Traffic Alert and Collision Avoidance System (TCAS).
Traffic Alert and Collision Avoidance Systems (TCAS) are progressively being fitted in all aircraft that fly in controlled airspace. A TCAS equipped aircraft uses a Mode S Transponder with two antennas; one mounted on top of the aircraft besides the normal antenna on the bottom, to scan its surrounding airspace for any 4intruding' aircraft. It then assesses the surrounding traffics' progress for any potential conflicts with its own flight path. These are shown on a display on the aircraft's instrument panel with the interrogating aircraft being placed in the middle of the screen.
There are several types of TCAS. These are summarised below:
This is a low-power, short-range system which was the first TCAS introduced. It only gives proximity alerts known as Traffic Advisories (TA), ie it locates other aircraft in its immediate vicinity to aid the flight crew in visual location of other aircraft in the surveillance area. It does not generate any suggested avoidance manoeuvre.
This is a. true avoidance system that generates TAs that informs the flight crew of aircraft present in the surveillance area, ie within approximately 30 nm of the aircraft. It can also generate vertical manoeuvre suggestions known as Resolution Advisories (RA) and will co-ordinate complementary manoeuvres with other TCAS II equipped aircraft, if necessary.
TCAS III is not yet fully defined, but it is intended to give the same functions as TCAS II and in addition, to be able to provide RAs in the horizontal plane, eg TURN LEFT', TURN RIGHT', etc.
TCAS II is the current standard and forms the basis for the rest of these notes on TCAS.
TCAS is designed to provide aircraft separation assurance, more commonly referred to, for obvious reasons, as collision avoidance. It does this by maintaining a surveillance area around the aircraft by using replies it receives from other aircraft Transponder systems. It maintains the surveillance within a sphere that is determined by the transmitter power and receiver sensitivity of the TCAS, figure 26.
The area in which a threat is imminent depends on the speed and path of the aircraft and the threat aircraft and is defined as Tau\ which represents the minimum time a flight crew needs to discern a collision threat and take avoiding action. The TCAS system determines the possibility of collision using algorithms that define the speed, and equations for solving a problem. In this case, the problem is whether the path and speed of two aircraft, or indeed many aircraft, will result in the aircraft passing within a predefined spacing of each other. If the answer to this problem is yes, the TCAS issues an advisoiy to the flight crew on the appropriate indicator.
The advisory message is split into two types:
* Traffic Advisory (TA) ~ which gives information on the position of other aircraft in the immediate vicinity. TA airspace is defined as within ± 1,200 feet and approximately 45 seconds distance, with respect to closure speed, of the aircraft.
* Resolution Advisory (RA) - which recommends a manoeuvre to increase vertical separation between aircraft. RA airspace is defined as ± 750 feet and approximately 30 seconds distance, with respect to closure speed, of the aircraft.
Fig. 26 TYPICAL SURVEILLANCE PATTERN
The aircraft's TCAS installation consists of the following component parts:
* Transmitter/ Receiver.
* RA and TA displays.
* One omni-directional antenna.
* Single omni-directional antenna.
* Single directional antenna.
* System controller.
Mode S Transponder with dual antenna installation.
Figure 27 shows how the system components interconnect in a typical aircraft installation.
The TCAS transmitter/receiver contains the processing circuits necessaiy to determine if the path of another aircraft is in conflict. It also handles the Mode S data link transmissions that are TCAS related and has monitoring circuits to check on its own integrity and of those providing data to it.
If other aircraft in its vicinity, ie intruding aircraft, are also TCAS equipped, the TCAS transmitter/receiver can also send a 'manoeuvre co-ordination' message to the intruding aircraft via the Mode S Transponder. This allows TCAS equipped aircraft to co-ordinate their RAs to prevent both aircraft performing the same action (ie the possibility of a climb RA being given to both aircraft). In this case one will be given a climb RA and the other a descend RA.
Displays can be dedicated TCAS instruments or incorporated as part of other system indicators, eg weather radar. Early analogue displays used modified versions of the aircraft's existing instruments to provide RAs and TAs.
The RA was incorporated as part of the VSI display (figure 28) which works as a normal instrument with the ability to show TCAS preventative and corrective resolution advisories as coloured lights around the display.
This type of display also requires an additional method of providing the TAs. On early systems, the TAs were incorporated as part of the Electronic Flight Instrument System (EFIS) Multi-Function Display (MFD) or weather radar display. Figure 28 shows a typical early EFIS MFD display, manufactured by Collins Avionics.
