What Is GPS

The Global Positioning System (GPS) is a system of satellites maintained by the US department of defence for navigation and security purposes. These satellites, which are located the earth is approximately 11,000 miles, follow known orbital paths and broadcast radio signals communicating their location at a given point in time. GPS receivers are able to determine location by tracking and comparing signals from multiple satellites.The receiver is able to determine how long each signal took to travel from the satellite to the receiver and translate this time interval to a distance measurement. By comparing the distances to multiple satellites the receiver is able to approximate location using the mathematical analysis.

This system requires extremely precise timing and communication between the satellites and the receivers. It also requires that the satellites be visible. Tree canopy, nearby cliff walls, buildings, etc. all can limit the ability of a receiver to accurately determine position. In addition, the configuration of the satellites themselves is sometimes better or worse for a given location. For example, if all of the satellites are directly overhead, it is very difficult to get accurate triangulation measurements. Conversely, if the satellites are extremely low on the horizon then their signal passes through so much of the earth’s atmosphere that the signal delay is unpredictable. What is desired is a large number of satellites, widely scattered and not too low (higher than 15°) across the visible horizon.

          Current receivers are able to determine horizontal location to within 5-20 meters, and vertical accuracy to within 50 meters. This accuracy can be improved by a method known as differential correction by readings from a field receiver are compared to those of a base station GPS receiver at a known location. Survey grade receivers, using high precision differential correction are able to produce sub meter accuracy in the field.

Most receivers can record points, lines or areas. For points, a longer duration of GPS recording will gives a more accurate location due to the power of averaging. For lines or areas, recording positions more frequently may produce a more accurate overall result.