Old 3rd Feb 2019, 13:08
  #996 (permalink)  
ATC Watcher
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Join Date: May 1997
Location: Europe
Age: 69
Posts: 2,602
Originally Posted by DaveReidUK View Post
I'm still struggling to understand how that can be so. Can you explain further ?

I don't see how it's possible to derive GS.

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You do not need Mode S to get GS . Any radar can give you GS, if you know the Rotation RPM of the antenna , the distance covered between 2 update and bingo : GS. I a multi radar environment , the GS is very accurate. It is often displayed on the labels of ATC radar displays .
What Mode S gives you is 25ft vertical increments ( i.s.o 100 with mode C ) , with that info you can calculate pretty accurately the ROD , and notice any change . i.e. if decreasing/ increasing or steady . When you lose returns close to the ground (or in our case sea, which is typically around or below 1000ft,) If you put that info with GS you can extrapolate when the line will hit the sea..
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No doubt that's true, but the estimate of final position could well have come from primary radar (possibly military?), rather than SSR.
Absolutely. I am pretty sure the BEA looked at a few PRI recordings to see if they match . But Primary will not give you any altitude nor Rate of descent info, and the primary returns will also stop at a certain point , generally above 1000 ft due garbling , and where the antenna is located , ( i.e. how far /how close away it is , obstacles, earth curvature , etc.. ) but with PRI you have no idea if the aircraft was descending at 200ft a min or 2000/ft/min or was just leveling off at 1000 ..., Mode S will give you that valuable info.
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