Learn about the technology in GPS systems.

The Global Positioning System (GPS) is a navigation system consisting of 24 satellites which orbit the earth every 12 hours and send information down to GPS units. The GPS is operated by the U.S. Government.  Each satellite is self contained with power run by solar panels, atomic clocks that provide precise time, and radio transmitters that send data back to Earth. The satellites transmit radio signals that contain the time the signal is sent and data that a GPS unit can use to determine the location of the satellite.

What is a GPS unit?

A GPS unit is an electronic receiver for the signals sent by the GPS satellites. The handheld models are about the size of a cell phone, although fixed-mount units are a bit larger. When the GPS unit is turned on with a clear view of the sky, it receives the signals and makes calculations to determine the difference between the time the signal was sent and the current time. With this information, the GPS unit can determine its distance from the satellites in view.

The GPS unit will be able to work with the following geographical features:

Latitude: The angular distance north or south of the earth’s equator, measured in degrees along a meridian, as on a map or globe.

Longitude: Angular distance on the earth’s surface, measured east or west from the prime meridian at Greenwich, England, to the meridian passing through a position, expressed in degrees (or hours), minutes, and seconds.

The Wide Area Augmentation System (WAAS) is a capability that produces a more accurate position fix.
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Using a GPS unit: How it works

By comparing the signals of at least three satellites, the unit can determine its three-dimensional position. A fourth satellite, however, is required for accuracy. This fourth satellite is needed because there is not an atomic clock in the GPS unit. At a minimum, the GPS unit will then provide the user with approximate latitude and longitude readings. Trees, buildings, and atmospheric conditions impact reception, but with optimum conditions a handheld unit can provide position information within 10 feet of anywhere on Earth.

Waypoint

A waypoint is simply the latitude and longitude of a place on the earth. Most GPS units will give you the ability to mark a waypoint. Mark your destination with a waypoint, and the GPS unit will show you the distance and directions to your destination.

Some units have features that will also help you plot which roads you should take to get there. Those that don’t will allow you to mark waypoints at the intersections of the roads you want to take, then have your unit connect the waypoints for a route. Routes can be planned ahead of time and stored in its memory. As you travel, your GPS unit may also show your track so you can back track to get home.

Features to consider when using a GPS unit

If you are going to buy a GPS unit, there are some features you should consider:

•     size of the display
•     detail of built in maps (you may wish to purchase additional maps)
•     built in compasses
•     amount of built-in memory
•     WAAS capability
•     battery life
•     number of routes you can store
•     waypoint capacity

Any of these features will give you higher accuracy.