In 1973 the U.S. Department of Defence launched the Navstar GPS network - a network of 24 satellites orbiting the earth every 12 hours and five ground stations.

This positioning system was made available for public use. With this capability, consumer location devices were produced to accurately determine location and other data such as current and average speed, directional heading, and elevation. These GPS devices need an unobstructed view of at least four satellites to provide a reliable 3D fix.

The GPS receiver overlays this location data onto map files stored on the unit, to give a current position on the map as well previous track. The receiver constantly recalculates position, giving real time position.

A typical GPS car navigation device contains:

12-channel receiver - the quality of the receiver determines how long it takes the device to acquire a 3D fix.
Antenna to capture satellite signals - positioned to get a clear view of the sky.
CPU to process the data and overlay on maps
DVD Hard-drive - where maps on DVD's or available online are uploaded and stored. Some cheaper units do not upload the maps, but reference them off the DVD or CD.
Display Screen - mostly color with handheld units using black and white
Voice Interface - more advanced units

How The GPS Device Gets A Fix

The first time you start your GPS device, its data store is blank so needs a to collect satellite information to determine your position. This is known as a cold start. Some units only take 30 to 45 seconds to acquire a 3D fix during a cold start, while others can take several minutes. Subsequent position updates only take 3 to 4 seconds. If you go out of range from losing line of sight, such as passing behind a large building or through a tunnel, a good receiver will instantly recover, whereas weaker units will require more time to reacquire a 3D fix.

How Different GPS Navigation Units Differ

Location of Antenna - A factory installed in-dash unit antenna is integrated into the dashboard where it has an unobstructed view of the sky. Many portable models have a suction-cup-mounting device to position the device on the windshield. Add-on antennas are also available. Regardless of the type of unit and antenna, the important thing is to keep the antenna visible to the greatest area of sky possible. Choose a unit where this can be done AT THE SAME TIME as being able to maintain a clear view of the screen.

Screens and Display - important to check how bright these are, and if they are clearly visible from the mounted position in bright day light. Onboard navigation systems are generally color screens, and portable units are black and white to save power. Larger screens and integrate better with other vehicle electronics.

Input Buttons - most enroute buttons are on the display screen. Ensure these are easy to use when driving; that is they are big enough and colored sufficently to see without causing a driving hazard.

Map Media - Earlier models were CD-based, requiring multiple discs to cover the entire United States. Newer in-dash systems are DVD-based; only 1-2 DVD's required for an entire country of maps.

Cost - In-dash systems are usually more expensive than portable counterparts. Aftermarket in-dash models usually require professional installation and can be just as expensive as the factory models.

Upgrading - always check how easy it is to upgrade the firmware and maps on your GPS unit. Some units detach a portion to be connected to the computer via USB, whereas others are done using a DVD. Those units which can be upgraded online, are much more convenient.

Added Features of GPS Units

Apart from giving you a current position, a number of GPS navigation devices can give you:

A track of where you have been - the number of tracks and waypoints stored varies from unit to unit. You may also want to save on part of a track for future use.
A path from your current position to your destination
Maintain commonly used navigation paths for reuse.
Points of Interest - user sets the types of points of interest, such as tourist, bank ATM, petrol stations, historical, accommodation, restaurants etc.
Real time traffic reporting to avoid traffic delays. This can also include road works.
Voice recognition to receive destination instructions, and voice guidance to give driving instructions
Weather updates
Street name navigation - instead of just turn left 200m it was say 'Turn left into Stanley St'
Integrated Multimedia players - MP3 players, image viewers, and audio books.
Onboard or Portable GPS Navigation

The downside of onboard GPS Navigation units are susceptible to theft; and you cannot take them with you to use in other vehicles or when travelling abroad. Portable units, such as the Garmin StreetPilot 2720, can be used in multiple cars; being easily moved from car to car. Depending upon the power supply and portability, portable and handheld units can be taken when travelling or used on cycles, boats, private aircraft etc.