keithl, don’t be too quick with the logical conclusion. My experiences were with older ILS (many still in service), but I suspect that even the latest equipment has many pitfalls for the unwary aviator.
I also note that at least one of the major manufacturers, whilst accepting joins from above, cautions against arming the automatic capture systems until the aircraft is safely clear of false GS or LOC signals. Although the human can deduce that the indications are reversed, automation just blindly follows the received signal.
The UK NATS give advice on the use of ILS in
AIC Pink 141 (AIC 34/1997) the link may have restricted access. If registration is required start with:
www.ais.org.uk
The following extracts are relevant:
“The GS coverage is 10 nm +- 8 deg from LOC centerline. Vertical coverage is between 0.45 x and 1.75 x GS angle (i.e. 1.35 deg to 5.25 deg above the horizon for 3 deg slopes)”. I assume this is the calibrated (guaranteed) signal, thus does not exclude false GS beams outside of these limits.
“Where special approach procedures have been devised in which the GS may be joined from above, pilots must be aware that false GS may exist at an angle of about an angle of 2 x GS angle (i.e. 6 deg for 3 deg slope). This false GS must not be used for descent guidance. Establishment on the promulgated GS should be confirmed by the relationship between aircraft height and the distance to runway threshold.” i.e. approx 300 ft/ nm for 3 deg GS.
For LOC the useable coverage is 25 nm for +- 10 deg and 17 nm for +-35 deg, both from the front course. (Does this imply no back course ILS in UK, I think so?) These values are less for steep approach ILS installations. There is a caution about the risk of early LOC capture even within these limits.