Over fifty GPS satellites
such as this NAVSTAR have been launched since 1978
For other uses of the acronym GPS, see GPS (disambiguation).
The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR GPS), is a satellite navigation system used for determining one's precise location and providing a highly accurate time reference almost anywhere on Earth or in Earth orbit. It uses an intermediate circular orbit (ICO) satellite constellation of at least 24 satellites.
The GPS system was designed by and is controlled by the United States Department of Defense and can be used by anyone, free of charge. The GPS system is divided into three segments: space, control, and user. The space segment comprises the GPS satellite constellation. The control segment comprises ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission by the satellites. The user segment consists of GPS receivers used for both military and civilian applications. A GPS receiver decodes time signal transmissions from multiple satellites and calculates its position by trilateration.
The cost of maintaining the system is approximately US$400 million per year, including the replacement of ageing satellites. The first of 24 satellites that form the current GPS constellation (Block II) was placed into orbit on February 14, 1989. The 52nd GPS satellite since the beginning in 1978 was launched November 6, 2004 aboard a Delta II rocket (see article in External links section, below).
The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were initially manufactured by Rockwell; the first was launched in February, 1978, and the most recent was launched on November 6 2004. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometres (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the orbital elements of their own motion, as determined by a set of ground-based observatories.
The receiver does not need a precise clock, but does need to have a clock with good short-term stability and receive signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time. The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange). It then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. The receiver does not have a very precise clock and thus cannot know the time delays. However, it can measure with high precision the differences between the times when the various messages were received. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 4 satellites yield 2 hyperboloids, whose intersection is a curve; it's impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude).
When there are n > 4 satellites, the n-1 hyperboloids should, assuming a perfect model and measurements, intersect on a single point. In reality, the surfaces rarely intersect, because of various errors. The question of finding the point P can be reformulated into finding its three coordinates as well as n numbers ri such that for all i, PSi-ri is close to zero, and the various ri-rj are close to C.Δij where C is the speed of light and Δij are the time differences between signals i and j. For instance, a least squares method may be used to find an optimal solution. In practice, GPS calculations are more complex (repeat measurements, etc.).
There are several causes: The initial local time is a guess due to the relatively imprecise clock of the receiver, the radio signals move more slowly as they pass through the ionosphere, and the receiver may be moving. To counteract these variables, the receiver then applies an offset to the local time (and therefore to the spheres' radii) so that the spheres finally do intersect in one point. Once the receiver is roughly localized, most receivers mathematically correct for the ionospheric delay, which is least when the satellite is directly overhead and becomes greater toward the horizon, as more of the ionosphere is traversed by the satellite signal. Since it is common for the receiver to be moving, some receivers attempt to fit the spheres to a directed line segment.
The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information which helps the receiver in estimating the correct speed of propagation. High-end receiver/antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays. Because certain delay sources, such as the ionosphere, affect the speed of radio waves based on their frequencies, dual frequency receivers can actually measure the effects on the signals.
In order to measure the time delay between satellite and receiver, the satellite sends a repeating 1,023 bit long pseudo random sequence; the receiver knows the seed of the sequence, constructs an identical sequence and shifts it until the two sequences match.
Different satellites use different sequences, which lets them all broadcast on the same frequencies while still allowing receivers to distinguish between satellites. This is an application of Code Division Multiple Access, or CDMA.
Several frequencies make up the GPS electromagnetic spectrum:
- L1 (1575.42MHz):
Carries a publicly usable coarse-acquisition (C/A) code as well as an encrypted P(Y) code.
- L2 (1227.60MHz):
Usually carries only the P(Y) code. The encryption keys required to directly use the P(Y) code are tightly controlled by the U.S. government and are generally provided only for military use. The keys are changed on a daily basis. In spite of not having the P(Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal (in a round-about manner) to increase accuracy and remove error caused by the ionosphere.
- L3 (1381.05MHz):
Carries the signal for the GPS constellation's alternate role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events.
- L4 (1841.40MHz):
Being studied for additional ionospheric correction.
- L5 (1176.45MHz):
Proposed for use as a civilian safety-of-life signal.
A minor detail is that the atomic clocks on the satellites are set to "GPS time", which is the number of seconds since midnight, January 6, 1980. It is ahead of UTC because it doesn't follow leap seconds. Receivers thus apply a clock correction factor, (which is periodically transmitted along with the other data), and optionally adjust for a local time zone in order to display the correct time. The clocks on the satellites are also affected by both special, and general relativity, which causes them to run at a slightly slower rate than do clocks on the Earth's surface. This amounts to a discrepancy of around 38 microseconds per day, which is corrected by electronics on each satellite. This offset is a dramatic proof of the theory of relativity in a real-world system, as it is exactly that predicted by the theory, within the limits of accuracy of measurement.
Sources of GPS measurement errors
Ideally, GPS receivers would easily be able to convert the C/A and P(Y)-code measurements into accurate positions. However, a system with such complexity leaves many openings for errors to affect the measurements. The following are several causes of error in GPS measurements.
