GPS tracks shifting land masses

The satellite-based Global Positioning System (GPS) first developed by the Defense Department to provide highly accurate position and location information has evolved into a technology with many more civilian applications than military.

Hikers consult $200 receivers to help them bushwhack across unfamiliar terrain farmers use GPS receivers integrated with computer-controlled seed drills to plant their crops and bus and truck dispatchers rely on GPS and geographic information systems to track the location of widely dispersed fleets.DOD developed GPS in part to accurately guide high-speed missiles to their targets. But it turns out that the Pentagon not only built a better yardstick it crafted a better millimeter stick. Civilian engineers and technicians have improved the accuracy of GPS to the level that it can track objects that are immense and move an imperceptible amount such as the inexorable shifts of the world's land masses.

Keeping track of the movement of these tectonic plates might seem akin to watching the grass grow unless you live in earthquake zones such as Southern California where every decade or so the results of the plates' slow-motion collisions caused by their millimeter crawl makes even a spectacular Hollywood special effect look minor.

Geodynamics researchers worldwide have embraced GPS as the technology of choice to monitor the shifts of tectonic plates to help predict earthquakes. The home page of the International GPS Service for Geodynamics (igscb.jpl.nasa) offers a chance to check in on the work of these monitors.

This Web site opts to provide a massive amount of information rather than glitz but it does offer continuously updated data from a worldwide network of GPS receiver-equipped monitoring stations as well as regional data centers and the Central Bureau at NASA's Jet Propulsion Laboratory in California.

The precision and accuracy of the monitoring stations make ordinary GPS receivers look like dime-store yardsticks. They use a bag of techno-tricks to turn the 10-meter accuracy of the ordinary civil 100-meter GPS signal into a precise tool that can track the 8-millimeter-per-year movement of the Southern California plates.

Besides operating the Central Bureau for the International Geodynamics Service JPL also runs the Southern California Integrated GPS Network (milhouse.jpl.nasa.gov) which runs a California monitoring network of 49 sites and offers the Web user a chance to tap into 72 other sites worldwide.

This Web page provides some compelling graphics that illustrate the Earth is not quite as solid as you would think. Click on the world map (sideshow.jpl.nasa.gov/mbh/series.html) and you end up with an image that illustrates the motion of continents. Another map (milhouse.jpl.nasa.gov/siteinfo/califmap.html) starkly illustrates the unifying fear of everyone who lives in San Diego: Sooner or later Los Angeles will quite literally end up in San Diego.

The National Oceanic and Atmospheric Administration also has adapted GPS technology to measure minute particles in this case water vapor. As NOAA explains on its GPS Water Vapor Measurements Web site (www4.etl.noaa.gov/gps.html) the ionospheric delay in GPS signals results from a combination of dry gasses and water vapor. NOAA has developed a formula to measure the delay subtract the dry gasses and come up with the total water vapor at a particular site.

Combine the results from the NOAA sites with the JPL tectonic-plate tracking system and someday forecasters might be able to predict whether Los Angeles will get hit with a rainstorm and an earthquake simultaneously.

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