[Geowanking] Low-cost GPS receivers and software for DGPS?
danlyke at flutterby.com
Sun Feb 13 07:04:28 PST 2011
On Sun, 13 Feb 2011 12:01:19 +0100
Stefan Keller <sfkeller at gmail.com> wrote:
> I'm just wondering about a fundamental configuration decision.
> My configuration idea is to have two low cost GPS receivers, where one
> of them is on a known position. Both issue just GPS position and log
> time data
So the point of your base station is twofold. First, to have a known
reference point with which to measure tropospheric delays for gross
adjustment to the beginning of the signal "chip" (a message bit, not
called a bit because there's bits that go into decoding that bit), the
second to do the same thing for relative carrier phase (the signal on
which the chips are broadcast).
For the first part, you're not going to get better than a two or three
meters anyway, so you may as well use reference stations that other
people are running. This is what an SBAS, like WAAS, does, and these
days that second tier of consumer devices receive SBAS signals anyway
and do their corrections in real-time.
If you have a device that doesn't, and you want to apply that
correction as a post-process, then NOAA CORS publishes reference data
that you can use.
To get better than a few meters, you need a receiver that can do
carrier-phase positioning and, as Brian Russo points out, log that data.
Consumer grade receivers don't do this. Furthermore, you need to know
the location of your base station sub-millimeter.
In my area the density of known base stations is high enough that people
use CORS or commercial providers for their reference stations for
centimeter level positining. In your case, since your receivers can't do
carrier-phase positioning, you don't care if you're a couple of hundred
miles from your reference stations.
Conceivably yes, you could put two consumer grade receivers down, one
at a known position, one mobile, and do your own augmentation. But
other people already publish free data from their reference stations
such that unless you need centimeter accuracy (in which case you need
to pay for carrier-phase capable receivers), there is a reference
station near you that someone else is already running that's better than
whatever you'll hack together.
And in most cases, if you're using the second tier of consumer devices
or better, they're already receiving and applying SBAS corrections.
Maybe if you shared your application we could suggest cheaper ways to
accomplish what you're trying to do?
More information about the Geowanking