The Global Positioning System (GPS) is a location system based on a constellation of satellites orbiting the earth at altitudes of approximately 12,500 miles (20,200 km). GPS satellites are orbited high enough to avoid the problems associated with land based systems, yet can provide
accurate positioning 24 hours a day, anywhere in the world.
Uncorrected positions determined from GPS satellite signals produce accuracies in the range of 7 to 10 meters. When using a technique called differential correction, users can get positions accurate to within 1 meter or less.
As GPS units are becoming smaller and less expensive, there are an expanding number of applications for GPS. In transportation applications, GPS assists pilots and drivers in pinpointing their locations and avoiding collisions. Farmers can use GPS to guide equipment and control accurate distribution of fertilizers and other chemicals. Recreationally, GPS is used for providing accurate locations and as a navigation tool for hikers, hunters, and boaters. The applications of GPS technology will continue to increase in the future, making this technology commonplace in the lives of millions of people.
The concept of GPS begins with two important develo
pments; the use of radio waves for navigation, and the space race of the 1950s and 1960s. The first use of radio waves for navigation came in the form of the Radio Direction Finder (RDF). A person using the RDF to find his position would simply rotate a highly directional antenna until he found the direction that the signal of a known radio station came in the strongest. If this could be accomplished with two different stations, a position could be determined by plotting lines on a map from the known radio station location in the direction of the strongest signal. The intersection of the two lines would be the navigator's position.
Another development in radio navigation originated in Germany in the 1930s when radio engineers devised a system to aid airplanes trying to land at night and during bad weather. In Germany, this new system was called Beams, but to the rest of the world it was referred to as Lorentz, which is the name of the company that manufactured the equipment. The system consisted of two highly directional radio trans
mitters set side by side that would transmit parallel radio signals. One signal would consists of a series of pulses in the form of a beep or dot. The other would consist of a dash or a longer pulse. This distinct difference between the left and right signal would give pilots flying into the signal the ability to determine if they needed to fly their plane left or right to align themselves for landing. If they where in the middle of the two signals, the dash and dot sounds produced from the two signals would combine to form a continuous tone. The pilot would then know he was on course.
During the late 1930s the German radio navigation system, Beams or Lorentz, was quickly adapted to aid in the war effort. It did not take long for German scientists to figure out how this new technology could increase the accuracy of the bombing raids they were conducting over England. The original design used two radio transmitters sending parallel pulses of tones. By adding another set of transmitters, operating at a different frequency and at right angles to the first set, a position could be established, most notably a desired target for the German Luftwaffe. The bomber pilot, equipped with two radio receivers, would navigate to the intersection of the two signals. When both of his radios where producing solid tones, he was directly over the target. Many variations of ground base radio navigation were developed and used including VHF Omni-directional Range (VOR) and Long-Range Radio Navigation LORAN.
History of GPS
The genesis of the modern GPS system has its roots in the "space race" that began with the launch of the Russian Sputnik satellite in 1957. Researchers at Johns Hopkins University's Applied Physics Laboratory (APL) realized that they could now use the Doppler shift in the signal broadcast by a satellite as an observation point to determine the exact approach time of a satellite. The ability to precisely determine a satellites orbit from the ground by using radio signals was the first step in developing an accurate space-based positioning system. The United States launched its first satellite, Explorer I, on January 31, 1958.Soon after the launch of Sputnik, the U.S. Navy began work on the modern predecessor to today's GPS. The Navy system was called the Navy Navigation Satellite System (NNSS), also called TRANSIT. It included six satellites orbiting the earth at altitudes of about 1,100 km and was used to determine the location of vessels and aircraft. The military did authorize civilian use of this system, prompting its use around the world for surveying and navigation.
The TRANSIT system had shortcomings, including significant time gaps in coverage and relatively low levels of navigational accuracy. Therefore, in 1973, the Department of Defense (DOD) officially approved the development of the GPS system and directed the Joint Program Office (JPO) to develop and deploy a system of satellites for spaceborne positioning. The Navy and Air Force programs were combined, forming the Navigation Technology Program, which eventually became the Navigation System and Ranging (NAVSTAR).
Development and testing of the system began following the first GPS satellite launch in 1974. The satellites were built by Rockwell Collins and launched by the Air Force.
