Avionics

• Avionics


  Electronic systems associated with flight
• Content of the module

  • Navigation Systems: GPS, inertial navigation, LORAN C
  • Communications: spread spectrum, PCM, satellite communications, communications link design, AM/FM radio
  • Radar and Aircraft Systems: Fundamentals, system design, radar cross-section, weather radar, ILS and VOR
  • Sensors: video, infrared thermal imagers and image intensifiers

  
• Navigation


  The process of planning, recording, and controlling the movement of an aircraft or a vehicle from one place to another

  
• Air navigation has additional restrictions: High speed of an aircraft – less time to calculate, Cannot stop mid-air! There is no in-flight rescue once you get lost!, Constant awareness of position is required!
• Navigation determination methods


  • Dead reckoning
  • Position fixing
  • Internal systems
  • External systems

  
• Dead reckoning


  The process of calculating one's current position by using a previously determined position, or fix, and advancing that position based upon known or estimated speeds over elapsed time and course
• Dead reckoning is prone to error accumulation
• Inertial navigation systems


  • Self contained system so does not rely on external radio signals
  • Dead reckoning system
  • Outputs velocity, position, heading and attitude based on measurements derived from inertial sensors mounted within the aircraft

  
• INS components


  • Accelerometer gives output signal proportional to force which acts on known mass attached to the aircraft
  • MEMS accelerometers are commonly used
  • Gyroscopic sensors provide output proportional to rotation angle to drive motors in order to keep platform in a constant orientation in inertial space
  • Gimbals arranged to provide 3 degrees of rotational freedom
  • Computer system
  • Torque motors activated by signals from gyros

  
• For gyro stabilised platform, no matter how the aircraft manoeuvres the platform maintains its orientation
• Rules of dead reckoning


  • DR at every course change
  • DR at every speed change
  • DR every hour on the hour
  • DR every time a fix is obtained
  • DR 3 hours ahead or for the next three expected fixes
  • DR for every line of position (LOP), either visual or celestial
• Position fix


  Occurs at the intersection of the lines of position (LOP)
• Line of position (LOP)


  The line at which the aircraft lies
• If LOPs are measured at different times, dead reckoning must be used to compensate for motion between observations
• Gyro stabilised inertial navigation platform


  Platform is decoupled from the airframe
• Difference between running fix and planned schedule are fed into auto-pilot
• Non-gyro stabilised inertial navigation platform


  Platform is not decoupled from the airframe and sensor incorrectly indicates acceleration in X direction instead of Z

  
• Position fixing using 2 angles


  Distance information gives circle centred at transmitter, Radius R = c tt - tr, Measurement of time difference between transmitted and received signals are translated into distance

  
• Block Diagram of a 2 Axis Gyro Stabilised Inertial Navigation System

  1. Gyros

  2. Accelerometers

  3. Axis transformation

  4. Integration after acceleration output signals resolved in navigation reference frame

  5. Output

  
• Position fixing errors


  Signal propagation velocity (nominally c = 3 × 10^8 m/s) varies in time and position, Change due to pressure and humidity, Precipitation changes refractive index above 10 GHz, Satellite receivers can correct for position-based atmospheric data
• Triple axis "StrapDown" INS

  • Inertial sensors are fastened directly to the aircrafts 3 main axis and not isolated from angular motion
  • Analytic System - computational technique combines outputs from accelerometer and gyros which gives attitude information
  • Reduces mechanical complexity - no motors/gimbals/bearings
  • Uses laser or fibre optics gyros instead of spinning wheel gyroscopes
  • Uses Digital technology
• Global Positioning System (GPS)

  Advantages: Global coverage, All weather fixing, Provides air traffic control over oceans, Disadvantages: Satellite, Replacement programme to replace failed satellites, Cost to maintain GPS is $400 million per annum (DOD funded)

  
• Triple axis "StrapDown" INS block diagram

  1. 3 Laser Gyros

  2. 3 accelerometers normally coincident with airframe axis

  3. Compute Attitude Vector

  4. Axis transformation

  5. Integration after acceleration output signals resolved in navigation reference frame

  6. Output

  
• Interferometer


  An instrument which utilises interference effects between two separate optical beams which travel in different directions
• GPS


  Uses the position fixing method to find the location in space, Needs 4 or more satellites to find out the position of the receiver
• Optical fringes


  Light and dark lines created by constructive and destructive interference of the two waves at the detector
• GPS position equation


  si – time sent by satellite, b -satellite clock bias, xi, yi, zi – are satellite coordinates, x, y, z – position of the receiver, t̃i – time when message received by onboard clock, c – speed of light

  
• GPS operation


  1. Receiver

  2. Range calculations

  3. Position estimator

  4. Satellite position data

  
• GPS configuration


  • Control Segment: System of tracking stations, Orbital models, Compute precise orbital data and clock corrections, Uplink clock and orbital data
  • User segment (aircraft): Receivers convert satellite data into x,y,z,t, velocity, Correlation time shift to provide synchronisation of pseudo random pattern data from satellite – velocity and range, Low cost crystal clock, Processes data using 4 equations ( minimum 4 satellites needed)

  
• Ring Laser Gyro


  • Uses interferometry to sense rotation
  • 2 counter rotating laser beams
  • Helium neon lasers reflect around hollow cavity
  • Optical path length of the two beams changes with rotation
  • Frequency difference between 2 beams causes fringing across detector
  • Rotation rate is proportional to the fringe rate
  • Fibre optics Gyros are more recent variation, operation principle is the same, but uses coils of fibre optics cable

  
• Laser gyro


  • Solid glass block case with 3 narrow tubes
  • No moving parts except dithering motor
  • Mean time between failures (MBTF) 10000 hours
  • Weight 10kg
  • Volume 20 cm3
• GPS Signal


  Uses spread spectrum modulation, Precise Positioning Service (PPS) – 1227 MHZ (L2) / 1575MHz (L1) for authorised users, Standard Positioning Service 1575MHz (L1) for general use

  
• GPS space segment


  24 Satellites in 6 orbital planes, 4 in each plane, 20,200 Km Altitudes, 55 degree inclination, 12 hour, over a given point on Earth each 24 hours, Each satellite transmits L1+L2 frequencies but unique code identifies the satellite, Message contains position and time info

  
• Differential GPS service (DGPS) can achieve typical accuracy better than 2m, depending on the method can reach centimeters
• Other satellite systems


  • Beidou – People's Republic of China's regional system
  • Galileo – a global system being developed by the European Union and other partner countries
  • GLONASS – Russia's global navigation system
  • IRNSS – A regional navigation system developed by the Indian Space Research Organisation
  • QZSS – A regional navigation system in development that would be receivable within Japan
• GPS vulnerabilities include spoofing attacks and jamming attacks
• Sagnac effect


  Δ𝜙 = 2𝜋𝑐/𝜆 Δ𝑇, where Δ𝑇 is the travel time difference, 𝜔 – platform angular velocity vector, A – oriented area, 𝜆 – wavelength of light

  
• A possible solution to GPS vulnerabilities is to use antenna systems with the ability to block signals from unwanted directions
• Fiber optics Gyro


  • Uses the Sagnac effect
  • Red and blue dots represent counter-propagating photons, grey dots represent molecules in the laser cavity
• Airborne INS interfaces


  Starting with initial position and acceleration as well as optional input from (Doppler NS, Black Box, GPS) inertial NS inform ABS, FMS, ADC, FCC,WXR