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 just before reentry.
Phase B of the project has been completed successfully in March 2019 with the Preliminary Design Review. Currently, the team is working hard towards the Critical Design Review scheduled for February 2020. The launch of SOURCE to low-earth orbit is planned for 2021.
Structural Design and Thermal Management
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. It also designs the deployment mechanisms for the solar panels and camera baffle.
The primary structure functions as the backbone of the satellite and houses all experiments as well as their supply systems. Each printed circuit board is mounted on four threaded rods secured by a clamping mechanism. Afterwards solar panels, antennas and experiments are attached to the outer surface. All electrical components are monitored by thermal sensors and kept in their respective operational thermal range with heaters. FEM and thermal simulations analyse the behaviour of the satellite during launch and on orbit. Currently, the primary structure and ground support equipment are prepared for production. An assembly and integration plan is being prepared as well. After assembly and integration of the prototype shaker tests to simulate the launch loads will be carried out.
Another task of the structural team is the design of the deployment switches. These are released when the satellite is ejected from the CubeSat deployer and then activate the satellite.
The SOURCE Payload can be separated into three groups: the camera system, the atmospheric- and re-entry sensors and the IRAS components.
The goal of the atmospheric sensors is to verify and improve simulations of re-entry scenarios with the software PICLas. To achieve this, several sensors are placed on the satellite which carry out measurements over the duration of the mission with a focus on the re-entry of the CubeSat. These sensors consist of two different types of heat flux density sensors, photodiodes, pressure sensors and FIPEX sensors for measuring oxygen developed by the IRS.
The camera system will demonstrate a star- and horizon tracker for attitude determination, as well as observe meteors to improve the understanding of the beginning of the solar system and test the system for future missions. Since the satellite will operate above the atmosphere, the light of meteors is not absorbed and more faint meteors can be observed. The second camera, called PRIma (PR-Imager), will be used for earth observation and to test the camera in space. The camera consist of “commercial off the shelf” (COTS) parts to verify these parts for operation in a spaceflight environment.
As the third payload component, the satellite will carry multiple technologies developed by the IRAS project. The goal of the IRAS-project is to reduce manufacturing costs for satellites by using commercial components. The payload of the IRAS project on SOURCE consists of an efficient type of solar cell by AzurSpace, a smart heater which self regulates the heat output and a multifunctional sandwich structure with built in radiometry sensors, inertial measurement units (IMUs) and strain gauges.
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.
At the moment work is being done to finalize all parts of the attitude control loop. The design of the magnetorquers has been completed and we are now looking for a supplier to make them. The attitude determination and sun sensor algorithms are being developed. All sensors are one by one connected to Arduino microcontrollers, tested and the software to control them is being written. Meanwhile work on the design of the circuit board that will hold all the components as well as the development of the star tracker continues.
Onboard Data Handling
The subsystem on-board and data handling is the central unit responsible for controlling and monitoring the CubeSat. Commands are distributed, the status of the entire satellite is monitored and events are logged. The on-board computer used to satisfy the requirements essentially consists of a microcontroller, different memory units and various interfaces for the communication with other subsystems.
The basic tasks which are assigned to the OBDH system are:
- the storage of housekeeping data.
- the storage and processing of payload data.
- the transition between the satellite modes.
- the execution of the attitude control algorithm.
- the detection and correction of device malfunctions.
- the execution of telecommands.
- the preparation of data for the transmission to the ground station.
As on-board computer with sufficient computing power and memory capacity for the reliable execution of all the tasks the iOBC (ISIS On-Board Computer) was selected in Phase A.
- 400 MHz ARM9 processor
- Volatile memory: 32MB SDRAM
- Mass data storage: 2x2GB SD cards
- Code storage: 1MB NOR Flash
- Critical data storage: 256KB FRAM
The figure above titled "Software Architecture" shows the fundamental structure of the on-board software for SOURCE. The AT91SAM9G20 as execution platform represents the physical layer in the architecture upon which the real-time operating system FreeRTOS is implemented. For the implementation of FreeRTOS, the microprocessor must only provide a timer that generates an interrupt each millisecond. FreeRTOS allows the separation of a program in several processes by managing the assignment of processor internal resources like processor time and memory to the various processes. In contrast to standard operating systems, a process in a real-time operating system (RTOS) with lower priority can never be executed before a process with higher priority. This is one of the crucial features of an RTOS and a prerequisite for ensuring the real-time requirements of the software.
