Article - Issue 11, February 2002
Microsatellites and nanosatellites: A brave new world
Professor Sir Martin Sweeting OBE FREng FRS
Within the 45 years of man's exploration of space, several distinct phases can be identified. At first, space was the preserve of technologically advanced nations, and the focus of national achievement. Then came commercial applications, again the preserve of very large corporations. Here Martin Sweeting describes a new phase, now nearly 20 years old but with major impacts in the last decade, of low-cost satellites making the exploration and exploitation of space available to smaller countries and organisations. This phase was started at the University of Surrey, which through its company Surrey Satellite Technology Limited, continues to lead the field.
Surrey’s first steps
My personal fascination with space and telecommunications in space began at the time when microelectronic devices were first appearing on commercial markets in the mid-1970s. These devices enabled complex and sophisticated functions to be performed with low mass and power requirements, and at low cost. The University of Surrey’s first two microsatellites, UoSAT-1 and UoSAT-2, were designed and built by a small team of research engineers, radio amateurs and academic staff. Successfully launched ‘free of charge’ by NASA in 1981 and 1984, respectively, they carried payloads for research and education, primarily to demonstrate the potential of such small satellites and also to investigate the suitability of emerging commercial off-the-shelf (COTS) microelectronics for use in space.
Much was learned quickly, firsthand, and sometimes the hard way through these first two missions: this was not just restricted to technical issues, but included management and financial matters. It became clear by 1984 that the UK government would not pursue a national satellite programme independent of the European Space Agency (ESA), and that ESA (and the established aerospace industries) were highly sceptical as to the relevance of microsatellites. A sustainable means of funding would be needed to continue the exploration of small satellites for affordable access to space and meeting the demands of real applications. Surrey Satellite Technology Ltd (SSTL) was formed in 1985 to generate regular income to sustain an activity in small satellite engineering at the University of Surrey from commercial activities and without dependence on government funding. SSTL provided a formal mechanism to handle the transfer of small satellite technologies from the University’s academic research laboratories into industry in a professional manner via commercial contracts.
Evolving a useful microsatellite
Whilst incorporating the latest COTS microelectronics, Surrey’s first UoSAT-1 and 2 microsatellites used a rather conventional physical structure – a framework ‘skeleton’ into which were mounted module boxes containing the various electronic subsystems and payloads, and requiring a complex three-dimensional interconnecting wiring harness. The first two missions demonstrated the need to accommodate a variety of payloads for different customers without having to redesign and re-qualify the satellite structure each time and to meet a standard launcher envelope. In addition, there were increased demands on packing density, electromagnetic compatibility, economy of manufacture and ease of integration. Together these catalysed the development during 1986 of a novel modular design of multi-mission microsatellite platform. This innovative 'modular' microsatellite has no ‘skeleton’ but rather a series of standard outline machined-aluminium module boxes stacked one on top of the other to form a body, onto which solar panels and instruments can be mounted. Each module box houses the various microsatellite subsystems: batteries, power conditioning, on-board data handling, communications and attitude control. Payloads are housed either in similar modules or on top of the platform alongside antennas and attitude sensors as appropriate. Aluminium provides radiation shielding and good thermal conductivity, and is inexpensive and easy to modify.
The microsatellite employs modern, sophisticated, commercial off-the-shelf, electronic circuits to provide a high degree of capability. Communications and Earth observation payloads require an Earth-pointing platform and so the microsatellite is maintained to within 0.3° of nadir by employing a combination of passive gravity-gradient stabilisation (using a six-metre boom) and closed-loop active damping using electromagnets operated by the onboard computer. Attitude determination is provided by the Sun, geomagnetic field sensors, and star field cameras. Orbital position is determined autonomously to within ±50 metres by an on-board global positioning system (GPS) receiver. Electrical power is generated by four body-mounted gallium arsenide solar array panels, each generating approximately 35 W, and is stored in a 7 Ah nickel–cadmium rechargeable battery. Communications are supported by VHF uplinks and UHF downlinks, using fully error-protected packet link protocols operating in conjunction with PC-based groundstation terminals.
It is the on-board data handling (OBDH) system that is the key to the sophisticated capability of the microsatellite. At the heart of the OBDH system is a 80C386 on-board computer (OBC), which runs a real-time multi-tasking operating system. In addition, there is a secondary on-board computer to share computing-intensive tasks and act as a complete back up.
