A Sat Nav system is this year's must-have gadget. The problem is knowing which satnav unit will serve you best.

A GPS system works through a series of satelites tracking your position and giving you information to help you get where you want to go. You enter your destination into your unit and it will give you commands either audibly and/or on the screen. Satnav is useful for long journeys to unfamiliar places.

There are different types of satnav systems:

1 Dedicated Systems
Transferable or dedicated satnav devices are self-contained GPS units that can easily be moved from one vehicle to another. They usually come with software preloaded and are ready to use straight from box. The Navman iCN520 is a typical example of this kind of GPS system. It is designed to be used in-vehicle, it has an inbuilt GPS receiver, attaches to the car windscreen and is powered by a cigarette charger or an integrated rechargeable battery. UK maps are pre-installed on the memory card which comes with it, providing voice and visual turn-by-turn instructions.

Alternatively there are transferable units that are supplied with internal hard drives. The TomTom GO 700 is an example. Everything you need for navigating is included. The GO 700 has a windscreen mount and also offers turn by turn instructions by voice and onscreen instructions. The maps are pre-installed on its 4GB internal hard drive. The GO 700 also has several extra features; it can even be used as a Bluetooth 'hands-free' kit for some compatible Bluetooth mobile phones.

Sometimes these units do require an extra external aerial to obtain satisfactory performance.

2 PDA or Pocket PC based solutions
People who already have or want a pda (personal digital assistant) or pocket pc, can find satnav solutions that integrate with these units.

The neatest solution is to use a PDA with an integrated GPS receiver such as the the Mitac Mio 168 or Acern35. These PDA units have a built-in GPS receiver which obtains all the positional information meaning there is no need for a separate GPS receiver so fewer cables are needed for recharging or powering the unit in the car. Also, as the pda has an internal battery, you are not limited to using it in-car.

Standard PDA's can be used with a 'Wired receiver'. This plugs in to the base of the PDA either directly or through a car mount. This is a very cost effective solution, but offers less flexibility. An example of this system is the TomTom HP Navigation Pack for iPAQ RZ1710 that includes a vehicle mount and a 128MB card with GB maps preloaded.

Evolving from this type of GPS comes the PDA GPS-mount solution such as the Holux cr-100. Like the PDA with integrated GPS, this solution removes the requirement for unnecessary cables with only one cable required to power both the PDA and the GPS mount. However once again flexibility is a problem as this GPS unit can only be used in-car.

As an alternative to these solutions there are GPS receivers that use a wireless form of communication called Bluetooth. Tom Tom utilise this form of GPS in their NAVIGATOR 5 Bluetooth product that includes the latest SiRF Star III Bluetooth GPS and detailed GB mapping with navigation software pre-installed on its 128MB memory card.

To use this type of receiver your PDA must be Bluetooth enabled. Bluetooth receivers from manufacturers such as Holux, Sysonchip and Fortuna give you a wide range of choices. There are many other examples of these component-based systems and the major feature of these is the flexibility it provides.