GPS

Want GPS for your car? Whether you're getting an in-dash device or an add-on, these are the features you should look for in a GPS navigator:

Display
Select a GPS model with a decent color screen that can be read in all lighting conditions. The larger the screen, the more expensive the unit will be, but even big screens can be difficult to read in direct sunlight. Also, check the viewing performance of the display to make sure you can read it from any angle.

Maps
Look for a model that includes street-level maps. Some manufacturers charge extra to unlock maps from their Web sites or CDs, while others include only partial regions. This can get expensive if you're planning a cross-country trip. Make sure you can update the unit's firmware and mapping data. On most portable models, a USB or serial port lets you connect the system to a PC, where you can upload the latest maps and system software as they become available. On in-dash models, maps are typically read from an integrated DVD or CD player and require only the latest discs to be brought up to date.

Form factor
Choose a GPS unit to fit your traveling habits. If you do a lot of long-distance driving, consider a model with a dedicated hard drive that stores maps of the entire country. Otherwise, a model that uses an SD card to store maps is a good bet, and you can buy additional cards to load more maps when necessary. If you want to share one unit between many vehicles, look for a model that is lightweight and easy to install and remove from your vehicle. Stay away from permanent mounting devices unless you plan on using the unit in just one vehicle.

If you decide on an in-dash model, pay the extra money to have it professionally installed. Unlike car stereo systems, which can be fairly easy to install, a GPS system requires careful placement of the antenna, and some systems require a special cable hookup to your vehicle's speedometer mechanism.

Navigation features
Like anything else, the more bells and whistles included in your GPS system, the more you'll end up paying, but there are certain features that are more or less standard equipment these days. Street-level maps with voice- and text-prompted driving directions are the foundation of any in-car GPS system worth its salt, and we're starting to see systems that use text-to-speech technology to deliver specific street names rather than more generic instructions, such as "Turn right in 0.5 miles." A comprehensive POI database containing airports, hospitals, dining options, shopping, service stations, and other landmarks is a must if you're traveling in unfamiliar territory, as is automatic routing from a POI, an address book, or your present location. Look for a device with touch screen controls, automatic rerouting when you veer off course, and variable map perspectives, such as 3D and bird's-eye views. As you get into the high-end models, look for big, daylight-readable screens and real-time traffic and weather alerts, which usually require an annual subscription fee.

Accessories
Most in-car navigation systems are ready to use right out of the box and come with everything you need to get up and running in a matter of minutes. Still, there are accessories available that will help you get the most out of your investment, including auxiliary antennas that can be mounted at the base of your windshield to improve signal reception and carry cases to protect your GPS unit when not in use. You can also pick up various mounting devices, including motorcycle-mounting kits and low-profile external antenna mounts that adhere to the trunk of your car. Additionally, you can purchase more maps on CD media or flash memory cards, as well as AC adapters that let you use your GPS unit at home.