Published by the American Geological Institute
and Trends in the Geosciences
Doug Walker received an e-mail message May 1 telling him that Selective Availability, the U.S. military’s degradation of the signals coming from Global Positioning System (GPS) satellites, had been shut off. The next day, Walker, who teaches geologic mapping at the University of Kansas, joined two colleagues and took his $200, hand-held GPS receiver outside. It read his position to within about 20 feet — it used to read it to within 100 feet.
“It’s going to make a huge difference,” says Walker, who has been developing methods for using GPS and other technologies that will allow geologists to create digital maps from the field. “I’m virtually positive now that GPS accuracy is going to be on the order you see on topo maps.”
The U.S. Department of Defense developed and launched the GPS satellites in the 1970s as a military tool for determining positions on Earth’s surface. Fearing potential U.S. enemies could use the system to target missiles, the military degraded the signals so that a single GPS receiver could only measure a given position to within 100 meters. With Selective Availability shut off, a receiver could potentially measure a position to within 10 meters.
The time it takes radio signals to travel from four of the 25 GPS satellites to a ground-based GPS receiver helps determine that receiver’s 3-D position. Thus the GPS satellites are ideal tools for measuring position and for navigating around the globe. According to a White House statement released May 1, the $8-billion market for commercial GPS applications should double in the next three years. “The decision to discontinue Selective Availability [SA] is the latest measure in an ongoing effort to make GPS more responsive to civil and commercial users worldwide,” the statement read.
Earth scientists are increasingly using GPS for a range of data collection, from measuring displacement of Earth’s crust over time to quantifying water vapor by the speed at which the signals travel through the atmosphere.
“The United States will stop the intentional degradation of the Global Positioning System signals available to the public beginning at midnight tonight,” the White House announced in the same statement May 1. “The decision to discontinue SA … is supported by threat assessments which conclude that setting SA to zero at this time would have minimal impact on national security.” The military will still jam GPS signals regionally if national security is threatened, it added.
Geoffrey Blewitt, a University of Nevada researcher who uses GPS for tectonic studies, also received an e-mail message May 1 with the news. He says the change is exciting for mapping, but doesn’t offer such a paradigm shift for using GPS to measure deformation of Earth’s surface.
Geoscientists have worked around the degradation by differencing the signals with two receivers, measuring positions to within millimeters of accuracy. Such precision is required for measuring displacement around faults or vertical displacement of volcanoes.
A single receiver can cost as little as $200, while a system of two receivers with radios for communicating data can run up to thousands of dollars. “It really affects users who use single receivers,” Blewitt says. With SA shut off, he adds, the dominant error for hand-held receivers comes from uncertainty in a signal’s speed as it passes through the ionosphere.
Single-receiver users range from designers of Geographic Information Systems to rental car companies directing lost drivers, says Thomas Herring, a Massachusetts Institute of Technology researcher who also uses GPS for tectonic studies. Still, he adds, the improved signal will make GPS data more robust because researchers will be able to accurately and directly pinpoint the locations of the GPS satellites. “Now we can actually use the satellite clock information to give us a better indication of whether or not the data are valid.” Without SA, a GPS satellite clock can be accurate to within 10 billionths of a second.