Over the Horizon Radar

[--JW]

How can OTH radar work? It bounces the waves off the ionosphere, but since the ionosphere changes every second wouldn't the radio wave be travelling on a different path on its way back from bouncing off a target?

[swansont]

How can OTH radar work? It bounces the waves off the ionosphere, but since the ionosphere changes every second wouldn't the radio wave be travelling on a different path on its way back from bouncing off a target?

What is the spatial extent of the signal? If it's large (and/or expanding), there will still be a return path for one part of the signal

 

How quickly does the ionosphere change when compared to the speed of light? How much distortion does that introduce? Signal return might be on order of a millisecond. Even if there is a change in the reflection profile in that time frame this would just limit the resolution.

[TonyMcC]

There are two basic types of Over the Horizon radar.

One called HF Sky Wave uses reflection from the ionosphere. It is important to realise that the beam from a search radar is not pencil shaped like the beam from a torch, but is fan shaped being being very thin in plan horizontally and very wide in plan vertically. Most of the time the height of the ionosphere is really quite small compared with the distance to a target that is over the horizon and so the actual height of the ionosphere will make little difference. Some of the fan shaped beam will bounce off the ionosphere and radiate the target, and some of the reflected energy will be returned as a signal.

However, there is another type called HF Ground Wave. This doesn't use the ionosphere to look over the horizon. It works on the principle of diffraction using the reducing density of the atmosphere with height. You actually see this at work when you look at the setting sun. When you see the sun just above the horizon it is actually just below the horizon!

[Enthalpy]

I believe the time resolution, if not precision, of the processed received signal is at least around 1µs, or 150m resolution on the range.

 

Sure, the atmosphere moves by more than that, and rather quickly.

 

But then, within the range of an OTH radar (10% of the Earth's surface!), you have many thousands of individual bright targets, which are point-like in a less reflective environment. This is all you need.

 

The rest of the job must be done by a software filter, probably a Calmann filter or adaptive filter. It adapts its coefficients permanently in order to get brilliant echoes from its deformed received signal. Once this is done on the most brilliant echoes, the filter also renders much fainter targets clear.

 

This must be necessary because the uneven atmosphere expectedly provides many transmission paths with different delays. Even a spherical atmosphere would allow different numbers of bounces, as is known from amateur radio using these decametric waves.

 

The theory behind a Calmann is less than trivial and needs insight in signal theory. But you can compare it with the now better known adaptive optics on telescopes. From a distorted image, they pick a few brilliant stars - or light points created by a laser from the ground if necessary - and deform a secondary mirror (similar to the Calmann filter) to obtain a clear image of these brilliant points. Then, fainter features get also sharp and can be discerned.