Registered as a company not for gain Registration 2016/111111/08
Listen to the weekly Satellite update on Amateur Radio Today
For the past year, in parallel with the development of |Kletskous), analogue CubeSat, AMSAT SA teams have been working on a digital software defined transponder named AfriCUBE. For many of its functions the digital version relies on a Raspberry Pi 3.
‘While the latest version of the analogue transponder performed well during a high balloon test flight into near space last October, it showed that further development was needed to improve the transponder’s receiver sensitivity. This prompted our decision to prioritise the digital SDR development”, Project team leader, Dr Hannes Coetzee (ZS6BZP) said. “Prioritising the digital transponder enhances our opportunity to have it considered for inclusion on the South African Space Agency’s EOSAT-1 project, a high-performance Earth-Observation satellite designed to produce data for a broad range of Earth observation applications.”
The main driver behind the AfriCUBE project is Anton Janovsky (ZR6AIC) who is assisted by a team of volunteers.
AMSAT SA have not abandoned work on the analogue transponder and are looking for budding engineers to join the team. Interested volunteers should email their details to firstname.lastname@example.org.
In a duplex configuration AfriCUBE receives on frequencies in the 435 MHz band and transmits on frequencies in the 145 MHz band. The theoretical bandwidth is 100 kHz but due to the limited spectrum available operation will be constrained to around 30kHz.
The 70cm SDR receiver is based on Mirics MSi3101 Chipset comprising two complex chips: the MSi001 wideband tuner and the MSi2500 analogue to digital converter. The MSi2500 also contains a USB interface, and uses the SPI bus to control the MSi001 tuner. The tuner contains a programmable frequency synthesizer and down converter which produces I/Q data at baseband or low-IF. Its front end is a series of low noise amplifiers optimised for various frequency bands between 150 kHz and 1,7 GHz. Each LNA has noise and gain figures appropriate to the band. The MSi2500 does analogue to digital conversion and signal processing. The ADC is nominally 12 bit, effectively 10.4 bit. Both chips run off 3.3V and use little power, which makes it a good candidate for AfriCUBE where most of the solar generated power is needed for the final transmit amplifier. The MSi2500 is preceded by bandpass filters.
The IQ Digital Signal Processing (DSP) required to implement the Software Defined Radio (SDR) functionality is performed by a Raspberry Pi 3A which also controls all the housekeeping functions on the CubeSat.
The Automatic Positioning Reporting System (APRS), Morse Code identification and IQ DSP transmission generation is done with Gnuradio which is a free and open-source software development toolkit that provides signal processing blocks to implement software radios. The transmit RF generation is with C library RF signals from IQ sources. The output stage utilises a RF amplifier using a Qorvo TQP7M9105 device. Output bandpass filters from Mini circuits (SXXBP – 150+) are incorporated to prevent any unwanted signals from being transmitted.
Common CubeSat modules
During the development of the analogue CubeSat, the team designed and built several modules that are common and will also be deployed in the building of AfriCUBE. These include the electronic power system (EPS) developed by Fritz Sutherland Jnr, ZS6FSJ, the space frame and stabilisation system. The development of the solar panel and antenna deployment systems are still in progress, but good headway is being made and will be reviewed and tested in the next few weeks.
SA designed spaceframe
While CubeSat space frames are commercially available from several manufactures, they are very costly and with the Rand/Dollar relationship of recent years, buying a space frame from overseas was not an option for AMSAT SA. The team opted to develop their own spaceframe. The local design has come a long way and last year entered the industrialisation phase which will scale up the manufacturing process.
Another motivation for developing a South African CubeSat Spaceframe was the inclusion of fold-out solar panels in order to have more power available. This is no mean task to achieve this in a 10x10x10 cm space and keeping the mass low to allow for maximum mass for the electronics inside the spaceframe.
Deon Coetzee, (ZR1DE) a keen DIY enthusiast, set out to develop the first space frame and built several prototypes. The last one which showed the most promise featured a fold-out mechanism for the solar panels and a release mechanism for the antennas but there was one problem, it weighed too much. With a mass of nearly 500 grams it took up 35% of the allowable mass of 1,33 kg.
