Avionics Chpt 8

• Primary radar system operate on an echo principle, when a pulse of energy strikes a target, which may be rain clouds, ground, or mountains, a portion of the pulse is reflected back to the radar receiver for analysis
• Principle of Airborn Weather Radar (AWR):

  • Most common primary radar used in aviation
  • A horizontal scanning antenna is fitted, usually in the nose of an aircraft, providing information relative to its heading
  • The antenna is behind a cover called radome, which is transparent to radar frequencies
• When the target fills the antenna beam the signal received is computed using equation below:

  Pr (R)= (KZ)/R^2

  Where,

  • Pr (R) = The received signal from a cloud formation of water or ice crystals
  • K = The radar parameter constant
  • R = Distance from the transmitter to the weather formation
  • Z or Z factor = Size & density of water drops or ice crystals
• Typical AWR consists of:

  • Receiver-transmitter
  • Indicator
  • Control panel
  • Antenna
• Receiver-Transmitter (RT):

  • It consists of 3 basic sections: transmitter, receiver & data processor
  • Self-monitoring circuits that check signal validity & performance parameters
  • The transmitter section produces a multi-pulse RF signal at X (8000-12000 MHz) or C (4000-8000 MHz) band frequencies
• Receiver-Transmitter (RT):

  • Sufficient power to illuminate targets at ranges of approximately 300 nautical miles
• Receiver-Transmitter (RT):

  • The receiver section contains the signal detection & amplification circuits necessary to process the RF return signal into a usable signal that can be processed by the data processing circuits
• Receiver-Transmitter (RT):

  • The data processing circuits use the received RF return signal to produce a data signal encoding the range bearing & target information that will be displayed by the system indicator
• Indicator:

  • Is a monochrome or colour CRT display presenting a 2-dimensional map of areas of precipitation (weather targets)
  • Contains the circuits & controls for adjusting the display brightness, display reference markers (range markers) & also monitor the performance of the indicator
• Control panel:

  • May contain all the controls & switches that allow the pilot to configure system operation
  • Required to relay this information to the RT, or the control panel may contain just switching circuits to select between weather radar systems
  • Provides for selection of system operation supplies the control information to the RT in the for of digital control words that are serially shipped to RT in the binary format
• Antenna:

  • Most common use today is the flat plate phased array (electronic) antenna
  • Before the advent of the flat-plate antenna, a parabolic antenna was a common sight in weather radar systems
  • The basic structure consists of a flat front & rear plates separated at waveguide dimensions by septum walls
• Antenna:

  • An antenna mounting is centred on the rear plate & the outer profile of the antenna is roughly circular
  • The size of the flat plate antenna determines the beam width of the transmitted signal
  • Narrower the beam width, the greater the resolution of the system
• E.g 8-1: An Airborne Weather Radar echo at 30 nm when the beam is level with the aircraft is lost when the centre of the beam is tilted upwards at an angle of 2°. How high above the aircraft is the top of the rain?

  
  

  Ans: tan (2°) = h/30

  h = 30xtan(2°)

  = 1.04 nm
• Working principle of weather radar:

  • The transmitted pulse is generated by the RT section by multiplication scheme at X-band or C-band frequencies
  • The transmit signal is then applied through a duplexer to the antenna
• Working principle of weather radar:

  • The antenna then will radiate the transmit signal away from the aircraft
  • The energy contained within the transmit pulse will be partially reflected to the antenna
• Working principle of weather radar:

  • The return reflected pulse is applied to the receiver section through the duplexer
  • The return signal is amplified & then mixed with are reference frequency to produce an intermediate frequency return signal
• Working principle of weather radar:

  • The intermediate frequency is amplified & decoded before being applied to a range filter
  • The range filter processes the return signal to range
• Working principle of weather radar:

  • The signal returns from azimuth bearing applied to azimuth bearing filter
  • The filtered digital return signals are encoded to a serial data that will be transmitted to the indicator
• Working principle of weather radar:

  • The indicator decodes the serial data word and stores the information in memory until it is used to generate the next raster scan
• Working principle of weather radar:

