Global Positioning System
The Global Positioning System (GPS), is a satellite-based navigation system owned by the United States government
It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth. The receiver must have an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the relatively weak GPS signals.
The GPS receiver does not transmit any data, and it operates independently of any cellphone or internet reception, (although these technologies can help the find your position).
The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver. The GPS project was started in 1973, with the first prototype spacecraft launched in 1978 and the full constellation of 24 satellites operational in 1993. Originally the use was limited to the United States military, civilian use was allowed from the 1980s after the Korean Air Lines Flight 007 incident.
During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; this was discontinued on May 1, 2000. The GPS service can selectively deny access to the system, as happened to the Indian military in 1999 during the Kargil War, or degrade the service at any time. As a result, several countries have developed or are in the process of setting up other global or regional satellite navigation systems.
- GLONASS - The Russian Global Navigation Satellite System - Some GPS devices are able to receive the GLONASS signal, making more satellites available and enabling positions to be fixed more quickly and accurately.
- BeiDou Navigation Satellite System - China's began global services in 2018, and was completely deployed in 2020.
- The European Union Galileo positioning system
- NavIC - India's Regional Navigation Satellite System.
- Japan's Quasi-Zenith Satellite System (QZSS) is a system to enhance GPS's accuracy in Asia-Pacific with satellite navigation independent of GPS scheduled for 2023.
Originally in 2000, GPS had about a 5m accuracy. Newer GPS receivers with the L5 band can have to within 30 cm - in May 2021 - 16 GPS satellites are broadcasting L5 signals, and the signals are considered pre-operational, scheduled to reach 24 satellites by approximately 2027.
Basic concept
Fundamentals
The GPS receiver calculates its own position and time based on data received from multiple GPS satellites. Each satellite carries an accurate record of its position and time, and transmits that data to the receiver.
The satellites carry very stable atomic clocks that are synchronized with one another and with ground clocks. Any drift from time maintained on the ground is corrected daily. In the same manner, the satellite locations are known with great precision. GPS receivers have clocks as well, but they are less stable and less precise.
Since the speed of radio waves is constant and independent of the satellite speed, the time delay between when the satellite transmits a signal and the receiver receives it is proportional to the distance from the satellite to the receiver. At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).
Receiver in continuous operation
Most GPS receivers have a tracking algorithm, also called a tracker, that combines sets of satellite measurements collected at different times. After a set of measurements are processed, the tracker predicts the receiver location corresponding to the next set of satellite measurements. When the new measurements are collected, the receiver uses a weighting scheme to combine the new measurements with the tracker prediction. In general, a tracker can
- improve receiver position and time accuracy,
- reject bad measurements, and
- estimate receiver speed and direction.
The disadvantage of a tracker is that changes in speed or direction are computed with a delay, and the direction becomes inaccurate when the distance travelled s very small. GPS units can use measurements of the Doppler effect of the signals received to compute velocity accurately and more advanced navigation systems use additional sensors like a compass to complement the GPS.
Although four satellites are normally needed to calculate a position, fewer are needed in special cases. If one variable is already known, a receiver can determine its position using three satellites. For example, a ship or aircraft may have known elevation. Some GPS receivers may use additional clues such as reusing the last known altitude, or including information from the vehicle computer, to give an approximate position when fewer than four satellites are visible.
Structure
The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.
The U.S. Space Force develops, maintains, and operates the space and control segments. GPS satellites broadcast signals from space, and each GPS receiver is the user segment - this uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.
Space segment
The space segment is made up of 24 to 32 satellites in medium Earth orbit. with six orbital planes with four satellites each. The six orbit planes have approximately 55° inclination (tilt relative to the Earth's equator) and are separated by 60° angle along the equator from a reference point to the orbit's intersection. The orbital period is 11 hours and 58 minutes so that the satellites pass over the same locations or almost the same location every day. The orbits are arranged so that at least six satellites are always in line of sight from everywhere on the Earth's surface (see animation at right).
Orbiting with an orbital radius of approximately 26 600 km, each satellite makes two complete orbits each day, repeating the same track each day.
As of February 2019 there are 31 satellites in the GPS satellite constellation, 27 are in use at a given time with the rest as stand-bys. A 32nd was launched in 2018, but it is still in evaluation. Decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations. With more satellites, the system is more accurate and this also improves reliability and system availability. With the expanded constellation, nine satellites are usually visible from any point on the ground at any time, ensuring considerable redundancy over the minimum four satellites needed for a position.
Control segment
The control segment (CS) is composed of:
- a master control station (MCS),
- an alternative master control station,
- four dedicated ground antennas, and
- six dedicated monitor stations.
