[time-nuts] Basic question regarding comparing two frequencies

Geoff vk2tfg at ozemail.com.au
Tue Jul 27 08:53:43 UTC 2010


On Tue, 27 Jul 2010 09:08:49 am Chuck Harris wrote:
> I suppose that you could always cheat?  Since you know where the
> transmitter is going to be, if you could get a timenut near to the
> transmitter to give you a beacon to measure 24hrs prior to the event,
> you could use the diurnal variations that you observed (observe?) on
> the beacon to predict the skywave offset due to Doppler at the time
> of the event.
>
> -Chuck Harris
>
> Murray Greenman wrote:
> > You guys are trying to crack a nut with a sledgehammer!
> >
> > For a start, as Didier says, you can't possibly read the frequency of a
> > sky-wave signal to 0.01Hz in any short time frame since the Doppler on
> > the signal can be as much as 1ppm (i.e. 10Hz at 10MHz). You can only
> > infer it closer than that by studying the frequency in the very long
> > term.
> >
> > In addition, you'll never know how much of the daily variation is
> > ionospheric, and how much is due to thermal changes at the source.
snipped

There is one possible way of getting an accurate reading from a sky wave 
signal over a short(ish) period. Plot a doppler shift curve with as fine a 
resolution as you can manage. Then look for a point of inflexion in the 
curve, that is a point where the second derivative of the curve function is 
zero. The frequency at that time will be that transmitted as at that instant 
the path length is not changing. You may have to examine your data set 
visually and mathematically examine a much smaller section. Of course if you 
don't get a point of inflexion you'll need much more data :-).

Cheers, Geoff vk2tfg.




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