[time-nuts] WWV Doppler Shift

Starting 1970 I used a modified Tracor 599H on WWVB with excellent results. It had a mechanical counter with 100 nsec, resolution. Noisy but perfect. Yes you have to take Ionosphere sunrise and sunset in to consideration and the hourly shift, but being a very early riser 4AM because of Europe no problem. Better than 2 E-11 per day and 4 E-14 per month.

In the 90 ties with my FRK having temperature and aging control frequency was better than 1 E-12 all the time.

Bert Kehren
In a message dated 11/19/2018 9:58:39 PM Eastern Standard Time, ***@gmail.com writes:

HF propagation of WWV or WWVH is horrible compared to VLF propagationof WWVB at 60 kc. In this video the 5 mc WWV signal from Ft Collins,Colorado is being received in New Jersey. It was compared against astable 5mc crystal source. You can see a shift of a few cycles persecond over a few seconds. This is due to the movement up or down ofthe ionosphere at a substantial fraction of the speed of sound. Iunderstand that it can be as bad as a part per million short term atsome frequencies. The 10 mc and 15 mc broadcasts may be better orworse at various times of day.
I have spent many hours watching WWVB in the days when they had theold format. It was good to a part per billion except just beforesunrise and just after sunset. Then it degraded to either + or -5parts per billion for about 30 minutes.
http://youtu.be/nf9KWfS9QBs
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jimlux

2018-11-20 16:04:56 UTC

Post by ew via time-nuts
Starting 1970 I used a modified Tracor 599H on WWVB with excellent results. It had a mechanical counter with 100 nsec, resolution. Noisy but perfect. Yes you have to take Ionosphere sunrise and sunset in to consideration and the hourly shift, but being a very early riser 4AM because of Europe no problem. Better than 2 E-11 per day and 4 E-14 per month.
In the 90 ties with my FRK having temperature and aging control frequency was better than 1 E-12 all the time.
Bert Kehren
HF propagation of WWV or WWVH is horrible compared to VLF propagationof WWVB at 60 kc. In this video the 5 mc WWV signal from Ft Collins,Colorado is being received in New Jersey. It was compared against astable 5mc crystal source. You can see a shift of a few cycles persecond over a few seconds. This is due to the movement up or down ofthe ionosphere at a substantial fraction of the speed of sound.

In general terms, the coherence time of the ionosphere is single digit
seconds - that is, there's essentially no correlation between
propagation path at one time and the propagation path 10 seconds later.

The "general length" of the path will be the same, but the details
different.

The actual ionization in the ionosphere can best be described as moving
"clouds" there's a fair amount of spatial inhomogeneity. In the same
sense that milk reflects light from a multitude of little fat globules.

Iunderstand that it can be as bad as a part per million short term
atsome frequencies. The 10 mc and 15 mc broadcasts may be better
orworse at various times of day.

Post by ew via time-nuts
I have spent many hours watching WWVB in the days when they had theold format. It was good to a part per billion except just beforesunrise and just after sunset. Then it degraded to either + or -5parts per billion for about 30 minutes.
http://youtu.be/nf9KWfS9QBs
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Donald E. Pauly

2018-11-20 22:35:51 UTC

That was the first time that I had seen an xy plot of WWV versus a
stable crystal oscillator. It is even worse than I thought. I had to
look up FRK to see that it is a rubidium standard. I talked to Jim
Maxton the chief engineer of WWVB many times around 1995. At the time
I was in Gila Bend 80 miles southwest of Phoenix. He had a Hewlett
Packard cesium standard at Ft Collins. They were using a dual view
GEOS Geostationary satellite to set the cesium to match the master
clock in Boulder. If the cesium was good to 10^-13, that is 8.6 μs
per day. I can't remember how close he tried to keep it or how often
he adjusted it. It looked like that I could determine the start of
the second to the individual transmitter cycle. Time transfer
accuracy was therefore limited to the height changes of the ionosphere
at sunrise and sunset.

