N25A-T019 TITLE: Passive Automatic Dependent Surveillance-Broadcast Validation to Support Collision Avoidance During Missions Requiring Emissions Control

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Sensing and Cyber

OBJECTIVE: Develop a passive means to validate Automatic Dependent Surveillance-Broadcast (ADS-B) tracks that is suitable for aircraft operating with no radio frequency emissions.

DESCRIPTION: The Navy desires a system that validates ADS-B independently of any active sensors. The use case for this STTR topic is a large U.S. military Unmanned Aircraft System (UAS) engaged in operations not conducted under International Civil Aviation Organization (ICAO) flight procedures. Procedures exist for military aircraft operations in international airspace consistent with the Convention on International Civil Aviation [Ref 1]. When following ICAO flight procedures is not practical and compatible with the mission, U.S. military aircraft must operate with due regard for the safety of all other aircraft including civilian aircraft consistent with DoDI 4540.01 [Ref 2]. Such a case might arise, for example, when the mission necessitates that radio frequency emissions be minimized or eliminated altogether.

The UAS commander must utilize all available resources and information in assessing an acceptable level of risk before conducting such operations [Ref 2]. At present, there is no technology by which a UAS might passively validate ADS-B tracks broadcast by other aircraft. The goal of this STTR topic is to create that technology and make new information available.

ADS-B is an aviation surveillance technology in which an aircraft periodically broadcasts its position and other related data, enabling it to be tracked. It consists of two distinct functions. "ADS-B Out" and "ADS-B In". "ADS-B Out" is active; it periodically broadcasts track information like identity, position, and velocity. "ADS-B In" is passive; it receives and processes "ADS-B Out" information transmitted by other aircraft [Ref 3]. Civilian flights in oceanic airspace must be conducted under Instrument Flight Rule procedures when operating at or above Flight Level 055 within the New York, Oakland, and Anchorage Oceanic Flight Information Regions. ADS-B is required under these rules. Oakland Center covers 18.7 million square miles of the Pacific Ocean, roughly 9.5% of the Earth's total surface area, making this the largest Area Control Center in the world by controlled surface area [Ref 4].

ADS-B tracks received by the UAS "ADS-B In" receiver inform collision avoidance maneuvers. Independent validation of the received data is necessary to prevent the UAS from responding to spoofed or erroneous tracks. In a permissive environment, Traffic Alert and Collision Avoidance System (TCAS) and radar could be acceptable means for validating ADS-B. Both of those are active so neither work when UAS operates under emission control. UAS with SIGNIT could conceivably validate the track’s relative bearing against the direction of arrival of the "ADS-B Out" broadcast but not all UAS have such systems. Modifying the ADS-B receiver to measure angle of arrival is cost prohibitive.

Comparing observed changes in received signal strength over time to the expected changes corresponding to changing distances between aircraft seems a promising approach to passively validating "ADS-B In." This is a variation of the approach in Reference 5 that estimates distance to the other aircraft but needs an initial calibration. The Navy desires to determine whether the change in received signal strength correlates to the propagation path change over time, so the initial calibration might not be needed. Reference 6 shows that the propagation path loss agrees with free-space propagation. That experimental effort included a method to remove the effect of the alternate transmission from the two transponder antennas on the aircraft of interest. Reference 7 demonstrates a means to remove co-channel interference that degrades the signal strength measurement accuracy.

PHASE I: Plan and execute a series of experiments to passively validate "ADS-B In" by correlating changes in reported separation with actual changes in received signal strength. Take received signal strength’s natural variability into consideration. While these experiments can be conducted with the receiver located on the ground consideration should be given to how the results may differ when implemented on aircraft.

PHASE II: Design and build a system that implements the techniques of Phase I and complies with flight critical software safety requirements. Conduct airborne experiments that demonstrate performance of the approach sufficient to certify that the system meets the requirement for independent validation.

PHASE III DUAL USE APPLICATIONS: Mature the capability sufficiently to transition as a software upgrade to airborne ADS-B system processing hardware. The technology is directly applicable to use by civil aviation particularly low-SWaP UAS incapable of supporting an active surveillance capability.

REFERENCES:

1. "Convention on International Civil Aviation." https://www.icao.int/publications/pages/doc7300.aspx

2. "DoDI 4540.01, 02 June 2015, Incorporating Change 1 22 May 2017

3. "Automatic Dependent Surveillance – Broadcast." Wikipedia. https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance%E2%80%93Broadcast

4. "ENR 7. Oceanic Operations. ENR 7.1. General Procedures." FAA Air Traffic Publications. https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_7.1.html

5. Naganawa, Junichi and Miyazaki, Hiromi. "Aircraft–Receiver Distance Estimation Using ADS-B Signal Strength for Position Verification Application." 2021 IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications (APWC), Honolulu, HI, USA, 2021, pp. 178-183. doi: 10.1109/APWC52648.2021.9539711

6. Naganawa, Junichi et al. "Evaluating Path Loss by Extended Squitter Signals for Aeronautical Surveillance." IEEE Antennas and Wireless Propagation Letters, vol. 16, 2017, pp. 1353-1356. doi: 10.1109/LAWP.2016.2635157

7. Naganawa, Junichi et al. "A Method for Accurate ADS-B Signal Strength Measurement Under Co-Channel Interference." 2018 Asia-Pacific Microwave Conference (APMC), Kyoto, Japan, 2018, pp. 354-356. doi: 10.23919/APMC.2018.8617413

KEYWORDS: Airspace access; Automatic Dependent Surveillance-Broadcast; ADS-B; emissions control; collision avoidance, safe separation; unmanned aerial systems; UAS

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