Satellite navigation device
A satellite navigation device, also called a satnav device or GPS/ GNSS device, uses satellites of the Global Positioning System or similar global navigation satellite systems using the four global satellite positioning systems to determine the user's geographic coordinates. It may also display the user's position on a map and offer routing directions.
, four GNSS systems are operational: the original United States' GPS, the European Union's Galileo, Russia's GLONASS, and China's BeiDou Navigation Satellite System. The Indian Regional Navigation Satellite System will follow and Japan's Quasi-Zenith Satellite System scheduled for 2023 will augment the accuracy of a number of GNSS.
A satellite navigation device can retrieve location and time information from one or more GNSS systems in all weather conditions, anywhere on or near the Earth's surface. Satnav reception requires an unobstructed line of sight to four or more GNSS satellites, and is subject to poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals bounce off structures, or are weakened by meteorological conditions. Obstructed lines of sight may arise from a tree canopy or inside a structure, such as in a building, garage or tunnel. Today, most standalone satnav receivers are used in automobiles. The satnav capability of smartphones may use assisted GNSS technology, which can use the base station or cell towers to provide a faster Time to First Fix, especially when satellite signals are poor or unavailable. However, the mobile network part of the A-GNSS technology would not be available when the smartphone is outside the range of the mobile reception network, while the satnav aspect would otherwise continue to be available.
History
As with many other technological breakthroughs of the latter 20th century, the modern GNSS device can reasonably be argued to be a direct outcome of the Cold War of the latter 20th century. The multibillion-dollar expense of the US and Russian programs was initially justified by military interest. In contrast, the European Galileo was conceived as purely civilian.In 1960, the US Navy put into service its Transit satellite-based navigation system to aid in naval navigation. The US Navy in the mid-1960s conducted an experiment to track a submarine with missiles with six satellites and orbiting poles and was able to observe satellite changes. Between 1960 and 1982, as the benefits were shown, the US military consistently improved and refined its satellite navigation technology and satellite system. In 1973, the US military began to plan for a comprehensive worldwide navigational system which eventually became known as the GPS.
In 1983, in the wake of the tragedy of the downing of Korean Air Lines Flight 007, an aircraft which was shot down while in Soviet airspace due to a navigational error, President Ronald Reagan made the navigation capabilities of the existing military Global Positioning System available for dual civilian use. However, civilian use was initially only a slightly degraded "Selective Availability" positioning signal. This new availability of the US military Global Positioning System for civilian use required a certain technical collaboration with the private sector for some time, before it could become a commercial reality.
The Macrometer Interferometric Surveyor was the first commercial GNSS-based system for performing geodetic measurements.
In 1989, Magellan Navigation Inc. unveiled its Magellan NAV 1000, the world's first commercial handheld GPS receiver. These units initially sold for approximately US$2,900 each.
In 1990, Mazda's Eunos Cosmo was the first production car in the world with a built-in satellite navigation system. In 1991, Mitsubishi introduced satnav car navigation on the Mitsubishi Debonair. In 1997, a navigation system using Differential GPS was developed as a factory-installed option on the Toyota Prius.
In 2000, the Clinton administration removed the military use signal restrictions, thus providing full commercial access to the US GPS satellite system.
As GNSS navigation systems became more and more widespread and popular, the pricing of such systems began to fall, and their widespread availability steadily increased. Several additional manufacturers of these systems, such as Garmin, Benefon, Mio and TomTom entered the market. Mitac Mio 168 was the first PocketPC to contain a built-in GPS receiver. Benefon's 1999 entry into the market also presented users with the world's first phone based GPS navigation system. Later, as smartphone technology developed, a GPS chip eventually became standard equipment for most smartphones. To date, ever more popular satellite navigation systems and devices continue to proliferate with newly developed software and hardware applications. It has been incorporated, for example, into cameras.
While the American GPS was the first satellite navigation system to be deployed on a fully global scale, and to be made available for commercial use, this is not the only system of its type. Due to military and other concerns, similar global or regional systems have been, or will soon be deployed by Russia, the European Union, China, India, and Japan.
Technical design
GNSS devices vary in sensitivity, speed, vulnerability to multipath propagation, and other performance parameters. High-sensitivity receivers use large banks of correlators and digital signal processing to search for signals very quickly. This results in very fast times to first fix when the signals are at their normal levels, for example, outdoors. When signals are weak, for example, indoors, the extra processing power can be used to integrate weak signals to the point where they can be used to provide a position or timing solution.GNSS signals are already very weak when they arrive at the Earth's surface. The GPS satellites only transmit 27 W from a distance of 20,200 km in orbit above the Earth. By the time the signals arrive at the user's receiver, they are typically as weak as −160 dBW, equivalent to 100 attowatts. This is well below the thermal noise level in its bandwidth. Outdoors, GPS signals are typically around the −155 dBW level.
Sensitivity
Conventional GPS receivers integrate the received GPS signals for the same amount of time as the duration of a complete C/A code cycle which is 1 ms. This results in the ability to acquire and track signals down to around the −160 dBW level. High-sensitivity GPS receivers are able to integrate the incoming signals for up to 1,000 times longer than this and therefore acquire signals up to 1,000 times weaker, resulting in an integration gain of 30 dB. A good high-sensitivity GPS receiver can acquire signals down to −185 dBW, and tracking can be continued down to levels approaching −190 dBW.High-sensitivity GPS can provide positioning in many but not all indoor locations. Signals are either heavily attenuated by the building materials or reflected as in multipath. Given that high-sensitivity GPS receivers may be up to 30 dB more sensitive, this is sufficient to track through 3 layers of dry bricks, or up to 20 cm of steel-reinforced concrete, for example. Examples of high-sensitivity receiver chips include SiRFstarIII and MediaTekʼs MTK II.
In aviation, the GPS receivers can be "armed" to the approach mode for the destination airport, so that when the aircraft is within, the receiver sensitivity will automatically change from en route and RAIM to terminal, and change again to ±0.3 nm at before reaching the final approach way point.
Sequential receiver
A sequential GPS receiver tracks the necessary satellites by typically using one or two hardware channels. The set will track one satellite at a time, time tag the measurements and combine them when all four satellite pseudoranges have been measured. These receivers are among the least expensive available, but they cannot operate under high dynamics and have the slowest time-to-first-fix performance.Types
Consumer GNSS navigation devices include:- Dedicated GNSS navigation devices
- modules that need to be connected to a computer to be used
- loggers that record trip information for download. Such GPS tracking is useful for trailblazing, mapping by hikers and cyclists, and the production of geocoded photographs.
- Converged devices, including satellite navigation phones and geotagging cameras, in which GNSS is a feature rather than the main purpose of the device. The majority of GNSS devices are now converged devices, and may use assisted GPS or standalone or both. The vulnerability of consumer GNSS to radio frequency interference from planned wireless data services is controversial.
Dedicated GNSS navigation devices
Other receivers, often called mobile are intended primarily for use in a car, but have a small rechargeable internal battery that can power them away from the car. Special purpose devices for use in a car may be permanently installed and depend entirely on the automotive electrical system. Many of them have touch-sensitive screens as input method. Maps may be stored on a memory card. Some offer additional functionality such as a rudimentary music player, image viewer, and video player.
The pre-installed embedded software of early receivers did not display maps; 21st-century ones commonly show interactive street maps that may also show points of interest, route information and step-by-step routing directions, often in spoken form with a feature called "text to speech".
Manufacturers include: