Timing Distribution in Mountainous Terrain
Ralph Smith wrote:
On Sep 9, 2010, at 8:01 PM, Rick Karlquist wrote:
I would like to point out that the environmental sensitivities of
the 5071A are unmeasureable, and the measurement threshold is
far below 5.8E-14. I would estimate that the 5071A (and ONLY the 5071A
among commercial clocks) could get the job done provided that you could
compare its frequency to GPS to the stated accuracy. This would
be using the 5071 as a secondary standard. You still need to
deal with the short term stability of the 5071A, depending on
your system needs. JPL uses H masers as flywheels.
I would imagine the cost of a 5071A per radio station would make the check writers swallow hard, and adding an H Maser into the mix would really get their attention. Especially if you need redundancy.
there's also the possibility mentioned earlier of using a lower quality
standard at each station and flying (driving) a higher quality clock
(5071) around often enough to keep them trued up.
If it's far enough in the future.. Hg ion traps have a lot of
potential.. smaller, lower power, etc. than Cs
I think, though, that some sort of self calibrating array using the
target of interest is a better scheme.. multiple receivers at each site
separated by some distance. Getting milliradian angular resolution is a
piece of cake.
Each pair of sites could maybe do two-way time transfer over VHF or
UHF meteor scatter. I don't know what the achievable resolution might
be; I suppose it would depend on the size of the scattering entity
(plasma cloud) and its geometry relative to the two sites. Sparse and
unpredictable measurement times shouldn't be a problem so long as
they're not too sparse. Even if 30 ns were unachievable single shot,
averaging over multiple events would help given reasonable flywheel
oscillators (say rubidium).
The common-view UAV or mountaintop ideas would be simpler, though, if
they pass muster in other respects. The mountaintop could also be
passive if that helps: pairs of antennas connected by a transmission
line and appropriately pointed.
Cheers,
Peter Monta
Jim is it possible you just gave a workable solution: Equip each site with a small radio telescope and watch pulsars.
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
Regards,
David Partridge
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of jimlux
Sent: 10 September 2010 06:14
To: richard@karlquist.com; Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Timing Distribution in Mountainous Terrain
Rick Karlquist wrote:
I would like to point out that the environmental sensitivities of the
5071A are unmeasureable, and the measurement threshold is far below
5.8E-14. I would estimate that the 5071A (and ONLY the 5071A among
commercial clocks) could get the job done provided that you could
compare its frequency to GPS to the stated accuracy. This would be
using the 5071 as a secondary standard. You still need to deal with
the short term stability of the 5071A, depending on your system needs.
JPL uses H masers as flywheels.
We also use the maser as a very low phase noise signal in the 10-1000 second tau range..
We multiply it up and send the (very clean) signal out to the spacecraft, it gets tracked by a loop with a few Hz BW, then sent back to earth where it's compared to the maser again to measure Doppler.
Basically we measure doppler and doppler rate over a few minutes, assuming that the transmitted signal is constant during that time (which the maser is, for all practical purposes). Over a longer time span (e.g. the time between transmit and receive, which could be many hours, implying that we are transmitting from one station and receiving from
another) I assume we use atomic standards and/or astronomical sources (pulsars).
time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
By using them in common view, though, any absolute error would drop
out. I'm not sure pulsar pulses are fast enough to do discrimination
at 30 ns time scales, though. VLBI with broadband sources (quasars)
would be fine here, but large amounts of data would need to be
exchanged, and the sources are weak, requiring large antennas.
Satellite laser ranging using LAGEOS and friends? But the original
poster said no satellites (not even passive rocks in MEO?), and
weather is a problem.
Cheers,
Peter Monta
Satellite laser ranging using LAGEOS and friends?
On second thought, this wouldn't work anyway (besides being too
expensive)---stations would have to be very close together to have
common view.
Cheers,
Peter Monta
Hi
Satellites appear to be out. Best case, pulsars would be a once a day thing. You would need a bit better than 30 ns on the transfer (10?) to get the system to perform.
