1.What Is GPS ?
The Global Positioning System - GPS, is a space age navigational system that can pinpoint your position anywhere on the globe, usually within a few 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, rafting, pack trips by horseback, hot air ballooning, general aviation, snowmobiling and skiing, search and rescue, emergency vehicle tracking, four wheeling, highway driving and a host of other outdoor activities where accurate positioning is required. |
2.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 3 different satellites, a GPS receiver can calculate a 2D positional fix, consisting of your latitude and longitude.
By locking onto a fourth satellite, the GPS can compute a 3D fix, calculating your altitude as well as your latitude / longitude position.
In order to do this GPS receiver makers use 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, and 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. |
3.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 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 D.O.D. 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, easily seen when you are on a .1 or .2 mile Plotter scale, 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.
Effective May 2, 2000 selective availability (S/A) has been eliminated. The United States Department of Defense (DOD) now has the technology to localize the control system to deny GPS signals to select areas. It is not often that your electronics products increase in value after you've purchased them. Now boaters, 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.
The decision to allow civilians so much accuracy in location information was finally completed 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. |
4.GPS Timing
| GPS uses the triangulation of signals from the satellites to determine locations on earth. GPS satellites know their location in space and receivers can determine their distance from a satellite by using the travel time of a radio message from the satellite to the receiver. After calculating its relative position to at least 3 or 4 satellites, a GPS receiver can calculate its position using triangulation. GPS satellites have four highly accurate atomic clocks on board. They also have a database (or almanac) of the current and expected positions for all of the satellites that is frequently updated from earth. That way when a GPS receiver locates one satellite, it can download all satellite location information, and find the remaining needed satellites much more quickly. |
5.Civilian Equipment And Consumer Users
Over the last several years, an increasing array of affordable GPS receivers and GPS mouse have been released for the average consumer. As the technology has improved, many additional features are added to these units, while the price and size continue to decrease.
Primarily, these are handheld GPS receivers and GPS Mouse that vary in price from USD$100 to $600. Almost all consumer GPS receivers are 12 parallel channel, and data capable. Most can even be connected to a laptop, and used with a street-level mapping software, for real-time automobile navigation such as GPS Software (GPSS).
Consumers have been using GPS technology for business and for outdoor adventures. These activities include hiking and biking, fishing and hunting, kayaking and boating, auto travel on vacation and on business trips, research and data collection. GPS is an excellent tool to help you locate a specific position, and to help you and your family stay on track, so that you don't get lost. (For those who are directionally challenged.) |
6.Military Applications
| The GPS system was initially set up for use by the military to provide precise postion information. GPS products are used by the military to coordinate and track the movement of soldiers and equipment in the field, to guide military ships at sea, and to provide position and navigation information to military aircraft. |
7.Commercial Applications
GPS products have been developed for use for many commercial applications. These include: surveying and mapping, aviation and marine navigation, vehicle tracking systems and mobile computer and cellular platforms.
Surveying and mapping consist primarily of the collection and processing of position information and usually requires specialized GPS equipment. In the surveying market, applications include construction and engineering surveying, route surveying (roads, pipelines, cable and utility lines) and geodetic research. Data can be collected for evaluation later in the office, or used real-time, in the field. Mapping applications use large amounts of position data in the development of Geographic Information Systems (GIS) databases, and natural resource mapping.
The FAA has adopted a policy in 1994 to establish GPS as the future standard for aviation navigation. Therefore many pilots are turning to GPS as a supplimental navigation aid for their aircraft. At sea, GPS receivers are used on recreational and commercial vessels to provide real-time latitude, longitude, time, course and speed information, and assist with coast-line and harbor navigation.
Many GPS products are being used by businesses and government agencies to track their vehicle locations using wireless communications. Some GPS receivers have been integrated into mobile radios, cellular phones and mobile data terminals to meet the needs of vehicle fleet managers.
Most of the commercial applications for GPS require equipment that exceeds the capability of the handheld GPS units that Adventure GPS Products offers. |
8.What is Differential GPS (DGPS)?
| Differential GPS, or DGPS, has been developed to improve GPS accuracy to within a few meters Originally initiated by the U.S Coast Guard. DGPS adds a land based reference receiver, located at an accurately surveyed site. Since 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.
The station can 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. |
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