DME X and Y-channels

[disctilt]

Can anyone explain what the definition of a DME X-channel is. I have searched the web for a few hours and have found varying definitions.

I am studying for JAA ATPL and have ground school notes that tell me:

An X-channel is where the response from the (X-channel) ground transponder is 63MHz higher than the interrogators request frequency

and that

a Y-channel DME responds 63MHz lower than the interrogators.

I have found evidence that this is incorrect and that it is in fact the other way around (i.e. that the X-Channel DME responds 63 MHz lower). Can someone please tell me which is correct

[forget]

DME frequencies are paired to VHF omnidirectional range (VOR) frequencies. A DME interrogator is designed to automatically tune to the corresponding frequency when the associated VOR is selected. An airplane’s DME interrogator uses frequencies from 1025 to 1150 MHz. DME transponders transmit on a channel in the 962 to 1150 MHz range and receive on a corresponding channel between 962 to 1213 MHz. The band is divided into 126 channels for interrogation and 126 channels for transponder replies. The interrogation and reply frequencies always differ by 63 MHz. The spacing of all channels is 1 MHz with a signal spectrum width of 100 kHz.

Technical references to X and Y channels relate only to the spacing of the individual pulses in the DME pulse pair, 12 microsecond spacing for X channels and 36 microsecond spacing for Y channels.

DME facilities identify themselves with a 1350 Hz morse code three letter identity. If collocated with a VOR or ILS it will have the same identity code as the parent facility. Additionally, the DME will identify itself between those of the parent facility. DME identity is 1350 Hz to differentiate itself from the 1020 Hz tone of the VOR or the ILS localizer.

http://en.wikipedia.org/wiki/Distance_Measuring_Equipment

[Luke SkyToddler]

That's as good an example as I've ever seen of why the JAA ATPLs are a raging-out-of-control bureaucratic eurocentric steaming pile of utterly disinteresting overly-technical and completely-irrelevant-to-the-skill-of-piloting load of bollocks :{:{:{

Honestly who thinks up this sh!t? And who in authority deems it necessary to examine up and coming commercial pilots on it? And does anyone have their address so I can go and give them a brisk smack around the earhole? :ugh:

[redsnail]

I could think up a better syllabus.

Logbook filling out. Module 1. CAA specific.
Logbook filling out. Module 2. What the employers want.

Fuel management. Putting on enough to keep the regs, the boss and you happy.

Allowances. How to get the most.

Roster manipulation. How to get the most allowances for the least amount of work.

Form filling. How to get it right for the office and the maintenance dept.

Leave. What are you entitled to and how to maximise it.

:E

[MrHorgy]

Don't forget:

MEL - What you can go with, what you probably should go with, and what you WANT to go with.

Cabin Crew - An introductory guide to the Venus Fly Trap.

Crewing - A 694 page volume on reasons they can use to get you report. Includes bonus guide on turning off most mobile handsets.

Horgy

[Whirlygig]

why the JAA ATPLs are a raging-out-of-control bureaucratic eurocentric steaming pile of utterly disinteresting overly-technical and completely-irrelevant-to-the-skill-of-piloting load of bollocks

:D:D:D
...and even more so for helicopters!

Cheers

Whirls

[Alex Whittingham]

Which would be a valid criticism.......except that the JAA syllabus does not require you to explain the difference between X and Y Channels.

[B2N2]

And here is the FAA equivalent;
You see this little box here? That's a DME..most of the time it's put together with a VOR. Now that's really handy, ' cause it will now show you the distance to the VOR. If you manage to fly in a straight line TO the VOR it will even show you GS fairly accurately and if you change this little knob here it will even show you the time.
:ok:

[BEagle]

And was there any explanation as to why the X and Y pulse spacings are different?

A lot of these tech sources tell you something, which can then be tested in an exam. But they fail to teach anything of practical use.

I recently researched this X and Y channel thing for air-to-air TACAN use. The client's aeroplane would give satisfactory information on Y-channels, but not on X-channels at closer ranges.

It turned out that the manufacturers of the TACAN had come up with a powerful Tx and sensitive Rx. Such was the pi$$ poor AGC response of their receiver that it couldn't tell when one X-channel pulse ended and the next began, so couldn't provide range. With AGC off it was even worse as it was simply overloaded.....

But the wider spaced Y pulses were acceptable to their system.

