Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Zefram]

Daniel R. Tobias wrote:
>length of the day, but it was shorter than the current day, which 
>would, I imagine, have an impact on timekeeping in this colony.

The magnitude of this difference is in the same ballpark as the present
difference between Terran and Martian days, so exactly the same range of
solutions are on the table as for colonisation of Mars.  We can expect
that the colonists would biologically synch to the local diurnal cycle
without excessive hassle.

Systematically generalising from current Terran timekeeping, we can
foresee distinct time scales for physical time and for planetary rotation,
but with the difference between them being too large to fudge.  The whole
culture would have to be conscious of the distinction, unless they go
native and define new time units that match the local planetary rotation.
I'm going to suppose a level of technological continuity that precludes
replacing the system of units.

There will be some tracking of proper time elapsed on the rotating geoid,
corresponding to the present TAI tracking TT.  At base this is a count
of SI seconds.  Whereas in present Terran timekeeping 86400 SI seconds
is very close to the length of a solar day, so it is possible to label
TAI times using something resembling the conventional calendar, the
Martian/Paleoterran situation would make such labelling nonsensical.
So the equivalents of TAI and TT would probably just be described in
the form of a linear count of seconds.

There will be some tracking of planetary rotation in the form of mean
solar time.  Ignoring the longitude issue, for which the concept of
timezones applies unchanged, this will correspond to the present UT1
(Terra) or AMT (Mars).  At base this is a count of mean solar days.
The integer part of the count would no doubt have a pure linear form
used for calculations, corresponding to MJD (Terra) and MSD (Mars).
This integer can also be described in terms of a calendar that relates
days to larger astronomical cycles, corresponding to the Gregorian
calendar (Terra) and Darian calendar (Mars).  The fractional part of
the day count, i.e., time of day, can be sensibly described either as
a decimal fraction or divided into 86400 equal parts in the 24:60:60
fashion.  Cultural continuity suggests that both forms would enjoy some
currency.  The risk of confusing 1/86400 of a mean solar day with an
SI second (of close but substantially different length) might provide
cultural pressure to switch to only using decimal fractions here, but
I'm inclined to think that change would not fully occur.

Where it gets really interesting is when we want to have days that closely
match mean solar days and to count time within the days in SI seconds.
This corresponds to present UTC.  I think the most viable approach is a
direct generalisation of UTC's 24:60:60 form.  In UTC, every day has the
same number of minutes (1440), every minute has an integral number of SI
(well, strictly TAI) seconds, and UTC modulates the number of SI seconds
in the minute in order to approximate the progress of mean solar time.
Present UTC has to achieve a mean minute length of something like
60.005 s, so it can get away with having almost all minutes being
60 s long and treating only a tiny fraction of minutes as opportunities
for extension to 61 s.  The obvious historical reasons behind that don't
apply in the Martian or Paleoterran situations, so unless you're very
lucky you'll need a lot more alternation of minute lengths.

The length of a Martian solar day is currently about 88775.24409 s
(per Wikipedia), yielding an average minute length for a UTC equivalent
of 61.6494751 s.  On Paleoterra you'll want an average minute length
somewhere between 58.0 s and 59.4 s.  So on Mars you'll want about
65% of minutes to be 62 seconds long and the rest 61 seconds long.
There'll have to be alternation of minutes within each day, which could be
mostly according to a regular pattern, but at least a small part of the
pattern will have to be variable on an observational basis.  Likewise,
on Paleoterra you'll need to alternate between 58 and 59 seconds or
between 59 and 60 seconds per minute, depending on era.

The frequency of leap seconds in these generalised-UTC systems implies
that it won't be viable to ignore leap seconds, in most of the places that
currently ignore them.  Wristwatches, for example, will have to get them
right at least most of the time, if they're trying to tick SI seconds
at all.  A watch could perhaps get away with not handling occasional
observation-based irregular leaps, but it'll have to get the regular
pattern right to remain in synch for days at a time.  An analogue watch
could get away with ticking mean solar seconds and not knowing anything
about the pattern of leap seconds, but then wouldn't be any good for
anything relating to the 1 Hz beat of TAI-equivalent.

