Operation of HMUs for RVSM

[cdb]

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

[BizJetJock]

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!

[PPRuNe Radar]

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

Roke Manor HMU