IEEE Spectrum - Dec 2017 - article on chip-scale atomic frequency reference
In the standards definitions that include "at sea level", the question these days is "which sea level?". As ocean temperature changes sea level will change (except maybe in Washington DC). Will the standards be amended to include something like "at sea level in 1990" or will the value being defined drift around with the changing sea level?
On 12/9/17 11:14 AM, Mark Sims wrote:
In the standards definitions that include "at sea level", the question these days is "which sea level?". As ocean temperature changes sea level will change (except maybe in Washington DC). Will the standards be amended to include something like "at sea level in 1990" or will the value being defined drift around with the changing sea level?
Sea Level is arbitrary anyway - what is usually meant is "zero elevation
relative to some specified geoid".
The Pacific and Atlantic oceans have different mean heights relative
to the geoid
In the United States, for a long time it was the North American Datum
(NAD) that was the reference, but now, it's probably WGS84 (I'm too lazy
to go look it up).
WGS84 has a very precise definition in terms of the semiaxes lengths,
and their orientation relative to stellar references. WGS 84 uses the
IERS reference meridian for longitude.
Flattening of 1/298.257,223,563
equatorial radius of 6,378,137 m
so polar radius of 6,356,752.3142 m
The Earth Grav Model (EGM96) defines the geoid, last revised in 2004.
that model defines the nominal sea surface with spherical harmonics.
There's something like 100,000 specific terms in that gravity model.
Sourceforge has a program that will tell you geoid height for a given
lat lon.
https://geographiclib.sourceforge.io/cgi-bin/GeoidEval
Near my house (34N, 119W), it appears that the EGM96 height is -37.17 m,
relative to the ellipsoid defined above.
It so happens that due to crustal movement, my house is gradually rising
about 1 cm/year, but I don't know if the local sea level also rises to
match, or if the beach is getting farther away.
One can measure this, in theory
https://www.unavco.org/education/resources/modules-and-activities/gps-california-plate-motion/gps-california-plate-motion.html
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Hi
I suspect that at the practical level, you define standard atmospheric pressure, standard
gravity, standard magnetic field ….. and on down the list. At some point “sea level” becomes
a redundant expression.
Bob
On Dec 9, 2017, at 2:14 PM, Mark Sims holrum@hotmail.com wrote:
In the standards definitions that include "at sea level", the question these days is "which sea level?". As ocean temperature changes sea level will change (except maybe in Washington DC). Will the standards be amended to include something like "at sea level in 1990" or will the value being defined drift around with the changing sea level?
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Hi,
On 12/09/2017 09:13 PM, Bob kb8tq wrote:
Hi
I suspect that at the practical level, you define standard atmospheric pressure, standard
gravity, standard magnetic field ….. and on down the list. At some point “sea level” becomes
a redundant expression.
The standard acceleration is internationally agreed at 3rd CGPM in 1901
to be 9.80665 m/s^2. So, that is "sea level". See SI brochure, I used
version 8 in english, page 143.
This is also the standard value I have in my calculators and used for
all my acceleration calculations.
In practice labs have their contributions into EAL/TAI corrected for
their deviation from "sea level" for proper frequency of TAI.
Cheers,
Magnus
Bob
On Dec 9, 2017, at 2:14 PM, Mark Sims holrum@hotmail.com wrote:
In the standards definitions that include "at sea level", the question these days is "which sea level?". As ocean temperature changes sea level will change (except maybe in Washington DC). Will the standards be amended to include something like "at sea level in 1990" or will the value being defined drift around with the changing sea level?
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On Sat, December 9, 2017 2:39 pm, Magnus Danielson wrote:
The standard acceleration is internationally agreed at 3rd CGPM in 1901
to be 9.80665 m/s^2.
So does that mean e.g. NIST and BIPM need to measure the acceleration at
their respective locations to within parts in 10^17 or 10^18 in order to
compare their frequency standards?
That seems not practical.
Chris Caudle
Hi,
On 12/09/2017 10:02 PM, Chris Caudle wrote:
On Sat, December 9, 2017 2:39 pm, Magnus Danielson wrote:
The standard acceleration is internationally agreed at 3rd CGPM in 1901
to be 9.80665 m/s^2.
