|Quoting Troubleshooter (Reply 3):
For an ILS approach you need an ILS receiver tuned to an ILS frequency. A VOR receiver can not receive or compute ILS signals. The ILS receiver is tuned to a localizer signal. Automatically a dedicated glideslope frequency is tuned. Both signals are received and a deviation is calculated (amongst other things) for display to the flight crew. A VOR receiver can not do this calculation! But a VOR antenna can receive the localizer signal as this is working in the same frequency range as a VOR.
Not entirely true...a VOR receiver, by design, will also receive the localizer portion of an ILS. So, as long as an ILS approach has an associated LOC (localizer only-left/right guidance), you may still shoot the approach using a VOR receiver (to localizer minimums, which are typically much higher than with LOC/Glideslope guidance). The phase shift principle used by the VOR receiver is the same for a localizer-the difference is that, when tracking a LOC, the receiver ignores the Omni Bearing Selector (OBS) setting.
LOC frequencies are tunable by every VOR receiver ever made (and, to the original poster, there is to transmit function on a VOR receiver
The only transmitting involved is if you are using DME
, or Distance Measuring Equipment, which is another beast entirely). The LOC will be received by the aircraft's VOR antenna, as LOC frequencies occupy the lower portion of the NAV frequency band (108.1 Mhz-117.9 Mhz). As I recall, in the US at least, 109.1-111.9 Mhz is reserved for localizers.
The Glideslope portion is automatically tuned by an ILS receiver, as there is a standard frequency pairing for glideslopes/localizer frequencies (and I believe, IIRC, the glideslope is up in the 300 Mhz frequency range...correct me as necessary). An ILS receiver is also, by nature, a VOR receiver. However, a VOR receiver, with no glideslope, would not provide you with full ILS capability (no glideslope-but you would be able to shoot many ILS approaches to LOC minimums).
A Localizer is much more sensitive than a VOR, however, and it has to be: it is trying to direct an aircraft towards a runway in some pretty bad visibility
[Edited 2006-11-08 19:36:26]