GPS is the US-operated Global Navigation
Satellite System (GNSS). It consits of 24
Satellites (SVs) orbiting at about 20,000km
above earth. Each SV constantly transmits
messages that include: the time of transmission,
precise orbital data (called ephemeris), and a coarse description of all other SV's orbits (called almanac). The ephemeris is only valid for 4 hours and a GPS receiver must first decode the ephemeris of at least 4 SVs - which can take 45s or longer - before it can calculate its own position on startup.
Assisted-GPS techniques were introduced to mitigate ephemeris acquisition delays.