The main disturbance was wind blowing the antenna. Ordinarily the
phase would jitter a few degrees per second. I could tell the wind
speed by the phase jitter without checking the Ft Collins weather. If
memory serves, the loaded Q at 60 kc was about 200. A half percent
tuning error caused a 45° phase error. I have seen a 45° excursions
on several occasions over a minute more than once. My receiver had a
slow lock mode that could spot them. It also had a 45° phase switch
on the 100 kc local oscillator to eliminate the station ID from 10 to
15 minutes after the hour. There was therefore no disturbance in lock
during it. I was never able to measure any error in the 45° phase
advance. One degree would have been obvious.

When I first got my receiver going, the phase would advance nearly 40°
at the start of the second when the power was reduced by 10 db. It
had been doing so for years and nobody noticed it. Maxton took an
unneeded condenser out of his time code generator which fixed most of
it. The new transmitter fixed the rest.

Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level. How did you correct for
altitude on yours? I presume that frequency is defined at sea level
but I don't know that. Sea level clocks at the North or South Poles
run fast relative to those at equator sea level by 1.192·10^-12.

WB0KVV
πθ°μΩω±√·÷Γλφ|Δ

In general terms, the coherence time of the ionosphere is single digit
seconds - that is, there's essentially no correlation between
propagation path at one time and the propagation path 10 seconds later.
The "general length" of the path will be the same, but the details
different.
The actual ionization in the ionosphere can best be described as moving
"clouds" there's a fair amount of spatial inhomogeneity. In the same
sense that milk reflects light from a multitude of little fat globules.

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Tom Van Baak

2018-11-20 23:30:54 UTC

An xy cycle of WWV is just 200 ns, about 80x shorter than the 16667 ns cycle of WWVB. So, yes the xy plot in the video seems to jump around a lot, but if that were WWVB it would be 80x less, barely a wiggle.

Does someone have a strip chart version of that video? Or, better yet, a raw data set of WWV (or WWVB) phase over a day or week? How hard would it be to use a hands-off SDR to produce a 5 MHz WWV phase data point every second?

Post by Donald E. Pauly
Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level.

Yes, an out-of-the-box cesium clock will be relatively fast by that amount. But NIST (and everyone else) uses UTC, which is based on the SI second, which is defined at sea level (and several other footnotes).

Which is to say that a national clock or radio transmitter (such as NIST, WWV, WWVB, or DCF77, or GPS for that matter) are adjusted in frequency so they tick SI seconds, and adjusted in phase so they align with UTC.

/tvb

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Tom Holmes

2018-11-21 00:02:16 UTC

So if the SI second is specified at sea level, and we know from Einstein and TVB's work that going up a mountain changes a clock's period, how would the second be affected at the center of the Earth ( ignore thermal problems, this is a conceptual discussion) where the net gravity vector might conceivably zero? Or for that matter, at a Lagrange point in space? We do have some data from those locations I would think.

A second question (no pun intended) is that given the Earth's elliptical orbit around the Sun, has there been observed an effect of the change in its gravity on atomic clocks?

Tom Holmes, N8ZM

-----Original Message-----
From: time-nuts <time-nuts-***@lists.febo.com> On Behalf Of Tom Van Baak
Sent: Tuesday, November 20, 2018 6:31 PM
To: Discussion of precise time and frequency measurement <time-***@lists.febo.com>
Subject: Re: [time-nuts] WWV Doppler Shift

Post by Donald E. Pauly
Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level.

Steve Allen

2018-11-21 04:06:10 UTC

Post by Tom Holmes
So if the SI second is specified at sea level, and we know from
Einstein and TVB's work that going up a mountain changes a clock's
period, how would the second be affected at the center of the Earth (
ignore thermal problems, this is a conceptual discussion) where the
net gravity vector might conceivably zero? Or for that matter, at a
Lagrange point in space? We do have some data from those locations I
would think.

Note that it is not at sea level, and not at the geoid, but as of
last week the rate is at a defined geopotential value. This makes
the rate of TAI insensitive to geological timescale changes of
the sea level. The astronomical time scale TT had adopted this
in 2000. It took 18 years for the CGPM to do the same for TAI.

Since the formation of earth the material at the center of the earth
has exprienced about two days less proper time than the material at
the surface.

GPS and Galileo and Beidou navigation satellites have onboard atomic
frequency standards. Their rate is tweaked so that the received
signals at the surface of the earth match the SI second here.

Post by Tom Holmes
A second question (no pun intended) is that given the Earth's
elliptical orbit around the Sun, has there been observed an effect of
the change in its gravity on atomic clocks?

