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Satellite Navigation SystemCompiled by Rohan Wickremasinghe Dip Mgmt., MBA (SGU), Dip B. (BPUSL), AIB (SL)

Satellite navigation systems use radio time signals transmitted by satellites toenable mobile receivers on the ground to determine their exact location. Therelatively clear line of sight between the satellites and receivers on the ground,combined with ever-improving electronics, allows satellite navigation systems tomeasure location to accuracies on the order of a few metres in real time.

History & Theory

An early predecessor were the ground based Loran and Omega systems, whichused terrestrial longwave radio transmitters instead of satellites. These systemsbroadcast a radio pulse from a known "master" location, followed by repeatedpulses from a number of "slave" stations. The delay between the reception and

sending of the signal at the slaves was carefully controlled, allowing the receiversto compare the delay between reception and the delay between sending. Fromthis 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 USmilitary in the 1960s. Transit's operation was based on the Doppler effect: thesatellites traveled on well-known paths and broadcast on a well knownfrequency. The received frequency will differ slightly from the broadcastfrequency because of the movement of the satellite with respect to the receiver.By monitoring this frequency shift over a short time interval, the receiver candetermine its location to one side or the other of the satellite, and several suchmeasurements combined with a precise knowledge of the satellite's orbit can fixa particular position.

Modern systems are more direct. The satellite broadcasts a signal that containsthe position of the satellite and the precise time the signal was transmitted. Theposition of the satellite is transmitted in a data message that is superimposed ona code that serves as a timing reference. The satellite uses an atomic clock tomaintain synchronization of all the satellites in the constellation. The receivercompares the time of broadcast encoded in the transmission with the time ofreception measured by an internal clock, thereby measuring the time-of-flight tothe satellite. Several such measurements can be made at the same time todifferent satellites, allowing a continual fix to be generated in real time.

Each distance measurement, regardless of the system being used, places thereceiver on a spherical shell at the measured distance from the broadcaster. Bytaking 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 ofthe signal moves as signals are received from several satellites. In addition, theradio signals slow slightly as they pass through the ionosphere, and this slowingvaries with the receiver's angle to the satellite, because that changes the

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distance through the ionosphere. The basic computation thus attempts to find theshortest directed line tangent to four oblate spherical shells centered on foursatellites. Satellite navigation receivers reduce errors by using combinations ofsignals from multiple satellites and multiple correlators, and then usingtechniques 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 ofweapons to targets, greatly increasing their lethality whilst reducing inadvertentcasualties from mis-directed weapons. (See smart bomb). Satellite navigationalso allows forces to be directed and to locate themselves more easily, reducingthe fog of war.

Satellite navigation using a laptop and a GPS receiver

In these ways, satellite navigation can be regarded as a force multiplier. Inparticular, the ability to reduce unintended casualties has particular advantagesfor wars being fought by democracies, where public relations is an importantaspect of warfare. For these reasons, a satellite navigation system is an essentialasset for any aspiring military power.

Satellite navigation systems have a wide variety of civilian uses:

navigation, ranging from personal hand-held devices for trekking, todevices fitted to cars, trucks, ships and aircraft

synchronization

location-based services such as enhanced 911

surveying

Entering data into a geographic information system

search and rescue

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Geophysical Sciences

Tracking devices used in wildlife management

Note that the ability to supply satellite navigation signals is also the ability to denytheir availability. The operator of a satellite navigation system potentially has the

ability to degrade or eliminate satellite navigation services over any territory itdesires. Thus, as satellite navigation becomes an essential service, countrieswithout their own satellite navigation systems effectively become client states ofthose which supply these services.

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

Current and proposed satellite navigation systems

The best known satellite navigation system is the United States' GlobalPositioning System (GPS), and as of 2002 the GPS is the only fully functionalsatellite navigation system. This consists of 24 to 27 satellites that orbit in sixdifferent planes. The exact number of satellites varies as satellites arereplenished when older ones are retired. They orbit at an altitude ofapproximately 20,000 km with an inclination of 55 degrees. The satellites aretracked by a world-wide network of monitor stations. The tracking data is sent toa master control station that continuously updates position and clock estimatesfor each satellite. The updated data is then uplinked to the satellite via one ofseveral ground antennas.

When it was first deployed, GPS included a "feature" called SelectiveAvailability (orSA) that introduced intentional errors of up to a hundred metersinto the publicly available navigation signals. Additional accuracy was available inthe signal, but in an encrypted form that was only legally readable by militaryunits. Using the encoded signal accuracies of about 10m horizontally and 30mvertically are possible. The inaccuracy of the civilian signal was deliberatelyencoded so as not to change very quickly, for instance the entire eastern USarea might read 30m off, but 30m off everywhere and in the same direction.

In order to improve the usefulness of GPS for civilian navigation, fixed GPSreceivers started broadcasting a signal "fixing" the inaccuracy. Known asdifferential GPS (or dGPS), the signals could be received on an FM receiverand plugged into many civilian GPS receivers, at which point they too gained10m accuracy. However this signal was available only at short ranges, making ituseless for enemies guiding long range missiles into the United States.

In the 1990s the FAA started pressuring the military to turn off SA permanently.This would save the FAA millions of dollars every year in maintenance and

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