Networked UAV Detection and Alerting of Ionospheric Anomalies within LADGNSS Navigation Framework
Sam Pullen, Stanford University; Jiyun Lee, Gihun Nam, and Andrew K. Sun, Korean Advanced Institute of Science and Technology (KAIST)
Date/Time: Thursday, Sep. 22, 11:48 a.m.
In previous research, the use of Local Area Differential GNSS (LADGNSS) to provide navigation and guidance to nearby Unmanned Aerial Vehicles (UAVs) has been explored (e.g., see [1,2]). LADGNSS ground stations would provide integrity based on the Ground-based Augmentation System (GBAS) architecture but with low-cost (and likely portable) equipment with greatly reduced siting constraints. The objective is to provide both decimeter-level 95% accuracy and position-domain protection levels of several meters to a probability of 1 – 10-7 per operation while mitigating the same threats as GBAS. The LADGNSS ground system and its datalink would be coupled with the command and guidance system that directs each UAV and coordinates UAV operations within 25 – 50 km of each installation.
Recent work on LADGNSS for UAVs has focused on the threat of anomalous ionospheric spatial decorrelation, which is also of primary concern to GBAS. In , modifications to monitors proposed for future dual-frequency, multi-constellation (DFMC) GBAS are evaluated. Its results show that worst-case undetected errors for LADGNSS can be limited to below 5 meters. However, monitors developed for GBAS are limited by the one-way nature of the GBAS VHF Data Broadcast (VDB) from ground station to aircraft. In contrast, LADGNSS supports two-way transmissions to facilitate UAV guidance and status reports. This feature will also support navigation integrity by relaying relevant UAV monitor statistics and protection levels back to the ground station.
This paper examines the use of UAV observations and UAV-to-ground-station transmissions as a means of “clearing the sky” of potential ionospheric threats. This would allow the LADGNSS ground system to limit the applicability of its ionospheric threat model to areas of the sky that had not recently been observed and cleared. UAVs in “fully cleared” areas would not need to consider unobserved anomalous ionospheric gradients in their protection levels. UAVs in “partially cleared” areas where anomalous gradients cannot be completely ruled out would still consider them but with smaller maximum gradients than would apply without observations.
The starting point in this analysis is the spatial representation of ionospheric spatial anomalies in GBAS as fronts where ionospheric delay suddenly changes that expand linearly over a region of coverage . Because this linear expansion model covers a great deal of ground, it is relatively easy to reject a large number of potential anomaly fronts by receiving monitor statistics from a single moving UAV. For this reason, in sensitivity studies, potential gradient fronts are assumed to curve away from the paths of UAV flights by a certain extent. The duration of assurance that a single UAV observation of nominal ionosphere can be relied on to verify the absence of a threat is another variable whose sensitivity is evaluated.
The primary analysis procedure is a simulation that combines different patterns of single and multiple UAV flights with anomalous ionospheric front patterns passing over from different directions, at different propagation velocities, and at different occurrence rates. The severe westward-moving ionospheric gradient structure observed over the U.S. Midwest on 20 November 2003 is one example, but conditions with multiple structures moving across the LADGNSS coverage area over the course of several hours (as can occur in low-latitude regions due to the effects of Equatorial Plasma Bubbles, or EPBs ) are also simulated. Sparse and dense UAV traffic patterns are simulated with different degrees of populating the azimuths around the LADGNSS ground station location.
The results of these simulations will show, under various conditions, the degree to which normal UAV traffic and monitor statistics transmitted back to a LADGNSS ground station can preclude the possibility of severely anomalous ionospheric gradients such that UAV protection levels need not be inflated to defend against them (in the manner of ). The benefits that these lower UAV protection levels provide to UAV operations at low altitudes are also discussed.
 S. Pullen, P. Enge, J. Lee, “Local-Area Differential GNSS Architectures Optimized to Support Unmanned Aerial Vehicles,” Proceedings of ION ITM 2013, San Diego, CA, Jan. 2013. http://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Pullen_IONITM_2013_ LADGNSSforUAVNetworksITM2013final.pdf
 D. Kim, J. Lee, M. Kim, J. Lee, S. Pullen, “High-Integrity and Low-Cost Local-Area Differential GNSS Prototype for UAV Applications,” Proceedings of ION GNSS+ 2017, Portland, OR, Sept. 2017. https://www.ion.org/publications/abstract.cfm?articleID=15110
 G. Nam, D. Min, N. Kim, J. Lee, S. Pullen, “Optimal Smoothing and Monitor Strategies for Dual-Frequency Dual-Constellation (DFDC) LADGNSS under Anomalous Ionospheric Conditions,” Proceedings of ION ITM 2022, Long Beach, CA, Jan. 2022. https://www.ion.org/publications/abstract.cfm?articleID=18166
 S. Datta-Barua, J. Lee, S. Pullen, M. Luo, A. Ene, D. Qiu, G. Zhang, P. Enge, “Ionospheric threat parameterization for local area global-positioning-system-based aircraft landing systems,” AIAA Journal of Aircraft, 47(4), pp. 1141–1151, 2010. https://doi.org/10.2514/1.46719
 M. Yoon, J. Lee, S. Pullen, J. Gillespie, et al., “Equatorial Plasma Bubble Threat Parameterization to Support GBAS Operations in the Brazilian Region,” Navigation, 64(3), pp. 309-321, 2017. https://doi.org/10.1002/navi.203
 J. Lee, J. Seo, Y.S. Park, S. Pullen, P. Enge, “Ionospheric threat mitigation by geometry screening in ground-based augmentation systems,” AIAA Journal of Aircraft, 48(4), pp. 1422–1433, 2011. https://doi.org/10.2514/1.C031309
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