Traffic Collision And Avoidance Systems (TCAS)

1.    INTRODUCTION

Aviation communities across the globe have always been focusing upon the reduction of aircraft accidents and mid-air collisions. It was necessary to develop an effective system to control aircraft activities and avoid dangers to the passengers. It was the role of the electronics industry to develop efficient airborne collision avoidance system. There was a need to develop a system which would work outside the radar coverage and independently of air traffic control (ATC). Concept of TCAS was introduced in 1970 and implemented in June 1981.^[1]

TCAS supports the concept of “see and be seen”. It can be called a virtual air traffic controller. This system provides the crew with the traffic alert information and advanced version of TCAS also provides with recommended action that needs to be taken. TCAS is a secondary radar resource which does not override the decision of ATC. 

2.    VERSIONS OF TCAS

There are mainly four versions of TCAS which are used in aircraft depending upon aircraft characteristics needs and passenger-carrying capacity.

2.1  TCAS I: It has a range of around 40 miles in radius and altitude and bearing information of other aircraft is provided. Its main aim is to alert a pilot regarding traffic. It does not provide any suggestion or measures that need to be taken. Pilot or air traffic control officer can decide about the action that needs to be taken.

2.2  TCAS II: It is the first system which can an alert pilot using vocalized instructions known as resolution advisory (RA). TCAS II advises the pilots the necessary actions that need to be taken e.g. ‘climb’ or ‘descend’ or ‘reduce speed’. The pilot has the authority to take decision irrespective of TCAS advisories. It automatically can detect the co-ordinates and take precautionary measures to ensure that maximum distance is maintained between two aircraft.  

2.3  TCAS III: It is a more enhanced version of TCAS II. It cannot only provide advisories but also resolve traffic conflicts. It uses both horizontal and vertical directives. This helps to separate aircraft horizontally as well as vertically. TCAS III did not prove to be useful because of the limited availability of horizontal resolution information of the flight. Hence TCAS III was no more used and replaced by TCAS IV.

2.4  TCAS IV: Additional information that is encoded by an aircraft is used by TCAS IV. It also uses some extra information obtained by GPS or navigation system. The information encoded by the air-to-air the data link is also used to find out about Horizontal advisories. TCAS IV was abandoned as ADS-B development was started. 

3.1  Computer unit

3.1.1       computer scans airspace, intruder aircraft, its location, altitude, vertical speed and threat. It is also used for resolution advice determination and generation of advisories.

3.1.2      To control collision avoidance, the processor utilizes radar altitude, pressure altitude, and status input from other aircraft.

3.1.3      These parameters are also used for calculating the protection envelope around the aeroplane.

3.2  Antennas:

3.2.1      TCAS operates on two antennas which are mainly located on the two faces of the aircraft, top face and bottom face.

3.2.2      Top face antenna is a directional antenna.

3.2.3      Bottom face antenna is an omnidirectional or directional antenna.

3.3     Cockpit Presentation:

4.3.1      TCAS uses two called as TA (Traffic Advisory) and RA (Resolution advisory).

4.3.2      Traffic advisory display displays the traffic and upcoming threat and also the proximity of the intruder aircraft. It also shows the distance of intruder aircraft and the altitude. The symbols used reflects the intensity of the threat.

4.3.3      Resolution advisory display is used for the suggestions given by the TCAS. It shows all the possible measures such as ‘climb’, ‘control speed’ to the    pilot.

4.3.4      The information displayed by this display is identical for every aircraft as the standards for both TA and RA display are predefined.

 

4.     TCAS OPERATION MODES

 

4.1  TCAS operates on four major modes that designed according to international standards by various aviation authorities.

4.2  Depending upon the needs and air traffic, modes can be alternated.

4.2.1 Stand-by mode

a)     This mode can also be called as off position of the TCAS.

b)     When TCAS is set to standby mode, it is electrically supplied but is not operative.

c)     ATC is normally operated in this case hence ATC continues responding to intruder interrogation.

4.2.2 Transponder mode

a)     In transponder mode, TCAS is set to standby but transponder operates in ‘ON’ condition

b)     The transponder can respond to the other interrogations and the signals received from the ground and TCAS of other aircraft.