A primary feature of the OBDH philosophy is that all the software on the microsatellite is loaded after launch and can be upgraded and reloaded by the control groundstation at will thereafter. Normally, the satellite is operated via the primary computer and the real-time multi-tasking operating system. All telecommand instructions are formulated into a ‘diary’ at the groundstation and then transferred to the satellite OBC for execution either immediately or, more usually, at some future time. Telemetry from on-board platform systems and payloads is similarly gathered by the OBC and either transmitted immediately and/or stored whilst the satellite is out of range of the control station. The OBCs also operate the attitude control systems according to control algorithms that take input from the various attitude sensors and then act accordingly. Thus it is this OBDH environment that allows such a tiny microsatellite to operate in a highly complex, flexible and sophisticated manner, enabling fully automatic and autonomous control of the satellite’s systems and payloads.
The latest SSTL microsatellite platforms have enhanced subsystems supporting higher frequency, higher data-rate communications, 3-axis attitude control using reaction and momentum wheels, autonomous navigation using on-board GPS receivers and cold-gas thrusters for orbit manoeuvres.
With regular (nearly annual) missions, latest generations of industrial components can be introduced into the satellites to provide leaps in capability – but, importantly, underpinned each time by the accumulated heritage of previously space-flown subsystems. The resulting layered architecture achieves high performance with operational redundancy, but by using alternative technologies rather than by simple duplication.
This modular microsatellite platform design was first flown in 1990 and has since been used successfully on 18 very different missions, each with diverse payload requirements, allowing the spacecraft to proceed from order to orbit in typically around 12 months! During the decade from 1990 to 2000, Surrey’s microsatellites steadily evolved their capabilities, achieved incrementally through regular launches. The emphasis was always on using carefully selected COTS components, coupled with a failure-resilient system design, an appropriately scaled total quality process, and a management ethos more typical of the IT industry.
But what can microsatellites actually do?
There are five major areas of applications we have entered as our satellites have become increasingly competent.
Communications Satellite communications have become synonymous with large geostationary satellites for transparent real-time wideband services. Satellites in low Earth orbits (LEO) are closer to the user and the consequent reduction in transmission loss and delay time are attractive, holding out the promise of less expensive ground terminals and regional frequency reuse. Nevertheless, the communications characteristics associated with a LEO constellation pose quite different and demanding problems, such as varying communications path and links, high Doppler shifts, and hand-over from satellite to satellite. SSTL pioneered the use of early Internet techniques to provide worldwide non-real-time digital data store-and-forward e-mail connectivity – especially to remote regions where the existing telecommunications infrastructure is inadequate or non-existent. Two microsatellites (HealthSat-1 and 2) were procured by SatelLife (USA) to provide routine e-mail communications for medical teams and aid workers in the developing countries and PoSAT-1 provided military e-mail communications for Portugal during the Bosnia crisis.
Space science Microsatellites can offer a very quick turnaround and an inexpensive means of exploring well-focused, small-scale science objectives (e.g. monitoring the space radiation environment, updating the international geo-magnetic reference field, etc.), or providing an early proof-of-concept prior to the development of large-scale instrumentation in a fully complementary manner to expensive, long-gestation, large-scale space science missions. This not only yields early scientific data but also provides opportunities for young scientists and engineers to gain 'real-life' experience of satellite and payload engineering (an invaluable experience for later largescale missions). A doctoral student can initiate a programme of researching, proposing and building an instrument, and retrieving orbital data for analysis and presentation for a thesis within a normal period of post-graduate study. Five of our microsatellites have payloads to monitor the near-Earth radiation environment. Ground-based numerical models are validated with flight data. Simultaneous measurements are made of the radiation environment and its induced effects upon on-board systems. A collaborative microsatellite mission with Chile (FASat-Bravo) carried UV-imaging cameras that provided unique data on the ozone concentrations and structure in the Earth’s polar regions.
Technology verification Microsatellites also provide an attractive, low-cost and rapid means of demonstrating, verifying and evaluating new technologies or services in a realistic orbital environment and within acceptable risk limits prior to a commitment to a full-scale, expensive mission. For example, satellites depend upon the performance of solar cell arrays for the production of primary power to support on-board housekeeping systems and payloads throughout their 7–15 years operational lifetime in orbit. Knowledge of the longterm behaviour of different types of cells in the radiation environment experienced in orbit is, therefore, essential. Unfortunately, ground-based, short-term radiation susceptance testing does not necessarily yield accurate data on the eventual in-orbit performance of the different cells and hence there is a real need for evaluation in an extended realistic orbital environment. UoSAT-5 carries a solar cell technology experiment designed to evaluate the performance of a range of 27 samples of gallium arsenide, silicon and InP solar cells from a variety of manufacturers.
