For the last quarter century Surrey Satellite Technologies Ltd has been manufacturing micro-satellites for paying customers. But with a flurry of activity at the nano-satellite end of the market, a new company project studies how small operational satellites might get while still performing useful tasks. SSTL project lead Shaun Kenyon explains...
Like the first amoebas crawling out of the ocean onto land, the early satellites started out small — though soon made up for it. “You could have picked up the early Explorers in one hand, and indeed even Sputnik was quite small,” noted Sir Martin Sweeting, founder of UK-based Surrey Satellite Technology Ltd. (SSTL), addressing last year’s Appleton Memorial Lecture in London.
Their compact nature was set by the limited nature of early rockets, Sir Martin recounted, “But as the launcher capacity grew so the satellites got fatter, and they carried multiple payloads. Alongside that however, the cost started to escalate, and the timescales for going from concept into orbit started to get very long, sometimes decades long. And as the satellites got more expensive so the ground infrastructure required to support them became correspondingly complex.”
There are very good reasons for this gargantuan tendency: Ensuring reliable operations in the unforgiving environment of space is far from easy, and onsite repair is seldom an option. But Sir Martin built up his world-beating company by acting upon an alternative vision — over the last quarter century, SSTL has launched more than 30 ‘micro-satellites’ within the 100kg class. These are minnows indeed in industry standard terms — with anything under one ton officially classed as a small satellite — but these missions, nevertheless, offer valuable communications and Earth observation services to paying customers. The flagships of the SSTL fleet are the washing-machine-sized spacecraft of the Disaster Monitoring Constellation, which operate together to provide Landsat-compatible surface coverage with an unbeatable revisit rate of just a few days.
Small equals successful in evolutionary terms, with the humble insect far outnumbering more conspicuous competitors such as dinosaurs in the past and human beings today. Sir Martin argues for a similar dynamic in the evolution of space missions.
Smaller satellites are quicker to build and cheaper to launch — giving the option of flying multiple small satellites for the price of a single standard unit. They also open up the possibility of employing the latest commercial-off-the-shelf (COTS) hardware instead of more costly, less advanced space-qualified items.
A lower price tag per satellite means that an increased level of risk becomes acceptable in return for accessing all the latest achievements of the terrestrial electronics industry, substantially boosting a mission’s capabilities.
Small Changes
The satellite industry keeps evolving: SSTL’s favored micro-satellites appear massive themselves next to the nanosatellites currently being built and flown by research institutions and universities worldwide. A new research project led by SSTL’s Mission Concepts team is examining what might be learnt from these miniature new entrants. How much smaller might a satellite be manufactured in the future while still remaining capable of doing a commercially useful job?
The company has made previous forays down to the nano-sat scale. In 2000, SSTL flew the 6.5 kg SNAP-1 (Surrey Nanosatellite Applications Platform), which demonstrated the potential of nanosats for observing larger space vehicles, successfully imaging the larger satellite that shared its flight to orbit. The mission was not followed up, though some of the hardware demonstrated during it ended up in larger SSTL missions.
In the meantime, SSTL’s academic counterpart, the Surrey Space Centre, prepared designs for ‘PalmSats’ — no bigger than a soda can — as well as a ‘PCB-Sat’, with a wafer-biscuit sized satellite based around a single printed circuit board. More recently, the Surrey Space Centre has been busy developing multiple CubeSat missions.
The CubeSat open standard has come to dominate the nano-sat domain. Developed by California Polytechnic State and Stanford universities, this standard is based on a standard satellite shape of 10 cubic cm. For any institution contemplating its own satellite, Cubesats take away the ‘where do I start?’ problem. Parts can either be manufactured in-house or bought in from a growing base of university spin-out suppliers, while the standard also incorporates design and manufacturing best practice. Low-cost rides are available using specially-designed dispenser mechanisms to piggy-back on existing launches.
Within these constraints all kinds of highly-innovative missions can be flown, with larger payloads supported by combining two or three Cubesats together.
Cubesats are not about to sweep away the rest of the industry, however. Their main uses are for education or experimentation, with their capabilities and working lifetime severely limited by their size. While this class of satellite might have a useful supporting role when it comes to in-orbit examination or even maintenance of larger missions, the SSTL Mission Concepts team are most interested in seeking out any potential ‘sweet spot’ in size between micro-satellites on the one side and current nano-sats at the other.
The laws of physics limit how far a Disaster Monitoring Constellation-type imaging mission could be shrunk down in practice. The payload is the real sticking point. A camera must maintain a minimum size of aperture to deliver the sought-after spatial resolution. The remaining area of investigation is to push down the volume and mass of spacecraft subsystems such as power generation, onboard data handling and attitude and orbit control.