5.2.3      Traffic advisory mode

a)     Traffic advisories are provided alerting aircrew about the possible dangers.

b)     TCAS does not provide with resolution advisories.

c)     The transponder is ‘ON’ in this mode and performs all the tracking activities normally.

5.2.4      Fully automatic

a)     This is fully efficient mode providing with traffic and resolution advisories both.

b)     Computer unit operates providing full information to the pilot.

c)     It also answers to the interrogations as well as interrogates to the intruder aircraft.

d)     The necessary course of action is provided to the pilot that needs to be taken to prevent mid-air collision.

5. WORKING OF TCAS

5.1) TCAS operates on the principle of interrogating with other transponder equipped aircraft and replying to any aircraft that interrogates.

5.2) TCAS creates an envelope of protected airspace that is about 3-meter diameter and 750 ft. deep. The interrogation is about 45NM ahead and 10NM behind.

5.3) TCAS operates on the frequency of 1030 MHz while interrogating with all other aircraft and 1090 MHz while replying to the interrogations by all other aircraft.

5.4) Computer categorizes the target depending on priority mainly into,

1) No Threat

2) Proximate threat

3) Potential threat

4) Immediate threat

5.5) The operation involves communication with all the aircraft in the area specified above. The figure shows target aircraft and the intruder aircraft equipped with the transponders.

5.6) These transponders are either mode ‘c’ to provide altitude information or mode ‘s’ 24-bit dedicated to each aircraft.  While communicating with each other it also negotiates avoidance manoeuvres between the aircraft.

5.7) TCAS uses complex formulas to determine the height, direction and speed of the flight.

6. COLLISION DETECTION LOGIC ALGORITHM ^[2]

        TCAS uses the principle of (tau) ‘τ’ to find time to the closest point of the approach between flights.

Where,

r = range over closure rate

r’ = range rate

Correction factors may be needed for problems such as low or high closure rate.  The formula is modified in such cases.

Where,

DMOD = Threshold reaction time for an intruder.

7. TRANSPONDERS:

Transponders are the devices responsible for receiving a radio signal and automatically transmitting a different signal.

In the aerospace industry, the transponder is an automatic transceiver that emits coded identifying signal in response to an interrogating received signal.

7.1) Modes of transponder

There are three modes of operation of the transponder.

7.1.1) Mode A – Mode A transponders are one dimensional. They only give the aircraft identification. No other information is given out. If the aircraft is equipped with a TCAS, it will only give traffic advisories and will call ‘traffic, traffic’ in the annunciation

7.2.2) Mode C – A mode C transponder gives both the identification and altitude of the aircraft. This is what we call two dimensional. The TCAS of the aircraft will now not only give TAs but RAs (Resolution Advisories) as well. Here the annunciation will warn the pilot to either descend or climb to avoid traffic conflict.

7.2.3) Model S (Mode select)- Reports location, altitude as well as the vertical speed of an aircraft. Avoiding over interrogation is an important function of this transponder.

 If two aircraft are equipped with a mode S transponder both will establish a discrete communication between them and give complementary RAs. That is, if the TCAS of the aircraft 1 says to descend, the TCAS of the aircraft 2 will say to climb.

7.3) Working of mode ‘S’ transponder:

	INTERROGATION	REPLY
FREQUENCY	1030 MHz	1090 MHZ
MODULATION	DPSK	PPM
DATA RATE	4 MBPS	1 MBPS

*DPSK: differential phase-shift keying

*PPM: pulse position modulation

8.3.1) Mode ‘S’ system consists of sensors, transponders and signal in space which link between them.

8.3.2) For interrogation with other aircraft, sensors maintain a file of recorded data with all the aircraft's in its range.

8.3.3) Each Mode the system is assigned with a unique 24-bit address permanently. These addresses include all the interrogations.

8.4.4) This system eliminates all unnecessary elements and minimizes interference. This results in the efficient functioning of the system

e) Each flight has its unique flight ID to be recognized.

8.5) Data flow pattern of transponder

8.5.1) data received from the computer is coded using the encoder into another code. An encoder is used for security purpose as well as quicker data transmission

8.5.2) The modulator modulates the encoded signal. For interrogation, DPSK is used while for reply PPM technique is used.

