Satellite navigation system

How they work

Modern satellite navigation systems require the reception of microwave signals from several satellites, all of which broadcast the satellite's location, and the precise time the signal was transmitted. The satellites carry atomic clocks. The receiver characterizes the time-of-flight of the signals from several satellites and computes its location.

An early predecessor was the ground based Loran system, which used terrestrial longwave radio transmitters instead of satellites. 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 satellite travels on a well-known path and broadcasts on a well known frequency. The received frequency will differ slightly 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.

The basic computation attempts to find the shortest directed line tangent to four spherical shells centered on four satellites. The radius of each shell is determined by the time-of-flight of the radio signal. A line has to be used because the measurements occur over a period of time, and navigation receivers usually move, sometimes very quickly. 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 changes the distance through the ionosphere. Almost all receivers correct for this effect. Satellite navigation receivers reduce the error 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.

Civil and military uses

The original motivation for satellite navigation was for military applications. Satellite navigation allows for hitherto impossible precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from mis-directed weapons. (See smart bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war[?].

In these ways, satellite navigation can be regarded as a force multiplier[?]. In particular, the ability to reduce unintended casualties has particular advantages for wars being fought by democracies, where public relations is an important aspect of warfare. For these reasons, a satellite navigation system is an essential asset for any aspiring military power.

Satellite navigation systems have a wide variety of civilian uses:

• navigation, ranging from personal hand-held devices for trekking, to devices fitted to cars, trucks, ships and aircraft
• synchronization
• location-based services[?] such as enhanced 911

Note that 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. Thus, as satellite navigation becomes an essential service, countries without their own satellite navigation systems effectively become client states of those which supply these services.

The same applies to the use of smart bombs: the operator of a satellite navigation system can effectively degrade the performance of smart bombs being used by other states using its satellite navigation system to that of gravity bombs, or even offset them from their targets in such a way as to render them useless.

Current and proposed satellite navigation systems

The best known satellite navigation system is the United States' Global Positioning System (GPS). As of 2002 the GPS is the only fully functional satellite navigation system. GPS had a "feature" called "selective availability" that introduced intentional errors of up to a hundred meters into the publicly available navigation signals. The satellites also generate high-precision data, believed to be accurate to about 4cm, which is encrypted for military uses. Some Japanese started to calculate and broadcast corrections by using an antenna on a geographic benchmark. This "differential GPS" erased the military advantage. "Selective availablility" ceased in 1999, but the frequency and other factors cause the civilian signals to be inherently less accurate than the military, to only about ten meters.

The Russian counterpart to GPS is called GLONASS and was used as a backup by some commercial GPS receivers. However the GLONASS constellation is currently (as of 2001) in very poor repair, rendering it almost useless as a navigation aid.

The European Union and European Space Agency have agreed (March 2002) to introduce their own alternative to GPS, called Galileo, pending a review in 2003. At a cost of about $ 2.5 billion (2.5×10⁹ dollars) the required satellites will be launched between 2006 and 2008 and the system will be working, under civilian control, from 2008.

As a precursor to Galileo, the European Space Agency, the European Commission and EUROCONTROL are developing the European geostationary navigation overlay system (EGNOS). This is intended to supplement the GPS and GLONASS systems by reporting on the reliability and accuracy of the signals, allowing position to be determined to within 5 metres. It will consist of three geostationary satellites and a network of ground stations and is intended to be operational in 2004.

China has started to launch a series of satellites intended to form a system called the Beidou navigation system[?].

Topics to be covered

• differential satellite navigation[?]
• WAAS[?]
• phase-counting differential satellite navigation[?]

External links

• ESA information on EGNOS (http://www.esa.int/export/esaSA/GGG63950NDC_navigation_0)
• Information on the Beidou system (http://www.astronautix.com/craft/beidou.htm)