Taking a Global Position

Do you have OnStar in your car? Have you had your property professionally surveyed lately? These are two examples of where Global Positioning System (GPS) technology is being used. In the 1960s, the Department of Defense was looking for a way to precisely locate their ballistic missile submarines and other ships on the ocean. Since then, the DoD has spent billions of dollars on developing GPS technology. GPS currently consists of 24 orbiting satellites, ground or base stations, and the GPS units (receivers) people hand carry or install in vehicles, boats, etc.

Western Pennsylvania Conservancy uses GPS in its Natural Heritage Program, Watershed Assistance Center and Land Protection Program. The GPS units carried by our staff are “mapping grade” and can calculate positions on earth within 2-5 meters with base station corrections. Some of the uses of GPS at WPC include precisely locating species of special concern for research and protection purposes, finding property boundaries, and documenting stream reaches affected by acid mine drainage.

How it works: the satellites constantly send out their unique radio signals. The GPS unit measures the travel time of the signal between the satellite and the unit. The distance from each satellite can be calculated by multiplying this time by the speed of the signal. For units that enable correction of this measurement for signal interference from atmospheric conditions, these measurements can be compared to those made by a local base station of known position to get a correction factor. By comparing the distances to at least 4 satellites, the unit can pinpoint its position on the earth.

Pictured above with hand held GPS units are WPC Staff Tami Campbell, and GIS Technician Matt Hoff.

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