[time-nuts] How does sawtooth compensation work?
kb8tq at n1k.org
Tue Jul 19 20:11:24 EDT 2016
> On Jul 19, 2016, at 7:41 PM, Michael <mikenet213 at 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.
The post process “quality” on L1 only depends a *lot* on just how close you are to a reference station. The ionospheric data is only just so good. The local reference with view of exactly the same sats at exactly the same time is what puts the finishing touch on things.
The other issue is “lanes” in carrier phase / code phase. Unless you have a very well surveyed location, it can be difficult to determine carrier phase directly with L1 only. You fall back to code phase, but that also repeats every so often (lanes).
> 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 :).
>> 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.
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts