Uncoiling an antenna in space


While the new SuperDARN digital radar has been installed in Antarctica, back on the home front SANSA is managing an experimental aspect of South Africa’s first CubeSAT mission. The experiment aims to determine how to broadcast a high frequency radio signal using an antenna on a satellite that can fit in the palm of a hand.

The TshepisoSAT satellite had to have an antenna of 10.5 m attached to it. That’s a big feat for a small satellite. Dr Robert van Zyl from the Cape Peninsula University of Technology (CPUT) devised a rolled up antenna which is coiled like a fishing rod reel.

Now TshepisoSAT has been launched, the 0.01 mm thick wire must be slowly uncoiled into a straight antenna. With a small tip mass placed at the wire’s end, the satellite will be put into a spin, using the Earth’s magnetic field and two small electromagnets fitted to the CubeSAT. This process should slowly uncoil the wire, which is similar to a piano wire in rigidity. If it bunches up or bends too much, it will fail.

Between two and ten centimetres of antenna wire will be uncoiled each day. If all goes well, the antenna should be straightened out over a period of several weeks. In early June, SANSA’s team had already uncoiled three metres of the antenna. Of course, this isn’t visible. The team use mathematical calculations to determine how much has been deployed, and once the signal is strong enough, they will be able to receive it as it is already being transmitted.

TshepisoSAT is the first nano-satellite to be constructed in South Africa. Funded by the Department of Science and Technology, it was designed and built by postgraduate students at the French South African Institute of Technology (F’SATI) at CPUT, in collaboration with SANSA. This approach offers students a unique learning experience and prepares them to participate in the South African space industry.

The satellite is currently healthy and with most tests completed it is ready to begin its experiment. When extended, the antenna will transmit a simple radio signal that can be received by the Hermanus HF/DF array and the SuperDARN radar in Antarctica, as well as the rest of the SuperDARN network. The experiment’s scientific investigation is led by SANSA’s Drs Ben Opperman and Pierre Cilliers.

This will ensure there is comparable data and enable the team to determine what effect the plasma in space is having on the travelling radio wave as it propagates through it. Once the effects of the plasma are known TshepisoSAT can act as an HF beacon, and since its location in space is accurately known, it can be used to calibrate the SuperDARN radar.

In late March and early June, TshepisoSAT had near misses with space junk. Since the satellite has no propulsion control, CPUT has no means to move it out of its orbit to avoid a collision with this kind of debris.

Morse code

The transmitted TshepisoSAT HF signal contains a simple message spelling the word CPUT in Morse code. This helps CPUT, SANSA and any other agencies assisting them to identify TshepisoSAT.

Once the warnings were issued that TshepisoSAT was in danger, SANSA determined the geographic position where the collision could occur in space and CPUT needed to find a partner agency close to this location to listen out for the signal from the satellite in order to establish whether it had survived the three respective close approaches. After a nail biting wait, a group at California Polytechnic informed the CPUT and SANSA teams after each approach that they were, in fact, misses, and that they could still hear the signal sent out by TshepisoSAT.

These close calls illustrated once again how big a concern space junk is. With both the satellite and the debris travelling at up to 14 kilometres per second, a collision would result in the complete destruction of a satellite. This, in turn, would just create more space debris which increases the chance of causing more collisions.

Space Junk

Space junk is uncontrollable debris in space – remnants of dead satellites and rockets. Most satellites and space vehicles use a low Earth orbit, which is where space junk is concentrated. But space debris presents an even bigger danger, known as the Kessler effect. Donald Kessler theorised in 1978 that space junk has the potential to cause a domino effect rendering space exploration and the use of satellites impossible. His theory is that with every collision, more space debris is created, increasing the potential for further collisions. This effect is replicated exponentially until the lower orbit around Earth is filled with an impenetrable layer of space junk.