1. VDL Mode 3 Cosite Interference Testing Summary

2. 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

3. Proposed Emissions Limits in GPS Band from VHF Transceiver
Current DO-271A CW Limit: -60 dBM
Proposed CW Limit: –70 dBM
Current DO-271A Broadband Limit: Not Specified
Draft ED-92A Broadband Limit: -60 dBM over a 100 kHz bandwidth
Proposed Broadband Limit: -68 dBM over a 100 kHz bandwidth

4. 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

5. Proposed Emissions Limits in DME Band from 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 MHz, not specified above 1000 MHz
Proposed Broadband Limit: -60 dBM over a 100 kHz bandwidth, 960 MHz – 1215 MHz

6. 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 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

7. 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

8. Maximum Receive Distance vs
Maximum Receive Distance vs. Frequency Separation Boeing Limited – Do Not Distribute Outside FAA

9. 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

10. 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

11. 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