This is sitting on the ground at Shoreham EGKA, and the DME should be reading about 0.0 or 0.1nm.
There are two specific issues here.
One is the one you see all the time after about time 0:18 which is a jumping around between 0.0/0.1 and some high value like 399. It has always done that – since new in 2002. I always thought it was just normal, and a consequence of being very close to the beacon. Once one is flying, and seeing higher values than 0.1, the reading is stable and accurate.
The other is what you see near the beginning, from about time 0.03, whereby the reading freezes at 383 and sits there for some seconds. I have never seen that before. Apparently the DME has recently been changed for a new old ex-Heathrow one. Again, in flight, it works fine, though that could be a temperature dependence of the fault (easy enough to test by sitting on the ground for much longer).
Is it just at Shoreham, or in other places too?
I ask because I remember one day about to taxi out at Cherburg I heard a strange repetitive clicking sound. Sometimes I get something similar when my headset batteries are about to go but they were fine. I didn’t want to take off without figuring out what the problem was. It’s always much easier to figure out on the ground.
I asked the other pilot with me if he could hear it too, and he said yes. We checked the intercom changed radio frequencies etc, and couldn’t figure out what was wrong. So I stopped and started to think what else I’d missed, and while thinking I stared outside……only to notice that every time the radar head pointed in our direction the noise came and it disappeared as soon as it passed us! The radar head was affecting the ANR circuits in both of our headsets (different brands).
Perhaps some radio equipment that is radiating from Shoreham is affecting it? If you also saw it on the ground elsewhere, that would disprove that theory.
Radar has a very large pulse power, compared to other emitters, so yes it can affect small analog audio signals, like the microphone of the ANR headset.
But I don’t think that’s the issue with Peter’s DME.
It looks to me that the Tx→Rx switching isn’t fast enough, so that it constantly looses lock and tries to reacquire.
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.
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.
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.
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.
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.
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 !
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 