The whole UTC-equivalent mechanism is conspicuously missing from present
Martian timekeeping.  Current and recent rover missions hav

Re: [LEAPSECS] Coding this week, and a trick for timeouts over leap seconds.

[Tony Finch]

Tom Van Baak  wrote:
>
> Try using clock() instead of gettimeofday_in_millisecs(). The former
> nicely increments with CLOCKS_PER_SEC resolution and is immune
> from UTC, timezones, and leap seconds. At least it does on windows.
> Can someone comment on unix/linux?

clock() is broken on Windows in order to be backwards-compatible with DOS.
It is supposed to return the time the process has spent running on the
CPU, which is traditionally rather slower than real time, though it may be
faster for a multithreaded process on a multicore computer. Since DOS
didn't multitask it just returned elapsed time since the process started,
and Windows has retained this braindamaged semantics.

Tony.
-- 
f.anthony.n.finchhttp://dotat.at/
Trafalgar: Variable or easterly, 3 or 4, increasing 5 to 7 for a time in far
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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Rob Seaman]

Sending this from the Super-80 on route to the Philadelphia UTC meeting.

On Oct 3, 2011, at 3:50 AM, Zefram wrote:

Lots of things I agree with.  A couple of comments below.

> Daniel R. Tobias wrote:
>> length of the day, but it was shorter than the current day, which would, I 
>> imagine, have an impact on timekeeping in this colony.

I caught some of that episode.  In particular the moon was absolutely colossal 
in the sky - more like what the Apollo astronauts saw looking back at Earth.  
But then once you posit time travel and alternate universes why not move the 
Moon around? :-)

> The magnitude of this difference is in the same ballpark as the present 
> difference between Terran and Martian days, so exactly the same range of 
> solutions are on the table as for colonisation of Mars.

An interesting equivalence.

> The length of a Martian solar day is currently about 88775.24409 s (per 
> Wikipedia), yielding an average minute length for a UTC equivalent of 
> 61.6494751 s.  On Paleoterra you'll want an average minute length somewhere 
> between 58.0 s and 59.4 s.  So on Mars you'll want about 65% of minutes to be 
> 62 seconds long and the rest 61 seconds long. There'll have to be alternation 
> of minutes within each day, which could be mostly according to a regular 
> pattern, but at least a small part of the pattern will have to be variable on 
> an observational basis.  Likewise, on Paleoterra you'll need to alternate 
> between 58 and 59 seconds or between 59 and 60 seconds per minute, depending 
> on era.

Seems rather unlikely :-)

Stepping back from all our old talking points, ignore UTC and TAI, etc.  
Arriving on a new planet it is always going to be the case that the local day 
is quite different than whatever clocks are onboard the spaceship, time 
machine, Tardis, vehicle.  What options are available for the colonists?  If 
not sexagesimal then what?  Could humans actually use a decimal counter 
productively?  Would the counter have to reset once a day?  Is there some way 
to indicate time without numbers?  Would they of necessity try to reconcile 
ship time with planet time (whatever either of these are)?

> Of course, they haven't yet had a need to link precise interval time on the 
> Martian surface to time of day, nor is there any Martian equivalent of the 1 
> Hz TAI-derived broadcast time signal.  My prediction is that the 
> UTC-equivalent will naturally arise after we land atomic clocks on Mars and 
> set up the Martian equivalent of TAI.

There are a lot of reasons that the Martian equivalent of TAI will be TAI.  
First off, all the equipment will be manufactured on Earth for the first few 
generations.  The Martian equivalent of UTC obviously won't be UTC, however.  
To accommodate both they will have to do something different than anything 
mentioned in the 1999 GPS World article.

I'm taken by your mention of Hertz as the unit of TAI.  That's an interesting 
way to break the artificial symmetry between the two meanings of the word 
"second".  Might be some play there.  The unit of atomic time is frequency, the 
unit of civil time is angles, fractions of a day.  Retire the unit of the 
"second" entirely.

Rob
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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Zefram]

Rob Seaman wrote:
>If not sexagesimal then what?  Could humans actually use a decimal
>counter productively?

