To find out how GPS works, lets start with the basics. GPS stands for Global Positioning System and is a way of locating a GPS receiver anywhere on Earth. GPS has so many different applications that many technologies and ways of using it are continually being improved upon.
So how does GPS work? Lets get technical.
Signals
In order for GPS to work, a network of satellites was placed into orbit around the Earth, each broadcasting a specific signal, much like a normal radio signal. This signal can be received by a low tech aerial, even though the signal can be very weak. The signals that are broadcast by the satellites carry data that is passed from the aerial, decoded and used by to the GPS tracking software.
The information is specific enough that the GPS tracking software can identify the satellite, along with it’s location in space and then calculate the time that the signal took to travel from the satellite to the GPS tracker. Using different signals from different satellites, the GPS tracking software is able to calculate the position of the receiver. The GPS tracking software performs a calculation using the known positions of the satellites in space, and measuring the time that the signal has taken to travel from the satellite to Earth.
The result of the tracking of at least three satellites enables the software to calculate, within a small margin of error, where the device is located in terms of its latitude (East-West) and longitude (North-South) and distance from the center of the Earth.
Timing & Correction
In a perfect world, the accuracy should be absolute, but there are many different factors which prevent this. Principally, it is impossible to ensure that the clocks are all synchronized. Since the satellites each contain atomic clocks which are extremely accurate, and certainly accurate with respect to each other, we can assume that most of the problem lies with the clock inside the GPS unit itself.
Keeping the cost of the GPS tracker down to a minimum is a key part of the success of any consumer device, and it is simply not possible to fit each GPS tracker with an atomic clock. However, GPS tracking devices are designed to work whether their clock is accurate or not.
Mapping
Knowing where the device is in space is one thing, but it is fairly useless information without something to compare it with. Thus, GPS mapping software is very important as it is how GPS works out possible routes, and allows the user to plan trips in advance. In fact, it is often the mapping data which increases the price of the GPS solution, as it must be accurate and updated reasonably frequently. There are, however, several kinds of map, and each is intended for different users, with different needs.
Road users, for example, require that their mapping data contains GPS traffic information about the road network in the region that they will be traveling in, but will not require detailed information about the lie of the land as they do not really worry about the height of hills and so forth.
On the other hand, hiking GPS users might wish to have a detailed map of the terrain, rivers, hills and so forth, and perhaps tracks and trails, but not roads. They might also like to improve their map with specific icons of things that they find along the way and that they wish to keep a record of, not to mention waypoints, or locations to make for on their general route.
Finally, marine users need very specific information relating to the sea bed, navigable channels, and other pieces of maritime data that enables them to navigate safely. Of course, the sea itself is reasonably featureless, but underneath quite some detail is needed to be sure that the boat will not become grounded.
Fishermen also use marine GPS to locate themselves and track the movement of shoals of fish both in real time, and to predict where they will be the next day. The advent of GPS fixing has also meant that co-operative fishing has become much easier, where there are several boats all relaying their locations to each other while they locate the best fishing waters.
Getting Started
There are a few solutions available to consumers, ranging from sophisticated in-car GPS systems, which feature voice guiding, powerful dynamic route calculation, and the possibility to update the map from a CD.
Then there are handheld GPS units, such as those from Magellan and Garmin, which store a map internally and are usually designed for outdoor use. Hand held GPS cannot generally perform dynamic route calculation since they do not have any road information, and leave the user to find their own way from A to B. However, you can buy a GPS handheld navigation system that does route calculations for you for use in both driving and walking.
Finally, a more recent solution has appeared on the market which uses the power of a PDA to run the actual GPS software, and uses the interface capability to attach a GPS receiver to the unit. These tend to be the solution for cheap GPS and are the easiest to use and most flexible solutions, but are only as rugged as the PDA to which they are attached.
Some recent systems can make use of a wireless Bluetooth connection so that they can be used both as a personal navigation aid and also inside a vehicle, by having several receives with Bluetooth capabilities, and a single PDA. You can now even buy a GPS watch, now that's portable.
