GPS is arguably one of the most important inventions of our time, and has so many different applications that many technologies and ways of working are continually being improved in order to make the most of it.
To understand exactly why it is so useful and important, we should first look at how GPS works. More importantly, looking at what technological achievements have driven the development of this fascinating positioning system.
Signals
In order for GPS to work, a network of satellites was placed into orbit around planet Earth, each broadcasting a specific signal, much like a normal radio signal. This signal can be received by a low cost, low technology aerial, even though the signal is very weak.
Rather than carrying an actual radio or television program, the signals that are broadcast by the satellites carry data that is passed from the aerial, decoded and used by to the GPS software.
The information is specific enough that the GPS software can identify the satellite, it’s location in space, and calculate the time that the signal took to travel from the satellite to the GPS receiver.
Using different signals from different satellites, the GPS software is able to calculate the position of the receiver. The principle is very similar to that which is used in orienteering – if you can identify three places on your map, take a bearing to where they are, and draw three lines on the map, then you will find out where you are on the map.
The lines will intersect, and, depending on the accuracy of the bearings, the triangle that they form where they intersect will approximate your position, within a margin of error.
GPS software performs a similar kind of exercise, 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 “trilateration” (the term used when distances are used instead of bearings) of at least three satellites, assuming that the clocks are all synchronized enables the software to calculate, within a 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.
Timmings & Corrections
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 technology 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 unit with an atomic clock costing tens of thousands of dollars. Luckily, in creating the system, the designers designed GPS to work whether the receiver’s clock is accurate or not.
There are a few solutions. However the solution that was chosen uses a fourth satellite to provide a cross check in the trilateration process. Since trilateration from three signals should pinpoint the location exactly, adding a fourth will move that location; that is, it will not intersect with the calculated location.
This indicates to the GPS software that there is a discrepancy, and so it performs an additional calculation to find a value that it can use to adjust all the signals so that the four lines intersect.
Usually, this is as simple as subtracting a second (for example) from each of the calculated travel times of the signals. Thus, the GPS software can also update its’ own internal clock; and means that not only do we have an accurate positioning device, but also an atomic clock in the palm of our hands.
By: Guy Lecky Thompson