So does that mean e.g. NIST and BIPM need to measure the acceleration at
their respective locations to within parts in 10^17 or 10^18 in order to
compare their frequency standards?
That seems not practical.
No, you have a large scale-factor so you really don't need that much of
precision to achieve it.
Cheers,
Magnus
Hi
If the frequency sensitivity is 1x10^-13 / G you don’t need a lot of precision
in your measurement of G. The same issues apply to things like magnetic
field and the rest.
Bob
On Dec 9, 2017, at 4:02 PM, Chris Caudle chris@chriscaudle.org wrote:
On Sat, December 9, 2017 2:39 pm, Magnus Danielson wrote:
The standard acceleration is internationally agreed at 3rd CGPM in 1901
to be 9.80665 m/s^2.
So does that mean e.g. NIST and BIPM need to measure the acceleration at
their respective locations to within parts in 10^17 or 10^18 in order to
compare their frequency standards?
That seems not practical.
Chris Caudle
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and follow the instructions there.
Mark,
In the standards definitions that include "at sea level", the question these days is "which sea level?".
Chris,
So does that mean e.g. NIST and BIPM need to measure the acceleration at
their respective locations to within parts in 10^17 or 10^18 in order to
compare their frequency standards?
Yes, all national timing labs do this to one degree or another. To operate and compare clocks at that level of precision you need to accurately know your geopotential, which is sort of like knowing the acceleration of gravity, or elevation.
But it's not one-to-one as you suggest. A 1 meter change in elevation corresponds to a frequency offset of about 1e-16. So for 1e-18 levels of performance you "only" need to know g, or your elevation to 1 cm accuracy.
That seems not practical.
It is practical, and necessary, and really cool!
Here are some papers that will give you an idea how much work it takes to make clocks at the 1e-16 and 1e-17 level. I mean, it's not like you just throw some cesium atoms in a bottle, rub the lamp, and out comes a genie singing 9192.631770 MHz.
These two examples describe the complexity of a primary cesium standard:
"Accuracy evaluation of the primary frequency standard NIST-7", 2001
http://tf.nist.gov/timefreq/general/pdf/1497.pdf
"Accuracy evaluation of NIST-F1", 2002
http://tf.nist.gov/timefreq/general/pdf/1823.pdf
In the first paper, see especially tables 1, 3 and 4 for an idea of the corrections they must apply. You'll notice that the largest correction is gravitational. Therefore part of their job in making a primary standard is to measure gravity at the exact point where the cesium atoms operate. And yes, that gets you in the dirty world of what's underground, what mountains are nearby, where's the water table this week, what shape the earth really is, and the phase of the moon, etc.
These two examples describe the complexity of precisely measuring gravity in order to calibrate an atomic clock:
"The relativistic redshift with 3 × 10−17 uncertainty at NIST, Boulder, Colorado, USA", 2003
http://tf.boulder.nist.gov/general/pdf/1846.pdf
"A re-evaluation of the relativistic redshift on frequency standards at NIST, Boulder, Colorado, USA", 2017
http://tf.boulder.nist.gov/general/pdf/2883.pdf
Really, all four papers are worth a quick read, even if you just look at the tables and photos.
/tvb
Citerar Mark Sims holrum@hotmail.com:
In the standards definitions that include "at sea level", the
question these days is "which sea level?". As ocean temperature
changes sea level will change (except maybe in Washington DC). Will
the standards be amended to include something like "at sea level in
1990" or will the value being defined drift around with the changing
sea level?
From the current Swedish vertical datum.
(http://www.lantmateriet.se/globalassets/kartor-och-geografisk-information/gps-och-matning/geodesi/rapporter_publikationer/rapporter/lmv-rapport_2007_4.pdf)
"This realisation was made using the Normaal Amsterdams Peil (NAP)
as zero level
in the traditional European way. [...] It has for instance been
questioned whether NAP is the most suitable way to fix the
zero level. Is it not better to wait for a so-called World Height
System (WHS), which is fixed using GPS and a global geoid model of cm
accuracy?"
Has the World Height System been agreed/released?
http://www.euref.eu/documentation/Tutorial2015/t-04-02-Ihde.pdf
Picture showing reference origin for vertical datums in Europe.
https://en.wikipedia.org/wiki/File:Vertical_references_in_Europe.svg
--
Björn