Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

Oops, units for TDEV are seconds, not nanoseconds!

On Wed, 20 Mar 2019 at 7:41 pm, Michael Wouters 
wrote:

> Hello Dan,
>
>
> As promised here is a comparison of the ZED-F9P and NEO-M8T
> sawtooth-corrected PPS. The PPS is measured against a 5071 with standard
> tube. Three days of data were used. The 5071 was measured against another
> 5071 and I divided the TDEV by sqrt(2). The 5071 data at shorter than 300s
> is limited by the counter resolution so I didn't show it. The ZED-F9P is
> significantly better than the M8T between 10 and 1000 s but much the same
> past 1s.
>
> Cheers
> Michael
>
> On Tue, 19 Mar 2019 at 8:02 am, Dan Kemppainen 
> wrote:
>
>> Bob,
>>
>> I recalled the plot you posted a few days ago. Nice plot on the F9P,
>> BTW. Thanks for posting that.
>>
>> I don't supposed you happened to be recording a similar plot with a L1
>> only Ublox part at the same time by chance, did you? I think it would be
>> interesting to compare a 6T or M8 part to the F9P. (I lack a standard
>> good enough to compare them to...)
>>
>> Thanks!
>> Dan
>>
>>
>>
>>
>> On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
>> > Message: 2
>> > Date: Mon, 18 Mar 2019 12:26:55 -0400
>> > From: Bob kb8tq
>> > To: Discussion of precise time and frequency measurement
>> >   
>> > Subject: Re: [time-nuts] Frequency Ensemble
>> > Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
>> > Content-Type: text/plain; charset=utf-8
>> >
>> > Hi
>> >
>> > There are a variety of GPS devices that put out a PPS. uBlox makes
>> some, there
>> > are a number of other companies that do so. The PPS comes out modulo
>> the local
>> > timebase. On a precision part, there is a ?sawtooth correction? message
>> that also
>> > comes out to further quantify the best guess time of that pulse.
>> >
>> > Noise wise, even with correction you are lucky to get a one second ADEV
>> in the
>> > 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like
>> the F9P,
>> > you might do a bit better than that.
>> >
>> > The ADEV of your other sources at short tau will be much better than
>> the GPS PPS
>> > noise. As you average out over long periods, the GPS will win the race.
>> >
>> > Bob
>>
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>> and follow the instructions there.
>>
>
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Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

Hello Dan,

As promised here is a comparison of the ZED-F9P and NEO-M8T
sawtooth-corrected PPS. The PPS is measured against a 5071 with standard
tube. Three days of data were used. The 5071 was measured against another
5071 and I divided the TDEV by sqrt(2). The 5071 data at shorter than 300s
is limited by the counter resolution so I didn't show it. The ZED-F9P is
significantly better than the M8T between 10 and 1000 s but much the same
past 1s.

Cheers
Michael

On Tue, 19 Mar 2019 at 8:02 am, Dan Kemppainen 
wrote:

> Bob,
>
> I recalled the plot you posted a few days ago. Nice plot on the F9P,
> BTW. Thanks for posting that.
>
> I don't supposed you happened to be recording a similar plot with a L1
> only Ublox part at the same time by chance, did you? I think it would be
> interesting to compare a 6T or M8 part to the F9P. (I lack a standard
> good enough to compare them to...)
>
> Thanks!
> Dan
>
>
>
>
> On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
> > Message: 2
> > Date: Mon, 18 Mar 2019 12:26:55 -0400
> > From: Bob kb8tq
> > To: Discussion of precise time and frequency measurement
> >   
> > Subject: Re: [time-nuts] Frequency Ensemble
> > Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
> > Content-Type: text/plain; charset=utf-8
> >
> > Hi
> >
> > There are a variety of GPS devices that put out a PPS. uBlox makes some,
> there
> > are a number of other companies that do so. The PPS comes out modulo the
> local
> > timebase. On a precision part, there is a ?sawtooth correction? message
> that also
> > comes out to further quantify the best guess time of that pulse.
> >
> > Noise wise, even with correction you are lucky to get a one second ADEV
> in the
> > 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like
> the F9P,
> > you might do a bit better than that.
> >
> > The ADEV of your other sources at short tau will be much better than the
> GPS PPS
> > noise. As you average out over long periods, the GPS will win the race.
> >
> > Bob
>
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[time-nuts] Frequency Ensemble

[Mark Sims]

Lady Heather can output RINEX (v2.xx and 3.xx files) for most receivers that 
output pseudorange and/or carrier phase data.   I have tested it with the Ublox 
4T, 5T, 6T, 8T, F9P, and F9T receivers.  Also the Venus RTK receivers, Trimble 
NetRS,  Ashtech Z12,, NVS08, Trimble timing receivers, and a few others.

With my horrible antenna location L1/L2 devices yield positions error ellipses 
around 10 mm. lat/lon and 50mm altitude  L1 only devices are around 100 mm.   
People with better antenna locations have reported error ellipses is the low mm 
range.  

