ILS Approaches?

Question

What is the difference between for example a CAT-1 ILS approach as compared to for example a CAT-4 ILS approach?

Answer 1

The last answer assumed a level of understanding and used a lot of terminology. Maybe this will be more helpful.

An crew and aircraft flying a CAT II/III ILS approach can begin the approach in conditions of lower visibility than a CAT I ILS. They can also continue the approach to a lower height above the ground before "waving off". To do this, they need an aircraft with special equipment and they need to be specially trained and certified to fly the lower-visibility approach. The runway also needs to have a published CAT II or CAT III approach.

Answer 2

There is no Cat-4 ILS. Cat 1 ILS is good to a minimum of 100 ft DH. The Cat 2 ILS requires a marker beacon and special certification and is good to a small RVR. Cat 3a & 3b ILS approaches require currency and special equipment such as a radar altimiter and can be shot to 0-0 weather with autoland.