The observed Doppler shifts thus represent the E-layer wind speed, taking into account the geometry of the reflection point relative to the signal path. However, the reflected signal fluctuates rapidly due to a number of factors - the variation with time of the ionisation, the variation in the trail geometry relative to the signal path, and the combination of signals from different reflection points with different Doppler shifts. So the result is both a centre frequency and a fading spectrum that varies with time. The effects that can be seen during the Leonids are quite different from normal MS propagation. Compare them with the plot showing reception earlier in the year of two 49.75 TV carriers about 30Hz apart.
Discrete pings are evident throughout the trace, with only a very weak background scatter signal.
Despite the much higher power used by the TV, the MS signal is not continuous. The continuity of the signals during the Leonids by contrast shows that much higher quantities of material are entering the earth's atmosphere, perhaps by a factor of 100 or more. The paths from IO91 are similar - the heading difference is only 15 degrees. We know that the ionisation intensity needed to sustain propagation is a function of the path length, so that shorter skip paths require higher ionisation. If there is a tendency for the scattering centres to be further off the direct path, this will also lead to more variation in the Doppler shifts.