SPACE AT SURREY SMALL SATELLITES FOR AFFORDABLE ACCESS TO SPACE

MN Sweeting

Director Surrey Space Centre

University of Surrey

1. ABSTRACT

Changing world politics and military emphasis has brought considerable pressure on space agency financial budgets and a shift to increasing commercialization of space activities. Budgetary pressure, coupled with the rapid advancement of commercial and consumer micro-electronics, has catalyzed the use of smaller and more computationally capable satellites as a ‘faster, cheaper, better’ means of realizing space missions - complementary to conventional large satellite systems. Affordable small satellites, however, require a very different approach compared with established space engineering techniques. Since 1979, the University of Surrey (UK) has pioneered cost-effective satellite engineering techniques for small satellites, and has developed a series of highly sophisticated, yet inexpensive, ‘microsatellites’ - built for around US$3M each. Fourteen such microsatellites have been launched by Surrey into low Earth orbit carrying a wide range of satellite communications, space science, remote sensing and in-orbit technology demonstration payloads both civil and military. Seven microsatellite missions have also provided the focus and means for highly successful satellite ‘know-how’ transfer and training programmes between Surrey and emerging space nations such as Korea, Portugal, Pakistan, South Africa, Chile, Singapore, Malaysia and Thailand and now P.R. China - leading to the launch of their first national satellites (e.g. KITSAT- 1&2; POSAT- 1, BADR- 1, TMSAT & FASat-B) - providing rapid response, low risk and affordable access to space. The Surrey Space Center offers a unique environment: combining both academic research and postgraduate teaching with commercial development & manufacturing of satellites through its spin-off company, Surrey Satellite Technology Ltd, and is almost entirely funded from commercial contracts. Surrey’s current and future space activities include a new 350 kg minisatellite (UoSAT-12) designed and built at SSTL for launch in April 1999 carrying high resolution EO, advanced digital communications and technology demonstration payloads within the same cost-effective satellite engineering philosophy. A 130 kg SSTL enhanced microsatellite will form the basis of a 6-satellite communications constellation, a network of 7 microsatellites for daily disaster monitoring, and will provide the basis for Surrey’s first low-cost interplanetary mission to the Moon for launch in 2001.

2. INTRODUCTION

Following the first space launch in 1957, satellites rapidly grew increasingly large and enormously expensive. Initially, the ‘space race’ was an effective catalyst for the development of advanced technology as the super-powers strove to out-do each other and gain the advantageous ‘high ground’ of space - irrespective of budget. However, as costs escalated and time scales lengthened with satellites generally taking many years to mature from concept to useful orbital operation, this process limited access to space to only a relatively few nations or international agencies. Changing world politics and military emphasis in the last decade has brought about a quiet revolution in space. Pressure on space agency financial budgets has increasingly meant that fewer (and bigger) satellites have been commissioned and new ideas, technologies and scientific experiments have found it difficult to gain timely access to space.

The staggering developments in micro-electronics, stimulated increasingly by the consumer market rather than military requirements, and the dramatic pace of consumer product development, caused space technology often to lag considerably behind that now taken for granted on Earth. The combination of reducing budgets for space and increasing capability of low-power micro-electronics have enabled a new breed of highly capable ‘smaller, faster, cheaper’ satellites to realize many space missions complementing the conventional large satellite systems still necessary for large-scale space science and communications services to small terminals. Indeed, in the field of commercial satellite communications, numerous constellations of small satellites have been proposed and are being built to provide a range of global services to hand-held terminals either for real-time voice or non-realtime data.

However, whether a particular satellite is ‘large’ or ‘small’ depends somewhat upon viewpoint. For instance the ,small’ satellites for the Iridium mobile communications system weigh in at over 600 kg each - whereas those for HealthNet email network are a mere 50 kg ! In view of this potential for confusion, the following classification has become widely adopted:

Class	Mass (kg)	Cost (£M)
Large satellite	> 1000	> 100
Small satellite	500-1000	30-100
Mini-satellite	100-500	7-20
Micro-satellite	10-100	2-4
Nano-satellite	1-10	0.2-1
Pico-satellite	<>	<>

Although there have been many examples of large, small and even mini-satellites, however it is only relatively recently that capable microsatellites have shown that it is possible to execute both civil and military missions very effectively, rapidly, and at low cost and risk, for the following applications:

Specialized Communications Services & Research;

Earth Observation & Remote Sensing;

Small-Scale Space Science;

Technology Demonstration Verification;

Education & Training.

