[HN Gopher] Future ultra-precise timing links to geosynchronous ...
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Future ultra-precise timing links to geosynchronous satellites
Author : raattgift
Score : 81 points
Date : 2023-06-22 17:29 UTC (5 hours ago)
(HTM) web link (www.nist.gov)
(TXT) w3m dump (www.nist.gov)
| dheera wrote:
| > Last year, scientists drove up Mauna Loa volcano on Hawai'i,
| aimed a laser at a reflector positioned on Haleakala peak on
| Maui, and beamed rapid pulses of laser light through 150
| kilometers of turbulent air.
|
| Stupid question ... why would they pick the tops of two volcanoes
| on two islands instead of two mountain peaks on the continent
| that have roads between them, overnight shipping for whatever
| components they may need, easier hiring, and no random lava flows
| destroying equipment?
| foota wrote:
| Why take a trip to the mountains in Boulder's backyard when you
| can go to Hawaii? :-)
|
| But also, I imagine it has to do with the Mauna Kea
| Observatory, where half the setup was.
|
| I'm not sure if it's for the observatory (e.g., it says they
| used a light source there, or possibly because they have some
| scientific equipment set up there already), or maybe because of
| the environment. This is what wikipedia says about the site
| "The location is near ideal because of its dark skies from lack
| of light pollution, good astronomical seeing, low humidity,
| high elevation of 4,205 meters (13,796 ft), position above most
| of the water vapor in the atmosphere, clean air, good weather
| and low latitude location." of course the astronomical parts
| don't matter, but some of the rest is likely relevant.
|
| Also, many mountaintops that high will be covered with snow.
| Mauna Kea has some, but probably not as much.
| throw0101c wrote:
| For the idea of latency, Grace Hopper explaining nanoseconds is
| always instructive:
|
| * https://www.youtube.com/watch?v=9eyFDBPk4Yw
| hinkley wrote:
| I wonder if one can get ahold of a Hopper Nanosecond today or
| if they all got identified as junk wire and thrown away.
| WJW wrote:
| Luckily, creating additional nanoseconds is cheap. The value
| was always in their symbolism, not in them being given out by
| admiral Hopper herself.
| hinkley wrote:
| Says you!
| WJW wrote:
| I do! If you're interested, I'll send you brand new
| nanoseconds including an Official Certificate stating
| that they are Officially Certified to be equally long as
| Hopper nanoseconds (within reasonable manufacturing
| standards tm of course).
| ortusdux wrote:
| I wonder how this compares to the precision of the GRACE-FO Laser
| Ranging Interferometers. Maybe this new comb method would allow
| for newer cheaper versions of the satellites.
|
| https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#GRACE_Follo...
| Terr_ wrote:
| Relevant for anyone who wants to know more about GPS, Bartosz
| Ciechanowski has this excellent interactive exhibit on how it
| works, from the basics of triangulation to orbital paths, signal
| structure, noise-avoidance, etc.
|
| https://ciechanow.ski/gps/
| refibrillator wrote:
| This is linked in the article but easy to miss, it has helpful
| visualizations of the "time programmable frequency comb":
| https://www.nist.gov/news-events/news/2022/10/break-new-grou...
|
| Some notable numbers from the paper:
|
| > the researchers' time programmable frequency comb is capable of
| operating at this quantum limit, where fewer than one photon in a
| billion reaches its target device. It worked even when the laser
| was sending out only 40 microwatts of power, or about 30 times
| less than a laser pointer uses.
|
| > the pulse time and phase are digitally controlled with
| +-2-attosecond accuracy
|
| > Over 300 km between mountaintops in Hawaii with launched powers
| as low as 40 mW, distant timescales are synchronized to 320
| attoseconds
|
| > at 4.0 mW transmit power, this approach can support 102 dB link
| loss, more than sufficient for future time transfer to
| geosynchronous orbits
| aftbit wrote:
| Very interesting stuff. One of the fun bits of being a time-nut
| is that there are somewhere around 15 orders of magnitude
| available to play with. Getting clocks synced to within a few
| milliseconds is child's play, even for an amateur with $15 of
| hardware. Getting clocks synced within a few dozen microseconds
| is possible with GPS and some minor effort. Getting clocks synced
| within a few nanoseconds requires a lot more effort. I have never
| attempted anything below the ns range, but this process seems to
| produce clocks in sync within hundreds of attoseconds! This is
| within around 10^-16 seconds. Quite amazing if you ask me.
