Ionosphere Gradient Monitoring (IGM) in a Ground Based Augmentation System (GBAS) using Double Difference Carrier Phase Measurements on short Baselines
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This thesis is about the Ionospheric Gradient Monitor (IGM), which is currently developed at Indra Navia, as a part of Ground Based Augmentation System (GBAS). GBAS is an aircraft approach/landing system using GNSS signals. In the GBAS concept, the responsibility of the ground station is to transmit corrections of the GNSS ranging signals, and estimates of the signal errors in addition to aircraft approach data, over a VHF data link to the airborne user. In this manner the aircraft position accuracy is improved, and integrity is added to the GNSS signals. The responsibility of the IGM, is to detect stationary or slowly mowing gradients in the ionosphere. Gradients within the magnitude range 300-500 mm/km have to be detected within the integrity and continuity requirements determined in the SARPS from the International Civil Aviation Organization (ICAO). Data has been collected from a test GBAS at FRA, and used to calculate bias matrices for further calibration. This method shows some consistence of these bias matrices for a given baseline in the period April-July 2014, but not enough to conclude if this calibration method is stable over time. Different baselines achieved different matrices, which implies that each baselines has to be calibrated separately. The bias matrices are based on monthly data, but other time frames could be better. Seasonal and annual variations should be further analyzed. Other atmospheric activity has lately been observed from the IGM, and are for different reasons assumed to occur in the troposphere. This has introduced the new phenomena known as tropospheric gradients, which are smaller and more short-lived than the ionospheric gradients. The tropospheric gradients are mostly observed in clear hot days, within the magnitude range 50-250 mm/km. They are observed to last between 1 and 3 minutes, which results in widths of 50-700 meters and velocities of 1-16 m/s. It is reported that the velocities of these events have similarities with the current wind speed. Even if the tropospheric gradients separately don't result in a ranging error threat for the aircraft, they can still harm the performance of the IGM. A two-dimensional IGM has been implemented in MATLAB, based on available literature. The method is based on a least squares estimation, and can be considered for airports with runways heading in different directions. The method is tested with various antenna configurations, and with different simulated ionospheric gradients. The implementation show promising results, with corresponding theoretical and processed values of the estimation uncertainties.