GPS Basics

GPS Basics

by Conrad & Judy Kreuter

The Global Positioning System (GPS) is a state of the art navigation system that can pinpoint your position anywhere on the globe. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. Data in the form of high frequency radio signals is transmitted from each satellite to the GPS receiver. This information is used to triangulate your position anywhere on earth.

The GPS receiver measures the travel time of the signals transmitted from the satellites it is sampling and multiplies them by the speed of light to determine exactly how far the receiver is from the satellite. A minimum of three satellite signals are required in order for the GPS receiver to calculate a 2 dimensional position fix, usually consisting of latitude and longitude. By obtaining a fourth satellite signal the GPS receiver can calculate altitude as well.

Most GPS receivers have 12 channels available to capture as many satellites as possible. By acquiring the signals from additional satellites the receiver can over resolve a solution which ensures reliable, continuous and uninterrupted navigation, even in adverse conditions.

The Department of Defense who originally conceived the system during the 1970’s has enhanced the overall accuracy of the GPS system. Accuracy, which is important to you, is now within 10 to 20 meters which means you can get back to your favorite fishing hole each and every time. Some GPS receivers now have enhanced capability as well. With the development of Differential GPS, which adds a land based reference receiver whose location is precisely known to the system. The non-moving DGPS reference station knows where the satellites are located in space at any given moment. The DGPS station calculates the error coming from the satellite and sends a correction signal to the GPS receiver, which further improves the accuracy of the entire system often within 3 meters or about 10 feet.

The first time a GPS receiver is turned on it will calculate its initial position. This may take a few minutes, as it must learn where it is, what the date is, and what the time is. This sequence allows the receiver to know which satellite is overhead, so that it can begin searching for them and to lock onto them. The receiver will achieve much faster lock-on the second and subsequent time the receiver is turned on.

Some less expensive GPS receivers show the position data as digital latitude and longitude information on a screen. Other GPS receivers have built in charts or chart modules covering your specific general location, and your exact position is displayed at the center of the chart. As you move, the chart moves with you always displaying your location at the center. GPS receivers can also provide location information to other devices such as autopilots to provide for steering control.