Both GPS satellites and receivers are prone to timing errors. Ground stations throughout the world monitor the satellites to ensure that their atomic clocks are kept synchronized. Receiver clock errors depend upon the oscillator provided within the unit. However, they can be calculated and then eliminated once the receiver is tracking at least four satellites.
The Ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. As a result, errors greater than 10 metres may arise. To compensate for these errors, the second frequency band L2 was provided. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated.
The antenna receives not only direct GPS signals, but also multipath signals: reflections of the radio signals off the ground and/or surrounding structures (buildings, canyon walls, etc). For long delay multipath signals, the receiver itself can filter the signals out. A variety of receiver techniques, most notably Narrow Correlator spacing, have been developed to mitigate multipath error contributions to pseudorange measurements. For shorter delay multipath signals that result from reflections from the ground, special antenna features may be used such as a ground plane, or a choke ring antenna. Shorter multipath signals from ground reflections can often be very close to the direct signals, and can greatly reduce precision.
In the past, the civilian signal was degraded, and a more accurate Precise Positioning Service was available only to the United States military, its allies and other, mostly government users. However, on May 1, 2000, then US President Bill Clinton announced that this "Selective Availability" would be turned off, and so now all users enjoy nearly the same level of access, allowing a precision of position determination of less than 20 meters.
Techniques to improve GPS accuracy
The accuracy of GPS can be improved in a number of ways:
- Using a network of fixed ground based reference stations. These stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and receiver stations may correct their pseudoranges by the same amount. This method is called Differential GPS or DGPS. DGPS was especially useful when GPS was still degraded (via the "Selective Availability" described below), since DGPS could nevertheless provide 5-10 metre accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries.
- Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision guided munitions such as the Joint Direct Attack Munition (JDAM).
- The Wide-Area Augmentation System (WAAS). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). The current WAAS system only works for North America (where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation System), which are virtually identical to WAAS.
- A Local-Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometre radius around the transmitter.
- Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially-equipped receivers.
- Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning sytem. In this approach, accurate determinination of range signal can be resolved to an accuracy of less than 10 centimetres. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and ambiguity resolution techniques via statistical tests - possibly with processing in real-time (real-time kinematic positioning, RTK ).
- Many automobile GPS systems combine the GPS unit with a gyroscope and speedometer pickup, allowing the computer to maintain a continuous navigation solution by dead reckoning when buildings, terrain, or tunnels block the satellite signals. This is similar in principle to the combination of GPS and inertial navigation used in ships and aircraft, but less accurate and less expensive because it only fills in for short periods.
The primary military purpose is to allow improved command and control of forces through an enhanced ability to accurately specify target locations for cruise missiles or troops. The satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System .
The system is used by countless civilians as well, who can use the GPS's Standard Positioning Service worldwide free of charge. Low cost GPS receivers (price $100 to $200) are widely available, combined in a bundle with a PDA or car computer .The system is used as a navigation aid in aeroplanes, ships and cars. The system can be used by computer controlled harvesters, mine trucks and other vehicles. Hand held devices are used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions.
On May 1, 2000, US President Bill Clinton announced that this "Selective Availability" would be turned off. However, for military purposes, "Selective Deniability" may still be used to, in effect, jam civilian GPS units in a war zone or global alert while still allowing military units to have full functionality. In reality, the shortage of military GPS units and the wide availability of civilian ones among personnel resulted in disabling the Selective Availability in the time of the Gulf War. However, European concern about the level of control over the GPS network and commercial issues has resulted in the planned GALILEO positioning system. Russia already operates an independent system called GLONASS (global navigation system), although with only 12 active satellites as of 2004, the system is of limited usefulness.
Military (and selected civilian) users still enjoy some technical advantages which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above). Commercial GPS receivers are also required to have limits on the velocities and altitudes at which they will report fix coordinates; this is to prevent them from being used to create improvised cruise or ballistic missiles.
Many synchronization systems use GPS as a source of accurate time, hence one of the most common applications of this use is that of GPS as a reference clock for time code generators or NTP clocks. For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately.
A large part of modern munitions, the so-called "smart bombs" or precision-guided munitions, use GPS. GPS jammers are available, from Russia, and are about the size of a cigarette box. The U.S. government believes that such jammers were used occasionally during the U.S. invasion of Afghanistan. Some officials believe that jammers could be used to attract the precision-guided munitions towards noncombatant infrastructure, other officials believe that the jammers are completely ineffective. In either case, the jammers are attractive targets for anti-radiation missiles.
Two GPS developers have received the National Academy of Engineering Charles Stark Draper prize year 2003:
On February 10, 1993, the National Aeronautic Association selected the Global Positioning System Team as winners of the 1992 Robert J. Collier Trophy, the most prestigious aviation award in the United States. This team consists of researchers from the Naval Research Laboratory, the U.S. Air Force, the Aerospace Corporation, Rockwell International Corporations, and IBM Federal Systems Company.The citation accompanying the presentation of the trophy honors the GPS Team "for the most significant development for safe and efficient navigation and surveillance of air and spacecraft since the introduction of radio navigation 50 years ago."
