BSc Thesis
You can find the full thesis here.
Introduction
Having dedicated a big chunk of my student life working on the AcubeSAT project, I chose to write my Bachelor thesis on AcubeSAT. If you are interested, you can read the abstract and a quick summary of the results on this page.
Abstract
The number of small- and nano-satellites being launched into orbit is expected to continue to grow in the coming years. In order to avoid the creation of space debris,spacecraft developers have to take action in order to reduce the severity of space debris. A widely used guideline is to ensure that no space system is left in the Low Earth Orbit environment for more than 25 years after the end of mission. Given this guideline, this thesis investigates 1U, 2U and 3U CubeSats in Low Earth Orbit using the General Mission Analysis Tool (GMAT) in order to determine the maximum allowable value of the semi-major axis so that the 25 year limit is met. This analysis results in orbital decay diagrams which can be used by CubeSat developers in order to evaluate different orbits early in the design phase. These findings are then applied to the AcubeSAT mission in order to perform an orbital analysis tailored to the mission. AcubeSAT is a 3U CubeSat that is currently being designed by students in the Aristotle University of Thessaloniki with the support of the Education Office of the European Space Agency, under the educational Fly Your Satellite! programme. The objective of AcubeSAT’s mission is to probe the expression of eukaryotic genes in the environment of Low Earth Orbit. AcubeSAT’s design calls for a very specific orientation to be achieved in order to downlink all images to the ground segment. This orientation is evaluated using the theory of spin-orbit coupling to determine if this orientation can be maintained in the event of a failure in the Attitude Determination & Control Subsystem of the spacecraft. Based on the findings, an alternative design solution is also proposed.
Main results
Orbital decay diagrams
To determine the maximum allowable altitude for a 3U CubeSAT, we followed the methodology presented in SMAD1, taking into account Solar Cycle historic data and predictions to model the upper atmosphere using the F10.7 index. Using the General Mission Analysis Tool (GMAT) and Python, we were able to re-create the orbital decay diagrams which can be found below.
Mission analysis
Having determined the maximum allowable attitude, we investigated three main orbits to be used for the AcubeSAT mission, namely:
- ISS Orbit
- SSO - 550 km
- SSO - 450 km
We concluded that both orbits could satisfy the mission requirements at a time. However, during the Critical Design Review of the spacecraft which took place in 2021, it was determined that the ISS orbit could not produce a positive power budget and it was therefore ruled out.
Spin orbit coupling problem
To downlink the payload data (which comes in the form of images with a total size of about 1.5 GB) from the spacecraft, AcubeSAT uses a patch antenna designed to operate at the S-Band (2.45 GHz). This antenna is placed on one of the long faces of AcubeSAT (+X), and this face has to continuously point towards nadir. However, our analysis indicated that this position is the unstable solution of the spin-orbit coupling problem in astrodynamics, and thus in case of an ADCS failure, the spacecraft cannot maintain the desired pointing. We therefore concluded that if we place the patch antenna on the -Z face and point this smaller face towards nadir we would use the stable solution of the spin-orbit coupling problem, resulting in a better chance of maintaining ground contact via the S-Band in case of an ADCS failure.
Full thesis and references
If you want to find the full thesis text, check out the code used or simply want to check our sources, be sure to find the full thesis here.
Wiley J. Larson and James R. Wertz, Space Mission Analysis and Design, 3rd edition ↩︎
