(meteorobs) RE: Doppler effect
Jack Meyer
jrmeyer at erols.com
Sat Oct 8 20:59:35 EDT 2005
Hi,
In response to the question about radio observation Doppler effects, my
experience is that very few meteor events have a noticeable Doppler
effect. There appear to be several reasons for this. First, the TV signal
reflection is from the ionization trail, not the meteor it self. This
trail is essentially stationary once it is formed. It appears to move
because it forms behind the meteor and then dissipates over a short period
of time. However, once formed a given section of the trail does not move
closer to or further from the TV transmitter and receiver, thus that
section will not produce a Doppler shift.
The strongest radio reflection from a meteor trail is that point on the
trail where the angle from the trail to the transmitter antenna is the same
as the angle from the trail to the receiver antenna. This point also
provides the shortest path from the transmitter to the receiver, thus it is
the point of zero Doppler shift.
My view is that noticeable Doppler shifts will only occur when a trail is
strong enough to provide useable reflections from parts of the trail where
the ideal reflection requirements are not met. On an audio spectrogram,
this would show up as a small bend in the normally horizontal spectrum line
at its start or finish.
In some locations, reflections from multiple distant TV transmitters with
the same channel and offset, can give the impression of a Doppler shift,
because the video carrier frequency can be off by a few 100 Hz.
Regards,
Jack Meyer
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