The Trimble receivers are currently a bit wonky since I have some issues 
calculating pseudoranges from the Trimble data.  I didn't have Trimble issues 
before CSRS-PPP "upgraded" their processing software.

The version of Heather that I am currently working on can apply sawtooth 
corrections to the PPS output measured by a counter such as the TAPR-TICC in 
order to make a better "paper clock".

--
>  As Bob says, the ublox chipsets from generation 6 onwards are probably
the best value as they can be configured to output the raw
measurements for precise position/time.
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Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

The f9p is about 3 times better than  the ublox m8t. Will post a plot
later..

Cheers
Michael

On Tue, 19 Mar 2019 at 9:00 am, Bob kb8tq  wrote:

> Hi
>
> ….. and here I was wondering if it had ever gone out :) Glad it made it.
>
> No, I did not run any single frequency units beside it. It is much better
> than any similar plot
> I’ve done in the past on an L1 only part.
>
> Indeed, as long as you just look at the first ~ 100 seconds, the
> “reference standard” is less of an issue.
> A fairly typical OCXO will hang in there at 2 ppt or so out to a couple
> hundred seconds.
> You can get a very good idea of what’s what simply by going out that far.
> Indeed you might spend
> some “quality time” finding a working example of something like a MT-260.
> That’s part of the fun ….
> Some of the better parts will be 5 MHz units. A HP 5335 can come in handy
> to turn 5 MHz into
> 10 MHz ….. there are a lot of ways to get that done.
>
> Yes LH and a TAPPR TICC would also help in the process …. still need those
> 25 orders :)
>
> Bob
>
> > On Mar 18, 2019, at 3:29 PM, Dan Kemppainen 
> wrote:
> >
> > Bob,
> >
> > I recalled the plot you posted a few days ago. Nice plot on the F9P,
> BTW. Thanks for posting that.
> >
> > I don't supposed you happened to be recording a similar plot with a L1
> only Ublox part at the same time by chance, did you? I think it would be
> interesting to compare a 6T or M8 part to the F9P. (I lack a standard good
> enough to compare them to...)
> >
> > Thanks!
> > Dan
> >
> >
> >
> >
> > On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
> >> Message: 2
> >> Date: Mon, 18 Mar 2019 12:26:55 -0400
> >> From: Bob kb8tq
> >> To: Discussion of precise time and frequency measurement
> >>  
> >> Subject: Re: [time-nuts] Frequency Ensemble
> >> Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
> >> Content-Type: text/plain;charset=utf-8
> >> Hi
> >> There are a variety of GPS devices that put out a PPS. uBlox makes
> some, there
> >> are a number of other companies that do so. The PPS comes out modulo
> the local
> >> timebase. On a precision part, there is a ?sawtooth correction? message
> that also
> >> comes out to further quantify the best guess time of that pulse.
> >> Noise wise, even with correction you are lucky to get a one second ADEV
> in the
> >> 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like
> the F9P,
> >> you might do a bit better than that.
> >> The ADEV of your other sources at short tau will be much better than
> the GPS PPS
> >> noise. As you average out over long periods, the GPS will win the race.
> >> Bob
> >
> > ___
> > time-nuts mailing list -- time-nuts@lists.febo.com
> > To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> > and follow the instructions there.
>
>
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Re: [time-nuts] Frequency Ensemble

[Bob kb8tq]

Hi

You can access an “NTRIP stream” from various free sites. There is some 
mumbo jumbo on some of them to get an account. There are streams dedicated
to “clock and orbit correction”. CLK11 and CLK93 are two fairly common ones.

Since NTRIP is a real time product (as in < 30 seconds delay) it can be used 
for 
“right now” sort of correction. RTKLIB is probably the most common way to get
the local receiver and an NTRIP stream combined. 

If you just want to play with it ntrip.itsware.net  
port 2101 is a free / no registration source
of the CLK11 stream. They also have various other “correction product” streams
you can play with. 

In theory (though I can in no way prove it yet) you should be able to reduce 
the 
timing errors associated with the broadcast clock and orbit estimates by about 
an order
of magnitude. It is *abundantly* unclear what that translates into in 
nanoseconds due
to a whole lot of layers everything goes through….. 

The NTRIP products are all aimed at surveying applications so there is a lot of 
translation 
involved to get to the sort of time numbers we like to deal with. It should 
help, the big
question is how much (10 is unlikely …. sqrt(10) maybe …. sqrt(10) / 2 … who 
knows ….). 

