[time-nuts] GPS 1PPS ultimate accuracy

Bob Camp kb8tq at n1k.org
Mon Jan 12 18:16:01 EST 2015


Actually it’s a bit worse than you might expect. 

The uncorrected sawtooth will give you about 20 ns of wander. At the one day level, GPS without some sort of ionosphere help (like a dual frequency receiver) will add another 10 ns or so to that. Net, your pps is spread over a 30 ns range. 

The output of the OCXO is at 5 MHz, the Rb is at 10 MHz. Maybe you double the OCXO to 10 MHz. It only has a zero crossing every 100 ns. (200 ns if you don’t double it).  You will have a 100 ns “dead zone” in your counter. That assumes it’s synchronous. If it’s a ripple counter, who knows what it will do. 

Net result, You have 30 ns of error, and a 100 ns resolution. Net is 130 ns. You will hit 1x10^-9 at  a bit over 100 seconds. You will get to 1x10^-12 at around 13,000 seconds. Since it’s a dead zone, averaging really does not help you much. In fact, long averages will mess up the ADEV computations.  If you have a goal that resolution should be 5X your data, then you get to 1x10^-12 data at around 80,000 seconds. For a good ADEV number, you would like about 100 samples. This gets you out to a 100 day run. Even for a minimalist number, you are running for > 10 days. Any time you have a power interruption, the process re-starts. 

With things like 5335’s running around for cheap prices, I would suggest doing this with a counter. You are going to spend a lot of days getting very much data. Your time’s got to be worth something …. 


> On Jan 12, 2015, at 9:10 AM, Attila Kinali <attila at kinali.ch> wrote:
> Ciao Andrea,
> On Mon, 12 Jan 2015 11:59:26 +0100
> Andrea Baldoni <erm1eaae7 at ermione.com> wrote:
>> The sampling interval could come from a (long time based on a) sawtooth
>> uncorrected PPS from a cheap GPS, a sawtooth corrected from a good one (perhaps
>> the Lucent GPSDO), or a computer using NTP.
> The GNSS Timing AppNote for the LEA6-T receiver[1] will give you an idea
> what jitter you get with GPS. Please be aware that these measurements
> were done with an antenna located at a _good_ position (ontop of a 4 story
> building with no other high buildings around). Unless you have a simlarly
> good location you will have worse performance.
> Said Jackson reported some time ago that he got around 1us of jitter for
> a GPS receiver (i presume it was either a LEA5-T or a LEA6-T) behind a
> window. After he averaged the position for a long time (several days)
> manually and stored that in the receiver he got much better performance
> (sorry i cannot find the mail at the moment, you have to look for it in
> the archives yourself).
> NTP will give you a jitter in the range of 1-100ms, depending on
> your internet connection and its conguestion. On a local network
> based NTP system, you can expect jitter in the range of 10-100us IIRC.
>> Each of these sources should reach a goal stability (say, 1 part in 10^13)
>> after averaging them on a different (and very high I suppose) number of
>> seconds (averaging them for an infinity number of seconds should give the
>> stability of the underlying reference clock, but I'm willing to stop sooner...).
>> I know there's no reason to go 1E-13 when the Milliren couldn't go that far,
>> but the DUT may be also something else like a FE-5680A).
> To get to 1e-13 with GPS (assuming 1-10ns jitter) you need somewhere around
> 10k to 100k seconds. At these time scales, the temperature dependent deviation
> of your OCXO is likely to dominate your measurement. I would rather do
> a two step measurement. If you have a FE-5680A measure its drift with a 
> tau in the 100ks-200ks region. Then use the FE-5680A as refrence to measure
> the drift of the OCXO in 10s-1000s timescales. If you do both continuously,
> you can apply some math and get out pretty good numbers (see three cornered
> hat method)
> Additional to GPS jitter you also have the deviation of GPS time in
> respect to TAI/UTC. This has been in recent years below 5ns (GPS vs UTC(USNO)).
> But because GPS time is steered to be close to UTC it will oscillate slightly
> around it. How much, i do not know. (But then the deviation between the
> different UTC realizations is larger) [2]
>> The sawtooth uncorrected GPS receiver may never yeld a good stability in the
>> short term, but in the long one it should as well because the internal clock
>> jitter would average results.
> It would average out if and only if the sawtooth correction would be completely
> independent of anything else. But it isn't. This results in effects where the
> cycle to cycle jitter is quite low, but there is a large offset in the sawtooth
> correction. This is know as "hanging bridges" in the GNSS world.
>> By the way, the stability of the TAI is known or, because it's
>> the reference one, it has zero deviation for definition (so you can reach
>> its ultimate stability through GPS really only at the infinity...)?
> There is an uncertainty number attached to TAI, but i dont know any numbers
> from the top of my head. I'm sure it is mentioned in the BIPM report somewhere.
> 				Attila Kinali
> [1] http://www.u-blox.com/images/downloads/Product_Docs/Timing_AppNote_%28GPS.G6-X-11007%29.pdf
> [2] "GPS time and its steering to UTC(USNO)", presentatin by Edward Powers, 2009
> http://www.gps.gov/multimedia/presentations/2009/09/ICG/powers.pdf
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
>                 -- Miss Matheson, The Diamond Age, Neil Stephenson
> _______________________________________________
> 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 mailing list