Hello Michael > Thinking out loud, I wonder how bad L1-only, post-processed, would be for > time-nuts use? Especially with a timing-grade antenna (e.g. the common > Trimble Bullet). Dual frequency is great when your receiver has the potential > to move, you have to resolve carrier phase ambiguity quickly, or you don't > have a reference station (CORS) nearby. (O.T. I was out hiking in Washington > state recently, and *accidentally* happened upon my local CORS station, so I > guess that's no issue for me :-)). But for many time-nuts, I wonder how badly > a timing-grade antenna, something with raw carrier phase output (which you > get very cheaply these days), and a stable enough local clock to allow you > average out local weather. >
There was a related discussion a month ago https://www.febo.com/pipermail/time-nuts/2016-June/098484.html and I posted a plot https://www.febo.com/pipermail/time-nuts/attachments/20160616/cb2801b0/attachment.png which might be helpful. As you can see, the improvement from raw 1 pps to a full carrier-phase solution with IGS rapid orbits and clock solutions is not enormous, just a factor of 4 in stability. BIPM does a bit better with TAI PPP, eg ftp://ftp2.bipm.org/pub/tai/publication/timelinks/taippp/1606/ maybe 30% or so. (To be completely fair, the raw 1 pps is from a $10K GPS receiver with a very good sawtooth correction so there may be a bit more of an improvement at averaging times less than 1000 s or so) There are a few more plots of L1 receiver performance at: http://www.openttp.org/scripts/blog.php > For time-nut use, I don't see any harm in using post-processing for > evaluation/measurement of clocks. This is exactly how most of the clocks contributing to UTC are linked together across the world. Cheers Michael On Wed, Jul 20, 2016 at 9:41 AM, Michael <mikenet...@comcast.net> wrote: > Thanks Tom, Bob, and Mark (wrote my response to Tom first, but didn't hit > send)! > > I've actually been collecting some *ancient* dual-frequency geodetic gear to > play with, some of which have external clock inputs (or should be hackable). > I've read a lot, but I wasn't sure what people were typically referring to on > this list. Thanks for the overview...that helps me connect the dots between > the time-nuts and survey/geodetic GPS worlds. > > For time-nut use, I don't see any harm in using post-processing for > evaluation/measurement of clocks. Won't get you something usable in > real-time, for sure, but if you're already collecting weeks of data, don't > see any harm in waiting for precise orbit and clock solutions to become > available for post processing. And it might tell me how far off you are in a > 24-hour PPP solution. Which I guess means you'd need to be very stable in the > <=24hr region. > > Thinking out loud, I wonder how bad L1-only, post-processed, would be for > time-nuts use? Especially with a timing-grade antenna (e.g. the common > Trimble Bullet). Dual frequency is great when your receiver has the potential > to move, you have to resolve carrier phase ambiguity quickly, or you don't > have a reference station (CORS) nearby. (O.T. I was out hiking in Washington > state recently, and *accidentally* happened upon my local CORS station, so I > guess that's no issue for me :-)). But for many time-nuts, I wonder how badly > a timing-grade antenna, something with raw carrier phase output (which you > get very cheaply these days), and a stable enough local clock to allow you > average out local weather. > > I guess while it's fascinating to me...wonder if it has any use in practice > compared to a simple, autonomous, real-time, L1-only receiver? I mean, I'm > interested in measuring my local tropospheric and ionospheric delays. But > then again, I am an aspiring time-(and maybe GPS)-nut :). > > Michael > >> Hi Michael, >> >> About #3 below... >> >> There are dozens of technical papers about all this in the PTTI, FCS, UFFC, >> EFTF journals. Google for words like: GPS carrier-phase dual-frequency >> time-transfer geodetic-receiver IGS precise point positioning PPP >> >> I don't have a link to a handy 1-page summary, but someone else on the list >> might. Otherwise skim the first ten papers you find and you'll pick up the >> concepts of high-precision time transfer. >> >> The basic idea is that high-end geodetic-grade receivers often have an >> external 10 or 20 MHz clock input (and maybe no internal clock at all). You >> give it your best lab clock and all then all GPS signal processing and SV >> measurements are based on your fancy clock. The output of the receiver is a >> stream of these measurements, not necessarily a physical 1PPS or 10 MHz (as >> with a GPSDO). >> >> So you can see there's no such thing as sawtooth error here, because you're >> not transferring some internal clock to some external clock via a TIC; there >> is only the one clock; your clock. >> >> All this measurement data is then post-processed, hours or days later, so >> that some of the learned errors in the GPS system can be backed out. This >> would include SV clock and orbit errors, as well as tropo/ionospheric >> errors. The goal in cases like this are to find out how good your lab clock >> is (was), not so much to steer anything in realtime. >> >> These receivers also tend to measure GHz carrier phase instead of (or in >> addition to) MHz code phase. And they often capture both L1 (1575.42 MHz) >> and L2 (1227.60 MHz) instead of L1, which not only doubles the effective >> number of SV received, but also is used to help compensate for >> speed-of-light variations through the ionosphere. With all this attention to >> precision, you then sometimes enter the realm of fancy temperature >> controlled antennas and special RF cables, maybe even temperature controlled >> receivers. It's all a very slippery slope. >> >> /tvb > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
