18th May 2007

GPS Glossary & F.A.Q

GPS stands for Global Positioning System. It’s an international navigation system that allows you to ascertain your location, navigate a course, or track the progress of others, from anywhere in the world.

Frequently Asked Questions

Q Does GPS work underground or inside?

A No. Like a mobile phone, it relies on a clear path of sight to its target. This means that it won’t work in some buildings, nor will it work underground. Certain units are more quick than others in ‘urban canyons’ (i.e. surrounded by tall buildings, yet outside), but all should function in these locations. GPS will, of course, work in a car or on a ship.

Q Does it cost money to use GPS?

A Apart from buying a receiver, GPS is free. You may be required to pay for maps or add-ons for your particular unit, but the GPS protocol itself requires no user fees, nor license.

Q How accurate is GPS, really?

A In the crudest terms, GPS will put you within at least 100 metres of where you really are. Usually, however, it’s much better than this: 15 metres’ discrepancy is the current benchmark for a standard GPS unit. Also, depending on the situation of the satellites you’re using, your GPS unit, and its software (for example, some road programs will automatically ‘lock’ you to the nearest road, rather than putting you 15 metres away inside a building!), you can actually be much closer than this. GPS is the most accurate electronic way of measuring your location, and while 100 metres’ maximum may sound like a large discrepancy, in actuality it is much better than this. Differential GPS helps even more, putting you usually within 5 metres of your location.



This is the most important of all information first sent from a satellite to a GPS unit when it’s switched on. This data file is sent from one satellite to the unit, and tells the unit information on all satellites, atomic satellite clocks and any atmospheric delay information. This allows the unit to get started with navigation.

Acquisition Time

See Almanac. This relates to how quickly your unit picks up a signal. Acquisition time of the almanac varies from unit to unit – select a fast one if you want to get information immediately without wasting power.

Differential GPS

Using a GPS receiver at a known (fixed) location – usually a reference station, this allows the roaming (user) receiver’s location to be narrowed down to within 5 meters of its true location. Due to the reliance on a ground station, this is not guaranteed in all locations, but it’s growing all the time. See also WAAS ( North America only.)


Your north/south location on the surface of the earth, measured in degrees from the equator to either pole, with the North at 90° N and the South at 90°S. This is further refined using Minutes (not minutes of the hour in the strict sense, although part of measuring Latitude comes from Greenwich Mean Time): The centre of Glasgow is around 55.83°N in latitude; the centre of Sydney is around 33.88°S (or -33.88°) in latitude.


Your east/west location on the surface of the earth, measured from the Prime Meridian of the Equator; that is to say, the Meridian that passes through the Royal Greenwich Observatory, which lies at 0° longitude. Glasgow ‘s centre is around 4.25°W (or -4.25) ; Sydney ‘s lies at 151.22°E.


Triangulation is the process of determining the location of a point by measuring the angle to that point from three or more known points, and using maths to determine the location based on these three angles.


Wide Area Augmentation System. This places you within 3 metres of your actual location on your GPS map or coordinates. This is only available in North America .

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