Re: [time-nuts] Positional accuracy of the M12+T
Position hold mode allows them to solve N(sat) equations with one
unknown instead of N(sat) equations with four unknowns.
That assumes you have good data for your location.
How good is the find-my-location code?
What happens to the timing results if the assumed location is off?
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In message 20070103183054.8B5F9BE01@ip-64-139-1-69.sjc.megapath.net, Hal Murr
ay writes:
Position hold mode allows them to solve N(sat) equations with one
unknown instead of N(sat) equations with four unknowns.
That assumes you have good data for your location.
How good is the find-my-location code?
It averages 10000 position samples and that is appearantly
good enough.
I played with plotting the per satelite residuals as a function
of a deliberately distorted set of coords for position hold
and it is quite insentitive.
The main effect of a slightly off position seems to be a diurnal
component to the estimated uncertainty more than an actual
component to the PPS output.
If you record the per sat residuals along with long+lat for the
sats and average the residual per az/elev gridpoint for a day
or two, you will be able to see a systematic effect but translating
this into a tangible delta to the held coordinates has so far
escaped me.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hal,
The M12M-T has a Self-Survey function that normally gets you down to a
meter or so. Unfortunately this function takes data for 10,000 fixes so
it takes about 2.5 hours to complete. This function was created by
Motorola back when SA was on and you really needed a long sample to get
a decent position. Nowadays I normally just let WinOncore12 or SynTaC
take data for 15-20 minutes, and the average positions on multiple runs
are always within a couple of meters of each other. You will see several
ns of 1PPS offset due to slight changes in the Position Hold Position,
but this is easily removed using the 1PPS offset commands if you are
trying to match to another receiver. In the big scheme of things these
offsets are pretty much below the noise level.
Randy Warner
Senior Applications Engineer
Synergy Systems, LLC
randy@synergy-gps.com
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Hal Murray
Sent: Wednesday, January 03, 2007 10:31 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Positional accuracy of the M12+T
Position hold mode allows them to solve N(sat) equations with one
unknown instead of N(sat) equations with four unknowns.
That assumes you have good data for your location.
How good is the find-my-location code?
What happens to the timing results if the assumed location is off?
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In message D6D8D27B5219D649A005529978F791371CCE0B@SBSERVER.Synergy-GPS.sbs, "
Randy Warner" writes:
You will see several
ns of 1PPS offset due to slight changes in the Position Hold Position,
but this is easily removed using the 1PPS offset commands if you are
trying to match to another receiver.
Are you sure these offsets are constant ? My impression is that they
depend on the satellite geometry ?
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
PHK,
You're right, they will move a little as the satellites move, but this
1PPS shift is really pretty minor compared to the fixed offset unless
you have horrible visibility. And for all practical purposes the diurnal
effects pretty well drown out any shifts unless your Position Hold
coordinates are REALLY off.
Remember, in the work I do I don't worry too much about single digit
nanoseconds, so I tend to gloss over little stuff like that. However, if
my 1PPS is flying back and forth +/-100ns or so, THEN I get
interested......
Randy
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Poul-Henning Kamp
Sent: Wednesday, January 03, 2007 11:26 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Positional accuracy of the M12+T
In message
D6D8D27B5219D649A005529978F791371CCE0B@SBSERVER.Synergy-GPS.sbs, "
Randy Warner" writes:
You will see several
ns of 1PPS offset due to slight changes in the Position Hold Position,
but this is easily removed using the 1PPS offset commands if you are
trying to match to another receiver.
Are you sure these offsets are constant ? My impression is that they
depend on the satellite geometry ?
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by
incompetence.
time-nuts mailing list
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From: "Poul-Henning Kamp" phk@phk.freebsd.dk
Subject: Re: [time-nuts] Positional accuracy of the M12+T
Date: Wed, 03 Jan 2007 19:26:02 +0000
Message-ID: 29724.1167852362@critter.freebsd.dk
In message D6D8D27B5219D649A005529978F791371CCE0B@SBSERVER.Synergy-GPS.sbs, "
Randy Warner" writes:
You will see several
ns of 1PPS offset due to slight changes in the Position Hold Position,
but this is easily removed using the 1PPS offset commands if you are
trying to match to another receiver.
Are you sure these offsets are constant ? My impression is that they
depend on the satellite geometry ?
Having looked at the way my position moves around during the day (the surveying
mode in the M12M-T windows software plots it live as it goes along) I am fully
convinced that it does move around due to geometry. A position error cranks out
as a different time error in the pseudorange depending on the bearing. Toss on
the multipath (which is my main error - got to move the antenna).
You need to look on the quality of the survey to determin if you need a longer
survey period. You need to measure for at least a certain period to be able to
be sure that the quality you requested it valid (confidence range is narrow
enought), but you should really maintain the averaging until you have reached
your surveying goal.
Cheers,
Magnus