IDENTIFYING COLLISION AVOIDANCE RESOLUTION ADVISORIES AND ANOMALIES IN AIRCRAFT AVIONICS GLOBALLY WITH SPACE-BASED ADS-B DATA OBSERVATIONS

Aireon’s nearly complete deployment of 66 payloads on the Iridium NEXT constellation has enabled global surveillance of tens of thousands of aircraft for the first time. Using 1090 MHz Automatic Dependent Surveillance-Broadcast (ADS-B), the Aireon system receives messages broadcast from equipped aircraft. As ADS-B equipage rises and mandates are enacted in regions around the world, observations of anomalies and outliers have increased. Aireon has been closely monitoring and analyzing the data to characterize the outliers in preparation for reporting to governing bodies such as the European Aviation Safety Agency (EASA), EUROCONTROL, the Federal Aviation Administration (FAA), and the International Civil Aviation Organization (ICAO). In addition to these outliers, the inclusion of traffic collision advisory data allows safety hotspots to be outlined. Non-compliant aircraft are commonly observed in the global ADS-B data set. Examples of noncompliant aircraft include aircraft transponders that do not adhere to the Minimum Operational Performance Standards (MOPS) and the rules outlined in ICAO Annex 10. Position outliers duplicate 24-bit addresses, and invalid aircraft identification data are examples of such observations. The transponder issues are probably not intentional and aircraft operators, Air Navigation Service Providers (ANSPs), Civil Aviation Authorities (CAAs), and ICAO may be unaware of many of the non-compliant transponders due to a lack of data and coverage and reliance on radar. The presence of duplicate 24-bit addresses is a safety concern that can lead to dropped tracks in surveillance systems and missed alerts in TCAS. [1] [2] In addition to non-compliant aircraft, Aireon has had the opportunity to analyze Traffic Collision Avoidance System (TCAS) Resolution Advisories (RAs) on a global scale using the RA broadcast message included in version two of the 1090 Extended Squitter ADS-B MOPS. This data can be used to better describe collision risk scenarios and identify areas with frequent RA activity. Some countries independently monitor TCAS via Mode S [3], but Aireon can monitor of TCAS data received via ADSB messages, including oceanic regions outside of radar coverage. EUROCONTROL’s December 2017 ACAS Guide states that there is no European-wide data on the frequency of RA occurrence. [4] The data collected from Aireon may be the first global look at RA occurrences. ICAO document 9863 notes that monitoring via controller reports is typically mandated by a state, pilot reporting can be used, and data from surveillance is voluntary. [5] A 2009 report concludes that only 48% of climb/descend RAs and 20% of other RAs are reported by pilots. [3] A global monitoring system could assist in filling in the gaps and correlating preventative and corrective TCAS RA events. This paper outlines examples of non-compliant aircraft from Aireon data, methods of detecting noncompliance, and proposes the implementation of global monitoring and reporting for regulatory agencies. In addition, the paper illustrates examples of preventative and corrective TCAS RAs using Aireon data. The exact date, position, and identity data are redacted for corrective RAs. I. Resolution Advisories TCAS is a specific implementation of the ICAO Airborne Collision Avoidance System (ACAS). [6] [7] While TCAS makes use of 1030 MHz and 1090 MHz for interrogation and reply, a copy of any resulting RA information is transmitted in specific ADS-B messages by aircraft equipped with MOPS version two (DO-260B/ED-102A) transponders. The 1090ES TYPE Code 28, subtype 2 message contains the TCAS Resolution Advisory and the data format is defined per ICAO Annex 10, Vol IV, § 4.3. 8.4. 2.2. 