TCAS and SSR

[twistedenginestarter]

Your radar is sweeping at let's say 1 revolution per second. As it comes around towards (say one twentieth of a second from)aircraft A, nearby aircaft B interrogates A with TCAS. A makes a return to B but also to you. Then your radar hits A and it makes a second return. How does your radar distinguish between the two returns 18 degrees apart from aircraft A?

[All Systems Go]

The message formats of TCAS and SSR will be totally different, and the reply will be different. By totally different I mean there will be a firld set somewhere within the message defining what type of message it is. Be it an SSR interogation by Radar head or A.N. Other. I'm not 100% sure about this, but that's how it would have to work. Also the SSR will be expecting a response within a pre-set amount of time - the time foir max. range of the radar multiplied by 2 plus a bit added for processing. Chances are the time for A/C Bs response would be outside this. And i'm also assuming there would be some sort of addressing going on, at a very basic level. I.e the SSR head would have an ID, the 2 A/C would have an ID which is independant of SSR code. Don't quite me but that's roughly how I understand the 2 work.

[HEATHROW DIRECTOR]

No ATC radar that I am famialir with has a 1 second sweep rate. The radars we use rotate at about 12-15 RPM.

[All Systems Go]

That all depends on the type of Radar being used HD. The Ground Movements radar used at airports will have a higher rpm than say a big 23cm HSA used for Area type applications, as well as different PRI (Pulse repition interval) and lots of other bits and bobs which help provide the desired resolution for the desired application. Standard 23cm area Radars, as you rightly say, rotate at between 8 and 15 rpm.

[HEATHROW DIRECTOR]

Maybe I should have said "no SSR radar I am familiar with has, etc, etc". I know about GM radars, having used them at Heathrow but I believe our corespondent was more concerned with a/c in flight and there sure aren't any surveillance radars turning at 60 RPM that I know of.

[twistedenginestarter]

HD

The speed of rotation of the radar I think is completely irrelevant. I was just giving a hypothetical figure to illustrate the argument.

The point is there is a zone where the trigger from ATC radar could be preceded or followed by a TCAS trigger so that the radar could not be sure geometrically which was the radar return.

All Systems says the transponder sends a different message to ATC compared to TCAS which of course would work but I thought TCAS would work of any old C150 with a Mode C transponder. Surely these would not have been designed with TCAS in mind?

Perhaps my real question is what is the format of a transponder message - does it receive a sender id and return that along with squawk and altitude? (Maybe it needed this to cope with pre-TCAS era conflicts between overlapping radars.)

[Scott Voigt]

HD;

Although not in ref to the TCAS issue, but to your statement about radars. We do have some radars here called PRM ( precision runway monitor ) which are phased array and we get one second updated of aircraft on final with it...

regards

[Iron City]

A secondary surveillance radar can be either the good old ATC Beacon Interrogator or the Mode S (for Select). TCAS is TCAS

If a ground sensor interrogates a Mode S equipped aircraft on the first time it either has the target and track information
handed off from another sensor or uses an all call. Subsequent interrogation responses are campared with the track data and single anomolous replies rejected. If the ground sensor is a ATCBI interrogator and the aircraft has the old ATCBI aboard the interrogator will receive the transponder transmissions caused by the TCAS interrogation as well as the ground sensor interrogation if the ground station antenna is pointed close enough to the aircraft replying to the TCAS interrogation. I don't recall what the beam widths and stuff for the old ATCBI-5 or 6 are. The Mode S . Through time (range) gating features in the sensor it will reject returns that are not from it's interrogations mostly. It will then provide the target information to the ATC facility which will then correlate the information with other track information. If the ground sensor is a Mode S sensor this correlation willl be done at the sensor site.

Mode S interrogations are either 56 bit or 112 bit and though I do not have the specifications for TCAS I believe they use a very similar data format with the same, or many of the same fields.

By the way, the Mode S specification describes performance using a 12.5 RPM scan rate.

Hope this helps

The question, of course has nothing to do with Mode S (selective) beacon interrogation or TCAS, which are essentially data radios with data links sending information on the aircraft and interrogation back and forth.

[twistedenginestarter]

Iron City

You seem to be saying that the ground radar receives both SSR responses - one for the radar and one for the TCAS. The ground system then 'resolves' the dual returns by computing which one is the most consistent with the previous accepted return with the same squawk.

