15 Jul HF/DF interferometer array construction
Electric plasma in space has always affected radio frequencies transmitted to Earth from man-made satellites – in the same way glass diffracts rays of light through windows. SANSA scientists aim to better understand just how plasma diffracts radio waves. This experiment brings together the SuperDARN radar at SANAE IV, the special coiled-up antenna mounted to TshepisoSAT and a high frequency direction finding (HF/DF) interferometer array under construction at the Hermanus facility under supervision of SANSA scientists, Drs Ben Opperman and Pierre Cilliers.
The project is a collaborative effort with the Cape Peninsula University of Technology (CPUT) and has started small – three separate antennas laid out in the shape of an ‘L’ have been constructed on an area of 10.5 m. Each antenna consists of two square loops one metre in diameter. The mini-array took six weeks to complete and there will ultimately be seven antennas, the last four of which will be built when the test signals from TshepisoSAT’s special antenna are confirmed as successful. The antennas are ready to receive the signal, and the software and cabling required are in place. All that remains is for electronics to be wired, which is managed by the team at the Cape Peninsula University of Technology (CPUT).
TshepisoSAT’s transmissions will be received from a known position in space by either SANAE IV or the Hermanus HF/DF array. The difference between the measured and the true incoming angles of the radio wave at either antenna locations should supply the team with comparable data. This data should be enough to determine just how the signal was refracted by the plasma in space. Experience from the project will be used to calibrate SANSA’s SuperDARN radar and the resulting data will be invaluable to gain a better understanding of how radio signals propagate through space.
A mirror experiment: South Africa and Canada
A very similar experiment to this is being conducted by a Canadian team from the University of Calgary. While SANSA’s beacon in space, on TshepisoSAT, sends a signal to the SuperDARN array, the Canadian team is sending a signal into space from their SuperDARN system. The Canadian satellite, CASSIOPE, carries a payload called e-POP. The Enhanced Polar Outflow Probe collets data from space storms and plasma flow, and receives the low frequency signal sent from the Canadian SuperDARN array.
This mirrored experiment is very useful to the SANSA team. The two teams will be able to compare data collected, analyse disparities and differences and learn more than their experiments would teach them in isolation. Once both experiments are running successfully, the teams will send signals between the two satellites to conduct an occultation experiment. The two will listen out for each other. What will be interesting is the data collected when one of the satellites moves behind the Earth’s horizon. The signal will then have to travel through the Earth’s atmosphere – particularly the ionosphere – and it will be diffracted by space plasma. How much diffraction occurs, and in what manner, is incredibly useful data that could inform many other studies.