We all slow down and speed up together, so we here looking at us here
see no effect. On the other hand, spacecraft tracking, VLBI, pulsar
timing, etc. can measure the effect.

--
Steve Allen <***@ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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Tom Van Baak

2018-11-21 15:49:17 UTC

Post by Tom Holmes
So if the SI second is specified at sea level, and we know from Einstein and TVB's
work that going up a mountain changes a clock's period, how would the second be
affected at the center of the Earth ( ignore thermal problems, this is a conceptual
discussion) where the net gravity vector might conceivably zero? Or for that matter,
at a Lagrange point in space? We do have some data from those locations I would think.

By convention, the SI second is defined at sea level.
A clock at infinity runs about 6.95e-10 faster.
A clock at the center runs about 3.48e-10 faster.
There's a useful diagram in [1]. Image attached. Just follow the green "gravity speedup" line.

If by "gravity vector" you mean the acceleration of gravity (as in "g") then yes, that's 0 at the center, also 0 at infinity and roughly 9.8 m/s^2 at the surface. If the Earth were homogeneous then g would drop by 1/r^2 outside and 1/r inside the surface. In reality the earth is far more interesting and complex. For a good time see [2] and also google: earth PREM

Post by Tom Holmes
A second question (no pun intended) is that given the Earth's elliptical orbit around the
Sun, has there been observed an effect of the change in its gravity on atomic clocks?

Right, an elliptical orbit means both velocity and distance will vary from a mean, so, yes, relativistic effects will also vary from their mean. For GPS the eccentricity is a mere 0.02 so the peak effect is only about 45 ns (this correction is done in GPS receiver software). For a wild satellite orbit like Molniya with eccentricity 0.7, the peak effect is 1.6 us. This data from the "Table 1" in [3]; a very useful paper. But you asked about earth/sun not gps/earth. I'll hunt or calculate those numbers.

/tvb

[1] https://en.wikipedia.org/wiki/Gravitational_time_dilation

[2] https://en.wikipedia.org/wiki/Preliminary_reference_Earth_model

[3] "Relativistic Time Transfer in the Solar System", Robert A. Nelson
https://ieeexplore.ieee.org/document/4319282
https://www.ietf.org/mail-archive/web/dtn-interest/current/pdfnEfIcI08jz.pdf

Tom Holmes

2018-11-21 16:04:45 UTC

Thanks Steve and Tom for helping me sort that out. Much appreciated.

Tom Holmes, N8ZM

-----Original Message-----
From: time-nuts <time-nuts-***@lists.febo.com> On Behalf Of Tom Van Baak
Sent: Wednesday, November 21, 2018 10:49 AM
To: Discussion of precise time and frequency measurement <time-***@lists.febo.com>
Subject: Re: [time-nuts] WWV Doppler Shift

Post by Tom Holmes
So if the SI second is specified at sea level, and we know from Einstein and TVB's
work that going up a mountain changes a clock's period, how would the second be
affected at the center of the Earth ( ignore thermal problems, this is a conceptual
discussion) where the net gravity vector might conceivably zero? Or for that matter,
at a Lagrange point in space? We do have some data from those locations I would think.

Post by Tom Holmes
A second question (no pun intended) is that given the Earth's elliptical orbit around the
Sun, has there been observed an effect of the change in its gravity on atomic clocks?

Attila Kinali

2018-11-22 23:38:38 UTC

On Tue, 20 Nov 2018 15:30:54 -0800

Post by Tom Van Baak
How hard would it be to use a hands-off SDR to produce a 5 MHz WWV phase
data point every second?

Fairly easy. If you go for one of the RTLSDR dongles, you will
have to do the direct-sampling-mod[1], as the tuner chips do not
go this low, and provide an external filter. Using the right
sampling rate (ie something odd) and both 60kHz and 5MHz will
end up somewhere nice. For additional goodness, I would either
lock the 28.8MHz crystal to a stable source, or use a third
RTLSDR dongle to provide the 28.8MHz clock for the other two,
and sample with that a convenient stable signal (eg. the harmonics
of an 10MHz signal).

Alternatively, and much simpler, bu5 more expensive, you can go
for an airspy[2] which already has an external reference input
and two ADC inputs. All you need are some pigtails with U-FL
connectors for the ADC inputs, and an MCX one for the reference
clock source.