Earth observation Microsatellites have really brought about a revolution in Earth observation. Conventional Earth observation and remote-sensing satellite missions are extremely costly – £300 million is not unusual. Thus there are relatively few such missions and the resulting data, whilst providing impressive spatial and spectral resolution, yield poor temporal resolution (revisit) of ground targets due to the small numbers of these spacecraft actually in orbit. A new opportunity for remote sensing using inexpensive small satellites has come with the availability of (i) highdensity 2-dimensional-array, semiconductor chargecoupled device (CCD) optical detectors (as used in consumer video and digital cameras), and (ii) low-power consumption yet computationally powerful microprocessors. In fact, UoSAT-1 and 2 both carried experimental firstgeneration 2-D CCD Earth imaging cameras. These paved the way for the first operational cameras on board UoSAT-5: the first privately owned Earth imaging satellite, which imaged the oil well fires in Kuwait resulting from the Gulf war. The Tsinghua-1 microsatellite launched in June 2000 provides remarkable 35-metre resolution images in four spectral bands (compatible with LANDSAT) with the capability of ±15° (±200 km) off-nadir imaging coverage upon demand – all at a total mission cost of £3 million, launched into orbit!
Military applications The demands of a military-style satellite procurement and the cost-effective approach to microsatellite engineering might, at first sight, appear incompatible. However, whilst retaining the essential characteristics of low cost and rapid response, a military version of the SSTL microsatellite platform with deployable solar panels has been developed to support various military payloads. The main differences between the 'commercial' and 'military' versions of the platform are in the specification and procurement of components and, particularly, in the amount of paperwork that traces hardware and procedures. An optimum trade-off between the constraints of a military programme and economy has been sought which results in an increase factor for cost and timescale of approximately 1.5 when compared with the ‘commercial’ microsatellite procurement process. The first use of the SSTL military microsatellite platform was on the Cerise mission, designed and built for the French Ministry of Defence (MoD), and launched into a 700 km low Earth orbit by Ariane in July 1995. After a year of perfect operations, Cerise made history as the first operational satellite to be struck by a piece of space debris (a rocket fragment), which severed its stabilisation boom. However, due to the flexibility of the microsatellite systems, SSTL engineers were able to restabilise Cerise by uploading new attitude control algorithms and return it to operations. A second microsatellite for the French MoD (Clementine) was launched into LEO in 1999 and a microsatellite (PICOsat) was launched successfully on 30 September 2001 for the USAF, carrying advanced technology payloads for the US Department of Defense.
Expertise transfer and training using microsatellites
The low cost, rapid timescale and manageable proportions of microsatellites are attractive to emerging space nations who wish to develop and establish a national expertise in space technology through an affordable small satellite programme. Ten highly successful international know-how transfer programmes have been completed by Surrey and SSTL (with Pakistan, South Africa, Portugal, Chile, Malaysia, Korea, Singapore, Thailand, China) and new programmes with Algeria, Nigeria and Turkey are currently under way.
Each know-how transfer and training (KHTT) programme is carefully structured according to the specific requirements or circumstances of the country or organisation concerned, but the first phase typically comprises: academic education through MSc/PhD courses; hands-on engineering training within satellite teams; installation of a groundstation in their home country; design, construction and test of a microsatellite and a know-how transfer package of documentation and software. Over 100 engineers have been trained through these in-depth KHTT programmes at Surrey, and a further 450 students from countries worldwide have graduated from the MSc course in satellite communications engineering unrelated to these KHTT programmes. Once developing space nations have mastered microsatellite technology, the more complex minisatellite provides a logical next step in the development of an increasingly capable national space infrastructure. Our ‘Surrey Space Club’ is the alumni association where the KHTT organisations meet, exchange ideas, share resources, build collaborative projects and learn from each other’s experiences.
But one size does not fit all
With the growing capability of microsatellites, some payloads demand greater power, volume and mass – but still within a small-scale financial budget. SSTL has invested its own funds in the development of an enhanced, modular, multi-mission minisatellite platform capable of supporting missions up to 400 kg and generating up to 1 kW of power.
Launched on a converted Russian SS- 18 ICBM in April 1999, the £6 million UoSAT-12 was funded by SSTL to demonstrate a new product capability in the small satellite marketplace. It represented a major step increase in capability. The UoSAT-12 minisatellite carries:
32-metre GSD 4-band multispectral and 10-metre resolution panchromatic CCD Earth cameras
frequency-agile VHF/UHF and L/Sband DSP regenerative transponders providing both real-time, and storeand- forward communications to small terminals
a comprehensive suite of 3-axis attitude determination and control sensors and actuators
both cold-gas and electrothermal propulsion for orbit manoeuvres.
