Over the horizon radar is an interesting idea but as anyone who has played around with amateur radio communications (back in the heyday) there are quite a few problems to be overcome. I'll admit I haven't done a lot of research on this type of radar but I remember back then wondering how the hell are they going to make this thing work, considering how variable simple point to point HF radio communications tend to be. These are a few of the problems I can think of.
The ionosphere is not a nice flat constant reflecting surface, but is constantly changing from day to night, on a yearly basis, and over the twelve year sunspot cycle.
One result of the above is that the optimum frequency of operation would have to be changed to to keep the desired ground area illuminated, if that is still possible in the worst case. The antennas would use a huge number of dipole elements which must all be resonant (or near resonant) at the operating frequency (this is how the signal can be concentrated into a narrow beam). Changing the operating frequency necessitates some way of changing the resonance of all those elements which is not a trivial exercise.
The ionosphere is not homogeneous, but could be likened to a cloud cover, so there will be many signal reflections from different parts of it, each having a different path length. The different path lengths alters the phase relationships between those signals which creates a lot of signal distortion when they are all combined back at the receiver.
Those clumps in the ionosphere are also constantly moving (turbulence) so the signal path lengths mentioned above are always changing. Due to the doppler effect, this causes small continuously varying frequency changes on each signal path. This could mask any desired doppler effect due to a moving target on the ground.
All of the above also makes it extremely difficult to scan a small strip on the ground. The RF beam would be scattered when going out and again when coming back. Compare this with a conventional millimetre radar which has a nice solid unchanging reflecting dish to do all the work.
Ionospheric reflection only works in the HF frequency ranges which have quite long wavelengths. We are talking tens of metres compared with millimetres for conventional radar. This limits the use to very large targets, if it can be made to work at all.
Did they ever claim to get these things working reliably?
BTW, this is a bit off topic so I'll try not to keep it going lol.
