[time-nuts] NIST

[jimlux]

On 8/13/18 6:10 AM, Bob kb8tq wrote:
> Hi
> 
> Ok, this is Time Nuts. We probably have a pretty good sample of those who use this and that as a source of time.
> We also are reasonably conscious about what we are doing. NIST’s claimed reason for running WWV (and WWVH) is to
> distribute accurate time and frequency.
> 
> Would / does anybody on the list actually use WWV as their *primary* source of accurate time or accurate frequency?
> 

I don't use WWV (or WWVH) as a source of time/frequency per-se - what I 
use it for is as a beacon at a known power, frequency, and antenna 
pattern, with (presumably) very good close in phase noise.

The ionosphere has a coherence time of a few seconds, so ADEV of the 
source at tau of 10 sec or longer isn't a huge thing for me.  But that 1 
Hz/1 sec time frame is of real interest.

As the receiver flies over, we get nice soundings "through" the 
ionosphere with good performance in the short term.

The usual ionosondes don't have anywhere near the same carrier purity, 
and, because they sweep, you have to have good time synchronization of 
your super het receiver to make sure you can tune the signal.

With WWV, I can set my center frequency to, say, 10.005 MHz, record 25 
kHz BW for several minutes as I fly over and make my measurements.  The 
"phase noise and ADEV" of my receiver position is very small - zipping 
along 500 km at the top of the ionosphere, there's not a lot of bumps in 
the road, so it's easy to model the position.

Not for my current spacecraft, but for a future one (SunRISE), we'll be 
measuring (post processed) spacecraft position and time to a few ns. 
The current one isn't that good - but for this one, we're interested in 
the lumps and bumps of the ionosphere, and that's a "over time spans of 
<3 seconds" kind of measurement.


That said, if WWV were to turn off tomorrow, I could probably build a 
ground beacon with adequate performance to do my science. I don't need 
kilowatts of radiated power - it's just convenient that WWV exists and 
someone else does it. And in reality, I'd rather have an antenna which 
radiates more "up" than "out" - WWV and WWVH are vertical dipoles - good 
for skywave propagation, not so hot for vertical sounding.