SOURCE (Stuttgart Operated University Research CubeSat for Evaluation and Education) is a nanosatellite with dimensions of approximately 10 * 10 * 30 cm^3 which is matching the 3U+ CubeSat Standard. The satellite is being developed in a corporation between KSat e.V., the Institute for Space System (IRS) at the University of Stuttgart and the IRAS Project. The main design work is conducted by KSat members and students of the University of Stuttgart. The student teams are supported and mentored by the IRS which has gained valuable knowledge through operating the ‘Flying Laptop’ small satellite.
IRAS (Integrated Research Platform for Affordable Satellites) is a corporation of industry and research groups to launch a development platform for satellites and new technologies. They provide multiple technology demonstrators as payload for SOURCE including solar panels, a composite sandwich structure and other sensors. Further payload is provided by the German Aerospace Center (DLR). The main monochromatic camera mounted on SOURCE ("MeSHCam") is also part of a future IRS mission and will be flight tested, functioning as an earth observation camera as well as a star tracker for attitude control. To prevent stray light from disturbing the images it is protected by a deployable baffle. There is also a smaller colour camera called PRIma (PR-Imager). Besides the space testing of these components there are multiple scientific payloads by the IRS onboard SOURCE. Two FIPEX Sensors will measure the level of atomic oxygen in altitudes below 200km. Heat flux sensors combined with pressure sensors will study the environment of the CubeSat upon reentry.
Phase A of the project has been completed successfully in July of 2018 with the Preliminary Requirements Review. Currently, the team is working towards the Preliminary Design Review in beginning of February 2019. The launch of SOURCE to low-earth orbit is scheduled for 2020.
The structural design team is in charge of the physical layout and the thermal management of the satellite. It is this team’s job to fit the components of all subsystems into the given dimensions and to make sure they stay within their operating temperatures.
The primary structure functions as the backbone of the satellite and houses all experiments as well as their supply systems. During launch this frame supports all additional mechanical loads. Each printed circuit board is mounted on four threaded rods and secured by 8 nuts. To keep all electrical component working at their optimal performance range a thermal management system is necessary. Ideally, this is realized by using passive heating elements and radiators. These systems are being designed right now and will be tested and improved during the coming phases of the project.
Attitude Control System
The Attitude Control System (ACS) is in charge of the pointing of the satellite. After the ejection out of the rocket it needs to stabilize the satellite. During later mission stages the solar panels need to be pointed towards the sun, the camera towards earth, the FIPEX sensors into the flight direction or the antenna towards the ground.
SOURCE uses magnetorquers to adjust its attitude. These coils create a magnetic field which aligns itself with Earth's magnetic field. This process introduces a turning moment which can be used to control the satellite. With only magnetorquers it is impossible to create moments around the magnetic field vector. This means that the satellite can only be controlled in two axes at any given time. However, as the satellite flies around the earth it travels through different regions of the magnetic field, allowing for full three-axis control. The magnetorquers on SOURCE are constructed as flat, thin coils. A 3.5 cm wide band of wire is wound in multiple layers to a thickness of 2 mm. The coils are bonded together by an epoxy resin and integrated into the satellites side panels and the PCB-stack.
To determine the satellites position and attitude, mainly sun sensors, magnetometers, GPS and gyroscopes are used. The sensors measurements are compared to a sun model and a magnetic field model. The Earth's magnetic field varies with the satellites position, which is measured with the GPS. All the sensors measurements are combined into a 3-axis attitude determination in a Kalman filter.
Besides these for CubeSats typical sensors the earth observation camera will be tested as a sensor for attitude determination. It will take pictures of the horizon and the stars which will then be compared to a model of earth or a map of the stars. The concept of star- and horizon trackers has been used on many satellites and is nothing new. However, using a commercial earth observation camera is a novel development. This is a technology demonstration which means that the system as a whole cannot be dependent on the camera as a sensor. If it works it promises much more accurate attitude information than the other sensors can provide.
Onboard Data Handling
The onboard data handling system is the main control unit of the CubeSat. It distributes commands to each component controlling and gathering information of the whole system. The on-board computer houses a micro controller, memory unit and multiple interfaces for communication with each subsystem.
The basic tasks include:
- Logging of housekeeping data
- Logging and processing of payload data
- Calculation of inputs for the attitude control
- Recognizing and fixing malfunctions of other modules
- Execution of commands from the ground station
- Processing data to be send to the ground station
As an on-board computer with enough computing power and storage capacity to execute the tasks reliably the iOBC (ISIS On-Board Computer) has been chosen.
- 400 MHz ARM9 processor
- volatile memory: 64MB RAM
- program memory: 1MB NOR Flash
- FRAM: 256KB
The software is currently being developed. It will be based on the real-time operating system FreeRTOS and flight proven software framework used by the ‘Flying Laptop’ small satellite.
Electrical Power System
The Electrical Power System (EPS) is in charge of the energy supply for the mission. This subsystem includes an energy generating unit, an energy storage unit and an energy distribution unit. An ideal EPS consists of efficient solar panels as energy generating unit, a battery for sufficient energy storage and a PCDU (Power Control and Distribution Unit) which is capable to supply the required power to each subsystem. For SOURCE the EPS is also in charge of the power up of the satellite and the unfolding of the solar panels.
The task of the communications system is to relay data for controlling the satellite, data generated by the experiments and telemetry data about the mission status. It upholds the connection between the ground station and the satellite. In order to achieve this, multiple components like tranceivers S-Band antennas need to be designed. The communications system is operating within the S-Band range which is typically used for larger satellites. This technology enables us to use the ‘Flying Laptop’ ground station at the University of Stuttgart and achieve larger transmissions data rates. Furthermore, we use inter-satellite communication to relay data during reentry if we are not directly above a ground station.
The testbed team builds a simulator of the whole SOURCE CubeSat. The onboard computer will be connected to the simulator to verify the software and conduct multiple tests.
Each component will be recreated as a model and connected to the real parts for further testing. The testbed helps to test and verify the whole satellite even before it is built. All component models will be replaced bit by bit with the actual hardware in this process.
Additionally, the simulator enables us to prepare the ground station and train the personnel using it for the new satellite.
The payload on board SOURCE can be separated into three groups:
The first group includes an atmospheric sensor system. It includes heat flux sensors, pressure sensors and FIPEX sensors used to determine the amount of oxygen in the atmosphere. The data generated will be used to verify atmospheric models developed at the Institute for Space Systems (IRS) at the University of Stuttgart.
The second group consist of a camera system which is tested for attitude determination using pictures of earth’s horizon and the stars and will also be used for other earth observation purposes.
The third group includes multiple components by the German Aerospace Center (DLR) and the IRAS-Project. These components have yet not been tested in the environment of space or are not specifically designed for this purpose. SOURCE offers the opportunity to test their function under realistic space conditions.