View Full Version : TCAS II and Mode S XPNDR

21st Jun 2004, 03:52
Hi all...can some one please enlighten me on the major differences between TCAS I and TCAS II and also the differences between Mode C and Mode S transponders....


21st Jun 2004, 09:37
Without going into what the states and other countries require when it comes to tcas how about i give you the basic differences.

TCAS 1 - gives TA's which mean traffic advisories, they make the crew look out and spot the traffic and alert you to traffic you cannot see. (first generation tcas)

TCAS 2 - gives RA's which mean resolution advisories, a voice telling you to climb or decend or monitor your vertical speed NEVER a turn left or right.

Now as far as mode c and mode s transponders.

If both aircraft have mode s transponders, they will talk to each other so to speak and coordinate the RA. For instance one aircraft will be told to climb the other told to decend.

Both aircraft dont need mode s for the RA to work as long as the the other aircraft at least has a mode c transponder (gives altitude) which compared to mode a which does not..

Its simplified but I hope this helps


Check 6
24th Jun 2004, 11:31
To go further, TCAS II (version 7) is the same as ACAS II. This system provides "coordinated RA's," i.e. the ACAS systems in both aircraft come to an agreement on who will climb and who will descend.

Check 6

24th Jun 2004, 11:53
Thanks guys.....howabout the envelopes ie 1200' and 6nm or the 20-25 sec point of impact for an RA or the 40-45sec for TA is that still the same?...looking at a XPNDR I have only been able to spot mode a and mode c switches...my q is if you place it in TA/RA does that mean you are in mode s?

Cheers again :ok:

Port Strobe
24th Jun 2004, 23:45
For some technical info on TCAS this is a great page


As for mode C and mode S tranponders I believe, although I'm open to correction by those who know a hell of a lot more about this topic than I do, one of the fundamental differences between the two is that a unique code will be assigned to a particular aircraft and it will stay with that aircraft for its life, or until the next best thing after mode S arrives, so ATC can ID a particular a/c. Hope this is of some use.

CJ Driver
26th Jun 2004, 13:50
There are significant internal and technical differences between Mode S and Mode A/C. A simplified analysis is as follows:

In Mode A/C the interrogator (what you usually think of as the ground radar site) is "dumb" - it just sprays interrogations into the sky, and it can only ask two questions - A or C. The Mode A/C responder (the transponder in your aircraft) replies with a 12 bit data burst which is either a squawk code (A) or your altitude (C).

In Mode S, the interrogator is much smarter - it squirts out a 56 bit (or sometimes 112 bit) burst of data. As you can probably imagine, this means that there's a whole lot more "questions" that the interrogator can ask. The transponder replies with a 56 or 112 bit response depending on the question, so the two boxes can have a fair old conversation. As mentioned by another poster, 24 of the data bits in the interrogation are usually used to select a specific transponder ID, so that the "questions" are directed to a specific box.

In an Elementary Surveillance Mode S transponder (usually in a light aircraft with simpler avionics) the only real conversation is to ask about squawk code, altutude, and a few other simple parameters, much like an old Mode A/C box. The main benefit of Mode S is that the interrogator can poll a specific aircraft much less often, because it can establish a connection based on unique ID, therefore there is less interference and it is easier for the ground radar to discriminate closely spaced aircraft.

In an Enhanced Surveillance Mode S transponder (usually in a more sophisticated flight deck) the interrogator can ask other questions like heading, airspeed, and a bunch of other useful things that cut down on R/T traffic and improve situational awareness for the controller. Obviously the set of data that can be requested depend on what is technically available on the data bus in your cockpit, but the principle is very powerful.

There are other "data link" possibilities being considered which also use the payload capability of Mode S interrogations and responses, such as clearance uplink, weather and traffic information, and so on.

Which brings us on to TCAS. TCAS puts an interrogator into an aircraft, and it polls other transponders just like a ground radar site.

