Equatorial Plasma Depletions observed over Brazil - Impact on safety critical GNSS Navigation

Kurzfassung

Equatorial Spread F is a common occurrence in the equatorial ionosphere that is associated with large depletions in plasma density that often cause scintillation, interference and delays in communication signals. These events are known as plasma bubbles and result from Rayleigh-Taylor instability. The conditions for its creation arise shortly after sunset, when the lower ionosphere rapidly decays and forms steep density gradients (Yokoyama et al, 2004). A perturbation here (for exampled caused by an atmospheric gravity wave) may lead to instability growth, resulting in the formation of plasma bubbles. These depletions are common near the magnetic equator and stretch poleward along magnetic flux tubes as they evolve. In general, due to the Rayleigh Taylor instability the plasma depletions become deeper as the bubble evolves. Plasma bubbles are especially of concern for differential navigation systems based on global navigation satellite systems (GNSS) that operate on a single frequency only, such as the ground based augmentation system (GBAS) for precision approach of aircraft. Here, large ionospheric gradients, i.e. a differential delay between the aircraft and ground station, can cause undetected position errors which may exceed the alarm limits for a given phase of flight and thus generate hazardous misleading information (HMI). This is most critical for the final approach phase where the aircraft operates close to the surface with limited terrain and obstacle clearance. Particularly with the current development of the GBAS approach service type D (GAST-D), which allows for zero visibility approach and automatic landing, these threats need to be very well understood. Moreover, since the plasma depletions propagate at nearly the same speed as the aircraft – contrary to most ionospheric fronts – the threat is even more profound since transmitted corrections are not valid at the aircraft (Luo et al., 2005). In this area, Saito et al. (2009), for example, have already modelled the impact of plasma bubbles on GBAS using the NeQuick ionosphere model and a rectangular shaped plasma bubble, but no comprehensive threat model has been published. For GBAS certification in equatorial regions, however, this needs to be taken in consideration to guarantee the safety of the system. Here we present ionospheric gradients derived from ionospheric plasma bubble measurements taken during the Spread F Experiment (Fritts et al., 2009) in Brazil in 2005. The velocity, height characteristics and temporal evolution of the plasma bubble have been characterized by Haase et al (2009) in conjunction with airglow images. We confirm these characteristics using an alternative analysis method by Mayer et al. (2008). Additionally, we apply the methods used by Mayer et al. (2009) to evaluate temporal and spatial gradients in the ionosphere and characterize the potential impact on GNSS navigation during all phases of flight with and without augmentations systems such as GBAS and SBAS. For this, we perform a worst case geometry screening near the magnetic equator and identify the most critical satellite constellation. Moreover, we test the existing code carrier divergence monitoring algorithms on their functionality while the plasma bubble propagates through the line of sight to a given satellite. This experiment is performed with DLR’s MASTER GNSS simulator. For a realistic GBAS setup, we use four GBAS ground station and an aircraft approaching the station for landing. We also consider a worst case combination of airplane velocity and direction, runway direction and bubble speed as investigated by Harris et al. (2009). Moreover, since plasma bubbles are a three-dimensional phenomenon the single-layer approximation of the ionosphere breaks down in the presence of a plasma bubble. Therefore we propose to set up an effective two-dimensional threat model of plasma bubbles representing worst-case ionosphere delays and gradients. Fritts, D. C. et al. (2009), Overview and summary of the Spread F Experiment (SpreadFEx), Annales Geophysicae, Volume 27, Issue 5, 2009, pp.2141-2155, J.S. Haase, T. Dautermann, M.J. Taylor, N. Chapagain, E. Calais, and D. Pautet; Propagation of Plasma Bubbles Observed in Brazil from GPS and Airglow Data, Advances in Space Research 2009, submitted Ming Luo, Sam Pullen, Seebany Datta-Barua, Godwin Zhang, Todd Walter, and Per Enge, “LAAS Study of Slow-Moving Ionosphere Anomalies and Their Potential Impacts”, Proceedings of the Institute of Navigation, GNSS Conference, Sept. 2005 Mayer, C.; Jakowski, N.; Borries, C.; Pannowitsch, T.; Belabbas, B, 2008.Extreme Ionospheric Conditions over Europe Observed During the Last Solar Cycle, ESA Navitec 2008 Mayer, Christoph and Belabbas , Boubeker and Jakowski, Norbert and Meurer, Michael and Dunkel, Winfried (2009) Ionosphere Threat Space Model Assessment for GBAS. ION GNSS 2009 , 22.-25. Sep. 2009 , Savannah, GA, USA. Matt Harris, Tim Murphy, Putting the Standardized GBAS Ionospheric Anomaly Monitors to the Test, ON GNSS 2009 , 22.-25. Sep. 2009 , Savannah, GA, USA. Yokoyama, T., S. Fukao, and M. Yamamoto, Relationship of the onset of equatorial F region irregularities with the sunset terminator observed with the Equatorial Atmosphere Radar, Geophys. Res. Lett., 31, L24804,doi:10.1029/2004GL021529, 2004.