GPS was originally designed to guide bombs, aircraft, soldiers, and sailors. In all cases, the GPS receiver was expected to be outside with a relatively clear view of the sky. The system was designed to require a start-up time of approximately 1 min,
and after that it would operate continuously. Today GPS is used for many more civilian than military purposes. Counterintuitively, the system demands of these civilian applications far exceed those seen before. GPS is now expected to work almost anywhere, even, sometimes, indoors; push-to-fix applications have emerged where a single position is expected almost instantly; and all of this must be delivered in a way that adds little or no cost, size, or power consumption to the host device. You probably recognize the ways that your car’s 3G DVR and telematics system track you for your own good. Their tracking gives you directions. It means you will be found in the event of a crash.
What can A-GPS do for you? The PLGR was the GPS receiver most widely used by the U.S. military. It is a five-channel, L1-only receiver, with a typical time to first fix of over a minute, and a cost of about $2,000. The PLGR receives encrypted P-code military signals, is waterproof, weighs three pounds and is far more robust than any modern mobile phone. But many of those mobile phones today have A-GPS, which can compute a position within a second and acquire satellites at signal levels 100 times lower than the PLGR, and adds less than $5 to the cost of the phone. Nowadays, GPS enabled phones are widely available and are viable options for a dedicated handheld or an in-car GPS. These requirements are what drove the development of A-GPS. Assisted GPS (GPS rear view mirror ) improves on standard GPS performance by providing information, through an alternative communication channel, that the GPS receiver would ordinarily have received from the satellites themselves.