8.5.3) The modulated signal is then transmitted to the transmitter. Transmitter filters noises i.e. unwanted signals and disturbances. It amplifies the signal. Suppressor bus suppresses unwanted signals.

8.5.4) Duplexer either passes the signal from the transmitter to the antenna or transmits a signal from the antenna to the receiver. As the name suggests, duplexer can select one out of two ways as shown in the diagram.

8.5.5) The receiver demodulates the received modulated signal and amplifies the demodulated signal. The signal is then forwarded to the decoder.

8.5.6) The decoder converts the encoded signal into its original form to make sure the communication is understood by the pilot.

8.5.7) Since TCAS is the cyclic process the signal is again transmitted to the encoder using feedback system from the control panel.

 

8.6) Characteristics of mode ‘s’ transponders

8.6.1      These transponders allow tracking aircraft when they are close to other aircraft.

8.6.2      It interrogates and replies to each airframe sequentially.

8.6.3      The transponder can even read out the data and interpret the signals so that aircraft can be identified by the pilot.

8.6.4      Error checking can also be done using this transponder using effective feedback systems.

 

9. TCAS PILOT COMMUNICATION SYSTEM

a)     Information is communicated to the pilot using various alerting techniques.

b)     Audio-Visual alert to flight crew is provided in case of emergency.

c)     Cockpit display shows all the traffic and provides audio alerts.

d)     Modern glass cockpits use navigational digital display or situation indicator. This cockpit uses electronics displays.

e)     Older cockpits included mechanical displays and instruments. The vertical analogue speed indicator is one of the examples of the mechanical display.

f)      As mentioned earlier information is communicated using two different displays mainly TA (Traffic Advisory) display and RA (Resolution advisory) Display.

9.1) Traffic/Resolution advisory display

9.1.1  The display present in the cockpit shows multiple aspects related to traffic using various illustration symbols.

9.1.2  Information such as aircraft distance, level of threat, altitude, climb/descend is shown on the display.

9.1.3  The display also provides resolution advisory to the pilot. The necessary course of action that is needed to be taken is shown on the display.

9.1.4  There are various symbols used as shown in the diagram.

    

9.1.1  The needle on the display shows the vertical speed of an aircraft. It also advises the required speed rate.

9.1.2  The red zone on the indicator signifies that the vertical rate needs to be avoided and the green zone depicts the required vertical rate.

9.1.3  Pilot or aircrew can get a detailed idea about the traffic using this display and can take all the safety measures to avoid a mid-air collision.

9.2     Audio alerts and warning

9.2.1      TCAS also provides audio alerts to the pilot in case the pilot loses his focus on the displays.

9.2.2      There are various types of audio alerts related to,

1)     Traffic advisory

2)     Resolution advisory

3)     Clear of conflict

9.2.3      TCAS mainly makes the sound such as ‘TRAFFIC TRAFFIC’ to provide traffic advisory to make the pilot alert. These alerts signify and require the pilot to be ready to manoeuvre in case of RA.

9.2.4      There are various types of resolution advisories such as ‘INCREASE / DESCEND CLIMB’, ’CLIMB’, ’TRAFFIC TRAFFIC’, DESCEND DESCEND’.

9.2.5      This messages guide the pilot to take necessary action.

9.2.6      Once the threat is no more ‘CLEAR OF CONFLICT’ signal is given by the TCAS.

9.2.7      This audio-visual advisory helps the pilot to avoid mid-air collisions.

11. REFERENCES

[1] Safety Topics, Traffic alert and collision avoidance system (TCAS); AIRCRAFT ENGINEERING 1988 (Pg 27-28)

[2] A TCAS II resolution detection the logic algorithm, James Chamberlian, NASA Langley research centre. (pg 3/12)

[3] TCAS experimental unit (TEU) Hardware description, Massachusetts Institute of Technology, Lincoln Laboratory

[4] TCAS traffic display redesign, IEEE systems and information, 2011, William P. Cleveland, Elizabeth S. Fleming, Gustavo Lee