GPS buying guide

How GPS mapping works

Originally developed in 1973 by the U.S. Department of Defense for military purposes, the NAVSTAR GPS network consists of approximately 30 satellites orbiting the Earth and a collection of 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, as well as where you’ve been, on the map. Since the receiver is constantly recalculating your position relative to the satellite’s position, the GPS unit can track your location in real time, as well as your speed and directional heading. 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.

Newer GPS devices feature newer receivers with upwards of 40-channels, which enables them to track more satellites for better accuracy. Other devices (such as GPS-enabled smartphones) use a system called assisted-GPS (A-GPS) which utilizes the fixed location of connected cellular towers or Wi-Fi hot spots to triangulate an approximate location, speeding up 3D fix times.

Older GPS devices featured bulky antennas, but newer units have slimmer profiles thanks to advanced integrated receivers.

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 1 to 2 minutes to acquire a 3D fix during a cold start, whereas 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; weaker units will require more time to reacquire a 3D fix. In rare 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.


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