Single DME read zero at both thresholds?
 

A quick question please..

I have recently come across an airport that states that a single transmitter, same frequency, same ident DME reads zero one threshold and zero at the other displaced threshold. It has been stated by the airfield operator that the DME will read zero as you transit from one end of the runway to the other.

I haven't come across this before, so can someone please explain how this is possible please?

Thanks in advance

Put a DME ground station close to the runway, midway between the thresholds. Reduce the receive-transmit time delay by a "1/2 runway length".
However, it will cause FMCs to disregard the DME data.

Either that or the DME is located alongside a very short runway.

Further to dusk2dawn,

DME ground stations have a built-in standard 50 microsecond delay between receiving the aircraft interrogation and transmitting the response. This is intentional, so the aircraft's DME transceiver "knows" about it and factors it in to the time-speed-distance calculation.

If you change the 50microsec delay, if will change the zero-DME point. For example, reducing the delay will cause the DME readout in the aircraft to be less than the true distance.

So rather than the DME referencing zero at the DME antenna itself, it will reference zero anywhere on the surface of an imaginary bubble extending around & over the DME antenna, a bubble which coincidentally intersects with the touchdown zone of the landing runway.

If you fly over an airfield and the DME reads a lesser distance than your altitude above the field (say, 0 DME at 4000'AGL), then you know that they've played tricks with the timing.

OKTAS, thanks for such a great explanation. Something I never knew. Now, while on the subject of DME, with modern ground stations these days, is it ever necessary to use the Over-ride function which I see installed in older aircraft units, but not all new ones?

This represents 100% of my knowledge of DME over-ride!

Not surprisingly, modern DME ground stations which have reached maximum capacity of interrogations, intelligently refuse to accept any extra aircraft interrogations. Which aircraft get excluded? The ones with the weakest (therefore most distant) signals.

So even if you do have DME over-ride selected, I'm guessing that the DME stations around Heathrow and Chicago probably aren't going to talk to you when you're 300nm away...