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[aprssig] differential corrections available to public (insome places)

Jim Lux jimlux at earthlink.net
Mon Mar 14 15:56:52 UTC 2005


----- 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
places)


> >>> 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
direction?)

Jim, W6RMK





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