Meeting the demands of space

Surrey Satellite Technology Ltd (SSTL) was formed in 1985 as a spin-off from the University of Surrey to commercialise the results of its innovative small satellite engineering research. The company specialises in the design, manufacture and operation of high performance small satellites, and as a result 'Surrey' is a name well known in the global space and satellite industry.

The company employs 230 staff and has launched 27 small satellite missions since it was formed. Typically, SSTL's satellites are used for imaging, GPS applications and communications, and its orders often come from foreign governments or organizations who seek a more affordable way to get into space. Despite this affordability, designing, manufacturing and sending a satellite to space is far from cheap, and the company's contracts are for millions or indeed tens of millions - relatively inexpensive when compared with a large satellite. The smallest satellite available weighs approximately 60-70kgs and is the size of a small domestic fridge - the largest offered by the company can be as big as a hatchback car. Due to the specificity of the missions, the satellites are almost always built bespoke, although some models share a number of key components.

SSTL's employees are based largely near the university in Guildford, Surrey, but the company will soon be officially opening a new facility in Sevenoaks, Kent, that is devoted to the design and manufacture of optical payloads for the satellites. This facility features three new cleanrooms.

The Space Centre in Guildford also houses three cleanrooms and an integration hall in addition to a full suite of design laboratories, and these key facilities can accommodate up to four microsatellites simultaneously.

Risks in satellite manufacture

SSTL's 'Clean Team' have assembled almost all of the printed circuit board for more than twenty missions, and the assembly of flight electronics takes place in the clean room suite in the Space Centre. The suite consists of a flight assembly area, containing surface-mount workstations, an integration hall with external access and an overhead crane and a small instrument room with blackout facility.

In these cleanrooms, the class of cleanliness does not need to be as high as in pharmaceutical or semiconductor manufacture, but a controlled environment is still of great importance and a Class 10,000 standard is necessary to achieve this. In the newly built optical payload cleanrooms, Class 100 is used, because of the increased delicacy of lenses and components in the imaging systems that will be sent out to space. Virtually none of the work is automated, and workers solder joints by hand.

Facility manager Tim Gilbert says that the risk of particle contamination is not massive when manufacturing the sub systems, but humidity is the biggest enemy and if it gets into components, creates moisture and causes irreparable damage that could jeopardise the success of the mission. Joints, (using surface mount technology to minimize vibration) can be corroded by excess humidity, and if they were to fail and their full adhesion was lost, the centre of the satellite may break-up, with disastrous consequences. Humidity inside encapsulated components can outgas in a vacuum environment such as space, releasing silicates and creating further problems.

In light of this, humidity levels in the cleanrooms are kept at around 40%, with 60% as an absolute maximum. Electrical components are also susceptible to damage from electrostatic discharge, and ESD-safe clothing, footwear, seats and flooring are in operation.

Regular precautions

Dust can taint components and the delicately soldered connections and is kept to a minimum by filtration systems and climate controls alongside regular validation and clean-downs, or “good housekeeping” as Gilbert puts it. SSTL use manufacturing and assembly guidelines from the European Space Agency (ESA), which are acknowledged to be stricter than is absolutely necessary, but is frequently a demand from customers. Throughout the manufacturing process (usually a period of 2-4 years depending on the project), the satellites are regularly tested to ensure they will be able to withstand the rigours of space - thermal cycling and vacuum testing are two ways in which tests are done. This testing may be done outside the cleanroom, where there are more facilities and greater room, but contamination is kept under control in transit. The tests are essential to a mission's success, says Gilbert: “We do our best to break the satellite down and exceed the environment it will experience in space, and in the process we can establish and find a solution to the vessel's weaknesses.”

High-tech facilities

The new facilities at Sevenoaks are designed around the manufacture of optical payloads, but the company was keen to make them as flexible as possible, in order that the cleanrooms and labs can be used for other projects. The July 2006 tender for construction was won by Hove, UK-based Cleanroom Solutions. With a higher standard of contamination control, (Class 100) the new cleanrooms use propulsion systems to cut down particle contamination. The facilities feature three dedicated darkrooms for optics, and having a cleanroom that is also a darkroom posed specific challenges - doors have validated seals to keep out light and contamination - working in the dark takes some getting accustomed to, says Richard Atkinson, commercial manager for the firm's Optical Payload Group.

Cleanroom space in the Sevenoaks facility is 200m² in a building of 1800m². A full body suit and eye protection is worn in these Class 100 environments; as the imaging systems become more and more advanced (with greater ground resolution), the standards needed to protect their components rise too.

Assembly and testing

In a tour of the Guildford cleanrooms and Assembly, Intergration and Test (AIT) area, cleanroom manager Andy Waites explains that the solar panels are often the most expensive and intricate part of the satellites, and as such it is essential to eradicate humidity and dust in order for the adhesives to work effectively, and for the electronics to stay intact. ESD precautions include the surfaces and benches mentioned earlier, but earthed wriststraps and foot straps are also employed to conform to the ESA's strict standards.

When the electronic modules are passed between the cleanrooms and the AIT area, or for bench testing in labs ESD bags and boxes are used. Waites says that the “mindset change” involved when one enters a cleanroom ensures the quality and diligence of the work and consequently pleases customers. The cleanrooms are independently validated every 6 months. There are no health and safety concerns particular to making satellites to take into account, but due to the considerable size of the vessels, care must be taken when they are moved or turned during assembly. Single-piece aluminium panels that encase the subsystems and electronic modules, whilst lightweight, still present health and safety risks due to their size and fragility. But carbon fibre casing is a new material occasionally being used for the panel of the spacecraft, says Ed Stevens, who is responsible for AIT, and this requires even greater diligence and less worker interference to ensure it is in the best possible condition for the mission.

Completing the mission

Currently, the team is constructing five satellites for the German company Rapideye, and launch dates are looming. After approximately six months of AIT, the spacecraft will have three months of environmental testing, and then they will be delivered to the launch site, usually in Russia or Kazakhstan. Delivery to launch is, of course a critical time, and attention to contamination control is just as vigilant in this period. Specially made transit cases are used, with the craft mounted inside to avoid shock damage in the cargo train or freight plane. Again, temperature and humidity are enemies here especially because the payloads (typically optical systems) are already installed, and high-tech sensor systems monitor the conditions inside the module boxes.

Waites and Stevens say that this stage is both very exciting and stressful, “a big event”, but a successful launch is the perfect release of this tension “It's what we're here to do”, says Stevens.