[747fulloftapes]

Most modern cellphones use AGPS, assisted GPS, to reduce the time to get their first fix. Basically, AGPS primes the GPS receiver with an accurate time of day and the GPS almanac (satellite ephemeris data). That's rather different from Differential GPS, which is basically corrections used to compensate for fluctuating conditions in the atmosphere.

I may be wrong, but I don't know of any system for cell-tower derived corrections being redistributed out as GNSS corrections. Could you clarify the specific system you described?

[bri3d]

A-GPS can run in Mobile Station Assisted (MSA) mode, where the device sends raw GPS data to the A-GPS network and the assistance server processes it. In this situation, the assistance server can use whatever precise cell site location and ionospheric correction data it wants in its processing, although this is more of an "exercise left to the implementer" and I'm not sure sure MSA mode is even very common anymore.

[ianburrell]

I think was only a brief window where MSA was used, when phone processors were weak and cheaper to have limited GPS chip. Now, GPS chip do everything onboard and are included on radio processor or main processor. Plus, phone processors are powerful enough to do GPS themselves.

[fortysixdegrees]

Not so much. Modern GPS will get a first fix pretty quickly regardless, you don't need to seed it.

AGPS is used when satellite coverage is poor, like urban canyons in a city, also a situation where there are a lot of cell towers and triangulation is somewhat useful

[bri3d]

AGPS refers to a specific set of technologies, MSB where the almanac and time seed is provided over the cellular network, and MSA where the unit sends GPS data to the AGPS server and the server solves for location using whatever data it would like (which can include a multitude of corrections).

I think what you're referring to with cell tower triangulation (and also WiFi location, etc.) is generally referred to as "hybrid location service" and is usually done at a higher level (ie - in software) using AGPS as one input to a sensor fusion algorithm.

[marsokod]

The time to first fix is mainly impacted by whether the receiver knows which satellites to listen to. This requires two things: having the ephemeris for the constellation (either 10min of listening to GPS signal, or getting that through AGPS), and it's approximate own position/time (done through cell antenna locations, and NTP) to know which satellites should be visible.

Once this is available, the receiver can then focus on the signals that are expected and will then be able to provide a 3D fix as soon as it receives something from 4 satellites, which would be in a few seconds.