Attila Kinali

[1] https://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/

[2] https://www.itead.cc/airspy.html?acc=cfcd208495d565ef66e7dff9f98764da

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

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jimlux

2018-11-23 00:12:47 UTC

Post by Attila Kinali
On Tue, 20 Nov 2018 15:30:54 -0800

Post by Tom Van Baak
How hard would it be to use a hands-off SDR to produce a 5 MHz WWV phase
data point every second?

Fairly easy. If you go for one of the RTLSDR dongles, you will
have to do the direct-sampling-mod[1], as the tuner chips do not
go this low, and provide an external filter.

The one from RTL-SDR (v.3) has the mod from the factory.

You *will* need a filter - there's a nice image; everything above and
below 14.4 MHz is replicated. The digital down converter inside works
just fine, so tuning to 5 MHz (or slightly off) works just fine.

It puts out 8 bit I/Q data - I use the rtl_sdr.c program (slightly
modified) to grab samples and dump them into a file, then post process.
You could probably also use rtl_tcp to stream to some utility hanging
off a socket.

I fire it off with something like this (with the rtl running connected
to a beaglebone):
ssh -f -q ***@beagle101 "rtl_sdr -D 2 -s 1e6 -f 5.001e6 -n 100000
/tmp/capture.bin 2>temperr.txt >temp.txt"

Using the right

Post by Attila Kinali
sampling rate (ie something odd) and both 60kHz and 5MHz will
end up somewhere nice. For additional goodness, I would either
lock the 28.8MHz crystal to a stable source, or use a third
RTLSDR dongle to provide the 28.8MHz clock for the other two,
and sample with that a convenient stable signal (eg. the harmonics
of an 10MHz signal).
Alternatively, and much simpler, bu5 more expensive, you can go
for an airspy[2] which already has an external reference input
and two ADC inputs. All you need are some pigtails with U-FL
connectors for the ADC inputs, and an MCX one for the reference
clock source.
Attila Kinali
[1] https://www.rtl-sdr.com/rtl-sdr-direct-sampling-mode/
[2] https://www.itead.cc/airspy.html?acc=cfcd208495d565ef66e7dff9f98764da

Bob kb8tq

2018-11-20 23:43:08 UTC

Hi

Having looked at WWV with a Carrier -> BFO -> audio card approach (and a radio
locked to an Rb standard …) you have dig a bit to find a situation that is
beyond a tenth of a ppm. If you average over minutes or tens of minutes (which
is exactly what you do with WWVB) the only time you get past 0.1 ppm is the
same sort of day/night propagation mode shift that drives WWVB nuts ….

Bob

Post by Donald E. Pauly
That was the first time that I had seen an xy plot of WWV versus a
stable crystal oscillator. It is even worse than I thought. I had to
look up FRK to see that it is a rubidium standard. I talked to Jim
Maxton the chief engineer of WWVB many times around 1995. At the time
I was in Gila Bend 80 miles southwest of Phoenix. He had a Hewlett
Packard cesium standard at Ft Collins. They were using a dual view
GEOS Geostationary satellite to set the cesium to match the master
clock in Boulder. If the cesium was good to 10^-13, that is 8.6 μs
per day. I can't remember how close he tried to keep it or how often
he adjusted it. It looked like that I could determine the start of
the second to the individual transmitter cycle. Time transfer
accuracy was therefore limited to the height changes of the ionosphere
at sunrise and sunset.
The main disturbance was wind blowing the antenna. Ordinarily the
phase would jitter a few degrees per second. I could tell the wind
speed by the phase jitter without checking the Ft Collins weather. If
memory serves, the loaded Q at 60 kc was about 200. A half percent
tuning error caused a 45° phase error. I have seen a 45° excursions
on several occasions over a minute more than once. My receiver had a
slow lock mode that could spot them. It also had a 45° phase switch
on the 100 kc local oscillator to eliminate the station ID from 10 to
15 minutes after the hour. There was therefore no disturbance in lock
during it. I was never able to measure any error in the 45° phase
advance. One degree would have been obvious.
When I first got my receiver going, the phase would advance nearly 40°
at the start of the second when the power was reduced by 10 db. It
had been doing so for years and nobody noticed it. Maxton took an
unneeded condenser out of his time code generator which fixed most of
it. The new transmitter fixed the rest.
Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level. How did you correct for
altitude on yours? I presume that frequency is defined at sea level
but I don't know that. Sea level clocks at the North or South Poles
run fast relative to those at equator sea level by 1.192·10^-12.
WB0KVV
πθ°μΩω±√·÷Γλφ|Δ

