Tcas
 

On the Swiss/French UIR Border this morning I heard an a/c at around FL330 report a same level TCAS at 5 miles, the reply was we have nothing on radar, must be military, followed bt a long and fragmented exchange re filing an "Airprox", during which ATC offered to obtain data from other radar sites, the decision was not to file since separation was 5 miles, fine. Imagine my surprise when in the security of Maastricht airspace later in the day when an a/c reported trafic at the same level at 5 miles and the reply was...Nothing on radar it must be military!!

If these returns are genuine military a/c then this is not the right time for un co-ordinated traffic and if they are not then

"BEAM ME SCOTTY"

'Stealth' military one presumes.

That's pretty unnerving. As a controller, I'd be a little concerned if things were flying around my airspace that I couldn't see.

Best keep your eyes peeled up there:eek: !

A couple of years ago I was flying an A320 in french airspace and requested a 2000 ft step climb, which was granted. On this particular airbus TCAS returns only showed in 40nm range. Just before starting the climb I reduced map range to 40 miles to find two contacts at my cleared level just 3 miles ahead. On querying the controller, I was told that they must be military traffic and was not under his control!

Though MIL may not have TCAS aboard, especially TACAIR, boggles the mind there is no Mode S or Mode C.


Then again I boggle easily

stealth aircraft? I dunno... I guy i know whos in the US military told me that stealth aircraft have to have a radar reflective shield, so that they can be spotted by Radar during Peace time operation. I assume this also includes training exercises.

I dont know how true it is, but it makes sense.

The nothing on radar line sounds like a crock.

With no transponder the TCAS can't see the other aircraft. Ergo if the TCAS can see the intruder so can ground based radar......

Must be UFO's!

Just seen Men in Black II and it had aliens - and if it is on filum it MUST be true!

Unless the traffic is operating a Mode C transponder you will not get an altitude read out. If it's Mode C ATC would see it too.

Well, strange things do happen.. I can recall several instances of aircraft refusing to take my headings "due to traffic dead ahead", for example, with absolutely nothing showing on radar. Bizarre, but it really does happen. Equally, we sometimes see returns on our radar which certainly look like a/c... and against which we have taken avoiding action only to find that there was nothing there!

can't vouch for europe, but stealth, and all other military are APPARENTLY required to be squawking in commercial airspace, and do so...ala the united incident in cal where ual followed a tcas ra due to f-117 fighter in proximity

As to the military aircraft near you but not within the 5nm. If you are flying in controlled airspace, military aircraft will route around that airspace whilst operating in the open FIR or their restricted airspace.

As far as military aircraft not squawking. In the 16 years that I was in the game, we would have a Mode 3A and C return (plus a bunch of others that you can't see). The squawk would either be assigned by ATC, Fighter Control or be a standard conspicuity code. In the UK we always used to have those nice VFR slots through airways but still normally took a RIS. Even when we were transiting the open FIR we would normally make use of a RIS until we got to our play area.

My guess would be that ATC are feeding you a line coz guess what even if the yanks were flying their new super-invisibility hyper-sonic space spy speeder plane through your airspace they would have a radar reflector attached so that all players could see a primary return. Think about it from their point of view, they don't want you to know what they know about what you want to know about what you would really see on the radar as it flies by.
:confused:

Ghost

;) I absolutely agree on this. I was "playing the game" on F16 for 12 years as well and I can only make the same comment with maybe a little extra:
One should know that in many circumstances, Tatical controllers do not have a full picture like most area/terminal controllers have. The reason is simple, in some areas (surrounded by airways, sometimes even below and above) the amount of traffic is too large and clutters the picture. Their responsability is mainly to check participants do not leave the working area whilst CIVILIAN controllers will monitor if their traffic is not converging towards an unwillingly exiting from the zone traffic. When this happens all phones become red in the tactical room and the military pilot is urged to manoeuvre sharply towards the working area. This working method only works if a positive radar contact is mainained==> mode 3/C always used. PS: I never flew without mode 3/C, even for some missions over Bosnia/Kosovo...

On top of this, military aircraft have an on board radar and as soon as a CIVILIAN traffic is called, the fighter pilot will both try to acquire it on radar and avoid at the same time, increasing the safety given by ground controllers.