The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Cape Canaveral, England, Argentina, Ecuador, Bahrain, Australia and Washington. The tracking information is sent to the MCS. Then 2 SOPS contacts each GPS satellite regularly with a navigational update. These updates synchronize the atomic clocks on board the satellites to within a nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model.
Satellite manoeuvrers are not precise —so to change a satellite's orbit, the satellite must be marked unhealthy, so receivers don't use it. After the satellite manoeuvrer, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again.
The operation control segment (OCS) currently is the control segment of record. It supports GPS users and keeps the GPS operational.
OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System (OCX), is functional. The GPS OCX program also will reduce cost, schedule and technical risk and provide four times the capability.
The GPS OCX program is part of GPS modernization and improve the current GPS OCS program.
- OCX will control and manage GPS legacy satellites and the next generation of GPS III satellites,.
- Built on a flexible architecture that can rapidly adapt to the changing needs of today's and future GPS users allowing immediate access to GPS data and constellation status through secure, accurate and reliable information.
- Provides a more secure, actionable and predictive information.
- Enables new signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
- Provides information improvements over the current program including detecting and preventing cyber attacks.
- Supports higher volume near real-time command and control capabilities and abilities.
User segment
The user segment (US) is made of tens of millions of civil, commercial and scientific users of the Standard Positioning Service.
In general, GPS receivers have:
- An antenna, tuned to the frequencies transmitted by the satellites,
A typical GPS receiver with integrated antenna. - Receiver-processors,
A typical GPS receiver module measuring 15x17 mm,
- a stable clock (often a[crystal oscillator).
- A display to provide location and speed information to the user.
- An interface with other devices using USB, or Bluetooth.
A receiver has a number of channels: (how many satellites it can monitor simultaneously). Originally limited to four or five, this has increased and now can typically have between 12 and 20 channels.
Applications
While originally a military project, GPS has civilian applications as well.
GPS has become a useful tool for commerce, scientific uses, tracking, and surveillance. GPS's accurate time facilitates everyday activities such as banking, mobile phone operations, and even the control of power grids.
Many civilian applications use one or more of GPS's three basic components: absolute location, relative movement, and time transfer.
- Atmosphere]: studying the troposphere delays (recovery of the water vapour content) and ionosphere delays (recovery of the number of free electrons).
- Automated vehicle: applying location and routes for cars and trucks to function without a human driver.
- Cartography: both civilian and military cartographers use GPS extensively.
- Clock synchronization: the accuracy of GPS time signals (±10 ns)
- Disaster relief/emergency services: many emergency services depend upon GPS for location and timing capabilities.
- GPS-equipped radiosondes and dropsondes: measure and calculate the atmospheric pressure, wind speed and direction up to 27km from the Earth's surface.
- Fleet tracking: used to identify, locate and maintain contact reports with fleet vehicles.
- Geofencing: vehicle tracking systems, person tracking systems, and pet tracking systems use GPS to locate devices that are attached to or carried by a person, vehicle, or pet. The application can track and send notifications if the target leaves a designated area.
- Geotagging: applies location coordinates to digital objects such as photographs and documents for creating map overlays showing where photos were taken.
- GPS aircraft tracking
- GPS data mining: It is possible to aggregate GPS data from multiple users to understand movement patterns, common trajectories and interesting locations.
- GPS tours: location determines what content to display; for instance, information about an approaching point of interest.
- Orbit of low-orbiting satellites with GPS receiver, such as GOCE,
- Recreational use: for example, Geocaching and Pokémon Go.
- Robotics: self-navigating, autonomous robots using GPS sensors, which calculate latitude, longitude, time, speed, and heading.
- Sport: used in football and rugby for the control and analysis of the training load.
- Surveying: surveyors use absolute locations to make maps and determine property boundaries.
- Tectonics: GPS enables direct fault motion measurement of earthquakes. Between earthquakes GPS can be used to measure movement of the earth cust and deformation to estimate seismic strain build-up for creating seismic hazard maps.
- Telematics: GPS technology integrated with computers and mobile communications technology in car navigation systems.
GPS technology for handsets has matured considerably, offering much better sensitivity, power consumption, size and price.
The major handset software platforms and operating systems are evolving, ensuring easier integration of GPS functionality for handset manufacturers and more powerful features for application developers. Along with the improving performance of handsets, in terms of screen size, processing power and memory size, current handsets thus provide much better platforms for location-enabled applications and services than before.
The GPS value-chain was reshaped considerably in 2007 as several specialist GPS technology developers were acquired by wireless chipset vendors. These transactions are likely to enhance the possibilities to meet handset manufacturers' demand for integrated connectivity solutions that include GPS at ever lower price points to enable true mass market deployment.