Avionics Chpt 9

• Secondary radar works on the principle of identification. When a target receives a radar signal, it would broadcast its signal, which could be received by the originating radar receiver.
• By coding this secondary signal, the aircraft equipment could indicate to the operator of the originating radar its identification.
• Distance Measuring Equipment (DME):

  • It automatically measures the distance in nautical miles (nm) between the aircraft & a selected ground station
  • The airborne DME interrogation facility transmits coded interrogation signals (pulse pairs) to the ground station
• Distance Measuring Equipment (DME):

  • The ground station receives the interrogation & returns a coded reply signal (pulse pairs) for each interrogation after a calibrated delay
• Distance Measuring Equipment (DME):

  • Single antenna is used for both transmission & reception
  • The airborne DME interrogator then computes the slant range distance To/From the ground station by using the equation below:
  • D = Tir - 50 μs/12.36
• Where,

  • D = Slant range distance in nm To/From the ground station
  • Tir = Time in ms between transmission of interrogation pulse pair & reception of corresponding replay pulse pair
  • 50 μs = Calibrated ground station delay
  • 12.36 = Time in ms for RF energy to travel 1 nm & return
• Distance Measuring Equipment (DME):

  • The airborne interrogator transmits pulses on one of 126 frequencies, spaced 1 MHz apart in the 1025 to 1150 MHz band
  • The pulses are in pairs, 12 μs or 36 μs apart, each pulse lasting 3.5 μs, with a pulse pair repetition rate ranging between 5 pulses pairs per sec to an average of 30 per sec
• Distance Measuring Equipment (DME):

  • Two modes of operation are used in DME
  • A high rate of interrogation is used for searching the selected ground station in the search mode
  • When a sufficient number of synchronous replies are received by the interrogator, it is said to be lock-on
• Distance Measuring Equipment (DME):

  • Track mode comes into operation and the interrogation rate greatly reduced to avoid overloading the ground station
  • The distance is derived from transmit to receive time in the airborne unit and displayed in the cockpit
• Distance Measuring Equipment (DME):

  • A military navigation aid called TACAN (Tactical air Navigation) uses a DME station to provide its range facility
  • VORTAC station is a combination of TACAN & VOR facilities transmit bearing information & identification signals
• Distance Measuring Equipment (DME):

  • VOR/DME ground station is a VORTAC station without the bearing information available from TACAN
  • An ILS/DME ground station provides facilities for positive distance to touchdown
• Distance Measuring Equipment (DME):

  • Airborne tuning involves only the selection of a VOR or ILS frequency
  • The associated DME frequencies are automatically selected DME Stations on Aviation Charts
• VHF NAV Fq (MHz) | DME Chn | DME Int (MHz) | DME Re (MHz)

  108.00 | 17X | 1041 | 978

  108.05 | 17Y | 1041 | 1104

  108.10 (ILS) | 18X | 1042 | 979

  108.15 (ILS) | 18Y | 1042 | 1105

  108.20 | 19X | 1043 | 980

  108.25 | 19Y | 1043 | 1106

  108.30 (ILS) | 10X | 1044 | 981

  108.35 (ILS) | 20Y | 1044 | 1107

  108.40 | 21X | 1045 | 982

  108.45 | 21Y | 1045 | 1108
• E.g 9-1: Find the slant range distance to a DME station from an aircraft, if the time interval between interrogation &reception is 100 μs

  Ans: D = 100 - 50/ 12.36 = 4.04 nm
• Airborne Installation:

  • The airborne DME consists of a Receiver-Transmitter (RT), a control unit, a distance indicator & an antenna
• Receiver-transmitter (RT):

  • The transmitter section of the RT unit contains all the necessary circuits to generate, amplify & transmit the interrogating pulse pairs
  • The receiver section contains the circuits required to receive, amplify, & decode the received reply pulses
  • Computing circuits then determine the validity of the reply pulses & calculate the distance
• Control:

