[time-nuts] nuts about position

J. Grizzard elfchief-timenuts at lupine.org
Wed Apr 25 16:57:18 EDT 2018

I think to really be confident about a position you really need the 
dual-frequency data (or that data from a nearby reference station), 
otherwise you could end up in a situation where you're consistent, but 
that consistency has a bias. IIRC, anyhow -- I'm not sure how the math 
actually works out.

Anyhow, I play around with PPP stuff on occasion, and the last run I did 
was in November using the Novatel OEM628 kit that was briefly available 
for cheap on eBay, and the included 702-GG antenna (which, conveniently, 
has calibrations available). Running a day's worth of data through 
CSRS-PPP produced sigmas (95%) of 0.004m latitude, 0.008m longitude, and 
0.024m in elevation. I've done some shorter runs since then that appear 
to fall in that same range ... I really need to do a few more full runs 
and see what kind of variance there is.

At any rate, theoretically you can get ^^^ that close, anyhow. CSRS even 
takes solid earth tides into account, though I didn't do that because I 
was never able to figure out which specific type of solid earth tide 
data I needed. I imagine there's still some issues with any given datum 
being somewhat imperfect, as far as altitude is concerned, and I don't 
really know how to correctly deal with that if exact altitude matters. 
Maybe we should all just agree to use XYZ/ECEF coordinates for 
everything and give up on this whole altitude thing altogether... ;)

(As an aside, I've been tempted to get someone to come professionally 
survey my antenna and tell me where it _actually_ is, so I could see how 
well I could actually do with my GPS kit, but I imagine it's pretty 
expensive -- anyone happen to know what getting that kind of thing done 
actually ends up costing?)


On 4/25/18 8:38 AM, Tim Lister 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.
>> Thanks,
>> /tvb
> Cheers,
> Tim
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