[time-nuts] Basic question regarding comparing two frequencies

[paul swed]

So on a 60 khz signal the long strip chart recorder is simply a super long
low pass filter averaging out the doppler somewhat. It really doesn't do
that well. The mark-1 eyeball does a better job. Right?

On Tue, Jul 27, 2010 at 4:53 AM, Geoff <vk2tfg at ozemail.com.au> wrote:

> 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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