Session B1: Atmospheric Effects, GNSS Remote Sensing, and Scientific Applications

Impact of Ionospheric Disturbances on NIC and NACp Degradation in ADS-B Messages
Toru Takahashi, Electronic Navigation Research Institute, National Institute of Maritime, Port and Aviation Technology; Anurak Pongpeaw, School of Engineering, King Mongkut’s Institute of Technology Ladkrabang; Susumu Saito, Tadashi Koga, Electronic Navigation Research Institute, National Institute of Maritime, Port and Aviation Technology; Pornchai Supnithi, and Jirapoom Budtho, School of Engineering, King Mongkut’s Institute of Technology Ladkrabang
Location: Beacon B

The Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance technology primarily used by aircraft and airport vehicles. It broadcasts messages that include position data derived from GNSS satellite positioning, barometric altitude, aircraft speed, and identity information. The messages also provide integrity and accuracy information, referred to as the Navigation Integrity Category (NIC) and Navigation Accuracy Category for Position (NACp). Degradation in NIC and NACp is sometimes observed, and it has been utilized to detect radio frequency interference (RFI), such as jamming and spoofing. While RFI is considered the major cause of this degradation, ionospheric scintillation, which is one of the ionospheric disturbances, could also degrade NIC/NACp. Therefore, to identify the cause of NIC and NACp degradation, it is necessary to distinguish between those caused by RFI or ionospheric disturbances.
Ionospheric scintillation is considered as a general threat to GNSS-based navigation systems. The International Civil Aviation Organization (ICAO) started the Space Weather Information Service in which advisories are issued when some impacts by space weather on aircraft operations are foreseen. The space weather advisories includes those for GNSS when ionospheric scintillations are expected in a certain region in a certain period. However, the relationship between the GNSS scintillation advisory and aircraft navigation performance in various flight phases has not been well investigated.
Therefore, this study aims to clarify the impact of ionospheric disturbances on the integrity and accuracy of aircraft positioning by analyzing ionospheric observation data during periods when NIC and NACp degradation in ADS-B was observed.
Southern Japan is located in the low geomagnetic latitude region, where amplitude scintillations associated with equatorial plasma bubbles are often observed. The Electronic Navigation Research Institute (ENRI) has installed five sets of GNSS receivers and an all-sky camera on Ishigaki Island to monitor ionospheric disturbances. The GNSS receivers used are Septentrio Pola5S, which are also utilized for the Ground Based Augmentation System (GBAS) at New Ishigaki Airport (24.4 deg. N, 124.2 deg. E), which is the southernmost airport with regular flights in Japan. The all-sky camera can capture ionospheric disturbances, such as plasma bubbles. The ADS-B receiver has also been installed at Ishigaki Island and received its message within almost 150 NM.
The ADS-B observation on Ishigaki Island has been operational since 2023. We observed that plasma bubbles and degradations in NIC and NACp values occurred simultaneously on November 30, 2023, and March 16, 2024. We calculated the Ionospheric Pierce Points (IPP) of GPS satellites observed by aircraft showing degraded NIC and NACp values. Plasma bubbles captured by the all-sky camera were projected onto the map. While amplitude scintillation is typically generated inside or at the edge of plasma bubbles, no IPP was located in those areas. This suggests that the NIC and NACp degradations did not appear to be related to plasma bubbles. Additionally, plasma bubble occurrences were more frequent than NIC and NACp degradations, suggesting scintillation did not cause significant changes in those values because of a wide range of those. We concluded that most of the observed NIC and NACp degradations were not caused by ionospheric irregularities. Instead, radio frequency interference (RFI) is likely one of the causes of these degradations. In this presentation, we will provide a detailed analysis of these observations.