Monitoring Volcano Ground Deformation with GPS
Global Positioning System (GPS)
The Global Positioning system consists of a constellation of 24 satellites. Each satellite orbits Earth twice a day at an altitude of about 20,000 km and continuously transmits information on specific radio frequencies to ground-based receivers. GPS was developed by U.S. Department of Defense as a worldwide navigation system and has been adopted by civilians for many other uses, including surveying, mapping and scientific applications. Relatively inexpensive GPS receivers like those used by pilots, boaters and outdoor enthusiasts can determine its position on the Earth‘s surface to within a few tens of meters. With more sophisticated receivers and data-analysis techniques, as shown on Mount St. Helens, we can determine receiver positions to less than a centimeter.
How does GPS work?
GPS satellites continuously transmit an estimate of the satellite’s position, a digital code, and the precise time of the signal. GPS receivers measure distance using the travel time of the radio signals. Distance is calculated by multiplying the time it takes the radio signals to reach the receiver times the speed at which the signals travel - approximately 186,000 miles/second (the speed of light). Knowing where the satellites are located when they transmit their signals, the receiver can calculate its position on Earth or in the air. The key is that receivers must simultaneously receive the signals from at least 4 satellites, in part because the clocks in the receivers aren‘t as accurate as the atomic clocks in the satellites. If the clocks in a receiver and satellite were out of sync by 1/1000th of a second, the distance measurement could be off by 186 miles! The fourth measurement essentially enables the receiver to correct its internal clock.
For an online overview of how GPS works, see Trimble Navigation's tutorial.
Using GPS Networks to Gain More Accurate Data
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.
Using GPS for Monitoring Volcano Deformation - Mauna Kea Volcano, Hawai`i
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.
