The DME is deaf for 50uS after the aircraft part has transmitted. 0 NM DME is = 50 uS
Could be due difference in 50uS timing in either the ground or aircraft part, or do to echo’s. Echo’s are replies on signals received by the ground part with a delay due to multipath.
Never had it on the G1000 or Avidyne setup. Must be King related.
Might have to do with the DME software as it might treat the distance as a negative value.
Many thanks A&C. Great to have this verified.
I am not sure I can get my head around why exactly this happens, because I would have thought a DME rigged to read zero when you are more than zero distance from it would simply return the packet sooner. So I guess it is the aircraft portion that can’t deal with a return delay of less than x.
I would hate to find out that I have a TB20 capable of 500+kt, only after I have sold it to somebody for €150k 
A few nights back I was inside the zero point of a DME station at a secret Oxfordshire aerodrome and the KN64 fitted to the aircraft gave much the same readings as Peter has illustrated except that the Cessna 152 managed to achive 740 Kts ground speed !
Peter, why do you complain? You are doing 496 kts !!! Pretty darn good for a TB 20 ;-)
The clicking or buzzing sound is normal in the Boses when relatively close to a radar head, I also get that in mine.
Even before the recent upgrade of the DME beacon I was able to pick up – if coming from the south and at a decent height – the Shoreham DME at about 80-90nm, which is pretty good for a TDME. I haven’t tested it from that “angle” recently.
All DME systems have a freeze system incase they momentaraly loose the lockon to the station this retains the distance for a few seconds while the system try’s to require the lock.
Occasionaly you outgoing signal will exactly match that from another aircraft and the DME will see this as a valid and reads some totaly random value for a second or two. I suspect that as you have a high power DME ( by GA standards) your random readings on the ground at EGKA are due to a combination of being too near the station and the signals bouncing around all the metal objects that are parked around you, this all exacerbated by your relitivly high output power.
I will have a look at what happens at EGKK when five very high power DME’s are tuned to a station that is calibrated for below zero at the aircraft position and report back my findings.
To be able to get the Zero reading for both thend of the runway they have adjusted the DME station to read Zero at a distance equivalent to the distance that the transmitter is located from the end of each runway.
Correct.
If you are located less than the distance the DME station is located from the end of the runway your DME Recever gets an invalid signal indicating less than zero and so gets confused.
I did wonder if that was the reason. I don’t recall seeing this behaviour elsewhere. What exactly is supposed to happen if a DME is adjusted to read zero at say 1nm and you are 0.5nm from the installation? It should read – 0.5nm, but it isn’t capable of displaying that.
However what concerns me more is that frozen reading at the start of the video. I have never seen that before.
The radar head was affecting the ANR circuits in both of our headsets (different brands).
Yes – this clicking (or a brief “buzzing” pulse) is common, at larger airports which have on-site radar. The Bose headsets always do it. At such close range you are getting zapped with massive RF pulses. But Shoreham has no radar.
From what I can see the DME at EGKA is used on two approaches 02 & 20 and the Jepps indicate that it reads very close to zero at both ends of the runway. So how come the same DME can do this ?
To be able to get the Zero reading for both thend of the runway they have adjusted the DME station to read Zero at a distance equivalent to the distance that the transmitter is located from the end of each runway.
If you are located less than the distance the DME station is located from the end of the runway your DME Recever gets an invalid signal indicating less than zero and so gets confused.
This is quite common practice and a technique used at a lot of single runway airports so that one DME station can be used for approaches to both ends of the runway, EGKK & EGGW are other examples of this.
