ghost targets on our 11 yr old eurocat

[radarbloke.ana]

Our radar has been seeing ghost targets lately. Yesterday i spotted a vfr traffic squawking 1200 holding over a nearby airport (CLARK) 50nm from Manila. Altitude readout:FL183 and right along my climb and descent area. This traffic is on my controlled airspace. MLA en route alt is from FL130 to unl.

When asked, Clark App reported having a traffic holding overhead their stn at 4500. MNL Approach Radar also confirmed seeing a target over Clark at 4500. One arrival and 1 departure traffic did not see the mysterious tffc on their tcas...except one: as my 3rd arrival was passing thru fl180 for fl160, he reported seeing a tfc on his tcas 300ft above, 12 o'clock.

My super asked Clark to adz their vfr tfc to turn off his transponder coz it's messing with our radar and some aircraft's tcas.

1. Is it smart to ask a pilot to turn off his transponder when something like that happens?
2. What's the best way to deal with this situation? (besides providing vertical separation)
3. Why do you think only our secondary radar (eurocat) plus 1 out of 3 aircraft saw this ghost target while two primary radars (clark's selex and Mnl app's nec) saw the target as it really was?
3.1 what do you think is common between my radar and the 3rd aircraft's tcas?

**hope you get what i mean to ask...english is not my primary language. Big Thanks!

[ayrprox]

we dont understand how they work, we're just mighty glad they do :E

[eglnyt]

There are a few engineers that visit this forum one of whom is far better qualified to answer than I but as he hasn't I'll suggest some possibilities for the technical parts. Question 2 is best answered by my ATC colleagues.

Within the UK we have had similar incidents since the new Mode S ground interrogators entered service. The problem in the UK is caused by some aircraft transponders provided incorrect responses to the ground interrogations. Typically this shows as an aircraft with a 7000 Mode A code operating around 2000 incorrectly supplying a Mode C response between FL180 and FL210 to the Mode S interrogators. It continues to provide the correct Mode C response to the older Mode A/C ground stations. That could be what is happening here but the details supplied about the radars in use are slightly confusing and the SSR radarbloke is using would have to be a Mode S interrogator. If it's 12 years old it probably isn't but that might just be the age of the whole system and bits of it may have been upgraded since. In the UK these corrupt responses don't show up on TCAS in other aircraft which continue to see the correct height.

If this is what's happening the only way to stop this is to ask the aircraft to switch off its transponder. Just switching off its Mode C doesn't work as we have seen cases of this happening with aircraft which are not Mode C equipped. It's an ATC call as to whether they prefer a label with a corrupt response or no label at all. There may be some conflict between the controller of the higher airspace who probably wants it switched off and the controller who actually has the aircraft in his airspace and a lot will depend upon whether or not either controller is using height filters and whether the rogue return is in conflict with anything..

[Tarq57]

This happens from time to time in my neck of the woods.
Simple (and I believe most likely) answer: The aircraft at 4500 FT had a faulty altitude encoder/mode c. The aircraft was at 4500FT, the transponder (not its tcas - it mayy not even have had tcas) - was mis-reporting the altitude.

Simple answer, when the actual altitude differs by more than the allowed tolerance, is to get the offending a/c to de-select mode C.

To answer your questions, in order:
1) "Is it smart to ask a pilot to turn off his tcas when something like that happens?"
Not appropriate, as it is the transponder that is faulty.
And no, it is never smart to get a pilot to turn off his tcas. In most cases the pilot would/should decline to do so.

2) "What's the best way to deal with this situation? (besides providing vertical separation)"
See above. The offending a/c should stop squawking charlie.

3) "Why do you think only our secondary radar (eurocat) plus 1 out of 3 aircraft saw this ghost target while two primary radars (clark's seles and Mnl app's thales) saw the target as it really was?"

Your radar was probably correctly displaying what was input to it. The other (primary) radars would have no way of reading the transponder signal, and would be reporting the (approximate) correct altitude of 4500FT, based on their slant ability/triangulation.

The other aircraft that saw the FL183 readout saw on its TCAS what was being signaled by the faulty transponder. The aircraft that did not record/report that were most likely far enough away in altitude or range that the signal did not trigger any kind of advisory. Can't really be sure of that without being there and seeing what you were seeing.

3.1 "what do you think is common between my radar and the 3rd aircraft's tcas?"

They both indicated the information that was transmitted to them. Chances are, your Eurocat gear is fine. The VFR a/c concerned needs to get its transponder fixed.

Hope that helps.

[radarbloke.ana]

1) "Is it smart to ask a pilot to turn off his tcas when something like that happens?"
Not appropriate, as it is the transponder that is faulty.
And no, it is never smart to get a pilot to turn off his tcas. In most cases the pilot would/should decline to do so.


oops. I meant to say TRANSPONDER instead of TCAS. But you understood it still the same. Thanks. :ok:

[Tarq57]

Vercingetorix
Tarq57
I note that you refer to Clark as having Seles radar.
Hi, not me.
I didn't even know what "Seles radar" was until I Googled it..it seems the name you might have meant was Selex Radar.

My deduction of the ability of the system to give an altitude readout was based on the OP's comment saying that the primary radar gave the altitude as 4500FT. From that I deduced that the primary radar was able to triangulate and calculate altitude, in much the same way as the Brazilian system, which was widely publicized- and especially on this forum- following the GOL/Legacy midair.
It makes sense to me that an area of such strategic importance to the US military would be likely to have this capability.
Whether it also has the ability to more accurately measure azimuth using a phased array, or localized microwave directional (3D) type of tech, I don't know, but it seems quite possible. Likely, even.
(Semi-educated guesswork, really.)

[Tarq57]

I understand.
What I posted was a c&p of the OP's questions, (in "") followed by what I thought the answers were likely to be.
Didn't realize the US had "left the building". But the radar gear they left behind is probably still operational/used by the local authorities, I guess.