TCAS 1 simply polls nearby traffic, and builds a model of the other aircraft trajectories. If it identifies a collision risk, it says "Traffic, Traffic" and lets the pilot figure out what to do next. Although most TCAS interrogators tend to be Mode S interrogators, it is quite possible to have an aircraft with a TCAS 1 transmitter, but simple Mode A/C transponders. That is not the case with TCAS 2. In TCAS 2, the TCAS computer will not only poll for nearby traffic, it will also propose an escape manouever if it thinks a collision is likely. To guard against the possibility that the threat aircraft has an identical TCAS computer and is going to come up with the identical escape manouever, all TCAS 2 installations combine the Mode S interrogator with a Mode S transponder. If both aircraft have TCAS 2, they will also both have Mode S transponders, and they use the data link capability of the Mode S interrogator/transponder link to agree that they will each propose a different escape manouever - such as one aircraft climbs while the other descends.

So, to answer the earlier question - if you've got TCAS 2, then you also have a Mode S transponder.

Finally, since it came up on an earlier thread, I should point out that there are backwards compatibility features built into Mode S interrogators and Mode S transponders. Although the Mode S system is quite different to the old Mode A/C system, both systems use the same transmit/receive frequencies. Mode S interrogators can also send out Mode A/C requests, and can process Mode A/C replies, so they intersperse Mode S transactions with Mode A/C transactions and can therefore track older aircraft. Similarly, a Mode S transponder can reply to old-style Mode A/C interrogations. As you might expect, there are some clever tricks so that when a Mode S interrogator and a Mode S transponder have recognised each other's capabilites, the transponder stops replying to the Mode A/C interrogations, otherwise some of the advantages of the Mode S system would be defeated.

29th Jun 2004, 13:57
CJ Driver

That is a great explanation of the system. Thanks

My observation of TCAS II returns is that the WX radar seems to fix the position of the traffic better than the TCAS return. The TCAS traffic display moves much more laterally than the WX return on traffic.

Is that a fair comment?

Captain Cautious
29th Jun 2004, 23:27
C J Driver,

You obviously know your stuff on this subject.

To broaden the question a little further if I may...

We are increasingly asked at major european airports - CDG, ZRH etc. to select mode S on the ground, for the benefit of ATC ground movement surveillance radar. This is a variation to our normal SOP of selecting SBY after landing, and selecting out of SBY normally to TA/RA when lining up for departure. As I understand it our equipment transmits mode S when in any mode except SBY.

The question is that if we were to change our SOP to leave the thing out of SBY when manoeuvering on the ground at all airports, would this still cause problems ( clutter on the radar), for other ATC units not equipped with such modern equipment?

CJ Driver
1st Jul 2004, 17:46

I am surprised that you are able to reliably identify other aircraft with your weather radar, but certainly it is true that if you have on-board radar that can paint another aircraft it will be more accurate in relative bearing than the TCAS receiver. TCAS depends on receiving the reply on a multipole antenna and then doing some maths to figure out an approximate bearing. The TCAS antennae are generally on the fuselage and at certain angles the local path distortion from the airframe will make the bearing "off" by a good chunk. That's why TCAS RA's are vertical modes - the accuracy of deciding a turn direction is not good enough.

Captain Cautious:

Mode S has a specified ground mode behaviour - briefly, when on the ground a Mode S transponder ignores Mode A/C interrogations and Mode S All-call interrogations, but still emits acquisition squitters and replies to directed interrogations. Part of the Mode S datalink protocol is to do with on-ground or in-air status. It is an important part of the ground safety picture and a useful defence against runway incursions. Providing your installation knows when it is on the ground or in flight (see later), it will not cause any clutter or other interference. The airports who have spent gazillions of euro installing modern ground radar installations are right to chide you for switching the thing off as you taxi clear of the runway!

Unfortunately there have been different styles of aircraft installations, and different manufacturers implementations, so SOP's have not always caught up with the equipment.

Some installations, like one of the aircraft I fly, have a squat switch input to the tranponders. Our SOP is therefore to just leave the thing switched on all the time. On the ground it is in ground mode (and has a little annunciation that says so) and at wheels up, it goes into airborne mode. On landing it goes back into ground mode. No pilot intervention required!

