You're quite right Chen. At the risk of annoy the moderator with off-topic
discussions, time-of-flight is not all that needs to be measured, but it is
necessary to a certain precision. The probe signal is quite a bit more
sophisticated. The signal bends in three dimensions, separates into counter
rotating waves of circular polarization that travel different paths and at
different propagation velocities, possibly reflects and diffracts from the gray
line region and Auroral oval boundaries. It is subjected to multipath and ISI,
and so on. It's a digital signal designer's heaven (and a propagation weenie's
heaven)! Competing theories predict different paths, as well as different
multipath, and differing propagation velocities. The ionosphere is complex, and
then you add the Earth magnetic field, and things get really interesting in a
hurry (think CRT). It's not like simple AFSK, where you just need to follow the
manufacturer's instructions.
73,
Kai, KE4PT
On 3/15/2014 12:23 AM, Kok Chen wrote:
On Mar 14, 2014, at 8:49 PM, Kai wrote:
We do however want to measure the propagation speed of signals in the
ionosphere...
There are more unknowns than measurables, Kai, if all you are measuring is the path
transit time. The problem is not the "baud rate" (we know for example that it
is easy to measure transit time to much more precise than T, using a pulse whose width is
T).
Think of a similar experiment where you use total internal reflection of a
prism in a reflex light path to measure the speed of a laser beam. You don't
know the size of the prism, nor the distance of the laser to the prism, and you
don't know the refractive index of the glass used to construct the prism. All
you can do is to measure the transit time as precisely as possible. With that
alone, you cannot determine the refractive index of the prism.
Now imagine the prism to be a simple model of the ionosphere. (In the case of
the ionosphere, the density is not even constant.)
73
Chen, W7AY
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