Testing continued into the 1980s when GPS satellites were to be among payloads carried by NASA Space Shuttle flights. The GPS program suffered a major setback when shuttle launches were suspended following the 1986 Challenger accident. Several years passed until modifications could be made to the Delta II launch vehicle, enabling it to carry GPS satellites. Into the early 1990s, the DoD continued to assemble the constellation through continued satellite deployments. In 1991, the general public was introduced to the usefulness and capabilities of GPS technology during Operation Desert Storm, referred to by some as the First Space War, due to its reliance upon a variety of satellites. This was the first time the system was used in a major military operation, and it proved invaluable for a variety of uses. Ground troops could navigate at night and during sandstorms, missiles could be guided to targets, and bombs could be dropped with precision.
The GPS system achieved Initial Operational Capability on December 8, 1993 when a constellation of 24 satellites, 21 operational
and three in reserve, became available. Not until July 17, 1995 did the system reach Full Operational Capability, whereupon the system had been fully tested by the DoD. In 1996, the White House released a directive "establishing a national policy for the management and use of the U.S. Global Positioning System."
Although developed by the DoD, and initially intended for military use, Congress, with the support and guidance of the President, directed the DoD to promote the civil use of GPS. In this way, the use of GPS technology moved increasingly into the civilian sector. According to the JPO, approximately
1.4 million civilian GPS receivers have been produced each year since 1997, creating an economic impact of $6.2 billion by the year 2000. Future expectations project that the economic impact of GPS technology may surpass $50 billion by 2010.
Civilian users therefore benefit from a taxpayer funded project that from 1973 - 2002 cost the Air Force approximately $6.3 billion to develop (not including military user equipment or launch costs). It also costs about $750 million annually to operate and maintain the constellation, including research and development, as well as procurement for and replacement of satellites.The Russians also developed a GPS system called GLObal'naya Navigatsionnay Sputnikovaya Sistema or GLONASS. More recently the European Union approved funding to develop a GPS system called Galileo, that could be operational by 2008.
Persons unfamiliar with GPS technology sometimes refer to the handheld receiver as the global positioning system. In reality, this "system" includes space, control, and user segments.
The space component is made up of a network or constellation of satellites. This constellation changes periodically as new satellites are launched and older models are decommissioned (Select this link to check the Constellation Status). Unlike television and most communications satellites, the constellation of NAVSTAR GPS satellites is not in geo-synchronous (stationary) orbit, but instead circles the Earth at much lower altitudes of about 10,900 nautical miles (approx. 12,500 statute miles). Each satellite in the constellation is on one of six orbital planes, inclined with respect to the equatorial plane by 55 degrees. A complete orbit of the Earth requires a period of slightly less than 12 hours (about 1.8 miles per second!) with satellites appearing on the horizon four minutes earlier each day. This configuration ensures that at least four satellites will be above the horizon anywhere on Earth simultaneously.
There have been four classes of GPS satellites, including the Block I, Block II, Block IIA, Block IIR. The next generation of satellites will be the Block IIF, scheduled to launch in 2006 . The current (March 2003) GPS constellation includes two Block II and eighteen Block IIA satellites built by Boeing, and seven Block IIR satellites built by Lockheed Martin, and fourteen Block IIRs remaining to be launched. To read more about the history of GPS satellites and the current constellation go to NAVSTAR's Space Division or GPS Overview webpage.The control segment is known as the Operational Control Segment (OCS), consisting of three components:
Master Control Station (MCS):
The MCS is located at Schriever Air Force Base (formerly Falcon Air Force Base) in Colorado and is responsible for the overall management of the control segment. The MCS collects tracking data from the monitoring stations and calculates any positional or clock errors. The results are transmitted to the ground control stations for upload to the satellites. The MCS is manned 24 hours per day, every day of the year. Monitoring stations are unmanned and remotely controlled by the MCS.Monitoring stations:
The five monitoring stations are located in Hawaii, Kwajalein (western Pacific Ocean), Ascension Island (South Atlantic Ocean), Diego Garcia (Indian Ocean), and Shriever Air Force Base (USA) (Click here to View Map of Stations). Each of these stations function as radio receivers, tracking each satellite that is within their view. They do not process the data, but send the unprocessed psuedo-range measurements back to the MCS for processing.Ground control stations:
These are also referred to as Ground Antenna, since they are unmanned installations operated remotely by the MCS. They are located at the monitoring stations on Ascension, Diego Garcia, and Kwajalein Islands. The ground control stations enable the MCS to communicate with and control the satellites.The user component includes an antenna and receiver that provides positioning, velocity, and precise timing measurements to a user located on the ground, in the air, or over water.
