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Date: 20 Nov 96 05:48:29
From: Ed Hahn <>
Subject: TCAS and Transponders [long] (was Re: India:747/Tu154 collision +TCAS) (Scott Odle) writes:

> >For the TCAS-III (in project) : The TCAS can generate manoeuvering in both
> >vertical and horizontal plan.
>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

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

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 Hahn | | (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.

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