An external GPS antenna lets you install your navigation system in an area of your vehicle where GPS reception is poor. Most navigation systems have a built-in antenna, but many can use an external antenna instead. A cable connects the GPS receiver to a separate antenna, which is mounted elsewhere in the vehicle.
The antenna portion is often square or rectangular and measures a little more than an inch on each side. Some attach to a window or your vehicle's roof using a suction cup. Others are designed to be permanently mounted with adhesive.