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[aprssig] differential corrections available to public (insomeplaces)

Brian Webster bwebster at wirelessmapping.com
Mon Mar 14 18:35:33 UTC 2005

	I think Jim has a good idea here, have the mobile units report their
observations of GPS signal data and let the client software apply the
differential corrections since it can connect to data sources on the
internet that have the correction information. There are many statewide
RINEX format compatible GPS stations out there and if someone could write
software to read that data and quasi post process/real-time process the
corrective factors it would give the accuracy on the mapping screen. This
information might also be passed back to findu.com and/or the internet data
stream (or ziplan if implemented) in some manor and identify it as corrected
data. A very interesting idea and it could create sub meter accuracy with
GPS units in the field that might be using differential correction receivers
and/or WAAS. Doing this at the client side would certainly reduce the over
the air information that needs to be exchanged. Go one step further and
apply fuzzy logic and you could put people on a map in the proper lane of a
highway based on their reported direction and/or a dead reckoning of their
position before. I know car mapping systems do this to keep you in the
proper lanes.
	An idea for an over the air correction factor might be to broadcast a
correction factor from a client station that reads the differential
correction from the internet and produces the data message with some sort of
PHG range circle that the mobile units would be able to discern how to use
it and at what point it might not be a good correction factor. If it were
put out as some sort of beacon at a reasonable interval it might not make
things to congested. If the mobile unit were able to keep this correction
factor in memory it would apply it to it's broadcast packets for a certain
time or until it received a newer update message. Of course I don't know if
there is anything in the APRS spec that could allow for this, I am just free
forming ideas.

Brian, N2KGC

-----Original Message-----
From: Jim Lux [mailto:jimlux at earthlink.net]
Sent: Monday, March 14, 2005 10:57 AM
To: Robert Bruninga; aprssig at lists.tapr.org
Subject: Re: [aprssig] differential corrections available to public

----- Original Message -----
From: "Robert Bruninga" <bruninga at usna.edu>
To: <jimlux at earthlink.net>; <aprssig at lists.tapr.org>
Sent: Monday, March 14, 2005 6:32 AM
Subject: Re: [aprssig] differential corrections available to public (insome

> >>> 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

But, if it's on a different frequency, then existing TNCs and radios, which
are single channel centric, wouldn't be able to gain the benefit of more
precise positions.  One hiccup might be the data rate for the RTCM104
corrections.  The commercial units used for wireless links are 9600 bps, but
perhaps a "reduced rate" correction stream might still provide useful data.

There's also the interesting possibility of having the mobile stations
report back their observables, rather than a lat/lon, and at the command
center, the differential corrections are applied. (This is more the
cellphone E911 model).  Same overall goal of providing a real-time tactical
display at a command center.

OTOH, maybe 10 meter precision is good enough for most conceivable APRS
applications, and there are other more worthy uses for the channel
bandwidth. (e.g. do you want precise positions, or do you want speed and

Jim, W6RMK

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