But you don't have to learn it! It's not in the syllabus and it is not examined. Its a little unfair to blame the JAA for the offences of an overenthusiastic (and mistaken) ground instructor.
For the record both X and Y channels pair high and low. For example, Channel 1X has an interrogator frequency of 1025MHz and a response on 962MHz, Channel 1Y has an interrogation on 1025MHz and a reply on 1088MHz, Channel 123X has an interrogation on 1147MHz and a response on 1210MHz whereas 123Y interrogates on 1147MHz and replies on 1084MHz. The thing that distinguishes X and Y Channels is not the frequency pairing but, as forget said in the first response, the pulse spacing. X Channels in DME/N (the normal en-route DME) space both interrogation and reply pulses by 12 microseconds, the Y Channels space the interrogation pulses by 36 microseconds and the reply by 30 microseconds. There are different spacings for DME/P, which is used with MLS and for the little used W and Z channels.
ICAO Annex 10 Volume Chapter 3 is your reference.
Last edited by Alex Whittingham; 16th Jul 2007 at 09:00.
Reason: To change ILS to MLS in the last line and X to Z