Browsing by Author "Christou, Nikolaos, T."

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    Evaluation of mathematical models for gyrocompass behaviour:: Error modelling and applications
    Christou, Nikolaos, T.
    Heading information is a fundamental parameter in ship’s navigation. Traditionally a gyrocompass is used as the primary sensors to provide heading reference on board ship. However, gyrocompass indicated headings are subject to a number of errors, which are functions of the ship’s motion and of the latitude of operation. The objective of this thesis is to investigate the gyrocompass behavior, study its deviations under different conditions of operation and develop suitable algorithms for the software compensation of these deviations. To meet this objective, mathematical models describing the gyrocompass behaviour are developed using different dynamic considerations. In particular, the gyrocompass equations of motion and their solutions are developed for the cases of a stationary, uniformly moving, and maneuvering ship. A general discrete-time model as well as a special model to represent a maneuvering ship are developed. Specific attention is drawn to the problem of high latitude behaviour of the gyrocompass. Simulation studies of the gyrocompass dynamic response are carried out using the mathematical models developed in this study. The simulation results indicate that transient errors of 1° are expected at latitudes of 30°, while errors in excess of 10° are likely to occur at latitudes of 70°. These errors may degrade considerable not only the gyrocompass performance, but also the performance of a multi-sensor integrated navigation system (e.g. introducing as much as 0.5 nautical miles error in a satellite fix), or they may introduce an error of as much as 2 mgals in real-time Eötvös correction calculations in precise sea gravimetry. An open-loop software compensation procedure of gyrocompass errors is proposed as an alternative to manual mechanical compensation traditionally used, to improve the gyroscope performance. The algorithm developed in this thesis is a function of the gyrocompass design parameters and of the particular dynamics of the ship’s motion. Finally, recommendations for future work include sea-trials of the developed software compensation algorithm, extension of the mathematical models to incorporate random disturbing forces, and evaluation of the dynamic response of modern marine gyrocompasses, such as, the Sperry MK 37 Gyrocompass Equipment.
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    Evaluation of mathematical models for gyrocompass behaviour:: Error modelling and applications
    Christou, Nikolaos, T.
    Heading information is a fundamental parameter in ship’s navigation. Traditionally a gyrocompass is used as the primary sensors to provide heading reference on board ship. However, gyrocompass indicated headings are subject to a number of errors, which are functions of the ship’s motion and of the latitude of operation. The objective of this thesis is to investigate the gyrocompass behavior, study its deviations under different conditions of operation and develop suitable algorithms for the software compensation of these deviations. To meet this objective, mathematical models describing the gyrocompass behaviour are developed using different dynamic considerations. In particular, the gyrocompass equations of motion and their solutions are developed for the cases of a stationary, uniformly moving, and maneuvering ship. A general discrete-time model as well as a special model to represent a maneuvering ship are developed. Specific attention is drawn to the problem of high latitude behaviour of the gyrocompass. Simulation studies of the gyrocompass dynamic response are carried out using the mathematical models developed in this study. The simulation results indicate that transient errors of 1° are expected at latitudes of 30°, while errors in excess of 10° are likely to occur at latitudes of 70°. These errors may degrade considerable not only the gyrocompass performance, but also the performance of a multi-sensor integrated navigation system (e.g. introducing as much as 0.5 nautical miles error in a satellite fix), or they may introduce an error of as much as 2 mgals in real-time Eötvös correction calculations in precise sea gravimetry. An open-loop software compensation procedure of gyrocompass errors is proposed as an alternative to manual mechanical compensation traditionally used, to improve the gyroscope performance. The algorithm developed in this thesis is a function of the gyrocompass design parameters and of the particular dynamics of the ship’s motion. Finally, recommendations for future work include sea-trials of the developed software compensation algorithm, extension of the mathematical models to incorporate random disturbing forces, and evaluation of the dynamic response of modern marine gyrocompasses, such as, the Sperry MK 37 Gyrocompass Equipment.
