Like many modern smartphones, Apple’s iPhone 3G and 3GS have built-in GPS satellite navigation systems and can show on a map where they are located.

This Tomtom program, downloadable for £60 from Apple’s App Store, turns it into a full-blown satellite navigation system for driving.

The program includes everything you would expect to find in a mid-range satellite navigation system. You can find locations by address or postcode – it has a full seven-digit postcode search – or look for points of interest such as a nearby petrol station.

Also, conveniently, you can navigate to any addresses stored in the phone’s contact list. Once the destination is found the software shows a 3D map of your location and provides spoken turn-by-turn warnings. As usual these can be given in a selection of voices and languages, and there’s a night mode for the display.

We tried several journeys with and without the car kit, and found that the Tomtom software worked well, guiding us to our destinations without trouble and recalculating reasonably quickly when we were forced off-route.

There are, however, a few extras required. For one you’ll need some kind of cradle to prop the phone up in a suitable position near the windscreen. Equally importantly you’ll need a car charger for the iPhone, as using the GPS sensor drains its battery enormously.

You can buy both together in the form of Tomtom’s own Car Kit. This includes a windscreen mount, a louder speaker and a charger. It also has an extra GPS sensor, which makes the application available to iPod Touch users, since the device doesn’t have a GPS sensor built in. The kit costs £100, though, making the complete app-plus-car-kit system fairly expensive at £160.

If you already have a suitable cradle and power supply for long journeys, the £60 price makes this software a bargain, against the price of buying a standalone satellite-navigation device. Add in the cost of the car kit if needed, though, and it begins to look a bit steep.