If this is so, it begs the question how the TCAS distinguishes the two returns. Presumably it carries out a similar guess computation although it is going to be more tricky as the TCAS is closer to the transponder and thus must cope with more rapid changes of angle and distance/time delay?

Note I am assuming TCAS simply interrogates transponders ie the only thing special is the TCAS receiver. There is no TCAS-to-TCAS dialogue.

[All Systems Go]

All:

I have it on good authority that we are all wrong in our own way. TCAS uses Mode S to interrogate another TCAS equiped A/C. Standard SSR radars, certainly in the UK only use Mode A (SSR code) and Mode C (Flight level). How does one TCAS distinguish from a ground-based Mode S interogation? Simple. Mode S has a huge amount of addressing available to it. Soneone said that there are enough different codes available for each A/C in the world to have its own unique Mode S address. So a pseudo-session is set up between TCASs all the time an RA is in progress where the 2 TCASs have a data link so that they are dealing with each other. Obviously your never going to get an RA between a rotating SSR head and a plane, unless the unthinkable happens. So the answer is (I'm not sure of the actual application of this - this is just a guess) each Mode S TCAS on an A/C has the ability to have an address. Seeing as Mode S and standard SSR type interogations from the ground are also achieved in slighlty different ways (to do with pulse spacing and pulse width) then there could be little room for a cross over.

Of course there are in the world some SSR radars that use Mode S, and again this will be ab addressing issue, as well as using something called Dopler Shift from the Radar pulses to determine of the track its picked up is moving (a static radar head will have no Dopler shift, whereas a moving Jumbo will), then it could differentiate between a static radar head type interrogation, and one from a moving A/C.

Hope that made sense. If not then I'll try and explain it again in English this time.

[twistedenginestarter]

All Systems

Many thanks for that. From what you are saying you can never pick up a TCAS alert from a passing C150 since it will have Mode C at best.

Just one minor point - doppler will not help as both radar and TCAS transmissions are coming from the same plane and the same antenna so will have the same doppler shift.

[All Systems Go]

Eye:

What I meant was an SSR Mode C/A interogation coming from the ground, from a stationary head, will have the same amount of dopler as the speed of the plane, whereas one from another plane will have a different dopler. I see your point and I agree about the plane and the TCAS and Mode C/A interogation however.

I'd be interested to know any more as and if you find it out. What I posted here was a little what I was told and a little that I figured out. I'm more than willing to be proved either wrong or right.

[Capt Pit Bull]

SSR and TCAS data formats.

A brief (non technical, approximate) description.

It IS possible for TCAS and SSR to garble one another - at the end of the day they are using the same freqs, so sooner or later two signals are bound to be in the same part of the sky at the same time.

Hence: TCAS / Mode S uses several techniques to try and minimise the ammount and power of the transmissions that are needed. These techniques are a key design constraint on the way the system works, lest the ATC SSR system grinds to a halt.

Mode A 'All Call'. Signal prompts transponder to reply with its 4096 code.

Mode C 'All Call'. Signal prompts transponder to reply with its encoded altitude data. If there is no altitude data, transponder still replies, but with just the 'frame' of the signal format, containing no data. Note that what people tend to call 'Mode A' transponders are actually 'A and C' without altitude encoding. I.E. its a misnomer, albeit very common.

ATC SSR Function: Alternating Mode A 'All Call' and Mode C 'All Call' gets 4096 code and altitude data.


Mode S. Loads of different 'uplink' and 'downlink' formats, many of which are not yet implemented.

Mode S 'Announcement'. (thats not its proper name, which I forget, but conveys what it does). The transponder radiates a signal announcing its presence, to other mode S capable radar equipment in the area, about once a second.

This signal contains a unique identifying 'Mode S Address'. This address is hardwired into the aircraft, and unique to the aircraft. I.E. no other aircraft, anywhere in the world, will have the same address.

Note that this announcement is made unprompted, i.e. the transponder does not wait to be interrogated, it just sends the signal anyway.

TCAS listens out for these announcement signals, and builds what is called a "Roll Call" of Mode S transponders in the area.