Although GPS receivers are sophisticated instruments, the basic principle used for determining Earth position is relatively simple. Precise clocks and the principle of trilateration (the word triangulation is sometimes used despite the fact that no angles are involved) are used to measure the distance between the user and a combination of three or more satellites.
Distances between satellites and receivers are measured according to the time needed for radio signals transmitted by the satellites to reach
a receiver. These radio signals are actually complex strings of random pulses, known as the pseudo-random code, that repeat every millisecond. The designers of the GPS system synchronized the satellites and receivers so that they generate the same code at the same time. When a receiver acquires a code it then looks back to see how long ago it generated the same code. The time difference is how long the signal took to travel from the satellite to the receiver. By multiplying this elapsed time by 186,282.3976 (the speed of light in miles per second), the receiver can determine an exact distance between its location and a satellite.
The challenge in determining the travel time of the signal is determining when the signal left the satellite. A receiver timing error as small as one thousandth of a second can cause a position error of nearly 200 nautical miles. Clock error is illustrated in this two-dimensional diagram, where lines show distances between a receiver and two satellites having perfectly aligned clocks. The receiver's correct position would be found where the lines intersect (point A). Another set of lines represent the distance measured when a receiver clock is running slightly fast. The intersection of these lines indicates a position that is farther away from the two satellites than is actually the case (point B).
To prevent errors in determining travel time of the signals, GPS satellites are equipped with four atomic clocks that are extremely accurate and cost about $100,000 apiece. Of course, the clocks found within GPS receivers are not as accurate or expensive! The clocks found inside GPS receivers are similar to quartz clocks found in wristwatches, necessitating the use of a fourth satellite distance measurement. For example, with only one satellite, the receiver is located somewhere on the surface of a sphere surrounding the satellite. Using two satellites, the receiver can narrow its location to a circle that represents the intersection of two spheres. A third satellite measurement is necessary to pinpoint the receiver's location at one of two points (Watch a simulation of "triangulation"). Most receivers can then eliminate one of these points as a ridiculous answer (it may be below the Earth's surface or it may be traveling at a high velocity). Nonetheless, to obtain a truly accurate position, GPS receivers need to make four measurements so that any clock error can be eliminated. Thus, three perfect measurements or four imperfect measurements can both locate a point in 3-dimensional space (Watch a simulation that demonstrates the use of four satellites).
GPS satellites transmit data to receivers using two radio carrier frequencies, the L-1 band (1575.42 MHz) and the L2 band (1227.6 MHz). A unique code assigned to each satellite allows the receiver to distinguish among satellites transmitting on the same L-band carrier frequency. These signals were modulated with two types of pseudo-random code, P-code (precision code), designated as the Precise Positioning Service (PPS) to facilitate the military's requirement for safeguarding a tactical advantage for friendly forces and C/A (coarse acquisition), designated as Standard Positioning Service (SPS) code for civilian use. The security was assured by random adjustments in mathematical formulas that only military-grade receivers could decrypt. P-code provides horizontal position accuracies of about five meters with a single hand-held receiver. Although a single C/A code receiver could provide an accuracy of only 20 to 30 meters, even this level of precision made the military uncomfortable.
To address military concerns, the DOD degraded C/A signals by introducing slight errors, known as "selective availability" (SA), in the clock timing or navigational messages sent by satellites. With SA in effect, the positions reported by a C/A receiver were degraded to +/- 100 meters. Although acceptable for some types of navigation, such errors were too large for most mapping and surveying purposes. The decision to turn off SA was a tremendous event in the history of GPS technology. President Bill Clinton made the official announcement and it went into effect on midnight May 1, 2000 when SA was actually turned off. GPS receivers instantly became significantly more accurate. These two graphs from the U.S. National Geodetic Survey demonstrate the immediate impact on accuracy. To view additional examples click here.
Another method to deny civilian users full use of the GPS is known as Anti-Spoofing. This allows the P-code to be turned off or to invoke an encrypted code (known as the Y-code) as a means of denying access to the system. The intent of Anti-Spoofing is to keep individuals or military adversaries form sending out false signals that could cause confusion or inaccuracies in positioning.
