R&N Mysterious Deletions
EGMA: "Re the comments on weather radar. It shouldn't be forgotten that radar cannot see through cells. It may be that AF avoided a cell only to find a massive cell hiding behind it."
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The above post at about 0119z remains, yet the following reply at 0240z was deleted: why? The mods are especially aggressive at deleting weather radar discussions, but seemingly not weather reports.
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That statement is true of the old X-band WXR designs prior to 1982. C-band radar does not have such a limitation. C-band will penetrate heavy precipitation better than X-band, but will not detect the lightest precip seen on X-band. (Airborne C-band wx radar is 5.4 GHz, and X-band is 9.3 GHz; your microwave oven is S-band, in the 2.2 GHz range.)
UAL had always chosen C-band radar up to that time, and continued with it into the Arinc 700 era, up until about year 2000. Air France was the only other major carrier to select C-band Arinc 700 WXR, which was discontinued about the time UAL abandoned it. Only Collins produced both C band and X band WXR in that era.
Collins pioneered PAC, Path Attenuation Compensation, in the 700 WXR, which would increase the receiver gain when it sensed reduced cell penetration. When it sensed it might not be able to detect the "cell behind the cell," it displayed PAC Alert, a yellow arc on the outer range ring behind the area of high attenuation.
RCA - (bought by Honeywell) Arinc 700 WXR had the same function they coined, REACT. I don't believe Bendix/Allied Signal/Honeywell RDR-4 series ever adopted that feature, but they may have.
In short, PAC compensated for the path attenuation of X-band weather radar, making C-band WXR almost redundant.
If the WXR is not pointing at the liquid part of the storm, path attenuation is not a factor, of course.
GB
Last edited by Graybeard; 18th Jun 2009 at 13:07.
Reason: Time Stamp