  The approximate frequency shift of a radar echo from a meteoroid is derived.
The origin of head echos observed during trail formation are discussed by
considering schematic models. The echo is either a reflection of the radar
signal from thermalized electrons in the meteoroid trail or from the plasma
electrons co-moving with the meteoroid itself. It is shown that in these
opposite models the frequency shifts of the echo relative to the radar emitter
frequency must be identical, even though the underlying physics differs.
Calculations were made that give the echo frequency as function of time,
allowing qualitative comparison with observations. Some typical observations
show a conspicuous absence of negative frequency shifts in the head echo in
forward scattering. Arguments are given that this can correspond to meteoroids
with the co-moving plasma in front.