So, sometimes it is useful to know this $hit, but I agree that the simple task of merely driving punters around in airliners does not require such knowledge. All you really need to know is that by having both X- and Y-channel DMEs, the frequency spectrum available for DME use can be used more efficiently - hence making more DME stations available. Which is very handy for those who rely on non-GPS DME-updated FMGC systems.

[disctilt]

Thanks for the responses so far!

Believe me I share the frustration here. I totally agree that the requirement to learn this type of information is illogical and makes the ATPL exams an exercise in who can regurgitate irrelevant facts and figures in a manner the JAA finds pleasing. :rolleyes:

Anyway.... can anyone tell me definitively whether X channels are 63MHz higher or 63 MHz lower.

Cheers!!

[Alex Whittingham]

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.

[paco]

I think also that frequencies requiring only one decimal place are known as X channels, and those with two are Y channels.

As for the exams, I believe that the JAA ones were originally supposed to be the equivalent of A BA degree, as you were regarded as entering a profession and, as with most degrees, they are crammed full of ****e to give them some form of credibility as most of the core stuff you can learn in one year (I did the 5-minute degree myself - studied what most people remember 6 weeks afterwards!)

Phil

[forget]

Alex W has to be right. There's no reason you should learn any of this. You've got more important things to study. Whoever asked you to swat up on it is wasting your time, and money. To paraphase Napoleon on the Metre - 'It's just tormenting your mind with trivia”.

[Alex Whittingham]

It is true that the paired VHF frequencies are arranged so that X Channels paired frequencies finish with a zero and Y Channels finish with a five, for instance the VHF pair for 123X is 117.60 and for 123Y is 117.65, but that has no affect on their principle of operation, they operate in UHF.

[snips]

Just had a quick scan through some old notes:-
X Y spacing allows a tighter geographical packing of the DME transponders on similar frequencies.
The different spacing help techniques employed within the DME transponder to reduce short range interference or long range interference.
:8

[forget]

Anyway.... can anyone tell me definitively whether X channels are 63MHz higher or 63 MHz lower.

3. DME Signal Characteristics.

DME transmission frequencies are 1 MHz apart. There are 126 aircraft transmission channels running from channel 1 at 1025 MHz through channel 126 at 1150 MHz. There are two transmission modes for each channel. In mode X the ground station transmission frequency is 63 MHz below the aircraft transmission frequency for channels 1-63 and 63 MHz above the aircraft frequency for channels 64-126. In mode Y the frequency spacing is reversed, i.e., the ground frequencies for channels 1-63 and 64-126 are 63 MHz above and below the aircraft frequencies, respectively. Hence, in mode Y the ground station transmissions are in the air-to-ground band. To avoid confusion between transmissions from aircraft and ground stations on the same frequency and to discriminate pulses from random interference, pulses are always transmitted in pairs. From the aircraft in mode X the pulses are 12 microseconds apart, and in mode Y they are 36 microseconds apart. From the ground transmitter the pulses are 12 and 30 microseconds apart in modes X and Y, respectively. Mode X is used much more frequently than mode Y.

However, from the same paper. Your DME may not work unless you understand this. :bored:


ABSTRACT
The neutral hydrogen 21-cm spectral line (1420.4 MHz) and the four 18-cm lines of the hydroxyl molecule (1612-1720 MHz) are observable at redshifts which put their measured line frequencies well below their protected frequency bands. Part of the redshift ranges (z = 0.171-0.477 for HI and z = 0.37-0.73 for OH) fall in the 960 to 1215 MHz band that is allocated to aircraft navigation. Most of the signals in this band are pulsed emissions of low duty cycle so much of the time between pulses is interference free. This paper outlines the structure and measured properties of signals in this band and demonstrates a signal processing strategy that is effective at removing the pulsed signals from spectra at sensitivities produced by several hours of integration.

[David Le Comte]

Can anyone explain what the definition of a DME X-channel is. I have searched the web for a few hours and have found varying definitions.

I am studying for JAA ATPL and have ground school notes that tell me:

An X-channel is where the response from the (X-channel) ground transponder is 63MHz higher than the interrogators request frequency

and that

a Y-channel DME responds 63MHz lower than the interrogators.

I have found evidence that this is incorrect and that it is in fact the other way around (i.e. that the X-Channel DME responds 63 MHz lower). Can someone please tell me which is correct

The ground based DME responder will respond to pulse pairs in the first 63 channels (1025-1088MHz) at -63MHz for X channels and +63MHz for Y. This reverses for the upper 63 channels (1089-1213MHz)