Scott Peabody (left) and Dean Heneghan, land surveyors with Heneghan and Associates, Engineers and Surveyors of Jerseyville, set up a GPS (Global Positioning System) instrument during a project at Pere Marquette State Park. The GPS instrument works by receiving readings from up to six satellites at one time. The satellites orbit the earth and allow the GPS instrument to efficiently measure distances and ground elevations over long distances and rough terrain.