SatNavs Explained

A Brief Guide to Sat Nav Vehicle Navigators

Satellite navigation brings Space Age technology into your daily life. Forget paper maps: through the wonders of the Global Positioning System (GPS), you can sit at the wheel of your car and be guided from A to B with the aid of a machine the size of a pack of playing cards. The new portable sat nav units work 'straight-out-of-the-box': just buy one, put it in your car, and away you go.

How it Works

Your little sat nav unit is a receiver linked to a network of satellites orbiting the Earth. The satellites help the receiver to pinpoint its precise position on the surface of the Earth, to within a few metres. Add maps and some computer technology, and your sat nav can guide you anywhere you want to go.


The Global Positioning System (GPS) was developed by the United States Department of Defense as a military tool for surveying and mapping the Earth's surface. The first satellite was launched in 1978, and there are now 29 in orbit, covering the entire surface of the Earth 24 hours a day. They emit constant signals relating to their position and the time. A GPS receiver on Earth uses these signals to determine the precise location of at least three of the satellites at any time. From this information, it can calculate its own latitude, longitude and altitude on the surface of the Earth, using essentially the old surveying technique of triangulation. With in-car sat nav systems, GPS is usually accurate to within about 10 metres, but some enhanced GPS receivers (used, for example, for mapping) are accurate to 10 centimetres or less. The GPS satellites are operated by the United States Air Force, but it allows free access to the system for civilian use - even though the system costs an estimated $400 million a year to maintain.

Sat Nav

Satellite navigation (sat nav) depends on GPS signals to establish the current position of the receiver. Your receiver (your in-car 'sat nav unit') also contains a set of road maps. Using GPS and the maps together, it can plan a route between your current position and any selected destination. It can also plot your progress along the route, by constantly updating your position through signals from the GPS satellites.

Fixed and portable sat navs

A sat nav unit may be built into a new car by the car-manufacturer (often sold as an option). Such 'hard-install' units are usually top-of-the-range in terms of technology and function, and relatively expensive. The alternative is a portable sat nav unit, which is fixed - by means of a suction pad - to a mount (or 'bracket', or 'dock', or 'cradle') on the windscreen or dashboard, and powered by a cable attached to the car cigarette lighter. A portable unit may also have battery power, which means it can be used outside the vehicle, for instance when walking to reach a destination in a town.

Inputting your destination

Before you begin your journey, you have to program your sat nav unit, to tell it where you want to go. You do this by identifying your destination, by keying in the address, usually by pointing to the keypad on the touch-screen. The more precise the address, the more precise the directions, and the best the way to identify an address is to use the full seven-digit postcode, if your sat nav unit will accept this. Once it knows your destination, the sat nav will calculate to quickest route to get there - but if you prefer, you can always choose another route by inputting various 'waypoints' that the route should take you to, or by identifying roads that you wish to avoid ('road exclusion').

Voice directions

When you get going, your sat nav unit will anticipate the manoeuvres that you have to make by talking to you: 'Take the next left', 'After three hundred yards, at the roundabout, take the third exit', and so on.

Map presentation

Meanwhile, the screen will show your changing position on a map of the immediate locality. This might be a 2D (two-dimensional) map, which gives you an overhead view, like a traditional paper map. But most sat nav units now use '3D mapping', which provides a schematic, angled view of the route ahead of you, as if seen from the air; this makes it easier to visualise your next move along the way. The screen will also constantly update written data about your journey, such as your next instruction, your speed, speed limits in the area, your estimated time of arrival, the name of the street you are currently in, and so on.

Dynamic route recalculation

If you take the wrong turning, or choose to take another route, many of the newer sat nav models will simply recalculate your route automatically, and revise all subsequent instructions accordingly. This is also known as 'dynamic re-routing'.

Itinerary planning

If you have a series of destinations to visit on your journey (for instance, if you are delivering goods to various addresses), you can input all the destinations in advance.

Handheld GPS navigators

Walkers, cross-country runners and mountain-bikers can also benefit from GPS technology. The receivers designed specially for their use are similar to in-car sat nav units, except that they are built for battery operation, contain Ordnance Survey maps (or equivalents) rather than road maps, and do not have voice instruction. With the help of land-based WAAS and EGNOS technology to augment GPS readings, they are accurate to within 7.6 metres (i.e. rather more precise than in-car sat navs). In a similar vein, some PDAs (Personal Digital Assistants) and smartphones can be upgraded with GPS software to provide rudimentary handheld navigation systems.

