Home > Satellite navigation system

1 History & Theory

An early predecessor were the ground based Loran and Omega systems, which used terrestrial longwave radio transmitters instead of satellites. These systems broadcast a radio pulse from a known "master" location, followed by repeated pulses from a number of "slave" stations. The delay between the reception and sending of the signal at the slaves was carefully controlled, allowing the receivers to compare the delay between reception and the delay between sending. From this the distance to each of the slaves could be determined, 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 traveled on a well-known paths and broadcast on a well known 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.

Modern systems are more direct. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite 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 with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time.

Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal 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 changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered 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.

2 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. Satellite navigation using a laptop and a GPS receiver In these ways, satellite navigation can be regarded as a force multiplierA force multiplier is a military term referring to a factor that dramatically increases (hence, "multiples") the combat-effectiveness of a given military force. Sometimes this factor may apply more or less under different cirmcumstances. Some common force. 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:

• navigationThis article concerns navigation in the sense of determination of position and direction on the surface of the Earth. See Navigation (disambiguation) for other meanings. There are several traditions of navigation . Polynesian navigation The Polynesian nav, ranging from personal hand-held devices for trekking, to devices fitted to cars, trucks, ships and aircraft
• synchronizationSynchronization is coordination with respect to time. It is an important concept in the following fields: Computer science Physics Telecommunication Cryptography Multimedia Photography Music ( rhythm) Synchronization has several subtly distinct sub-concep
• location-based serviceA location-based service (or LBS in a cellular telephone network is a service provided to the subscriber based on her current geographic location. This position can be known by user entry or a GPS receiver that she carries with her, but most often the ters such as enhanced 911Enhanced 911 service or E911 service is a North American telephone network feature that automatically associates the physical address with the calling party's telephone number. This is generally done by a form of reverse telephone directory that is suppli
• surveyingSurveying is concerned with the accurate measurement and position of points on the Earth's surface, and to the establishment of boundaries. It basically achieves this by measuring the relative horizontal and vertical position of points on the ground, aide
• Entering data into a geographic information systemA geographic information system (GIS) is a specialized form of an information system. In the strictest sense, it is a computer system capable of assembling, storing, manipulating, and displaying geographically-referenced information in a relational databa
• search and rescue
• Geophysical Sciences
• Tracking devices used in wildlife management

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.