Satellite navigation (often known as SatNav) is a facility offered for cars. On many high end Japanese cars (eg the Lexus) this is a standard piece of kit, but it's available as an option on much lower cars as well (eg the Toyota Yaris). It's also sometimes available as a bolt-on extra, using a PDA as the display unit.
What does it do? Essentially exactly what it says. It has a complete map of the country you're in (or countries / continent in some systems), and a GPS tracking system so the car knows where it is.
To set up a guidance route, you simply tell the system where you want to go to - this can usually be by a postcode / zip code or sometimes simply a street address. Addresses you go to can sometimes be programmed in and stored. The system then calculates the best route to the destination and gives you directions how to get there. If you make a mistake, or have to take a diversion, at first the system may ask you to turn around and get back on the route, but if you don't do that, it will simply recalculate a new route based on where you are.
There are a few variations in the type of system.
- DVD vs CD-ROM based - this is simple. DVDs have about 6 times the capacity of CDs, and therefore can store more roads. For example, a CD based system may just have UK roads in it, whereas a DVD system may have all the roads in Europe in it.
- Moving Maps - The simpler systems have a small display which gives you an arrow and distance to the next turn, and often audio directions such as "turn left 100 yards". More advanced systems have a full colour LCD display which displays a moving overhead map of your current location, and on some newer systems a perspective view as well.
- Integrated vs options - As mentioned above, many modern high end cars, which already have colour LCD displays to control their systems, can have the system fully integrated into the car. Or, you can get systems for PDAs which attach to the dashboard and provide the same functionality.
Are they worth it? Depends what you do. If, for example, you're a salesman, driving around lots to places you're not familiar with, they are worth their weight in gold. On the other hand, when my father in law offered getting a SatNav system for my mother in law's car, her answer was "I know the way to the supermarket".
Oh, and with most SatNav systems you can subscribe to the provider so that they send you a new CD / DVD every quarter or six months with updates to the road network. With a new car that has a fully integrated SatNav, this service may be included.