Spinning-in New Technologies
The Mission Concepts team plans to attempt this in the traditional SSTL manner: By spinning-in the latest tried and tested terrestrial technologies as much as they can.
The latest solar cells have reached 30 percent efficiency, for instance, which means more power generation for a decreased surface area. Shrink a satellite and its maximum solar array size goes down — to rule out deployable mechanisms that would be bulky and potentially trouble-prone — but the overall power budget needed is likely to shrink, too, in a broadly scalable way. The current generation of ARM or Intel PC chips need much less power (and by extension much less thermal management of their waste heat) than just two years ago.
Another innovation seized upon by the terrestrial industry is Micro-Electro-Mechanical systems (MEMS) technology. These complete devices on a chip are mass-produced in their millions, their applications ranging from anti-lock braking systems to inkjet printer heads. MEMS devices have already made it to orbit: Sweden’s PRISMA double-satellite formation flying mission includes a MEMS-based cold-gas micro-thruster with less than milli-Newton range precision that is reported to be responding as expected despite some concerns about gas leakage in August.
PRISMA is also notable for employing a non-toxic high-energy alternative to hydrazine in its main thruster system. Known as HPGP (High Propulsion Green Propellant) it could turn out to be as an important enabler for smaller satellites — current SSTL satellites also use non-toxic propellents, such as butane, xenon, and even water. They are, however, lower energy than HPGP.
Propellant tanks are less capable of being miniaturized, of course, although as satellite mass decreases, less propellant will be required throughout its operational lifetime. The main issue here is compliance with orbital debris regulations — will a smaller satellite with less propellant still be capable of de-orbiting at the end of its working lifetime? Such satellites could therefore fly in lower orbits — as with most Cubesats at the moment — so they are naturally swept out of orbit by upper atmospheric drag. Or to increase their operational reach, they could be fitted with so-called ‘terminator sails’, being worked on by the SSC among other several institutions, which deploy to increase their overall area and hasten their orbital decline.
Among other promising technologies, RF (radio-frequency) systems on a chip offer a means of shrinking the communications subsystem but also enhanced attitude ability via GPS navigation, and wireless inter-satellite links for putting multiple satellites to work in formation.
Operating such a cluster of satellites might be a way of getting around one of the main limitations of smaller satellites — maintaining adequate communications with Earth. A shrunken platform means a lower diameter antenna, and lower bandwidth downlinking. For the amount of imagery produced by a medium-resolution multispectral imager of the DMC type, data bottlenecking could prevent end-users getting timely hold of the latest images.
Advanced data compression might be part of the answer, but there are also some promising operational techniques. The European Space Agency currently supports a project called GENSO (Global Educational Network for Satellite Operations). The GENSO project is looking at the highly-collaborative operation of multiple ground stations, each one run by a local educational institution, providing a satellite with multiple chances for downlink every orbit instead of waiting for downlink opportunities at a single ground station. A similar concept could be used in a commercial application, and with such a wealth of downlink opportunities, mission data could rain down gradually and continuously as the satellite circles the Earth.
Fractionated Satellites: One Out Of Many
In addition, a cluster of satellites could work together via inter-satellite links, sharing resources as needed. So a single satellite within the cluster might be dedicated to downlinking the data gathered by its companions. This, in essence, is the model called for by DARPA (Defense Advanced Research Projects Agency) in their System F6 (Future, Fast, Flexible, Fractionated Free-flying Spacecraft) project.
Intriguingly, DARPA plans to proceed on an open source basis. Following the same basis as the CubeSat open standard, not to mention the DARPA-fostered Internet, the Agency will make all the interface standards and operating systems developed for F6 available to the general public. This should allow any interested party to prepare their own satellites that will be fully compatible with the F6 network, and capable of joining and extending their cluster.
This open architecture opens the way to a collaborative approach, F6 project manager Paul Eremenko explained in a press release, August 2010, “An explicit program goal is to enable multiple payloads supplied by different agencies, services or even countries to share common infrastructure at multiple levels of security. It is a unique architectural approach to enhancing the adaptability, survivability and responsiveness of future space assets — and really changing the dynamics of the space industry by lowering the barrier to entry.”
Reflecting this approach, DARPA will team with other institutions and possibly international partners to develop the individual elements of their planned demonstration. The fit with Sir Martin Sweeting’s own biological metaphor is clear enough. Out of all the world’s insects, those that come together to swarm or form hives are among the most successful.
The Mission Concepts study on a new smaller platform will not use the F6 concept as a starting point, but it will be included “in the mix” of considerations, along with new terrestrial technologies, and heritage SSTL products and know-how.