Comparing phase of physically distant signals

On a sunny day (Sat, 03 Aug 2013 00:45:41 -0700) it happened Don Y wrote in :

This sounds like a GPS case, but really without knowing _numbers_ and other parameters no way to tell.

This whole question smacks of one of those questions that ought to be "unasked".

The concept of having "as single time everywhere" is erroneous in general, but under limited circumstances there are useful approximations. You do not give enough information to indicate whether the approximations would be valid and/or useful.

If the time-of-flight of messages between the two ends is much less than the time resolution to which synchronisation is required, then effectively a "single time" /is/ possible. Otherwise, not. As an extreme illustration of that, consider somebody orbiting Proxima Centauri so that the one-way time-of-flight is 4.2 years. Clearly it doesn't make any sense to ask the other person what the date is! Reduce the distances, and similar principles apply with smaller time differences.

As a separate issue, non-line-of-sight radio channels are continuously varying in length and thus in propagation delay (and even LoS ones to a lesser extent). Ask any ham, or consider how multipath is *required* for cellular phone systems.

Have you studied NTP and IEEE 1588-2008 ?

Agreed when thinking of the theory of relativity.

For ground bound or slowly moving (v

In GPS time transfer, the distance from the satellite to the atomic clock is known within a meter as well as the distance from the end user to the satellite. It is certainly possible to synchronize clocks within nanoseconds. I am not talking about consumer GPS devices with 1 PPS outputs :-).

VLBI with quasars have been used to measure continental drift in millimeters/year, with known time between different continents. With known distances VLBI could be used to synchronize clocks.

If there is a phase coherent transponder on a (hypothetical) planet around Proxima Centauri, the distance to that transponder can be measured with a fraction of a wavelength from the propagation delay (2x4.2 years) and the velocity from the doppler.

To make sure that both clocks are the same, 3x4.2 years might be needed.

A somewhat quicker and more accurate situation would be that both stations monitor the phase of a particular quasar billions of light years away. To solve the phase ambiguity, monitoring some nearby pulsars should help.

IMHO, it is quite relevant to ask the date and time on a planet orbiting Alpha Centauri, of course the answer might not be too usable after 8.4 years :-).

If the response is sent back at _exactly_ the same frequency, the propagation delay is the same in both directions and hence satisfy the NTP approximation in both directions. However, if a duplex system is used, using different frequencies for the up- and downlink, the multipath pattern might be quite different and the NTP equal propagation delay assumption do not apply.

Dropping this to the situation for a colony on Mars, there are no technical problems maintaining the "same time" concept on Earth as well as on Mars, but if "Mars Universal Time" would track the UTC, the length of the second or the number of seconds a day needs to be varied.

Velocity is irrelevant to the issue - although velocity would introduce extra complications.

All true, but misses the point.

All true, but misses the point.

Having identical frequencies is possible. Having the same time isn't.

That's almost the point! Take it one stage further and ask in what way it would be meaningful/useful to have April

Completely false. The path *changes* over time, so what is true now won't be true in the future. Look up and understand the concept of "coherence distance" and its dual "coherence time".

True, but there are more fundamental problems.

Extra complications that obscure the fundamental point.

While I have no idea, what Don Y had in mind (might be whatever :-), but at least for any ground based system, having "the same time" is not that alien concept.

in the North East North America, there was a severe power black out in Aug 2003 as well as in Central Europe in Nov 2006, which were caused by individual link failures, which propagated to larger and larger parts of the continent.

In order to solve the root cause of these events, various SequenceOfEvent (SoE) analysis were performed later on, i.e. post mortem dumps. Each protection relay will record the event with a time stamp from the local clock.

In order to be useful for SoE analysis, the event must be time stamped from a reliable local clock, which must be traceable to same local time standard, ultimately back to IAT.

Modern protection relays on an electric switchyard are synchronized to some local GPS time receiver via NTP and the event messages are stamped with time with 1 ms resolution.

Thus, for millisecond level SoE analysis, about 1 ms continent wide synchronization is needed.

And below that resolution the concept of "common time" has no useful meaning. It all depends on the distance; inside an ic it is much smaller, but still there :)

I ought to add that comparing the *phase* of repetitive signals is entirely possible and useful to much less than time-of-flight resolution and accuracy. But not the *time*.

Installers will hate this.

Installers will love this. But why different flight times? Where is this stuff going to be located? Down a borehole?

Aside from using GPS, WWV or some other reliable transmitter you could have a very stable oscillator on each module. Such as a TCXO. Then you have a transceiver on each. You perform a loopback echo locally, via an RF switch. That gives you the latency of each radio (should be pretty much the same for each module). Now only the path adds in which should normally be reciprocal.

If the distance is small you can use a cheap transceiver chip or module. If using a chip and rolling your own hardware you must usually go through the radio cert for "intentional radiator" at the EMC lab which adds NRE.

Sounds like a use case for NTP (network time protocol) which already keeps millions of computers synchronized to a few milliseconds across the whole planet.

?-)

Don hasn't given us any specs yet but if this has to be in the sub-microsecond class then real terrestrial RF may be needed. Even old Loran could do that back in the 80's, over lots of miles:

According to the paper you referenced, it was not sub-microsecond.

Also, it discusses only the surface wave. Reflections may occur at other frequencies and upset the path length.

John S

Under "Results" it says so.

It does fall apart when you get over 100 miles or really bad terrain. But I doubt Don needs that much.

Loran is (or was) amazing. It literally saved the life of a guy at my university. He sailed his fathers large boat, alone, in the French Atlantic at the end of summer. Woe to whom who gets into a storm there. Long story short he got into a lull and then the mother of all storms rolled in. At night. He headed for a port but knew that it had a very narrow opening of just a couple hundred feet, with massive concrete and rock structures on either side. Structures that would have busted his boat into smittereens in that storm. He was not religious but turned to his navigation screen and prayed. The Loran coverage wasn't even that great. Many hours later his boat went right through the middle, he saw the two massive structures pass by on either side. He needed the bathroom, fast.

Of course! Hence NTP, PTP, "let's synchronize our watches, gentlemen", etc.

Exactly. If two different "entities" do or observer "things", then you need some way of deciding which event is the chicken an which the egg.

Yes. I.e., any sense of time *finer* than this is ambiguous. Saying something happened at t=4.035ms and something else happened at t=4.036ms doesn't allow you to declare the ACTUAL ordering of these events.

OTOH, if one occurs at t=4ms and the other occurs at t=6ms you can confidently claim which followed the other.

But, there are (obviously) costs to getting more and more precision. And, being able to *measure* that when the two signals that you are comparing are physically distant (too far for scope leads to simultaneously monitor).

Move down a few orders of magnitude :>

But, were not talking about "continent level synchronization". Just comparing a locally derived clock in one room with another locally derived clock in another room on a different floor or in a different "wing" of a large building (e.g., office complex).

My mistake. I will read the article again.

You are correct.

Nice story. I longed to sail the sea, and in fact, I still do. But, at my age, I could not handle it.

Cheers

That will work, if you can get cables or fibers long enough. The propagation delays can be measured, but tempcos will matter if you're concerned about nanoseconds or less.

What are your numbers? Distance? Accuracy expectation?

There is a technique for absolute-time-syncing clocks that measures the round-trip cable (or fiber) delay in both directions and does the math to take it out. I don't recall what it's called.

Can't GPS deliver absolute time ticks? Wasn't that used in the recent bogus FTL neutrino experiment?

You can physically transport an atomic clock from site to site to align absolute times. You need relativity compensation to get really good.