There are various systems on board that an aircraft must have servicable on an auto approach, they must have at least two autopilot systems (usually three) all monitoring each other, the aircraft will have auto throttles although on the aircraft I fly it is possible to autoland using manual thrust control.
As I mentioned before the majority of actions that the aircraft goes through on the approach e.g. aligning the nose with the runway centre line and flaring into the landing attitude are all done with reference to the Radio altimeter which measures height above the ground directly below the aircraft, so it goes without saying that these must be fully servicable.
This leads on to your next point about Jersey becoming cat 2 or 3.
I think that for a start the runway might not be of sufficient width ( not 100% on that) and more importantly as Pointer mentioned the signal emitted from the ILS ground installation must be protected from other aircraft, namely those ones sitting at the holding point waiting for departure. Large chunks of metal, such as aircraft are capable of bending the beam of the ILS possibly resulting in an undesirable low level fly by of St Hellier!!.
Jersey as as it stands at the moment does not have a holding point far enough from the runway to achieve this ILS protection.
Back to the Radio altimeter point, the approach to both runways at Jersey comes in over rapidly undulating terrain.The problem that this creates is that the height that the radio altimeter is indicating ( and therefore the point that the aircraft flares etc) is not correct, this is particularly the case when landing on the easterly rwy that approaches over the beach. one minute the Rad-alt is indicating 2000ft then in a matter of seconds it decreases to less than 100 ft and by this point the aircraft systems have not done a number of things that they should have.
Therefore we reference all our cat 1 approaches on height above sea level (altitude) which is not affected by terrain.
So in answer to your question an upgrade to Cat 2 0r 3 would for a start only be possible on to the westerley runway and even then new taxiways would be required with new holding points and even then I am not sure if the terrain below the approach is adequately uniform.
I have probably only confused the situation, but I hope this helps a little.
[ 26 August 2001: Message edited by: spoilers yellow ]