Thread: Theory about why GPS performs poorly
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lardman is offline lardman
2008-02-07 , 12:19
Posts: 2,102 | Thanked: 1,309 times | Joined on Sep 2006
#97
A-GPS, AFAIK, only works, now, over GSM networks that support it.
Again AFAIK, those networks have GPSes on their repeaters (or know exactly where those repeaters are) and provide the repeater's position along with the list of visible satellites in that area, to the cellphone's GPS chip over GSM/UMTS/whatever.
More or less that should be the data sent.
This helps in calculating the fix (as you narrowed a lot already your position).
Sort of; the AGPS server (there are a few different types of AGPS service that can be implemented) can take fragmentary data received from the satellites by the GPS in the device and try to use its processing power to determine where the GPS would need to be to receive such data, then after it's worked this out, it can supply almanac/ephemeris data for that location.

Another method, called AGPS LTO (Long time orbit) supplies high quality ephemeris data to the phone (it must presumably also provide almanac data - or perhaps it does away with these terminologies and just supplies a single set of high quality data).

That doesn't work on the tablet. As no-one else knows where you are and what satellites are in view in that area.
Well yes, we need some other technique to establish a rough location, for gsm phones this is the cell ID (which we could forge and try passing to supl.nokia.com) but otherwise we could use some other geoclue type service for this. How we generate, or obtain the accurate almanac/ephemeris data is then the next step