Speaker
Mr
Sven Schaeff
(Astos Solutions GmbH)
Description
Telecommunication satellites located in the Geostationary Equatorial Orbit (GEO) are typically not directly placed there by the launch vehicle. The satellites are often injected in a Geostationary Transfer Orbit (GTO) and then transferred to the GEO using their own onboard propulsion system. State of the art for the GTO to GEO transfer is still the chemical propulsion.
Just recently few satellites transferred or are transferring to GEO using Electric Propulsion (EP), since it is very attractive to exploit their high specific impulse reducing the propellant mass of the orbit transfer. Since the total spacecraft mass is reduced this yields launch vehicle cost reductions. Further, Electric Orbit-Raising (EOR) is now available for most telecommunication satellite platforms or at least under development.
But electric orbit-raising requires much more complex maneuver sequences than what is needed for pure chemical transfers. Since EP provides only small thrust magnitudes in comparison to chemical propulsion, the transfer lasts many months. A careful planning of the spacecraft attitude maneuvers is required in advance to fulfill this mission. In recent years, many software tools have been developed for the preliminary assessment of low-thrust orbit transfers. Unfortunately, most tools lack both maturity and accuracy necessary to fully exploit the capabilities of electric orbit-raising. For example, during the transfer any crossing of the GEO ring poses a certain collision risk with high value assets. Thus, the precomputed transfer trajectory has to avoid crossings of the GEO belt. Further, ground station visibility might be considered for transfer planning as well as limitations and constraints related to different spacecraft subsystems, such as eclipse handling, power generation, storage and consumption, or EP firing limitations in general. Other possible limitations are related to the attitude of the spacecraft or consider environmental aspects like the radiation dose.
Using Non-Linear Programming (NLP) to optimize the attitude profile in combination with detailed modelling of complex mission constraints and limitations of the spacecraft model is essential, especially under consideration of tight accuracy and fidelity requirements for achieving optimality in sense of propellant consumption and transfer duration. Besides optimization, many aspects have to be analyzed in more details. It encompasses subsystem issues for example of the Attitude and Orbit Control System (AOCS) as well as station visibilities.
In the newest version of the low-thrust optimization and analysis tool LOTOS (Low-thrust Orbit transfer Trajectory Optimization Software) all aforementioned features are available. But the tool is not only limited to electric orbit-raisings; it also supports hybrid transfers where chemical maneuvers are followed by the low-thrust transfer. Another feature of the software is the support of spacecraft operations. This mode identifies the spacecraft location on a pre-computed reference trajectory and uses its attitude profile for re-optimization. Such a processing is required due to deviations of the real-flown spacecraft trajectory from the nominal one.
A full overview of the software capabilities and features will be given in this paper, such as hybrid transfers, 6 degrees of freedom attitude control and verification of trajectories. Highlights of the Graphical User Interface (GUI) will be presented as well. It includes automatic user defined analyses and customizable reports to support mission analysis engineers and to relieve them from repeating tasks. For example, the reports provide full access to all data of the user defined scenario and may include tables and plots.
| Applicant type | First author |
|---|
Primary author
Mr
Sven Schaeff
(Astos Solutions GmbH)
Co-author
Mr
Martin Juergens
(Astos Solutions GmbH)
