When analyzing WSPR signals in the 4 m band, signals can be spotted that are near the carrier wave of WSPR signals transmitted by other stations. These "ghost signals" are caused by aircraft reflecting the WSPR signal. Because the aircraft are moving a dopplershift is introduced.
This morning I caught a WSPR signal from PA0TBR on 70.0926 MHz with an aircraft reflection. I used the nice program SpecLab to have a close look at the signals:
The dopplershift between the carrier and the signal on the right (coming from the aircraft) is approximately 50 Hz. With this you can calculate the speed of the aircraft:
v= 50 * ((300/70.0926)/2) = 107 m/s = 385 km/h
Above you see a screenshot of WSPR March 2012. Measured dopplershift here was about 90 Hz which represents an aircraft speed of 695 km/h. A normal speed for an airliner jet at cruizing altitude.