Fig. 29 COLLINS COMBINED TA/MFD
Fig. 30 COMBINED VSI/TA/RA DISPLAY
Later, dedicated VSI/TA/RA displays were introduced as standalone instruments as illustrated in figure 30.
With these TA displays, other aircraft in the vicinity are shown as various symbols, depending on their proximity and the extent of their threat potential. Next to each symbol numbers indicate the altitude of surrounding aircraft above or below the TCAS equipped interrogator. For example, +07 equates to an aircraft 700 feet above and -01 equates to an aircraft 100 feet below the interrogator aircraft. If an arrow appears next to the symbol, pointing up or down, it indicates whether the aircraft is climbing or descending at greater than 500 feet/second.
Aircraft with no threat potential appear as hollow blue diamonds, but when the TCAS detects an aircraft closing on the interrogating aircraft as a potential threat, it changes the symbol to a solid yellow circle and the TCAS can also output an audio TA call out to the aircraft's Audio Integrating System (AIS) as 'Traffic, Traffic'. If the situation worsens and the approaching aircraft becomes an immediate threat, a RA is given and the target aircraft symbol becomes a red square and an audio warning commands evasive action by calling out 'Climb, Climb' or 'Descend, Descend' as appropriate. The modified VSI also automatically gives a recommended climb or descent rate by illuminating the coloured annunciators around its display dial. Some of the more sophisticated TCAS may also give other command instructions, eg 'Do not descend', *Limit vertical speed' as appropriate.
A TCAS equipped aircraft may have up to six L-band antennas; two for each Mode S Transponder; an omni-directional antenna on the bottom and a directional antenna on the top of the aircraft; or two directional antennas for the TCAS system. The omni-directional antennas are used in the conventional way for Mode A, C and S transmissions, while the directional antenna is used to determine the altitude and bearing of aircraft responding to the interrogations of the TCAS transmitter/receiver.
Mode S Transponder
As discussed above, the Mode S Transponder provides the communication capabilities, ie data-link, required for TCAS, as well as the standard Mode A and C Transponder.
TCAS controllers come in all shapes and sizes. Some are only used for controlling the TCAS, while others are combined with the Mode S Transponder controller in one LRU. Figure 31 shows a combined controller manufactured by Gables.
With reference to the numbered items in the drawing (figure 31).
1. ALT RP - This selects the normal Transponder Altitude Reporting function, but TCAS will not operate if it is selected OFF.
2. Transponder Mode A code window - Displays the selected 4-digit Transponder code as dictated by ATC.
3. TRAFFIC Selector - Selects the parameters for the TCAS displays as follows:
* AUTO - Normal operating position that displays RAs and/or TAs along with conflicting traffic when an intruder aircraft is detected.
* ON - In this position, the display shows all traffic within range of the TCAS and with + 2,700 feet vertically of the aircraft.
4. IDENT - This push-button switch controls the standard Transponder 'IDENT function.
5. XPDR FAIL - This amber light will illuminate when the selected Transponder system has failed. It will also illuminate if the altitude source information fails when ALT RP function is on.
6. TCAS/XPDR Mode selector - Selects the Transponder and TCAS modes.
* 1 - STBY - 2 - Selects the Transponder in use; TCAS is not operating.
* TA - Selects Traffic Advisory mode; TCAS only provides TAs.
* TA/RA - Selects Traffic Advisory and Resolution Advisory modes. This is the system's normal operating mode.
* TST - When pressed and held initiates the TCAS self-test function.
As shown in figure 26 above, a TCAS equipped aircraft is, in effect, surrounded by three imaginary envelopes of radar-monitored airspace that are defined by Tau, ie the flight time associated with the closure speeds of other aircraft in the vicinity. Perhaps the best way to discuss TCAS operation is to use another diagram (figure 32).
In figure 32, aircraft A' transmits a squitter signal, at a rate of once per second, at 1090 MHz from its Mode S Transponder that identifies it as the transmitting aircraft. Any other TCAS equipped aircraft in the vicinity (aircraft 'C'), continually monitors the 1090 MHz frequency. When aircraft 'C' receives a valid identification squitter, it adds the transmitting aircraft to a list that it will interrogate. This list is called a 'roll-call'. In addition, aircraft 'A' will also add aircraft 'C' to its roll-call.
The TCAS II transmitter/receiver of aircraft 'A' will also transmit interrogations to any standard mode A / C Transponder equipped aircraft in the area.
