Buying Guide | GPS Units
Before we dive into the types of navigation systems and buying advice, it's always good to get a little background on the technology and have a basic understanding of how it works. Originally developed in 1973 by the U.S. Department of Defense for military purposes, the NAVSTAR GPS network consists of 30 satellites orbiting the earth every 12 hours, and five ground stations that monitor the satellites' position in space and operational status. To determine your location and other data accurately, such as current and average speed, directional heading, and elevation, GPS devices use a receiver to acquire signals from at least four of these satellites. This is known as a 3D fix and it's why GPS antennas require an unobstructed view of the sky to work correctly.
Armed with your precise latitude, longitude, and other location data, the GPS receiver can overlay this information onto map files stored on the unit, revealing your current position on the map as well as where you've been. Since the receiver is constantly recalculating your position relative to the satellite's position, the GPS unit can track your location in real time. A typical GPS device contains a 12-channel receiver and an antenna to capture satellite signals, and a CPU to process the data. The quality of the receiver and your geographic location will determine how long it takes the device to acquire a 3D fix. For example, it's harder for the receiver to lock onto and hold a signal if you're traveling through a dense forest or an urban area with tall buildings.
The first time you fire up your GPS, it collects certain satellite information to determine your whereabouts. Known as a cold start, the receiver is essentially blank and needs to know what time it is, where the satellites are in their orbital patterns, and where it is in relation to the satellites. Most systems take around one to two minutes to acquire a 3D fix during a cold start, while some can take a few minutes. Thereafter, it can take as little as 3 to 4 seconds to lock in since the unit already has your coordinates and a general location of the satellites. A good receiver will instantly recover from a complete signal loss when you drive through a tunnel, for instance, while weaker units will require more time to reacquire a 3D fix. In some cases, you'll have to stop the car to give the receiver a chance to lock on to the requisite signals.
How well a GPS unit will work in your car depends on the location of the antenna. If your vehicle has a factory installed in-dash unit, chances are the antenna is integrated into the dashboard in a place where it has an unobstructed view of the sky, which is ideal. Many portable models are designed to be positioned directly on the windshield via a suction cup mounting device, giving the antenna a wide sky view. There are also add-on antennas available for GPS units that allow you to keep the receiver close to the front seat for easy viewing without sacrificing signal quality.