Earth observation has gained importance in the last decades to support daily life and better understand natural processes, driven by the need to study global climate changes. Most space agencies around the globe have developed sophisticated systems to collect data about planet Earth based on microwave, optical or magnetometer sensors. Currently, several missions carry synthetic aperture radars (SAR) or interferometers to measure key parameters in the microwave range. However, most of these missions, such as Cloudsat, MetOp, SMOS, SMAP and many others, are based on a monolithic satellite architecture. This approach limits the potentiality to improve data quality and spatial resolution to monitor natural events: with a monolithic approach, this can only be obtained by employing rather large antennas. The introduction of constellations and distributed systems promises a significant improvement in data quality and coverage. In the context of active SAR, few mission concepts have been developed based on distributed systems improving scientific data quality by observing the same terrain area from different platforms. One example is the TanDEM-X - TerraSAR-X mission launched in 2010 by the German Aerospace Center. On the other hand, no distributed missions carrying passive radiometers have been designed and launched, and few studies are currently available to design such missions. There are several research and technological challenges connected to these mission concepts. First, vehicles should be separated by a few tens of meters to perform passive interferometry with a swarm/formation, which indeed triggers many difficulties in operating such space systems. Second, precise navigation and control techniques are required, even for the real-time relative guidance, navigation, and control sub-system. In this thesis, we start from these challenges and assess to what extent distributed missions can be realistically implemented to improve the spatial resolution of passive interferometers. Consequently, the research questions regard how we can design guidance, navigations and control techniques to support distributed systems compliant with the need for on-board autonomy and robust control techniques to fill the current gap in the literature. This work proposes a preliminary mission design technique to enable future distributed multi-satellite systems for high-resolution interferometry and to understand how operational and payload constraints can be included in the design from the early phases of the process. To this end, first, the needs for future mission concepts for microwave observations are identified, and different cluster geometries are analysed in terms of performances achievable by the combined scientific instrument. Then, the generation of guidance profiles for the maintenance and reconfiguration of a formation is addressed. The latter is designed based on a convex description of the fuel optimal problem employing a continuous control law. Such a computation of the relative trajectory is then embedded into a relative guidance, navigation and control framework to simulate in a reliable, robust, and fast way the overall achievable performances of the distributed system, including operational requirements. Finally, the methodology is applied to test case studies involving active and passive microwave-distributed systems. As a result, this research provides a possible baseline for mission analysis design and preliminary outcomes of passive microwave applications, including typical control and navigation errors. This dissertation shows that indeed distributed systems can be designed to support future missions carrying microwave antennas and future cluster studies. In addition to the results associated with the specifically investigated microwave architectures, the significance of this study relies on the flexibility of the developed methodology, which can be applied to several multiple-spacecraft formation concepts, not only in the Earth observation field. Furthermore, the proposed approach can be extended to other orbital regions and space applications, opening the path for new passive microwave remote sensing applications.
Negli ultimi decenni, l'osservazione della terra è diventata fondamentale in supporto alla vita quotidiana e allo studio dei processi naturali, guidati dalla necessità di comprendere i cambiamenti climatici. La maggior parte delle agenzie spaziali ha sviluppato dei sistemi complessi per raccogliere dati con su sensori a microonde, ottici o magnetici. Attualmente, svariate missioni sono dotate di sistemi radar ad apertura sintetica (SAR) o di interferometri per misurare diversi parametri scientifici. La maggior parte di queste missioni, come Cloudsat, MetOp, SMOS, SMAP e molte altre, sono basate su un'architettura monolitica, che può limitare la qualità dei dati e la risoluzione spaziale per monitorare eventi naturali che non si potrebbero osservare altrimenti. Con un'architettura monolitica, una qualità migliore si può ottenere solo sviluppando antenne più grandi. L'introduzione delle costellazioni e dei sistemi distribuiti ha dimostrato un miglioramento significativo nella qualità dei dati e nella copertura. Nel contesto di SAR attivi, alcune missioni distribuite hanno dimostrato questa capacità osservando la Terra da diverse piattaforme combinate. Un esempio è la missione TanDEM-X - TerraSAR-X lanciata nel 2010 dall'agenzia spaziale tedesca (DLR). D'altro canto, nessun sistema distribuito con interferometri passivi è stato disegnato e lanciato; e fino ad ora, pochi studi sono disponibili per progettare questo tipo di missioni. Ci sono diverse sfide connesse a questo tipo di missione: i satelliti devono volare a distanza di poche decine di metri per permettere l'interferometria combinata, e questo richiede lo sviluppo di tecniche per controllo e navigazione accurata e precisa. In questa tesi, vogliamo investigare in quale modo le missioni distribuite possano essere progettate per migliorare la risoluzione spaziale dell'interferometria passiva. La domanda alla base di questa ricerca vuole comprendere come si possano sviluppare tecniche di guida, navigazione e controllo in supporto a sistemi distribuiti, considerando autonomia a bordo e