Choosing a Sat Nav

There are dozens of sat nav models to choose from. Some of them simply stick to their job of getting you to where you want to go; others are virtual in-car entertainment and communication systems, with a price tag to match.

Portable or fixed?

By and large, the fixed (or 'hard-install') sat nav units that are fully installed by car manufacturers, or by specialist fitters, are considerably more expensive than the portable models. They have the advantage, however, of being more sophisticated, and less vulnerable to theft and loss; the voice instructions are also co-ordinated with, and override, the sound from any in-built radio or CD player. With a portable sat nav, you have the advantage of being able to take the unit with you wherever you go, and use it in any car - even hire cars abroad. You can even take it for a walk.

Beware of heat-reflecting windscreens

Some newer models of cars, particularly those of the French manufacturers, have been fitted with heat-reflecting windscreens. These have sometimes been found to interfere with the ability of sat navs to receive the GPS signal. If in doubt, test a sat nav in your vehicle before you buy it. Otherwise, sat navs can be used in any vehicle.

Need and aerial?

Not usually: most sat navs have a built-in, internal antenna. However, if you opt for live traffic updates, you may need to install an extra aerial.

Sat navs for bikers

Some sat navs are designed specifically for bikers. Waterproof and especially robust, they fit on a mount on the motorcycle handlebars. The touch-screen is responsive to gloved fingertips, and the voice instruction is relayed to speakers inside the helmet. Models using Bluetooth technology also offer hands-free telephony.

Map coverage

Make sure that the map coverage that comes with your unit is sufficient for your requirements. If you are not sure of your future needs, check that you can easily extend the coverage (e.g. to Europe or North America) at an affordable price.

Points Of Interest (POIs)

What kind of information do you need when you are on the move? Do you want to know about car parking, or galleries and museums? Would you like a list of recommended restaurants? See what kind of POIs are pre-installed in your package, and see if you can add sets of POIs that are specifically tailored to your needs. But maybe you barely need any of these at all: just how often are you in an unfamiliar place, where you can't easily find this information for yourself?

Safety camera alerts

If you feel you are in constant danger of being caught speeding or going over red lights by the increasing number of safety cameras that line our streets, then perhaps you need a system that warns your about their location - and regular updates of that information. Other drivers may feel that they can simply observe the rules without additional aids.

Live traffic updates

For frequent road-users who are constantly frustrated by congestion, accidents and roadworks, it may well be worth considering paying the extra cost of a sat nav that receives live traffic alerts. Such services are usually available by monthly subscription, but some sat nav manufacturers offer free subscription to live updates with their more upmarket models.

Bells and whistles

Sat navs offer numerous add-ons. The more you pay, the more you get - and the more complicated the unit becomes to operate. You have to decide whether what you need is really just a car navigator, or whether it would really be an advantage to have a sat nav that also doubles as a hands-free mobile phone, SMS message reader, MP3 player, DVD player, digital photo album, calculator and currency converter. Don't you have other gadgets that do these things already?

Jargon Buster

What kind of coverage do you want, and can you download more POIs? If you don't have an auto-dimmer, can you always see the 3D mapping? And should you insist on dynamic re-routing? This jargon buster should help you find out.

Anti-glare - See Auto-dimming.

Auto-dimming - A system, using a built-in sensor, that automatically reduces (or increases) the amount of light emitted by the screen to take account of the light conditions outside, to aid clarity.

Bluetooth - Wireless technology that allows mobile phones, computers etc to communicate with each other over short distances, without cables.

Car navigator - Another name for a sat nav.

Coverage - See Map coverage

Cradle - Another term for the mount by which the sat nav unit is fixed to the windscreen of dashboard.

Custom POIs - Specialist companies gather and publish sets of Points of Interest (POIs), which you might be able to purchase and load onto your sat nav. These are known as Custom POIs. See Point of Interest.

Dock - Another name for a mount (to attach the sat nav unit to the windscreen or dashboard).

Dynamic re-routing - If you miss a turn, or decide to change your route while driving, the sat nav unit will automatically recalculate your route from your present position. Also known as 'dynamic route recalculation'.

EGNOS - European Geostationary Navigation Overlay Service, a land-based navigation system created to augment GPS accuracy, and used in the newer handheld GPS receivers. The American equivalent is WAAS.

Embedded flash memory - A memory device that stores information when the power supply is turned off.

ETA - Estimated Time of Arrival; also referred to as 'time to destination'.

Expansion slot - A slot in the sat nav unit that allows you to expand its capabilities or data by inserting a card.