The decimal day is quite practical.  It doesn't catch on on Earth, of
course, because of the network effect favouring interoperability with the
existing sexegesimal division.  On a new planet, if you're deliberately
constructing a new timekeeping system, you'd logically end up with
decimal subdivision, because in our present technical culture decimal
is the default way to subdivide anything.  Sexegesimal subdivision is
a relic of an older technical culture.

I think the question here is which approach would dominate in practice:
copy the old mechanism or devise a new one.  Pure sexegesimal subdivision
*is* applied to Mars at present, but you can attribute this to the fact
that the time-of-Martian-day users are also time-of-Terran-day users.
The situation might be different for one-way colonists, and especially
if there's no communication with (present day) Earth.

>Would the counter have to reset once a day?

Not sure what you mean here.  I think if you do tabula-rasa time-of-day,
without reference to a local atomic time scale, then you get pure decimal
subdivision with no irregularity.  The count gets to . just before
wrapping to . at midnight.  Are you imagining something like counting
seconds within the day, denoted in decimal, and resetting to zero at
midnight?  So on Mars, with about 88775 s per day, the count would go
:88773, :88774, :0 (midnight).  I think such an awkward radix would
be avoided at this level.  You might get that in a lower-order component
of time-of-day (see my last suggestion below).

>Is there some way to indicate time without numbers?

Sure, but numbers are awfully convenient.

>Would they of necessity try to reconcile ship time with planet time
>(whatever either of these are)?

I think if the tech level is high enough to use atomic clocks on the
planet's surface, and the diurnal cycle is of great practical importance,
then there would naturally be an attempt to reconcile the local atomic
time scale with planetary rotation.  That's what UTC does for us.

>There are a lot of reasons that the Martian equivalent of TAI will
>be TAI.

It can't be, once we've got precise synchronisation requirements
for Mars-local applications.  Relativistic effects mean that TAI
(or TT) as perceived on Mars is substantially non-uniform.  It's a
practical necessity for the base technical time scale, reference point
for radio frequencies and the like, to be uniform as perceived in a
planet-centred-planet-fixed reference frame.  So I foresee us relatively
soon defining a Martian equivalent of TAI based on atomic clocks located
on the Martian surface.

>First off, all the equipment will be manufactured on Earth for the
>first few generations.

Sure, but location of manufacture doesn't dictate which reference
frame it'll tick with.  Location of operation mainly determines that.
Are you using this as an argument that the clocks will tick SI seconds
rather than some Mars-sensitive unit?  I'm expecting the Martian TAI to
use SI seconds, but in any case it's a trivial transformation to read
a different unit from the same equipment.

>I'm taken by your mention of Hertz as the unit of TAI.  That's an
>interesting way to break the artificial symmetry between the two meanings
>of the word "second".

There's some resemblance in your suggestion to the old controversy from
the early days of atomic clocks.  I recall reading that some astronomers
denied that the atomic clock was a timepiece at all, claiming that it was
merely a frequency standard and that Earth rotation was the true time.
Relatedly, there was the proposal that the atomic second should be named
"essen" rather than "second".  If you're building a new timekeeping
culture from scratch, I guess you get to make those decisions differently
if you like.

So, you can use unambiguous terms "essen" and "hertz" to refer to physical
time.  Then continuous time-of-day uses the unit "day", corresponding
directly to the angular unit "circle".  On Mars you'd plan your schedule
in centidays, during each of which the mean sun moves one centicircle
(3.6 arcdegrees) around the sky.  A native would know that the duration
of the centiday is about 887.75 essens, in the same way that ey'd know
that the duration of the year is about 668.6 days.

Working from this relatively fresh start, free of sexegesimals, I think
you might still end up with a UTC-alike, but with a looser resemblance.
You could divide the calendar day into calendar centidays, and the
calendar centiday into (an integral number of) essens.  Some centidays
are 887 essens long, and others are 888 essens long.  There's an
obvious regular pattern: three centidays of 888 essens followed by one
of 887 essens, repeating throughout the day.  The last centiday of the
day, exceptionally, would vary between 887 and 888 essens day by day.
Anti-leap-second activists would rail against the irregular ".99

Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Warner Losh]

On Oct 3, 2011, at 8:15 AM, Rob Seaman wrote:
> I'm taken by your mention of Hertz as the unit of TAI.  That's an interesting 
> way to break the artificial symmetry between the two meanings of the word 
> "second".  Might be some play there.  The unit of atomic time is frequency, 
> the unit of civil time is angles, fractions of a day.  Retire the unit of the 
> "second" entirely.

