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GPS Tutorial

An Introduction to Global Positioning System Technology 
The Global Positioning System (GPS) is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few yards or meters. This amazing technology is available to everyone, everywhere, day and night, and best of all, at no cost for use of the navigational data. GPS uses a constellation of 24 satellites in precise orbits approximately 11,000 miles above the earth. The satellites transmit data via high frequency radio waves back to Earth and, by locking onto these signals, a GPS receiver can process this data to triangulate its precise location on the globe.

GPS operates 24 hours a day, in all weather conditions, and can be used worldwide for precise navigation on land, on water and even in the air. Some of its many current applications include: boating, fishing, hunting, scouting on land or from the air, hiking, camping, biking, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, 4 wheeling, highway driving and a host of other outdoor activities where accurate positioning is required.

How GPS Determines Your Position
GPS uses satellite ranging to triangulate your position. In other words, the GPS unit simply measures the travel time of the signals transmitted from the satellites, then multiplies them by the speed of light to determine exactly how far the unit is from every satellite it's sampling.

By locking onto the signals from a minimum of three different satellites, a GPS receiver can calculate a 2D (two-dimensional) positional fix, consisting of your latitude and longitude. By locking onto a fourth satellite, the GPS can compute a 3D (three-dimensional) fix, calculating your altitude as well as your latitude/longitude position.

In order to do this Eagle uses a 12 parallel-channel receiver in all of its current products. Three of the channels lock on to satellites for triangulation. Another channel locks on to a fourth satellite for 3D navigation, which lets the unit calculate altitude in addition to latitude and longitude. These four channels continuously and simultaneously track the four satellites in the best geometrical positions relative to you. The additional eight channels track all other visible satellites, then add this data to the data from the original four satellites. The unit then over-resolves a solution, creating an accuracy-enhanced reading. The additional channels also ensure reliable, continuous and uninterrupted navigation, even in adverse conditions such as valleys or dense woods.

Accuracy of GPS
GPS was conceived in the 1970s, and is controlled by the United States Department of Defense. Although GPS was initially envisioned for military use, the Government realized early on that there would be numerous civilian applications as well. Subsequently, the Department of Defense (DOD) created two transmission codes; the P code (Precision code) for military use, and the C/A code (Civilian Access code) for civilian use.

The highest accuracy levels were to be reserved for the military so as to prevent hostile enemy attacks against the U.S. using our own navigational system. However, once in operation, the civilian GPS receivers using the C/A code proved to be more accurate than the DOD had intended. Consequently, the military developed a system for randomly degrading the accuracy of the signals being transmitted to civilian GPS receivers. This intentional degradation in accuracy is called Selective Availability or S/A. This reduced the civilian GPS accuracy levels to being within 100 meters or less, 95% of the time. However, typical accuracy for most users averaged between 20 and 50 meters the majority of the time. You could easily see the effects of S/A on a GPS receiver when you were not moving. Typically, there would be random movements in speed, altitude and position readings, along with slow position "wandering" on the plotter trail. This was easily seen when you were on a .1 or .2 mile zoom range and not moving. For example, while parked at the dock in your boat, you would see unexplainable changes in your digital speed readings up to a few miles per hour, even though you were not moving.

Plot of position accuracy using standard Eagle GPS receiver (stationary). Note the differences in scale.

5.5 hour period immediately prior to shutoff of selective availability

longitude meters
8 hour period immediately after shutoff of selective availability

longitude meters

Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense now has the technology to localize the control system to deny GPS signals to selected areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, aviators, drivers, hikers, hunters, and outdoor enthusiasts of all types can locate their position up to ten times more precisely (within 10 to 20 meters) and navigate their way through unfamiliar terrain. Anglers can now return to their favorite spot on a lake or river instead of just their favorite area. A Lowrance GPS receiver in combination with advanced technology of today's GPS management will take you anywhere you want to go.

The decision to allow civilians so much accuracy in location information was finally made because GPS is continually playing a more important role in the lives of people around the world - it's becoming a national utility. GPS is the global standard in navigation because it is completely free of charge to the public.

Differential GPS (DGPS)
Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters. DGPS was originally initiated by the U.S. Coast Guard to counter the accuracy degradation caused by Selective Availability. Even with S/A now eliminated, DGPS continues to be a key tool for highly precise navigation on land and sea. DGPS technology adds a land-based reference receiver – located at an accurately surveyed site – to the other GPS components. This non-moving DGPS reference station knows where the satellites are located in space at any given moment, as well as its own exact location. This allows the station to compute theoretical distance and signal travel times between itself and each satellite. When those theoretical measurements are compared to actual satellite transmissions, any differences represent the error in the satellite's signal. All the DGPS reference station has to do is transmit the error factors to your DGPS receiver, which gives the information to the GPS receiver so it can use the data to correct its own measurements and calculations.

The two most common sources of corrective DGPS signals currently are: (1) Coast Guard, land-based beacon transmitters, broadcasting the data at no charge to the public, covering all coastal areas and much of the inland USA as well; and (2) FM radio sub carrier transmissions available both in coastal and inland areas, but limited to paid subscribers. In order to receive DGPS correction data from Coast Guard beacon transmitters, a mobile GPS unit requires a separate beacon receiver. And to receive FM sub carrier DGPS signals from local subscriber radio stations, the GPS unit requires a separate FM receiver, normally the size of a pager. Naturally, your GPS unit must have the capability to both receive and process DGPS data.

Wide Area Augmentation System (WAAS)
GPS is plenty accurate for route navigation, but the U.S. Federal Aviation Administration has special need for aircraft traffic control that go beyond basic GPS. The FAA has a plan under way to boost GPS performance even further with its Wide Area Augmentation System, or WAAS. This GPS add-on will include a time control element that will help airliners fly closer together while avoiding collisions. In addition to carefully spacing airplanes along travel corridors, WAAS will eventually make instrument landings and takeoffs more accurate as it replaces existing aviation navigation systems.

Non aviators can use WAAS signals to make their GPS navigation even more accurate. However, WAAS has some limits you should know about.

First, the U.S. government has not completed construction of the WAAS system, so it is not yet fully operational. The ground stations are in place, but only a few of the needed WAAS satellites have been launched.

WAAS can boost the accuracy of land GPS navigation, but the system is designed for aircraft. The satellites are in a fixed orbit around the Equator, so they appear very low in the sky to someone on the ground in North America. Aircraft and vessels on open water can get consistently good WAAS reception, but terrain, foliage or even large man-made structures frequently block the WAAS signal from ground receivers.

You'll find that using your GPS receiver without WAAS is both easy and amazingly accurate. It's easily the most accurate method of electronic navigation available to the general public today. Remember, however, that this receiver is only a tool. Always have another method of navigation available, such as a map or chart and a compass.