  • Provides the necessary controls & switching circuits for the airborne DME-RT
  • Provide the frequency selection for navigation
• Indicator:

  • The distance indicator displays the aircraft distance in nm from the ground station
  • The distance indicator may also be a part of another indicator such as a Horizontal Situation Indicator (HSI)
• Antenna:

  • The antenna is a single L-band for both transmit & receive with an Omni-directional radiation pattern
• Working principle of DME:

  • Distance measurement begins with the selection of a VHF navigational frequency on the frequency control
  • The VHF frequency is a coded serial data word that is applied to the frequency control unit of the airborne DME equipment
• Working principle of DME:

  • The channel selected can be any one of 252 channels. Once the frequency is selected, RF signal is generated to produce a transmitter dive signal
  • A tuning voltage is also generated to tune the preselector to the proper receive frequency
• Working principle of DME:

  • The airborne DME interrogation begins with a pair of RF pulses being transmitted
  • The length of the interrogation period depends upon the airborne DME mode of operation i.e. search or track
• Working principle of DME:

  • Because of the greater number of pulse pairs per second in search (90 pp/s), the interrogation period is shorter than when RT is in the track mode
• • Working principle of DME:
  • Whenever a pulse pair is transmitted, another pulse pair is sent to the transponder, which will be protected from damage by airborne DME
• Working principle of DME:

  • After the interrogation pulse pair has been transmitted, the receiver portion of the airborne DME-RT becomes active and looks for reply pulses
• Working principle of DME:

  • For the transmitted pulse pairs of another aircraft not to be mistaken for reply pulses, the transmitted pulse rate of the airborne DME is varied randomly
  • In any case, no two airborne DME’s will be transmitting at the same rate
• Working principle of DME:

  • When the reply pulses occur at regular intervals, DME will lock on & decrease interrogation state of the DME to the track mode
• Working principle of DME:

  • The signal from the ground station is routed to preselector where it is mixed with oscillator frequency to produce a 63 MHz IF signal
• Working principle of DME:

  • The IF signal is amplified and decoded to produce video signal and applied to the range computer
  • Video signal contains 1350-pp/s station identification signal (IDENT) which is applied to the aircraft audio system for the pilot to track the station selected
• Working principle of DME:

  • In the range computer, the decoded video signal is applied to circuits that measure the elapsed time from interrogation to reception & solve range equation
  • The computed distance is applied to external indicator for display
• Working principle of DME:

  • The DME system always display valid information to the pilot, any failure of the system is monitored by self-checking circuits
  • Under normal circumstances, a DME station can transmit ranges to 100 aircrafts locked on to it
• System interface:

  • The interrogator (RT) is tuned by frequency data from the VOR/DME control unit
  • If desired, Flight Management Computer (FMC) may provide tuning data, in which case, the control unit frequency selection will be ignored
• System interface:

  • Distance data output is given to VOR/DME-RMI for readout display & FMCs for aircraft position updating purposes
  • Pilot can monitor the Morse code identity of the tuned DME ground station via the audio-integrating system
• ATC:

  • Classified as secondary radar system
  • The primary surveillance (watching) radar used bythe ATC ground station, provides the ground station operator with a symbol on his surveillance radar scope for every aircraft in his area
• ATC:

  • The primary surveillance radar is a reflection-type radar system not requiring any response from the aircraft
• ATC:

  • The primary & secondary surveillance radar antennas are on the same rotating mounting, & therefore both always look in the same direction at the same time
• ATC:

  • The secondary surveillance radar system uses what is called an ATC transponder in the aircraft
  • The main function of an ATC transponder is to response to an interrogation from the ground station secondary surveillance radar system
• ATC:

  • The aircraft reply can include a special code which identifies that particular airplane on the display scope
  • If the pilot receives instructions from the ground station, he presses ident button on the control panel
• ATC:

  • It will result in the display on the radar scope changing so that the ground station operator can be positive of particular aircraft location on the radar scope
• ATC:

  • The altitude and identification of the airplane are also transmitted to the ground station by on-board transponder