Navigation computers, now sold by most car-makers, cost $ 2,000 and up. No surprise, then, that they are most often found in luxury cars, like Lexus, BMW and Audi. But it is a developing technology — meaning prices should eventually drop — and the market does seem to be growing. Even at current prices, a navigation computer is impressive. It can guide you from point to point in most major cities with precise turn-by-turn directions — spoken by a clear human-sounding voice, and written on a screen in front of the driver. The computer works with an antenna (天线) that takes signals from no fewer than three of the 24 global positioning system (GPS) satellites. By measuring the time required for a signal to travel between the satellites and the antenna, the car’’s location can be pinned down within 100 meters. The satellite signals, along with inputs on speed from a wheel-speed sensor and direction from a meter, determine the car’’s position even as it moves. This information is combined with a map database. Streets,　landmarks and points of interest are included.Most systems are basically identical. The differences come in hardware — the way the computer accepts the driver’’s request for directions and the way it presents the driving instructions. On most systems, a driver enters a desired address, motorway junction or point of interest via a touch screen or disc. But the Lexus screen goes a step further: you can point to any spot on the map screen and get directions to it. BMW’’s system offers a set of cross hairs (瞄准器上的十字纹) that can be moved across the map (you have several choices of map scale) to pick a point you’’d like to get to. Audi’’s screen can be switched to TV reception. Even the voices that recite the directions can differ, with better systems like BMW’’s and Lexus’’s having a wider vocabulary. The instructions are available in French, German, Spanish, Dutch and Italian, as well as English. The driver can also choose parameters for determining the route: fastest, shortest or no freeways (高速公路) , for example.