TCAS indication for Mode A traffic
 

hi, would like to clear my 'simple & stupid' doubt .

What will the tcas II indicate if the traffic is on Mode A. When will it announce 'traffic'?

thanks for your inputs.

Hi, we had this happen today. The TCAS displays the appropriate symbol without any altitude information. It treats the intruder aircraft as being at all altitudes so it'll give a "TRAFFIC" warning when the intruder is calculated to be a threat regardless of the actual height of the intruder (obviously the TCAS unit doesn't know how high the Mode A aircraft is and so acts on the worst case scenario.)

We had a TRAFFIC alert today from a Mode A helicopter 4,500" below us.

Strictly speaking it won't, because inspite of what most books say, TCAS doesn't interrogate Mode A.

What you probably mean is a Mode A & C transponder with altitude reporting not fitted or deselected.

In this case TCAS will issue a TA if the flight past is assessed to be a threat horizontally. This assessment is based on looking at closing speed, and the rate it is changing, to estimate the miss distance. If the miss distance is small then the alerting condition is met. (The value for the miss distance depends on closing speed but also bnacked up by an absolute minimum value that is related to altitude / radio altitude)

When the horizontal condition is being met, a TA is generated 20-48 seconds prior to the predicted closest point of approach.

pb

Capt Pit Bull is spot on. One other point is that since TCAS determines the closest intercept by the extrapolating from current range and rate of closure, if you are reasonably high above the traffic that is not reporting altitude you will sometimes see it be alerted as "traffic" and then the alert clears before you actually catch up to the traffic, because as the slant range moves to more vertical than horizontal the rate of closure reduces towards zero.

Resurrecting this thread as, in my dotage, I appear to have become mistaken for a subject matter expert on various aviation matters ;-). A lot of ATPL theory books, and question banks, are under the impression that aircraft with Mode A transponders are detected by TCAS II but can only provide TA's due to the lack of altitude information. Sounds reasonable until you glance at the Eurocontrol ACAS guide which states:
'Aircraft equipped with only Mode A transponders are not tracked nor detected by TCAS II because TCAS II does not use Mode A interrogations.' It then goes on to describe how Mode A/C transponders are detected,when Mode C is disabled, using mode c only 'all call'.

This appears to backed up by Mr Honeywell too. Interestingly a document provided by Airbus contradicts this so we're into the 'who plagiarised who' scenario and from what source.

My understanding is that TCAS II detects aircraft with Mode A/C transponders and if there's no altitude infomation provided by Mode C it will track them but only provide a TA (below FL155). Aircraft equipped with only a Mode A transponder aren't detected?

I think it depends on whether or not the aircraft is in radar airspace. The TCAS II may not be using Mode A interrogations but the Secondary Surveillance Radar is and TCAS II will see the replies to those interrogations.

That’s interesting, I don’t suppose you have a source for that info?

Hi Ahramin
I don't understand what information can be gained from the mode A reply, by a unit that didn't transmit the interrogation signal?
Cheers
TeeS

You can look in Eurocontrol ACAS guide March 2022 page 30, under “two antennae”

https://www.eurocontrol.int/sites/de...4-1.pdf#page30

Thanks but I can't see how that paragraph implies that TCAS II can receive replies from Mode A transponders which have been interrogated by ATC. The same document states that Mode A transponders cannot be seen by TCAS II

Please see page 5 of this https://www.faa.gov/documentlibrary/...%20booklet.pdf

TCAS 1&2 interrogates other transponders the same way SSR does.

Yes but Page 17 of that document states: ' Since TCAS does not use Mode A interrogations, the Mode A transponder codes of nearby aircraft are not known to TCAS.' The next paragraph goes on to explain how Mode A/C (Not Mode A only) transponders with inoperative altitude encoders are tracked by using the blank altitude data field framing pulses.

My understanding is that a basic (cheap) Mode A transponder will not be seen by TACAS II but a Mode A/C transponder that doesn't have a height encoding altimeter will be seen.

This is correct, the uplink message generated by the TCAS transponder is in mode C format, so it transmits on 1030 Mhz with a 21 microsecond delay between the P1 and P3 pulses. The mode A uplink format (only by ground stations in the civil world) is on the same frequency, however it has a 8 microsecond space in the between the P1 and P3 pulses. The reply from the transponder to the uplink message is on 1090 Mhz. Ground stations transmit alternate Mode A and Mode C uplink messages, TCAS uses a mode C uplink.

The format of the packet in response by the transponder to an uplink message is exactly the same regardless if it was a Mode A or Mode C uplink message.

So if the transponder received a Mode C transponder uplink message and pulses in the reply was
A1 A2 A4 B1 B2 B4 C1 C2 C4 D1 D2 D4
0 1 1 1 0 1 0 1 0 0 0 0

That would be decoded as 10,000 ft.

However if it was a Mode A uplink, and the transponder reply was
0 1 1 1 0 1 0 1 0 0 0 0

That means squawk code of 6520, exact same pulse format as 10,000 ft.

The transponder replies are the same, how the packet is decoded depends on what the ground station was expecting. In the civil aviation world, only ground stations use mode A uplink messages, so only ground stations expect squawk codes and decode the packets accordingly.

On the military side, there are lot more modes. Some military aircraft can uplink and decode squawk codes, this is known as military mode 3.

Thanks SWH that's a very good explanation of the technical aspects of transponder replies.
So, in pilot terminology, when a TCAS II mode S transponder shouts 'is anybody there' an obsolete mode A only transponder won't hear the call so won't respond whereas a nearly obsolete Mode A/C capable transponder with no altitude encoder will say 'I'm here but I've no idea of what my altitude is'?

Mode A transponders are to conform to a minimum of TSO-C74b, ever since the previous 1967 version of this standard (TSO-C74a) it has been a requirement under that “2.4 Interrogation. The equipment must accept and reply to interrogations on at least Modes 3/A and C”

So that any Mode A transponder SHOULD accept Mode C uplinks, very very very old transponders that predate this TSO are technically illegal even for VFR use unless they have been modified to meet the minimum Mode A standard. Those transponders are so old, I doubt they would still be serviceable.

TSO-C74a https://drs.faa.gov/browse/excelExte...300669076.0001
TSO-C74b https://drs.faa.gov/browse/excelExte...107150505.0001

I notice that most up to date documents to do with ACAS II only refer to Mode A/C or Mode S transponders, there's never any reference to a 'Mode A' transponder (other than to point out that they're invisible to TCAS II). Given the information that you've provided it appears that even the most basic transponder nowadays should be referred to as Mode A/C, even if there's no altitude encoding equipment fitted, because it can respond to a Mode C interrogation by leaving the altitude field blank in the reply.

That is correct, any civil TSOed transponder should reply to a mode C uplink message from a TCAS unit.