Operation of HMUs for RVSM
 

Can anyone explain how the HMUs (e.g. at Strumble and Geneva) work (the boxes that measure RVSM compliance). I was told that it is a differential mode S ariel array - but wouldn't that limit operation to mode S equipped aircraft? Surely a vertical radar would be more useful?

Thanks

cdb

I'm afraid I don't know how the HMU's actually operate, but it wouldn't surprise me if they use mode S. When you look into all the other bits of rules you find that 99% of aircraft that are going to be RVSM approved are also required to have TCAS, and the TCAS equipment includes mode S transponders. Therefore there are very few aircraft needing a height accuracy check that don't have mode S. For those few there are alternative methods of accuracy checking using recording GPS units.
Hope this helps!

There is basic info on how it works in this link.

Roke Manor HMU