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.
The spheres around each satellite that have the calculated distance as their radius overlap in exactly one point — the receiver’s location. Theoretically, three satellite signals are enough for a pretty good guess: while three spheres overlap in two points, only one of them is likely to be on Earth.
How accurate are they?
- Normal GPS can be up to about 20 m 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 picking up Differential GPS (DGPS) — a network of stationary GPS receivers with known locations that constantly compare their 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.
How do they work?
They consist of:
- A Global Positioning System (GPS) receiver that picks up satellite signals that allow it to determine your exact position.
- A map database (all the tested models have maps for most of Australia on internal flash memory or an SD flash memory card). This includes a large number of ‘points of interest’, such as schools, police stations, car parks, petrol stations and hospitals.
- A computer processor to calculate routes, distances and times.
- A screen displaying maps and route instructions.
- A loudspeaker for verbal instructions.
- You enter where you want to go. Using the map data, it then plots a route, calculates the travel distance and estimated time of arrival, and displays the route on a map. You can usually choose between the fastest or the shortest (geographically) route, or specifically exclude toll roads or highways.
- 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 pre-recorded or computer-generated voice.
- The portable units in our test have a mount with a suction cap that can be attached to the windscreen. They plug into the cigarette lighter for power, 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.
- The tested models cost between $280 and $799, which is much cheaper than the fully integrated systems that come as standard or optional equipment on some (usually more upmarket) cars. However, 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, such as in a tunnel.
- All the tested models work with the same satellite signals and use Australian mapping data from one of two providers: Sensis or Navteq.
However, the software each brand uses to let you turn all this into information that guides you from A to B can vary significantly.
Go to Wikipedia on GPS.
The Victorian Department of Sustainability and Environment has produced the 20-page guide GPS — a guide for users.
- 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 city centre 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. When we let the units search for destinations or plot routes in other states, our randomly selected addresses and points of interest showed weaknesses in the mapping data and points of interest, especially in rural areas.
- Don’t blindly trust your navigation system. There were a few occasions during our test when the voice instructions were wrong but the displayed information correct. And there may be traffic situations that are simply too tricky to handle for a navigation system — for example, if some quick lane-change manoeuvres are required.
- Some models have a ‘walking’ option that lets you plot a pedestrian route ignoring one-way streets and using, for example, walkways through parks. However, none of them is suitable for guiding you on bushwalks.
Welcome to Camden — California!
While the technology that enables users to successfully get from A to B based on a satellite that pinpoints a position on the globe is amazing, car GPS units are not infallible. One of our test models (a Garmin) announced correctly that the tester had arrived at the University of Sydney, Camden; however it also announced that our tester was in California.
Other models insisted that the tester make a sharp right turn on the M5 in Sydney, which was impossible as it meant going through a concrete dividing wall, then proceeded to set a new route of 38 km to get to a destination instead of merely turning left.
Locked in to the map
The two main map choices available to car GPS users in Australia are Whereis and Navteq. The car GPS you choose determines which map you get, so you can’t use a Whereis map in a GPS that uses Navteq and vice versa.
You can purchase map upgrades to keep up to date on the inevitable changes in roads, traffic directions and speed limits. However, these upgrades cost anywhere from about $100 up to $180 (see table for upgrade pricing), so you might want to check with the company to clarify its map upgrade policy. Some companies offer to upgrade the car GPS to the latest maps if a new version is released within a month of purchase, other companies include one upgrade to the next map version when it arrives and some offer nothing.
The GPS units with Navteq maps performed marginally better than units that use Whereis maps in the city, however, the models that use Navteq map data scored significantly worse in our country driving test. However, the quality of the information provided to the user is not only dependent on the map provider but also the companies that make the car GPS units. This is why our CHOICE tester observed several differences in map accuracy on models using supposedly the same map data.