[Xastir-dev] Position accuracy

[Jack Twilley]

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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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