2. Assisted GPS
GPS works well once your phone finds three or four satellites, but that may take a long time, or not happen at all if you're indoors or in an "urban canyon" of buildings that reflect satellite signals. Assisted GPS describes a collection of tools that help to solve that problem. One reason for the wait is that when it first finds the satellites, the phone needs to download information about where they will be for the next four hours. The phone needs that information to keep tracking the satellites. As soon as the information reaches the phone, full GPS service starts. Carriers can now send that data over a cellular or Wi-Fi network, which is a lot faster than a satellite link. This may cut GPS startup time from 45 seconds to 15 seconds or less, though it's still unpredictable, said Guylain Roy-MacHabee, CEO of location technology company RX Networks.
3. Synthetic GPS
The form of assisted GPS described above still requires an available data network and the time to transmit the satellite information. Synthetic GPS uses computing power to forecast satellites' locations days or weeks in advance. This function began in data centers but increasingly can be carried out on phones themselves, according to Roy-MacHabee of RX, which specializes in this type of technology. With such a cache of satellite data on board, a phone often can identify its location in two seconds or less, he said.
4. Cell ID
However, all the technologies that speed up GPS still require the phone to find three satellites. Carriers already know how to locate phones without GPS, and they knew it before phones got the feature. Carriers figure out which cell a customer is using, and how far they are from the neighboring cells, with a technology called Cell ID. By knowing which sector of which base station a given phone is using, and using a database of base-station identification numbers and locations, the carriers can associate the phone's location with that of the cell tower. This system tends to be more precise in urban areas with many small cells than in rural areas, where cells may cover an area several kilometers in diameter.
Wi-Fi can do much the same thing as Cell ID, but with greater precision because Wi-Fi access points cover a smaller area. There are actually two ways Wi-Fi can be used to determine location. The most common, called RSSI (received signal strength indication), takes the signals your phone detects from nearby access points and refers to a database of Wi-Fi networks. The database says where each uniquely identified access point is located. Using signal strength to determine distance, RSSI determines where you are (down to tens of meters) in relation to those known access points. The other form of Wi-Fi location, wireless fingerprinting, uses profiles of given places that are based on the pattern of Wi-Fi signals found there. This technique is best for places that you or other cellphone users visit frequently. The fingerprint may be created and stored the first time you go there, or a service provider may send someone out to stand in certain spots in a building and record a fingerprint for each one. Fingerprinting can identify your location to within just a few meters, said Charlie Abraham, vice president of engineering at Broadcom's GPS division, which makes chipsets that can use a variety of location mechanisms.