Bob

> On Mar 18, 2019, at 1:32 PM, Anders Wallin  
> wrote:
> 
> If you search for "GNSS time transfer" you will find a lot of papers etc.
> For example these might get you started:
> https://www.bipm.org/ws/CCTF/TAI_TRAINING/Allowed/Fundamentals/Training-2012-GNSS-Defraigne.pdf
> https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7909843
> 
> I tried to collect some tools for PPP post-processing on github:
> https://github.com/aewallin/ppp-tools
> I am not sure what (open) software exists for common-view analysis...
> PPP uses satellite clock-corrections and orbit-corrections from an IGS
> data-centre. They have "ultra rapid" and "rapid" products (=downloadable
> files) that are available with some days or hours of delay.
> The "final" products can have up to two weeks (?) of delay.
> With a dual-frequency receiver the ionosphere delay can be removed
> ('ionosphere-free L1/L2 linear combination') and my understanding is the
> troposphere-delay (water content) is one of the larger remaining
> uncertainties.
> 
> AW
> 
> 
> On Mon, Mar 18, 2019 at 5:03 PM Rodger via time-nuts <
> time-nuts@lists.febo.com> wrote:
> 
>> 
>> Regarding your comments on collecting raw time data from GPS and post
>> processing it.  Can you provide any reference info, links, etc. with more
>> detail on that topic?
>> Clearly I'd need a GPS that outputs the proper raw messaging and the
>> software for processing it.  I'm somewhat familiar with the techniques
>> involved to improve GPS position data, but hadn't thought about it as much
>> for timing.
>> 
>> Thanks,
>> 
>> Rodger
>> 
>> 
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Re: [time-nuts] Frequency Ensemble

[Bob kb8tq]

Hi

….. and here I was wondering if it had ever gone out :) Glad it made it. 

No, I did not run any single frequency units beside it. It is much better than 
any similar plot
I’ve done in the past on an L1 only part. 

Indeed, as long as you just look at the first ~ 100 seconds, the “reference 
standard” is less of an issue. 
A fairly typical OCXO will hang in there at 2 ppt or so out to a couple hundred 
seconds. 
You can get a very good idea of what’s what simply by going out that far. 
Indeed you might spend
some “quality time” finding a working example of something like a MT-260. 
That’s part of the fun ….
Some of the better parts will be 5 MHz units. A HP 5335 can come in handy to 
turn 5 MHz into
10 MHz ….. there are a lot of ways to get that done. 

Yes LH and a TAPPR TICC would also help in the process …. still need those 25 
orders :) 

Bob

> On Mar 18, 2019, at 3:29 PM, Dan Kemppainen  wrote:
> 
> Bob,
> 
> I recalled the plot you posted a few days ago. Nice plot on the F9P, BTW. 
> Thanks for posting that.
> 
> I don't supposed you happened to be recording a similar plot with a L1 only 
> Ublox part at the same time by chance, did you? I think it would be 
> interesting to compare a 6T or M8 part to the F9P. (I lack a standard good 
> enough to compare them to...)
> 
> Thanks!
> Dan
> 
> 
> 
> 
> On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:
>> Message: 2
>> Date: Mon, 18 Mar 2019 12:26:55 -0400
>> From: Bob kb8tq
>> To: Discussion of precise time and frequency measurement
>>  
>> Subject: Re: [time-nuts] Frequency Ensemble
>> Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
>> Content-Type: text/plain;charset=utf-8
>> Hi
>> There are a variety of GPS devices that put out a PPS. uBlox makes some, 
>> there
>> are a number of other companies that do so. The PPS comes out modulo the 
>> local
>> timebase. On a precision part, there is a ?sawtooth correction? message that 
>> also
>> comes out to further quantify the best guess time of that pulse.
>> Noise wise, even with correction you are lucky to get a one second ADEV in 
>> the
>> 1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like the 
>> F9P,
>> you might do a bit better than that.
>> The ADEV of your other sources at short tau will be much better than the GPS 
>> PPS
>> noise. As you average out over long periods, the GPS will win the race.
>> Bob
> 
> ___
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe, go to 
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.


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Re: [time-nuts] Frequency Ensemble

[Dan Kemppainen]

Bob,

I recalled the plot you posted a few days ago. Nice plot on the F9P, 
BTW. Thanks for posting that.


I don't supposed you happened to be recording a similar plot with a L1 
only Ublox part at the same time by chance, did you? I think it would be 
interesting to compare a 6T or M8 part to the F9P. (I lack a standard 
good enough to compare them to...)


Thanks!
Dan




On 3/18/2019 1:00 PM, time-nuts-requ...@lists.febo.com wrote:

Message: 2
Date: Mon, 18 Mar 2019 12:26:55 -0400
From: Bob kb8tq
To: Discussion of precise time and frequency measurement

Subject: Re: [time-nuts] Frequency Ensemble
Message-ID:<88460ea2-6034-48d1-a77a-fed1f824d...@n1k.org>
Content-Type: text/plain;   charset=utf-8

Hi

There are a variety of GPS devices that put out a PPS. uBlox makes some, there
are a number of other companies that do so. The PPS comes out modulo the local
timebase. On a precision part, there is a ?sawtooth correction? message that 
also
comes out to further quantify the best guess time of that pulse.

Noise wise, even with correction you are lucky to get a one second ADEV in the
1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like the F9P,
you might do a bit better than that.

The ADEV of your other sources at short tau will be much better than the GPS PPS
noise. As you average out over long periods, the GPS will win the race.

