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
Interesting. I didn't know that.
There's a comment on another thread that someone just made: 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
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.
https://www.microsemi.com/products/timing-synchronization-sy...
http://www.oewaves.com/media-events/item/50-oewaves-to-devel...
http://www.gps.gov/governance/advisory/meetings/2016-05/lutw...
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.
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
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.