Satellite navigation


Satellite navigation or satellite positioning is the use of artificial satellites for navigation or geopositioning. A global navigation satellite system provides coverage for any user on Earth, including air, land, and sea. There are four operational GNSS systems: the United States Global Positioning System, Russia's Global Navigation Satellite System, China's BeiDou Navigation Satellite System and the European Union's Galileo. Furthermore, there are two regional navigation satellite systems in the form of Japan's Quasi-Zenith Satellite System, and the Indian Regional Navigation Satellite System.
A satellite-based augmentation system is a system that is designed to enhance the accuracy of the global GNSS systems. The SBAS systems include Japan's Quasi-Zenith Satellite System, India's GAGAN, and the European EGNOS, all of them based on GPS.
Satellite navigation devices determine their location to high precision using time signals transmitted along a line of sight by radio from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver. The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as Positioning, Navigation and Timing. Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated.
Global coverage for each system is generally achieved by a satellite constellation of 18–30 medium Earth orbit satellites spread between several orbital planes. The actual systems vary, but all use orbital inclinations of >50° and orbital periods of roughly twelve hours.

Classification

GNSS systems that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows:
  • ' is the first generation system and is the combination of existing satellite navigation systems, with satellite-based augmentation systems or ground-based augmentation systems. In the United States, the satellite-based component is the Wide Area Augmentation System ; in Europe, it is the European Geostationary Navigation Overlay Service ; in Japan, it is the Multi-Functional Satellite Augmentation System ; and in India, it is the GPS-aided GEO augmented navigation. Ground-based augmentation is provided by systems like the local-area augmentation system.
  • ' is the second generation of systems that independently provide a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity monitoring necessary for civil navigation; including aircraft. Initially, this system consisted of only Upper L Band frequency sets. In recent years, GNSS systems have begun activating Lower L Band frequency sets for civilian use; they feature higher aggregate accuracy and fewer problems with signal reflection. As of late 2018, a few consumer-grade GNSS devices are being sold that use both. They are typically called "Dual-band GNSS" or "Dual-band GPS" devices.
By their roles in the navigation system, systems can be classified as:
As many of the global GNSS systems use similar frequencies and signals around L1, many "Multi-GNSS" receivers capable of using multiple systems have been produced. While some systems strive to interoperate with GPS as well as possible by providing the same clock, others do not.

History

Ground-based radio navigation is decades old. The DECCA, LORAN, GEE and Omega systems used terrestrial longwave radio transmitters which broadcast a radio pulse from a known "master" location, followed by a pulse repeated from a number of "slave" stations. The delay between the reception of the master signal and the slave signals allowed the receiver to deduce the distance to each of the slaves, providing a fix.
The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler effect: the satellites travelled on well-known paths and broadcast their signals on a well-known radio frequency. The received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Satellite orbital position errors are caused by radio-wave refraction, gravity field changes, and other phenomena. A team, led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970 to 1973, found solutions or corrections for many error sources. Using real-time data and recursive estimation, the systematic and residual errors were narrowed down to accuracy sufficient for navigation.

Principles

Part of an orbiting satellite's broadcast includes its precise orbital data. Originally, the US Naval Observatory continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO sent the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain its most recent ephemeris.
Modern systems are more direct. The satellite broadcasts a signal that contains orbital data and the precise time the signal was transmitted. Orbital data include a rough almanac for all satellites to aid in finding them, and a precise ephemeris for this satellite. The orbital ephemeris is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission of three or four different satellites, measuring the time-of-flight to each satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time using an adapted version of trilateration: see GNSS positioning calculation for details.
Each distance measurement, regardless of the system being used, places the receiver on a spherical shell centred on the broadcaster, at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where the shells meet, a fix is generated. However, in the case of fast-moving receivers, the position of the receiver moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that angle corresponds to the distance which the signal travels through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centred on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlators, and then using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity.
Einstein's theory of general relativity is applied to GPS time correction, the net result is that time on a GPS satellite clock advances faster than a clock on the ground by about 38 microseconds per day.

Applications

The original motivation for satellite navigation was for military applications. Satellite navigation allows precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons.. Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war.
Now a global navigation satellite system, such as Galileo, is used to determine users location and the location of other people or objects at any given moment. The range of application of satellite navigation in the future is enormous, including both the public and private sectors across numerous market segments such as science, transport, agriculture, etc.
The ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires.

Global navigation satellite systems

In order of first launch year:

GPS (1978)

The United States' Global Positioning System consists of up to 32 medium Earth orbit satellites in six different orbital planes. The exact number of satellites varies as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is the world's most used satellite navigation system.

GLONASS (1982)

The formerly Soviet, and now Russian, Global'naya Navigatsionnaya Sputnikovaya Sistema,, is a space-based satellite navigation system that provides a civilian radionavigation-satellite service and is also used by the Russian Aerospace Defence Forces. GLONASS has full global coverage since 1995 and with 24 active satellites.