Bob


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Re: [time-nuts] Frequency Ensemble

[Tim Lister]

On Mon, Mar 18, 2019 at 8:03 AM Rodger via time-nuts
 wrote:
>
> Tom,
>
> Thanks for the explanation of clock ensembles.  That answered a few
> questions I've had for a while.
> Regarding your comments on collecting raw time data from GPS and post
> processing it.  Can you provide any reference info, links, etc. with more
> detail on that topic?
> Clearly I'd need a GPS that outputs the proper raw messaging and the
> software for processing it.  I'm somewhat familiar with the techniques
> involved to improve GPS position data, but hadn't thought about it as much
> for timing.

As Bob says, the ublox chipsets from generation 6 onwards are probably
the best value as they can be configured to output the raw
measurements for precise position/time. There is info on configuring
these at OpenStreetMap and rtklibexplorer's wiki and blog respectively
and I also added a howto on my blog on doing this
(https://adventuresinprecision.space/howtos/precise-gps-positions/)

The OpenTTP project has code for capturing the raw data (ublox M8's
are supported right now, I am working on adding support for -6T and
the main developers are looking at adding support for the new ublox F9
chips) and software for doing the GPS Common View processing. I am
just starting to dig into this but need to get the last bit of the
ublox6 support working in order to give it some valid data to play
with. The other easily available GPS receiver that OpenTTP supports is
the Trimble Resolution T which I think has been "end-of-lifed" by
Trimble which has pros of being available cheap on ebay and cons of
not being able to buy a brand new "official" board unlike with the
ublox 8 and 9 receivers.

>
> Thanks,
>
> Rodger
>

Cheers,
Tim

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Re: [time-nuts] Frequency Ensemble

[Michael Wouters]

This project www.openttp.org provides software for post-processing of raw
data for time-transfer from some currently available single frequency
receivers. It provides CGGTTS and RINEX, the latter for use with the
various PPP services. There are some tools in there too for doing eg common
view and all-in-view comparisons. If you want to try it out, the develop
branch in the GitHub repo is the place to start. I'll be talking about it
at IFCS-EFTF.

Cheers
Michael


On Tue, 19 Mar 2019 at 5:08 am, Anders Wallin 
wrote:

> If you search for "GNSS time transfer" you will find a lot of papers etc.
> For example these might get you started:
>
> https://www.bipm.org/ws/CCTF/TAI_TRAINING/Allowed/Fundamentals/Training-2012-GNSS-Defraigne.pdf
> https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7909843
>
> I tried to collect some tools for PPP post-processing on github:
> https://github.com/aewallin/ppp-tools
> I am not sure what (open) software exists for common-view analysis...
> PPP uses satellite clock-corrections and orbit-corrections from an IGS
> data-centre. They have "ultra rapid" and "rapid" products (=downloadable
> files) that are available with some days or hours of delay.
> The "final" products can have up to two weeks (?) of delay.
> With a dual-frequency receiver the ionosphere delay can be removed
> ('ionosphere-free L1/L2 linear combination') and my understanding is the
> troposphere-delay (water content) is one of the larger remaining
> uncertainties.
>
> AW
>
>
> On Mon, Mar 18, 2019 at 5:03 PM Rodger via time-nuts <
> time-nuts@lists.febo.com> wrote:
>
> >
> > Regarding your comments on collecting raw time data from GPS and post
> > processing it.  Can you provide any reference info, links, etc. with more
> > detail on that topic?
> > Clearly I'd need a GPS that outputs the proper raw messaging and the
> > software for processing it.  I'm somewhat familiar with the techniques
> > involved to improve GPS position data, but hadn't thought about it as
> much
> > for timing.
> >
> > Thanks,
> >
> > Rodger
> >
> >
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>
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Re: [time-nuts] Frequency Ensemble

[Anders Wallin]

If you search for "GNSS time transfer" you will find a lot of papers etc.
For example these might get you started:
https://www.bipm.org/ws/CCTF/TAI_TRAINING/Allowed/Fundamentals/Training-2012-GNSS-Defraigne.pdf
https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7909843

I tried to collect some tools for PPP post-processing on github:
https://github.com/aewallin/ppp-tools
I am not sure what (open) software exists for common-view analysis...
PPP uses satellite clock-corrections and orbit-corrections from an IGS
data-centre. They have "ultra rapid" and "rapid" products (=downloadable
files) that are available with some days or hours of delay.
The "final" products can have up to two weeks (?) of delay.
With a dual-frequency receiver the ionosphere delay can be removed
('ionosphere-free L1/L2 linear combination') and my understanding is the
troposphere-delay (water content) is one of the larger remaining
uncertainties.

