Satellites
A system of 24 satellites orbiting Earth creates the basis of the GPS, or global positioning system. Each satellite orbits Earth twice a day on one of six separate orbital paths. The system was created by the Department of Defense originally for military purposes. In 1983, it was authorized for civilian uses and has since seen many practical applications stem from its infrastructure.
Information
All the satellites in the global positioning system transmit signals down to Earth. Each satellite is programmed to send the signals at the same time as the rest of the satellites. The signal contains digital information about the satellite's identification and position. It contains the very important piece of information of the transmission time. Each satellite has an atomic clock, so this time is extremely precise. As long as your car's satellite GPS receiver has a clear view of the sky not blocked by any solid objects such as buildings or mountains, it is able to receive these satellite transmissions.
Signal Reception and Distance
When your GPS unit receives a satellite signal, there is obviously a time difference from when the signal was sent because it took time for the signal to reach your receiver. This time delay allows the receiver to automatically calculate the distance to the satellite by the simple formula of Speed x Time = Distance. Because satellite transmissions all travel at the speed of light, the speed is already a known constant, so the calculation is quite simple.
Trilateration
If you get the distance to just one satellite, you know where you are in relation to it, but this won't tell you where on Earth you are. In fact, all it tells you is that you are somewhere within a sphere that's centered around the satellite. If you're having trouble understanding, imagine that you are in the middle of nowhere, with no idea of where you are. Someone informs you that you are 300 miles from St. Louis. You now have a better idea of where you are, but you don't know which direction St. Louis is in, so this really doesn't help you figure out where you are; you could be anywhere in a circle with a 300-mile radius to St. Louis. The same is true of your location in relation to the satellite, but because it's in space, you're dealing with three dimensions (sphere rather than a circle). For this reason, GPS must use trilateration, which equates to using the distance to at least three satellites to determine your position. Three satellites are able to convey an accurate two-dimensional fix, giving you your longitude and latitude coordinates, and four satellites give a three-dimensional fix that includes elevation. The more satellites that are used, the more accurate the location is. GPS satellite orbits were designed in such a way that you will always have an effective, clear path to at least four satellites, so that you can always get an accurate fix.
Mapping
When you're in a vehicle, knowing your coordinates is not very helpful. No one uses coordinates to give directions or to navigate highways and roads. That's why car GPS units include mapping software. Just like a paper map, these maps include detailed information on the roadways and features of a given area. The more detailed and updated the map, the better the directions. Map companies such as TeleAtlas and Navteq are constantly updating their digital maps to provide the most accurate mapping possible. With the map data, the car GPS unit is able to overlay your location onto the map and provide turn-by-turn directions. Your GPS is constantly receiving satellite signals and updating your location so that it is able to convey your movement on the road.
Other Functions
In addition to location and directions, car GPS offers a variety of other helpful features, including updated traffic information and information on nearby businesses such as gas stations and restaurants. Other non-navigational features such as music player may also be included on a given GPS navigation system.
Tags: Each satellite, your location, able convey, four satellites, global positioning