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Assisted GPS (abbreviated generally as A-GPS and less commonly as aGPS) is a system that is often able to significantly improve the startup performance, or time-to-first-fix (TTFF), of a GPS satellite-based positioning system. A-GPS is extensively used with GPS-capable cellular phones, as its development was accelerated by the U.S. FCC's 911 requirement to make cell phone location data available to emergency call dispatchers. ==Description== Standalone/self-ruling GPS units depend solely on information from satellites. A-GPS augments that by using cell tower data to enhance quality and precision when in poor satellite signal conditions. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit multipath propagation where signals skip off structures, or are weakened by meteorological conditions or tree canopy. Some standalone GPS navigators, when used in poor conditions, will be unable to fix a position because of satellite signal fracture and must wait for better satellite reception. A GPS unit may require as long as 12.5 minutes (the time needed to download the GPS almanac and ephemerides) to resolve the problem and be able to provide a correct location.〔() NavCen GPS User. 3.5.3 Almanac Collection〕 An assisted GPS system can address these problems by using external data. Utilizing this system can come at a cost to the user. For billing purposes, network providers often count this as a data access, which can cost money depending on the plan.〔 To be more precise, "A-GPS" features are mostly dependent on an internet network and/or connection to an ISP (or CNP, in the case of CP/mobile-phone device linked to a Cellular Network Provider data service). A mobile (CellPhone/SmartPhone) device featured with "A-GPS" (no additional "S-GPS"/Standalone-GPS feature to be selected as alternative, or there is no "Hybrid GPS" as a complete A-GPS/S-GPS hybrid features in one device) can work only when there is an internet link/connection to an ISP/CNP - it is useless in areas with no coverage of internet link (or no BTS towers nearby, in the case on CNP service coverage area) to connect to those A-GPS servers (that are usually provided by CNPs). On the other hand, an "S-GPS" (including "Dedicated GPS") device/feature requires no connection to internet/network to obtain GPS data since it connects directly to GPS satellites overhead. Assistance falls into two categories: # Mobile Station Based (MSB): Information used to acquire satellites more quickly. # * It can supply orbital data or almanac for the GPS satellites to the GPS receiver, enabling the GPS receiver to lock to the satellites more rapidly in some cases. # * The network can provide precise time. # Mobile Station Assisted (MSA): Calculation of position by the server using information from the GPS receiver. # * The device captures a snapshot of the GPS signal, with approximate time, for the server to later process into a position. # * The assistance server has a good satellite signal, and plentiful computation power, so it can compare fragmentary signals relayed to it. # * Accurate, surveyed coordinates for the cell site towers allow better knowledge of local ionospheric conditions and other conditions affecting the GPS signal than the GPS receiver alone, enabling more precise calculation of position. As an additional benefit, in mobile station assisted implementations, the amount of processing and software required for a GPS receiver can be reduced by offloading most of the work onto the assistance server. A typical A-GPS-enabled receiver will use a data connection (Internet or other) to contact the assistance server for aGPS information. If it also has functioning autonomous GPS, it may use standalone GPS, which is sometimes slower on time to first fix, but does not depend on the network, and therefore can work beyond network range, and without incurring data usage fees.〔(Watch out for data charges on a GPS phone ). CNET, August 2007〕 Some A-GPS devices do not have the option of falling back to standalone or autonomous GPS. Many mobile phones combine A-GPS and other location services including Wi-Fi Positioning System and cell-site multilateration and sometimes a hybrid positioning system.〔(Networking iPhone A-GPS ) Hybrid system〕 High Sensitivity GPS is an allied technology that addresses some of these issues in a way that does not require additional infrastructure. However, unlike some forms of A-GPS, high-sensitivity GPS cannot provide a fix instantaneously when the GPS receiver has been off for some time. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Assisted GPS」の詳細全文を読む スポンサード リンク
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