[Xastir-dev] Position accuracy

[Gerry Creager (N5JXS)]

I'll respond to this one after the 0830 meeting where I act like a 
meteorology prof...

73, gerry

Jack Twilley wrote:
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>>>>>>"Gerry" == Gerry Creager <N5JXS <gerry.creager at tamu.edu>> writes:
> 
> 
> Gerry> My post last evening should have noted that i"m the pedantic
> Gerry> academic with regard to these things...
> 
> But of course. :-)
> 
> [... Oncore spits out 38.0124733 -122.1346611 ...]
> 
> Gerry> That's decimal degrees.  It's a term of convenience.  The
> Gerry> Motorola binary format uses ms. of arc.
> 
> I knew it was decimal degrees, but I didn't know about the
> milliseconds of arc bit.
> 
> Gerry> Are you asking what unit to work in?  I'd do it in
> Gerry> milliseconds.  Or more precisely, I'd end up transforming to a
> Gerry> cartesian system and working meters.  And then back.
> 
> [... xastir configuration file uses 3800.749N 12208.024W ...]
> 
> Gerry> And we're complying with the wishes of WB4APR who strongly
> Gerry> believes||believed (ymmv) that APRS entries should be in
> Gerry> degrees and decimal minutes.
> 
> We can still comply with the wishes of WB4APR by only *transmitting*
> degrees and decimal minutes, while using milliseconds of arc or
> whatever for internal use and configuration files, right?
> 
> Gerry> Garmin muddied the waters early with either 3 or 4 places of
> Gerry> precision. The NMEA spec calls for a max of 4 places of
> Gerry> precision; Trimble started using the idea that they'd give 4
> Gerry> places when they had differential lock, or WAAS lock, 3 with an
> Gerry> autonomous position.
> 
> My GPS II+ and GPS III+ both display microdegrees.  I haven't looked
> into whether that translated to milliseconds of arc.
> 
> Gerry> That's all nice, but either one exceeds the achievable accuracy
> Gerry> of the system for autonomous positions and tends to give the
> Gerry> user a false sense of security.
> 
> [... useful tips for site surveys ...]
> 
> [... datum set to WGS84 ...]
> 
> Jack> I haven't changed the datum, I've left it at the default.
> 
> Gerry> Probably still set to WGS84.  Good.  reduces problems with
> Gerry> conversion error.
> 
> [... sample every fifteen to thirty seconds ...]
> 
> Jack> I'm using SynTac, the software that came with the M12, to
> Jack> perform the site survey.  ntpd does it the way you describe it
> Jack> should be done, but ntpd won't do it for the M12 for a reason
> Jack> which isn't clear to me.
> 
> Gerry> I'll try to remember to pulse Randy and Art about this.
> 
> It's probably something about the firmware.  
> 
> [... require good satellite geometry ...]
> 
> [... use a multiple of 1024 samples for the average ...]
> 
> Jack> The software recommends setting the sample average to 86400.
> 
> Gerry> Implying they want a full day's worth of 1-sec signals.  More'n
> Gerry> likely, they're averaging them.  The noise will reduce your
> Gerry> chances of a good answer...
> 
> They're using a running average, according to the documentation.
> 
> Gerry> "Surveys" done with a single code-phase receiver with nothing
> Gerry> sitting simultanelously on _NEARBY_ geodetic control to act as
> Gerry> a QC mechanism are frought with peril, and performed
> Gerry> daily... often by folks who are not pessimistic enough.
> 
> Jack> That's all well and good for those who can afford a Leica AT-504
> Jack> and the like, but for the rest of us, this is the best we can
> Jack> do.  I'm aware that I won't get a rock-solid location of my GPS
> Jack> antenna down to the meter with 100% reliability.  I would like
> Jack> to get the best possible position, because that combined with
> Jack> DGPS should permit me to create a pretty darned accurate time
> Jack> source.
> 
> Gerry> If you can promote another receiver and do a simultaneous
> Gerry> survey on, say, a nearby NGS HARN monument, if same monument is
> Gerry> within about 20 km of where you're doing the truly autonomous
> Gerry> survey, I'd not feel too uncomfortable telling you that you
> Gerry> could simply remove the mean North, East and Up errors from the
> Gerry> control point survey, from your autonomous survey.  Think of it
> Gerry> as an attempt at a spatially correlated result, sort of a poor
> Gerry> man's double difference.
> 
> Not likely to happen in this practical world. ;-)
> 
> Gerry> Pedantically yours, Gerry 73, gerry
> 
> Jack.
> - -- 
> Jack Twilley
> jmt at twilley dot org
> http colon slash slash www dot twilley dot org slash tilde jmt slash
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University
Office: 979.458.4020  FAX: 979.847.8578
Cell: 979.229.5301    Pager: 979.228.0173