Sunday, November 26, 2000

Pinpoint precision: Geographic locators are accurate to within tenths of an inch

Published in the Antarctic Sun

Several scientists in the U.S. Antarctic Program use specific measurements and locations on the surface of the Earth as key elements in their research. They watch many processes, including
the movement of glaciers, growth or shrink rates of ice sheets and rock layers and the melting of patches of snow in the Dry Valleys.

These researchers use the Global Positioning System, originally created for combat use by the U.S. Defense Department, to locate themselves and their study areas very specifically. At McMurdo Station each summer are GPS experts who provide equipment and training for about 20 science groups on the continent.

“We’re supporting grantees who are using GPS for their field research,” said project leader Bjorn Johns, of the University NAVSTAR Consortium (UNAVCO), a group of 100 academic institutions, including the National Science Foundation, promoting the use of high-accuracy GPS for scientific research.

Many people on the Ice and in the U.S. have their own handheld GPS units, which cost around $200. “It’s become a national utility,” Johns said.

Commercial handhelds provide accuracy to within about fifteen feet of an actual location, Johns said. By contrast, the equipment Johns and his colleague Chuck Kurnik issue are accurate to within tenths of an inch, cost around $15,000 and involve a plattersize antenna and laptop computer-size receiving box.

GPS is based on a group of satellites orbiting Earth and several ground stations monitoring them. The satellites broadcast their position in space and the exact time from an on-board atomic
clock. By receiving the signals from several satellites, a GPS unit on the ground can calculate its location.

But that can be difficult at high latitudes because the satellites don’t pass directly overhead, which would give the best possible readings. “They’re all low on the horizon in the polar regions,” Johns said.

All of the positions calculated are relative to other, fixed, known locations. To be precise, measurements need to be compared very carefully with the exact trajectories of the satellites at the time of the reading.

“That typically means collecting and post-processing data,” Johns said. That process can take a couple of days, he said. Some groups need Johns and Kurnik to do GPS portion of their work, while other researchers need technical assistance or data-processing help.

Johns and Kurnik also install both permanent and temporary stationary GPS stations to monitor ongoing geologic processes and to improve accuracy of nearby readings.

This season, they put a station on Mount Erebus to watch how underground activity changes the volcano’s surface. “If there’s any inflation or deflation of the volcano relative to McMurdo we’ll see that,” Johns said. If anything significant happened on Erebus, or anywhere else with a permanent GPS monitoring station, the data would be valuable for scientists.

“When an event occurs, you’ve captured it, with pre- and post-event data,” Johns said.

Another important element is fixing the exact antenna position to the ground. If a measurement is accurate within fractions of an inch, a human error in antenna placement for observation could
appear to be a large fluctuation in surface movement.

To provide a stable platform, Johns and Kurnik sink a metal rod into the rock or ice and affix a leveling platform to the rod. The antenna screws onto the platform.

Each reading, then, is taken from the same location relative to the rod. If a location change is measured, it means the rod has moved, and therefore the rock or ice surrounding the rod has moved.

This type of measurement is possible around the world using base stations and satellite readings anywhere on the surface of the Earth. But Johns said Antarctica is where GPS gets used most heavily. He and Kurnik may support five science projects during the rest of the year, and more than 20 during the summer field season on the Ice.

The GPS work helps influence future research, Johns said. This season at Icestream C, a group wanted to drill an ice core in an area where the glacier isn’t moving very quickly. Because of GPS
surveying last year, they knew where one was.

GPS is also used to map the atmosphere. Since GPS uses radio waves, which behave differently as atmospheric conditions change, GPS readings at known locations can show variations in
the ionosphere and troposphere through changes in radio waves along different paths.

Johns and Kurnik don’t directly interact with the atmospheric mapping projects, which are not based in Antarctica, but help people use GPS in all kinds of ways. “Everyone has something they want measured,” Johns said.