[time-nuts] Re: Clock displays -- eye response

[Lux, Jim]

On 12/10/21 12:31 PM, Brooke Clarke via time-nuts wrote:
> Hi Hal:
>
> There has been some recent research into illusions related to sight 
> and sound.
> https://en.wikipedia.org/wiki/McGurk_effect - related to speech
> and search "audio optical illusion"
>
> I like a crisp "tick" for clock human synchronization.
>
> I wonder why there has not been more done with military "Have Quick" 
> for time synchronization?
> https://en.wikipedia.org/wiki/HAVE_QUICK

HAVE QUICK was a very early hopping spread spectrum system and was long 
since superseded by SINCGARS and other systems in the 1980s.  I don't 
know that it is a "source" of time.

There was a whole set of systems for loading "time" into these frequency 
hopping systems, since all manner of their synchronization behavior 
depended on time.  And in usual DoD/NATO fashion, forward and backward 
compatibility was often required, so you'd have a way to sync any two 
random radios in a battlefield situation.  Cryptographic keying is also 
often time based, as are frequency nets (local interference, keeping the 
other guy guessing, or propagation changes)

Synchronization is the "hard part" of most spread spectrum systems both 
Direct Sequence (PN codes) and Frequency Hopping, and that's where most 
of the classified stuff is - how do you synchronize reliably, how do you 
prevent the synchronization from being spoofed or jammed. A naive FH 
approach is to have a "hailing channel" and the first person transmits 
there, and the other person hears it, listens to a sync pattern, and 
then commence hopping together. This works for point to point between 
two stations, but doesn't work very well when you have multiple 
stations, not all of which can hear each other.

GPS would have been a godsend back then (although it's not very good 
from an Anti Jam standpoint).


Dixon, in the seminal tome "Spread Spectrum Systems" kind of makes an 
offhand comment that synchronization is the challenging part, and then 
moves on "assuming we have synchronized".


> It's been a part of the PLGR and DAGR GPS receivers and I expect also 
> for the military embedded versions for a long time.
> https://prc68.com/I/PLGR.shtml#Time
> https://prc68.com/I/DAGR.shtml#HQ1PPS
> Also things like the O-1814/GRC-206 Reference Frequency Rb Oscillator 
> make us of it.
> https://prc68.com/I/O1814.shtml
>
I don't know that they make use of it, rather, they can provide sync TO 
a HAVE QUICK radio.