[time-nuts] nuts about position

Tim Lister listertim at gmail.com
Sun Apr 29 20:45:51 EDT 2018


On Wed, Apr 25, 2018 at 8:38 AM, Tim Lister <listertim at gmail.com> wrote:
> On Wed, Apr 25, 2018 at 7:56 AM, 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.
>
> Hi Tom, list, as another researcher who is also interested in
> telescope positions (!) I have done this for personal use at home with
> a ublox 6T and 53532A antenna to see what I got. I was logging in the
> UBX binary format with the raw (carrier phase) measurements turned on
> and then converting it to RINEX and using the NRC's CSRS-PPP online
> service which is one of the few that will take single frequency L1
> only data. The results based on approx. 41.5 hours of data and which
> were post-processed 21 days later (so that they used the IGS Final
> products rather than the Rapids or Ultra Rapids) were Sigmas(95%) of
> 0.105 m, 0.089 m, 0.217 m in latitude, longitude and ellipsoidal
> height respectively. I was quite impressed with the results without
> use of the L2 frequency to correct for the ionosphere etc.

It has taken me quite a bit longer that I had hoped but I have finally
published a writeup and howto of collecting raw ublox data, converting
it to RINEX and how to do (or get the NRC experts to do) the
post-processing. It's at a new website I have setup:
https://adventuresinprecision.space/howtos/precise-gps-positions/
Please let me know any comments or suggestions you have to improve it
or make it more comprehensible and comprehensive.

I have also been doing some experiments with RTK solutions with local
precise base stations and receiving NTRIP correction messages over
RTCM. This is looking very promising but I need to do some more
experiments (and as detailed in the howto, wait for some more precise
products to be available to compare it with...)

Cheers,
Tim



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