1. [8] [9] A sample of RA broadcasts received by Aireon over multiple days is illustrated below in Figure 1. Not every point on the map represents a critical event, or even a loss of separation as the algorithms used for collision avoidance can generate unnecessary or nuisance alerts. [7] Figure 1: Sample of TCAS RAs (Preventative and Corrective) Based on ADS-B Data Observed by Aireon The higher density of RA events reported via ADS-B over United States airspace is likely due to the equipage rate of version two transponders (given the January 2020 mandate) as opposed to a reflection of total RA occurrences. [10] Therefore, the measurement of RAs with ADS-B will become more accurate as version two equipage increases. Using Aireon data from August and September 2018, a heat map illustrating the percent of version two (vs. version 0 and version 1) aircraft per tile shows a higher population of version two targets over the United States. This plot is shown in Figure 2 below. Figure 2: Density of DO-260B (version 2) Aircraft An RA event observed using Aireon ADS-B data in early 2018 is detailed below in Figure 3 and Figure 4. The two wide-body targets illustrated were flying over the ocean on the same track in opposite directions and it is unknown if the targets were within radar contact. The first target, flying at 37,700 feet with a ground speed of 520 knots, was in a climb while the second target, flying at 38,000 feet with a ground speed of 427 knots, maintained its flight level. The two targets were separated by less than nine nautical miles with a lateral closure rate of approximately 947 knots, or about 0.26 nautical miles per second as shown at 03:46:07. Figure 3: Resolution Advisory , Over Ocean, Before Corrective Action At 03:46:08, Aireon received an ADS-B RA broadcast report from the first target (in blue) in the above example. Table 1 details the contents of the RA message which indicates that the aircraft should descend. The target descended at 1665 feet per minute and the second target climbed at 2880 feet per minute in response to the RA. Table 1: Example 1 RA Broadcast Received from the First Target Subfield Bits Set [9] ARA -Only one threat -RA is corrective -Downward sense RA has been generated -RA is not increased rate -RA is not a sense reversal -RA is not altitude crossing -RA is positive RAC -Do not pass above RAT -ACAS is currently generating the RA indicated in the ARA subfield MTE -The resolution logic is processing one threat TTI -TID contains a Mode S transponder address TID -24-bit address of the second target This reaction by both aircraft resulted in separation to satisfy the instructions from the TCAS RA, shown in Figure 4. Figure 4: Resolution Advisory, Over Ocean, After Corrective Action (30 seconds after Figure 3) Encountering high-altitude RA events is assumed to be uncommon. Using terrestrial surveillance data, Olson and Olszta reported that 97.4% of RAs in the United States occur below 18,000 feet and 86% below 10,000 feet. [11] Between 28 August 2018 and 3 September 2018, Aireon observed ADS-B RA reports from 462 unique targets. Though the sample set is relatively small compared to other TCAS studies, Aireon found that 78% of RAs occur below 10,000 feet. A histogram of altitude per RA is shown in Figure 5. Figure 5: RA Distribution by Altitude A more common RA example is one occurring in the approach and departure areas in the airspace surrounding a busy airport. Such an event from summer 2018 was located and is illustrated in Figure 6 and Figure 7. The two targets in were both equipped with ADS-B version two, and each reported corrective RA reports via ADS-B. Figure 6: Resolution Advisory, on Approach Figure 7: Resolution Advisory, on Approach (21 seconds after Figure 6) II. Mode S / 24-Bit ICAO Addresses Over a five-day period, from 28 August 2018 to 1 September 2018, Aireon observed 47,684 individual 24-bit addresses. A small subset of the addresses are duplicates, a condition when the same address is broadcast by two different aircraft. EUROCONTROL states that the presence of duplicate 24-bit addresses can jeopardize the safety of aircraft and must be avoided at all times. [1] These duplicates not only affect ADS-B, but also affect radar and TCAS and can cause missed TCAS RAs. [2] Common duplicate addresses include 0x000001, 0x123456, and 0xA00000. A listing of unique aircraft identifications (ACIDs) per duplicate is detailed below in Table 2. Table 2: Unique ACIDs for Common Duplicate Addresses Date Range Common Duplicate 24-bit ICAO Address Unique ACIDs Observed 2018-08-28 to 2018-09-01 0x000001 32 0x123456 23