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    On the space-time ocean current variability and its effects on the length-of-day
    Christou, Nikolaos, T.
    Earth Rotation (polar motion and length-of-day) studies have embarked on a new era with the contributions from space geodesy observation techniques (high accuracy, higher temporal resolution) and the availability of new, global data bases of atmospheric and oceanic observables. Irregular variations in the earth’s rotation rate (length-of-day – LOD – variations) on time scales of 5 years or less are associated with changes in the angular momentum of the solid earth. The problem of transfer of angular momentum between the Earth System components (consisting of the solid earth, the oceans, and the atmosphere) has emerged todays as a problem of great scientific interest, because of the geophysical and environmental implications associated LOD variations. This thesis research chiefly investigates the time-dependent perturbations of LOD on time scales of about two years or less, associated with the space-time fluctuations of the global ocean circulation. Estimates of the space-time variations of ocean currents were derived from both in-situ oceanographic data and two years of satellite altimetry observations from the GEOSAT Exact Repeat Mission. A new technique for extracting ocean current variability from satellite altimetry was developed. The technique is based on the gradient operator and offers advantages over previously existing techniques on the recovery of oceanic variability in that it is conceptually simple and computationally efficient. Through the analysis of ocean current variability, it is established that the oceanic excitation of LOD is at least at the level of 0.1 milliseconds. Variations in the oceanic excitation of ΔLOD have been identified and quantified at periods of 1 year and 0.5 years. At the annual period, the results indicate that the oceanic excitation appears large enough to account for the existing discrepancy between the observed (non-tidal) ΔLOD and the atmospheric contribution. No significant oceanic excitation of ΔLOD is found at the semi-annual period. The results indicate that there exist statically significant higher frequency variations at approximately 121-day, 107-day, and 89-day periods. The analysis of the GEOSAT altimetry data generated many new questions regarding the oceanic behaviour and its dynamical link to LOD variations that require further investigations.
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    On the space-time ocean current variability and its effects on the length-of-day
    Christou, Nikolaos, T.
    Earth Rotation (polar motion and length-of-day) studies have embarked on a new era with the contributions from space geodesy observation techniques (high accuracy, higher temporal resolution) and the availability of new, global data bases of atmospheric and oceanic observables. Irregular variations in the earth’s rotation rate (length-of-day – LOD – variations) on time scales of 5 years or less are associated with changes in the angular momentum of the solid earth. The problem of transfer of angular momentum between the Earth System components (consisting of the solid earth, the oceans, and the atmosphere) has emerged todays as a problem of great scientific interest, because of the geophysical and environmental implications associated LOD variations. This thesis research chiefly investigates the time-dependent perturbations of LOD on time scales of about two years or less, associated with the space-time fluctuations of the global ocean circulation. Estimates of the space-time variations of ocean currents were derived from both in-situ oceanographic data and two years of satellite altimetry observations from the GEOSAT Exact Repeat Mission. A new technique for extracting ocean current variability from satellite altimetry was developed. The technique is based on the gradient operator and offers advantages over previously existing techniques on the recovery of oceanic variability in that it is conceptually simple and computationally efficient. Through the analysis of ocean current variability, it is established that the oceanic excitation of LOD is at least at the level of 0.1 milliseconds. Variations in the oceanic excitation of ΔLOD have been identified and quantified at periods of 1 year and 0.5 years. At the annual period, the results indicate that the oceanic excitation appears large enough to account for the existing discrepancy between the observed (non-tidal) ΔLOD and the atmospheric contribution. No significant oceanic excitation of ΔLOD is found at the semi-annual period. The results indicate that there exist statically significant higher frequency variations at approximately 121-day, 107-day, and 89-day periods. The analysis of the GEOSAT altimetry data generated many new questions regarding the oceanic behaviour and its dynamical link to LOD variations that require further investigations.