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