Follow-up to my first question: Why do you have a the CAT-2-approach a DH of 100 ft, but a RA of 95 ft?

Question

I always thought that, if there is written DH=100 ft, you put this number in the radar altimetry, and when you pass this altitude, you make the decision. So, why do you have there a RA below 100 ft?

http://i213.photobucket.com/albums/cc115/PilotFerdi/26r.jpg

Answer 1

RA is what your radar altimeter would read when the airplane is at the DH on glideslope.  This is isn't measured from the TDZE (touchdown zone elevation) like DH... It's measured straight down from the point where the center of the glideslope is 100 feet above TDZE.  Differences in terrain make up for this difference.

Answer 2

On the nose, Coffeebuzz!

Answer 3

im not sure about this but from what i can under stand is that they have differnt levels of radar to keep track of the planes at different levels and going in different directions to prevent any damage

Answer 4

As you can see your airport elevation is 122' and your RWY threshold elevation is 117'. That's a difference of 5' and is due to  the different reference points of each one and this is where the discrepancy comes from. For a cat II approach you set RA on your minimums.