I have also seen installations where the transponder has a specific GND setting, between SBY and ON. That's for people who don't have the squat switch wiring, so the pilot has to tell it whether you are on the ground or in the air. I suspect that has proven to be too challenging for some pilots, because there have been further proposals in the industry to add features like (1) always respond to ground acquisition and directed messages, even when switched to SBY and (2) if the airspeed is greater than 90 knots go into airborne mode, even if the switch is set to GND, and (3) always go to GND when the ground speed is less than 10 knots, even when the switch is ON. Some or all of these features may have been implemented in some boxes, but off-hand I'm not sure what the current state of play is with the particular vendors.

Since ATC have complained, it sounds like your installation does not respond to ground interrogations when the switch is in the SBY position. If there is no obvious ON GND / IN AIR switch on the control panel, it is likely that your transponder has a squat switch input, and should switch modes automatically. In which case - just leave it on whilst taxying.

1st Jul 2004, 21:03
The problem, as more Mode S Interrogators get installed around airports, is weeding out the aircraft with faulty squat switches or problems with the installation. For example, many still respond to the All Call on the ground. Are there any ramp checks that should normally pick up this kind of thing on Mode S? (It's quite rare to find an aircraft still responding to A/C on the ground.) If so how often are they normally performed?

A number of aircraft leave the thing left on when they get to the gate and sit there responding to Roll Call for ages. Is it normally in the SOP to do so and they've just forgotten?


2nd Jul 2004, 12:29
Our main base & company had been involved in the BETA (http://www2.dlr.de/beta/) programme. Prior to that, SOPs and practice was to (de)select STBY when entering / exiting RWY. Then we were asked to keep'em on for taxi. Given the fact (single hub ops) that 50% of our movements were in this environment, it found its way to SOPs. Of course, only after was confirmed that the installations are correct and modes A,C would not misbehave on ground, also this makes sure we do not fuss at other airports.

I turn the anticollision light on before pushback and select TA/RA on the squawkbox in sequence. Similar applies for shutdown. Unless one gets distrupted enough to omit, but then that's what checklist are for. :ok:

Note: the boxes have neither "OFF" pos, nor "S". Our installations come with "TEST, STBY, ON, ALT RPTG OFF, TA only, TA/RA" along a dial selector. Given the pinpoint description provided above, it is pretty much evident what they are for without further explanation. Air/ground logic provides for proper behaviour on ground.


4th Jul 2004, 07:50
thanks all for your replies.....i didnt know how little i actually knew about this topic until i read all your posts....keep'em comin'


Shore Guy
5th Jul 2004, 14:59
There is also in progress at some airports a procedure to use transponders not just to respond/ident to radar interrogations on the ground, but are used in a “multilateration” system to provide a surface movement system. (KSDF – Louisville, Ky., for example). This surveillance system uses triangulation from multiple sites to pinpoint aircraft and/or vehicle position for a surface map. The KSDF system also uses transmitted ADS-B data from aircraft and ground vehicles.

I have read, but cannot confirm, that the AMASS system requires use of transponders, but cannot confirm this (CJ Driver – do you know anything of this?).

Great, informative thread.

25th Jul 2004, 14:13
Hello all (CJ Driver inn particular!)

A question please. Most aircraft manufacturers have as a standard procedure that the TCAS should be changed to "TA only" when you have an engine failure (even 4-engined aircraft).

I have a problem with that for a number of reasons. The type I fly is an A340, and this aircraft does not have a performance problem that would limit the climb in the event of a "climb" RA (50% chance of a descent command anyway!).

In addition, the flight control laws prevent the aircraft from stalling.

Airbus insists that the "other aircraft" will manoeuvre based on your TA signal, ans the S band signal tells the other aircraft you are on TA only. My argument is that in Africa, perhaps 10% of the aircraf have TCAS, thus limiting this argument.

Am I missing something in being uncomfortable setting up TA ONLY in a tricky environment?


25th Jul 2004, 15:48
And one more question for you CJ, coming from someone who really should know the answer but I can't find it anywhere in my manuals. Does TCAS2 know the performance limits of the aeroplane it is fitted to or is it possible (for example) to get a climb RA that would take you above max certified altitude?