AW


On Mon, Mar 18, 2019 at 5:03 PM Rodger via time-nuts <
time-nuts@lists.febo.com> wrote:

>
> Regarding your comments on collecting raw time data from GPS and post
> processing it.  Can you provide any reference info, links, etc. with more
> detail on that topic?
> Clearly I'd need a GPS that outputs the proper raw messaging and the
> software for processing it.  I'm somewhat familiar with the techniques
> involved to improve GPS position data, but hadn't thought about it as much
> for timing.
>
> Thanks,
>
> Rodger
>
>
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Re: [time-nuts] Frequency Ensemble

[Bob kb8tq]

Hi

There are a variety of GPS devices that put out a PPS. uBlox makes some, there 
are a number of other companies that do so. The PPS comes out modulo the local
timebase. On a precision part, there is a “sawtooth correction” message that 
also
comes out to further quantify the best guess time of that pulse. 

Noise wise, even with correction you are lucky to get a one second ADEV in the 
1 to 2 ppb range with a typical L1 receiver. With a L1 / L2 device like the F9P,
you might do a bit better than that. 

The ADEV of your other sources at short tau will be much better than the GPS PPS
noise. As you average out over long periods, the GPS will win the race.

Bob

> On Mar 18, 2019, at 9:56 AM, Rodger via time-nuts  
> wrote:
> 
> Tom,
> 
> Thanks for the explanation of clock ensembles.  That answered a few
> questions I've had for a while.
> Regarding your comments on collecting raw time data from GPS and post
> processing it.  Can you provide any reference info, links, etc. with more
> detail on that topic?
> Clearly I'd need a GPS that outputs the proper raw messaging and the
> software for processing it.  I'm somewhat familiar with the techniques
> involved to improve GPS position data, but hadn't thought about it as much
> for timing.
> 
> Thanks,
> 
> Rodger
> 
> -Original Message-
> From: time-nuts  On Behalf Of Tom Van Baak
> Sent: Sunday, March 17, 2019 3:00 PM
> To: Discussion of precise time and frequency measurement
> 
> Subject: Re: [time-nuts] Frequency Ensemble
> 
>> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 
>> Rb clock together and take the average to control 1 "master clock" and
> have better stability ?
>> like what BIPM or NIST doing.
>> I have search about ensemble system but I have no idea how much 
>> advantage I get from some clock that I already have.Thank You Anton
> 
> Anton,
> 
> The rule-of-thumb is that, *under the right conditions*, N clocks will
> perform sqrt(N) better than 1 clock.
> 
> So yes, NIST, USNO, PTB, BIPM -- all the big boys -- use ensemble
> techniques. But the key is that they mostly use cesium clocks, not OCXO or
> Rb clocks from eBay. Laboratory cesium standards don't suffer from frequency
> drift. The other key is that the clocks are independent. Under these
> conditions one can obtain sqrt(N) advantage.
> 
> The problem with using cheap OCXO or Rb clocks is that they drift, and this
> drift may depend on make / model / environment; all of which are possibly
> common mode for you. This means the full sqrt(N) assumption is likely not
> valid.
> 
> The problem with using GPSDO is that they are not independent clocks. In
> fact, they aren't clocks at all: they are just noisy radio receivers,
> implementing "time transfer" from the USNO GPS master clock, which is
> related to but not equal to UTC(USNO) which is related to but not equal to
> UTC itself. There's a lot of common mode error amongst a set of GPSDO. This
> means the full sqrt(N) assumption is likely not valid.
> 
> Those who use GPS for highest accuracy tend not to use GPSDO. Instead they
> just collect raw timing information and post-process it some hours to weeks
> later. That is, they want to know
>what time-it-was-precisely
> rather than
>what time-it-is-approximately.
> A GPSDO only does the latter.
> 
> /tvb
> 
> 
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Re: [time-nuts] Frequency Ensemble

[Rodger via time-nuts]

Tom,

Thanks for the explanation of clock ensembles.  That answered a few
questions I've had for a while.
Regarding your comments on collecting raw time data from GPS and post
processing it.  Can you provide any reference info, links, etc. with more
detail on that topic?
Clearly I'd need a GPS that outputs the proper raw messaging and the
software for processing it.  I'm somewhat familiar with the techniques
involved to improve GPS position data, but hadn't thought about it as much
for timing.

Thanks,

Rodger

-Original Message-
From: time-nuts  On Behalf Of Tom Van Baak
Sent: Sunday, March 17, 2019 3:00 PM
To: Discussion of precise time and frequency measurement

Subject: Re: [time-nuts] Frequency Ensemble

> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 
> Rb clock together and take the average to control 1 "master clock" and
have better stability ?
> like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much 
> advantage I get from some clock that I already have.Thank You Anton

Anton,

The rule-of-thumb is that, *under the right conditions*, N clocks will
perform sqrt(N) better than 1 clock.

So yes, NIST, USNO, PTB, BIPM -- all the big boys -- use ensemble
techniques. But the key is that they mostly use cesium clocks, not OCXO or
Rb clocks from eBay. Laboratory cesium standards don't suffer from frequency
drift. The other key is that the clocks are independent. Under these
conditions one can obtain sqrt(N) advantage.

The problem with using cheap OCXO or Rb clocks is that they drift, and this
drift may depend on make / model / environment; all of which are possibly
common mode for you. This means the full sqrt(N) assumption is likely not
valid.

