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[aprssig] APRS resolution

Wes Johnston, AI4PX wes at ai4px.com
Fri Oct 5 14:09:13 UTC 2007


When we started with gps units when selective availablility was on, my GPS
would "wander" within a 25 to 35 meter circle.  At that time, 20 meters for
an aprs position made sense.

The day SA was turned off, my GPS started wandering in a 5 meter circle.

WAAS came along and my gps wanders in a 2 or 3 meter circle.

So at a glance it makes sense to have aprs resolution go to 2 meters.
(accuracy and precision belong to the GPS, _resolution_ belongs to the
protocol used to convey the information). While, yes it's true that my gps
can't accurately get to less than one meter - but I'm sure someone has a
survey grade GPS that can... or a differential GPS on a tractor that can.

But wait there's more...

What if I want to place objects that precisely.  The aprs protocol should
allow me to place objects at a level of precision far below what an ordinary
GPS can provide.  Let's not forget that aprs is not synonomous with GPS.
Tactical objects are also in the mix.

For what it's worth....
I use the machine readable derivation of the !DAO! when I convert garmin
rino positions to aprs objects using my script for xastir.  Try explaining
to a lay person why an object placed in a parking lot is within a 60ft
box.... which is two rows over and several parking spaces away from where
the object actually is.

Wes


On 10/5/07, Ken H> <sailingtoo at gmail.com> wrote:
>
> Hey guys - this whole thing about 1 foot accuracy vs 1 foot resolution
> is sorta silly - GPS just doesn't do that good.  The whole thing is a
> lot like measuring with a tape measure, then writing the results to
> 0.001 inch - just is not there.
>
> IF you go to differential GPS you do get to a few feet - but NOT with
> the normal GPS being used for APRS.... OK, I guess there are a few
> folks using differential GPS... but how many?
>
> I've just been involved with APRS for a couple of months and find the
> whole APRS thing amazing.  It's really neat.  I've been involved with
> GPS for many years - from Loran A, to Loran C, to SatNav, then to GPS
> with SA, then FINALLY to GPS with no SA. Is deleting SA the best thing
> that happened during the Clinton years?  Well, other than all the good
> jokes {grinning}
>
> 73 all.
> Ken K9FV
>
> On 10/3/07, Steve Noskowicz <noskosteve at yahoo.com> wrote:
> > Since I know the difference, I take that as a yes.   (:-)  73, Steve
> >
> >
> >
> > Tapio Sokura <oh2kku at iki.fi> wrote:
> > Steve Noskowicz wrote:
> > > I'm no expert on GPS, but what about the random wander
> > > of GPS. Does this level of accuracy need differential
> > > GPS with a local GPS reference transmitter?
> >
> > There's a difference between accuracy and resolution. I'm not going to
> > repeat the definitions, google can do that for you. Anyway your accuracy
> > can be limited by resolution, which is the case with the regular 2
> > decimals of minute APRS. Of course good resolution doesn't guarantee
> > accuracy (or precision), but at least good enough resolution doesn't
> > prevent you from reaching good accuracy.
> >
>
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-- 
In theory there is no difference between practice and theory.
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