Wouldn't that be imperial ounces vs US ounces all over again?  It would also 
break the conversion from the frequency domain to the time domain and vice 
versa.  It is a quaint notion, but I don't think that dog would hunt.

Warner

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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Steve Allen]

On 2011 Oct 3, at 07:15, Rob Seaman wrote:
> Sending this [...] on route to the Philadelphia UTC meeting.

ditto

> the moon was absolutely colossal in the sky

They must have arrived in September and that was the harvest moon
One hopes that by year 2149 we'll have a good answer for the topic
of this list.

That whole sequence was to establish geek cred for the characters,
along with their utter disregard for the authorities they left ahead.
I suspect this tells us much more about the hangups of Braga, Echevarria,
Fury, Spielberg, etc. than it has to do with technical issues.

That same may be true for the topic of this list.

--
Steve Allen WGS-84 (GPS)
UCO/Lick Observatory  Natural Sciences II, Room 165  Lat  +36.99855
University of California  Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064  http://www.ucolick.org/~sla/   Hgt +250 m

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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Rob Seaman]

On Oct 3, 2011, at 9:20 AM, Warner Losh wrote:

> On Oct 3, 2011, at 8:15 AM, Rob Seaman wrote:
>> I'm taken by your mention of Hertz as the unit of TAI.  That's an 
>> interesting way to break the artificial symmetry between the two meanings of 
>> the word "second".  Might be some play there.  The unit of atomic time is 
>> frequency, the unit of civil time is angles, fractions of a day.  Retire the 
>> unit of the "second" entirely.
> 
> Wouldn't that be imperial ounces vs US ounces all over again?

Volume is volume.  Time is many things.

> It would also break the conversion from the frequency domain to the time 
> domain and vice versa.

No, it would recognize that what we call time-of-day is an angle, and only 
approximately either frequency or time.  As has been said innumerable times, 
astronomers are power users of high precision timescales and appreciate the use 
of atomic time for frequency standards and precise time intervals.  This does 
not describe civil timekeeping.  (And that statement is not support for 
advocating being able to do any damn thing we want :-)

> It is a quaint notion, but I don't think that dog would hunt.

I'll take a quaint dog.  One of the tenets of problem solving is not to shoot 
down ideas too early in the brainstorming phase.

Rob


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[LEAPSECS] What timekeeping system should the Terra Nova settlers use

[Hal Murray]

> How would the people on this list who advocate for various treatments of
> future timekeeping in our own world deal with that situation?

Fun question.  Thanks.

I'd setup two separate timekeeping systems.

For measuring time (and frequency) in scientific experiments, I'd use some 
fixed unit and give it a new name to avoid confusion.  I wouldn't tie it into 
dates, just use kilo and mega and giga for longer time spans.

For solar time of day and calender dates, I think you either need rubber 
seconds or something like leap seconds.  Are there any other options?

If leap seconds happened more often the software would get debugged.  If leap 
seconds were really rare, you could ignore them as long as the mechanisms for 
distributing UT1-UTC could handle the drift for some reasonable time span.  
Your great-great-...-grandchildren will have to sort it out, but that won't 
be a problem if they all escape to a new planet first.

If you want to start over, I'd use 100 seconds per minute, 100 minutes per 
hour and 10 hours per day.  (no AM/PM)  Or something like that.

---

What do the people at JPL do when they want solar time on Mars?  I assume 
they have two clocks on the wall like some places have a second clock running 
on a different time zone.

Humans naturally sync to Earth solar days.  What happens if a "day" is half 
or twice as long?  Years ago, I worked a 6 day week without any troubles, but 
I get tried if I try to stay up for 2 days.





-- 
These are my opinions, not necessarily my employer's.  I hate spam.



