23 May, 2019
50 MHz Doppler shift from planes
The two stations are LA9PJA (1415 Hz) and LB6D (2477 Hz). Both of them are located in the same square as me (JO59). The other weaker signals are British and French. One can see the direct signal which has a constant frequency, and then a time-varying frequency on top of that.
Let me try to estimate the velocity. The largest change is around 2500 Hz where frequency has increased by about fD = 50 Hz. The scenario is like in bistatic radar, with two Doppler shifts, on the up-link and the down-link:
0 = 50.313 MHz and c is the speed of light. Since I cannot find the two angles between the up-link beam and the flight path of the plane and similarly on the down-link, I use the right-hand expression. Thus the velocity component observed is v=150 m/s or about 540 km/h. This is a projection of the actual speed along the lines of propagation, so the actual speed can be larger. Anyway, only a plane can be this fast which is not so unexpected as we are not very far from Oslo airport.
One can see how the Doppler-shifted signal starts out at a higher frequency and then falls to a lower one implying that the plane passed overhead of us somewhere between me and LB6D who lives only 18 km from me. It is also interesting to note that both signals, at 1400 and 2500 Hz, have two different Doppler-shifted versions, implying that two different planes at the same time bounced off the signals.
This blog post first appeared on the LA3ZA Radio & Electronics Blog.