Just a clarification, aGPS won't work unless you have a N810w. The "a" is for assisted, like.....cell tower assisted.

...but I just use my LD-4W anyway, it gets a lock in nothing flat where I've only gotten a lock with the built-in GPS twice...ever.

No, this isn't a clarification. agps-ui uses two things, IP-based locationing for when you're connected to the internet and tap-on-the-big-map locationing for when you're offline.

It does not require an N810W.

i thought that A-GPS always needs to download information from internet. does it really work when you are offline?

Thanks for the clarification. Forgot about the geolocation.

That's what I thought too... and here is my (uninformed?) reasoning:

When you point your location on the map, you're just telling the software where you are (roughly). Based on that information the app connects to dedicated A-GPS server that tells back what satellites are visible at that given date/time. The benefit of A-GPS is that GPS itself doesn't need to search for satellites. It already knows what satellites are there. Now the GPS only needs to receive satellite signal and calculate fix.

I just used my N810 GPS yesterday after about two weeks pause. When I was still home I had my WiFi connected, did go outside and fired up MaemoMapper. Immediately the GPS status was showing ~10 visible satellites... but calculating fix took another 2 minutes. Without A-GPS searching satellites alone takes ~1-2 minutes...

Exactly, except for the bit about an "A-GPS server"; those calculations are easily done from a fixed database of orbits, so there's no need to query an external server. Telling the GPS where it is is sufficient.

I think this is the piece of information I'm missing: If it is enough that the GPS "knows" where it is... why the A-GPS needs internet connection? What data does the N810 get from the internet? If it is only the fixed database... why don't we just download the database and save it to the tablet once... and that's it?

The database of orbits and the current time is sufficient to tell you where the satellites are in an absolute sense, but what's needed is a relative position, so you also need (an estimate of) your current location; if you have an internet connection, it can use the SSID of the access point to look up the location of the AP (if that's known) in an on-line database of APs; this is probably a better estimate of your position than clicking on a map, but either one helps...

hmm, are we sure about this. Afaiu agps-ui allows you to tap on the map and send the lat/long to a supl server which will work out the ephemeris data for that location.

It might be possible to calculate Almanac data on-device (though I've not seen any sign of this) and I think this data contains more than a simple geometric projection of the satellite locations knowing a prior position, but I think Ephemeris data would be rather time consuming (and would require outside assistance - to obtain corrections supplied by someone as to the ionosperic conditions and satellite path variances).

Therefore, afaik, to use AGPS you require some sort of connection - this can be in the form of a wifi connection (in which case you tap on the map and the lat/long are used to give you an approximate set of Ephemeris data) or via your mobile phone, which should be able to automatically tell the supl server which base tower it's connected to (which is more accurate than tapping on a map).

Just my thoughts of course,

Cheers,

Simon

P.S. Regarding using SSIDs to work out positions, this is probably the next step - to use GeoClue (in with whatever backend) rather than agps-ui. I hope so anyway