Autonomous GPS
In strong environments GPS receivers are able to extract all the information required to calculate position and timing solutions directly from satellite signals, including ephemeris, almanac and ionospheric corrections. This is often referred to as either conventional or autonomous GPS.
Assisted GPS or A-GPS
In weak environments, receivers can not extract the ephemeris, almanac and ionospheric corrections due to the relatively high levels of noise in comparison to the satellite signal strength. To reduce the time to establish accurate positioning and timing solutions, receivers must be provided this information, or assistance, via an alternative path, such as over the internet or cell phone network.
Assistance must be current, regularly updated at least every 4 hours and relevant to satellites currently “visible” in the same geographic region as the GPS receiver to be assisted.
The best source of assistance is from satellites themselves. Specialized GPS receivers, termed gatherers, in strong signal environments collect assistance from “visible” orbiting satellites in their region. Gatherers forward their extracted assistance to central servers for collation and distribution to AGPS receivers.