The problem with using GPSDO is that they are not independent clocks. In
fact, they aren't clocks at all: they are just noisy radio receivers,
implementing "time transfer" from the USNO GPS master clock, which is
related to but not equal to UTC(USNO) which is related to but not equal to
UTC itself. There's a lot of common mode error amongst a set of GPSDO. This
means the full sqrt(N) assumption is likely not valid.

Those who use GPS for highest accuracy tend not to use GPSDO. Instead they
just collect raw timing information and post-process it some hours to weeks
later. That is, they want to know
what time-it-was-precisely
rather than
what time-it-is-approximately.
A GPSDO only does the latter.

/tvb


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Re: [time-nuts] Frequency Ensemble

[Attila Kinali]

On Sun, 17 Mar 2019 10:58:49 -0400
Bob kb8tq  wrote:

> The first thing you need (after all the standards) is a way to do precision 
> comparisons of all your devices. There are an infinite number of ways to do
> this. Let’s say you buy 26 TICC’s to do the job ( TAPPR needs 25 ordered to
> get the next batch going so that would solve two problems at once :) ). A
> PPS from a single source with good short term stability (maybe an OCXO) goes
> into one side of all of them. A pps from a DUT goes in the other side (yes
> there are other ways …TAPPR needs that order ….).
> You then have 26 devices each reporting how a single standard compares to 
> the “main OCXO”. 


I would use here a variant of DMTD: Instead of comparing pairs
of standards, have one offset frequency generator feed multiple
mixers, one for each standard. This way, the standards can be
compared to eachother more easily. Downside of this is, that
the precision of this comparison depends on the stability of
the distribution of the offset frequency signal. Though, under
the assumption that ground loops can be kept in check, this
should be easier to ensure than having pair wise comparisons
not drifting away too much. When using TICCs, with their high
measurement rate, I would also go for an offset frequency in
the order of 1kHz instead of the customary 1-10Hz. This will
help getting away from the flicker noise region and also give
a much higher slope of the signal to work with, reducing the
white noise of the measurement. Additonally, this should
also give higher precision at tau = 1s, as the system is now
averaging down from 1ms instead of 100ms (when using 10Hz),
which could potentially give a boost of a factor of sqrt(100ms/1ms) = 10.

Attila Kinali

-- 
The bad part of Zurich is where the degenerates
throw DARK chocolate at you.

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Re: [time-nuts] Frequency Ensemble

[Tom Van Baak]

> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb clock
> together and take the average to control 1 "master clock" and have better 
> stability ?
> like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much advantage
> I get from some clock that I already have.Thank You Anton 

Anton,

The rule-of-thumb is that, *under the right conditions*, N clocks will perform 
sqrt(N) better than 1 clock.

So yes, NIST, USNO, PTB, BIPM -- all the big boys -- use ensemble techniques. 
But the key is that they mostly use cesium clocks, not OCXO or Rb clocks from 
eBay. Laboratory cesium standards don't suffer from frequency drift. The other 
key is that the clocks are independent. Under these conditions one can obtain 
sqrt(N) advantage.

The problem with using cheap OCXO or Rb clocks is that they drift, and this 
drift may depend on make / model / environment; all of which are possibly 
common mode for you. This means the full sqrt(N) assumption is likely not valid.

The problem with using GPSDO is that they are not independent clocks. In fact, 
they aren't clocks at all: they are just noisy radio receivers, implementing 
"time transfer" from the USNO GPS master clock, which is related to but not 
equal to UTC(USNO) which is related to but not equal to UTC itself. There's a 
lot of common mode error amongst a set of GPSDO. This means the full sqrt(N) 
assumption is likely not valid.

Those who use GPS for highest accuracy tend not to use GPSDO. Instead they just 
collect raw timing information and post-process it some hours to weeks later. 
That is, they want to know
what time-it-was-precisely
rather than
what time-it-is-approximately.
A GPSDO only does the latter.

/tvb


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Re: [time-nuts] Frequency Ensemble

[John Ackermann N8UR]

Just a couple of other measurement options on top of Bob's excellent 
description.


You could half the number of TICCs by using them in timestamp mode with 
a common 10 MHz clock serving as the reference -- in that mode you can 
do two simultaneous measurements with each TICC.  There are some 
hardware and software hooks built into the TICC to allow multiple units 
to operate synchronously.  (But we still need 25 orders :-) ).


Second, as Bob mentions any PPS measurment is likely to be noisier at 1, 
10, or 100 seconds than the sources you want to measure.  If you decide 
to do one round of measurements every 1000 seconds, it's thoroughly 
practical to use some sort of switch matrix to measure each of the DUT 
in turn with a single TICC or other counter.  In other words, once each 
1000 seconds switch each DUT in turn to the TICC START input long enough 
to get one measurement.


That's what I designed the TAPR TASS coax switch system for -- it lets 
you switch 8 DUT to one common output under USB control, and the 
controller supports up to 4 switches so you can make a really big matrix 
if you want (my setup at home has jacks for 24 DUT and 8 references).


