
Comparison of 4 different NTP stratum 1 clocks - zdw
http://www.febo.com/time-freq/ntp/stats/index.html
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rektide
Major hard on for this work, amazingly awesome geekery going on here.

Given the clock drift, it seems like our man could use a PPS injector/killer
alike what Google uses to slowly shift their time-synchronous data-centers
through situations like leap seconds. Something's got to be done about the
drift, but making a big event out of it would be, IMO, generally a bad thing,
whereas taking a couple days to undrift off the added milliseconds would be
ideal. Just- don't stop logging drift, in absolute time too.
[http://www.wired.com/wiredenterprise/2012/11/google-
spanner-...](http://www.wired.com/wiredenterprise/2012/11/google-spanner-
time/)

Superb work. Would love some pictures of this setup! Some of the coolest 486's
in use!

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panic
"Major hard on"? Really?

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rektide
Gracious no, but to be safe I've contacted the author & he's agreed that a
restraining order is the best course of action for us.

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jlgaddis
I occasionally read up on some of the Arduino stuff that's happening, projects
people are coming up with, etc. and I've noticed that vendors are selling GPS
receivers and related items[0] that can be used in various projects.

I also have a fairly elaborate NTP setup within our datacenters and throughout
our networks and have wondered if it would be possible to basically build my
own stratum 1 server out of some of this equipment. I still haven't figured it
out yet, but I hope that someday soon it is possible; as a geek, I think it
would be very awesome to have my own stratum 1 timeservers.

[0]:
[https://www.sparkfun.com/categories/4](https://www.sparkfun.com/categories/4)

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jcurbo
You could build your own or you can buy them from vendors, it is actually very
common practice to hook up a GPS-powered stratum 1 time server to a network.
(e.g.
[http://www.spectracomcorp.com/ProductsServices/TimingSynchro...](http://www.spectracomcorp.com/ProductsServices/TimingSynchronization/NetworkTimeServers/tabid/112/Default.aspx))

~~~
dbarlett
CDMA is another option for stratum 1 [http://www.endruntechnologies.com/gps-
cdma.htm](http://www.endruntechnologies.com/gps-cdma.htm)

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jrockway
K1JT has collected some interesting data regarding the propagation from WWV
and WWVH:
[http://www.physics.princeton.edu/pulsar/K1JT/HFTOA_1.pdf](http://www.physics.princeton.edu/pulsar/K1JT/HFTOA_1.pdf)

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adestefan
Joe Taylor's work in weak signal propagation is phenomenal. His protocols are
the basis of all the weak signal modes. You can use his methods to make
contacts all around the world on milliwatts of power. The work started as a
way to improve earth-moon-earth contacts. Yes, that's bouncing radio waves off
the freaking moon and the path loss is around 275 dB for VHF frequencies that
are commonly used.

~~~
ISL
Joe Taylor's work is, in general, impressive. Among scientists, he's in a
fairly exclusive club:

[http://www.nobelprize.org/nobel_prizes/physics/laureates/199...](http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/)

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adestefan
If you're interested in timing systems, then this article about upgrading the
clock system at JHU APL, which is one of the UTC ensemble systems, will
interest you. It's pretty damn stable since the upgrades.

[http://techdigest.jhuapl.edu/TD/td3201/32_01-Miranian.pdf](http://techdigest.jhuapl.edu/TD/td3201/32_01-Miranian.pdf)

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majke
~1ms drift for a GPS clock sounds like a lot to me. Have I missed something?

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ajross
It's graphing "offset", not "drift". The actual fluctuations are more like
0.1ms it seems. The DC bias is presumably inherent in the measurement
mechanism (asymmetric IP delays, etc...).

Best guess, anyway. Honestly the data looks surprisingly uninteresting. There
isn't much there in the way of notable observation or correlation with other
effects...

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majke
Even 0.1ms drift sounds like an awful lot of time for a serious clock.

[edit] if 1ns is 11.8 inches of wire (at the speed of light) then 0.1 ms is 30
km of wire!

[1] [http://highscalability.com/blog/2012/3/1/grace-hopper-to-
pro...](http://highscalability.com/blog/2012/3/1/grace-hopper-to-programmers-
mind-your-nanoseconds.html)

~~~
VLM
300 KM is not too unbelievable WRT multipath distortion thru the ionosphere.

You can game it a bit by averaging or building a clock that "knows" it cannot
move or that its oscillator is more stable than ionospheric scintillation.

Fundamentally the whole system works by having a constellation of satellites
with precision clocks squirting out the "exact" time and where they think they
are WRT each other. So you're 0.01 light seconds from satellite 3 and 0.005
light seconds from satellite 4 blah blah repeat at least 4 times and draw some
circles of a certain light second diameter and there you are. But its going to
be inherently sensitive to anomalous propagation, the kind of thing ham radio
operators actually like but drives GPS nuts. It would work a lot better if
radio waves traveled at a constant speed, but if your system design demands
nine decimal places of constant speed and the world won't deliver, well..

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mikeash
Typical real-world GPS accuracy is on the order of 10m, so that means that the
error in the time _differences_ cannot be more than about 30ns typically.

To see anything close to a 0.1ms/300km offset, you'd need _every signal_ to be
shifted by almost the same amount. To maintain 10m position accuracy while
suffering from 300km time error, all of the signals would have to be delayed
to within 0.003% of each other. That's an extremely tall order for a natural
phenomenon, considering that the satellites are positioned such that signals
can come from any direction in the sky.

It's vastly more plausible that this is simply a result of troublesome
hardware _after_ the core GPS clock hardware gets done doing its thing.

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mrottenkolber
Fascinating. I don't have a real need for super accurate clocks but was always
fond of the idea of having one. Be it for measuring concurrent behaviour or
just being able to judge the truth value of the clocks we trust every day.

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ChuckMcM
Old joke, "A man with one watch always knows what time it is, a man with two
watches never does."

Chris Kuethe
([http://www.ualberta.ca/~ckuethe/gps/](http://www.ualberta.ca/~ckuethe/gps/))
did some work on making accurate time out of the 1pps signal from a GPS
receiver, which I replicated at home (using a tossed out Garmin receiver)
which allows me to provide accurate time to my local network of machines.

