The GPS System
The Global Positioning System (or GPS) is a collection of satellites owned by the U.S. Government that provides highly accurate, worldwide positioning and navigation information, 24 hours a day. It is made up of twenty-four NAVSTAR GPS satellites which orbit 12,000 miles above the earth, constantly transmitting the precise time and their position in space. GPS receivers on (or near) the earth’s surface, listen in on the information received from three to twelve satellites and, from that, determine the precise location of the receiver, as well as how fast and in what direction it is moving.
It’s a question of 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.
Even with highly accurate atomic clocks, certain errors do creep into the process of determining your position. Selective Availability (SA) is the program implemented by the U.S. Department of Defense that makes GPS less accurate for non-military users for security reasons. With SA in effect, the accuracy of your position may be within 50 to 100 meters. Even without SA, other errors will be encountered. The most significant of these errors is due to variations in the earth’s ionosphere, which effects the speed of GPS radio signals. Another source of error is from water vapor in the troposphere. Both of these errors are fairly small. The accuracy of GPS can be improved with DGPS (differential GPS) capabilities, which is the ability of your receiver to read signals from a nearby DGPS beacon receiver. (Also see GPS RECEIVER PERFORMANCE section on the HOW TO USE GPS page)
In May 2000, the Selective Availability affect was discontinued, and now typical accuracy of most units is 20-50 ft. (up to 15 meters).
In 2001, manufacturers started making GPS receivers that are capable of getting WAAS (Wide Area Augentation System) corrections. Basically, it’s a system of satellites and ground stations that provide GPS signal corrections, giving you even better position accuracy. If you are in an open location and can get the WAAS signal, you can improve your position accuracy to within 3 meters most of the time.
Civilian equipment and consumer uses (Fun stuff)
Over the last several years, an increasing array of affordable GPS receivers 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. What was state-of-the-art consumer GPS in 1996, is now relegated to the least expensive units, while today’s latest-and-greatest units have features only wished for years ago.
Primarily, these are handheld GPS receivers that vary in price from $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.
Some specialized GPS receivers for consumers include:
— handheld GPS receivers that have background moving maps (of highways) for car navigation (with and without mini-cartridges),
— GPS receivers that provide automatic routing and navigation for drivers
— GPS receivers that attach to PDAs (personal data assistants)
— handheld and mounted GPS receivers for boating and fishing,
— aviation GPS receivers with built-in Jeppensen airport information,
— GPS receivers that combine with radios, or e-mail, into one unit.
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.)
The GPS system was initially set up for use by the military to provide precise position 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.
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 supplemental 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. However, if this is something you might need professionally, and you cannot find it on the Internet, contact me and I will try to send you in the right direction.