LMFS wrote:I thought this effect required very long wavelengths to operate. But maybe modern processing can take advantage of both high and low band components of the return and get the same effect as an OTH-B?? When you say "tomography", you refer professing all the frequencies received right? Otherwise I cannot make sense of the term.kvs wrote:All of these early warning "radars" look over the horizon by design. They bounce their signal off the ionosphere and the surface. This way they outgoing signal
can follow the curvature of the Earth for thousands of kilometers. The reflected signal does the same thing, bouncing between the surface and the ionosphere.
With modern computer resources, it makes sense to design such "radars" with a set of frequency bands and doing "tomography" on the backscatter.
In the 1950s and 1960s it was just the analog return signal that they could use. That is why there was a preference for a few frequencies.
The reflection from the ionosphere is due the the presence of ions and is not fundamentally frequency dependent. Some frequencies will scatter
more efficiently. But that is neither here nor there. Modern high sensitivity amps (GaN) together with computer processing allows real time
analysis of backscatter from a wide range of frequencies with varying efficiencies of scatter. This is a species of tomography since tomography
is the reconstruction of some scanned shape from either transmitted or scattered waves. Seismic tomography is an example.
User "SOC" was arguing in the Voronezh thread some years ago that this radar could not be a OTH-B (backscatter) radar but maybe OTH-SW (surface wave) radar that could be made smaller but had nevertheless the disadvantage of having shadow zones caused by mountains...
All radars work on backscatter. SOC has to be more specific before being credible. A passive detector requires a strong signal from the target or from a source
behind the target (e.g. solar occultation). Phased array early warning radars are not passive detectors. They use a more efficient EM wavelengths to
detect far objects. But that range is not as limited these days as it was back in the 1960s.