He took the problem to Dr G A Oosthuizen in the Industrial Engineering department at Stellenbosch University, just in time for the allocation of final year projects for their students. Francois Oberholzer was allocated the project and set out examining Coetzee’s design, investigating how the space frame could be made lighter but would still have the required mechanical strength to withstand the harsh launch conditions on its journey into space.
To design an effective manufacturing process, it is important to investigate the material to be used. Oberholzer evaluated various materials including titanium but settled on aluminium 7075 which is the material used for the prototype. Although titanium may be stronger, it is a very difficult material to machine, which greatly increases the tooling cost. Aluminium 7075 proved to meet all the strength requirements while being lighter than titanium.
The next step was to design the manufacturing process. Oberholzer was part of the Industrial Engineering Department’s Rapid Prototype Development Lab, where extensive research is done on manufacturing processes and machining of alloys. The work he was doing was ideally suited to develop a lighter Kletskous space frame. His brief was to develop a design that could be replicated by other satellite builders.
“It was fantastic to have a student as enthusiastic as Francois on the team,” Coetzee said at the time. “At one stage I began to believe that we had embarked on project impossible, but now it all is coming together”.
Students graduate and move on to launch their own careers. “AMSAT SA is proud of having worked with Francois Oberholzer and motivating him to achieve a Masters’ degree with the development of a spaceframe”, Coetzee said.
However, more development was needed. Frik Wolff (ZS6FZ) took over the project and made several improvements on the design including the further development of the foldout solar panel and antenna launch mechanism.
AMSATSA is now at the stage where industry must become involved to industrialise the manufacture. Frik Wolff approached Dr Gary Immelman, CEO of Deckel Sportslids, an automotive component manufacturer in Vereeniging, to evaluate the work done to date and take it through an industrialisation process and manufacture a finished spaceframe that would be fully reproducible and meet all the specifications. Dr Immelman (ZS6YI), himself a radio amateur, handed the project to one of the company’s senior engineers, Juan-Pierre Menezes (ZR6YI) The company produced three spaceframes which are now being fitted with all the electronics and the proto-type solar panel and antenna deployment mechanism for further testing.
In addition to Amateur Radio, AMSAT SA is also focussing on Citizen Science, a concept that promotes science as a hobby to learners and students as well as the man in the street. While the primary payload is an amateur radio transponder which requires an amateur radio license to utilise, the telemetry can be received freely. It is this part of the project that will create interesting science observation opportunities.
Anton Janovsky ZR6AIC
Various components of AfriCUBE.
Some of the team members discussing the transponder. L - R Nico van Rensburg ZS6QL, Anton Janovsky ZR6AIR, Hannes Coetzee ZS6BZP and |Fritz Sutherland Jnr ZS6FSJ
Deon Coetzee ZR1DE with one of the first space frame prototypes
Frik Wolff ZS6FZ demonstrating the fold out antenna and solar panel
Francois Oberholzer who worked on the development of the space frame as part of his Masters' degree at the Stellenbosch University
In 1999, California Polytechnic State University (Cal Poly) and Stanford University in the USA developed the CubeSat concept to promote and develop skills necessary for the design, manufacture, and testing of small satellites intended for low Earth orbit (LEO) deployment. CubeSats, typically 10 x10x10 cm and a mass of around 1.3kg, were instantly popular among Radio Amateurs, Academic institutions and later also in the commercial satellite world.
One of the contributing factors to their instant popularity was the ease with which they can be launched into orbit as secondary payloads on commercial satellite launches and from the International Space Station. Cal Poly also developed a Picosatellite Orbital Deployer (P-POD) which can release 3 CubeSats into orbit. In addition, the P-POD’s design allows several deployers to be mounted together on one launch vehicle. During the deployment sequence the CubeSats ride on rails built into the corners of the tube and a simple spring provides the force to push the CubeSats out of the deployer with a linear velocity of approximately 0.3m/s. Deployment is initiated by the release of the P- POD’s spring loaded door. To ensure a smooth ejection from the P-Pod, the spaceframe must be within the allowable tolerance specified making the spaceframe the most important part of a CubeSat.
Kletskous team members
AMSAT SA acknowledges the following persons who made a major contribution to AMSATSA CubeSat projects
Leon Lessing ZS6LMG