ILS signal strength dilemma?
 

Hey guys, just a quick question;
We can auto-land with CAT I minimums in use normally but always i have a doubt about that.

If an airport/runway has both CAT I and III minimums (capable of both), and the weather is nice, (so we suppose to execute CAT I landing), as far as know we can will auto-land. But is there any signal strength difference between CAT 1 and CAT 3 landings? I mean whether we land by 1 or 3, is the signal same? (Assuming both CAT 3 areas are clear)

My understanding is that the if an ILS is CAT III then it radiates CAT III at all times irrespective of weather, unless Notam'd otherwise.

If autoland is carried out in good weather and CAT III protected area is confirmed clear then the radiated signal should not be affected.

If CAT III protected area is not guaranteed to be clear then the ILS CAT III signal integrity may be affected.

All I can say is that throughout my ATPL and subsequent flying career, I have never heard of variable ILS strength.

I cannot really think of a good reason for it either - it would add more complexity and cost to the ILS installation and would add another potential human error (leaving the ILS on low strength for Cat lll ops).

As you allude to; making an autoland approach on an unprotected ILS can result in signal and path errors, because aircraft will not be holding at the CAT lll holding points on the airfield.

The question appears to relate more to the standard of ground equipment opposed to the published approach minima.
Cat 2 – 3 ILS beams are generally built and checked to a higher standard than Cat 1. In many installations a Cat 1 ILS is not as accurate as Cat 2 – 3, but most aircraft will be able to autoland on a Cat 1 beam (check with aircraft manufacturer). The autopilot performance could be degraded by the accuracy of ground installation and lack of LVP protection – interference / reflected beams.
Many airports require the crew to notify ATC if an autoland (or HUD flare) is to be flown for practice on any ILS type without LVP. If this is not done then the possibility of ILS beam interference and guidance variability require the crew to carefully monitor the automatic approach and landing.
In addition, Cat 1 ILS installations may not meet the required ‘flat’ ground profile before or at the runway threshold which could change the approach and flare performance if based on Rad Alt.

Most CAT II and all CAT III localizers are dual frequency. There are two signals sent out, just a few hz above and below what is dialed in the box. The freqs are close enough to the published freq that the nav system will lock on to the stronger one. If it fails, the nav system will immediately lock on to the other signal and you can continue the approach. But you are only using one signal at a time.