- Beacon Software
Orbits and Controls
Jon Marc O'Kins (Space Systems)
The orbits and controls (O&C) system is in charge of all the orbital and down linking simulation for the mission operations of the space craft. Our team also runs the on orbit power simulations. In addition to these simulations, the orbits and controls team has designed, and is currently in the process of testing a passive attitude control system designed to force the space craft to point in a predictable direction.
Since each group of cubesats is typically launched as a secondary payload, its precise orbital trajectory is dictated by the requirements of the primary mission. However, basic historical trends can be observed between each significant cubesat launch, which can be used to establish an approximate “reference orbit” to aid in mission design.
Based on previous cubesat launch trends, a sun-synchronous reference orbit has been chosen with an inclination of 98 degrees and an altitude of 650 km. A sun-synchronous orbit has a polar orbital plane that remains fixed with respect to the Sun. This reference orbit is a design guide constructed from historical averages, and additional worst-case figures must be considered whenever possible throughout the mission design process. For instance, the “local time” of previous sun-synchronous cubesat orbits, or the orientation of the orbit plane relative to the Sun (θ), has no apparent trend. As a result, the implications of every local time must be considered during the design process. Each of the four large cubesat groups has been launched into a sun-synchronous orbit of this type. The upcoming cubesat group launch in March of 2008 is also planning a sun-synchronous orbit. While a sun-synchronous orbit is very likely for our mission, it is only an educated assumption until our specific launch vehicle is confirmed.
Passive Attitude Control
M-Cubed utilizes a passive magnetic attitude control system to achieve a proper orientation for Earth-imaging. The system consists of a single permanent magnet aligned on one cubesat body axis, along with additional magnetic hysteresis materials aligned on each additional perpendicular body axis. In this configuration, the permanent magnet aligns one body axis of the cubesat with the local Earth magnetic field direction. Since the magnet still permits cubesat rotation about this single axis, the hysteresis materials are added to dampen unwanted rotation. Chosen for their high magnetic permeability, the Hymu80 hysteresis materials create internal current as they are rotated through the local magnetic field. This dissipates rotational energy as heat, effectively damping the rotational motion of the cubesat. If each magnetic component of the passive attitude system is properly sized, a controlled spin rate can be achieved about the local magnetic field direction.
In practice, this passive attitude control system will allow for Earth-imaging throughout only a designated portion of the M-Cubed orbit. Ideally, the camera will continuously point in the nadir direction or straight down towards Earth. Since the camera is aligned along the permanent magnet axis, however, its direction relative to nadir is dictated by the cubesat’s orbital position
Due to this Earth magnetic field configuration, the passive magnetic control system will allow for ground coverage over a significant portion of the Northern Hemisphere. As M-Cubed passes over the North Pole, the permanent magnet and camera will be aligned in the nadir direction, due to the vertical direction of the local Earth magnetic field. As the M-Cubed orbit continues toward the Southern Hemisphere, the camera-nadir angle will increase until the Earth leaves the camera field of view. The Earth will then reenter the camera field of view after M-Cubed crosses the equator into the Northern Hemisphere. On average, this control strategy will allow for approximately 15 picture opportunities of the Northern Hemisphere per day (once per each 90 minute orbit). Although the Northern Hemisphere will remain in the camera field of view for approximately 40 minutes during each overpass, the window of opportunity will vary depending on ground lighting conditions.
Although limited in performance, this type of passive control system was chosen for several reasons. When compared with active attitude control systems, such as magnetic torque coils, passive systems of this type require less mass and no power consumption. Furthermore, passive attitude systems offer a robust, simple control strategy that boasts extensive flight heritage in similar Earth-imaging cubesat missions.