To put an order of magnitude on the difficulty:
I believe that 20 ns is in the same range as the error national standards labs hold relative to UTC.
http://tf.nist.gov/pubs/bulletin/nistusnoarchive2009.htm
That's with a lot more effort than any rational system is going to put into timing. Also that's with GPS available to allow precision time transfer.
Bob
On Sep 10, 2010, at 5:15 AM, Peter Monta pmonta@gmail.com wrote:
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
By using them in common view, though, any absolute error would drop
out. I'm not sure pulsar pulses are fast enough to do discrimination
at 30 ns time scales, though. VLBI with broadband sources (quasars)
would be fine here, but large amounts of data would need to be
exchanged, and the sources are weak, requiring large antennas.
Satellite laser ranging using LAGEOS and friends? But the original
poster said no satellites (not even passive rocks in MEO?), and
weather is a problem.
Cheers,
Peter Monta
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
David C. Partridge wrote:
Jim is it possible you just gave a workable solution: Equip each site with a small radio telescope and watch pulsars.
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
Pulsars are pretty stable.. the problem is whether propagation is
stable enough. Maybe with a flywheel..
GIMF (pulsar time stability) and it turns up a paper by Petit on DTIC
(the research was done in 1995 at BIPM in France)
http://www.google.com/url?sa=t&source=web&cd=1&sqi=2&ved=0CBcQFjAA&url=http%3A%2F%2Fwww.dtic.mil%2Fcgi-bin%2FGetTRDoc%3FLocation%3DU2%26doc%3DGetTRDoc.pdf%26AD%3DADA502306&rct=j&q=pulsar%20time%20stability&ei=-DOKTLXwKouqsAO2vNHZBA&usg=AFQjCNEH_epMW_tthuGiX50n93HH4xPxfg&cad=rja
Covers performance and all the perturbations (gravitation effects from
stars alongside the propagation path, propagation through atmosphere, etc.)
take home value: for tau of few years, ADEV of few parts in 1E15.
OTOH this paper also says the measurement uncertainty is 1 microsecond..
Also, one might look at:
tf.nist.gov/general/pdf/2166.pdf
Relativity and Timing in X-ray Pulsar Navigation
This paper is about XNAV (think GPS in deep space, using X-ray pulsars
as the S/Vs)... it seems to show that ModifiedADEV is 1E-13 (an order of
magnitude worse than "atomic clock"; presumbably Cs) at tau of 0.1 year.
ANother chart shows the relative performance of a whole raft of clocks
(and you can see why everyone is excited about JPLs Hg-ion clock...) In
any event PSR 1937+21 is about 1E-12 at tau of 500,000 seconds..
probably not good enough for the OP's need. (PSR 1937+21 has a period of
about 1.5 msec, by the way.. if pulsars are really a spinning star,
that's mighty impressive)
thesis.library.caltech.edu/3590/
describes work on another millisecond pulsar.. the abstract says 100ns
uncertainties, and long term (3yr) residuals of 500 ns.
Peter Monta wrote:
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
By using them in common view, though, any absolute error would drop
out. I'm not sure pulsar pulses are fast enough to do discrimination
at 30 ns time scales, though. VLBI with broadband sources (quasars)
would be fine here, but large amounts of data would need to be
exchanged, and the sources are weak, requiring large antennas.
Satellite laser ranging using LAGEOS and friends? But the original
poster said no satellites (not even passive rocks in MEO?), and
weather is a problem.
Hmm.. here's an idea
look for the 217 MHz signals reflected from LEO satellites and
NAVSPASUR.. big illuminating fence, fairly easily detectable signals,
LEO satellite so ionosphere isn't a big deal. You get many satellites
per hour crossing the fence.
Actually, if any satellites are available, how about using TV signals
from GEO relays? (or is the presumption that everything above 50,000 ft
MSL has been wiped out?) Some folks at JPL have used such signals to do
microwave holography on big antennas (big signal, essentially plane
wavefront, etc.)