In general terms, the coherence time of the ionosphere is single digit
seconds - that is, there's essentially no correlation between
propagation path at one time and the propagation path 10 seconds later.
The "general length" of the path will be the same, but the details
different.
The actual ionization in the ionosphere can best be described as moving
"clouds" there's a fair amount of spatial inhomogeneity. In the same
sense that milk reflects light from a multitude of little fat globules.

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John Ackermann. N8UR

2018-11-21 00:38:25 UTC

A few years ago I did some measurements of WWV Doppler shift, measured by a 0.1 Hz resolution you get in an HP 3586C selective voltmeter. It's not quite a phase record but does show the significant shifts that occur.

See https://www.febo.com/pages/hf_stability/

John
----

Post by Bob kb8tq
Hi
Having looked at WWV with a Carrier -> BFO -> audio card approach (and a radio
locked to an Rb standard …) you have dig a bit to find a situation that is
beyond a tenth of a ppm. If you average over minutes or tens of minutes (which
is exactly what you do with WWVB) the only time you get past 0.1 ppm is the
same sort of day/night propagation mode shift that drives WWVB nuts ….
Bob

Post by Donald E. Pauly
That was the first time that I had seen an xy plot of WWV versus a
stable crystal oscillator. It is even worse than I thought. I had

Post by Donald E. Pauly
look up FRK to see that it is a rubidium standard. I talked to Jim
Maxton the chief engineer of WWVB many times around 1995. At the

time

Post by Donald E. Pauly
I was in Gila Bend 80 miles southwest of Phoenix. He had a Hewlett
Packard cesium standard at Ft Collins. They were using a dual view
GEOS Geostationary satellite to set the cesium to match the master
clock in Boulder. If the cesium was good to 10^-13, that is 8.6 μs
per day. I can't remember how close he tried to keep it or how often
he adjusted it. It looked like that I could determine the start of
the second to the individual transmitter cycle. Time transfer
accuracy was therefore limited to the height changes of the

ionosphere

Post by Donald E. Pauly
at sunrise and sunset.
The main disturbance was wind blowing the antenna. Ordinarily the
phase would jitter a few degrees per second. I could tell the wind
speed by the phase jitter without checking the Ft Collins weather.

Post by Donald E. Pauly
memory serves, the loaded Q at 60 kc was about 200. A half percent
tuning error caused a 45° phase error. I have seen a 45° excursions
on several occasions over a minute more than once. My receiver had a
slow lock mode that could spot them. It also had a 45° phase switch
on the 100 kc local oscillator to eliminate the station ID from 10 to
15 minutes after the hour. There was therefore no disturbance in

lock

Post by Donald E. Pauly
during it. I was never able to measure any error in the 45° phase
advance. One degree would have been obvious.
When I first got my receiver going, the phase would advance nearly

40°

Post by Donald E. Pauly
at the start of the second when the power was reduced by 10 db. It
had been doing so for years and nobody noticed it. Maxton took an
unneeded condenser out of his time code generator which fixed most of
it. The new transmitter fixed the rest.
Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level. How did you correct for
altitude on yours? I presume that frequency is defined at sea level
but I don't know that. Sea level clocks at the North or South Poles
run fast relative to those at equator sea level by 1.192·10^-12.
WB0KVV
πθ°μΩω±√·÷Γλφ|Δ

Starting 1970 I used a modified Tracor 599H on WWVB with excellent

results. It had a mechanical counter with 100 nsec, resolution. Noisy
but perfect. Yes you have to take Ionosphere sunrise and sunset in to
consideration and the hourly shift, but being a very early riser 4AM
because of Europe no problem. Better than 2 E-11 per day and 4 E-14 per
month.

In the 90 ties with my FRK having temperature and aging control

frequency was better than 1 E-12 all the time.

