Assisted GPS explained

A smaller search

Assistance helps the receiver find the right signals sooner.

A receiver starting with no useful context may have to search a wide range of satellite identities, code positions and frequency shifts. Assisted GNSS provides starting information through a source outside the satellite signal itself. On a phone, that source is commonly a network service reached over Wi-Fi or mobile data.

A rough location and time tell the receiver which satellites are likely to be above the horizon and what signal conditions to search. Orbit and clock information can reduce how much navigation data must first be decoded from a weak satellite transmission. The result can be a shorter time to first fix, particularly after a cold start or in a constrained environment.

Assistance changes the acquisition job. It does not make the satellite signal stronger and cannot create an open view through a cliff, roof or dense structure.

Useful prior knowledge

Broad constellation data and precise orbit data do different work.

An almanac provides coarse orbit and status information across a constellation. It helps a receiver predict which satellites may be visible, but it is not precise enough for the final range calculation. Ephemeris data describes a particular satellite's orbit in the detail needed for positioning and has a shorter useful life.

Assistance standards can carry several kinds of information. Depending on the system and implementation, these can include reference time, approximate position, acquisition parameters, satellite orbit and clock data, or corrections. A phone does not need to expose this transfer to the user.

It is inaccurate to reduce A-GPS to “downloading a map of the satellites”. Assistance can narrow multiple parts of signal acquisition and measurement, while the receiver still has to observe usable radio signals and calculate or contribute to a solution.

Three separate services

Network location, satellite assistance and satellite position are not interchangeable.

• Network estimateWi-Fi or mobile infrastructure can help produce a rough location without a fresh satellite solution.
• Assistance dataA connection can give the GNSS receiver prior knowledge that reduces its acquisition work.
• GNSS measurementsThe receiver measures satellite signals to form or support the positioning solution.

A phone location service may blend these stages into one stream, so the on-screen dot does not identify its source. Losing mobile coverage after a strong fix does not switch off the satellites. Conversely, having a fast data connection does not guarantee a useful GNSS position under a roof or beside a severe obstruction.

Before coverage ends

Use assistance early, then prepare the parts it cannot supply.

Open the navigation or recording session while a connection and a reasonable sky view are available. Let the current position develop before moving into a valley, forest or built-up start. This gives the receiver the best chance to collect both network assistance and satellite data.

That preparation does not download the route or its map. Save those separately, including enough surrounding mapping for route context. If a watch will work away from the phone, confirm that its own route and map data are present rather than assuming the phone's assisted start transferred everything.

A receiver can acquire without network assistance, but the starting conditions may be less favourable. Test the exact setup offline in a familiar place. The result tells you how that device, app and saved data behave together, without turning A-GPS into a dependency it may not be.