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TCAS is obviously a fantastic piece of kit, but I was wondering: exactly how does it work?

What I mean is, how does it determine whether to tell a pilot to climb or descend when there's traffic?

Also, can you give an overview of the various types of TCAS, and how planes agree a resolution?

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TCAS works by sending interrogations to other aircraft's transponders. The transponder will reply to the interrogation in a similar way it responds to radar. From the time difference between the interrogation and the reply, the distance to the other aircraft is calculated. The reply itself contains the altitude of the other aircraft.

The explanation below gives an insight in the basic working of the TCAS algorithm. In reality it is more complex to deal with all kinds of exceptional situations.

The distance and the altitude difference with the other aircraft is tracked to identify a trend. From successive distance measurements the closure rate is determined. With the closure rate and the current distance (slant range) an approximation is made of the time to the closest point of approach (CPA). This is done by simply dividing the range by the closure rate, the result is called 'range tau'. The same is done in the vertical plane. Dividing the difference in altitude by the vertical speed difference leads to the vertical tau.

If both tau's are less than certain threshold, a Traffic Alert (TA) is raised. When the tau's are less that another (lower) threshold, a Resolution Advisory is given. The TA is a 'heads up' indication, the RA is an instruction that must be followed by the pilot to reduce the collision risk. The threshold times depend on the altitude, ranging from 20 seconds (<1000 ft AGL) to 48 seconds (> FL200) for TA's and from 15 seconds (<2350 ft) to 35 seconds (> FL200) for RA's. Below 1000 ft AGL, RA's are inhibited.

Originally TCAS did only give Traffic Advisories in the form of a "Traffic, traffic" annunciation. There was no avoiding action indicated.

With the introduction of TCAS II in the second half of the '80s the Resolution Advisory made its entrance.

RA selection

In selecting a resolution advisory, there are two basically two steps. The first step is to select a sense, either upward or downward. This is based on a calculation of how much altitude difference can be achieved at the CPA by either starting a climb or descent, assuming the target will maintain its vertical rate. In the calculation the reaction time of the crew and acceleration and climb / descent rate of the aircraft is assumed (5 seconds, 0.25g, 1500 fpm).

The second step is to select a magnitude. The algorithm is designed to be the least disruptive to the flight path, whilst still achieving a minimum vertical separation.

RA coordination

When a RA is selected, it is transmitted to the other aircraft. When the other aircraft receives that message, it will only use the opposite sense for its own RA. In the rare case that both aircraft transmit their RA intent at the same time, the aircraft with the higher Mode S address will give in and reverse its RA sense if it is conflicting with the other.

Algorithm versions

Over the years various improvements are made to TCAS II. The initial TCAS II systems had version 6.0 of the algorithm, later replaced by 6.04a in the mid 90's. Further system evaluation lead to the development of version 7.0, implemented from 1999. Following a near mid-air collision in Japan in 2001 and a mid-air collision over Uberlingen, Germany further changes were made leading to version 7.1.

Changes that were made include:

  • the wording of the RA's
  • logic to limit the vertical path deviations
  • logic to prevent nuisance alerts in RVSM airspace and on closely spaced parallel runway operations
  • logic to detect non-compliance to an RA
  • improved RA sense reversal logic

TCAS & ATC

TCAS RA's take precedence of ATC instructions. The fact that one of the two aircraft followed ATC instructions, and the other TCAS RA's contributed to the Uberlingen accident. In case a pilot is getting a TCAS RA he should notify ATC.

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The FAA has produced an Intro into TCAS II v7.1 which provided some interesting reading on the topic.

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    $\begingroup$ The RA is not negotiated. The two system carry out the same calculation from the data they both know, which basically means just the altitude of both aircraft which are broadcast. Remember, it needs to work with mode C transponders that are rather dumb. $\endgroup$ – Jan Hudec Feb 23 '14 at 21:46
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    $\begingroup$ @JanHudec As of TCAS II, they do indeed negotiate a solution (if they are both properly equipped) that prevents both aircraft from coming up with the same solution and making the problem worse. $\endgroup$ – Lnafziger Feb 23 '14 at 21:57
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    $\begingroup$ @DannyBeckett simply put climb is called when this plane is higher than the other plane and vice versa. Also I believe the TCAS RA takes priority over traffic control (as part of See-And-Avoid) $\endgroup$ – ratchet freak Feb 24 '14 at 12:00
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    $\begingroup$ @ratchetfreak When responding to an RA that has priority over ATC (you are correct) and you'll make a call "Approach, N1234 TCAS Climb" to inform them you are complying with an RA. $\endgroup$ – casey Feb 24 '14 at 15:01
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    $\begingroup$ @ratchetfreak It is not simply which aircraft is higher. The algorithm will calculate if that would work, and if it does it is certainly the preferred solution. But if the aircraft above is descending, it might be instructed to descend faster if reversing to a climb would take too long. $\endgroup$ – DeltaLima Feb 24 '14 at 23:08
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The TCAS standard uses the standard transponder, having the interrogator on board in addition to the transponder itself.

The aircraft periodically broadcasts the interrogation signal. The power is less than from the ground radar, but sufficient to trigger response from aircraft that may be less than a minute of flight.

The distance to the other aircraft is determined from the timing the signal, closure rate from two consecutive measurements of distance and pressure altitude is part of the transponder response. Position can only be determined approximately by using directional antenna, so it is only used for showing the other aircraft on the nav display, but not for determining the resolution.

The system activates if the closure rate is such that collision could possibly happen in some specific time. That does not mean that the aircrafts are actually on colliding courses; the TCAS system does not evaluate headings or positions. It is just that the momentary closure rate is high enough.

If the aircraft could collide in 40 seconds, the system gives out a "Traffic Advisory". This is just a warning that there is a nearby traffic and that an action may need to be taken soon.

If the distance reduces so that the risk of collision gets to 25 seconds, a "Resolution Advisory" will be given. This is an instruction to climb, descend, climb/descend faster or level out. The pilot is required to comply in 6 seconds, the instruction has priority over ATC instructions.

Which advisory to give is derived by a set of rules from altitude and rate of climb of this and the other aircraft. Since the altitude is sent in the transponder response and rate of climb is simply difference of consecutive altitude measurements, both TCAS have the same information and therefore will always come to the same conclusion. Basically it will tell whichever aircraft is higher to climb, climb faster, or if descending to not descend and whichever is lower to descend, descend faster or if climbing to not climb.

There is also similar system for light aircraft called FLARM. The principle is similar. It uses lower power radios, because gliders and ultra-lights don't have enough power for the standard transponder. It's messages also include position and heading, so it can also give lateral (turn left/right) instructions. It is however incompatible with TCAS.

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    $\begingroup$ TCAS has nothing to do with the flight rules, it works regardless of IFR/VFR $\endgroup$ – Steve Kuo Aug 28 '14 at 0:39
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TCAS I: only needs Mode-A transponders, only provides "Traffic Traffic". TCAS II: needs Mode-C transponders, provides "Climb Climb" etc. TCAS III: needs bigger computers, provides "Climb Left" etc.

TCAS III has never been produced - TCAS II is good enough to do the job, and nobody's worked out a good algorithm for it yet!

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    $\begingroup$ Please consider providing citations for your statements. $\endgroup$ – Federico Oct 6 '17 at 11:06
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    $\begingroup$ TCAS I requires Mode C (TSO C118, RTCA DO-197). TCAS II requires Mode S (TSO C119, RTCA DO-185). $\endgroup$ – Gerry Oct 6 '17 at 12:55

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