Golden eye in the sky: deployable antennas for satellite systems
Image credit: Oxford Space Systems
If you crossed Goldfinger with Moonraker, you might come up with an image like that pictured above. What this actually shows is a computer-generated image of a deployable antenna, woven in gold, for use with communications satellites.
Using gold-plated wires thinner than hair, a team at Nottingham Trent University (NTU) is working in collaboration with Oxford Space Systems (OSS) with a view to developing a new type of deployable antenna for satellite systems.
Satellites are pretty expensive to build, but they are very expensive to get into orbit. Although recent events could indicate a drop in costs, the typical estimation for putting a kilogram of payload into low-Earth orbit (LEO) is in the order of $10,000. LEO varies in definition but is generally regarded as being an altitude in the region of 160km to 1,800km.
If a satellite weighed, for example, 1,000kg, it costs $10m just to get it into its operating environment. Size is also important. The more compact a unit is the more you can get on the launch rocket, so costs can be shared. For this reason satellite makers like to keep their creations compact, and one way of doing this is to use deployable structures.
The most familiar image of a deployable unit is that of solar panels fanning out from a satellite, but there are several other devices, that benefit from being remote from the spacecraft. One such device is the antenna, which can take up considerable space once deployed without being very heavy.
OSS specialises in deployable structures and, although it has only been around since 2013, already has flight-proven technology under its belt. One interesting aspect of its development process has been the involvement of a ‘professor of origami’ to help it develop optimal folding techniques – an important part of its technology.
The metal mesh being developed with NTU is intended to work with two types of deployable antenna that OSS is working on. The first is an expanding ‘outer-ring’ parabolic antenna for large diameters (4m to 14m diameter). The second is a ‘wrapped-rib’ antenna that uses OSS’s proprietary flexible composite material to unfurl and tension the metal mesh for antennas between 0.5m and 4m.
Mike Lawton, founder and CEO of OSS, explains that these antennas could be used on a wide range of satellites. “It can be anywhere from a microsat weighing from 75kg and the size of a domestic fridge in LEO, through to satellites weighing 6,500kg and the size of a large transit van in geostationary orbit. In LEO, large deployables will be used for Earth observation radar systems and for detecting very weak signals from ‘Internet of Things’ transducers. In geostationary, large deployable antennas are used for data comms and satellite TV.”
Researchers from the Advanced Textiles Research Group (ATRG) are using a proprietary ‘knitting’ technique to create the antenna reflectors from high-performance gold wire.
“Few people associate knitting with high-end space technology,” says project lead Professor Tilak Dias, head of the ATRG in NTU’s School of Art & Design. “However due to the advancements in knitting technology we can now knit an antenna which is extremely lightweight, cost-effective and robust enough to withstand solar radiation.”
One of the interesting aspects of this project is that it is building the space sector’s supply chain within the UK. The government has pledged that the UK will win a 10 per cent share in the global space market by 2030 and building a supply chain will be one of the factors that determines if its goal can be achieved. Having high-performance space materials is one link in that chain.
The wire is less than half a millimetre thick and is gold-plated to make it more resistant to the harsh operational environment of space. By using advanced knitting techniques, the researchers aim to produce membrane-like structures that can be shaped into 3D parabolas resembling the ideal reflector antenna geometry. The targeted first flight date is 2020.
Researcher and senior lecturer Will Hurley says: “By applying novel knitting techniques, we can eradicate waste from the manufacturing process and save on valuable resources. When you consider that knitted gold wire can cost hundreds of thousands of pounds per square metre, waste is something we have to be very mindful of.”
So why use gold? Is it the best and only material for the job? Lawton takes up the story: “The wire used for flight surfaces is actually gold-plated molybdenum or gold-plated tungsten. A gold plating is used as it plastically deforms under tension, so that when wires are pulled against each other, they ‘mould’ into each other to give a high-quality electrical connection and an electrically neutral overall parabolic surface.”
It is a high-value product but the cost of on-orbit recycling means that it is still more expensive than the cost of flying new. Lawton believes that could change: “Changes are coming. In the same way that SpaceX is leading the way in re-using/recycling external booster stages, so many entrepreneurial companies are exploring on-orbit recovery of space hardware for reuse.”
In fact, the relatively low cost of SpaceX for delivering to space (around $1,700 per kilo) might further make the use of reclamation equipment in space viable.
Antennas
At last year’s IET Innovation Awards, the winner in the Communications category was a single chip from Xilinx that could enable the implementation of 5G wireless networks. Here is an overview of the technology it developed.
5G networks must carry larger numbers of subscribers and support faster data rates than 3G or 4G. Among the several advanced techniques that are used to achieve this, massive-MIMO (Multi-Input/Multi-Output) implements many more antennas in 5G base stations than has been the case in 3G or 4G equipment. These large antenna arrays need to be connected to the base station electronics.
In prototype networks and early field tests, this has been done using large and power-hungry circuitry that is not suitable for use in consumer-grade networks. Although this circuitry was state-of-the-art when work on 5G began, the industry now needs a smaller and lower-power solution for building these interfaces in commercially deployable networks. Xilinx’s RFSoCs are the industry’s first to deliver the technical leap needed to implement massive-MIMO interface circuitry to within practicable constraints on size and power consumption.
One important breakthrough has been in integrating communication-grade RF-sampling data converters, alongside the digital processing, within the same integrated circuit. These RF-sampling data converters translate the radio signals received from subscribers’ handsets into digital data that can be moved through the network or stored on the subscriber’s cloud.
Large numbers of converters are needed to connect massive-MIMO antenna arrays. To create the space-saving RFSoC, Xilinx developed advanced signal-processing techniques including Direct RF sampling, which enables the entire conversion to be done on one CMOS chip instead of a complex network of discrete analogue components. Moreover, integrating the entire circuit on the same silicon eliminates signal distortion introduced by copper interconnects.
In addition, because RFSoCs are all-programmable, engineers can implement new algorithms when needed simply by reprogramming, without having to wait for new chips to become available. This provides important flexibility, as the GSM Association’s 5G standards are continually evolving.
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