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How does GPS work?


Active antenna

An antenna that amplifies the GPS signal before sending it to the receiver.

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A wireless connection used for everyday equipment like computers and phones. It only connects over a short distance, basically replacing the electrical wire and socket. This makes it especially useful for equipment used outdoors, such as mobile phones and portable satellite navigators.

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A GPS (Global Positioning System)with a Bluetooth connection.

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The skill and technique of making maps or charts. Many of the early maps were also great works of art, just as the maps you find on your satellite navigator are the result of highly complex technology.

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Traditionally, one of the 88 groups of stars we see in the sky at night that our ancestors named after mythological characters (e.g. Aries), animals (e.g. The Great Bear) or everyday objects (e.g. The Plough). Since ancient times, stars have been a key navigational tool.

In the context of GPS, ‘constellation’ refers to either a specific set of satellites used when calculating your position, or all the satellites visible to a GPS receiver at any one time.

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Control segment

A name for the chain of 5 monitoring stations around the world that control and manage the GPS satellite constellation.

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A set of numbers, or references, that identify your position anywhere on or above the Earth. Your coordinates are usually stated in terms of latitude and longitude. So by knowing that the coordinates of New York City are 40° 47’ latitude and 73° 58’ longitude, you can pinpoint New York’s exact location on a map or globe.

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A nautical term meaning the direction you’re following between two points, or waypoints, on your journey. A course is usually measured in degrees, though sometimes in radians or mils.

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Dead reckoning

A simple navigational technique using time and distance. You work out how far you’ve travelled by multiplying the speed you’ve been going by the length of time you’ve been travelling. (Distance = speed x time). If you also know your course, you can work out your position.

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An imaginary line around the centre of the Earth at zero degrees latitude.

The equator is 40,075.16 kilometres (24,901.55 miles) long and passes through 13 countries. Strangely enough, Equatorial Guinea is not one of them! To this day, the ancient tradition continues whereby sailors who cross the equator for the first time undergo an initiation ceremony.

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The ‘speed’ of radio waves. Or, to be more precise, the number of waves that pass a specific point within a set period of time. Frequency is measured in Hertz (cycles per second). Radio signals can be transmitted anywhere between about 3 kilohertz and about 300,000 megahertz.

Different frequencies are used for different applications. So submarine signals are sent out at a very low frequency (3-30 Hz), AM radio stations signals at 300-3000 kHz. This is in the middle of the range, hence the term ‘medium wave’. FM radio stations signals travel at a much higher frequency (30-300 MHz), which is why the reception quality tends to be better than with AM radio. GPS signals are sent over various signals, but the C/A Code used by commercial navigation systems is at 1575.42 MHz.

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Europe's own satellite navigation system, sometimes also known as GNSS (Global Navigation Satellite System). GALILEO is due to be completed and operational in 2008/9.

Read more about GALILEO.

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Global Orbiting Navigational Satellite System. The Russian counterpart to GPS. GLONASS also provides worldwide coverage, but is more effective in northern latitudes.

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General Packet Radio Service. A system for transmitting high-speed wireless information. It is used for the internet, mobile phones, computers and other data. GPRS is the world’s main system for transmitting mobile phone data.

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Global Positioning System. A system of 24 space satellites used to give the precise location of any point on Earth.

For everything you’d want to know about GPS, go to GPS.

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A layer of the atmosphere about 50 to 1000 kilometres above the earth's surface. The Ionosphere contains electrically charged particles (ions) that interfere with radio signals as they pass through the Ionosphere. This is called Ionospheric Delay. A GPS receiver calculates this delay in so that the information on your position is still accurate.

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The distance north or south of the equator of a point on the earth's surface. Latitude is measured in degrees, minutes and seconds.

Lines of latitude run horizontally across a globe or map. The equator is at zero degrees, +90 degrees is the North Pole and -90 degrees the South Pole. 1 degree of latitude equals 60 nautical miles and 1 minute of latitude equals 1 nautical mile. Because lines of latitude circle the earth parallel to one another they’re also referred to as ‘parallels’.

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The distance east or west of the prime meridian of a point on the earth’s surface. Longitude is measured in degrees and the prime meridian is at zero degrees longitude - an imaginary circle that passes through the north pole, south pole and Greenwich, England. The maximum longitude value is 180 degrees and longitudes are usually given as positive for easterly longitudes and negative for westerly longitudes (so +67 degrees = 67 degrees East and -43 degrees = 43 West).

But why Greenwich? It would make a lot more sense to have the prime meridian run through as little land as possible, and the line through Greenwich in South London certainly doesn’t do that. The Greenwich Meridian was in fact not agreed upon until as recently as 1884, at a conference of 25 countries in Washington DC. Sensing they weren’t going to win, the French delegation abstained from the vote and for years afterwards French maps continued to use the Paris Meridian.

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2-D Mode

Two-Dimensional Mode. A GPS position fix using only horizontal coordinates and not elevation. To get a position fix using 2D Mode, you need at least 3 visible satellites.

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3-D mode

Three-Dimensional Mode. A GPS position fix using horizontal coordinates and elevation. To get a position fix using 3D Mode, you need at least 4 visible satellites.

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Monitoring stations

5 stations around the world that receive and monitor satellite signals for the GPS system.

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Interference to GPS reception caused by a signal reaching a GPS antenna by 2 or more different paths. Often because the signal has bounced off a nearby building.

Inaccuracies caused by Multipath is called Multipath Error.

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NAVigation Satellite Timing and Ranging. The name given to GPS satellites.

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The first global navigation system. A ground-based system, OMEGA was introduced in 1971 with 8 stations around the world. But with the introduction of GPS it became obsolete and ceased operations in 1997.

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Location of a point defined by its latitude, longitude and altitude.

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An object that finds itself in an orbit around a larger object, such as a planet, or is actually revolving around it. There are natural satellites like the moon, and artificial satellites like the ones that are used for GPS. An artificial satellite consists of three basic elements: a communication system to send information back to earth, a powerstation and a control system to execute specific tasks.

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Selective Availability

Selective Availability, or ‘SA’, was the policy of the US Defence Department to intentionally reduce the accuracy of GPS satellite signals for civilian users. The policy was ended on May 1, 2000 and SA is now turned off.

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Space segment

Those parts of the GPS system that are in space. In other words, the satellites orbiting earth and their signals.

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A method for calculating your position from other known reference points. Read more about how Trilateration works in practice.

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User segment

A general term in GPS to cover GPS user equipment, applications and operational procedures.

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A checkpoint along a course, measured in coordinates of longitude and latitude.

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