Spacecraft formation flying, i.e. the maintenance of a specific relative motion between two or more spacecraft, is considered to be a new and enabling technology in space engineering. However, it is almost as old as spaceflight itself. It has been performed since the American Gemini missions in the 1960s. However, what sets the "new" formation flying missions apart from the "old" formation flying missions is that the "new" missions rely on a certain amount of autonomy for the spacecraft, enabling them to maintain the formations with little to no human intervention. This autonomy saves cost in ground operations and enables missions that are not feasible when formation control needs to be performed using ground operators. Spacecraft formations are a subclass of distributed space systems. In these systems, multiple spatially separated platforms are used to fulfill the mission objectives. Performing space missions using spacecraft formations brings several advantages over the more traditional monolithic spacecraft design, but also comes with new challenges. It enables for instance challenging missions where a very high angular resolution is required of the object to be studied or where single-pass interferometric measurements are needed. In addition, missions can be made intrinsically redundant or failure tolerant: If one spacecraft in a formation fails, the mission is not necessarily lost. Spacecraft formations can also be considered for missions where a formation is not strictly necessary but which would require a very complex, and thus costly, monolithic spacecraft (e.g., Envisat). Such a complex spacecraft with many different payloads that all have conflicting requirements could be split intomultiple smaller spacecraft that are optimized for one or two payloads. However, formation flying also adds complexity atmission level since multiple spacecraft need to be built, tested, and launched. Once in orbit, these spacecraft also need to be brought and kept in a certain configuration for the entire mission duration. This also requires additional subsystems on each spacecraft in order to determine and maintain the positions and velocities of the spacecraft relative to each other and to avoid spacecraft collisions. In the last decade, much research has been performed in the field of spacecraft formation flight. However, much of this research has focused on specific missions, the modeling of the orbital dynamics, or efficient formation control and collision avoidance algorithms. In the field of navigation for formation flying spacecraft, several important aspects still need further investigation. For instance, it is not known how the ranging accuracy, intersatellite distance, and antenna baseline (i.e., the distance between two antennas on the same spacecraft) impact the accuracy in the estimation of the relative positions and velocities, or ’state’, of the spacecraft. If the ranging accuracy is increased by a factor two and the inter-satellite distance is also increased by a factor two, will the accuracy in the estimation of the relative state remain the same or not? This is important to know since spacecraft formations should preferably consist out of relatively small spacecraft (otherwise the total mission costs will quickly become prohibitive) and these small spacecraft are limited in terms of power, mass, and volume. Furthermore, it is also not known if the type of relative orbit has an impact on the estimation of the relative state. Perhaps one type of relative orbit will lead to intrinsically better relative state estimates than another type of relative orbit. Lastly, it is known that a large relative out-of-plane motion of the spacecraft leads to more accurate relative state estimates because of the favorable viewing geometry. In fact, a very small relative out-of-plane motion will even cause the estimation process to fail. However, it has never been quantified how small the relative out-of-plane motion can be before the estimation process will fail. In order to improve the knowledge in the areas discussed in this paragraph, the following research questions have been formulated at the start of the research. These research questions are answered in this thesis.
|Qualification||Master of Science|
|Award date||2 Sep 2014|
|Publication status||Published - 2014|