Interferometric orbit determination for geosynchronous earth sensing missions
(English) Future Geosynchronous Synthetic Aperture Radar (GEOSAR) missions will provide permanent monitoring of continental areas of the planet with revisit times of less than 24 h. Several GEOSAR missions have been studied in the USA, Europe, and China with different applications, including water c...
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| Tipo de recurso: | tesis doctoral |
| Fecha de publicación: | 2023 |
| País: | España |
| Institución: | Universitat Politècnica de Catalunya (UPC) |
| Repositorio: | UPCommons. Portal del coneixement obert de la UPC |
| Idioma: | inglés |
| OAI Identifier: | oai:upcommons.upc.edu:2117/400811 |
| Acceso en línea: | https://hdl.handle.net/2117/400811 https://dx.doi.org/10.5821/dissertation-2117-400811 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Enginyeria de la telecomunicació |
| Sumario: | (English) Future Geosynchronous Synthetic Aperture Radar (GEOSAR) missions will provide permanent monitoring of continental areas of the planet with revisit times of less than 24 h. Several GEOSAR missions have been studied in the USA, Europe, and China with different applications, including water cycle monitoring and early warning of disasters. GEOSAR missions require unprecedented orbit determination precision in order to form focused SAR images from Geosynchronous Orbits (GEO). A precise orbit determination technique based on microwave interferometry is proposed. An interferometric orbit determination system has been designed and implemented in order to acquire experimental phase observations from which the trajectory of the satellite can be estimated. The technique presents the capacity for tracking illuminators of opportunity, allowing interferometry to be tested without launching any new spacecraft. Multiple experimental campaigns are carried out using the signals of opportunity from the satellites of the Astra 19.2oE constellation. Since the measurement sensitivity of the interferometer increases with the length of the baseline, a signal relay strategy is proposed in order to extend the baseline while keeping all the receivers together, sharing a common clock reference. The use of passive reflectors in the near field of the secondary receivers forms a compact interferometric baseline of 15~m. The interferometer uses correlation techniques to operate with arbitrary transmitted signals. The correlator's high processing gain allows the system to effectively track weak signals well below the noise level. Thus, some buildings found in the vicinity of the campus are used as reflectors of opportunity, providing a weak satellite signal scattered in the direction of the receiver antennas, forming large interferometric baselines of up to 1200 m. An error model is developed in order to estimate the expected orbit determination precision from a set of interferometric phase observations. It considers the most important perturbations affecting a microwave interferometer, such as the tropospheric and ionospheric perturbations, the transmission line errors, the system's temperature differences, the absolute phase ambiguity, and the uncertainty in the receivers positioning. This model allows to establish the requirements and to configure the settings of the tracking system according to the needs of the space mission under consideration. The relative phases measured between the receivers are used to estimate the satellite position. The experimental results prove the technique is able to track GEOSAR satellites based on their transmitted signals. An interferometer with three baselines of 15m, 300 m, and 1200 m uses the shorter baseline to solve the absolute phase ambiguity and the longer ones to retrieve the trajectory of the satellite with precision. The resulting orbit determination error with respect to the true trajectory is estimated to be in the order of 400 m in an urban environment due to the uncertainty of the position of the distributed reflectors of opportunity. The precision would improve substantially with the installation and precise geodetic location of dedicated metallic reflectors. This result cannot be confirmed due to the lack of an accurate ground truth, but it is consistent with the expected orbit determination precision computed with the error model. Finally, the echoes resulting from the scattering of GEO satellite transmissions on the surrounding urban structures and mountains are processed in order to form SAR images of Barcelona. The images were formed with the estimated trajectory information provided by the large baseline interferometric orbit determination system, proving their performance is valid for GEO remote sensing missions to form focused SAR images. |
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