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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Warner Losh]

On Oct 3, 2011, at 11:03 AM, Rob Seaman wrote:

> On Oct 3, 2011, at 9:20 AM, Warner Losh wrote:
> 
>> On Oct 3, 2011, at 8:15 AM, Rob Seaman wrote:
>>> I'm taken by your mention of Hertz as the unit of TAI.  That's an 
>>> interesting way to break the artificial symmetry between the two meanings 
>>> of the word "second".  Might be some play there.  The unit of atomic time 
>>> is frequency, the unit of civil time is angles, fractions of a day.  Retire 
>>> the unit of the "second" entirely.
>> 
>> Wouldn't that be imperial ounces vs US ounces all over again?
> 
> Volume is volume.  Time is many things.

Well, Yes and no.  US ounces can either be volume or weight.

>> It would also break the conversion from the frequency domain to the time 
>> domain and vice versa.
> 
> No, it would recognize that what we call time-of-day is an angle, and only 
> approximately either frequency or time.  As has been said innumerable times, 
> astronomers are power users of high precision timescales and appreciate the 
> use of atomic time for frequency standards and precise time intervals.  This 
> does not describe civil timekeeping.  (And that statement is not support for 
> advocating being able to do any damn thing we want :-)

But it would throw a grande into all that pesky math you have to do with time. 
For Electrical Engineering, nobody cares about earth angles, but they do care 
that all seconds are the same length.  Astronomers have the luxury of being 
able to define time that's convenient for them, but that doesn't make it 
convenient for others.

And again, civil time is just an approximation of solar time of day. There are 
many ways to approximate this with differing degrees of luck.

Then again, if astronomers are such power users of time scales, why would they 
need to force everybody else to use something that's convenient for them and 
would cost them lots of cash to retool?  If they are so studly, shouldn't they 
have higher burdens than those less studly, and therefore less capable of 
handing the burdens :)

>> It is a quaint notion, but I don't think that dog would hunt.
> 
> I'll take a quaint dog.  One of the tenets of problem solving is not to shoot 
> down ideas too early in the brainstorming phase.

True, but you also don't want to spend too much time on the lemons either.  
Balance also dictates that you not waste time on losing ideas...

Warner
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Re: [LEAPSECS] preprint about timekeeping for neutrino experiment

[Zefram]

Rob Seaman wrote:
>An interesting discussion of a difficult measurement:
>
>   http://arxiv.org/pdf/1109.6160

I've found a description of the Time-Transfer Device that is the
subject of that paper.  The original OPERA paper doesn't actually
say that a TTD was used for synchronisation, it says that the
synchronisation was "independently verified", by PTB, using a TTD.
 is a note
from PTB describing their part in the affair, and it's a very limited
part.  Their job was only to compare corresponding parts of the timing
gear at the two labs, checking the delay between GPS signals and the
resulting PPS signal at the input to a timestamping unit.

PTB's note explains why they used a portable device, and it's got nothing
to do with actual time transfer.  There's a problem that units of the same
species have individual variations, so measurements made with different
units are not directly comparable at the finest precision.  So they
transfer one unit between the labs in order to perform corresponding
measurements with the same unit, so that the unknown biases in the unit
cancel themselves out.  The unit does not maintain its own time scale, so
the path it took between the labs is irrelevant.  Contaldi's assumption
that "this device [is] a transportable atomic clock" turns out to be
wide of the mark.  (He redeems himself by the footnote deploring the
lack of sources on this point.)

The synchronisation may still have gone awry in other areas not covered by
PTB's work, which are not described by anything I've seen yet.  There's
also a clear error in that the OPERA paper treats time as Newtonian:
the two labs are synched to GPS time, hence to TAI, and there's no
discussion of the difference between this time scale (SI seconds on the
geoid) and time along the neutrino path (varying between 1 km above and
30 km below the geoid).

 discusses the
geodesy in reasonable detail.  Down at the bottom of it, the endpoints
are ascribed coordinates in ETRF2000, and Pythagoras's theorem is used
to determine the path length.  At the precision stated, gravitational
length contraction must make this calculation invalid.

Do the gravitational time dilation and the gravitational length
contraction cancel each other out, when viewed from a suitable reference
frame?  I'm out of my depth here.

-zefram
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Re: [LEAPSECS] What timekeeping system should the Terra Nova settlers use?

