A positioning system is a mechanism for determining the position of an object in space. Technologies for this task exist ranging from worldwide coverage with meter accuracy to workspace coverage with sub-millimetre accuracy.
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In navigation, position fixing or positioning is the determination of the position of a vehicle or person on the surface of the Earth. Position fixing uses various visual and electronic methods including:
- Position lines and position circles
- Celestial navigation
- Radio navigation
- Satellite navigation system
Generally speaking a position fix is calculated by taking into account measurements (referred to as observations) of distances or angles to reference points whose positions are known. In 2D surveys observations of three reference points are enough to compute a position in a two dimensional plane. In practice observations are subject to errors resulting from various physical and atmospheric factors that influence the measurement of distances and angles.
A practical example of obtaining a position fix would be for a ship to take bearing measurements on three lighthouses positioned along the coast. These measurements could be made visually using a hand bearing compass, or in poor visibility electronically using radar or radio direction finding. Since all physical observations are subject to errors the resulting position fix is also subject to error. Although in theory two lines of position (LOP) are enough to define a point, in practice 'crossing' more LOPs provides greater accuracy and confidence, especially if the lines cross at a good angle to each other. Three LOPs are considered the minimum for a practical navigational fix. The three LOPs when drawn on the chart will in general form a triangle, known as a 'cocked hat'. The navigator will have more confidence in a position fix that is formed by a small cocked hat with angles close to those of an equilateral triangle.
It is not true to say that the navigator's true position is 'definitely' within the cocked hat on the chart. The area of doubt surrounding a position fix is called an error ellipse. To minimize the error, electronic navigation systems generally use more than three reference points to compute a position fix to increase the data redundancy. As more redundant reference points are added the position fix becomes more accurate and the area of the resulting error ellipse decreases.
The process of combining multiple observations to compute a position fix is equivalent to solving a system of linear equations. Navigation systems use regression algorithms such as Least squares in order to compute a position fix in 3D space. This is most commonly done by combining distance measurements to 4 or more GPS satellites, which orbit the earth along known paths.
Interplanetary-radio communication system not only communicate with spacecraft, but are also used to determine their position. Radar can track targets near the Earth, but spacecraft in deep space must have a working transponder on board to echo a radio signal back. Orientation information can be obtained using star trackers.
Global navigation satellite systems (GNSS) allow specialized radio receivers to determine their 3-D space position, as well as time, with an accuracy of 2–20 metres or tens of nanoseconds. Currently deployed systems use microwave signals that can only be received reliably outdoors and that cover most of Earth's surface, as well as near-Earth space.
The existing and planned systems are:
- Global Positioning System – US military system, fully operational since 1995
- GLONASS – Russian military system, fully operational since October 2011
- Galileo – European Community, fully operational since December 2019
- Beidou navigation system – a planned project in China
- Indian Regional Navigation Satellite System – a planned project in India
Networks of land-based positioning transmitters allow specialized radio receivers to determine their 2-D position on the surface of the Earth. They are generally less accurate than GNSS because their signals are not entirely restricted to line-of-sight propagation, and they have only regional coverage. However, they remain useful for special purposes and as a backup where their signals are more reliably received, including underground and indoors, and receivers can be built that consume very low battery power. LORAN is such a system.
A local positioning system (LPS) is a navigation system that provides location information in all weather, anywhere within the coverage of the network, where there is an unobstructed line of sight to three or more signaling beacons of which the exact position on earth is known.
Unlike GPS or other global navigation satellite systems, local positioning systems don't provide global coverage. Instead, they use (a set of) beacons which have a limited range, hence requiring the user to be near these. Beacons include cellular base stations, Wi-Fi and LiFi access points, and radio broadcast towers.
In the past, long-range LPS's have been used for navigation of ships and aircraft. Examples are the Decca Navigator System and LORAN. Nowadays, local positioning systems are often used as complementary (and in some cases alternative) positioning technology to GPS, especially in areas where GPS does not reach or is weak, for example, inside buildings, or urban canyons. Local positioning using cellular and broadcast towers can be used on cell phones that do not have a GPS receiver. Even if the phone has a GPS receiver, battery life will be extended if cell tower location accuracy is sufficient. They are also used in trackless amusement rides like Pooh's Hunny Hunt and Mystic Manor.
Examples of existing systems include
Indoor positioning systems are optimized for use within individual rooms, buildings, or construction sites. They typically offer centimeter-accuracy. Some provide 6-D location and orientation information.
Examples of existing systems include
These are designed to cover only a restricted workspace, typically a few cubic meters, but can offer accuracy in the millimeter-range or better. They typically provide 6-D position and orientation. Example applications include virtual reality environments, alignment tools for computer-assisted surgery or radiology, and cinematography (motion capture, match moving).
Multiple technologies exist to determine the position and orientation of an object or person in a room, building or in the world.