Post Processing/Differential Correction
To further increase accuracy a postprocessing technique called differential correction can be carried out to remove error. Differential GPS (or DGPS) is accomplished by using two GPS receivers
simultaneously. One unit, called the base, is placed over a precisely known location such as a surveyor's monument and is set to continuously collect data. The second receiver, called the rover, is used in the field to collect the data desired by the user. Errors in positions collected by the rover receiver can be removed because the location of the base unit is already known and errors collected by the base and rover receivers will be identical for any given moment in time. Although positional accuracies will vary depending on the receivers used, in most cases DGPS improves horizontal position accuracy to between two and five meters. More sophisticated and expensive survey-grade receivers can yield accuracies of one centimeter. A Nationwide DGPS program has been initiated by the U.S. Coast Guard and is expected by 2005. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. To receive the corrected signals, a user must have a differential beacon receiver and antenna. An example is Trimble's Beacon on a Belt.
Another technology developed to increase the accuracy of GPS is the Wide Area Augmentation System (WAAS). This system of is being created by the Federal Aviation Administration (FAA) and the Department of Transportation (DOT) to assist pilots in precision landing. WAAS is a network of approximately 25 widely spaced wide area reference stations that receive GPS signals. These stations relay the GPS signal data to a wide area master station (WMS) where errors are processed and corrections calculated. The WMS then sends this information to one of two Geostationary Earth Orbit (GEO) satellites. These satellites broadcast the WAAS message on the same frequency as GPS (L1, 1575.42MHz) to users equipped with WAAS enabled receivers. Using WAAS can improve GPS signal accuracy from 20 meters to approximately 1.5-2 meters, both horizontally and vertically. WAAS will eventually cover the entire Continental U.S. and Alaska (Map of WAAS service area). See also WAAS.Of course, a still common and important question is: How accurate is my GPS unit? The answer to this question is somewhat elusive since it is based upon a variety of factors including the number of satellites available, the satellite geometry of those satellites, and the inherent accuracy of the unit used. To test the accuracy of your unit see this link.
Although the turning off of SA greatly improved accuracy, there are other sources of error in GPS measurements. Small amounts of error may be generated by ionospheric or atmospheric propagation delays as GPS signals pass through various layers of the atmosphere. The particles in the ionosphere affect the speed of the GPS radio signals. To correct this some receivers produce "dual frequencies" where the difference between the two frequencies is used to deduce the amount of error. This is called "ionspheric-free solution" and is available on more advanced receivers. Atmospheric error, caused by water vapor, is also a factor but it is nearly impossible to correct.
Another source of error is the satellite geometry, or the position of the satellites in relation to each other and the receiver. The satellites the receiver uses to fix a position, and the location of those satellites impacts accuracy. Point dilution of precision, or PDOP, is a general indicator of the accuracy of a GPS measurement as determined by satellite geometry. Low PDOP values are reported by a receiver when satellites are widely spaced in the sky above the user. The best possible satellite geometry, one that yields the lowest PDOP, can be found when one satellite is directly overhead and the remaining three satellites are equally spaced around the horizon. High PDOPs occur when satellites are close together or when they form a line in the sky. Only in very rare cases are satellites arranged in a configuration that prevents the receiver from obtaining a position.
Multipath error, another common source of inaccuracy, occurs when satellite signals bounce off solid objects, such as buildings or cliffs, before reaching the receiver; producing error in much the same way as ghost images seen on television screens. The signal does not go directly to the receiver, taking a more circuitous path, and thus taking longer to reach the receiver. Multipath is an important factor when collecting data around tall buildings, sometimes referred to as "urban canyons", and when collecting in mountainous areas.
Finally, the atomic clocks in the satellites may have slight variations. The DoD monitors and corrects errors, but slight inaccuracies may occur. Receivers may also make mistakes by rounding off mathematical calculations or electrical interference might cause incorrect calculations.
Mission planning refers to the setup of a GPS project prior to fieldwork. The extent and focus of mission planning varies depending upon the type of receiver used, the objectives and intent of the data collection, and the accuracy sought. For example, for someone collecting data for personal recreational use, mission planning may only include making sure the receiver being used functions properly. On the other extreme, someone involved in a research project in a remote area, will want to maximize time spent by creating a detailed plan prior to fieldwork.
