Active antennas draw their power from the GPS unit through a power jack, which puts extra drain on the GPS' batteries. These are almost always fixed on the exterior of the vehicle. Your GPS unit's internal antenna will turn off automatically, provided the exterior antenna is drawing a suitable amount of satellite signals.
Passive units do not include a Low Noise Amplifier (LNA) that active units have to amplify weak signals, and therefore they do not require power. However, this means the cord running from the antenna to the GPS receiver must be 1 meter in length or less.
Reradiating antennas are composed of two antennas, one which gathers signals on the exterior of the car, and another which rebroadcasts the signal inside the vehicle. These require a power connection and are the only option for GPS units lacking an antenna jack.
Article Written By Jonathan Croswell
Jonathan Croswell has spent more than five years writing and editing for a number of newspapers and online publications, including the "Omaha World-Herald" and "New York Newsday." Croswell received a Bachelor of Arts degree in English from the University of Nebraska and is currently pursuing a Master's of Health and Exercise Science at Portland State University.