TCAS False alerts
 

Q. Does or can TCAS give false indications from wx radar stations?

Recently I have been flying with a portable TCAS or more correctly a PCAS, Zaon MRX to be specific. I have noted during AWK around Ceduna that the unit displays a target which appears at or near ground level, there is no level traffic.

My initial thought was that the unit is faulty as the MRX manual states that false indications are impossible.

However today I heard a REX 340 trying unsuccessfully to make contact with what they believed to be low level traffic near Ceduna. Couldn't get much comment from the crew as they were busy, but it appears their TCAS was showing same false TA.

Anyone experienced false indications, particularly from wx radar?

Regards
M

Was the indication definitely over land or could it have been over water?

False indications are possible if the unit is faulty. I have seen this as a target that just appears, seemingly from nowhere, at 1000' directly below and remains fixed in that position for the remainder of the flight.

It might be possible that there was a target aircraft and either they couldn't be contacted on the radio or did not wish to respond.

Some military vehicles (such as warships) use some sort of SSR that can generate a TCAS return at surface level.

I think you'll find that the transponders found on ships and other surface vessels are not Mode C equipment, and only return a Mode A signal (why would a ship have an altimeter?)

Consequently, return from these will be Other, Proximate or TA only, and will not have a level assigned to them on the TCAS display.

I would not expect a WX radar to result in a TCAS return, for two reasons:

1) Different frequency bands (Transponders on 1030MHz, Weather radar > 4 GHz)
2) Transponders work on the PRF of the response in order to segregate signals. Weather radar is not going to generate PRF of the same nature that a Transponder will.

That's the limit of my radio navigation wisdom - without further info on your unit or the specific circumstances, my only conclusions can be that it was an aircraft transponder being operated Mode C on the ground, or your unit has a fault.

Icarus

RENURPP,

The false indications I have experienced are all over land, can't speak for the REX's inbound track from AD but suspect they were over land.

Zaon MRX PCAS, works on the basis of receiving a signal (1090 Mhz) from an interrogated transponder unit within 5nm and 5000' (can be zoomed down if required). The other parties transponder needs to be interrogated from either a ground based ATC radar within approximately 150nm or from another aircrafts TCAS, typically overflying jets, within 40nm.

The typical indication I am getting is various ranges from 0.5 nm to 5 nm and below and indicating at or about ground level. There is no traffic I am 100% on that. No azimuth info on this unit, only altitude difference between you and them, range and whether they're climbing or descending relative to you.

This all sounds just like what the REX flight seemed to experience with their TSO'd system.

No warships in visible range, two gypsum/grain ships docked at the wharf though.

M

Hi Youngmic

Heres another two possible answers to your question

1. The Ceduna ADS-B station came on line (or maybe is still on test) in the last few days. Now ADS-B is not suposed to transmit on 1090, but that is a possible cause of the interfearence.
Note: The AsA map is a bit hard to read, and the ADS-B station may not be in Ceduna, but it is in that general area.

2. Someone or something is transmitting a signal on 1090. There are several possible sorces like the warship thingy.

If the signal remains stationary on your unit, i would sugest interfearance from something in your aircraft causing a false signal.
Can the unit be used outside the aircraft, if so try that and see if the signal goes away.

richo