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All British satellite project nears completion

Proving technology in space is not a cheap business. 'Heritage' technology – that which has been flown successfully in space before – is essential for companies requiring guaranteed performance when their own technology is committed to space. The nature of space is that you just can't afford to get it wrong.

A British venture is hoping to progress a number of technologies by giving them the opportunity, at a relatively low cost, to both prove the technology and give it valuable flying time. The project, called TechDemoSat, is led by Surrey Satellite Technology (SSTL). Its platform will carry eight payloads, along with a stack of its R&D, into orbit later this year.

Project manager at SSTL is Victoria O'Donovan: "It is a really interesting project because normally a satellite just has one payload, one aim of what it wants to be – an imager or GPS or a science mission. This has everything. It's got eight payloads instead of one and all sorts of product development; from the onboard computers to techniques for laying down solar cells."

The project started when SSTL had its frustrations trying to test product developments and suspected others shared the same frustrations. In 2009, it approached the Technology Strategy Board and the now defunct South East England Development Agency with a view to developing a programme that could deliver benefits across the UK's aerospace industry. Grant applications were submitted in 2010 and the project kicked off in October 2010. In order to keep costs down, TechDemoSat is an auxiliary load on the launch vehicle, which means the launch date will be determined by the primary load, but it is expected to be in orbit by Q3.

When proposals were invited for payloads, it was massively oversubscribed. An independent consultant VEGA Space (now Telespazio VEGA) determined the successful bids and this has evolved to form the list of eight that will now fly in the satellite. SSTL was not part of this process, it just needed to know the basic technical requirements – how big, how much power and data, earth or space pointing. After this information was presented to SSTL, it could decide how to fill the remaining payload capacity.

The payloads selected are:
• SSTL's Sea State Payload that will demonstrate how GPS signals reflected off the ocean's surface can be used to determine ocean roughness and help shipping plan more efficient routes.
• MuREM, a miniature radiation environment and effects monitor supplied by the Surrey Space Centre.
• The Charged Particle Spectrometer, a radiation detector developed by the Mullard Space Science Laboratory that can perform simultaneous electron-ion detection.
• The Highly Miniaturised Radiation Monitor from Rutherford Appleton Laboratory and Imperial College.
• The Langton Ultimate Cosmic Ray Intensity Detector (LUCID). Developed by the Langton Star Centre, part of a sixth form college, the detector can characterise high energy particles.
• A Compact Modular Sounder system, an infrared remote sensing radiometer unit, provided by Oxford University's Planetary Group and Rutherford Appleton Laboratory.
• SSBV's CubeSAT ACS payload, which will provide three axis attitude determination and control.
• The Cranfield de-orbit sail, designed by Cranfield University, will move the satellite to burn up quickly in the Earth's atmosphere at the end of its life.

The satellite itself is the SSTL 150 – a 150kg satellite used on a previous SSTL mission called RapidEye, which was used as the starting point for TechDemoSat. O'Donovan explained how the equipment on the TechdemoSat had been assembled. "The primary string is our proven avionics that has flown on other satellites. You would then have a redundant, which is your second string." Under normal circumstances, this second string would also have to be space proven, but this is not the case with TechDemoSat.

O'Donovan continued: "The secondary string for the platform is all SSTL developments, so there are new onboard computers, new solid state data storage, new battery charge modules and solar cell lay-down techniques – it's all on the platform. For data, we have an S Band rf link as our primary and a slightly more capable X Band on the secondary."

According to O'Donovan, her biggest technological challenge has dealing with so many payloads. "We had all these different technologies arriving and had to work out how this development talk to that one, what happens when this draws power and so on – it is quite challenging because a typical satellite only has one payload and we have eight. You might normally have one piece of new development which you don't know exactly how it will work, and on this we have 15 to 20 new SSTL developments. So the challenge is to get it all to work together in this very small system in a small space of time with a limited budget.

"We communicate using the CANbus and that was a specification when we invited people to submit for a payload. We couldn't have lots of different buses – everyone had to use the same thing. It is a protocol that I don't think many people use, but it is typical in satellites."

CAN nodes on the spacecraft use an SSTL proprietary protocol, known as CAN Spacecraft Usage, where the most significant byte in the arbitration field is used as a destination node address. Each module connected to the CAN bus has a unique node address and SSTL spacecraft may support up to 250 nodes (certain node addresses are reserved). In addition, there are two physically separate CAN buses, primary and secondary. All units communicate initially on the primary CAN bus on power-up, then switch to the redundant bus if they do not receive CAN messages within five minutes.

TechDemoSat is scheduled to be in space for three years. The first month will be taken up by stabilising the flight and getting the platform in stable operation. This will be followed by two months of commissioning the new technology, followed by a seven month period during which the payloads share resources on an eight day cycle (two days each) to gather all of the information required to satisfy their objectives.

Unlike an ordinary commercial operation, once that initial phase has been completed, the gloves are off, as O'Donovan explains: "After the first year, and until the end of the three years, is what we call 'extended operations'. It is continuing data collection – the eight day cycle – but when everyone has got what they want and we know how the platform is behaving, we might be able to do something a bit funkier. So they have their standard operations and there is scope at the end to try them out in anger and see what they are really capable of!"

At the end of the three years, after waiting patiently in the sidelines, the final payload, Cranfield's de-orbit sail, will be deployed and bring TechDemoSat back into the earth's atmosphere.

Author
Tim Fryer

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