On Target GPS Tutorial
Revised March 2010
A Brief Introduction to GPS Technology
History of GPS
We have always been concerned about where we are and where we are going. We have created maps that use landmarks and paces, and sextants that use angles and starts. During the cold war, the U.S. wanted to be able to destroy missile silos in communist countries. In order to target enemy missile silos, the U.S. ships had to know their own exact location.
The solution... Global Positioning System (GPS).
The Global Positioning System consists of a 24 satellite constellation orbiting about 12,500 miles above the earth (the moon is 250,000 miles away). Each of the satellites is 17 feet and takes 12 hours to orbit the earth.
The Theory of GPS
The method a GPS uses to pinpoint a location is called trilateration, which is similar to triangulation except no angles are involved. Each satellite measures distance. The following list explains how satellites pinpoint location.
1) Satellite gives a sphere. 2) Satellites give a circle along the intersection of the spheres. 3) Satellites give two points where the 3 spheres intersect. 4) Satellites give exact location and time.
How do the satellites measure distance?
Each satellite transmits a code to the receiver at the speed of light (186,000 miles/second). If the satellite and receiver have the exact same time, the time shift in signal is multiplied by the speed of light to give us distance. An example would be if you are driving 60 miles/hour for 1 hour, you have gone 60 miles.
How do the satellites keep track of time?
The atomic clocks on the satellites must be extremely accurate. If clocks are off by one-thousandth of a second, the distance error would be almost 200 miles. The clocks on the satellites cost between $50,000 and $100,000. The receiver must have the same accuracy. It does this by using the fourth satellite to estimate exact time.
How accurate is GPS?
- Selective Availability on: 100 m
- Selective Availability off: 15 m
- DGPS: Claim under a meter, but actually 1-5 m
- Wide Area Augmentation System (WAAS): < 3 m
- Real Time Kinematic: 1-2 cm
Errors affecting GPS position?
- Satellite Clocks
- Orbit of satellites
- Receiver Noise
What is a GPS used for?
- Location (weeds, rocks, fishing spots)
- Navigation (planes, cars, tractors)
- Tracking (cows, trucks, fertilizer application, pesticides)
- Mapping (surveying, fires, damage to crop from chemical drift)
- Timing (TV and radio stations, meal times, meetings)
This tutorial was authored by Dennis Wright, Chod Stephens, and V. Philip Rasmussen for the USU Geospatial Extension Program. Illustrations were designed by Jolyn Keck.