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

Gerry Creager N5JXS gerry.creager at tamu.edu
Mon Mar 14 09:07:55 CST 2005

The data are pretty well obsolete now.

TAPR had a DGPS kit that worked pretty well.  I've used it successfully 
for various implementations of Cows In Space.

One thing that's pretty useful is the DGPS-IP project.  It was one of 
the first IP-distributed Differential Corrections packages, predating... 
and if I were to check completely, probably the basis for the commercial 
systems that started in Europe and were bought by Trimble and the other 
big survey-GPS firms.

The requirements for data transmission for the GPS-Survey commuity, 
however, are not the same as those for the AVL or Automatic Cow Location 
community.  The surveyors tend to use the RTK (real time kinematic, 
sometimes called real-time surveying) corrections sentences.  These are 
longer and transmitted more frequently.  Note that while it's still 
RTCM-SC-104 compatible, it's *NOT* the same sentences most consumer 
receivers will understand.

And, putting out the ones consumer receivers will understand will not be 
beneficial to the Survey community.

I use an (old) Trimble 4700 and an Ashtech Z-Surveyor to produce 
RTCM-104 RTK sentences.  They work well.  We've been doing this in the 
College Station community for years.  I intend to continue 'til there's 
no longer a need, or until my receivers die (when I'll either replace 
them or not, depending on existing funding).

Rich's comment on the 60-mile range for DGPS is both  correct and 
incorrect today.  In older receivers and using older algorithms, that'd 
be right.  Today, the algorithms are better and the receivers will apply 
PRC and PRRC to the observables (correctly) when they can.

Setting up a local DGPS network would give you, usually, submeter 
accuracies providing the DGPS antenna is correctly sited, positioned, 
and the data accurately entered.  A poorly surveyed DGPS receiver can 
degrade positioning results significantly.

If there's interest, I can carry on about how to appropriately survey 
your location.  It's not too difficult but it is also not quite as 
simple as just getting a long-term average of GPS positions.


Robert Bruninga wrote:
>>>>jimlux at earthlink.net 3/13/05 2:50:18 PM >>>
>>Some thought should be given how to put these [differential]
>>corrections on the 2m air in a useful fashion and for
>>future GPS/TNC widgets to accept corrections and feed 
>>them to the GPS.
> We were doing this back in 1995 time frame in APRS in
> the Washington DC area and APRSdos has the hooks 
> built-in.  I have not looked at it in years, but It is documented 
> towards the end of the  APRSdos README\GPS.TXT file.
> That may all be old or obsolete, but it is how we were
> recommending it back then.  I think we were suggesting
> that the RTCM104 be on a different 2m freq so that it
> couild be real-time full operations without any QRM to 
> 144.39.
> de WB4APR, Bob
> 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
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Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843

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