For that matter, LEO satellites aren't that far away, and your Lband
interrogator for the transponder could probably light them up and
reflect some power back. You'd have to run the radar equation and see
how much power you get back... I have no idea about the RCS of a LEO
satellite, but I'll bet its "many" square meters (if you count ISS as a
candidate.. thousands of square meters RCS) ISS passes over several
times a day, and while its orbit isn't particularly well known or
controlled (at least not to a scale of meters), it might be good enough
for you.
Hi
An event that totally takes out every single GPS sat probably takes out everything else in orbit. A single GPS sat, no longer under ground control would be fine for timing a system like this. You don't need a full constellation or ground segment steering.
About the only non-end of the world 6 day no GPS situation would be a jammer. Directional antennas could take care of terrestrial sources. You might need a steered dish, but those are available items (you can all look at the same sat at the same time). That get you to fleets of orbital jammers sync'd to the GPS birds. Pretty far out....
Bob
On Sep 10, 2010, at 10:02 AM, jimlux jimlux@earthlink.net wrote:
Peter Monta wrote:
Aren't pulsars a reliable accurate time source or do they not provide the 30nS over ten days accuracy?
By using them in common view, though, any absolute error would drop
out. I'm not sure pulsar pulses are fast enough to do discrimination
at 30 ns time scales, though. VLBI with broadband sources (quasars)
would be fine here, but large amounts of data would need to be
exchanged, and the sources are weak, requiring large antennas.
Satellite laser ranging using LAGEOS and friends? But the original
poster said no satellites (not even passive rocks in MEO?), and
weather is a problem.
Hmm.. here's an idea
look for the 217 MHz signals reflected from LEO satellites and NAVSPASUR.. big illuminating fence, fairly easily detectable signals, LEO satellite so ionosphere isn't a big deal. You get many satellites per hour crossing the fence.
Actually, if any satellites are available, how about using TV signals from GEO relays? (or is the presumption that everything above 50,000 ft MSL has been wiped out?) Some folks at JPL have used such signals to do microwave holography on big antennas (big signal, essentially plane wavefront, etc.)
For that matter, LEO satellites aren't that far away, and your Lband interrogator for the transponder could probably light them up and reflect some power back. You'd have to run the radar equation and see how much power you get back... I have no idea about the RCS of a LEO satellite, but I'll bet its "many" square meters (if you count ISS as a candidate.. thousands of square meters RCS) ISS passes over several times a day, and while its orbit isn't particularly well known or controlled (at least not to a scale of meters), it might be good enough for you.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On 09/10/2010 07:17 AM, jimlux wrote:
Ralph Smith wrote:
On Sep 9, 2010, at 8:01 PM, Rick Karlquist wrote:
I would like to point out that the environmental sensitivities of
the 5071A are unmeasureable, and the measurement threshold is
far below 5.8E-14. I would estimate that the 5071A (and ONLY the 5071A
among commercial clocks) could get the job done provided that you could
compare its frequency to GPS to the stated accuracy. This would
be using the 5071 as a secondary standard. You still need to
deal with the short term stability of the 5071A, depending on
your system needs. JPL uses H masers as flywheels.
I would imagine the cost of a 5071A per radio station would make the
check writers swallow hard, and adding an H Maser into the mix would
really get their attention. Especially if you need redundancy.
there's also the possibility mentioned earlier of using a lower quality
standard at each station and flying (driving) a higher quality clock
(5071) around often enough to keep them trued up.
If it's far enough in the future.. Hg ion traps have a lot of
potential.. smaller, lower power, etc. than Cs
Commercial availability is somewhat limited. A problem with Hg ion traps
would be ROHS, unless they can be exempted or assumed to be within the
telco exempt, which would be a legal twist on the commercialisation aspect.
Another aspect I have been wondering about is the trap hold-length, I
think I recall that there was some issues relating to that...
I think, though, that some sort of self calibrating array using the
target of interest is a better scheme.. multiple receivers at each site
separated by some distance. Getting milliradian angular resolution is a
piece of cake.
That moves the expense, and I don't think the available receivers have
that option. They intend the spatial separation to be in kms and not m.
Cheers,
Magnus