MikeMal, I think I can answer your question for you. If you are happy that you can follow a CLIMB RA in whatever engine configuration your aircraft is in, then there is a case for keeping RA selected. The big danger of course is that (as you do not know the required vert spd until the RA is commanded) if you are single engine (on a twin jet) and you are given an RA that you can't comply with because of reduced performance then the system falls down and a collision is more, rather than less, likely. Similarly, in a fully serviceable aircraft in a very busy TMA for eg (and particularly on parallel Rwy operations) it might be necessary (and at some airports I believe it is compulsory) to select TA only to avoid "nuisance" RAs. Hope this helps.

25th Jul 2004, 22:44
Normally the TCAS climb will only be a few hundred feet, however it can be much more than that. (recently my Co had a TCAS climb over 1000ft to avoid traffic and ended up having to make a turn as the other traffic decided not to respond to their command)

You may be on a one engine out cruize, close to your performance limit. To assume that you will always have the performance to 'outclimb' the other aircraft may not be correct.

I would assume on one engine out, I would have a 100%chance of being asked to do the TCAS climb...Murphy's..

I find it reasonable to let the other aircraft do the manuovering should it be required. By turning the TCAS to TA only lets the other acft know that if a climb/decent is required, then they will do that.


26th Jul 2004, 01:48
Fair point. I should add that our Boeing QRH does state that for an engine failure you do select TA only. As you dont know the rate of climb that may be required for an RA it does seem a fair call.

26th Jul 2004, 07:27
Hi BananasB

Thanks for your answer. Of course you are correct about the performance constraints. This is understood, and the hypothesis has ben tested in our simulator. Looking at the maximum deviation commanded by the TCAS II, it looks like we can comply with an engine out, even at 2000' above max recommended altitude with one engine inop.

One other reason Airbus gives for their rationale is "fleet commonality", and we take this to mean the mix of Fly-by-wire and non- FBW aircraft. This does not affect us, flying just the 340.

To answer your question, we have been battling to try and ascertain whether performance is taken into consideration by the TCAS, and have no conclusive evidence from either the manufacturer not the TCAS vendor.

However, Airbus, in one of their conferences, confirmed that the TCAS can command a climb through max alt without any repercussions (FBW aircraft).

Hope this helps.


Hi Ibthesius

One problem we have with the rationale to let the other aircraft do the manoeuvre is that 90% of our conflicting traffic will not be fitted with TCAS, so they will not get out of your way.

This leaves two alternatives - one, try to visually acquire the traffic (IMC?) - not recommended by Airbus, or get flown into by the other aircraft.

IN addition, in the Quick Reference Checklist (QRH), Airbus states "Do not manoeuvre based on a TA alone". This has severe repercussions, asn any manoeuvre based on a RA would be in contravention of AIrbus emergency procedures.


26th Jul 2004, 13:11
Hi mikemal

Quick Reference Checklist (QRH), Airbus states "Do not manoeuvre based on a TA alone". We have similar wording in the Boeing manuals.

I would say that if you see the other traffic, and manoeuvring is required, then this is not on TA alone. If you can find space on the radio, (not always easy) you 'should' have 40 secs to enquire with ATC, ask for a hdg or what ever you deem to be neccessary.

From Memory, somewhere in my Boeing training, I was told performance is not taken into account for the TCAS command. That doesn't of course mean what was told is the actual truth.

Engine failures are rare events. RAs are rare events. I guess murphy says you can have a really bad day and combine them on one day. I wonder however if this is part of Boeing/Airbus risk assessment and management. Regulatory authorities do not consider compounding failures in some other areas.

What happens to the TCAS commands (other acft) if you decide during a manoeuver to switch from TARA to TA as you have run out of performance?

Still plan to follow the QRH


26th Jul 2004, 19:34

Valid observations, thanks for sharing them with me.

BTW, we have added a one-liner to the TCAS training, related to the engine out situation. "Pilot discretion" instead of a concrete order to go to TA only. This will, hopefully, add a bit of flexibility to allow us to operate into hi-density non- TCAS airports with a little more confidence.