[Rob Seaman]

On Oct 3, 2011, at 10:46 AM, Warner Losh wrote:

> For Electrical Engineering, nobody cares about earth angles, but they do care 
> that all seconds are the same length.

It depends what project the electrical engineers are working on.  Astronomical 
projects employ plenty of engineers.

The fundamental issue is precisely the attempt to take two different things and 
pretend they are one and the same.  The conceptual model is broken.

> And again, civil time is just an approximation of solar time of day. There 
> are many ways to approximate this with differing degrees of luck.

And the ITU proposal doesn't pursue any of them.

> Then again, if astronomers are such power users of time scales, why would 
> they need to force everybody else to use something that's convenient for them 
> and would cost them lots of cash to retool?

Oh please.  Use GPS for intervals and UTC for angles.  The ITU proposal is to 
take universal time away and replace it with something equivalent to GPS.

> Balance also dictates that you not waste time on losing ideas...

Who said balance is a requirement?  And who determines whether the time is 
wasted?

"Losing" is not a description of the idea, but of the point of view of some 
observer.  It also implies a zero sum game.  Both TAI and UTC can coexist.  We 
have proof of this because it is the status quo.  Some may want to tweak the 
way they coexist.  By all means discuss modifications.  Drowning UTC in the 
bathtub is not a description of "balance".

Returning to the original question: how can a timekeeping system millions of 
years in the past or future or on another planet like Mars function?  How will 
a standardized chronometer significantly different that any round fraction of 
the solar day be coordinated with the need to keep a rational time-of-day?  
Ignore UTC and TAI.  Ignore the ITU politics.  Focus on this issue.

Rob

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Re: [LEAPSECS] preprint about timekeeping for neutrino experiment

[François Meyer]

On Tue, 4 Oct 2011, Zefram wrote:


Rob Seaman wrote:

An interesting discussion of a difficult measurement:

http://arxiv.org/pdf/1109.6160


I've found a description of the Time-Transfer Device that is the
subject of that paper.  The original OPERA paper doesn't actually
say that a TTD was used for synchronisation, it says that the
synchronisation was "independently verified", by PTB, using a TTD.
 is a note
from PTB describing their part in the affair, and it's a very limited
part.  Their job was only to compare corresponding parts of the timing
gear at the two labs, checking the delay between GPS signals and the
resulting PPS signal at the input to a timestamping unit.

PTB's note explains why they used a portable device, and it's got nothing
to do with actual time transfer.  There's a problem that units of the same
species have individual variations, so measurements made with different
units are not directly comparable at the finest precision.  So they
transfer one unit between the labs in order to perform corresponding
measurements with the same unit, so that the unknown biases in the unit
cancel themselves out.  The unit does not maintain its own time scale, so
the path it took between the labs is irrelevant.  Contaldi's assumption
that "this device [is] a transportable atomic clock" turns out to be
wide of the mark.  (He redeems himself by the footnote deploring the
lack of sources on this point.)

The synchronisation may still have gone awry in other areas not covered by
PTB's work, which are not described by anything I've seen yet.  There's
also a clear error in that the OPERA paper treats time as Newtonian:
the two labs are synched to GPS time, hence to TAI, and there's no
discussion of the difference between this time scale (SI seconds on the
geoid) and time along the neutrino path (varying between 1 km above and
30 km below the geoid).


In my view the two clocks are just synced to each other as far as this 
experiment
is concerned. That does not change the point about gravitation though.



 discusses the
geodesy in reasonable detail.  Down at the bottom of it, the endpoints
are ascribed coordinates in ETRF2000, and Pythagoras's theorem is used
to determine the path length.  At the precision stated, gravitational
length contraction must make this calculation invalid.


This you cant say unless you estimate the order of magnitude of 
gravitational effects. For example :

http://adsabs.harvard.edu/full/1994A&A...286..971P
gives an exhaustive review of what terms should be accounted for.
Errors excepted, I found that the only term exceeding 1 ns is due to 
sagnac effet and it is about 2 ns. The other terms are below 1 ns.


If those figures are correct then the newtonian approach could be
considered sufficient.

--
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Observatoire de Besancon - BP1615 - 25010 Besancon cedex - FRANCE
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