Updated Aviation Assessment of Interference in the L5/E5A Bands from Distance Measuring Equipment

RTCA, Inc.’s Special Committee 159 is updating its assessment of interference to GNSS in the 1164 – 1215 MHz band (L5/E5a) to support interference requirements for next-generation airborne GNSS equipment standards. These standards are now in development by RTCA and the European Organization for Civil Aviation Equipment (EUROCAE). The previous interference assessment was published in 2004 within RTCA document DO-292. One of the primary interference concerns for L5/E5a signal processing is from Distance Measuring Equipment (DME)/Tactical Air Navigation (TACAN) ground-based beacons. The assessment showed that at certain geographic locations an aviation user could observe multiple DMEs and experience aggregate pulsed interference with duty cycles in excess of 50%. This led to suggestions for aviation receiver implementation details such as more stringent radiofrequency (RF) filtering and pulse blanking. The purpose of this paper is to update this portion of the assessment with additional DME/TACAN modeling and actual flight and ground RF data collections. The GNSS RFI Environment Evaluation Tool (GREET) developed at MITRE is used to assess expected DME/TACAN interference at various flight altitudes across the conterminous United States. This tool has the capability to emulate deployed and postulated DME installations. It also simulates a representative aviation receiver and antenna design to assess the pulsed interference impacts. The paper evaluates the outputs of this tool against RF data collected during recent flight tests conducted on FAA flight inspection and test aircraft. These aircraft have been modified with top-mounted GNSS antennas with performance characteristics consistent with requirements expected in next generation aviation standards. The RF signals from these antennas were recorded digitally for replay in a laboratory environment and for RF spectral analysis to identify individual DMEs. Replay of these data sets was conducted with a NovAtel WAAS G-III reference receiver, which has the stringent RF filtering and pulse blanking mentioned above. The G-III receiver has a 24 MHz processing bandwidth, matching the widest receiver bandwidth currently envisioned for RTCA’s new dual frequency equipment standard. The blanking implemented in this receiver is configurable, allowing different threshold settings as well as no blanking to be evaluated. The testing with the G-III receiver also included a digitally filtered version of the recorded data to emulate signal reception with a 12 MHz processing bandwidth. This narrower bandwidth should reduce DME/TACAN interference and some receiver manufacturers have indicated an interest in the new standard supporting such a configuration. Lastly, the paper characterizes some interesting pulsed interference observed by the WAAS ground segment in the L2 and L5 bands. The WAAS network uses the G-III receiver mentioned above and many reference stations are located adjacent to DME/TACAN ground beacons and FPS-117 surveillance radars. The pulsed interference from space-based systems is also observable in the L2 band and is characterized and presented for general awareness.