Originally Posted by idg
RZ,
I am not an expert on the technology but this is what I know!
The weather radar can 'see' reflective particles (usually water droplets) and graduate their size based on the microwave energy that is returned. With modern digital electronics the radar can also now detect moving targets using doppler shift principles. This target shift can be quantified on detection and since windshear will always result in movement (usually vertically but also sometime horizontally), the radar can now be used to detect it.
The 'alert' formula is this F= Wh/G - Vz/ TAS
where
Wh = rate of airspeed loss
G = Gravitational Force
Vz = Vertical downdraft in knots
TAS = A/C true airspeed.
F is the alert level and NASA/FAA studies found that this should be set to 0.15 if memory serves. Above this value the warnings are generated.
Note that in 'dry' conditions the wx radar cannot see a downburst.
During take-off the antenna scans in a figure of 8 pattern (laid on it's side) so that the antenna is always looking up and down and left and right to see if there's windshear about.
I wanted to share a practicle experience with you, well i work on the 737-NG and experimented by opening up the radome, and pushing the throttle levers full ahead, as most of us will be knowing, when the throttle levers are set beyond 53deg on the ng the radar automtically, begins a PWS Scan, well the scanner never did make the pattern of eight....