Avionics Chpt 10

• Traffic Alert & Collision Avoidance System (or TCAS) is a computerized avionics device that is designed to reduce the danger of mid-air collisions between aircraft
• It monitors the airspace around an aircraft, independent of ATC & warns pilots of the presence of other aircraft that may present a threat of Mid Air Collision (MAC)
• Airborne Collision Avoidance System mandated by International Civil Aviation Organisation (ICAO) to be fitted to all aircraft over 5700 kg or authorized to carry more than 19 passengers
• In modern aircraft, the TCAS display may be integrated in the Navigation Display, in older cockpit aircraft TCAS display is Vertical Speed Indicator (VSI) which indicates the speed with which the aircraft is climbing or descending
• TCAS Operation:

  • Involves communication between all aircraft equipped with an appropriate transponder
  • Equipped aircraft interrogates all other aircraft in a determined range about their position (via 1030 MHz), & all other aircraft reply to interrogation (via 1090 MHz)
• TCAS Operation:

  • Interrogation-and-response cycle may occur several times per second, by which the TCAS system builds a three-dimensional map of aircraft in the airspace, incorporating their bearing, altitude & range
• TCAS Operation:

  • By extrapolating current range and altitude difference to anticipated future values, it determinesif a potential collision threat exists
  • Primary Surveillance Radar (PSR) was first used to support ATC in the late 1940s
• TCAS Operation:

  • PSR displays to the controller the plan view position of aircraft, however, it does not know the identity or altitude of aircraft
• TCAS Operation:

  • Secondary Surveillance Radar (SSR) interacts with a Transponder carried on each aircraft. SSR interrogates the transponder to downlink Identity(Mode A) and Altitude (Mode C) information or more comprehensive data including Identity & Altitude (Mode S)
• TCAS Operation:

  • TCAS is a mini SSR on aircraft It is perpetually scanning the sky around own aircraft searching for proximate aircraft to ensure sufficient time for avoidance action in a high speed head to head encounter
• TCAS Operation:

  • Has a minimum surveillance range of 40 nm
  • Top & bottom antennas are used to ensure threat aircraft above and below are detected
• TCAS Operation:

  • The static, electronically steered antennas used to measure the position of proximate aircraft with a range accuracy of 1/125 nm & a bearing accuracy of 3°
  • Antennas operate at temperatures between -600 °C & 1000 °C at air speeds up to 600 knots
• TCAS Operation:

  • TCAS is constantly interrogating Mode C (Altitude) transponders in proximate aircraft respond with altitude information
• TCAS Operation:

  • The range is determined from the time elapsed between interrogation & reply, bearing is determined by the directional antenna & altitude is received in the reply from the other aircraft’s altitude encoder
• TCAS Operation:

  • It can be seen that TCAS will not detect aircraft which do not have a functioning Mode A/C or Mode S transponder
• TCAS symbols - Non-threat traffic:

  • Is not a threat will appear initially as an open white diamond on display, It means that traffic is more than 6 nm range or more than 1200 ft vertically separation from given aircraft
• TCAS symbols - Proximate traffic:

  • Is the traffic within 6 nm range indicated by solid diamond with relative altitude annotation, but the computer calculates it is still not a threat
• TCAS symbol - Traffic advisory (TA):

  • If the computer calculates that the intruder is potentially hazardous, the symbol will change to a solid yellow circle A TA voice is annunciated as ‘Traffic Traffic’
• TCAS symbol - Resolution advisory (RA):

  • About 10-15 seconds later, if the intruder is assessed as an actual collision threat, the symbol will change to red square. A voice command ‘Descend Descend’ with computed RA will be shown on the cockpit display
• 3 main elements of the human machine interface (HMI) are:

  • Plan Position Display of traffic & threat aircraft
  • Aural Prompts for the required action
  • Display of Required Avoidance Manoeuvre
• HMI - Pilot response to TA & RA:

  • In response to a TA, the Pilot is expected to attempt visual acquisition & prepare for any RA
  • In response to an RA, the Pilot is expected to recognize & enact the RA within 5 seconds. This requires a very high degree of trust in TCAS
  • Pilots are generally trained to obey ATC, however in the specific case of TCAS-RA & ATC instruction being in conflict, the RA must take priority
• An encounter - The pilot's view:

