Analysis and Detection of Ionospheric Depletions over the Indian Region in the Context of Satellite Navigation
Satellites have revolutionized navigation by making it more universal, accessible and ac- curate. Global Positioning System (GPS) is the most widely used satellite navigation system in the world. However, it is prone to errors from various sources such as the ionosphere, troposphere and clock biases. In order to make the system very accurate and reliable, especially to meet the requirements of safety-critical applications, Satellite Based Augmentation Systems (SBAS) have recently been designed in various countries to augment the GPS by providing corrections for its errors. An Indian SBAS called GAGAN (GPS Aided Geo Augmented Navigation), developed for the Airports Authority of India (AAI) by Indian Space Research Organization (ISRO) is currently being installed and proven for aviation and other use. The uncertain propagation delay of signals through the ionosphere is the most important contributor of error in GPS positioning, its maximal elimination is a major task of SBAS overlays. Ionospheric delays have steady, cyclic, and irregular components. The last types are of particular concern because they are unpredictable. This thesis deals with ionospheric depletion, an important phenomenon of this class that is specific to tropical regions like India and hence have not been well studied in the context of other SBAS systems of the world which cover mid-latitude domains. Depletion is an ionospheric phenomenon in which the density of electrons dips suddenly and then returns close to the previous value. It poses a challenge to the model adopted for ionospheric delay estimation since it may not be detectable by ground systems be- cause of its localized nature, and its occurrence and intensity cannot be predicted. In this work we have analyzed the depletion characteristics over the Indian region such as its distribution, frequency of occurrence, and depth and duration parameters. We have then studied and implemented an existing algorithm to detect a depletion from the Total Electron Content (TEC) data. This algorithm has been found to be inaccurate for estimation of depletion duration, and we have proposed an improved algorithm for depletion detection and shown it to be more suitable for the Indian SBAS, GAGAN. The algorithm utilizes multiple thresholds for depletion detection in order to improve performance in the presence of irregularities including noise. These thresholds are determined by analyzing real TEC data containing depletion events over the Indian region. The detected depletion events are those that have a strong likelihood of contributing large range errors and degrading GAGAN's reliability. The thresholds include depletion parameters such as the depth, duration, rate of change of TEC, and the rate of change of slope of the TEC curve. The characterization of depletion events over the Indian region yielded useful insights into the behaviour of the phenomenon. It was observed that the depletion events were invariably present post-sunset, between 1900 and 0200 hrs. This observation is consistent with the other studies on plasma bubbles so far. The average depth of the depletion was found to be about 3.31 meters of propagation delay while the strongest depletion corresponds to about 5.04 meters of delay. The latter observation impresses upon the need to detect and study the phenomenon of depletion since it is capable of causing a significant loss of accuracy and reliability to the system. The duration of the depletion was found to range from about 10 min to 2.35 hours. In addition, a statistical study of the relationship among the different parameters and a study devoted to now-casting of depletion was made to get a more quantitative insight into the phenomenon of depletion. Scintillation is another phenomenon occurring in the ionosphere which causes rapid fluctuations of phase and amplitude of the signal due to TEC variations in the ionosphere. The occurrences of depletion were observed to be accompanied by scintillation, as also noted in previous studies. The correlation of depletion and scintillation was studied using the data available for this research. A spatial characterization of the depletion events was also investigated using the same temporal TEC data from neighbouring stations which were relatively close to each other. This study addressed the movement of the plasma bubble with respect to the advection speed and direction with definite results. Attention was also devoted to the spatial dimension of the bubble as observed from various stations. Contributions to this variability in the apparent spatial extent comes from the observation of the depletion event from varying lines-of-sight corresponding to different GPS satellites which are also moving, and the differential `slicing' effect because of the location of the stations with respect to the plasma bubble, in addition to the evolution of the bubble during transit. The detection of depletion and its temporal characterization, in addition to the knowledge of its spatial extent and motion, can provide very useful insights on the behaviour of a depletion event and over the ionosphere in general. This knowledge and the mechanism for detection can help to improve the quality and dependability of the information provided by SBAS systems, in particular the Indian GAGAN system, for improved navigation in this part of the world. The present thesis aims to make a significant contribution in this direction.
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