GPS for private and commercial use
The GPS system is free for everyone to use, all that is needed is a GPS receiver, which costs about $90 and up (March 2005). This has led to widespread private and commercial use. Examples of private use is the popular activity Geocaching where a GPS unit is used to search for objects hidden in nature by travelling to the GPS coordinates. Commercial use can be land measurement, navigation and road construction.
GPS for the visually impaired
The projects of the navigation system using GPS for the visually impaired have been conducted quite a few times. GPS was introduced in the late 80ís and since then there have been several research projects such as MoBIC, Drishti, and Brunel Navigation System for the Blind, NOPPA, BrailleNote GPS and Trekker.
MoBIC means Mobility of Blind and Elderly people Interacting with Computers, which was carried out from 1994 to 1996 supported by the Commission of the European Union. It was developing a route planning system which is designed to allow a blind person access to information from many sources such as bus and train timetables as well as electronic maps of the locality. The planning system helps blind people to study and plan their routes in advance, indoors.
With the addition of devices to give the precise current position and orientation of the blind pedestrian, the system could then be used outdoors. The outdoor positioning system is based on signals and satellites which give the longitude and latitude to within a metre; the computer converts this data to a position on an electronic map of locality. The output from the system is in the form of spoken messages.
Drishti is a wireless pedestrian navigation system. It integrates several technologies including wearable computers, voice recognition and synthesis, wireless networks, Geographic information system (GIS) and GPS. It augmentes contextual information to the visually impaired and computed optimised routes based on user preference, temporal constraints (e.g. traffic congestion), and dynamic obstacles (e.g. ongoing ground work, road blockade for special events).
The system constantly guides the blind user to navigate based on static and dynamic data. Environmental conditions and landmark information queries from a spatial database along their route are provided on the fly through detailed explanatory voice cues. The system also provides capability for the user to add intelligence, as perceived by the blind user, to the central server hosting the spatial database.
Brunel Navigation System for the Blind
Franjo Cecelja from Brunel university introduced a 10 year project on a navigation system for blind and visually impaired people. The system is based on the integration of state of the art current technologies, including high-accuracy GPS positioning, GIS, electronic compass and wireless digital video transmission (remote vision) facility with an accuracy of 3~4m. It provides an automated guidance using the information from daily updated digital map datasets e.g. roadworks. If required the remote guidance of visually impaired pedestrians by a sighted human guide using the information from the digital map and from the remote video image provides flexibility.
The difficulties encountered includes the availability of up to date information and what information to offer including the navigation protocol. Levels of functionality have been created to tailor the information to the userís requirements.
NOPPA navigation and guidance system was designed to offer public transport passenger and route information using GPS technology for the visually impaired. This was a three-year (2002~2004) project in VTT Industrial Systems in Finland. The system provides an unbroken trip chain for a pedestrian using buses, commuter trains and trams in three neighbour citiesí area. It is based on an information server concept, which has user-centred and task oriented approach for solving information needs of special needs groups.
In the system, the Information Server is an interpreter between the user and Internet information systems. It collects, filters and integrates information from different sources and delivers results to the user. The server handles speech recognition and functions requiring either heavy calculations or data transfer. The data transfer between the server and the client is minimised. The user terminal holds speech synthesis and most of route guidance.
NOPPA is currently able to offer basic route planning and navigation services in Finland. In practice, the limits are map data can have outdated information or inaccuracies, positioning can be unavailable or inaccurate, or wireless data transmission is not always available.
The BrailleNote GPS device is developed by Sendero Group, LLC, and Pulse Data International in 2002. It is like a combination of a personal digital assistant, Map-quest software and a mechanical voice.
With a receiver about the size of a small sell phone, the BrailleNote GPS utilizes a network of 24 satellites to pinpoint a travelerís position on earth and nearby points of interest. The personal computers receive radio signals from satellites to chart the location of users and direct them to their destination with recorded voice commands. The system uses satellites to triangulate the carrierís position, much like a ship finding its location at sea.
Visually impaired people can encode points of interest such as local restaurants or any other location, into the computerís database. Afterward, they can punch keys on the unitís keyboard to direct themselves to a specific point of interest.
Victor Trekker, designed and manufactured by Canada-based company VisuAid, was launched on March 2003. It is a personal digital assistant (PDA) application operating on a PocketPC with WinCE, adapted for the blind and visually impaired with talking menus, talking maps and GPS information. Fully portable (weights 600g), it offered features enabling a blind person to determine position, create routes and receive information on navigating to a destination. It also provided search functions for an exhaustive database of point of interests, such as restaurants, hotels, etc.
It is fully upgradeable, so it can expand to accommodate new hardware platforms and more detailed geographic information.
Trekker and Maestro, which is the first off-the-shelf accessible PDA based on Windows Mobile Pocket PC, are integrated and available in May 2005.
For a list of other systems, see satellite navigation system.
Last updated: 07-31-2005 02:54:17
Last updated: 09-12-2005 02:39:13