Radar Detection and The Horizon Distance

While reading some of the posted items about the our Peacock-class Corvette’s radar and gun range, I just came across the problem of detection by Radar because the “Horizon Distance”.

A typical Radar system is limited in terms of detecting objects close to the surface of the earth because of the curvature of the Earth. It is ideal for detecting aircrafts flying at altitude, but for ships, or very low flying aircrafts, its detection range would be limited by the height of the object being observed, and the height of the Radar Transmitter/Receiver (T/R).

A typical formula for Horizon Distance I found in the web (click here) is HD = SQRT(H)*112.88 where:
HD = Horizon Distance in km.
H = Height of the Observer
112.88 = Formula Constant

So, let’s say that a typical Radar mast on a ship has a height of 30 feet or 10 m from the ground, then the theoretical Horizon distance would only be about 11 km.

To me, this means a couple of things:

* Ships using conventional Radar will have limited range with which to detect other ships, or aircraft flying close to the surface.

* In order to increase the detection range, Radar T/R will have to mounted at a certain height, the higher, the better. This can be done either thru the use of masts, or aircraft (rotary or fixed wing).

* Due to the limited detection range, medium or long range Ship-to-Ship-Missiles (SSMs) would not be feasible unless there is support from the air in terms of detecting surface crafts. In this case, a ship to ship engagement would be more or less limited to gun range unless one or both do not have air detection support. Our Peacock’s guns would then be just as effective, or arguably even more so in a ship-to-ship confrontation without air detection support.

* Surface ships without air detection support would also seem to be more vulnerable to sea-skimming Anti-Ship Missiles (ASMs), and the faster, the better. A supersonic “Yakhont”, for example, flying at near sea level would take only about less than a minute to cover the distance of 11 km at Mach 1 (around 700 km/hr at sea level). That means a ship will only have that much time to detect, determine the location of the ASM and react to the threat, a relatively short amount of time to counter the threat effectively.

‘Apaches Against Radar Installations’
The Radar Horizon is probably the main reason why the US has been relying a lot on the AH-64 Apaches to take out Ground Based Radar Control Stations in the last couple of major wars (i.e., Iraq 1, Iraq2). Apaches fly very low, only a couple of do zen feet off the ground, ensuring that detection of them by Radar will be no more than 11-13 km away (based on a 20-30 ft. Radar T/R height).

With a good knowledge of the terrain around the target Radar installation, they can follow the ground using the terrain as cover to cut down even more on that detection range. PLUS, you also have ECM that could degrade the detection ranges even more!

All an Apache has to do, is get to within 5-6 km of a target Radar installation. That is the range of their Hellfire missiles, and then they can effectively take out the targets.

I remember that during Dessert Storm, Apaches were among the very first ones into Iraq, along with the F-117s to take out Radar installations & defenses.

‘B-1B Lancer’s Effectiveness’
Its also one of the main reasons adding to the effectiveness of the B-1B Lancer in terms of penetrating enemy Radar defenses. According to one of the graphs showing the detection ranges of aircrafts posted above, a B-1B flying at around 100-200 ft.off the ground will have a detection range of below 20 NM against a Radar T/R with a height of about 25-50 ft.

Add in items like the B-1B’s semi-stealthy shape and radar absorbing paint, that detection range will be cut down even more. You add in ECM and terrain masking, god knows how much that’ll rob a Radar off it’s detection range.

The 2nd to the last factor in the mix would be the B-1B’s speed. Flying at supersonic speed at low level, then you have an aircraft that will give any opposing Radar systems only around a minute of detection time before it ends up literally on top of them.

The last factor would be the B-1B’s standoff missile capability. It doesn’t even have to be on top of any target to destroy it. Depending on the weapon used, all it has to pop up at no less than a dozen kilometers from its target to launch it’s standoff weapons, and that’s it.

But all of this starts off with the ground-based Radar’s limitation of not being able to “see” over the horizon. It was only after the Russians
started having their own AWACs and fighters with look-down, shoot-down capability that the B-1B’s effectiveness was severely compromised. Owing to the airborne Radar’s height, it overcame the horizon limitations of ground-based Radars, allowing the B-1B to be detected at much longer ranges.

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