There’s a particularly interesting discussion emerging on our forum on the subject of Ionosondes – this is where a transmitter sweeps all or part of the HF frequency range, transmitting short pulses. These pulses are reflected at various layers of the ionosphere, at heights of 100–400 km, and their echos are received by the receiver and analyzed by the control system. The result is displayed in the form of an ionogram, a graph of reflection height (actually time between transmission and reception of pulse) versus carrier frequency.

Join in the discussion on – there must be lots of scope for analysing such emissions passively and having your own perspective on real time propagation!