[time-nuts] nuts about position

Ole Petter Ronningen opronningen at gmail.com
Thu Apr 26 05:11:46 EDT 2018


I just ran a quick test - a Ublox 6T was configured to output the RXM-RAW
messages every second and logging for some 14 hours using U-Center. The
.ubx-file was converted to RINEX using teqc.exe, and uplaoded to NrCAN PPP.
NrCAN PPP will process single frequency observations and correct using
ionospheric maps I *believe*. (If I am wrong, ignore this post!)

I also continously log observations from a Trimble NetRS dual frequency GPS
receiver, hooked up to the same antenna. I trust the PPP-calculated
position accuracy of this receiver to within low double digits to high
single digit millimeters.

(For comparison, the report from NrCAN gives a 95% error ellipse on the
UBlox as semi-major: 2.494dm, semi-minor: 1.760dm. The corresponding
numbers for the NetRS is semi-major: 1.245cm, semi-minor: 0.760cm - the
reports are usually below 10 mm in both axis. Note difference in units.)

Comparing the calculated ECEF coordinates from the ppp-results using
Pythagoras gives me a distance of 76 cm - 53 cm if we simply ignore height.

This is with a survey grade antenna in a good location. It might be
possible to shrink this to 30 cm and still have confidence in the results,
but I think it would not be easy. I would guess a lot more data would be
required, and it is also possible that delaying processing until more
accurate ionospheric maps are available could help.

I will reprocess in a couple of days and see if theres much of a difference.


On Wed, Apr 25, 2018 at 4:56 PM, Tom Van Baak <tvb at leapsecond.com> wrote:

> List -- I had a recent query by a researcher who would like to pinpoint
> the location of his telescope(s) within 0.3 meters. Also (he must be a true
> scientist) he wants to do this on-the-cheap. He may have timing
> requirements as well, but that's another posting.
> So I toss the GPS question to the group. Surely some of you have crossed
> the line from precise time to precise location?
> How easy, how cheap, how possible is it to obtain 0.3 m accuracy in 3D
> position?
> When we run our GPSDO in survey mode how accurate a position do we get
> after an hour, or even 24 or 48 hours? And here I mean accurate, not
> stable. Have any of you compared that self-reported, self-survey result
> against an independently measured professional result or known benchmark?
> Do you know if cheap ublox 5/6/7/8 series receivers are capable of 1 foot
> accuracy given enough time?
> If not, what improvement would -T models and RINEX-based web-service
> post-processing provide?
> It that's still not close enough to 0.3 m, is one then forced to use more
> expensive multi-frequency (L1/L2) or multi-band (GPS, GLONASS, Galileo) to
> achieve this level of precision? If so, how cheaply can one do this? Or is
> the learning curve more expensive than just hiring an survey specialist to
> make a one-time cm-level measurement for you?
> Something tells me 1 foot accuracy in position is possible and actually
> easier than 1 ns accuracy in time. I'm hoping some of you can help
> recommend solution(s) to the researcher's question or shed light on this
> interesting challenge.
> Thanks,
> /tvb
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