4 ms·
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 d
by leggomylibro 9y ago
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?
- ajross 9y agoMIL-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.
- GCU-Empiricist 9y agoAs 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.
- leggomylibro 9y agoI 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.
- grkvlt 9y ago1/3 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?
- cocoablazing 9y agoUSN ballistic missile submarines deploy the most accurate INS in the world (ESGN), and that system is used in conjunction with another advanced gyro.
- GCU-Empiricist 9y agoHere is a 2 RU atomic clock on ebay for context: https://www.ebay.com/itm/Antelope-Audio-Isochrone-10M-Rubidium-Atomic-Master-Clock-Rack-Unit/292465831262?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D1%26asc%3D49924%26meid%3Dd0f3bb7e07324300b3aa0f1462206b9b%26pid%3D100011%26rk%3D1%26rkt%3D3%26sd%3D183045907785%26itm%3D292465831262&_trksid=p2047675.c100011.m1850 https://www.ebay.com/itm/Antelope-Audio-Isochrone-10M-Rubidi...
- grkvlt 9y agoNot 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%20Proceedings/RTO-MP-SET-104/$MP-SET-104-KN2.pdf https://www.sto.nato.int/publications/STO%20Meeting%20Procee... 1. http://users.cecs.anu.edu.au/~Jonghyuk.Kim/teaching/Inertial_Where%20to%20now_DraperLab.pdf http://users.cecs.anu.edu.au/~Jonghyuk.Kim/teaching/Inertial...
- simonh 9y agoIt 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.
- leggomylibro 9y agoGood point - hindsight is 20/20...
- grkvlt 9y ago> 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.