A project like this is ripe for all sorts of hardware hackery!

John


On 3/17/19 10:58 AM, Bob kb8tq wrote:

Hi

The answer is (of course) yes. The somewhat more detailed answer is that it 
actually is a practical basement sort of thing *if*
you have the space. I was headed off to do this and changed course. That’s just 
my lack of focus rather than it being an
un-doable sort of thing. What’s below is sort of a random walk through doing it.

The first thing you need (after all the standards) is a way to do precision 
comparisons of all your devices. There are an infinite
number of ways to do this. Let’s say you buy 26 TICC’s to do the job ( TAPPR 
needs 25 ordered to get the next batch going
so that would solve two problems at once :) ). A PPS from a single source with 
good short term stability (maybe an OCXO) goes
into one side of all of them. A pps from a DUT goes in the other side (yes 
there are other ways …TAPPR needs that order ….).
You then have 26 devices each reporting how a single standard compares to the 
“main OCXO”.

As. this chugs along you get a whole bunch of timestamps telling you how far 
your main OCXO is from each and every one of your
“standards”. Since 10 of them are GPSDO’s they likely will be doing some very 
similar things. For the moment let’s ignore that
and assume they are independent / uncorrelated sources. Since things like the 
Rb’s are free running data comes in spread out
over the second, collecting data and comparing it isn’t exact.

Once a second, you round up all the data and make a guess about what is 
correct. If everything is independent and equally
noisy and … and … and … your guess could be sqrt(N) better than any individual 
source. With 26 sources, you would be a bit
over 5X better than what you started with. You then diddle the EFC on an OCXO 
to put it inline with that estimate (yes that
may mean a 27th TICC).

Stepping back, there was a bit of hand waving going on there :

One assumption is that the TICC measurement noise at one second is better than 
the noise of the sources. That probably is
only true at much longer time spans (say >100 seconds). You can either upgrade 
the measurement or accept the longer time
span. (TICC is about 1x10^-10 at 1 sec, 1x10^-12 at 100 sec, and 1x10^-13 at 
1,000 sec)

The “independent / uncorrelated sources” part is very suspect with a group of 
(possibly same manufacture) GPSDO’s in the mix.
A burp here or there in the way GPS L1 (I’m guessing they are L1) is behaving 
can easily swing them all at the same time.. Things
like temperature (Rb’s have temperature dependence) also can get into the mix.

One alternate that might actually be easier to deal with: Run 10 free running 
OCXO’s and the 16 Rb’s. Add some number of
GPS modules (with PPS outputs) to the mix. That way you will not be constantly 
fighting the unknown loop dynamics of the
GPSDO’s.

Next, somebody is likely to raise their hand and ask if the noise really *is* 
gaussian (and thus goes down as fast as the square root).
The same things that get into making the sources correlated (and some other 
things) contribute to them being non-gaussian in
this regard. Simple answer is you likely will not quite get to the 5X 
improvement advertised above. One thing you *can* do for
  some effects is try to learn them via cross comparison. That gets into the 
guts of your software and how you decide to do this.

What the final result is at any specific tau will be very much a “that depends” 
sort of thing. The Rb’s have an ADEV curve that
should drop by sqrt(tau). ( 10X better ADEV at 100X tau). Most GPSDO’s fairly 
flat ADEV out to some “hump” and then they
follow GPS down from that point. If you actually steer an OCXO, the control 
loop will get into the act as well.

You might also look into ganging up the TICC’s 

Re: [time-nuts] Frequency Ensemble

[paul swed]

Anton
Others will reply but my sense is that there is little advantage in doing
that.
Assuming everything is the same in all 10 systems then you would be
measuring the receiver behaviors at a given moment. I simply am unclear
that there is an advantage.
Regards
Paul.

On Sat, Mar 16, 2019 at 11:00 PM Anton Moehammad via time-nuts <
time-nuts@lists.febo.com> wrote:

> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb
> clock together and take the average to control 1 "master clock" and have
> better stability ?like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much advantage
> I get from some clock that I already have.Thank YouAnton
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Re: [time-nuts] Frequency Ensemble

[Bob kb8tq]

Hi

The answer is (of course) yes. The somewhat more detailed answer is that it 
actually is a practical basement sort of thing *if* 
you have the space. I was headed off to do this and changed course. That’s just 
my lack of focus rather than it being an
un-doable sort of thing. What’s below is sort of a random walk through doing 
it. 

The first thing you need (after all the standards) is a way to do precision 
comparisons of all your devices. There are an infinite
number of ways to do this. Let’s say you buy 26 TICC’s to do the job ( TAPPR 
needs 25 ordered to get the next batch going
so that would solve two problems at once :) ). A PPS from a single source with 
good short term stability (maybe an OCXO) goes
into one side of all of them. A pps from a DUT goes in the other side (yes 
there are other ways …TAPPR needs that order ….).
You then have 26 devices each reporting how a single standard compares to the 
“main OCXO”. 

