[Xastir-dev] Position accuracy

[Gerry Creager N5JXS]

I've read this twice and my brain now hurts...  Generally, one works in 
milliseconds for the OnCore, but that's not burned in stone. 
Conversions are conversions.  Mind the rounding error.

Are you asking what unit to work in?  I'd do it in milliseconds.  Or 
more precisely, I'd end up transforming to a cartesian system and 
working meters.  And then back.

For the sake of accuracy, make sure the following are happening...

1.  Datum should be set to WGS84.  I don't care what map you're using 
now, that reduces the chances of rounding error on the OnCore.
2.  I'd be collecting a position no more frequently than ever 15 sec. 
and more likely, I'd do so every 30 sec.  Don't bother collecting it 
every second.  You will add more noise to the equation than you need. 
GPS data are strongly autocorrelated in the time domain anyway.  Any 
decoupling you can do helps the statistics.
3.  Make sure no data are collected unless HDOP is less than 4, and 
preferentially, GDOP is less than 4.  Using PDOP is also acceptable.
4.  I'd want some multiple of 1024 samples for the average.  This should 
help smooth some of the noise out as you're looking at the chipping rate 
for L1 C/A code at that point.

"Surveys" done with a single code-phase receiver with nothing sitting 
simultanelously on _NEARBY_ geodetic control to act as a QC mechanism 
are frought with peril, and performed daily... often by folks who are 
not pessimistic enough.

73, gerry

Jack Twilley wrote:
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> I'm setting up my Motorola M12 Oncore to perform a site survey so I
> can use it for my stratum-1 time server at home.  A pleasant
> side-effect of this operation is getting a much more accurate location
> for my home station, on the order of hundreds of nanodegrees.
> 
> The map window's position is given in microdegrees, which gave me hope
> until I went to configure my station's position to find I was limited
> to milliminutes (nice unit, eh) for resolution.  Now, since one minute
> is one-sixtieth of a degree, there are 60000 milliminutes to each
> degree.  That's almost as good as 100000 microdegrees to each degree,
> but it got me thinking.
> 
> It's obvious that milliminutes was chosen to make it easier on
> transmitting and the like, but it might be better to use either
> decimals of arbitrary precision or raw xastir coordinates.  It will be
> potentially more precise and will probably save time, code and memory
> in error-checking and conversion.
> 
> Thoughts?
> 
> 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