Radio Meteor Head Echoes

Meteors generally meet the Earth's atmosphere at speeds between 12 kilometres per second and 71 kilometres per second (that's between 43,200 kilometres per hour and 255,600 kilometres per hour). This is fast by most standards, but even the fastest meteors are traveling at less than 0.025% of the speed of light.

Despite this, radio meteor observers do see Doppler-effects when listening to radio reflections from meteors. Occasionally radio meteor observers record a head-echo which, unlike the more common specular reflections, is a reflection directly from the head of the meteor. The head echo produces a descending tone heard just prior to the meteor passing through the point of closest approach for the observing system.

The changing tone is due to the change in apparent radial velocity that is observed when the meteor is moving with a relatively constant linear velocity across the observer's line of sight.

The following spectrum and audio files are provided courtesy of Felix Verbelen and were recorded from the VVS beacon.

Listen to head echo (441Kbyte)

There is an excellent article by James Richardson and Werfried Kuneth, 'Revisiting the Radio Doppler Effect from Forward-scatter Meteor Head Echoes'.

Abstract: Following an introduction to the radio meteor head echo and its historical aspects, a PC-based technique is described whereby the radio Doppler effect from meteor head echoes can be used to make rough meteor range and speed measurements, employing a commercial AM or CW transmitter in a forward-scatter link. The technique is then applied to four known shower meteors, two Leonid and two Geminid, which provided measurable head echoes in addition to specular trail reflections.