Bert Kehren
In a message dated 11/19/2018 9:58:39 PM Eastern Standard Time,
HF propagation of WWV or WWVH is horrible compared to VLF

propagationof WWVB at 60 kc. In this video the 5 mc WWV signal from Ft
Collins,Colorado is being received in New Jersey. It was compared
against astable 5mc crystal source. You can see a shift of a few
cycles persecond over a few seconds. This is due to the movement up or
down ofthe ionosphere at a substantial fraction of the speed of sound.

Post by jimlux
In general terms, the coherence time of the ionosphere is single

digit

Post by jimlux
seconds - that is, there's essentially no correlation between
propagation path at one time and the propagation path 10 seconds

later.

Post by jimlux
The "general length" of the path will be the same, but the details
different.
The actual ionization in the ionosphere can best be described as

moving

Post by jimlux
"clouds" there's a fair amount of spatial inhomogeneity. In the

same

Post by jimlux
sense that milk reflects light from a multitude of little fat

globules.

Post by Donald E. Pauly
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Post by Donald E. Pauly
and follow the instructions there.

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Tom Van Baak

2018-11-21 17:12:06 UTC

Post by Donald E. Pauly
Ft Collins is at 5,003 ft and clocks there run fast by 1.663·10^-13.
(g/c^2)/meter) compared to sea level. How did you correct for
altitude on yours? I presume that frequency is defined at sea level
but I don't know that.

Yes. Standard time & frequency is defined at sea level.

But in the context of a WWV/WWVB thread it's best not to say that the clocks in Ft Collins "run fast". The clocks at the transmitter site are set and maintained to keep standard time. They don't run fast or slow; they tick SI seconds and they report UTC time. They are more like "UTC-disciplined oscillators" than stand-alone cesium clocks.

What you may be thinking of is that if you built a cesium clock and turned it on in Ft Collins it would run fast, faster than a similar clock running at sea level. That is true. But it wouldn't be UTC then. That's why all the national timing labs coordinate their clocks so they tick the same rate, in spite of the actual elevation of the lab.

Post by Donald E. Pauly
Sea level clocks at the North or South Poles
run fast relative to those at equator sea level by 1.192·10^-12.

No. Clocks at sea level all tick at the same rate.

You might be thinking that because the earth spins, clocks on the equator run slower due to SR. But remember the earth is not a sphere, but an oblate spheroid. So clocks on the equator are also farther from the center of the earth and thus run faster due to GR. The two effects neatly cancel each other (not by accident). In fact that is one definition of sea level -- the point where all clocks run the same, minimum rate.

See also my reply to N8ZM earlier. Clocks run faster both as you go below or as you go above the surface. So altitude is the key factor, not latitude or longitude. I can go into this in more detail if you want.

/tvb

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jimlux

2018-11-21 17:18:58 UTC

Post by Tom Van Baak
You might be thinking that because the earth spins, clocks on the equator run slower due to SR. But remember the earth is not a sphere, but an oblate spheroid. So clocks on the equator are also farther from the center of the earth and thus run faster due to GR. The two effects neatly cancel each other (not by accident). In fact that is one definition of sea level -- the point where all clocks run the same, minimum rate.

I'm not sure I understand why the slowing due to spin happens to exactly
match the speedup from altitude.

The spheroidness of the Earth is, indeed, mostly due to the rotation,
but that would be related to the overall material properties and
rotation rate, wouldn't it? Would a blob of water and a blob of rock
both converge to the same shape (water would move faster, but over
billions of years, rock would get there)

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Steve Allen

2018-11-21 17:59:35 UTC

Post by jimlux
I'm not sure I understand why the slowing due to spin happens to exactly
match the speedup from altitude.
The spheroidness of the Earth is, indeed, mostly due to the rotation, but
that would be related to the overall material properties and rotation rate,
wouldn't it? Would a blob of water and a blob of rock both converge to the
same shape (water would move faster, but over billions of years, rock would
get there)

The important aspect is that mean sea level (or the geoid) is an
equipotential. That is saying "from where I am there is no downhill".
Getting from any point on the equipotential to Infinity requires the
same amount of energy.

If the earth were alone in space and not rotating and of uniform
density everywhere then the surface of the ocean would be a sphere.
If it were rotating then an oblate sphere.

