Most aftermarket navigation systems have a built-in antenna that usually works pretty well, but here are a few situations where an external antenna might be required:

• some windshields have built-in heating systems or coatings that block GPS signals
• mounting the GPS near the roof (instead of on the dash) might place the GPS in a location where most GPS signals are blocked.

External antennas can be located away from the GPS display, in an area of the vehicle where GPS signals are easier to receive. Some antennas can even be be mounted outside the vehicle.

Article from The RoadmapGPS Team