Re : 'Stealth & UFO'

Daventry Sector - Yesterday. FL240 (level capped)

TCAS target left wingtip - 1nm, solid diamond, indicating '00'
Followed us for about 10mins. altered Heading & Level with us.
Suddenly disappeared, no warnings !!

Is this a 'ghost' generated by TCAS ? if so how ?
Capt. PB care to comment ?

Seen this phenomonen before.

TCAS is not infallible.

Very wise remark fmgc...

as to the "UFOs," have you never heard of ( e.g. Air defence ) 2 digit familly codes being filtered out civil ATC systems.?
France does it, I know of at least 2 other countries doing it....

Here is what could have happened:
 

As stated above, it is true, that normally, if you get something on TCAS, it should be on ATCs radar as well. I can think of two instances where it is not so.

1. The aircrafts transponder is partly broken and so weak that the signal cannot reach the ATC radar 100 miles away, but can reach the other aircraft a few miles ahead.

More probable in this case though:

2. A low altitude aircraft has an altitude encoding problem (this is very common - have you never been told in low altitude something like "your mode charley indicates FL145, please switch to mode alpha"). If there is a low altitude aircraft 5 miles away from you, and his transponder by mistake indicates same FL as you are flying in, TCAS will always see his direction in azimuth, which is independent of altitude, and his mode charley will be used to tell altitude.

There you go, an aircraft way below you triggers your TCAS, and ATC doesn't see him if he is below radar range. Two minutes after he is "gone", because his altitude encoding indicates something else, that is away from the TCAS range.

Also, some bored person might find a transponder with altitude encoder and set it to a certain flight level, just to see if he can trigger some other aircrafts TCAS. Why ? There are idiots "down there" who gives false clearences for whatever reason !

Therefore, any such ghost TCAS instance should be reported with accurate position, FL, time etc, and if it is the same place, I would send someone out to search for a ground based TCAS transponder !!!

Ground PRIMARY radar performance depends on age and condition of ground equipment and range of A/C and A/C radar reflectivity.

Ground SECONDARY radar performance depends on age and condition of ground equipment and range and strength of A/C transponder transmission.

Not all radar heads have both primary and/or secondary radar and some primary displays are fed secondary info by landline from a differently located secondary radar site. To my knowledge this happens alot at rural/regional French airports.

So it is possible for an aircraft to be outside the performance criteria for the primary radar and if ground has no secondary it would be "invisible" for all modes of transponders setting, if switched on. Or that ATC centre might be receiving a secondary feed from a site not "transponding" with the target A/C.

With 2 airborne A/C in relative close proximity they have an ideal "line of sight" transponder environment. If the primary "invisible" contact has a weak transponder strength it may not show on the ground secondary receiver approaching the limit of the ground performance envelope.

And then of course there's the military. Rumour (and I stress rumour) has it that they can vary the strength of their transponder so as not to be visible to ground radars for training and excersice but still be a good "TCAS" rule observer.

One thing that I dont know, and perhaps someone out there can supply, is is military IFF compatible with civil TCAS? Because IFF is not the same system, to my knowlegde, it's more complex and more secure.

Can anyone tell me, is the upgrade to TCAS 7.0 a software change, or a box change? And is there any way to identify one's TCAS as being 7.0 without hearing the (slightly) different verbiage on the warnings?

Thanks in advance.

ShoreGuy,

6.04->7.0 should be a software update. According to the following FAA document, it can even be done in the field:

http://www.faa.gov/AVR/AFS/FSAW/FSW0102A.DOC

The document above also shows the difference in callouts. Another way to tell software version is if you get a "traffic" TA between FL 310 and 420. Version 6.04 will give you one, 7.0 will not (RVSM).

TCAS I = TA only
TCAS II = TA/RA
TCAS III = TCAS II with azimuth advisories

Regarding TCAS III, it is not yet implemented. TCAS II only gives pitch advisory/resolutions, whereas TCAS III also can give azimuth (turning) advisories.

