VDL Mode 3 Cosite Interference Testing Summary

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VDL Mode 3 Cosite Interference Testing Summary
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GPS Emissions

• DO-271A MOPS Insufficient to Ensure
  Non-Interference to GPS
• Existing Mode 2 Transceiver Emissions Measured
  Very Small Margin of Safety With Some
  Transceivers
• Boeing Recommends Revising DO-271A to Reduce
  Allowed VHF Emissions in GPS Band

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Proposed Emissions Limits in GPS Bandfrom VHF
Transceiver

• Current DO-271A CW Limit -60 dBM
• Proposed CW Limit 70 dBM
• Current DO-271A Broadband LimitNot Specified
• Draft ED-92A Broadband Limit -60 dBM over a 100
  kHz bandwidth
• Proposed Broadband Limit-68 dBM over a 100 kHz
  bandwidth

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DME Emissions

• DO-271A MOPS Insufficient to Ensure
  Non-Interference to DME
• Existing Mode 2 Transceiver Emissions Measured
  Very Small Margin of Safety With Some
  Transceivers
• Boeing Recommends Revising DO-271A to Reduce
  Allowed VHF Emissions in DME Band

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Proposed Emissions Limits in DME Bandfrom VHF
Transceiver

• Current DO-271A CW Limit -16 dBM
• Proposed CW Limit 60 dBM
• Current DO-271A Broadband Limit-36 dBM over a
  100 kHz bandwidth,960 MHz - 1000 MHz, not
  specified above1000 MHz
• Proposed Broadband Limit-60 dBM over a 100 kHz
  bandwidth,960 MHz 1215 MHz

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Comparison of GPS and DME Emissions, Mode 2 and
Mode 3

• MOPS for Mode 2 and Mode 3 have equivalent
  emissions limits Mode 3 transmitting on a
  particular frequency is expected to have
  equivalent interference to Mode 2 on the same
  frequency
• Mode 3 is expected to operate anywhere in the
  comm band, whereas Mode 2 is currently operating
  only on 136.975 MHz
• Harmonics of the existing Mode 2 frequency do not
  fall in the GPS receive band, but Mode 3 is
  expected to use frequencies with harmonics in the
  GPS receive band

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VHF to VHF Interference

• Analysis, testing indicate insufficient isolation
  exists between VHF antennas to preclude
  interference
• Receive distance is reduced when another VHF
  radio is transmitting on the same airplane
• Receive performance degradation depends on RF
  circuitry in radio and frequency separation
• Squelch breaks occur on adjacent channels

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Maximum Receive Distance vs. Frequency
SeparationBoeing Limited Do Not Distribute
Outside FAA
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Other Considerations in Evaluation of VHF to VHF
Interference

• Flight crew coordination in traditional DSB-AM,
  interference occurs as a result of keying
  microphone other pilot can request a halt to
  transmissions if interference is noticed
• Relatively few digital (automated) communication
  frequencies in current environment automated
  digital systems are unaware of interference and
  the flight crews do not have direct control over
  when the interference occurs

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Other Considerations in Evaluation of VHF to VHF
Interference

• Difficult to recognize degradation in receive
  performance flight crew may not hear ATC
  clearance due to simultaneous transmission to a
  company frequency or ACARS datalink transmission
  on board the airplane ATC standard practice of
  repeating the clearance may result in no
  awareness by the flight crew that ATC clearance
  was blocked

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Mode 3 Polling Response

• Two manufacturers of transceivers tested using
  800 kHz channel spacing and antenna isolation
  equivalent to two antennas on same side of
  aircraft
• No interference noted from one manufacturer, but
  noticeable interference sounding like pops or
  clicks were heard when testing the other
  manufacturer
• Speech was intelligible in presence of
  interference