GPS is the oldest one from the frameworks available to the large audience. It isn’t the first positioning system, but it is the one with the most success and longevity. It started as a military project in 1957. Because of the cold war, once the Russians sent to space the first man-made satellite, a team of scientists started monitoring its radio transmissions. The crucial discovery was that, because of the Doppler Effect, the signal transmitted frequency increases as the satellite approached, and lowers as it continued moved away from them. This added to the fact that they knew their exact location on the globe, the logical conclusion was that one could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
Since then lots of GPS signal emitting satellites were launched, now the entire surface of the Earth being covered by them. Since 1983, it became also available to civilians, but with lower precision (but still 50 m were quite good for that time) and higher times for getting the current position.
The mechanism of its functioning is simple: you need three satellites to get your precise position in the device line of sight, unobstructed for entire period of measurement, or four if you want also the elevation. The device receives high precision clock signals (atomic clock precision) in order to measure the delay of the wave between the satellites and earth. Then, using triangulation, the location can be pinpointed on the Earth surface by doing some math calculus. The entire process takes few minutes in a device start and less if the device is readjusting the location.
Since its creation, it was used in all domains: aviation for getting to the correct location, help and rescue for climbers and excursionists which missed their path in mountains, sea navigation and now more and more for city navigation. Since the receivers became so miniaturized that they can be easily fit into smart phones, one can use it to get to a newly recommended cafe or theatre without having to carry with it the map of the city.
The recording devices that use GPS can be split into three categories:
1. Data loggers – log data as it arrives and here we can include almost all devices with user interface, no matter that this device is a cell phone or a standalone device. They simply receive the GPS signal, record it and show it to the user.
2. Data pushers – the kind of devices using for tracking logistic fleet in realtime- they read the GPS data but instead of only show it to the user, they also send it via GSM connections or radio to a central location where it is processed. Then the data is analyzed and the merchandise path to the client can be optimized.
3. Data pullers – almost the same as the pushers, but instead of sending by their own at specific intervals data to the central location, these ones can be queried as needed. One of the most important usages is placing one of these devices inside of a valuable device and if stolen it can send its position once queried for it.
The main drawback of this technology is the fact that it requires clear line of sight to at least three satellites. In crowded urban areas or in mountains, this is a luxury. New technologies help minimizing these facts, like Assisted GPS which relies also on GSM towers to get its location done.
Note: This is an excerpt from my work during Agile Lab and Seminar in Fall 2009