ALINX – Empowering innovation from Earth to Space

1 SpaceDot and AcubeSAT

SpaceDot is a non-profit, interdisciplinary student team supported by the Aristotle Uni-versity of Thessaloniki (AUTh). The team was founded in January 2021 with a singular goal: to pave the way for innovative research in space applications. SpaceDot aims to make space more accessible to the broader scientific and academic community and the public. To achieve these aspirations, the team's efforts are amplified by its open-source, open-science stance; almost all research documentation, results, and implementation details are available for everyone to explore.

As of today, the team consists of more than 80 student members from AUTh and other universities across Greece and Europe, spanning various scientific disciplines including Electrical Engineering, Mechanical Engineering, Physics, Biology, Medicine, and many more.

The main project of the team is the design and manufacturing of the AcubeSAT satellite, a CubeSat-type satellite that will host a biological experiment in space. AcubeSAT was selected as one of the three projects of the “Fly Your Satellite!” 3 program in February 2020. In the summer of 2021, the design of the nanosatellite and the mission were fi-nalized and approved by ESA experts. AcubeSAT is now entering the manufacturing and testing phase of its lifecycle, aiming for a launch in 2026. The second project undertaken by the team is PeakSAT nanosatellite. It aims to demonstrate the capability of perform-ing laser links with Optical Ground Stations in Greece.

2 AcubeSAT Qualification

The AcubeSAT nanosatellite has already started the qualification phase, with the first parts undergoing testing to simulate space conditions. Initially, the spacecraft parts de-signed by the team members will be tested for functionality and resilience under these conditions. Following this, the satellite system will be assembled for a complete func-tional test. Environmental testing is conducted in facilities that simulate the stresses the spacecraft will endure. The spacecraft parts will experience vibration testing, to mimic the forces exerted by the launch vehicle and thermal cycling in high vacuum con-ditions, to simulate the orbit around the Earth.

To date, the SpaceDot team has completed testing campaigns for three out of four in-house developed subsystems of the satellite: the Antenna Deployment Mechanism (ADM), the On-Board Computer (OBC)/Attitude Determination and Control Subsys-tem (ADCS) board and the Communications (COMMS) board. The final component to be tested will be the mission's payload, which contains the purpose of the satellite: a biological experiment! Testing these subsystems is crucial to ensure the mission's suc-cess.

3 Communications Subsystem and the Role of SatNOGS

A critical aspect of the AcubeSAT mission is the advanced communication system, which ensures reliable data transmission between the satellite and the ground station. The board takes advantage of both the UHF and S-bands for managing the satellite's teleme-try, telecommands, and the scientific data generated by the biological payload respec-tively. The use of both bands is necessary due to the different requirements of each data category in speed and reliability. More specifically the UHF band falls in the radio ama-teur frequencies, giving us the ability to get precious insight as to the state of the satellite from the community and also adhere to the teams philosophy of open-sourceness and giving back to the scientific community.

(Figure 1: SatNOGS COMMS Board)

The AcubeSAT mission decided to use and contribute to the SatNOGS COMMS board, an open-source design by the Libre Space Foundation (LSF). The decision to incorporate the SatNOGS communications board, which includes the Alinx AC7Z020 FPGA System-on-Module (SoM), was influenced by the need for flexibility, global connectivity, and ad-vanced processing capabilities that the hardware can provide. The SatNOGS board’s open-source nature allowed our engineers to customize both the hardware and soft-ware to suit AcubeSAT’s specific needs. The inclusion of the Alinx FPGA SoM on the SatNOGS board further strengthens the system by providing substantial computational power, enabling the efficient undertaking of complex Digital Signal Processing (DSP) tasks required by the mission.

The Alinx FPGA SoM is central to the AcubeSAT mission, playing a key role in the satel-lite's communication subsystem. This FPGA is responsible for the parallel execution of the DSP tasks that are essential for the satellite's communication operations, such as modulation, demodulation, and the encoding and decoding of signals for error correc-tion. The choice of the Alinx FPGA SoM was driven by its high processing power, flexi-bility, and reconfigurability, which are crucial for adhering to the demanding restrictions required by the mission. Additionally, the FPGA’s ability to be reprogrammed even af-ter deployment offers significant advantages, as it allows the satellite’s functionality to be updated and optimized post-launch. This reconfigurability is particularly valuable in space missions, where the ability to adapt to new challenges or improve performance without physical intervention is a substantial benefit.

