GPS data to be deleted for Cold start

Question

Can anyone tell me what all the GPS data (almanac, epheremis, time, sa data etc) gets deleted on a cold start? And how long will it take to get a fix?

Answer 1

A cold start is just that... no data available. On unassisted GPS only, it can take as long as 5-7 minutes to start getting a fix.

Phones are a bit different... the cell network data assists, which is why you can get a GPS fix so fast.

Answer 2

A pure cold start would require about 12 minutes of uninterrupted GPS signal to download the almanac alone. Supposing that the chip can process enough channels, you should have the ephemeris of the 4 satellites by the time the almanac is downloaded.

The thing is, in reality, you're unlikely to get uninterrupted GPS signal but, more importantly, really cold starts do not exist. As long as the phone has even a weak EDGE signal (I think EDGE introduces this, could be GPRS, though), it would have the visible towers' locations, which gives a constraint on the calculations, allowing an incomplete signal to still give a fix. Also, if there's a data link, the phone will likely download the almanac over that instead of waiting for 12 minutes.

Lastly, I am not entirely sure whether you can get a fix without the almanac (should be possible by iterating over the list of satellites - which should be built-in - and trying to find its ephemeris signal). In that case, you might be looking at a time < 12 minutes but any further fix will require a comparable time until the almanac is available.