[FOSS-GPS] DGPS for newbies?

[Lauri Hahne]

2009/1/16 Timo Jyrinki <timo.jyrinki at gmail.com>:
> What would be the best resource for information about DGPS? I gather
> that DGPS is the term that is usually used about GSM base station id
> collections, to help getting approximate coordinates, or are there
> other meanings?

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.

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

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. Usually it takes about 30 seconds to get the ephemeris of a
satellite after you've found its signal and the data is valid only for
a few hours. Almanac, on the other hand, takes 12 minutes to download
from a satellite and it tells the approximate position of each and
every gps satellite. Information from the almanac is used to find
signals of visible satellites and the ephemeris data is thereafter
used for actual navigation as almanac is way too approximate for that.

The ephemeride and almanac are usually accompanied by an approximate
location of your handset which is derived from cell-id or something
similar. This allows the device to interpret from the almanac which
satellites it should look for and not to waste time by searching for
satellites which aren't visible.

Some cheapest devices only have the radio part of a gps receiver and
can't do the positioning on their own. These devices record a part of
the gps signal and upload it to their network operator for the actual
positioning. This takes time and isn't very usable in navigation and
other real time applications but the yankee e911 system uses this
extensively to locate low-end handsets.


-- 
Lauri Hahne