| comboy wrote:
| I cannot comprehend. Napkin math says if you move like 50
| nanometers within one second this is enough acceleration over
| time to make these clocks out of sync because relativity. I
| mean, do points on earth do not move that much by themselves if
| they are far enough?
| dogline wrote:
| I didn't realize that a "time-nut" was a thing. Never thought
| about people doing this as a sort of hobby. Congrats!
| hotpotamus wrote:
| https://en.wikipedia.org/wiki/Clock_of_the_Long_Now
|
| Jeff Bezos might be among them since he has funded this
| project.
| fanf2 wrote:
| You can find a community of time nuts via
| http://www.leapsecond.com/time-nuts.htm
| superdug wrote:
| ok, but....
|
| If you set two clocks to the same time and put one at the bottom
| of the ocean and one at the top of a mountain ... after time,
| they will drift apart ... so is this ultra precise time in space
| making up for gravity time distortion as well?
| bequanna wrote:
| The question is which time do we consider to be the "correct"
| time. Turns out, we've decided to use a clock in Colorado as
| the time of record and then occasionally sync that clock with
| GPS satellites.
|
| https://timeandnavigation.si.edu/satellite-navigation/gps/sy...
| fanf2 wrote:
| It's several layers more complicated than that.
| https://dotat.at/@/2023-05-26-whence-time.html
|
| The USNO Alternate Master Clock at Schriever SFB is not the
| clock of record. It is synchronized to the USNO Master Clock
| in Washington DC.
|
| The USNO Master Clock generates the US DOD's official time,
| but it is also not the clock of record. There is also NIST's
| clock, which is the official time for civilian use in the
| USA. And the NPL's clock in Teddington for the UK. And ESA's
| clock in Noordwijk for Galileo. And the PTB's clock in
| Braunschweig for Germany. etc. usw.
|
| All these clocks and many more contribute their measurements
| and cross-comparisons to the BIPM in Paris on a regular
| schedule. The BIPM calculates a consensus timescale from
| these measurements, which takes the form of retrospective
| corrections published in BIPM Circular T.
|
| Circular T is the time of record. But it is not the most
| accurate time available because of its relatively short
| averaging time.
|
| The best time is TT, terrestrial time, a uniform timescale
| that ticks at the same rate as the SI second as measured on
| the rotating geoid, i.e. the notional surface of equal
| gravitational potential which is the general relativity
| equivalent of mean sea level.
|
| Well, not TT itself, but TT(year). The BIPM periodically
| publishes retrospective corrections going back several
| decades, saying what the error in TT was back then based on
| their best understanding now.
| colechristensen wrote:
| It's much worse than that, last I heard we could measure the
| differences in time passage separated by only a few vertical
| feet.
|
| Ultra precise time in space absolutely has to account for
| relativity changing clock rates based on how deep you are in
| the gravity well. GPS would be all but useless without it.
| raattgift wrote:
| The Nature paper corresponds with
| https://arxiv.org/abs/2212.12541
| pikrzyszto wrote:
| See also White Rabbit Project, i.e. how to synchronize clocks
| over the internet with sub-ns accuracy
| https://en.wikipedia.org/wiki/White_Rabbit_Project
| aftbit wrote:
| Thanks for the link. I had not heard of this project, though I
| knew about CERN's experiments with synchronous ethernet. Tiny
| nit though - the plan was never to sync over the internet, with
| its variable latency and multiple PHY formats, but instead to
| provide an ethernet network with links up to 10km long that can
| provide a timing and phase reference for the LHC.
| Zenst wrote:
| Could you, in effect with this level of precision, get a bonus of
| a gravity wave detector?
| colechristensen wrote:
| LIGO detects the changes in distance between mirrors down to
| 1/10000th of a proton
|
| 1 atto-light-second is a few hydrogen atoms long. So still
| seems like quite a few orders of magnitude needed for gravity
| wave detection but perhaps with the lengths involved?
|
| Some more expertise is needed. I would guess probably not but
| also not so far off as to be crazy.
| nashashmi wrote:
| * * *
| throwway120385 wrote:
| I wonder -- if you can use this to synchronize clocks with very
| low power signals, could you use this to transmit data with very
| low power signals? If so, you could conceivably transmit data
| with very little power over vast distances.
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