Time of flight
Time of flight systems determine the distance by measuring the time of propagation of pulsed signals between a transmitter and receiver. When distances of at least three locations are known, a fourth position can be determined using trilateration. Global Positioning System is an example.
Optical trackers, such as laser ranging trackers suffer from line of sight problems and their performance is adversely affected by ambient light and infrared radiation. On the other hand, they do not suffer from distortion effects in the presence of metals and can have high update rates because of the speed of light.
Ultrasonic trackers have a more limited range because of the loss of energy with the distance traveled. Also they are sensitive to ultrasonic ambient noise and have a low update rate. But the main advantage is that they do not need line of sight.
A spatial scan system uses (optical) beacons and sensors. Two categories can be distinguished:
- Inside out systems where the beacon is placed at a fixed position in the environment and the sensor is on the object
- Outside in systems where the beacons are on the target and the sensors are at a fixed position in the environment
By aiming the sensor at the beacon the angle between them can be measured. With triangulation the position of the object can be determined.
The main advantage of an inertial sensing is that it does not require an external reference. Instead it measures rotation with a gyroscope or position with an accelerometer with respect to a known starting position and orientation. Because these systems measure relative positions instead of absolute positions they can suffer from accumulated errors and therefore are subject to drift. A periodic re-calibration of the system will provide more accuracy.
This type of tracking system uses mechanical linkages between the reference and the target. Two types of linkages have been used. One is an assembly of mechanical parts that can each rotate, providing the user with multiple rotation capabilities. The orientation of the linkages is computed from the various linkage angles measured with incremental encoders or potentiometers. Other types of mechanical linkages are wires that are rolled in coils. A spring system ensures that the wires are tensed in order to measure the distance accurately. The degrees of freedom sensed by mechanical linkage trackers are dependent upon the constitution of the tracker's mechanical structure. While six degrees of freedom are most often provided, typically only a limited range of motions is possible because of the kinematics of the joints and the length of each link. Also, the weight and the deformation of the structure increase with the distance of the target from the reference and impose a limit on the working volume.
Phase difference systems measure the shift in phase of an incoming signal from an emitter on a moving target compared to the phase of an incoming signal from a reference emitter. With this the relative motion of the emitter with respect to the receiver can be calculated Like inertial sensing systems, phase-difference systems can suffer from accumulated errors and therefore are subject to drift, but because the phase can be measured continuously they are able to generate high data rates. Omega (navigation system) is an example.
Direct field sensing
Direct field sensing systems use a known field to derive orientation or position: A simple compass uses the Earth's magnetic field to know its orientation in two directions. An inclinometer uses the earth gravitational field to know its orientation in the remaining third direction. The field used for positioning does not need to originate from nature, however. A system of three electromagnets placed perpendicular to each other can define a spatial reference. On the receiver, three sensors measure the components of the field's flux received as a consequence of magnetic coupling. Based on these measures, the system determines the position and orientation of the receiver with respect to the emitters' reference.
Magnetic positioning is an IPS (Indoor positioning system) solution that takes advantage of the magnetic field anomalies typical of indoor settings by using them as distinctive place recognition signatures. The first citation of positioning based on magnetic anomaly can be traced back to military applications in 1970. The use of magnetic field anomalies for indoor positioning was instead first claimed in papers related to robotics in the early 2000.citation needed]. It offers accuracy without any hardware requirements and a relatively low total cost of ownership. According to Opus Research magnetic positioning will emerge as a “foundational” indoor location technology.
Because every technology has its pros and cons, most systems use more than one technology. A system based on relative position changes like the inertial system needs periodic calibration against a system with absolute position measurement. Systems combining two or more technologies are called hybrid positioning systems.
Hybrid positioning systems are systems for finding the location of a mobile device using several different positioning technologies. Usually GPS (Global Positioning System) is one major component of such systems, combined with cell tower signals, wireless internet signals, Bluetooth sensors, IP addresses and network environment data.
These systems are specifically designed to overcome the limitations of GPS, which is very exact in open areas, but works poorly indoors or between tall buildings (the urban canyon effect). By comparison, cell tower signals are not hindered by buildings or bad weather, but usually provide less precise positioning. Wi-Fi positioning systems may give very exact positioning, in urban areas with high Wi-Fi density - and depend on a comprehensive database of Wi-Fi access points.
Hybrid positioning systems are increasingly being explored for certain civilian and commercial location-based services and location-based media, which need to work well in urban areas in order to be commercially and practically viable.
Early works in this area include the Place Lab project, which started on 2003 and went inactive in 2006. Later methods let smartphones combine the accuracy of GPS with the low power consumption of cell-ID transition point finding.
- 3D scanning
- Assisted GPS
- Dynamic positioning
- Eye tracking
- Handheld tracker
- Indoor positioning system (IPS)
- Local positioning system
- Mobile phone tracking
- Motion capture
- Real-time locating system
- Resection (navigation)
- Virtual reality
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