Car owners guide to: Satellite Navigation > In-Car Sat Nav

Portable Sat Nav devices can be a good choice if you find yourself having to hire cars at home or abroad, or if you have to drive a variety of vehicles in your job, as they allow you to carry the device from car to car. However, they fall short in matching the accuracy, ease of use, display clarity (especially on road junctions) and cosmetic integration of the dedicated In-Car Navigation devices.

If you've read our short introduction to Satellite Navigation you will know that these devices make use of the Global Positioning System to calculate the position of your vehicle with a good degree of accuracy. As long as the car navigation receiver can read the position of at least three satellites, then by a process of mathematical triangulation it can work out where you are.

GPS is at the core of nearly all these devices, but added features improve both the accuracy and useability of navigational aids in the car, and it's these features that set apart the various types of Sat Nav devices now on the market.

Fixed or 'embedded' Sat Nav systems

Most fixed In-Car Satellite Navigation systems consist of a miniature computer connected to a CD, DVD or Data Card reader; a gyroscope; and a GPS receiver and antenna. A feed from the vehicle's electronics known as the 'speed pulse' supplies additional data to the navigation computer. The speed pulse feed and gyroscope are features that considerably improve the accuracy and reliability of fixed Sat Nav systems compared to the portable devices.

The fixed systems tend to have large, high-quality displays, often touch-sensitive and fold-away. This allows the device to neatly fit into the same space as a conventional in-dash CD player. Most will come with a remote controller and sophisticated mapping with advanced search facilities and a huge number of Points of Interest (POI) - these are useful places like petrol stations and bank ATMs, and leisure spots such as restaurants.

Inputting your destination is easy, either by speaking the pre-assigned name you've given to the address, or by entering the address or post code on the remote control, or by touchscreen. They all support the screen instructions with clear spoken directions delivered through the vehicle's own speaker system for best clarity.

Most fixed car navigation systems can also retrieve and decode realtime traffic information (via a TMC data feed - TMC stands for Traffic Message Channel) and automatically re-route you around traffic trouble spots.

Systems that use CD or DVD to deliver their mapping data will usually double as an entertainment system. Once the required mapping data is read into memory for a particular route or area, the reader is free to play other discs.

All-in-all these are the most accurate and easy-to-use Sat Nav systems for the car, but they do require professional installation. They are relatively expensive, but don't forget that in most cases you're also getting a CD or DVD player for entertainment too. And the whole system will fit perfectly into your car's environment - no trailing cables, no ugly and sometimes unreliable mounting brackets.

What's the benefit of the Speed Pulse feed and Gyroscope?

These are the two most significant aspects of fixed In-Car Sat Nav systems that set them apart from the portable devices in getting you from A to B with minimum fuss. Receiving speed pulse information from the engine management system (or a speed pulse generator in the case of some older or diesel vehicles), allows the navigation device to know how far along the road the car has travelled.

Of course the triangulation of the satellite signals will allow the device to track the vehicle, as long as it can see the satellites, but having the speed pulse information provides even greater accuracy. And if the vehicle passes through areas where vertical visibility is restricted, such as between highrise buildings or steep hillsides, you won't be left stranded.

While the speed pulse data tells the system how far the vehicle has travelled, the Gyroscope provides information on which direction you're headed. Again, this gets added to the data from the satellites or, if there's no line of sight to the satellites, then the data from the Gyroscope and speed pulse does a good job of tracking you on the digital map until satellite reception is restored.