Summary on Radar Cross Section

Message posted by Dr Ron Milione on April 16, 2002 at 8:47:42 PST:

First, when a radar pulse is directed toward an object, it hits the object with a certain amount of radar energy. The amount that hits the object is equal to the cross sectional (or projected) area of the object. So an object with a cross section of, say 10 square meters, will absorb the same amount of energy be it a sphere, flat plate, cube, or whatever. It's the object's cross section that matters as to how much energy is absorbed.

The second part of the explanation is where it gets interesting. After absorbing a radar pulse, the object immediately re-radiates the energy as an echo. This energy is primarily radiated at right angles to the object's surface. In the case of a sphere, it radiates the echo equally in all directions. But in the case of a flat plate, it all radiates off the front and back flat surfaces. If one of those surfaces happens to be facing the radar transmitter, a huge signal will be radiated (or "bounced", if you prefer) back, tremendously larger than a sphere of the same cross sectional area. Depending on the frequency of the radar in use, the return from a flat plate can be 1000 times larger than that of a sphere of the same cross section, and thus seems much "larger" to the radar receiver. To make things more complicated, the RCS values will change depending what frequency they are being measured at. But you can easily see the tremendous value of angling surfaces so that the radar beam is directed away from its source.

Just for chuckles, here are some typical RCS values for different objects from the "Radar Engineer's Sourcebook" by Morchin:

Object RCS in Square Meters
B-52 125

B1-B 1

Cessna 180 1.5

Cabin Cruiser 10

Navy Cruiser 14,000

Pickup Truck 200

Automobile 100

Bicycle 2

To interprete the table, let's use the bicycle as an example. Based upon our definition, the bicycle has a radar return the same as that of a sphere with a cross section of 2 square meters. To spare you the math, that would be a sphere about 1.6 meters in diameter.

The very essence of stealth technology is to shape the object is such a manner that incoming radar beams are radiated every which way but back toward the radar receiver. It's an amazingly simple concept, but the trick is its implementation.

Every object, even stealth aircraft, can be picked up on radar. The important thing is how close the object has to be before the signal it's reflecting toward the radar receiver is strong enough to register. Our bus would likely be spotted a hundred miles out, leading to early deployment of anti-bus missiles. But a stealth fighter might only start registering a mile away, leaving the radar operator only enough time to put down his coffee cup before his facility is destroyed.

So, the goal is to minimize the RCS of our aircraft to ensure their survivability. That's where RCS facilities get into the act.

The most reliable way to find out how much radar signal a new aircraft design will reflect is to simply try it and find out. And there's no need to use an actual aircraft, a smaller size model will work just fine. Reduced to basics, what RCS facilities do is to put a model on a pole a few thousand feet away, and bounce radar pulses off it to see how much comes back.

Of course there's much more to it than just that. The pole, properly known as the pylon, must itself not bounce any signal back. If it did, it might be confused with the signal from the model. To prevent this, the pylons have beveled faces that radiate the radar pulses away from the receiver. Also, the pylons lean toward the radar receiver, the more lean the better (The amount of forward tilt is usually limited by the pylon's structural considerations). In effect, the pylons are stealth shapes, the very earliest implementation of the concept.

The model also needs to be able to rotate on the pylon. Getting back to our flying Greyhound, the amount of radar reflected from the bus when it's pointed head on toward the radar unit will be less than if the bus is sideways. There's simply less area to reflect the signal. By rotating a model on the pylon, the radar reflectivity can be measured from all angles, thus identifying any problem areas that might require reshaping.

An RCS facility must allow for testing of models at different frequencies. Sometimes these frequencies are also referred to as lettered bands. Long range search radars operate at lower frequencies and targeting radars operate at higher frequencies. This is because higher frequency radar establishes the position of the object with much greater accuracy. The drawback to higher frequencies is that the atmosphere tends to absorb them more, so their range is not nearly as great as lower frequency radar. RCS facilities usually have a broad range of testing frequencies available. It's possible to get an idea of what types of frequencies are in use simply by looking at the radar dishes. The high frequency targeting radar dishes will be small, while the long range, low frequency radar require much larger dishes.

The ground between the radar transmitter and the model is a concern too. As the radar pulse leaves the antenna and shoots downrange toward the target, a portion of the beam bounces off the horizontal surface of the range and also strikes the target. This is called the "ground plane effect" and can create problems in some instances. Designers of radar ranges either try to eliminate this ground plane bounce, or incorporate it into the overall operation of the facility.

In most cases, designers simply pave the area with asphalt. This will also prevent the growth of vegetation, a big no-no. The paving gives an RCS range the appearance of a very strange landing strip. But a landing strip it is most definitely not.. The paving is typically only a few inches thick, too thin to support anything heavier than a very small light plane. Then there are the pylons sticking up in the middle of the range, providing quite a surprise for an incoming pilot.

Security considerations at RCS facilities are a major concern. State of the art stealth shapes must be kept from prying eyes on the ground, as well as spy satellites. This can be accomplished in a number of ways. It's possible to simply inflate a large opaque plastic dome over the model while it's on the pylon. The plastic is generally transparent to the radar beam, and what little signal is returned from the plastic bubble can be factored out. Some RCS facilities have large buildings (also known as target shelters) on tracks with rollup sides. These can quickly scoot over the pylon and hide the model if necessary. These buildings also have the added benefit of internal hoists to place the model on the pylon. Finally, the most high-tech of the RCS facilities have sophisticated mechanisms that retract the pylon and model into the ground. The model comes to rest in an underground room where it can be worked on and maintained.

The ultimate in security is provided by indoor ranges, where the entire facility is enclosed in a very large building. However this type of facility can be quite a challenge to make perform well, and is usually quite smaller than the outdoor ranges. For most purposes, outdoor ranges are preferred.


Best regards, Ron


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