How does flying low keep you off the radar?
In: Technology
When using radar to find flying objects, you are limited by what is known as line of sight. This means that you cannot see through obstructions, such as the terrain. The lowest point your radar signal is restricted to is the horizon from where you are. Since the Earth is round, that point gets lower the farther away it is from you.
So when an airplane is flying low enough at a distance, you won’t be able to see them as the curvature of the Earth prevents your signal from reaching them.
This is why many nations try to place their radar sites on higher elevations and some countries even have specialized aircraft with powerful radars on them.
What others have said regarding terrain is true. That said, radar design is all about tradeoffs. If they’re good at differentiating target altitude, then they’re poor at something else, or at the very least they are more complex and expensive. Air defense radars are designed to detect targets in the air, and designers must often design them to be effective against “most” targets, or perhaps against the most dangerous targets. So, to cover the effective altitude of “most” targets, a radar might be designed to cover medium to high altitudes very well, but the trade-off is reduced detection at low altitudes.
The terrain does block the line of sight of the radar. It literally can’t see your plane when it is behind a mountain. Because of this air surveillance likes to put radars on top of mountains and towers.
Also, and this is an equally “blinding” factor:
The terrain isn’t plain, isn’t made of the same material isn’t standing still (like trees moving in the wind or water waves on the ocean) and isn’t quiet in electromagnetic emissions means (radio signals, TV signals, WiFis, other emisisons on every possible frequency band).
On top of this all these factors literally mutilate the radar signal. Every edge, every different shape and every different material all reflect, refract, shape shift the radar’s EM waves so much that it is impossible for the radar’s detector to distinguish between a plane flying in all these “noise” and the noise itself.
The higher you fly, the more homogeneously spread the noises energy is in the sky and the higher the possibility to dedect a single object. IIRC the safe to detect height is about 5m on open seas and 15m on land. Thus youd have a hard time “sneaking up” below the radar without crashing.
If you were trying to creep up on someone in open ground who was looking out for you, one of the things you’d do would be to flatten yourself to the ground and crawl towards them using tufts of grass and bushes etc for cover to block their line of sight.
It’s the same with radar: the ground is covered with ‘clutter’: hills, buildings, towers, trees etc, and if you can keep your aircraft down close to them, you can either hide in their radio shadows (radar doesn’t go through hills) or at least be confused with the background of other stuff and harder to spot as a result.
Assuming a ship at sea and an aircraft flying low and coming towards you, can you pick up the vibrations from the water by a sonar device?
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