[time-nuts] GPS 1PPS, phase lock vs frequency lock, design

[Bob kb8tq]

Hi

A little about the “why” of all this ….

Few of us have ideal antenna locations. Even what we consider to be “really good” is still
quite a ways from ideal. A concrete tower 50’ above everything else with a clear view of 
the sky down to zero degrees in every direction is “ideal”.  Due to it’s location in view of 
the Rocky Mountains, NIST simply can’t put up an “ideal” antenna ….. (they have pointed
this out a number of times …). 

If you are talking about a commercial product. The best guess is that it will be set up with
an antenna location that is utter junk in some cases. There simply will not be any other 
location available. 

The result of this is that we get signals that come in from multiple paths. Each one has a 
delay associated with it. There is no antenna magic that will reject them all. Signal strength 
is *not* a good indicator in terms of multipath. Using a single satellite does not eliminate the
problem. 

Since these mulitpath signals inherently are “at the wrong time”, they mess up the timing solution.
They also mess up navigation. The receiver tries to deal with them as best it can. That 
process can be a bit random. Even the best signal you can get is still pretty low SNR. 

One would *assume* that the firmware makes decisions about how good a signal is and
weights it accordingly into the solution. How accurate this process is (if it is even present) 
is unclear. There’s at least a couple of million lines of code going into that module. Sorting
out what it all does …. yikes ….

Bob

> On Jun 3, 2019, at 2:55 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> I think you may be missing the most likely primary contributor.
> 
> Each GPS receiver (and, thus, each GPSDO) tracks a constantly-changing "constellation" of satellites.  Each rx switches constellations as it sees fit, depending on reception conditions as it sees them, and no two receivers will track the same constellations, switching at the same time, even if they are fed from the same antenna.  Most GPS receivers switch constellations quite frequently (at least several times per minute, sometimes much more frequently) even with strong signals.  At each switch, "GPS time" as computed by each rx changes by a few nS (maybe more, depending on the quality of the unit's "time solution" algorithms and the signal environment).  You can see this dynamically if you run each receiver into a separate instance of LH (there seems to be some finite latency in LH's constellation reports, but I'm not sure how much -- perhaps Mark will comment).
> 
> So, for short tau (averaging times), there is quite a bit of jitter on each receiver's time solution, which is *not* correlated between receivers even if they are fed from the same antenna.  (I.e., the jitter is almost all differential, very little common-mode.)
> 
> Of course, we already knew that raw GPS data at low tau has bad jitter (compared to the jitter after averaging for 1000+ seconds, which is what we think of naively as the precision of GPS), so all this should come as no surprise).
> 
> Some GPS receivers let you switch into "reduced switching" or even "single satellite" modes, but this turns out to be much less helpful than you might think with real-world signals.
> 
> Hanging bridges can cause significant phase jumps, but they should be much less frequent than most of the changes you are reporting.
> 
> Best regards,
> 
> Charles
> 
> 
> ed wrote:
> 
>> I think I have a setup that exemplifies this situation, and some
>> anecdotes. A while back, I acquired two "identical" GPSDO boards, and
>> boxed them up together, with common environment, power supply, and GPS
>> signal via a splitter. I've mentioned this thing a couple of times here,
>> and had planned to do some experimenting to see how they track each
>> other, if crosstalk at the front-ends may have effects, etc. I haven't
>> done any of this yet beyond looking at the relative phase of the 10 MHz
>> outputs on a scope, over various periods from minutes to days.
>> 
>> I had expected them to agree quite closely after enough running time,
>> and be quite stable, but was disappointed. The phase drifts up and down,
>> sometimes very, slowly, over an hour or so, and sometimes quickly,
>> noticeable over a few minutes observation time. After some pondering on
>> why identical units with the same GPS signal should drift like this, I
>> realized that besides possible front-end interactions, and noise, that
>> this was likely mostly from the sawtooth effect - the discreteness of
>> phase comparison of the 1 PPS vs 10 MHz counting, and discreteness of
>> OCXO tuning voltage via the DACs. They each responded differently, for a
>> number of reasons. More time and voltage resolution would help, of
>> course, but they will never perfectly agree, even in this idealized
>> setup with identical units. Virtually identical, that is - there's no
>> such thing as truly identical units, and operating in identical conditions.
> 
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.