[FOSS-GPS] DGPS for newbies?

[Timo Jyrinki]

2009/1/16 Lauri Hahne <lauri.hahne at gmail.com>:
> That isn't dgps. That's agps.
>
> DGPS is used in high-precision applications where you have a base
> station in a location which is known very precisely. This base station
> compares it's known position to one calculated from gps signal and
> broadcasts the error to near-by gps devices. This makes it possible
> for the gps devices calculate their position even in centimeter
> precision.

Right, so DGPS is "information service provided by base stations".

> As for AGPS, there are a few common uses to term which actually are
> different variations of the same idea.

That's what I've gathered, yes.

> The most basic one is that the receiver downloads the ephemeride and
> almanac of the satellites from a server which makes it faster to get a
> fix.

Yep, like I mentioned I'm downloading the ephemeris (ca. 3kB) often
when using the GPS from a cold boot. But when asking those from u-blox
server I need to know the approximate coordinates of my phone so that
I don't need the whole almanac or anything.

> The ephemeride and almanac are usually accompanied by an approximate
> location of your handset which is derived from cell-id or something
> similar.

Yes, that was the point: are there databases linking the cell-id to
approximate locations, and what kind of databases (libre or not). I
don't think the base stations provide any coordinates themselves to
phones automatically like you mentioned as a possibility?

-Timo