Transponder double echo
 

I had a 1 hour scenic charter in controlled airspace and atc a couple of times said we had unidentified traffic 100 feet below which was disconcerting. Eventually it was decided it was a phantom transponder echo. We did a quick transponder recycle but that didn't fix it.ATC seemed to say they were seeing our mode c and another mode c 100 feet lower with 3 different code digits from our code.

After leaving controlled airspace, the problem was fixed by powering down the transponder for 30 seconds and re enterting the code and hey presto it worked fine.

Are these phantom echoes common?

Nothing that I've ever seen. I've observed radar reflections which briefly produce a duplicate contact miles away from the original, but nothing like described here. Are you sure there wasn't actually traffic there? Or maybe the observed contact was a reflection of something elsewhere on the display.

Some military aircraft have an ECM fit which can do this.

The only military traffic within say 10nm was a PC9 trainer at low level. I guess it could have been a fault with the ATC display but still think it was our transponder that was faulty.

Where were you flying and what make is your transponder? At times the mode S radar sensors can be particularly choosey about the data that they accept and how they receive it. Some transponders can cause this issue. This can result in incorrect returns being displayed, displaced both in both horizontal and vertical planes. It's not very common, however I have seen it a few times with Military traffic in WA.

Perth, and its an old Narco AT150 TSO Mode C.

I've seen it at LATCC enroute pre-'83. Resolved by asking crew to 'Squawk Low' (Low sens). Also seen SSR Mode A 'ring-around', again solved by squawking 'Low'. Not sure if TXPDRs have 'Low' setting any more. Narco AT150 hasn't.

Some, not all, instances of this particular transponder do not play well with modern Mode-S interrogators. In the UK this has generally been seen as the aircraft returning a Mode C at about FL190 when the aircraft is actually around about 3000 feet. When the UK still had Mode A/C interrogators as well as Mode-S the aircraft would be shown at the correct height for controllers using those radars and at the incorrect height for those using the newer radars.

It is probably your transponder but you will probably find when checked out in the shop they can find nothing wrong with it because it can meet all its original specifications and still cause this issue. There are others who frequent this forum who know far more about this particular issue than I but I believe it may have had something to do with the transponder sending a Mode A reply in response to a Mode C interrogation.

As eglnyt says it could well be the old Narco transponder not behaving. There have been problems with them in the past. It would be interesting to know what your Mode A squawk was at the time to see how that would translate as a height but that doesn't explain why you Mode A code was different as well. Another factor affecting this is the Mode S interrogator mode interlace pattern as because of the different turn-around delay for an SSR and Mode S transponder the false target is often seen at a different range (e.g. 10 miles closer).

It could have been a phantom transponder echo caused by multipath but such effects do not tend to last for many scans and modern interrogators normally have protection against this. You would also expect the false target to be at a slightly different range (around 0.2 to 1.0 miles longer).

If this is ever seen again in another location it's likely to be your transponder but if not probably multipath.

RR

Are you fitted with ADS-B?
 

I am not an Engineer so I don't know the reason why but I have seen many times (particularly with Gulfstreams) a duplicated and slightly offset vertically second surveillance return. Causes havoc with the STCA alerts as system is unable to recognise that it is the same aircraft. Each time it has been concluded that it was due to ADS-B returns not being accepted as the same aircraft by the multi-radar tracker and so displays two targets. We were able to ascertain this by detaching the ADS-B input from the RDP.

Hope that helps.

One of the parachute aircraft at Pinjarra used to do that regularly. It was on 1200 though and both returns had the same code ( I think ).