| Author: | Li, Wang |
| Title: | Ionospheric monitoring for dual-frequency multi-constellation Ground Based Augmentation System (GBAS) |
| Advisors: | Jiang, Yiping (AAE) Wen, Chih-yung (AAE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Global Positioning System Aids to air navigation Ionosphere Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Aeronautical and Aviation Engineering |
| Pages: | xxiii, 246 pages : color illustrations |
| Language: | English |
| Abstract: | For navigation in safety-critical application such as aviation, stringent performance regarding accuracy, integrity, availability, and continuity are required. The standalone Global Navigation Satellite System (GNSS) is not sufficient to meet these requirements during precision approach of the aircraft. Therefore, the Ground Based Augmentation System (GBAS) is designed to augment GNSS by providing differential corrections and integrity information via Very High Frequency (VHF) radio data link. Common errors including satellite errors and atmospheric errors between GBAS ground system and aircraft are mitigated by the differential corrections. However, due to the spatial decorrelations (gradients) of the ionosphere, residual ionosphere errors remains after the correction and need to be considered in the GBAS design to ensure the protection of the aircraft user. Under nominal conditions, the residual ionosphere errors are taken into account into the protection levels which bound the true position error. The standard deviation of vertical ionospheric gradients that bounds ionospheric spatial gradients under nominal conditions are used in the protection levels computation. The first dissertation contribution is to assess the nominal ionospheric gradients in Hong Kong and provide an accurate standard deviation of vertical ionospheric spatial gradients. An improved time-step method is developed to accurately estimate the ionospheric spatial gradients with temporal components removed. Based on the estimation results, a seasonality trend is revealed with larger values occurring in spring and autumn equinoxes. To account for this trend and achieve flexible GBAS performance, two quadratic polynomial expressions as functions of day of the year are designed to bound the standard deviation of vertical ionospheric spatial gradients. Under ionospheric anomaly conditions, ionospheric spatial gradients can reach several hundreds millimeters per kilometers, which can pose dangers to the aircraft user by inducing unacceptable position errors. The ionospheric threat model is defined to describe the parameters of the threatening ionospheric spatial gradients. Since the ionospheric dynamic driven by the largest spatial gradient varies between regions, this thesis establishes an ionospheric threat model for Hong Kong based on GNSS data collected from Hong Kong Satellite Positioning Reference Station Network (SatRef). In order to mitigate the threat induced by the large ionospheric gradients, ionospheric monitors are implemented in the GBAS ground and airborne subsystems. The performances of these monitors regarding meeting the allocated integrity requirements are assessed based on the established ionospheric threat model. In addition to the ionospheric monitors, the geometry screening method is applied to screen out potentially unsafe satellite geometries affected by the undetected ionospheric anomalies. This requires the computation of maximum value of the ionospheric error at landing threshold point computed by the integrity parameters transmitted by the ground subsystem. This thesis establishes a linear expression of the undetected maximum ionospheric error to help determine the integrity parameters for each runway threshold. The geometry screening method conservatively assumes the worst-case ionospheric gradient defined by the threat model is always present. This conservative assumption is required to guarantee the required level of safety can be met, while it leads to a loss of availability even in days where no ionospheric anomalies are present. In addition, the worst-case gradient is based on the threat model derived from historical data, which cannot cover a larger gradient that might occur in the future. Therefore, this thesis develops a real-time ionospheric monitoring approach based on the scintillation index derived from stations around GBAS facility. The proposed methodology aims to reduce the conservatism of the geometry screening method and aims to implement GBAS in low-latitude regions such as Hong Kong. With multi-frequency signals introduced into the civil aviation, using multi-frequency signals to mitigate the ionospheric effect becomes possible. This thesis evaluates the GBAS performance with dual-frequency measurements applied to the positioning and ionospheric monitoring. However, current dual-frequency ionospheric gradient monitor suffers from the non-Gaussian residual tropospheric error induced by the tropospheric anomaly. This thesis developed a dual-frequency ionospheric gradient monitor with tropospheric error constrained using the geometry-free combination of carrier phase measurements. The analytical performance of the proposed monitor is demonstrated to meet Category III precision approach requirements. |
| Rights: | All rights reserved |
| Access: | open access |
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