Some common radar myths

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Dave Saxton
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Some common radar myths

Postby Dave Saxton » Tue Aug 05, 2014 4:05 pm

While reading up on the issue of using longer wave radar to detect LO targets on blogs..ect.. I’m surpised by some common myths about radar, pertaining to wavelength especially, that seem to be accepted as common facts.

Myth: Even if UHF (decimetric 12cm to 100cm) and VHF/HF (metric 100cm +) radar can detect a stealth target they lack the accuracy and resolution to accurately target it.

Truth: No-mostly. With shorter wave length, bearing resolution is improved all other things remaining the same. UHF radar can attain the same bearing resolution as S band(9cm) by using a large enough antenna, however. Accuracy for both bearing and range has nothing to do with wave length. Range accuracy is a function of how precise the elapsed time can be measured. Bearing accuracy is a function of the lobing/scanning technique used. Resolution for range has nothing to do with wavelength. It is a function of how finely the time/distance measurement can be segmented. UHF can attain the same accuracy and resolution as S-band and X-band (3cm).

Myth: The shorter the wavelength the less ground and sea clutter.

Truth: Actually the opposite is true up to a point. X-band (3cm ) is much more affected by clutter than S-band (9cm) and S-band is much more affected by clutter than UHF. However at sea, VHF is significantly affected by sea clutter because the waves and swells act as reflective dipoles to those wave lengths. This is why the early 150cm Type 286 could hardly detect surfaced U-Boats on the high seas.

Myth: Vertical polarization has more sea clutter than horizontal polarization.

Truth: It depends on the grazing angle. For ship to ship radar the grazing angle is less than 3*. Vertical polarization has slightly less sea clutter than horizontal polarization at less than 3* grazing angle. The difference at these low grazing angles is so small that either can be used in practice. At more than 5* grazing angle up to almost 90* grazing angle (as in airborne radar) vertical polarization has significantly more sea clutter. At 90* grazing angle vertical polarization has less sea clutter.

Myth: The shorter the wavelength the smaller the object that can be detected.

Truth: Actually the longer the wavelength the greater the radar cross section of the object. If the myth was true then how could Wuerzburg, at 53cm, detect a strip of metal foil 2cm wide and 25cm long? Yet that little strip of foil could appear as big as a Lancaster bomber on the Wuerzburg’s screen. What can have a tiny radar cross section to X-band may have a large RCS to UHF radar. This is why UHF and VHF can be used to detect LO aircraft. The German Navy found during WWII that 27cm was far better for tracking aircraft than 9cm. The modern USN uses 27cm to track jet aircraft because it works better vs jet aircraft than 9cm or 3cm.

Myth: the shorter the wave length the greater the range attainment

Truth: Yes and no. Actually all other things the same; the shorter the wavelength the lesser the range attainment. The radar range equation puts the wavelength into the numerator (or frequency into the denominator). Indeed, millimeter wave radar has very short range attainment because the energy is absorbed by water vapor in the air to the point that mega power levels can not over come it. However, an important factor for range attainment is antenna gain. Antenna gain is much greater the shorter the wavelength given the same size antenna. So at S-band there may be greater range attainment than at UHF to a given object. Nonetheless, the detection range attainment of X-band to a given object is usually less than that of S-band. Raytheon tests of surface search naval radar at X-band and S-band have proven that X-band will often miss detection of a ship when S-band will detect the same ship, especially if a sea clutter filter is used, or if it’s raining or snowing.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Steve Crandell
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Re: Some common radar myths

Postby Steve Crandell » Tue Aug 05, 2014 10:27 pm

OK, I will bite.

Why are virtually all fire control radars X-band or above?

Why are aircraft ASW search radars K-band? Their primary purpose is to detect periscopes.

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Dave Saxton
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Re: Some common radar myths

Postby Dave Saxton » Wed Aug 06, 2014 12:04 am

Fine bearing resolution is much easier to obtain when using shorter wave lengths. It's mostly about size. For example, in 1951 the Linclon Laboratory built two special UHF ground radars for the USAF. These radars featured bearing resolution the same as WWII Mk-8, but at 75cm. The performance was excellent but they required 10 meters wide antennas. In an aircraft, or even aboard ship, and especially aboard a submarine, a 10 meters wide antenna is hardly practical. There are always trade offs. With a limited or fixed antenna size, one will go with the shortest usable wavelength to gain the best possible bearing resolution. The trade off is better bearing resolution in exchange for some of the nicer aspects of UHF.

Aircraft can use Synthetic Aperture Radar. This allows a huge virtual antenna size regardless of the wavelength. We usually used X-band and it provided a resolution cell of 15cm from 40,000 feet for mapping. However, to penetrate jungle folage or ice we found L band UHF worked better in a mapping application. A particular wavelength band may have certain attributes for certain jobs. The shorter the wavelength the more it reflects off the sea. X-band in particular reflects off of rain droplets and therefore shell splashes well. It's possible with modern digital signal proccessing to filter out the clutter leaving only the object with little or no clutter registering.

Shorter wavelength may offer a better way to detect LO objects at sea, as oppossed to LO aircraft, by creating a high clutter back ground which highlights a low clutter object.

Its not that any one wave band is better than another; it is what is best suited for the job at hand.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.


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