What is GPS?
The Global Positioning System (GPS), developed by the U.S. Department of Defense, is a satellite-based system used to locate positions anywhere on the earth, 24 hours a day, in all weather. GPS can be used for navigation, mapping, surveying, timing, military activities, and many other applications. The MSU GPS Lab in the Department of Land Resources and Environmental Sciences (LRES) at Montana State University uses GPS technology for teaching, research and service.
How Does GPS Work?
A GPS receiver on the ground tracks radio signals from four or more NAVSTAR GPS satellites which are orbiting the earth, and calculates it's position based on the time it takes for the signal to arrive at the receiver antenna.
How Accurate is GPS?
The accuracy that can be achieved using GPS depends on the type of equipment used, the time of observation, and the positions of the satellites being used to compute positions. In general, recreational and mapping grade receivers using C/A code without differential correction are accurate to between 5 and 15 meters. Many people using recreational grade receivers don't realize they cannot get highly accurate readings using them autonomously (without differential correction).
Most mapping and recreational grade receivers with differential correction can provide from about 1 to 5 meter accuracy. Some receivers use what is called "carrier-smoothed code" to increase the accuracy of the C/A code. This involves measuring the distance from the receiver to the satellites by counting the number of waves that carry the C/A code signal. These receivers can achieve 10 cm to 1 meter accuracy with differential correction. Dual frequency survey grade receivers using more advanced network survey techniques can achieve centimeter to millimeter accuracy.
Some people wonder why GPS is better than Loran or other systems that use ground-based transmitters. The accuracy of ground-based location systems such as Loran, which uses low frequency radio signals, is affected by signal distortion, varied terrain, local atmospheric disturbances and limited coverage. Since GPS signals come from satellites, the problems common to ground-based systems can be avoided.
What Should I know Before Purchasing a GPS Receiver?
Before investing in GPS equipment, it is important to clearly define your needs in terms of accuracy level required and end results expected. Do you simply want to be able to navigate in the woods, or do you want to map out points, lines and areas that can be differentially corrected and imported into a GIS (a computer mapping system)? Do you need real-time differential GPS for any reason?
Is 15 meter accuracy good enough? If so, you don't have to worry about differential correction. If you want to make a map from your data, is 1-5 meter accuracy sufficient, or do you need sub-meter accuracy for your application? Remember that more accurate equipment is more expensive. If you decide you need high accuracy, be sure you can justify the added expense.
In addition, consider your needs for durability and weather resistance, and details such as whether or not an external antenna can be connected to the receiver, and its size, weight and suitability for your method of survey (e.g., will it be used in a backpack, mounted on a vehicle, or carried in your hand?).
Identifying your requirements ahead of time will help you determine which type of receiver to purchase, and specific features you will need in order to accomplish your objectives. It will help you avoid purchasing a receiver that you will be disappointed with later because it can't perform the way you expect it to. A good strategy is to clearly outline your project requirements and then contact several GPS equipment manufacturers with your specifications. As you research available equipment and ask questions, you will gain an understanding of what kinds of equipment are currently available and will meet your needs.
What about Differential Correction?
The effects of atmospheric and other GPS errors can be reduced using a procedure called differential correction. Differential GPS (DGPS) requires a second GPS receiver at a known location (a base station) to act as a static reference point. The accuracy of differentially corrected GPS positions can be from a few millimeters to about 5 meters, depending on equipment, time of observation and software processing techniques. Even though SA (previously the largest source of GPS error) has been removed, differential correction is still required to obtain accuracies of better than about 10-15 meters
What is Selective Availability (SA)?
Up until May 1, 2000, the government scrambled GPS signals for National defense reasons. This intentional signal degradation was called selective availability or SA. Because of SA, the positions computed by a single GPS receiver were in error by up to 100 meters. Because of pressure from the civilian GPS user community and other reasons, the government agreed to remove SA. It was discontinued as of midnight on May 1, 2000.