GBAS Ionospheric Threat Analysis using DLR’s Hardware Signal Simulator

Kurzfassung

The verification and certification of Global Navigation Satellite System (GNSS) integrity is based on the analysis of extremely rare events which can lead to large positioning errors. These events are generally only dealt with on a theoretical basis. The most important of these effects are large ionospheric delays for absolute positioning, strong ionospheric gradients for relative positioning, signal deformation, satellite and receiver clock irregularities and strong antenna multipath. Anomalies in these domains occur with such a low repetition rate that field testing alone does not usually include them during the testing period. However, the ability to simulate these events in RF measurements would allow hardware to be tested against these threats, thereby greatly increasing our confidence in the robustness of the navigation system. This paper describes our implementation of comprehensive ionospheric threat generation within the German Aerospace Center’s (DLR) Master GNSS Simulator that can be used for comprehensive receiver testing in hardware. Augmentation systems can correct GNSS navigation errors induced by the ionosphere on a local or regional scale. In these systems, normal ionospheric behavior has a limited impact on the position error. In particular, the confidence interval of the user position is fully acceptable for CAT I precision approach supported by Ground Based Augmentation Systems (GBAS) or LPV precision approach supported by Satellite Based Augment Systems (SBAS). Unfortunately, the ionospheric medium is sometimes subject to unpredictable perturbations due to the temporal and spatial variability of the ionospheric plasma. Severe anomalous ionospheric behavior has been discovered in the Conterminous U.S. in April of 2000 and October – November 2003. While severe anomalies occur rarely (per haps 2 – 4 times in 10 years) in mid-latitudes, they can be a serious threat to GNSS integrity. Existing GBAS architectures cannot fully mitigate these effects by monitoring; thus mitigation includes real-time exclusion of potentially unsafe geometries (“geometry screening”) in addition to monitoring. In order to use the proposed mitigation algorithms in different geographical regions, anomalous ionosphere threat models must established for the relevant regions. DLR has defined a model for Germany and nearby regions. These models are implemented in the MASTER GNSS simulator and are used for testing and verification. The Multi-output Advanced Signal Test Environment for Receivers (MASTER) is a unique and powerful hardware simulation tool for testing and quality assessment of global navigation satellite system receivers. At the heart of the system are two modified Spirent GSS7790 Galileo and GPS constellation simulators which provide all GPS and Galileo satellite signals at RF, IFand digital baseband level. The user has full control over the satellite orbit definitions and over all major effects influencing the quality of real navigation satellite signals: orbit and clock errors, ionosphere, troposphere, multipath, and RF interference. For the verification of differential systems consisting of ground and airborne stations individual but coherent and consistent signals can be generated for both entities. In order to test receivers and GBAS ground system architectures with respect to robustness under the influence of ionospheric irregularities, we developed a comprehensive set of scenarios based upon the threat models mentioned above. In these scenarios, ionospheric “fronts” travel across the area in which reference and user receiver ionospheric pierce points are located. These scenarios include “typical” anomalies with pre-selected parameters as well as “worst-case” anomalies with parameters (front gradient, speed, width, and direction) which when cause the maximum position error for a given user satellite geometry. In addition to the standard ionospheric front model that applies in mid-latitude regions, we can simulate several types of plasma depletion structures (plasma bubbles) that propagate through the ionosphere. Plasma bubbles result from the gravitational Rayleigh-Taylor instability of the interaction between the charged plasma and the Earth’s magnetic field. At the rising and falling edge of the depletion, strong scintillation can cause GNSS receivers to temporarily lose lock on the satellite and thus degrade user satellite geometries such that approach services may become unavailable. Using the MASTER facility, we will test a simplified GBAS hardware system under typical and worst-case ionospheric conditions. We will compare the results of these tests to the errors predicted for these scenarios by software simulations and the resulting broadcast parameter inflation factors needed for geometry screening. We will present the resulting performance in terms of ionosphere-induced errors, hardware impacts, and the resulting availability of CAT I and future CAT III GBAS.