GPS stands for Global Positioning System and it allows users to determine their location on land, sea, and in the air around the Earth. It does this using satellites and receivers. There are currently 24 satellites in orbit operated by the US Department of Defense that provide worldwide coverage 24 hours a day, 7 days a week, in all weather.
How the system works is by the satellites sending information to receivers. This information includes time, position, and satellite strength among other things. The receivers pick up this information and use it to determine the users location. Using the signals from at least 4 satellites, a receiver can determine latitude, longitude, and elevation. Some receivers can then convert the latitude and longitude into other coordinate system values.
The accuracy of GPS depends on several factors such as which receiver is being used, the surroundings it's being used in, and Selective Availability. Selective Availability is the Department of Defense deliberately interfering with the satellite signals to reduce positional accuracy to around 30m - 100m. With Selective Availability receivers are divided into two types: precise positioning systems (PPS) and standard positioning systems (SPS). PPS receivers are used by the military and are not affected by Selective Availability. Currently there are efforts under way to end the use of Selective Availability.
Differential GPS uses position corrections to attain greater accuracy. It does this by the use of a reference station. The reference station (or base station) may be a ground based facility or a geosynchronous satellite, in either case it is a station whose position is a known point. When a station knows what it's precise location is it can compare that position with the signals from the GPS satellites and thus find the SA error. These corrections can then be immediately transmitted to mobile GPS receivers (real time DGPS), or the receiver positions can be corrected at a later time (post processing).
The use of DGPS can greatly increase positional accuracy (in general, the better it is the more expensive it is). Some surveying systems can give subcentimeter readings. There are a lot of different differential providers that supply real time and post processing corrections, many are private companies. The availability of these services varies greatly depending on what part of the country you are in, but the US Coast Guard covers the US coastline and the number of private and governmental providers is increasing, so I imagine that someday the entire US will be covered.
THE STARLINK DGPS PAGES --This site is currently under construction, but it does have a description of how real time DGPS works, along with a couple graphics.
USCG DIFFERENTIAL GPS --This site gives a brief introduction to GPS including: how it works, how it is used, the Coast Guard DGPS system, and the latest US and Canadian Coast Guard status reports. Check out the NAVCEN homepage for links to lots of documents related to GPS.
THE GLOBAL POSITIONING SYSTEM FROM ELIRIS --This site gives a brief introduction that covers the space segment, the control segment, the user segment, SPS/PPS, and has a few graphics.
THE GLOBAL POSITIONING SYSTEM (GPS) --A brief overview with some information about receivers for boaters.
GPS BY CAPTAIN ROD AND SUSIE STEBBINS --This site is boating oriented and gives an overview that includes: how it works, accuracy, DGPS, monitoring and control, information sources, a map datum explanation, and navigation responsibilities. Check out the Suncoast Boating link to find help in up/downloading Magellan receivers to PC's.
K&L GPS AND DIFFERENTIAL GPS --This site gives an introduction to GPS and covers the space segment, the control segment, and the user segment. It also has a link to a page of GPS related links and differential providers.
NAVIGATIONAL SYSTEM FOR THE VISUALLY IMPAIRED --This site covers an introduction, satellite triangulation, timing, frequencies, DGPS, and has some graphics. It is part of a project using GPS to aid the visually impaired.
DIFFERENTIAL GLOBAL POSITIONING SYSTEMS --A site from the Ontario Ministry of Agriculture, Food and Rural Affairs that gives an overview of DGPS with an emphasis on satellite and differential signals. It also has some information about differential providers, such as their addresses, phone numbers, hardware, signal formats, and receiver prices.
GLOBAL POSITIONING SYSTEM INFORMATION --This site from the US Naval Observatory covers GPS policy, signal characteristics, selective availability, system segments, system time, system transfer, and the current constellation.
THE GLOBAL POSITIONING SYSTEM (GPS) --A great site full of information from the Department of Geography at the University of Texas. It covers just about everything and has links from the table of contents to the rest of the document, graphics, and a bunch of links to other sites.
RADIONAVIGATION --This site is part of a book for community emergency services planners by Rex Buddenberg. This page gives an introduction to GPS, LORAN, DGPS, and covers advantages, limitations, and integration of multiple systems. It also has a few graphics.
INTRODUCTION TO GPS APPLICATIONS --This is an excellent site by John Beadles that is currently under construction (but is color coded to give construction status). It has many links and covers how GPS works, policy issues, applications, vendors, equipment, services, and related sites.
Differential GPS is currently being used for many things, and it is a growing technology. One of it's more popular applications is in air navigation. By using it a pilot can receive constant information about where the plane is in 3 dimensions. It is also becoming a hot topic in precision farming. (MORE) (MORE) Farmers can use DGPS to map out their crops, map crop yields, and control chemical applications and seeding. It is also proving to be useful in ground and hydrographic surveying. Another application is in weather forecasting, where atmospheric information can be gained from it's effects on the satellite signals. There has also been at least one experiment where it was used for beach morphology and monitoring.
DGPS can also be used for train control for such things as avoiding collisions and routing. There is even been research into using it to help the visually impaired in getting around in cities. There is also at least one project that is working on using DGPS for car navigation. In the sports world it is finding a place in balloon and boat racing. I think that the future of DGPS applications will only be limited by imagination and money, and I predict that it will eventually become an integral part of much of our technology.
GLOBAL POSITIONING SYSTEM (GPS) RESOURCES
Submitted by Mark Bohrer 4/22/96