Factual Information
On 21 October 2020, between 01:00 and 04:00 UTC, all aircraft operating within 10 km of the UDA reporting point on airways Y520, A310, and A575 in the Central sector of the Regional Air Traffic Control experienced loss of ADS-B symbols. The senior controller notified the Communications, Navigation, and Surveillance (CNS) shift engineer via telephone. The engineer checked the Ulaanbaatar, Khentii, Sainshand, and Ikh-Ulgiy ADS-B ground stations and reported them to be operating normally. The controller then asked an aircraft that had lost ADS-B detection, and the aircraft reported a loss of GPS signal. At that time, ADS-B detection for aircraft in other areas was normal. Radar was working normally, and no difficulties in air traffic control were reported.
ADS-B System
ADS-B (Automatic Dependent Surveillance–Broadcast) is defined by ICAO as a surveillance system that automatically provides aircraft address, four-dimensional position, and other data from on-board navigation systems via data link. The system relies on GNSS signals for position information and broadcasts continuously to ground stations.
Implementation
The Director of the Civil Aviation Authority approved the ADS-B implementation plan on 3 April 2014. The CAA purchased five single-channel ADS-B ground stations from INDRA (Spain) in 2013, five dual-channel stations in 2014, and three full-configuration stations in 2016. These were installed at 12 locations across Mongolia. Additionally, contracts were signed for full dual-channel stations from Becker Aero (Germany), with initial units tested. According to AIP SUP 01/18, ADS-B was used for situational awareness from 18 March 2016, for ATC surveillance without requiring aircraft equipment from 21 June 2018, and was to become mandatory for ATC surveillance from 17 June 2021.
Maintenance
Maintenance of ADS-B ground stations followed the Technical Operation Instruction (TOI). In 2020, a total of 12 faults occurred: two total system failures (Khovd and Dalanzadgad) and nine cases of reduced reliability. The causes included six component faults, two software issues, one unknown cause, and two external factors (optic cable company).
Incident Details
Radar ATM recordings from 01:00 to 04:20 UTC on 21 October 2020 showed loss of ADS-B for specific aircraft. Flight CCA932 (B77W, type H) on airway A308, 26 km from TANAN at flight level FL1070, lost ADS-B at 01:16:30 UTC, restored at 01:17:36 UTC, lost again at 01:20:58 UTC, and restored at 01:21:37 UTC. The recording also indicated another aircraft on airway Y520, 3 km from SERNA, but further details are not available in the source.