  • Non-threat traffic is shown with open white diamond in relative position to own aircraft on the TCAS display
• An encounter - The pilot's view:

  • Proximate Traffic is displayed similarly to non threat traffic but with the diamond is now filled. The aural warning ‘Traffic, Traffic’ is annunciated. Relative altitude is displayed next to the traffic, in this case the traffic is 300 ft higher than own aircraft & level
• An encounter - The pilot's view:

  • Resolution Advisory symbol changes from a white diamond to a filled red square. The aural advisory ‘Descend, Descend’ is annunciated. A red trapezoid indicating forbidden aircraft pitch is displayed on the Flight Director. By changing the aircraft pitch to be outside the forbidden region, the required descent rate is achieved
• An encounter - The pilot's view:

  • Descend Established pilot has initiated & established the required descend rate, indicated by the ‘nose’ of the aircraft now pointing down on the Flight Director
• An encounter - The pilot's view:

  • Level Off descent has achieved the required vertical separation & own aircraft will pass safely below the threat aircraft
• An encounter - The pilot's view:

  • Clear of conflict two aircraft have passed and the distance between them is growing, the threat aircraft symbol becomes yellow. The aural advisory ‘Clear of Conflict’ is annunciated. Pilot will now initiate a climb to return the aircraft to the ATC assigned Flight Level
• International standardization:

  • The Collision Avoidance System is required to work anywhere in the world & hence systems must operate to agreed standards
• International standardization:

  • TCAS is installed in some 20 000+ aircraft worldwide & has proven to add a significant reduction in collision risk
  • The commercial value of safety is reflected in the reduced insurance premium for TCAS equipped aircraft
• GPS:

  • Used to establish a position at any point on the globe
  • Developed by US department of defence
  • Used by both civilians & military personnel
• • Standard positioning service (SPS) - can be used freely by general public
  • Precise positioning service (PPS) - only for authorized government agencies
• The first satellite was placed in orbit on 22nd February 1978, & there are currently 24 constellations of satellites (21 active + 3 spare) orbiting the earth at a height of 20,000 km on 6 different orbital planes
• Working principle of GPS - the following 2 values can bedetermined anywhere on Earth:

  • One’s exact location (Longitude, Latitude & Altitude) accurate to within a range of 20 m to approx. 30 mm
  • Universal Time Coordination (UTC) accurate within a range of 60 ns to approx. 5 ns
  • Speed & Direction of travel can be derived from these coordinates as well as the time
• Working principle of GPS:

  • Satellite orbits are inclined at 55° to the equator, ensuring that a least 4 satellites are in radio communication with any point on the planet
• Working principle of GPS:

  • Each satellite orbits the Earth in approx. 12 hours & has 4 atomic clocks on board
  • Exact position of each satellite is known at any given time and the distance to each satellite can be calculated by GPS receiver
• Working principle of GPS:

  • Exact satellite position data is transmitted to the receiver as part of the satellite message
  • Since distance equals velocity multiplied by time, the receiver need only measure the time it took for the GPS signal to reach the receiver
• Working principle of GPS:

  • The speed at which the signal travelled to the receiver is a constant 3×10^8 m/s. Using time & velocity to derive distance is known as the time of arrival ranging
• E.g 10-1: 2D model of GPS user segment:

  • In this case, all satellites are located in 1 geometric plane, knowing the distance from just 2 satellites would provide the location of aircraft
• E.g 10-1: 2D model of GPS user segment:

  • Aircraft must be located somewhere on a circle with a radius of 30 km from satellite A, & somewhere on a circle with a radius 40 km from satellite B
  • In this 2D model the aircraft can be in 1 of 2
• E.g 10-1: 2D model of GPS user segment:

  • To further define the locations of our aircraft in the 2D model, a 3rd GPS satellite is added
  • If the aircraft is 30 km from satellite A, 40 km from satellite B, and 20 km from satellite C, the aircraft must be in position #1