ADS-B and GPS-squitter are some of the terms used in the development of a system, where a GPS is added to the collosion avoidance system. This will allow especially smaller aircraft to "participate" in TCAS, as it is a lot cheaper, more precise and less space and weight consuming.

The major problem here is agreeing on a standard, because as usually there is a faster and cheaper way, and then there is a more expensive and slower path to follow, but the latter being more capable.

Read more about ADS-B, TCAS and GPS squitter implementation:

http://www.faa.gov/and/and300/datalink/navsur/gps.htm

http://www.faa.gov/and/and300/datalink/navsur/cdti1.htm

http://web.mit.edu/6.933/www/Fall200...ouilhet2a.html

http://web.mit.edu/6.933/www/Fall2000/mode-s/links.html

http://www.honeywelltcas.com/faq/whi...ointpaper.html

TCAS history/background: http://www.caasd.org/proj/tcas/

Most important,

I would still say we should take it very serious as to find out whether certain screwd individuals are setting up transponders on the ground and faking the mode charley to reply with FLXXX returns, so any aircraft crossing that FL will get a TA or RA - which ATC will not see.

Just a small correction.
For the FAA and the Mitre corporation ( designers of the software ) version 7.0 will be last one. This was announced
TCAS III was abandonned about 4 years ago and its replacement TCAS 2000 was also scrapped.

I stand corrected (I think it is called)

What then - will the GPS/TCAS implementation still continue ? It is still relevant to separate small<>small as well as small<>large ! A 727 hits a C172 on downwind, and both crash.

But the azimuth TCAS version (III), I didn't know it was abandoned - do you know the reason why ?

Do not Know much about the plans of the FAA regarding the GPS/TCAS baby. But I think that right now it is not on the top of their priority list.

TCAS III was abandonned for 2 reasons : first it proved extremely more difficult to resolve a conflict in the horizontal plane than vertical, both in reality and in software terms. mainly due to the very short time frame in which TCAS operates. .
Secondly current TCAS tracking in azimuth is very poor , and a new version would have necessitated very large changes and very large additional costs. Seen the current price tag of a new TCAS II version 7.0, the project was deemed not cost-efficient.
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I am currently reading the safety study made by the Mitre corporation in 1997 on version 7.0. Interesting to read that their positive risk ratio was derived asuming all pilots will follow all RAs all the time, and that the TCAS does worse when European [airspace] data is included in the model.. (as the system was initially designed for the US airspace model...

OK, thank you for the information. After what you said, I found this, written by Ed Hahn | [email protected] back in 20 Nov 96:


There will be no TCAS III. For whatever reason it will be designated TCAS IV.

The reason it is designated TCAS IV is that the method for determining the correct horizontal resolution manuever will be entirely different than initially envisioned in TCAS III.

One of the results of TCAS II experience has been that the directional antenna used by the TCAS processor to assign a bearing to a received transponder reply is not accurate enough to generate an accurate horizontal position, and thus a safe horizontal resolution.

TCAS IV will use additional position information encoded on an
air-to-air data link to generate the bearing information, so the
accuracy of the directional antenna will not be a factor.

In order to compare and contrast the two, the decision was made to refer to the new system as TCAS IV, to prevent confusion between technologies.

In summary:

TCAS I: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and "Traffic Advisory" (TA) for nearby targets.
This TA is used to help pilots visually locate nearby
co-altitude traffic (Mode C) or unknown altitude traffic
(Mode A).

TCAS II: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and a TA for nearby targets.

For target aircraft with Mode C or S transponders, the TCAS
display can generate a "Resolution Advisory" (RA), which
commanded vertical manuever (climb/descent) to avoid nearby
co-altitude traffic.

For target aircraft with Mode S transponders *AND* TCAS II
equipment, RAs will be coordinated between aircraft
(e.g. the two TCAS processors will cooperatively agree to
send one aircraft in a climb and the other in a descent.)

Note: aircraft equipped with TCAS II must have Mode S
transponders installed.

TCAS III: Attempts to use the TCAS directional antenna to assign a
bearing to other aircraft, and thus be able to generate a
horizontal manuever (e.g. turn left or right).

Judged by the industry to be unfeasible due to limitations
in the accuracy of the TCAS directional antennas. The
directional antennas were judged not to be accurate enough
to generate an accurate horizontal-plane position, and thus
an accurate horizontal resolution.

