ESA sets clock by distant spinning stars | Hacker News Reader

ESA sets clock by distant spinning stars

esa.int

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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

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

It’s possible but but not easy.

https://www.rtl-sdr.com/detecting-pulsars-rotating-neutron-s...

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.

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.

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

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?

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.

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.

> 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.

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

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

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.

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.

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

First you need a peripheral that can see the pulsar.

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

I was only half joking. This is actually being looked into by NASA - https://www.livescience.com/62309-galactic-positioning-syste...

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

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.

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