Contact: Nick Houtman; [email protected]

Merger of U.S. and Russian Satellite GPS

A satellite-based positioning system used by hikers, farmers, pilots and scientists could double in size if Alfred Leick, University of Maine professor of spatial information science and engineering, can solve a problem stemming from the Cold War.

Leick teaches courses on the theory and use of satellites in surveying and is the author of GPS Satellite Surveying, a widely used textbook.Last fall, he spent a sabbatical leave in California working on a method for integrating two satellite positioning systems, one built by the United States (known as the GPS or Global Positioning System) and the other built by Russia (known by its acronym GLONASS or Global Navigation Satellite System).

"In surveying, our goal is always to make things faster and more accurate," says Leick."If you want to measure a baseline from Orono to Bangor to within a centimeter, you can do this with GPS under good circumstances. You need to have a certain number of satellites. The more satellites the better.

"Sometimes we have five or six, even seven. Then you have good solutions, almost instantaneous. But that doesn't happen very often. GPS was not designed for this purpose. It was not designed to please the surveyors." The U.S. and Russian systems were designed for military purposes during the Cold War and are still operated by military authorities. Nevertheless, civilian applications are expanding rapidly. A decision by the U.S. government to open its system to civilian use has contributed to a boom in the production of hand-held satellite receivers.

Under the right conditions, these units can determine the user's position and elevation at any point on the earth. By using a component of the satellite signal known as codes, the receivers can achieve accuracy to within 150 meters. In relative positioning, when two satellite receivers record data at the same time, they can be accurate to within 1 meter.

In Maine, the DeLorme Mapping Company of Freeport sells a GPS receiver which works with the company's road atlas software to determine a vehicle's location within the state. Surveyors working for the James Sewall Company, Old Town, use GPS routinely, as do airline pilots flying into Bangor, Portland and other communities. Hikers and sea kayakers have been known to use GPS units, although most orienteering is still done by map and compass.

At present, says Leick, the necessary conditions for an accurate determination do not always occur. At least four satellites must be above the horizon and accessible to the user, but buildings and mountains can obstruct the signals the satellites emit. While Leick and his colleagues can't do anything about the terrain or other objects that block signals, they may be able to make more satellites available by merging the U.S. and Russian systems.

"With the addition of the GLONASS satellites, you can have eight or nine satellites visible at a time, and then you can determine a position baseline that is instantly accurate. If you wanted to determine a position of a plane as it flies, you can do this now with GPS. It works, but with more satellites, it would work better. It would be more reliable."

Satellite receivers make two types of meaurements, Leick explains. One type uses codes carried on the satellite signal, much like music that is carried on a radio station signal. Using the GLONASS system, those codes can be used to find a position with an accuracy of about 25 meters.

The other type of measurement uses part of the signal known as the carrier phase. Just as light travels in waves, so does the carrier phase. Measuring that wave and comparing measurements from two or more satellites and two receiving stations makes precision possible to within a centimeter. Leick's goal is to make that type of measurement with GLONASS.

"The main problem is that every GLONASS satellite uses its own specific frequency. That makes certain techniques for precise positioning more difficult. The different frequencies cause biases in the carrier phase. For precise positioning at the centimeter level, we need to measure the carrier phase accurately. However, as a result of the problem with the frequencies, we don't get the true carrier phase. Moreover, that error changes from frequency to frequency, and it changes with temperature."

With grant support from the U.S. National Imagery and Mapping Agency (formerly the Defense Mapping Agency), Leick has developed an algorithm which accounts for different frequencies and other sources of error. During his sabbatical in California, he collected GLONASS data and over the next few months, plans to use standard surveying techniques to test the quality of that information.

The Agency's goal, Leick says, is to build a GLONASS monitoring station in the U.S. That effort reflects the increased attention which is now given to GLONASS by US agencies and industry. Beyond the technical dimension, a number of political issues still must be resolved, Leick points out, including who exericises ultimate control and whether or not other frequencies will be dedicated to civilian use.

Civilian control could be important, for example, during times of conflict. "Imagine landing an airplane using the global system and having the switch turned off," he says.

Leick maintains a World Wide Web page dedicated to information about GPS and GLONASS developments. It can be seen at http://www.spatial.maine.edu/~leick/gpshome.htm.

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