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Global Positioning System constellationThe global positioning system (GPS) baseline constellation consists of 24 slots insix orbital

planes, with four slots per plane. Three of the slots are expandable and can hold no

morethan two satellites. Satellites that are not occupying a defined slot in the GPS

constellation

occupy other locations in the six orbital planes. Constellation reference orbitparameters

and slot assignments as of the defined epoch are described in the fourth edition ofthe

GPS Standard Positioning Service Performance Specification, dated September

2008. As

of that date, the GPS constellation had 30 operational satellites broadcastinghealthy

navigation signals: 11 in Block IIA, 12 in Block IIR and 7 in Block IIR-M.

HOW DOES GPS WORK?

The Global Positioning System (GPS) is a network of about 30 satellites orbiting

the Earth at an altitude of 20,000 km. The system was originally developed by the

US government for military navigation but now anyone with a GPS device, be it a

SatNav, mobile phone or handheld GPS unit, can receive the radio signals that thesatellites broadcast.

Wherever you are on the planet, at least four GPS satellites are visible at anytime. Each one transmits information about its position and the current time at

regular intervals. These signals, travelling at the speed of light, are intercepted by

your GPS receiver, which calculates how far away each satellite is based on howlong it took for the messages to arrive.

Once it has information on how far away at least three satellites are, your GPS

receiver can pinpoint your location using a process called trilateration.

Imagine you are standing somewhere on Earth with three satellites in the sky above

you. If you know how far away you are from satellite A, then you know you mustbe located somewhere on the red circle. If you do the same for satellites B and C,

you can work out your location by seeing where the three circles intersect. This is

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just what your GPS receiver does, although it uses overlapping spheres rather thancircles.

The more satellites there are above the horizon the more accurately your GPS unitcan determine where you are.

PERFORMANCE OF GPS:

A GPS receiver calculates its position by precisely timing the signals sent by GPS

satelliteshigh above the Earth. Each satellite continually transmits messages thatinclude

the time the message was transmitted

satellite position at time of message transmission

The receiver uses the messages it receives to determine the transit time of eachmessage and computes the distance to each satellite using the speed of light. Each

of these distances and satellites' locations define a sphere. The receiver is on thesurface of each of these spheres when the distances and the satellites' locations are

correct. These distances and satellites' locations are used to compute the location ofthe receiver using thenavigation equations. This location is then displayed,

perhaps with a moving map display orlatitudeandlongitude; elevation

information may be included. Many GPS units show derived information such asdirection and speed, calculated from position changes.

In typical GPS operation, four or more satellites must be visible to obtain anaccurate result. Four sphere surfaces typically do not intersect. Because of this, it

can be said with confidence that when the navigation equations are solved to findan intersection, this solution gives the position of the receiver along with the

difference between the time kept by the receiver's on-board clock and the truetime-of-day, thereby eliminating the need for a very large, expensive, and power

hungry clock. The very accurately computed time is used only for display or not atall in many GPS applications, which use only the location.

System architecture:

The GPS system is based on three segments:

the space segment, consisting of a constellation of satellites that emit the

navigation signals,

http://en.wikipedia.org/wiki/Satelliteshttp://en.wikipedia.org/wiki/Satelliteshttp://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equationshttp://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equationshttp://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equationshttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Global_Positioning_System#Navigation_equationshttp://en.wikipedia.org/wiki/Satellites

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the ground segment, which monitors and controls the space segment. In

particular it provides the satellites with their orbital parameters for

redistribution to the users,

the user segment, consisting of all the GPS receivers which calculate their

position, velocity and time (PVT) using the signals received.Space segmentThe space segment of the GPS system is specified as nominally consisting of

24 satellites distributed evenly across 6 circular orbital planes at an altitude of

20,184 km, spaced at 60 intervals and with an inclination of 55 to the

equatorial plane. Additional positions have been allocated for when the

number of satellites in the constellation exceeds 24.

Ground segmentThe GPS satellites are permanently controlled by a network of five control

stations, with the Master Control Station being located in Colorado Springs.The ground segment has several roles:

To recalibrate the satellites atomic clocks.

To generate the data that enable the user to calculate a position (satellite

ephemeris data, clock corrections).

To load the previous data onto the satellites for distribution to users.

To control and command the satellites.

User segmentThis segment consists of the GPS receivers. It is important to bear in mind thata GPS receiver only monitors signals sent by the satellites and does not

establish any contact with them. Therefore, a GPS receiver cannot be used by a

third party to find out a users position without his knowledge.

GBAS (Ground Based Augmentation System):

GBAS is a local augmentation system to GNSS used and standardised by ICAO

(International Civil Aviation Organization) for precision approach and landing

operations, with a high level of integrity. Its principle is similar to that of

DGPS. GBAS is made up of a ground subsystem comprising two to four GNSS

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reference receivers and an airborne subsystem. Using data from reference

receivers, the ground-based subsystem calculates corrections to the

pseudoranges for all visible satellites. The ground subsystem also monitors

the quality of the information transmitted to the airborne subsystem by

performing a large number of tests on the differential corrections andpseudoranges. These corrections are transmitted to the aircraft using the VDB

(VHF Data Broadcast) system. A GBAS system provides its services to all

aircraft present in its coverage area of up to 23 nautical miles. GBAS is

designed to respond to the problems posed by the most demanding of

operations (all-weather precision approach). The civil aviation community is

currently working towards standardising GBAS for category II and III

precision approach, which is likely to be operational as of 2015-2020.

RTK (Real-Time Kinematic):

This technique is based on a principle similar to that of DGPS with a single

reference station and a means of communication between the receiver and the

station, but in this case it is not corrections that are transmitted but raw data.

These raw data then enable specialised receivers to calculate the satellite-to-

receiver transit time based on the phase of the wave received and not on the

code sequence. This method, which requires more complex receivers, makes it

possible to achieve accuracy of roughly 3 to 5 cm, conditional upon being

within a distance of up to 100km from the reference station. It also takes

considerable time to initialise and requires dual-frequency receivers. A

variant of this method known as interpolated RTK makes it possible toachieve even greater accuracy by using a denser network of reference stations

(in France, for example the Teria, Orpheon and Sat-Info networks). In this

case, the errors in the receiver measurements are interpolated with

measurements carried out by the stations situated around the user.

PPP (Precise Point Positioning):

The Precise Point Positioning method (PPP) is a different approach which

makes use of undifferentiated code and phase observations from a single or

dual-frequency receiver. This method is principally used in deferred timesince it requires correction data to be received. PPP uses these precise orbital

data and clock corrections to calculate an extremely accurate absolute

position (static or kinematic) to the decimetre or even centimetre in

kinematic mode using precise IGS products, available with 3 weeks delay.

Unlike with RTK, common errors (the effect of tides or ocean loading, for

example) are not eliminated. Obtaining a position that is both absolute (that

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