The results from this mission have been spectacular when compared with the cost. The Earth-imaging cameras using COTS optics and sensors have returned over 1000 high-quality images worldwide and have been used extensively for flood monitoring in South-East Asia. This minisatellite platform is now the basis of the RapidEye commercial 6.5-m GSD multispectral Earth observation constellation to be built by SSTL for Germany and launched in 2004. A 400 kg minisatellite is being designed for a collaborative project with Nigeria to provide real-time Ku-band communications from geostationary orbit for West Africa.
The experience gained from UoSAT-12 has been used to develop an intermediate-class of 100 kg enhanced microsatellites that are targeted for use in constellations and high-resolution Earth observation missions. The first use of the enhanced microsatellite platform will be on the disastermonitoring constellation (DMC) of six satellites in collaboration with Algeria, China, Nigeria, Thailand, Turkey and the UK, and for the TOPSat 2.5-metre resolution panchromatic imaging systems being built by SSTL for the UK MoD. The enhanced microsatellites have on-board gold gas propulsion to maintain the individual satellite’s position in the constellation.
A tiny, yet highly sophisticated, 6.5-kg ‘nanosatellite’, SNAP-1, designed and built at Surrey in under one year was successfully launched in June 2000. SNAP-1 is a highly integrated and complex spacecraft carrying:
advanced micro-miniature GPS navigation
propulsion and attitude control technologies.
SNAP's primary payload is a machine vision system (MVS) enabling the inspection of other spacecraft in orbit. The MVS consist of three ultra-miniature wide-angle and one narrow-angle solidstate video cameras, together with sophisticated image processing electronics. The MVS has also been used to provide medium resolution images at 500 m ground resolution in the near infra-red of the Earth from SNAP’s 650 km altitude, near-polar orbit. SNAP-1 attracted great interest after it demonstrated the ability to image a Russian military satellite in orbit (inset photo above) and then rendezvous some months later with its companion Tsinghua-1 microsatellite following orbit manoeuvres using its on-board GPS receivers and tiny butane propulsion system. Future applications for the nanosatellite are for remote inspection of satellites and the international space station, the monitoring of deployment mechanisms in orbit, and carrying small space science instruments requiring formation flying to yield measurements with spatial diversity.
We need more than just low-cost microsatellites
The story is not complete without further important areas not covered here in any detail. Low cost must extend from the satellite design to the total mission, including launch and in-orbit operations. Initially, our launches have piggy-backed on the launch of large satellites. Most recently, we have had access to demilitarised intercontinental ballistic missiles in the former Soviet Union, where launches can be obtained for around £10,000 per kg, keeping the launch costs to typically around 15% of the satellite cost. Our satellites are designed for a five-year operational lifetime, although UOSAT-2 is still functioning after 17 years in orbit. Conventional ground tracking would cost about £1 million over five years, or 40% of the construction plus launch costs for a microsatellite. Our highly capable satellites are linked to PC-based groundstation computers and achieve a high degree of operational autonomy, with minimum human intervention. At our Surrey Mission Control, a single operator manages some 14 satellites in orbit and is alerted by automatic alarms should anything unexpected occur. The management of our projects is also a low-cost feature: small teams work in close proximity with good communications, with well-informed and responsive management, and a flexible, committed ‘can-do’ culture where individuals assume full responsibility for quality and rigour.
We have many projects in hand, but the future looks even more exciting. Current micro-electromechanical system (MEMS) technology, driven by commercial and consumer requirements will open up the prospect of satellites on-a-chip or ‘femptosats’! One such femptosat is of little use, but a cloud of such satellites with coherent intercommunications and precise knowledge of relative position promises a resilient reconfigurable and highly adaptable entity in orbit, capable of communications, remote sensing by radar or optical observations. In many ways we have already created the PC in space, and our future technology may resemble closely a wireless internet!
Sir Martin Sweeting is Chief Executive of Surrey Satellite Technology Ltd and Director of the Surrey Space Centre. Over the last 20 years, he has pioneered low cost but highly capable 'microsatellites' – originally considered (at best) a curiosity, small satellites have since become a significant factor in space, especially enabling developing countries to access the benefits of space at an affordable level. Sir Martin's team has launched 20 mini-, micro- and nanosatellites for international civil and military customers – and is now constructing its first constellation of EO microsatellites, an interplanetary minisatellite, and researching towards a 'credit-card' pico-satellite.