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Comment by rich_sasha | Hacker News Reader

I guess timekeeping is relatively easy? These systems would only operate independently for a few hours tops. I would imagine even a standard quartz movement would be accurate enough.

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Depends on what you're using time for. If you are doing advanced anti-jamming for comms for instance, you want extremely accurate timing (more accurate means you can frequency hop faster and do better anti-jamming).

> I guess timekeeping is relatively easy...... would imagine even a standard quartz movement would be accurate enough.

Good Lord! How wrong can you get!

Very precise timing (often taken from GNSS for convenience) is needed for much of the modern word, from IP, cellular and DAB networks, to AC phase matching the electrical mains grid. Quartz clocks are nowhere near accurate enough for these purposes.

This government report makes very sobering reading: https://www.gov.uk/government/publications/satellite-derived...

TLDR: Our dependence on GNSS for timing almost dwarfs that for navigation. And we urgently need to consider using backups (be that local atomic clocks, or long wave time signals).

I mean, considering celestial navigation was a thing long before we had accurate clocks… I’d venture they aren’t wrong at all. Or did you forget that people have been doing celestial navigation by hand for over two millennia?

Celestial navigation actually drove the development of accurate clocks

https://timeandnavigation.si.edu/navigating-at-sea/longitude...

Quartz clocks didn't overtake chronometers in terms of accuracy until the mid 20th century, and chronometers will still beat regular crystals like you'd find in cheap electronics.

> Celestial navigation actually drove the development of accurate clocks

That's true, but that still doesn't change the fact that you don't need nanosecond precision for this purpose. At the equator, 1 second precision gives you roughly 500m accuracy, which is already much higher than what the celestial imagery allows here (4km in the paper).

Clearly this method isn't limited by clock accuracy at all.

1 second precision is a lot. A typical quartz resonator will drift by about .5 seconds per day at ambient conditions. In this paper they set the clock with GPS right before flight and they only fly for a few hours, so it's tolerable. But in a GPS denied environment where you can't set the clock right before flight, ie exactly where you are using this instead of gps, clock accuracy will become the dominant factor affecting your accuracy after a few days.

> But in a GPS denied environment where you can't set the clock right before flight

First of all I don't think the use-case involves the drones operators being deprived of GPS, but even if they were: you don't need GPS to get sub-second accurate time, any internet connection will do it thanks to NTP. Sure it's not as accurate as GPS, but it's still way more accurate than what you need for this to work. Heck, even sharing time through a phone call would work well enough.

In the context of position keeping I think it's not too bad.

If we focus on longitude, where timing I guess matters more, the equator moves at a speed of about 0.46 km/s. So I guess being out by 1 second translates to precisely 0.46km error. That's second order compared to the stated error of 4 km, and it will be smaller still away from the equator.

I'm working off the assumption that such a drone can sync up to an accurate time source at launch, and then only needs maintain good timekeeping for its time in the air. I guess without the accurate initial time source, it gets bad. Being a minute out is suddenly 30km of latitude direction away.

Plus I think most decent quartz oscillators have a drift measured in single-digit PPM (or less) so even 100ms error over a single sortie would be surprising.

> Our dependence on GNSS for timing almost dwarfs that for navigation.

Galileo satellites also now sign the timestamp (IIRC) via a Merkle tree so you know it isn't spoofed.

Some clueless downvoting here!

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