In the above example, aircraft B will respond to an A or C interrogation, but only aircraft C will be able to respond to a Mode S All-CalV interrogation. Figure 33 shows the interaction between aircraft 'A' and 'C'.
'A' SQUITTER SIGNAL
Fig. 32 TCAS OPERATION
Aircraft 'A' interrogates each of the aircraft appearing on its roll-call, and replies are received at the TCAS antennas. The directional antenna contains several receiving elements that provide the transmitter/receiver with phase information relating to the actual bearing of the intruder aircraft and in addition, if the replying Transponder is capable of altitude reporting, the altitude of the replying aircraft is also received and used by the TCAS.
The TCAS computes the range of any intruding aircraft by using the round-trip time between transmission of the interrogation and reply reception, ie the fact that a radar mile equates to 12.36 µSeconds.
Once several replies have been received, the altitude, altitude rate, range and range rate of each aircraft in the vicinity is computed by tracking the replies to each interrogation.
The TCAS then uses computer analysis of the replies, to determine which aircraft represent potential collision threats and provides the appropriate aural and visual advisory indications to the flight crew. If the TCAS determines that the flight path of an aircraft within the region will pass within 1200 feet, an advisory is issued.
ATC SSR Interrogation 1090 1030
ATC SSR interrogation
Each threat aircraft within the area is processed individually to allow selection
Fig. 33 INTERROGATION AND REPLY BETWEEN TCAS EQUIPPED AIRCRAFT
of the minimum safe resolution advisory and to provide co-ordination with other TCAS equipped aircraft. If the threat aircraft is equipped with TCAS, a coordination procedure is generated via the air-to-air Mode S data link.
Displayed Resolution Advisories are of one of two types:
* Corrective advisories - that instruct the pilot to deviate from the current flight path.
* Preventative advisories - that advise the flight crew to avoid certain manoeuvres to prevent collision.
A Traffic Advisory informs the flight crew of a potential threat aircraft present within the surveillance area, and they are notified visually on the display screen of the potential threat. In addition, if the traffic poses a threat, an aural announcement of TRAFFIC, TRAFFIC is sent to the AIS. If the threat continues and evasive action becomes necessary, then the TA is followed by a RA, which will give a corrective action that could include an aural announcement of 'CLIMB, CLIMB' or 'DESCEND, DESCEND.
This action, ie climb or descent, will also be displayed on the VSI scale. When the threat clears, the display continues to show the now non-threatening target and will also provide an audio announcement of 'CLEAR OF TRAFFIC, CLEAR OF TRAFFICThere are currently 12 possible aural announcements, and these are listed in table 5.
In the future, the aircraft's TCAS will have the capability to provide to the ATC SSR ground station, through the Transponder's Mode S data link, the RA given to the flight crew, giving the ATC controller an indication of what the aircraft is about to do.
The traffic alert and collision avoidance system (TCAS) helps the flight crew maintain safe air traffic separation from other ATC transponder equipped airplanes. TCAS is an airborne system and operates independently of the ground-based ATC system.
TCAS sends interrogation signals to nearby airplanes. These airplanes which are equipped with an air traffic control radar beacon system (ATCRBS) transponder or an air traffic control (ATC) mode S transponder respond to these interrogations. TCAS uses these response signals to calculate the range, relative bearing, and altitude of the responding airplane. If a responding airplane does not report altitude, TCAS cannot calculate the altitude of that airplane. Airplanes tracked by TCAS are called targets.
Using the information from the response signals and altitude of own airplane, TCAS calculates the relative movement between own airplane and the target. TCAS then calculates how close the target will be to own airplane at the closest point of approach (CPA).
Targets are classified as one of these four types depending on the separation at CPA and the time it will take until CPA occurs:
· Other traffic
· Proximate traffic
Each type of target has a different symbol on the display.
If the separation at CPA is within certain limits, TCAS provides advisory messages to the flight crew. TCAS provides two levels of advisories to the flight crew, traffic advisory (TA) and resolution advisory (RA). The type of advisory is determined by a combination of altitude, the time to CPA, and the separation at CPA. The TA shows for relatively longer times to CPA and relatively larger separation at CPA and is for intruder targets. The RA shows for relatively shorter times to CPA and relatively smaller separation at CPA and is for threat targets.
The TA shows the range, bearing, and relative altitude (if relative altitude is known) of the intruder target. The RA also gives visual and aural commands to the flight crew to make sure there is safe vertical separation from the threat target. TCAS also communicates with other airplanes that have TCAS to coordinate the flight movement to prevent a collision.