
ESA sets clock by distant spinning stars - based2
https://www.esa.int/Our_Activities/Navigation/ESA_sets_clock_by_distant_spinning_stars
======
watersb
Ten years ago, I was a software engineer at the National Radio Astronomy
Observatory [0]. We used the Very Long Baseline Array telescope to calibrate
the rate at which the Earth's surface moves, relative to distant pulsars.

Atomic "clocks" are more like CPU "clock speed". Atomic clocks give a very
precise measure of a time interval.

But they don't tell you what time it is. The time of day needs to express both
the idea of the rate of time, and also the position of the Sun and stars,
relative to the position of the Earth.

Like, we use "number of seconds since midnight, 1 January 1970" and expect
that number to map to a time like "2018-12-25 20:37:44 UTC", and we expect
that value to map onto a time about four days after the Northern Hemisphere
winter solstice.

Pulsar surveys make sure that those two concepts line up. Most of the time.

[0]: [https://nrao.edu](https://nrao.edu)

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remcob
Ever since I heard about the first Pulsar clock in Dansk [1] I've been
wondering what it would take to DIY one. Unfortunately, documentation on the
Dansk clock is nonexistent, giving me little to start on.

I hope ESA documents and open sources the work. If the required
antenna/receiver sensitivity is not too ridiculous, this might make a cool
project.

[1]
[https://en.wikipedia.org/wiki/Pulsar_clock](https://en.wikipedia.org/wiki/Pulsar_clock)

~~~
jcims
It’s possible but but not easy.

[https://www.rtl-sdr.com/detecting-pulsars-rotating-
neutron-s...](https://www.rtl-sdr.com/detecting-pulsars-rotating-neutron-
stars-with-an-rtl-sdr/)

I think some of the newer SDRs with 12-16 bits per sample and 20+ MHz
bandwidth might be substantiated more suited to the task.

~~~
remcob
The graph in the article was made using a 30m dish, which I would not consider
DIY territory. The referenced article is a study on what size dish is required
with a RTL-SRD. Their conclusion is a minimum of 6m dish is required with
simple post processing. They conclude that with improved hardware and better
post processing 3.5m dish should be usable.

I'm thinking phased array. It's a more scalable method to getting large
apertures and it can constructed to target multiple pulsars at once.

~~~
jcims
Agree on phased array as a good option to get necessary aperature.

My thinking on the better hardware was that getting 10Mhz bandwidth allowed
them to reduce the dish size to 3.5m. Better dynamic range and even more
bandwidth might get you to <3m, which could possibly be in old C-band dish
antenna range.

Edit: More recent exercise -
[http://hawkrao.joataman.net/pulsar/results/index.html](http://hawkrao.joataman.net/pulsar/results/index.html)

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fyfy18
How do they take into account the relatavistic effects of time dilation? Just
from Earth orbiting around the solar system, the relative velocity could
change by up to 60km/s. I guess that's relativity easy to deal with as it's
known, but what about our orbit around the galaxy, or the pulsar's own orbit?

~~~
nraynaud
I think you can do everything from the perceived time.

GPS system need to reconstruct the time of the satellites because we know they
tick in sync, but those star tick independently it’s really about observing
the blinking.

~~~
throwawaymath
Yes but satellites revolve around us consistently. A pulsar's distance from
Earth is dynamically changing according to several distinct systems: the
Earth's orbit, the Sun's orbit and the pulsar's orbit.

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saagarjha
> The gradual diversion of pulsar time from ESTEC’s UTC time can therefore be
> tracked – anticipated at a rate of around 200 trillionths of a second daily.

If you were curious how much this clock drifts. I think we might have more
accurate atomic clocks, though, so this still isn’t practical for actual
precise timekeeping at that level.

~~~
0xfaded
It would be useful for measuring the angular velocity decay rate though.

~~~
saagarjha
Of pulsars? For sure. But the clock you’d use for that would be better than
this one, obviously.

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supernova87a
I think this is kind of gimmicky, or at least very far from practical
applications aside from the satellite calibration they're talking about.

Their radio telescopes are located in Europe. How would you have constant time
signals available? I would guess the errors in synchronizing / matching the
signals across telescopes in the ground would be by far the larger error
compared to the signal itself.

But who knows, I'm just an outside observer.

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jessriedel
Key paragraph:

> “A timescale based on pulsar measurements is typically less stable than one
> using atomic or optical clocks in the short term but it could be competitive
> in the very long term, over several decades or more, beyond the working life
> of any individual atomic clock.

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starbeast
Awesome. So when can I purchase a Galactic Positioning System running off
pulsars?

~~~
nraynaud
Never, they don’t control the local phases of those clocks.

~~~
starbeast
I was only half joking. This is actually being looked into by NASA -
[https://www.livescience.com/62309-galactic-positioning-
syste...](https://www.livescience.com/62309-galactic-positioning-system-
nasa.html)

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sandGorgon
Is there a NTP server that exposes this time ? How do people use this ?

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betimsl
This is wrong. In Europe, the clock is set by KEK (Kosova Energy Corporate.)

For people who rush to down vote: this is just a joke. Obviously ESA should
decide how we measure time.

