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Before we discuss what an assisted GPS unit is, we have to describe what a standalone GPS unit is. Essentially, conventional GPS units only use the radio signals from satellites to triangulate position. Fragmented signals, signals that have bounced off of buildings, or even just weak reception all contribute to a poor locating device. It may take up to 40 seconds of a clear signal in order to triangulate position! Obviously, this is not optimal for a navigational device.
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A-GPS: Filling in The Holes
That's where the assisted part of A-GPS comes into play. A-GPS units use an assistance server, or basically, information from outside the device and the satellite.
For instance, the server may supply orbital data for the GPS satellites, so that a GPS device can locate the satellites more quickly. By using a third party system to aid in heavy computation, A-GPS systems may take advantage of fragmentary signals and other difficult-to-process data, as well as simply being faster for those GPS devices that do not have particularly fast processors. Some A-GPS devices also use surrounding cell towers to further triangulate and clarify the position.
A-GPS technology is constantly improving as engineers come up with more ways to use third party information to make your GPS devices more accurate, so keep an eye out!
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Disadvantages to A-GPS
A-GPS has a few downsides, though. Use of an assistance server usually requires some sort of subscription, for instance to a particular cell phone plan, and often cost extra. Others require wi-fi or other wireless Internet access, which is obviously not available globally. Many A-GPS devices are so dependent on the outside server that they may not be able to function without it, or simply do not have a standalone option. There are also some privacy concerns, as a third party server thus knows your exact location (though cell phone companies know this anyway, as required by the U.S. FCC's 911 mandate. Indeed, much of A-GPS development was spurred on by this mandate!)
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Where A-GPS is Used
Most devices that use A-GPS technology are not immediately labeled as such: increasingly, it's just the way that GPS navigation is done. Most of today's GPS-enabled cell phones use A-GPS technology to some degree or another, the assistance server being provided by the network, though this often requires additional data charges.
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High Sensitivity GPS & Hybrid Positioning Systems
High sensitivity GPS is another field that is working to make GPS units faster and more accurate. While it is not the same as A-GPS systems, being independent of outside servers, it works along the same lines.
Hybrid positioning systems are another allied technology. In such systems, a device uses any available information from any network, be they global networks such as GPS, local satellite position networks, cell networks, a wireless Internet connection, whatever it takes to locate you. Such systems can work with any combination of the above. However, they are still very much in development, and haven't become completely feasible for consumer use.
For more information, check out this extensive series of articles on A-GPS from GPS World.