4 ms·

Also, in the case of a rebroadcast it need not be delayed by more than something on the order of a microsecond or so. That only works if the spoofer is very cl

Also, in the case of a rebroadcast it need not be delayed by more than something on the order of a microsecond or so. That only works if the spoofer is very close by. One microsecond is 1000 feet. (Recall the "Grace Hopper nanosecond" as a start.) So, if the spoofing signal rebroadcast originates 5 miles away, that's 26 microseconds of delay right there. If the receiver has some sort of out-of-channel time reference that is accurate to nanosecond levels I suppose that could be used as a check, but that sort of thing takes an atomic clock and doesn't fit in a wristwatch. You're way off in your accuracy estimate. Oven controlled crystal oscillators have been around for at least 50 years. Probably a lot more. And they're dirt cheap. All they are is some temperature stabilization around an ordinary crystal oscillator. Quoting from the (always highly accurate) :) Wikipedia: The short term frequency stability of OCXOs is typically 1x10−12 over a few seconds, while the long term stability is limited to around 1x10−8 (10 ppb) per year by aging of the crystal https://en.wikipedia.org/wiki/Crystal_oven#Accuracy https://en.wikipedia.org/wiki/Crystal_oven#Accuracy An OCXO isn't practical in a tiny drone or a wristwatch, but it's highly practical in any military instrument that weighs more than a few pounds.

jhayward 9y ago
> That only works if the spoofer is very close by. One microsecond is 1000 feet. (Recall the "Grace Hopper nanosecond" as a start.) So, if the spoofing signal rebroadcast originates 5 miles away, that's 26 microseconds of delay right there. I made a mistake in ambiguously referring to two things: processing delay at the spoofer, and speed-of-light based time difference between spoof signal and direct broadcast at the receiver. However, I'm not aware of any crystal based clocks that are suitable for high dynamic range G environments that would be easy to use as an independent reference for any reasonable amount of time. They all suffer in shock/vibration conditions, and need external conditioning to maintain longer term stability. Maybe the military has 'em, but I guarantee they cost a lot more than a GPS rec.eiver
- PhantomGremlin 9y ago
  They all suffer in shock/vibration conditions Interesting. I didn't know that. There's a comment on another thread that someone just made: https://news.ycombinator.com/item?id=15007006 https://news.ycombinator.com/item?id=15007006 And now you can buy a genuine atomic wristwatch. Here's a few sentences from the linked website: In each atomic physics unit is a small vessel of Caesium 133, an oven to heat it to 130°C, a laser to excite the atoms and a microwave resonator that resonates at the hyperfine transition frequency of the atoms, 16,546,737,186,000 vibrations per hour, with an accuracy of one 5 parts in 10-11. https://www.hoptroff.com/pages/about-us https://www.hoptroff.com/pages/about-us The same shock/vibration limitation probably applies to it. Still, it exists! I think. I wouldn't spend that much money to find out if it was real.
  - jhayward 9y ago
    Some links you might find interesting: https://www.microsemi.com/products/timing-synchronization-systems/time-frequency-references/chip-scale-atomic-clock https://www.microsemi.com/products/timing-synchronization-sy... http://www.oewaves.com/media-events/item/50-oewaves-to-develop-an-all-optical-integrated-micro-primary-atomic-clock-ao-impac-for-darpa http://www.oewaves.com/media-events/item/50-oewaves-to-devel... http://www.gps.gov/governance/advisory/meetings/2016-05/lutwak.pdf http://www.gps.gov/governance/advisory/meetings/2016-05/lutw... https://www.darpa.mil/program/micro-technology-for-positioning-navigation-and-timing https://www.darpa.mil/program/micro-technology-for-positioni... I haven't seen much (haven't been looking either tho) since the Micro-PNT program which was in 2008. Sometimes that means that things are getting interesting, other times it's a dry hole. I'm interested in any later developments if you know of them.

jhayward 9y ago

> That only works if the spoofer is very close by. One microsecond is 1000 feet. (Recall the "Grace Hopper nanosecond" as a start.) So, if the spoofing signal rebroadcast originates 5 miles away, that's 26 microseconds of delay right there. I made a mistake in ambiguously referring to two things: processing delay at the spoofer, and speed-of-light based time difference between spoof signal and direct broadcast at the receiver. However, I'm not aware of any crystal based clocks that are suitable for high dynamic range G environments that would be easy to use as an independent reference for any reasonable amount of time. They all suffer in shock/vibration conditions, and need external conditioning to maintain longer term stability. Maybe the military has 'em, but I guarantee they cost a lot more than a GPS rec.eiver

PhantomGremlin 9y ago
They all suffer in shock/vibration conditions Interesting. I didn't know that. There's a comment on another thread that someone just made: https://news.ycombinator.com/item?id=15007006 https://news.ycombinator.com/item?id=15007006 And now you can buy a genuine atomic wristwatch. Here's a few sentences from the linked website: In each atomic physics unit is a small vessel of Caesium 133, an oven to heat it to 130°C, a laser to excite the atoms and a microwave resonator that resonates at the hyperfine transition frequency of the atoms, 16,546,737,186,000 vibrations per hour, with an accuracy of one 5 parts in 10-11. https://www.hoptroff.com/pages/about-us https://www.hoptroff.com/pages/about-us The same shock/vibration limitation probably applies to it. Still, it exists! I think. I wouldn't spend that much money to find out if it was real.
- jhayward 9y ago
  Some links you might find interesting: https://www.microsemi.com/products/timing-synchronization-systems/time-frequency-references/chip-scale-atomic-clock https://www.microsemi.com/products/timing-synchronization-sy... http://www.oewaves.com/media-events/item/50-oewaves-to-develop-an-all-optical-integrated-micro-primary-atomic-clock-ao-impac-for-darpa http://www.oewaves.com/media-events/item/50-oewaves-to-devel... http://www.gps.gov/governance/advisory/meetings/2016-05/lutwak.pdf http://www.gps.gov/governance/advisory/meetings/2016-05/lutw... https://www.darpa.mil/program/micro-technology-for-positioning-navigation-and-timing https://www.darpa.mil/program/micro-technology-for-positioni... I haven't seen much (haven't been looking either tho) since the Micro-PNT program which was in 2008. Sometimes that means that things are getting interesting, other times it's a dry hole. I'm interested in any later developments if you know of them.