[aprssig] differential corrections available to public (in some places)

[Jim Lux]

There's a growing interest in the surveying community in having state run
high accuracy reference networks (HARN) for GPS surveying. The state goes
out and places high quality receivers and antennas in carefully surveyed
places, and then they distribute the data from those receivers to the
public. The basic idea is that instead of every state contractor having to
go out and setup their own reference stations for each job (which takes not
only money but adds a week or so to the job), they might as well share ones
installed by the state.

What's of particular interest (to APRS using GPS) is that some of these
networks are broadcasting (either by internet, or other "free to receive"
means) standard RTCM-104 corrections which many receivers can use for
differential GPS corrections. (Some are even broadcasting the data via
802.11b wireless, though I don't know the details)  Some thought should be
given how to put these corrections on the 2m air in a useful fashion and for
future GPS/TNC widgets to accept corrections and feed them to the GPS.

One might want to take a look at: http://www.mdotcors.org/ for the state of
Michigan, as an example, although I couldn't find any info on RTCM104
corrections there, but they do give Rinex format data for post processing.

Here's another interesting source of differential correction data:
http://www.wsrcc.com/wolfgang/gps/dgps-ip.html
http://www.rlageosystems.com/dgps.htm has many links to data, not sure which
have realtime data available.

Anyway, folks who have a suitable receiver and a fixed station location
could do real good for those near them by setting up to put RTCM104
corrections out there.  Bob? Gerry? Suggestions on how to do this within
APRS without overflowing the capacity.  I don't know enough about
differential correction to know how often you want to push corrections out,
and how to manage the distance issue (you want corrections from someone
close to you, but, on the other hand, for APRS kinds of applications, we're
probably not interested in centimeter accuracy.. meter is good enough).

And, since a lot of people have given the whole prospect of broadcasting GPS
corrections over wide areas a fair amount of thought, reports such as
http://www.lvvwd.com/assets/pdf/wolftuc99_low_res.pdf might save a bit of
"reinventing the wheel" or "inventing a new incompatible wheel" problems.

Jim, W6RMK