  • In most weather radar systems, the indicator will also display the system operation parameters in addition to the 2-dimensional display of weather targets
• System interfaces:

  • The control unit selects the transceiver for active operation besides providing control data
  • The waveguide switch positions itself in accordance to the transceiver RF port to the antenna RF port
• System interfaces:

  • Waveguide runs route the RF between the transceiver, waveguide switch & antenna
  • Attitude signals are provided by IRS (Inertial Reference System) for antenna stabilization
• System interfaces:

  • Range selection is made at EFIS control panel and transceiver sends reflectivity data formatted to selected range to EFIS for display on the CRTs
• Doppler navigation:

  • Is a self-contained dead reckoning (act of calculation) system
  • Desired Doppler information obtains through a measurement of aircraft velocity (both laterally & longitudinally) using Doppler radar
• Doppler navigation:

  • It may be explained where, an aircraft transmits a signal in the form of a narrow beam of radio waves such that the radiation will hit the ground
• Doppler navigation:

  • On reaching the ground the energy will be scattered & the airborne receiver will pick some up
  • On reception, the frequency shift of the reflected wave is measured and the relative velocity (Vr) is determined by
  • Vr = fdλ/2
  • Where fd is Doppler frequency and λ is transmitted signal wavelength
• Doppler navigation:

  • To determine the true aircraft velocity the angle of depression of the beam must be in the calculation
  • To ensure accuracy with the system, the aircraft would have to be flown straight and level at all times
• Doppler navigation:

  • Which is not possible, an effective method would be to transmit a second beam Aft at the same angle of depression as the forward beam
• Doppler navigation:

  • By comparing the frequency shifts of forward & backward beams any error due to pitch angle can be eliminated
  • This technique is called Janus configuration
• Doppler navigation:

  • Due to cross winds velocity measurement derived from a single forward beam would be in error so, a third beam is therefore introduced to provide a further velocity measurement
• Doppler navigation:

  • The three beams are often arranged in the shape of the Greek letter lambda (λ), two beams being radiated forward symmetrically about the centre line of the aircraft and the third pointing aft, 1800 displacement from one or other of the forward beams
• Doppler navigation:

  • Alternatively, four beams may be radiated in an ‘X’ configuration
  • Although this fourth beam is not strictly necessary, it does give the advantage of redundancy which permits automatic monitoring
• Doppler navigation:

  • The velocity measurements from each of these beams are fed to the computer
• Doppler navigation:

  • The relative bearing of each of the beams relative to the axes of the aircraft, and comparing with a heading reference such as a gyro compass, performs a mathematical calculation deriving the velocities along the track, of drift & height variation
• E.g 8-2: An aircraft using a single beam Doppler system to measure its ground speed is travelling at 300 m/s. Depression angle is 600. Transmission frequency is 10GHz. What will be the Doppler shift?

  
  

  Ans: Doppler shift = 2 x Ground speed x Freq x cos0/ speed of light

  = 2x300x10x10^9xcos60°/3x10^8

  = 10 kHz
• Parameters for Doppler navigation system:

  • The sensitivity of a Doppler radar (Hz/knot) increases with frequency
  • As frequency increases the beam width of the antenna will be narrower & also absorption, back scattering effects & rain reflections become increasing problems
• Parameters for Doppler navigation system:

  • The practical operating frequency for Doppler airborne radar equipment is
  • 8.8 GHz - 9.8 GHz (X-band)
  • 13.25 GHz – 13.4 GHz (Ku–band)
• Airborne installation:

  • An airborne Doppler navigation system is FM-CW system operating at the X-band frequency of 8800 MHz at a power of approximately 1 watt
• Airborne installation:

  • The transmitter-receiver generates the 8800 MHz transmit signal & send it to the antenna while the receiver portion detects the Doppler frequency IF signal received from the mixer through the antenna
• Airborne installation:

  • The antenna is moving, slotted–waveguide type that produces four-beam pattern necessary for the derivation of ground speed, drift & pitch angle information