tecniche di controllo robuste per migliorare lo stato dell'arte attuale. Questo lavoro propone un approccio per il disegno preliminare di missione per rendere possibili sistemi basati su satelliti distribuiti per interferometria ad alta risoluzione e per comprendere come i requisiti operativi e dello strumento possano essere inclusi nel progetto. Per questo motivo, come prima cosa sono stati identificati i requisiti dei sistemi a microonde, e diverse geometrie di formazione di satelliti sono state analizzate in base alle prestazioni dell'interferometria combinata. Successivamente, a partire dalle tecniche disponibili, il mantenimento della formazione e le manovre sono state sviluppate con un sistema di controllo ottimo convesso basato su spinta continua. Infine, questa metodologia è stata integrata in un framework di guida, navigazione e controllo per simulare in modo robusto e efficiente i sistemi distribuiti, includendo i requisiti operativi. La precedente metodologia è stata applicata a diversi casi studio considerando voli in formazione con sistemi a microonde attivi e passivi. Questa ricerca fornisce un punto di partenza per il disegno di missione e lo studio preliminare delle prestazioni di strumenti a microonde, includendo le incertezze del controllo e navigazione di bordo. Questa tesi mostra come sistemi distribuiti possano essere sviluppati in supporto a mission future, con strumenti a microonde attivi e passivi, ed a studi futuri di voli in formazione. L'importanza di questo studio si basa sulla flessibilità del modello sviluppato, che può essere applicato a diversi studi, non solo nel campo dell'osservazione della terra. Per concludere, questa ricerca può adattarsi allo studio in altre regioni orbitali e a differenti applicazioni spaziali, aprendo nel contempo la strada per approcci innovativi per il telerilevamento con sistemi passivi a microonde.
multiple satellites formation flying for earth observation applications in low earth orbits
SCALA, FRANCESCA
2022/2023
Abstract
Earth observation has gained importance in the last decades to support daily life and better understand natural processes, driven by the need to study global climate changes. Most space agencies around the globe have developed sophisticated systems to collect data about planet Earth based on microwave, optical or magnetometer sensors. Currently, several missions carry synthetic aperture radars (SAR) or interferometers to measure key parameters in the microwave range. However, most of these missions, such as Cloudsat, MetOp, SMOS, SMAP and many others, are based on a monolithic satellite architecture. This approach limits the potentiality to improve data quality and spatial resolution to monitor natural events: with a monolithic approach, this can only be obtained by employing rather large antennas. The introduction of constellations and distributed systems promises a significant improvement in data quality and coverage. In the context of active SAR, few mission concepts have been developed based on distributed systems improving scientific data quality by observing the same terrain area from different platforms. One example is the TanDEM-X - TerraSAR-X mission launched in 2010 by the German Aerospace Center. On the other hand, no distributed missions carrying passive radiometers have been designed and launched, and few studies are currently available to design such missions. There are several research and technological challenges connected to these mission concepts. First, vehicles should be separated by a few tens of meters to perform passive interferometry with a swarm/formation, which indeed triggers many difficulties in operating such space systems. Second, precise navigation and control techniques are required, even for the real-time relative guidance, navigation, and control sub-system. In this thesis, we start from these challenges and assess to what extent distributed missions can be realistically implemented to improve the spatial resolution of passive interferometers. Consequently, the research questions regard how we can design guidance, navigations and control techniques to support distributed systems compliant with the need for on-board autonomy and robust control techniques to fill the current gap in the literature. This work proposes a preliminary mission design technique to enable future distributed multi-satellite systems for high-resolution interferometry and to understand how operational and payload constraints can be included in the design from the early phases of the process. To this end, first, the needs for future mission concepts for microwave observations are identified, and different cluster geometries are analysed in terms of performances achievable by the combined scientific instrument. Then, the generation of guidance profiles for the maintenance and reconfiguration of a formation is addressed. The latter is designed based on a convex description of the fuel optimal problem employing a continuous control law. Such a computation of the relative trajectory is then embedded into a relative guidance, navigation and control framework to simulate in a reliable, robust, and fast way the overall achievable performances of the distributed system, including operational requirements. Finally, the methodology is applied to test case studies involving active and passive microwave-distributed systems. As a result, this research provides a possible baseline for mission analysis design and preliminary outcomes of passive microwave applications, including typical control and navigation errors. This dissertation shows that indeed distributed systems can be designed to support future missions carrying microwave antennas and future cluster studies. In addition to the results associated with the specifically investigated microwave architectures, the significance of this study relies on the flexibility of the developed methodology, which can be applied to several multiple-spacecraft formation concepts, not only in the Earth observation field. Furthermore, the proposed approach can be extended to other orbital regions and space applications, opening the path for new passive microwave remote sensing applications.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/202797