It is my understanding (and observation) that with certain GP transmiiters, a false GP lobe will exist at 6 degrees, and a weaker one at 9 and 12. The strength and stability of the various false lobes varies from installation.
(OzExp..From memory, Rwy 14 at POM had a noticeable false GP at 6 degrees, a point missed by the advocates of the higher-is-safer school of Instrument approach flying!).
Certainly, in specific instances, the 6 degree GP can be strong and steady enough to lure the dopey pilot into flying it. MInd you, one would think that being 6000' above aerodrome elevation at 10 miles should be a decent clue....but on a dark and stormy night, its never quite that easy is it?
Does anyone know if the new technology ILS installations have this problem beaten?
False GP's and Erroneous GP's, I think are not necessarily the same thing. False GP's are a fact of life and are an unfortunate side effect of GP design - but they are always steeper than than 3 degrees, occuring, as I remember, in mutiples of 3 degrees (bear with me - its been a long long time)
In a past life, I have observed that monitors have been disabled by the techies because of spurious warnings.
No problemo!!... until the day comes that the GP does actually drift off completely - as it will on a rainy night at 2 am. Since then, I have always been very careful to check GP against altimeter and distance to run on every ILS I fly...I cannot reccomend highly enough that everyone does his own personal GP integrity check at 10 miles.