As. this chugs along you get a whole bunch of timestamps telling you how far 
your main OCXO is from each and every one of your
“standards”. Since 10 of them are GPSDO’s they likely will be doing some very 
similar things. For the moment let’s ignore that
and assume they are independent / uncorrelated sources. Since things like the 
Rb’s are free running data comes in spread out
over the second, collecting data and comparing it isn’t exact. 

Once a second, you round up all the data and make a guess about what is 
correct. If everything is independent and equally 
noisy and … and … and … your guess could be sqrt(N) better than any individual 
source. With 26 sources, you would be a bit
over 5X better than what you started with. You then diddle the EFC on an OCXO 
to put it inline with that estimate (yes that
may mean a 27th TICC). 

Stepping back, there was a bit of hand waving going on there : 

One assumption is that the TICC measurement noise at one second is better than 
the noise of the sources. That probably is 
only true at much longer time spans (say >100 seconds). You can either upgrade 
the measurement or accept the longer time
span. (TICC is about 1x10^-10 at 1 sec, 1x10^-12 at 100 sec, and 1x10^-13 at 
1,000 sec)

The “independent / uncorrelated sources” part is very suspect with a group of 
(possibly same manufacture) GPSDO’s in the mix.
A burp here or there in the way GPS L1 (I’m guessing they are L1) is behaving 
can easily swing them all at the same time.. Things 
like temperature (Rb’s have temperature dependence) also can get into the mix. 

One alternate that might actually be easier to deal with: Run 10 free running 
OCXO’s and the 16 Rb’s. Add some number of 
GPS modules (with PPS outputs) to the mix. That way you will not be constantly 
fighting the unknown loop dynamics of the 
GPSDO’s. 

Next, somebody is likely to raise their hand and ask if the noise really *is* 
gaussian (and thus goes down as fast as the square root).
The same things that get into making the sources correlated (and some other 
things) contribute to them being non-gaussian in
this regard. Simple answer is you likely will not quite get to the 5X 
improvement advertised above. One thing you *can* do for 
 some effects is try to learn them via cross comparison. That gets into the 
guts of your software and how you decide to do this.

What the final result is at any specific tau will be very much a “that depends” 
sort of thing. The Rb’s have an ADEV curve that 
should drop by sqrt(tau). ( 10X better ADEV at 100X tau). Most GPSDO’s fairly 
flat ADEV out to some “hump” and then they
follow GPS down from that point. If you actually steer an OCXO, the control 
loop will get into the act as well. 

You might also look into ganging up the TICC’s to run 4 channels in one block. 
Unfortunately that would get the total order 
below what TAPPR needs … :) That would let you compare three standards against 
the master in one block and cut the 
total down to nine “pods” of two each. 

Indeed a fun project. There are a lot of papers out there on various aspects of 
it. Jim Barnes and David Allan authored a lot
of them, either together or independently. Since they both worked at NIST, the 
papers are in the public domain. 

Have fun 

Bob



> On Mar 16, 2019, at 9:16 PM, Anton Moehammad via time-nuts 
>  wrote:
> 
> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb 
> clock together and take the average to control 1 "master clock" and have 
> better stability ?like what BIPM or NIST doing.
> I have search about ensemble system but I have no idea how much advantage I 
> get from some clock that I already have.Thank YouAnton 
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Re: [time-nuts] Frequency Ensemble

[Mike Cook]

> Le 17 mars 2019 à 02:16, Anton Moehammad via time-nuts 
>  a écrit :
> 
> Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb 
> clock together and take the average to control 1 "master clock" and have 
> better stability ?like what BIPM or NIST doing.

I think the answer to this is probably no but it would make a nice project. I 
say no because your GPSDO will already be benefiting the from the clocks in GPS 
constellation that are being steered, probably indirectly) by the UTC(NIST) 
clocks, steered by the AT1 time scale, created from a whole bunch of cesium , 
maser and optical clocks  , so your GPSDO is the equivalent of a master clock. 
This means that you only need one…well three to verify that one is not going on 
the blink. A GPSDO or Rubidium stability will probably be in the range of a few 
parts in 10^11 - 10^12. 
Measuring the phase offsets of a bunch go those and applying corrections to a 
another free running clock might buy you a zero but it is questionable I think. 
You are probably constrained by the quality of the chosen ‘master’. If there 
was a distinct advantage I would expect to see products on the market using 
this approach. I haven’t heard of any.

> I have search about ensemble system but I have no idea how much advantage I 
> get from some clock that I already have.Thank YouAnton 
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"Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une 
petite et provisoire sécurité, ne méritent ni liberté ni sécurité."
Benjimin Franklin


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[time-nuts] Frequency Ensemble

[Anton Moehammad via time-nuts]

Hi Everyone,I like to know if it possible to run let say 10 GPSDO, 16 Rb clock 
together and take the average to control 1 "master clock" and have better 
stability ?like what BIPM or NIST doing.
I have search about ensemble system but I have no idea how much advantage I get 
from some clock that I already have.Thank YouAnton 
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