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