TCAS IV: Uses additional information encoded by the target aircraft
in the transponder reply (i.e. target encodes it's own position
into the transponder signal) to generate a horizontal
resolution to an RA.

Obviously, this requires the target aircraft to have
some data link capability at a minimum.

In addition, some reliable source of position (e.g. GPS) is
needed on the target aircraft in order for it to be
encoded.

Mode A: A transponder which can encode a number into the reply
signal. This code is a four digit octal number XXXX, with
each digit having the value 0-7. The famous "1200" VFR
transponder code is an example of a Mode A code.

Mode C: A transponder which can encode its altitude into the reply
signal. This code is known as the "Grey Code", and it
encodes 100 ft. increments into 12 bits. Note that Mode C
transponders can also encode Mode A, and that ground
radar typically alternates which information it asks for on
successive sweeps.

Mode S: A transponder which can be selectively interrogated (hence
Mode S = Select), which can also encode additional
information into the data stream. This transponder
essentially gives a basic data link capability, which in TCAS
II is used to coordinate RA manuevers.

TCAS IV could use Mode S data link capability to encode
position information into TCAS replies.

TCAS IV development is still underway, but it is not likely to be
fielded in the next year or so, as there are still technical and
institutional issues to resolve.

Also, new trends in data link such as Automatic Dependent Surveillance - Broadcast (ADS-B) have popped up recently, and have pointed out a need to re-evaluate whether a data link system dedicated to collision avoidance such as TCAS IV should be incorporated into a more generic system of air-to-air data link for additional applications. These issues are being worked by the government and industry in groups such as RTCA.

Note that I am writing this posting to provide information only. This post does not intend to endorse the merit of any particular solution for collision avoidance or other application. Any errors in the above are mine.

Hope this helps,
ed

-------- Ed Hahn | [email protected] | (703) 883-5988 --------
The above comment reflects the opinions of the author, and does not constitute endorsement or implied warranty by the MITRE Corporation.

Really, I wouldn't kid you about a thing like this.

Moleslayer, just out of interest, did you mention the TCAS target to ATC to see if they had anything on radar?

On the German midair thread, there is mention that one of the reasons behind the procedural reason for obeying an RA vs. a controller command is the slow refresh rate of enroute radars. What is considered the “refresh rate” (how often to the transponders interrogate each other) of TCAS?

No
 

Shore Guy:

No.

The reason for following the RA instead of the controller is that an RA is an instruction issued after an agreement or "contract" has been reached between the two aircraft's TCAS's. One has agreed to descend and the other has agreed to climb and each will instruct its pilots accordingly.

If the a/c that has been told to climb decides to descend it will descend into the airspace that the a/c that has been instructed to descend into is descending into. Hence the recent collision.

It should be noted that TCAS is the last resort.

With respect to some early comments on this post about military traffic. I can assure you that on a daily basis we are witness to and have reported many situations of unknown traffic conflictions, whether they have their transponders on or not, which on some occassions they do not!! This has been confirmed by visual sightings and when the transponders are on, TCAS information. With the extensive military ops still occurring in Afghanistan and probing activity to the south towards Somalia etc., there is a lot of conflictions and potential conflictions. Add to this the fact that 70% of this traffic (majority US military) is 1) not talking to any appropriate controlling agency, 2) are flying across very busy domestic and oceanic air routes and 3) at non-conformal levels, the receipe for disaster is there. For example, the route from the operational area (for entry into to Pakistan/Afganistan) to Diego Garcia crosses all the Indian Oceanic Airways to the west at almost right angles and with no liason with either Middle Eastern or Indian agencies!

So the moral to the story is don't just look at the squizzy kit which relies on transponders being switched on

Heads up, eyes out, day or night, save that fright!

:o

YNSL,

Well, I know where you are coming from, but its not quite as cut and dried as that, since not all RAs are coordinated.

The radar update rate versus TCAS update rate is an important issue, albeit not the major one, that goes into the pot when deriving TCAS strategy.

Shore Guy.

An oft quoted figure for TCAS is once per second.

CPB