(Figure 2: Overview of FPGA’s Modules)

Furthermore, the AcubeSAT mission must abide to several stringent communication con-straints, particularly concerning bandwidth and data rates. The satellite operates within specific frequency bands, namely the UHF (435-438 MHz) and S-bands (2400-2450 MHz). These bands are regulated by international agreements, limiting the available bandwidth for communication. Within the UHF band, AcubeSAT must operate within a narrow bandwidth, which significantly constrains the data rate that can be achieved. This limited bandwidth requires efficient use of modulation schemes and error correction techniques to maximize the amount of data that can be transmitted without exceeding the allocated spectrum, whilst at the same time properly recovering the data intact.

The data rate is another critical constraint. Given the narrow bandwidth, the UHF down-link data rate is typically limited to a few kilobits per second (kbps). This low data rate necessitates careful planning of the data transmission strategy, ensuring that essential telemetry and payload data are prioritized during transmission windows. In contrast, the S-band allows for higher data rates, potentially reaching up to hundreds of kbps, which is used primarily for downlinking the large volumes of scientific data in the form of com-pressed images generated by the payload. More detailed information about our mission can be found in our doumentation.

4 SatNOGS COMMS Board Campaign

An environmental campaign is our way to test the readiness of the subsystems to be part of the nano-satellite. During this one-week-long testing we get to push our board to its limits by conducting functional tests in what is going to be the environment the cubesat is going to be living in, space! These tests take place in appropriately designed areas called cleanrooms, so as to avoid contamination and damage to vital and sensitive electronic parts. In our case the test facilities were provided for by ESA Education at ESEC-Galaxia, Transinne, Belgium.

At the first phase of testing, the boards undergo Vibration (VIBE) tests to mimic the launch. First of all, the board is vibrated in various frequencies in all three axis, X, Y and Z. During, as well as after the vibration tests vital information about the structural integrity of the board was being collected, in order to ensure that the board had not been damaged or altered during the test. Upon completion of each axis, a visual verification of the board is conducted to ensure no damage had been sustained throughout the process.

(a) X and Y Axis

(b) Z Axis

(Figure 3: VIBE Tests)

Finally, besides visually checking the board, the operating engineers ensure it behaves as expected with a series of functional tests.

(Figure 4: Functional Tests During TVAC)

After successful completion of the above, it was time for the Thermal Vacuum Cham-ber (TVAC), which creates an environment similar to that of space. This small chamber serves to both remove the atmospheric pressure, as well as change the temperature of the surroundng environment. The TVAC fluctuates the temperature from approximately -20 degrees to +80 degrees celcius, during what is known as a complete thermal cycle. Since the functionality of the board is susceptible to change during the whole thermal cy-cle, our team of engineers collect data and conduct functional tests, so that we can are once more ascertain no damage was sustained and everything functions as it should.

(Figure 5: COMMS Board Inside TVAC)

Part of all the above tests was of course the SoM from Alinx, based on the xc7z020 chip from AMD (former Xilinx). The industrial grade of the product allowed us to conduct the tests, even when reaching the extreme temperatures of -20 all the way up to +80 degrees celcius. Through those tests, not only did we verify the structural strength of the SoM, but also the resilience of its functionality under the significant stress added by the simulated environmental conditions. In particular, during our testing, we success-fully used the Low Voltage Differential Signaling (LVDS) protocol to send In-Phase and Quadrature data to the AT86RF215 IC, for it to add the carrier to the signal and transmit as RF through the antennas. Throughout the whole duration of all the tests we were also successfully conducting healthchecks through Universal Asychronous Receiver Trans-mitter (UART).

5 Conclusion

In conclusion, our team's efforts create new knowledge in space research, due to the unique payload structure and experiment. The interdisciplinary and open-source nature promoted by our team and team members is enabling a larger audience to engage in space exploration and more specifically in the cubesat industry.

Central to the success of the AcubeSAT mission is the communications subsystem, based off of LSF'S advanced design of the SatNOGS Comms Board featuring the Alinx FPGA SoM. This powerful System-on-Module has proven indispensable in the mission’s com-munications subsystem, enabling the satellite to perform all its difficult tasks with pre-cision and reliability.

The tests conducted, both before as well as during the environmental campaign of our communications board, highlighted the robustness of the design as well as of the AC7Z020 SoM. The successful execution of these tests, especially under the aforementioned ex-treme conditions, underscores the critical role that the FPGA plays in the mission's suc-cess in terms of performance and reliability. As AcubeSAT moves closer to the launch of 2026, the integration of such high-performance solutions not only enhances the satel-lite's capabilities but also sets a new standard for future nanosatellite missions with its innovative payload structure. The dedication of the team, supported by advanced de-signs and products, like the afforementioned, further promotes the cubesat and aerospace industries