motions much more accurately and conveniently, and also to estimate vertical motions in the same survey. Thus, our use of GPS in monitoring ground-surface deformation on volcanoes has increased dramatically in the past few years.

GPS offers several advantages compared to volcano surveys that use electronic distance meters. GPS does not require lines-of-sight between benchmarks so they can be located almost anywhere as long as the site has a clear view of the sky. This is a big advantage on most volcanoes, where steep slopes at stratovolcanoes like Mount Rainier or broad, gently-sloping ones at shield volcanoes like Mauna Loa often get in the way of line-of-sight between benchmarks. Another advantage of GPS is that measurements can be made in almost any weather condition. Both horizontal and vertical changes in position can be measured to an accuracy of a few millimeters (horizontal) to several millimeters (vertical). Finally, GPS receivers are portable, require only one person to set up the equipment, and can transmit data in near real time and operate unattended for several months on batteries and solar panels.

The current constellation of satellites provides the GPS user with 5 to 8 satellites in view from anywhere on Earth, if one has an unobstructed view of the sky in all directions. With this much information, a GPS receiver can very quickly determine its position to within a matter of meters. On volcanoes, however, an accuracy of a few centimeters or less is extremely important for detecting the build up of stress and pressure caused by magma rising toward the ground surface. To obtain this kind of accuracy in our measurements, we need to take other factors into account, including the variation in the speed of the signal transmitted from the satellite as it travels through the atmosphere and the uncertainty in the position of the satellite.

A common way of eliminating these potential errors is to set up GPS receivers over several volcano benchmarks at the same time so that we can simultaneously collect data from the same satellites. Since most of the error associated with the delay of the signal through the atmosphere and the location of the satellites becomes the same for all sites, we can determine their positions relative to one another to less than a centimeter. For the greatest accuracy, we collect GPS data for 8 to 24 hours and then calculate the position of the benchmark utilizing more precise satellite locations and modeling the atmospheric delay.