Car navigation systems buying guide

What to look for in a car GPS.
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01 .How they work

Please note: this information was current as of November 2006 but is still a useful guide to today's market. For more recent information, see our 2012 Car GPS review.

A car navigation system consists of: Car GPS

  • A Global Positioning System or GPS receiver that picks up satellite signals which allow it to determine your exact position.  
  • A map database This includes a large number of ‘points of interest’, such as schools, police stations, car parks, petrol stations and hospitals. Most car navigation systems available in Australia use the same mapping data. So the difference between the tested models is how they let you use and display the data.
  • A computer processor to calculate routes, distances and times.
  • A screen displaying maps and route instructions.
  • A loudspeaker for verbal instructions.

You enter into the system where you want to go. Using the map data, it then plots a route (you can usually choose between the fastest or the shortest route — which aren’t necessarily the same — or specifically exclude toll roads or highways), calculates the travel distance and estimated time of arrival, and displays the route on a map.

Using satellite signals, it keeps track of your position and guides you along the plotted route with travel instructions on the display and verbally via a computer-generated voice.

Portable units sit in a cradle with a suction cap that can be attached to the windscreen. They plug into the cigarette lighter, but also have a battery that provides a few hours of operation. They’re installed within moments, and can therefore easily be moved from car to car.

Integrated systems are usually connected to the car’s electronics, and can overcome some of the limitations of portable units. For example, they can use speed information to keep calculating your position when there’s no satellite signal (for example, in a tunnel).

General limitations

  • No satellite signal, no guidance. Car navigation doesn't work when you're in an underground car park or tunnel. And even high-rise buildings in a CBD can block the view of the sky to an extent that leaves navigation systems lost or at least handicapped.
  • The systems can only be as good as the mapping data allows. Currently there are still big gaps outside major centres.
  • Don't blindly trust your navigation system. There may be traffic situations that are simply too tricky to handle for navigation.

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  • Do you need one? Before you buy a car navigation system, consider how often you usually have to use a street directory or map, and whether it justifies spending hundreds of dollars.
  • Data entry: Check how easy it is to enter addresses and routing preferences.
  • Display: This should be large and glare-free, and show the information you want to see — such as a two- or three-dimensional map view, distance to the next turn, current street name, time of arrival, and distance to destination.
  • Installation and portability: Check how easy the system is to install, but also how easy it is to remove and carry — you’re likely to take it with you when you park the car somewhere, as it could be an attractive target for thieves if you leave it prominently displayed on the windscreen.

Features they (almost) all have

  • Li-ion battery.
  • SD memory card slot.
  • Australian map data (to street number level) and points of interest.
  • You can buy map data for other countries
  • USB PC connection.
  • Touchscreen.
  • Allow you to store your home address.
  • Allow you to select a location on the displayed map.
  • Option to calculate fastest or shortest route.
  • Option to exclude highways and toll roads.
  • Display car speed, distance to destination, and estimated time of arrival.
  • Display street name and distance to the next turn.
  • Option of two- or three-dimensional map display.
  • Volume control for voice instructions.
  • Points of interest: schools, police stations, car parks, fast-food outlets, post offices, petrol stations, airports, railway stations, hospitals.
  • Option to search for a point of interest.

Extra features

  • Walking option: You can make the system plot a route for pedestrians — for example, ignoring one-way streets, or using walkways through parks.
  • Languages: You can select a number of other languages for the display and voice instructions (such as French, Spanish, German, Italian, Swedish, Danish and Dutch).
  • Fuzzy look-up: If the system can’t find the street name in the suburb you’ve entered, it’ll search for it in neighbouring suburbs.

Video: What to look for - GPS

Want a new GPS for the car? Here's what you should look for.

GPS stands for Global Positioning System. It was developed by the US military, but is available free of charge to the general public. It has many commercial uses, from land, sea and air navigation to land surveying and map-making.

GPS consists of 24 satellites that orbit Earth exactly twice a day at an altitude of about 20,200 km. The orbits are aligned so that at least four satellites are ‘visible’ at any time from most places on Earth.

The satellites carry highly accurate atomic clocks and constantly send coded time signals to Earth. A GPS receiver can read these signals and use the time delay between the send and receive times to calculate its distance from the satellite (assuming the signal travels at the speed of light).

A receiver uses a method called three-dimensional trilateration to then calculate its position on Earth:

  • It calculates the distances to four of the satellites.
  • Each calculated distance is the radius of a sphere around each satellite. The four spheres intersect at exactly one point — the receiver’s location.
  • Theoretically, three satellite signals are enough: while three spheres intersect at two points, only one of them is likely to be on Earth.
  • If your GPS receiver doesn’t get a signal from at least three satellites (for example, if you’re in a tunnel, or if the signals are blocked by high-rise buildings), it can’t calculate your position.

How accurate?

  • Normal GPS can be up to about 20m out – the satellites' signal may be slowed down by the atmosphere, the signals may bounce off high-rise buildings or other structures, or the receiver itself may have its limitations (quality of the antenna, etc).
  • More expensive GPS receivers can correct at least for atmospheric inaccuracies by using Differential GPS (DGPS) — a network of stationary GPS receivers that constantly compare their known position with the position calculated from satellite signals. They can then calculate a correction factor, which is sent to DGPS-capable receivers in the area. DGPS increases accuracy to a few metres.

More information

If you want to read more about GPS, there's lots of information on the web - for example:

  • Go to Wikipedia and search for ‘GPS’.
  • VicRoads has a guide that explains the basics.
  • If you need help with unfamiliar routes only occasionally, there are websites that let you plot a route with turn and distance instructions, and print it out for you to take it on your trip. Try Whereis or NRMA Travel.
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