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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 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, we can determine receiver positions to less than a centimeter.
The GPS satellites continuously transmit an estimate of their position, digital codes, and a precise time signal. A GPS receiver uses an internal clock and the codes to determine the distances to at least 4 satellites. 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, which is 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 to 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.
Norris Geyser Basin is the hottest and most dynamic of Yellowstones hydrothermal areas. Many hot springs and fumaroles here have temperatures above the boiling point (199 F). Its features change daily because of water fluctuations and seismic activity.
Earthquakes occur frequently due to the intersection of three major faults beneath the Norris area. These faults combine with the basins primary rock type (welded tuff) to create a setting for dynamic and even explosive change.