Mode S 'Selective Address'. (after all, the S does stand for Selective!). This is an interrogation format that includes a mode S address. It prompts that specific aircraft, and no other, to reply. Note also that Mode S doesn't reply to Mode A or Mode C 'All Calls'.

(IIRC) Mode S 'All Call'. I'm guessing now, because I studied mode S from an airborne TCAS point of view, not ATC, but I would guess that ATC radars send mode S all call signals. But don't quote me.



TCAS Function:

TCAS works its way through the Roll Call, sending Selective Interrogations to those aircraft. This gives it a picture of all Mode S aircraft in the vicinity. As aircraft move out of range, their announcement signal eventually gets weak enough to be ignored so they are dropped from the roll call.

It then does a Mode C "All call" sweep, to catch non mode S aircraft. Note, it only cares about altitude, so its sends Mode C interrogations only - However, remember the note above - non altitude encoding aircraft still reply to Mode C calls, just with an empty frame, so the existence and range to the target can still be derived.


Other Mode S / TCAS tricks for interference reduction.

Whisper / Shout.

TCAS sends its mode S interrogations at variable power levels. In a nutshell, it 'Whispers' at low power to the target, if no reply is received it transmits progressively more power, eventually 'Shouting'. Its uses a successful transmission as a benchmark for its next attempt, and progressively adjusts power to the minimum required to maintain contact.


TCAS interference Limiting Mode.

Lengthy topic, more pilot oriented, will post if anyone interested (Its already written for an email query I received, so no trouble).




CPB

[twistedenginestarter]

Capt Bull

Excellent post - many thanks.

[Duke of Burgundy]

Capt Pit Bull - I think the word you were looking for is "squitter".

[RaRadar]

CPB - Good Post, just to clarify a couple of points.

At present TCAS and Mode S ground stations operate completely independently. A ground station would see TCAS squitter or air to air surveillance as interference and visa versa.

Mode S ground stations do not use the squitter data but transmit an All Call interrogation to find out who's out there which will also include its interrogator identity code. This then gets included in the transponder reply. All Mode S interrogations and replies are parity encoded so that the data is only used if it contains the expected aircraft address or interrogator code.

The way that Mode A/C systems reject interference is to interrogate a number of times and only believe the reply if it gets the same information more than once (There's also some clever decoders that correlate replies on amplitude and angle of arrival.)

At the moment the only link beween TCAS and ground based Mode S is that the transponder will generate a message to the ground station that contains the RA information if one is generated. This is of limited use to co-ordinate actions, it wouldn't arrive in time, but could be used to clearly identify the aircraft concerned on the display. At present there's not many operational Mode S stations outside the US and even these don't use this feature. However, most of Europe will be updating to Mode S in the next couple of years.

Lots more proposed for Mode S in the future, see the Eurocontrol web-site for info on 'Enhanced Suveillance' and ADS-B using 1090 extended squitter.

[Capt Pit Bull]

RaRadar / Duke

Thanks for the info.

Now that you've reminded me about 'squitter' I remember why I forgot!

It sound horribly biological to my (admittedly) rather juvenile sense of humour...
;)

Was my recollection that Mode S transponders ignore Mode A or C All Call correct? If so, how do pre-mode S ground stations see Mode S boxes?

CPB

[rickity]

Given that TCAS will issue an RA (resolution advisary?) because of a dialogue between 2 or more aircraft TCAS's in close proximity i.e. it can tell one aircraft pilot to climb and another pilot to descend, what does the pilot do if instructed differently by ATC ( who can't see or hear an RA) is it mandated that the pilot should follow the TCAS?

Roll on ADS B and handling Mode S data on the ground at least more than just the 4096 code and height.

[RaRadar]

CPB

There are three different flavours of Mode S All Call:
1. Mode S Only All Call - Mode S transponders reply, A/C do not (normally use this one)
2. Mode A/C Only All Call - Mode S transponders do not reply, A/C reply with normal A/C response. (Can use this mixed with 1)
3. Mode A/C/S All Call - Mode S transponders reply with All Call, A/C reply with normal A/C response (not currently proposing to use this one)

For now Mode S stations will probably use a mix of 1,2 and normal A/C interrogations.

The Mode S transponder is fully backwards compatible with Mode A/C and will look like any other A/C transponder to a non-Mode S ground station.

RR