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Comment by leggomylibro | Hacker News Reader

I could be reading this wrong, but 1/3 of a second within 100 hours seems really good, like something you'd get from a temperature-controlled crystal oven.

I don't mean to second-guess them in an area I know so little about, but if that was enough to cause a serious issue in the span of only a few days, shouldn't the devices be designed with a separate synchronization system, at least as a backup? Maybe GPS?

Which brings up a sort of interesting question...would a Patriot missile system even have receivers for a weak public signal like GPS, or is it all self-contained?

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As a former submariner who has had used clock for inertial navigation or for similar weapons systems 1/3 of a second over 100 hours is terrible.

I mean, it's not an atomic clock, but I'm comparing it to the 32.768KHz RTC crystals I use with consumer microchips. If super-precise isolated accuracy were actually important, I assume they would use a rubidium or cesium oscillator.

1/3 of a second in 100 hours is basically 1ppm, or TXCO levels of accuracy, so pretty good i'd have thought, even for a submarine INS?

USN ballistic missile submarines deploy the most accurate INS in the world (ESGN), and that system is used in conjunction with another advanced gyro.

Here is a 2 RU atomic clock on ebay for context: https://www.ebay.com/itm/Antelope-Audio-Isochrone-10M-Rubidi...

Not sure about that. ESGN is old technology, since submarines don't need that much accuracy. For example, space probes, ballistic missiles, smart artillery shells/rockets/missiles and so on would all appear to have multiple orders of magnitude better accuracy than submarines, in the fractional ppb ranges, rather than tens of ppm. [0][1]

0. https://www.sto.nato.int/publications/STO%20Meeting%20Procee...

1. http://users.cecs.anu.edu.au/~Jonghyuk.Kim/teaching/Inertial...

It wasn’t clock drift, it was an error in calculation leading to separate parts of the system, that were calibrated to the same common clock, to drift out of synchronization. Using a different clock, like GPS wouldn’t help with this.

But the rest of your point boils down to ’if you know your system has a flaw why not mitigate it’? But of course at design time they didn’t know it had this flaw.

Good point - hindsight is 20/20...

MIL-SPEC was indeed famous for overspecified components. So it's not terribly shocking that the oscillator on that board would operate really well as an isolated system. You probably don't need temperature control per se, a temperature compensation circuit could probably do that.

> weak public signal like GPS

i'd assume it could, since GPS is military, and a mobile missile system is exactly the sort of thing that wants to know where it is, so would have the keys to the (higher resolution) encrypted GPS signals as well.

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