X band radar tips?
 

TS season is upon us here in the US. I am a newcomer to the wonders of X band - up until last year I have always been using C band radar.

I wonder if anyone with some time on this equipment would be willing to share with me (and many lurkers) their thoughts on operating differences. I am pretty fed up with my companies lack of guidance and background info on this system - one would think that there is an ink shortage going on!

So how about tilt management etc.? Any significant differences? What are other airlines publishing about this equipment and SOPS?

Ta very much

JJM

X-band radar is not all that different than C-band. Essentially, C-band will attenuate a bit less, but the tradeoff is that you do not have as clear definition. Since weather radar is an avoidance tool, and not a penatrating one, X-band is generally considered the better wavelength for thunderstorm avoidance.

I would personally recommend you contact Dave Gwinn and enroll in his radar course. I would not recommend you waste your time on Archie Trammel's course. Also, check this web link: http://bluecoat.eurocontrol.fr/repor..._99_Radar.html

Presumably the dish size/beamwidth figures in that article are for C band? I'd expect X band to generate a much tighter beam for those dish sizes.

R

Our X band RDR-4A (Allied Signal) has a 3 degree beam width with a 30" antenna (when properly calibrated).


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Antenna type vs dish size & beam width

Size X-band C-Band

10" 10°    NA
12" 8°     NA
18" 5°     NA
24" 4°     6.5°
28" 3.5°    6°
30" 3°      5°

[This message has been edited by Checkboard (edited 23 April 2000).]

Obviously the tighter beam width is an advantage in getting a better picture, so long as you move the beam enough to not miss something. That leads one to expect X band to be the better system for that reason alone.

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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Tech Log forum moderator

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

Thanks Checkerboard and others for the info. I am still getting used to it, and the thoughts are welcome. I have been very pleased with it in high level cruise. In teh terminal area I have found it tends to paint the sky as falling: a very "busy" picture - it paints far more weather than is really there. I have tried tilting well up to make sure I am not getting ground returns. If there is shower activity in the area, it seems to paint it as a higher torm level than it really is. First time I saw this I was asking for deviations around stuff that was really nothing other than light/moderate rainfall. It painted up as yellow/red return, whereas C would not have been more than green. Other pilots concur on the phenomena.

Prof2MDA, I disagree about Archie Trammel's radar course, I bought it (with my own, tax deductable, money) and thought it was OK :)

Jurassic Jet Man,

The differences you are experiencing are not due to the differences in propagation of c-band and x-band radar signals, they are due to a different philosophy in radar presentation with the "newer" radars (post late 70's).

Your older C-band was designed back in the days before legal departments got involved in radar design. The radar display was calibrated to show the different reflectivity of a weather system, measured in decibels (dbz). The reflectivity is a function of precipitation density.

Beginning in the 1930s research pilots have penetrated hundreds of storm cells in instrumented aircraft, as radar scientists concurrently measured the radar reflectiveness of the cells.

This data first became available 25 years ago in a benchmark report prepared by J.T. (Jean) Lee of the US National Severe Storms Laboratory. Since that time radar displays have been calibrated to show the "red" area to indicate storms with a reflectivity above 40 dbz. Eariler radars used the contour hole to show this level of precipitation.

In the 70s, when tort law in the US began to drive everything, radar engineers were told to modify their STC (Sensentivity Timing Control) curves so that the closer a pilot gets to an area of rain the redder it gets. The STC desensitises the radar reciver to acount for the strong returns recieved from close echos, typically within about 30-40 miles (but up to 250 miles on some sets).

In short, your newer X-band has a purposely misdesigned STC arrangement, giving you a false picture at close ranges. A bad situation, but you have to fight the lawyers to change it (or keep your old radar!).