In a Nutshell:

The GPS receiver calculates its position, the GPS satellites broadcast the information regarding their position and time is sent back to the GPS receiver.  The GPS receiver than utilizes a wireless cellular modem, installed in the unit, to send the location, speed and timestamp data to the Datacenter. 

GPS Technology in Detail:

The Global Positioning System (GPS) is a government technology consisting of constellation of satellites that orbit the earth transmitting time and position information. With a GPS receiver, users can determine their location anywhere on the Earth. Detailed mapping and navigational programs can be used in conjunction with GPS technology to track the movement and location of vehicles.

US Department of Defense Development developed the $10 billion GPS satellite navigation system to provide continuous, worldwide positioning and navigation data to US military forces. However, it became increasingly clear that GPS technology had an even broader civilian and commercial application. To meet these needs, GPS offers two levels of service: one for civilian access and the second encrypted for exclusive military use. The GPS signals are available to an unlimited number of users simultaneously, and there is no charge for using the GPS Satellites.

 

Using the distance measurements of at least three satellites in an algorithm computation, the GPS receiver arrives at an accurate position fix. Information must be received from three satellites in order to obtain two-dimensional fixes (latitude and longitude), and four satellites are required for three-dimensional positioning (latitude, longitude and altitude). GPS uses what is commonly referred to as “triangulation” to find location. This term is confusing, however, since there are no actual angle measurements involved in the process. It would more accurately be described as “trilateration” because the vehicles latitudinal and longitudinal distance from waypoints is what determines their position, and trilateration is what makes real-time vehicle tracking possible.

Vehicle tracking is one of the fastest-growing GPS applications. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times. Not only can vehicle tracking help improve efficiencies and productivity in mobile businesses, but they can also help overcome common problems such as getting lost and vehicle theft.