Individuals seeking to optimize time, money, and effort spent in the field, along with obtaining the greatest degree of accuracy will want a well thought-out plan. The following are the series of steps often used for mission planning:
Field Reconnaissance: When appropriate, a visit to the site where data will be collected can greatly enhance accuracy and efficiency of field data collection. Visiting the site will assist in identifying features you may want to collect, the order to collect them, and the attribute information collected for selected features. Visiting the area prior to data collection also allows you to identify features which may obstruct, slow down, or impact the accuracy of data collection. Field reconnaissance greatly assists the building of appropriate data dictionaries and avoiding inaccuracies related to obstructions in the field.
Data Dictionary: The creation of a data dictionary for the GPS, and pre-field consideration of satellite geometry for scheduling fieldwork (satellites can be poorly positioned or in insufficient number for your receiver) is extremely important. Often the company which built the receiver also includes software that enables the user to create a data dictionary that streamlines and organizes data collection. For example, Trimble® provides software called Pathfinder Office, that includes a Data Dictionary Editor which includes a series of dialog boxes that help you define the feature types and attributes of features before entering the field.
Identify the Best Collection Times: Accurate data collection is dependent upon having at least four satellites in a good geometric configuration. To increase accuracy, the best times to collect data need to be identified for the area where data will be collected. Again, many companies that produce GPS receivers include software such as Trimble's Pathfinder Office that includes QuickPlan, which identifies times when satellite geometry is the best for a certain geographic location.
Computer Workspace: For those wanting to integrate GPS and GIS technologies, mission planning should include the setup of a workspace directory in the computer. This should include appropriately named and organized folders and subfolders that will contain the files collected and created during the projects duration.
Equipment Setup: The last step prior to field collection should be properly configuring the GPS receivers. Make sure the batteries are fully charged and the unit is functioning properly.
Select the links below to view the websites of companies that manufacture GPS units or related technology:
Selecting a GPS unit is dependent upon two basic criteria; how much money you would like to spend and what you would like to do with the receiver. The first may be easier to answer than the second, since your intended uses may change once you purchase a unit. Selecting a GPS receiver has become somewhat more difficult due to their popularity, with hundreds of models available from a variety of manufacturers. To simplify the process you can divide receivers into three categories: recreational, mapping, and survey. The following describes the uses and accuracy of each:
Recreation Grade: These are the most common types of receivers used by the general public. Hikers, boaters, hunters, and anglers all use these to navigate or mark waypoints. Recreation grade receivers vary in price with the least expensive models at about $100, with higher end models costing as much as $1200. Lower end models do not come with built in maps nor can maps be downloaded to them from CD-ROM. Higher end models usually come with a built in map as well as the ability to download additional points-of-interest maps from CD-ROM.
Mapping Grade: Mapping grade receivers are more accurate than recreational grade receivers and are used for a variety of applications. These types of receivers generally can achieve sub-meter accuracy and are therefore useful in creating accurate maps. Mapping grade receivers also cost considerably more than recreation grade recievers.
Survey Grade: Survey grade receivers cost considerably more than either mapping or recreation grade receivers. These models may cost $40 thousand and achieve accuracies within several centimeters.
These links will provide you with a variety of websites that demonstrate how GPS technology is being used by numerous industries:
Books:
Hofmann-Wellenhof, Bernhard, Lichtenegger, Herbert, and Collins, James. Global Positioning System: Theory and Practice, 4th ed. (New York: Springer-Verlag/Wien, 1997).
Hurn, Jeff. GPS: A Guide to the Next (Sunnyvale, CA: Trimble Navigation Ltd., 1989).
Hurn, Jeff. Differential GPS Explained (Sunnyvale, CA: Trimble Navigation Ltd., 1993).
Websites:
The Geographer's Craft, the University of Colorado at Boulder
A GPS Tutorial: Basics of High-Precision Global Positioning Systems (PDF). From Topcon website.
GPS Acronyms, from University of Colorado website.
GPS History, Chronology, and Budgets (PDF). From Rand website.
2000 IGS Annual Report (PDF). IGS Central Bureau, eds. Pasadena, CA: Jet Propulsion Laboratory, 2002.
2002-2007 IGS Strategic Plan (PDF). IGS Central Bureau, eds. Pasadena, CA: Jet Propulsion Laboratory, 2002.
GPS World Buyer's Guide (PDF). From GPSworld website.