[Xastir-dev] Position accuracy

[Gerry Creager N5JXS]

My post last evening should have noted that i"m the pedantic academic 
with regard to these things...

Jack Twilley wrote:
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>>>>>>"Gerry" == Gerry Creager <N5JXS <gerry.creager at tamu.edu>> writes:
> 
> 
> Gerry> I've read this twice and my brain now hurts...  Generally, one
> Gerry> works in milliseconds for the OnCore, but that's not burned in
> Gerry> stone. Conversions are conversions.  Mind the rounding error.
> 
> Generally.  SynTac is outputting numbers like this:
> 
> 38.0124733 -122.1346611
> 
> Now, it's quite possible that the decimal representation of whatever
> the Oncore's using internally is whacked.  I'm ignoring that
> possibility because I don't see how to answer that question.

That's decimal degrees.  It's a term of convenience.  The Motorola 
binary format uses ms. of arc.

> 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.
> 
> This is what the xastir configuration file uses:
> 
> STATION_LAT:3800.749N
> STATION_LONG:12208.024W
> 
> This is also what the configure window uses, although the configure
> window is slightly prettier.
> 
> This is *not* what xastir uses internally for maps and the like.
> 
> The point of my post is that we're using different units at different
> times and that it might be easier and nicer to use the same units at
> all times if at all possible.

And we're complying with the wishes of WB4APR who strongly 
believes||believed (ymmv) that APRS entries should be in degrees and 
decimal minutes.

Garmin muddied the waters early with either 3 or 4 places of precision. 
  The NMEA spec calls for a max of 4 places of precision; Trimble 
started using the idea that they'd give 4 places when they had 
differential lock, or WAAS lock, 3 with an autonomous position.

That's all nice, but either one exceeds the achievable accuracy of the 
system for autonomous positions and tends to give the user a false sense 
of security.

> Gerry> For the sake of accuracy, make sure the following are
> Gerry> happening...
> 
> Gerry> 1.  Datum should be set to WGS84.  I don't care what map you're
> Gerry> using now, that reduces the chances of rounding error on the
> Gerry> OnCore.
> 
> I haven't changed the datum, I've left it at the default.

Probably still set to WGS84.  Good.  reduces problems with conversion error.

> Gerry> 2.  I'd be collecting a position no more frequently than ever
> Gerry> 15 sec. and more likely, I'd do so every 30 sec.  Don't bother
> Gerry> collecting it every second.  You will add more noise to the
> Gerry> equation than you need. GPS data are strongly autocorrelated in
> Gerry> the time domain anyway.  Any decoupling you can do helps the
> Gerry> statistics.
> 
> I'm using SynTac, the software that came with the M12, to perform the
> site survey.  ntpd does it the way you describe it should be done, but
> ntpd won't do it for the M12 for a reason which isn't clear to me.

I'll try to remember to pulse Randy and Art about this.

> Gerry> 3.  Make sure no data are collected unless HDOP is less than 4,
> Gerry> and preferentially, GDOP is less than 4.  Using PDOP is also
> Gerry> acceptable.
> 
> Mmm.  I've been unable to examine or change these settings.

Failure of the software to allow adequate configuration, or at least, if 
they're doing it well, failure to tell you they're doing right!

> Gerry> 4.  I'd want some multiple of 1024 samples for the average.
> Gerry> This should help smooth some of the noise out as you're looking
> Gerry> at the chipping rate for L1 C/A code at that point.
> 
> The software recommends setting the sample average to 86400.

Implying they want a full day's worth of 1-sec signals.  More'n likely, 
they're averaging them.  The noise will reduce your chances of a good 
answer...

> 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.
> 
> That's all well and good for those who can afford a Leica AT-504 and
> the like, but for the rest of us, this is the best we can do.
> 
> I'm aware that I won't get a rock-solid location of my GPS antenna
> down to the meter with 100% reliability.  I would like to get the best
> possible position, because that combined with DGPS should permit me to
> create a pretty darned accurate time source.

If you can promote another receiver and do a simultaneous survey on, 
say, a nearby NGS HARN monument, if same monument is within about 20 km 
of where you're doing the truly autonomous survey, I'd not feel too 
uncomfortable telling you that you could simply remove the mean North, 
East and Up errors from the control point survey, from your autonomous 
survey.  Think of it as an attempt at a spatially correlated result, 
sort of a poor man's double difference.

Pedantically yours,
Gerry
> 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	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843