Editing Assisted GNSS (section)

==Background==
Every [[GPS navigation device|GPS device]] requires orbital data about the [[GPS satellite|satellites]] to calculate its position. The data rate of the satellite signal is only 50 bit/s, so downloading orbital information like ephemerides and the almanac directly from satellites typically takes a long time, and if the satellite signals are lost during the acquisition of this information, it is discarded and the standalone system has to start from scratch. In exceptionally poor signal conditions, for example in urban areas, satellite signals may exhibit [[multipath propagation]] where signals skip off structures, or are [[Radio propagation#Absorption|weakened]] by meteorological conditions or tree canopies. Some [[GPS navigation device|standalone GPS navigators]] used in poor conditions can't fix a position because of satellite [[signal fracture]] and must wait for better satellite reception. A regular GPS unit may need as long as 12.5 minutes (the time needed to download the [[GPS signals#Navigation message|GPS almanac and ephemerides]]) to resolve the problem and be able to provide a correct location.<ref>{{cite web|url=http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|title=NavCen GPS User. 3.5.3 Almanac Collection|website=Navcen.uscvg.gov|access-date=2017-04-02|archive-date=2008-09-10|archive-url=https://web.archive.org/web/20080910184805/http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|url-status=dead}}</ref>