Furthermore, the software is based on a Flight Software Framework (FSFW) developed at the Institute of Space Systems (IRS) which is already being used successfully for the software of the small satellite Flying Laptop. The FSFW provides several building blocks and templates for software components in the form of abstract classes promising a reduction in development time and an increase in reliability for new flight software. As an example, the FSFW contains a sub‑framework facilitating the programming of PUS services. PUS stands for Packet Utilization Standard and is a protocol developed by ESA. This standard specifies a set of mission‑independent services for satellite operation and defines the associated packet structure resulting in communication compatibility between several ground and on-board systems.
For software prototyping, the AT91SAM9G20-EK development board is used which is based on the same microprocessor as the iOBC. Currently, the software consists of FreeRTOS, the FSFW and several PUS services programmed during phase C1. Furthermore, the lwIP stack was implemented and the Ethernet interface of the AT91SAM9G20-EK was configured which allows uncomplicated testing of the PUS services. For the testing, a python script was written that automatically sends different telecommands and verifies their processing through the returned data.
In phase C2, the integration of all the device drivers into the software is planned. This includes the programming of the interface drivers as well as implementing device‑specific components such as communication protocols or troubleshooting measures.
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 communications subsystem has three tasks:
- Receive commands to control the satellite
- Send status information to the ground station
- Transmit payload data
The first two are most critical for the mission, since an erroneous command can easily let the satellite malfunction. Payload data, on the other hand, focuses more on the achievable data rate. Up to 1 Mbit/s is planned.
All data is sent by electromagnetic waves. The direct communication between the IRS ground station and SOURCE is conducted in the S-band (2 – 2.4 GHz). This can also be used to communicate with partner ground stations worldwide. S-band technology is more typical for larger satellites but is now finding its way into CubeSats. Additionally, SOURCE can communicate to commercial satellite constellations in the L-band (1.6 GHz). This allows SOURCE to transmit data regardless of its current position. Such abilities are rare for CubeSats.
The satellite receives signals through two patch antennas. This antenna type can receive from any direction. The signals are then processed by a transceiver, which converts them into computer-readable data. The reverse path, towards the ground station, works exactly the same. This defines the transceiver: it combines a receiver and a transmitter. The currently planned transceiver is supplied by Syrlinks.
All transmitted data is packed into protocols for each layer. Additionally, certain coding algorithms are employed to increase the reliability of the transmission. With these the system can detect and correct errors in any received data. SOURCE uses international standards as defined by the CCSDS. FPGAs provide the computing power to code or decode messages.
The eventual reentry – which will destroy the satellite – may happen over an area without ground stations, for example the Pacific Ocean. To preserve the important atmospheric data that is gathered during this phase, a secondary communications system is installed. It communicates to the commercial Iridium satellite constellation in the L-band (1.6 GHz). Iridium operates a constellation of 66 satellites; primarily for satellite telephones and internet. The transceiver on SOURCE was, originally, meant for regular satellite telephones. Through this network short messages, similar to SMS can be sent.
Operations and Ground
The Operations and Ground subsystem deals with both the ground station and the operations of the satellite. SOURCE will use the ground station of the IRS, which is already in operation and being used for the satellite "Flying Laptop". However for this it is necessary to revise the existing software of the ground station, so that it can also be used for SOURCE. This concerns in particular the flight dynamics tool, which is responsible for the analysis of the navigation data, but also the mission planning tool and the mission control software (MCS). The aim is to automate as much as possible in order to simplify the later operation of the satellite.
In addition, the subsystem is responsible for the operation sequence of the satellite. Various scenarios must be simulated and a time schedule devised in order to ensure the safe operation of the satellite and thus the success of the mission.
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.