(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


More information about the Meteorobs mailing list