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Old 24th Apr 2000, 11:03
  #7 (permalink)  
Checkboard
 
Join Date: Aug 1998
Location: Ex-pat Aussie in the UK
Posts: 5,215
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As mentioned above, the basic difference that will concern you as a pilot is that C-band radar has a greater penetration of storms than the 3 centimetre wavelength X-band radar. With X-band radar, you will see more radar shadows and precip attenuation will be greater - the major consequence of this is that it is possible to be mislead by attenuated returns, such as a thin line of "red" weather, with a seemingly clear area behind it (which is actually a shadow). X-band gives good precip. returns. Wet & dry hail will only provide a return when its diameter is greater than about 8/10 of the radar wavelength (say, hail better than one inch in diamter for X-band) so X-band is pretty poor for hail detection and you will need to use your tilt to obtain returns from the "wet" part of the storm in the lower levels if manoeuvering closely.

C-band radar is actually the better system, however the C-band wavelength requires a relatively large antenna (as can be seen from the table above, C-band radar is not available with the smaller antennae.) The definition loss in negligible, with the beam widths used in airbourne weather radar, so the real downside is that C-band is heavier and more expensive.

Distance attentuation is inversely proportional to the square of the distance when the target is larger than the cross-section of the beam, and inversely proportional to the 4th power of the distance when the target only partially intercepts the beam. This means that X-band, with its smaller beam width suffers slightly less attenuation (for large targets) however, as these distances are within the STC (Sensitivity Timing Control) range, the effect is also negligible.

With the entry of high performance light jet and turbo-prop aircraft X-band radar underwent a surge in sales (as the smaller antennae allowed it to be installed in smaller aircraft), but the real change came with the entry of digital technologies in the late 70s and early 80s. Digital processing allows better screen resolutions, and you can begin to play with the attenuation effects, using a fourth color for turbulence and shadow predicition. This mostly off-sets the disadvantages of the cheaper X-band, so it is becomming very common in jet transports these days.

S-band provides the best returns, and is used by the met guys on the ground, however the antenna size required precludes it being used in aircraft. L-band is used by ATC, and is very poor at providing weather returns, which is why ATC weather advice is pretty poor, unless they have a weather screen as well as an ATC one.

HF (3 to 30 MHz)
VHF (30 to 300 MHz)
UHF (300 to 1,000 MHz)
L band (1,000 to 2,000 MHz)
S band (2 to 4 GHz)
C band (4 to 8 GHz)
X band (8 to 12 GHz)
K band (12 to 40 GHz)

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[This message has been edited by Checkboard (edited 24 April 2000).]
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