Add in continents and that becomes a distorted surface with geoid
undulations.

Add in sun and moon and there are diurnal tides which happen because
the water can see that "right now downhill is over there".

But as of last week TAI is not defined by sea level nor the geoid.
It is defined as the rate of the cesium resonance on an equipotential
surface at a depth of W_0 = 62636856.0 m^2 s^-2
This is the same change that the IAU made 18 years ago in response to
the IAG defining the potential of the geoid. See pages 12 and 33
https://www.bipm.org/utils/en/pdf/CGPM/Convocation-2018.pdf

The fixed value of the potential means that the rate of TAI is now
defined not to change over geologic time scales as sea level changes.

--
Steve Allen <***@ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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Tom Van Baak

2018-11-22 04:01:18 UTC

Post by jimlux
I'm not sure I understand why the slowing due to spin happens to exactly
match the speedup from altitude.

Right. It's not obvious to me either. I've been looking some time for the right book, article, or web page to hand out when people ask that question. The same goes for a rotating planet made of foam vs. water vs. diamond question. It's possible that separating "SR" and "GR", as newcomers to relativity (me too) often do, is itself the problem in this case. When two things magically equal or cancel there's usually a deeper reason.

Let me encourage you ask around JPL over the months and see if any of your relativistic geodesy friends has a clean answer. Web searches on topics of relativity tend to have poor SNR. That said, to start with, see if one these links helps more than it hurts:

https://arxiv.org/pdf/gr-qc/0501034.pdf

https://physics.stackexchange.com/questions/126919/does-time-move-slower-at-the-equator

https://thatsmaths.files.wordpress.com/2012/11/two-clocks1.pdf

If you (and Steve, Donald, Bob, et al.) have half an hour to study these, let me know what you think. I've been down this rabbit hole myself.

The good news in this case is that we know what the right answer is. Any number of legacy and modern experiments show altitude is a factor in atomic clock rate but not latitude. The bad news is I / we don't have a convincing one sentence explanation for it.

/tvb

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Steve Allen

2018-11-22 04:10:34 UTC

The gravitational redshift is your depth in the gravitational
potential well. That is how far down you are from Infinity.
The geoid is an equipotential surface.

--
Steve Allen <***@ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855
1156 High Street Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m

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Bob kb8tq

2018-11-21 17:43:54 UTC

Post by Tom Van Baak

Yes. Standard time & frequency is defined at sea level.
But in the context of a WWV/WWVB thread it's best not to say that the clocks in Ft Collins "run fast". The clocks at the transmitter site are set and maintained to keep standard time. They don't run fast or slow; they tick SI seconds and they report UTC time. They are more like "UTC-disciplined oscillators" than stand-alone cesium clocks.
What you may be thinking of is that if you built a cesium clock and turned it on in Ft Collins it would run fast, faster than a similar clock running at sea level. That is true. But it wouldn't be UTC then. That's why all the national timing labs coordinate their clocks so they tick the same rate, in spite of the actual elevation of the lab.

Gravity is not the only thing you need to “standardize” if you are building a Cs clock from scratch
in your basement. Magnetic field also quickly gets its nasty fingers into things as well. There are other
environmental impacts, even on a Cs standard. For “best” performance you do indeed need to sweat
what seem like really minor details.

This ultimately gets back to a never ending debate about depending on one design for all of your standards.
Even if a *really* good job was done - how can you be sure? Having multiple this and that in your comparison
“pool” is the answer to that concern.

Bob

Post by Tom Van Baak

Post by Donald E. Pauly
Sea level clocks at the North or South Poles
run fast relative to those at equator sea level by 1.192·10^-12.

No. Clocks at sea level all tick at the same rate.
You might be thinking that because the earth spins, clocks on the equator run slower due to SR. But remember the earth is not a sphere, but an oblate spheroid. So clocks on the equator are also farther from the center of the earth and thus run faster due to GR. The two effects neatly cancel each other (not by accident). In fact that is one definition of sea level -- the point where all clocks run the same, minimum rate.
See also my reply to N8ZM earlier. Clocks run faster both as you go below or as you go above the surface. So altitude is the key factor, not latitude or longitude. I can go into this in more detail if you want.
/tvb
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Magnus Danielson

2018-11-24 11:49:21 UTC