What is GPS?
The Global Positioning System (GPS) is a navigation and positioning system developed by the U.S. Department of Defense for use by U.S. and Allied military forces. Like many technological advances developed for the military, civilian applications for GPS were soon recognized. Today, GPS is used by (among others) surveyors, mappers, foresters, archeologists, geologists, hikers, hunters, pilots and boat captains.
Most simply, GPS can determine your location on the earth. Or more correctly, the position of the antenna. To do so it uses either latitude/longitude or some other coordinate system to display or store your location.
GPS receivers determine direction and rate of travel by continuously determining position as the user moves.
By storing the coordinates of some distant place, most GPS receivers can direct you to that place from nearly any start point on earth. The accuracy with which your GPS can guide you to the place of interest, regardless of whether it is across the street or across the country, depends on the type of receiver you have.
The Global Positioning System consists of basically three components.
- First is the ground based control component that tracks the status of the satellite component. The government logs the health and location of individual satellites. Periodically, this data is uploaded to all of the satellites. Receivers download this data to allow them to calculate their positions with respect to the satellite positions.
- The second component of the system is approximately 24 satellites that each complete approximately two revolutions of the earth every day at an altitude of 20,200 km. These satellites transmit time-coded signals on two frequencies that can be received by antennas in the third component.
- The third component is the receiver that you or I use here on earth. Basically, GPS determines location by trilateration. (Trilateration deals with lengths, whereas triangulation deals with angular measurements.) The GPS receiver determines the distance from 4 or more satellites in orbit. The distance from these satellites, whose positions are known, intersects at a point that locates the user's position. This distance is calculated by measuring the length of time it took for the signal travel from the satellite to the receiver. A variety of conditions may introduce error when they influence the time it takes the signal travels down to earth. Also, the DoD is currently introducing clock error intentionally to degrade the utility of GPS positions to potential hostile users.
As used by civilians, GPS comes in several "flavors."
- The most basic GPS receiver is the "handheld" unit available in many sporting goods and consumer electronic stores. These units are capable of locating positions with an accuracy of several hundred feet. These units will not give pinpoint accuracy, but will allow you to return to your vehicle or camp after walking all day. The receiver may tell you "Stop, you've reached your camp" when you can see it 100 feet away.
- Next comes professional, "mapping" grade receivers, which uses better electronics and antennas to produce better accuracy. An additional calculation will allow users of this equipment to determine locations within several feet. This extra step is called differential processing and requires a second GPS receiver located on a known point. Accuracy of hundreds of feet is improved to several feet by comparing the error in the receiver in the known point with the signals received at the unknown point.
- "Survey grade" or Phase based receivers achieve much better accuracy, within inches, by using a different portion of the GPS signals. They also use differential processing similar to the mapping grade receivers. There are also receivers today that can use both signals, dual frequency, to quickly obtain very accurate positions. These GPS units can efficiently replace conventional survey techniques in locations involving long distances without point-to-point visibility.
Several factors affect GPS users:
- All GPS users must understand and accept the error inherent in their equipment. As with any measuring tool, error must be within the reasonable error acceptable for your application. For example, a tape measure won't give you the accuracy required to measure engine components. We would never use a handheld receiver to survey property lines.
- GPS requires an unobstructed view of the sky. To receive signals, the sky must not be blocked by either trees, buildings or nearby hills. You may not have the required number of satellites visible in the portion of clear sky available where you want to GPS. GPS navigation under a forest canopy can be frustrating. The canopy may be just open enough to allow a fix one moment and then block signals from one or more satellites the next. For this reason, always carry a compass when navigating with GPS. Your compass will still work under a dense forest canopy, your GPS probably won't.
- GPS signals can be reflected. Buildings, and sometimes natural features, can reflect GPS signals. This changes the time it takes the signals to travel from the satellite to you. This error, called multi-path, can introduce additional errors to the calculations performed to determine position.
- On May 1, 2000 the government ended Selective Availability (S/A) and made GPS recievers 2-3 times more accurate. While this immediately benefits all users, mapping and surveying applications still use differential correction to get sub-meter and sub-centimeter precision.
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- Because the GPS satellites revolve around the earth, not geo-stationary like most communication satellites, their position is constantly changing. This changes the number and position of the satellites you may receive at any one time. Accuracy depends on satellite geometry. Stated simply, there are good and bad times to collect GPS positions. Professional software packages allow users to plan data collecting missions for optimum conditions. Technically, this 12-hour period gives the ground control station twice daily access to each satellite for data upload and downloads. Tactically, this helps protect the military satellites from attack.
- For reasons related to satellite geometry, elevation error is generally one and one half to twice as great as that in the horizontal positioning. This type of error means that handheld and mapping grade receivers are not used for determining elevation.
Do you have a mapping project that could benefit from the application of GPS technology?
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