Fuel and park buttons - Dedicated buttons on the sat nav that tell you where the nearest petrol stations and car parks are located.

Global Positioning System (GPS) - The network of 29 US orbiting satellites that operate in unison to locate the longitude, latitude and altitude of any point on Earth.

GNSS - Global Navigation Satellite System.

GPRS - General Packet Radio Service: this upgrades 2G (second generation) mobile telephones to 3G (third generation).

GPS - Global Positioning System.

Handheld battery-operated unit - Some sat navs are designed for walkers and runners (as opposed to vehicle use), and are powered by batteries. Car sat navs can perform the same function when removed from the car, if fitted with batteries.

Hard install - Sat nav units that are fixed into the car, often by the car-manufacturer; the opposite of 'portable sat navs'.

Itinerary planning - This allows you to plan a whole a journey, involving a sequence of destinations, by inputting the data into your sat nav unit before setting out.

Li-ion battery - A lithium-ion battery: a common form of rechargeable battery.

Live traffic information - A live link to traffic reports, supplying up-to-date information about road conditions and traffic jams ahead; some sat navs will automatically change your route to take account of any problems.

Map coverage - The geographical area covered by the maps that are supplied as part of your sat nav package.

MP3 capability - The sat nav is able to transmit sound from an MP3 player, such as an iPod, if linked by a cable.

Navigation audio prompts - The spoken directions given by your sat nav to guide you; the same as voice direction/instruction.

POI - See Point of Interest.

Point of Interest (POI) - Most sat navs also supply details of the location (usually represented by icons) of various places that could be useful to you on your route. These are known as Points of Interest (POIs). They might include: petrol stations, car parks, vehicle repair centres, cashpoints, shops, restaurants, hotels, museums, post offices, emergency services, parks, cinemas and other places of entertainment.

Portable sat nav - A sat nav unit that you can take out of the car, carry around with you, operate on batteries, and use in any vehicle you choose.

Pre-loaded safety camera locator - A programme, already installed in your sat nav unit, which alerts you to upcoming safety cameras (speed and red-light cameras) along your route.

RDS-TMC - Radio Data System - Traffic Message Control: a service that provides the delivery of traffic information to an international standard.

Real-time traffic information - See Live traffic information.

Receiver - Your sat nav unit contains a receiver capable of receiving signals from orbiting satellites.

Red-light camera - A traffic control camera stationed at traffic lights to record infringements of the red-light instruction.

Road exclusion - The facility to exclude certain roads or places along your route when requesting an itinerary from your sat nav unit (for instance, when you know of congestion on a local road caused by roadworks, or want to avoid a toll bridge).

Route update - See Dynamic re-routing.

Safety camera - The polite term for a speed camera or a red-light camera.

Sat nav - Satellite Navigation. The term is now also generally used to refer to the sat nav unit itself.

Screen size - The size of the screen measured diagonally across it. Screen sizes are typically 3.5" (inches) or 4", or 4.5" for wide-screen models.

SD memory cards - Secure Digital memory cards, a standard form of memory card used in digital devices.

Searchable postcode digits - One way to find a destination on your sat nav unit is to input the postal address, with the street name; but it is far quicker and more precise to use a postcode, if your unit permits this - and best if it allows you to input the full postcode, when all seven of the postcode digits are used (and hence 'searchable').

Speed camera - A camera that monitors traffic speeds, and records infringements.

Stand-alone battery time - The amount of time that a sat nav unit will operate when powered only by the battery (e.g. when you take it out of the car and use it to guide you to your destination on foot).

3D mapping - 2D (two-dimensional) maps give you an overhead view of a location, like a traditional paper map. 3D maps give you a schematic, angled view of the route ahead of you, as if seen from the air above you. 3D mapping makes it easier to visualise your next move along the way.

Touch-screen - A technology that allows you to input instructions by using a fingertip to select data on the screen. This is rather easier that using the buttons on a keyboard.

Traffic alerts - Warnings about traffic problems ahead. See Live traffic information.

Voice direction/instruction - The spoken directions given by your sat nav to guide you.

WAAS - Wide Area Augmented System, a US land-based navigation system created to augment GPS accuracy, and used in the newer handheld GPS receivers. The European equivalent is EGNOS.

Waypoints - Points in your itinerary that you may wish to specify in order to get your sat nav unit to select the route that you want to take.


+1 #1 Kumar 2014-12-10 01:24
This is a great article thanks for sharing this informative information.... I will visit your blog regularly for some latest post. Sat Nav .
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