3. SURREY MICROSATELLITES

The University of Surrey has pioneered microsatellite technologies since beginning its UOSAT programme in 1978. From very modest experimental beginnings, its space-related research, post-graduate teaching and international commercial activities are now housed within a purpose-built Surrey Space Center - with over 150 academic and professional staff and postgraduate research students. The objectives of the Center’s programmes are:

• to research cost-effective small satellite techniques;
• to demonstrate the capabilities of micro/minisatellites;
• to catalyze commercial use of micro/minisatellites;
• to promote space education and training.

Over the last decade, Surrey has established an international reputation as pioneers of innovative small satellites in a uniquely combined academic research teaching and commercial environment.

Surreys’ first experimental microsatellites (UoSAT-1 & 2) were launched free-of-charge as ‘piggy-back’ payloads through a collaborative arrangement with NASA on DELTA rockets in 1981 & 1984 respectively. Since then, a further ten low cost yet sophisticated microsatellites have been placed in low Earth orbit using Ariane, Tsyklon and Zenit launchers for a variety of international customers and carrying a wide range of payloads.

Two new (50 kg) microsatellites for Chile and Thailand have been launched on Zenit from the Baikonur Cosmodrome during July ‘98, and three new microsatellites for Malaysia, France and the US-DOD and an experimental (350 kg) minisatellite are currently being prepared for launch in 1999

Microsatellite	Launch	Orbit	Customer	Payloads
UoSAT-1	1984-D	560 km	UoS	Research
UoSAT-2	1984-D	700 km	UoS	S&F, EO, rad
UoSAT-3	1990-A	900 km	UoS	S&F
UoSAT-4	1990-A	900 km	UoS/ESA	Technology
UoSAT-5	1991-A	900 km	SatelLife	S&F, EO, rad
S80/T	1992-A	1330 km	CNES	LEO comms
KitSat-1	1992-A	1330 km	Korea	S&F, EO, rad
KitSat-2*	1993-A	900 km	Korea	S&F, EO, rad
PoSAT-1	1993-A	900 km	Portugal	S&F, EO, rad
HealthSat-2	1993-A	900 km	SatelLife	S&F
Cerise	1995-A	735 km	CNES	Military
FASat-Alfa	1995-T	873 km	Chile	S&F, EO
FASat-Bravo	1998-Z	835 km	Chile	S&F, EO
Thai-Phutt	1998-Z	835 km	Thailand	S&F, EO
TiungSAT-1	1998-Z	1020 km	Malaysia	EO, comms
UoSAT-12	1999-S	650 km	SSTL & Singapore	EO, comms
SNAP-1	1999-S	650 km	SSTL	Technology
PicoSAT	1998-?	650 km	USAF	Military
Clementine	1999-A	735 km	CNES	Military
Tsinghua-1	1999-LM	750 km	PR China	EO, comms

University of Surrey Microsatellite Missions

*built in Korea using SSTL platform & KAIST payload

D=Delta; A= Ariane; T=Tsyklon; Z=Zenit,; S=SS18/Dnepr; LM=Long March

UOSAT- 1 & 2 both used a rather conventional structure - a framework ‘skeleton’ onto which were mounted module boxes containing the various electronic subsystems and payloads with a complex 3-dimensional inter-connecting wiring harness.

Following UoSAT-1 and UoSAT-2 (in 1984), the need to accommodate a variety of payload customers within a standard (ASAP) launcher envelope (400x400x600 mm and 50 kg), coupled with increased demands on packing density, electromagnetic compatibility, economy of manufacture and ease of integration, catalyzed the development at Surrey during 1986 of a novel modular design of multi-mission microsatellite platform. This modular microsatellite platform has been used successfully on seventeen missions, each with different payload requirements, and allowing the spacecraft to proceed from order-to-orbit in typically 10- 12 months!.

The SSTL modular microsatellite has no ‘skeleton’ but rather a series of identical outline machined module boxes stacked one on top of the other to form a body onto which solar panels and instruments may be mounted.

Each module box, houses the various microsatellite subsystems – e.g. 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.

Electronically, the microsatellite uses modern, sophisticated, but not necessarily space-proven, electronic circuits to provide a high degree of capability. These are underpinned by space-proven subsystems - resulting in a layered architecture that achieves high performance with operational redundancy by using alternative technologies rather than by simple duplication.

Communications and Earth observation payloads require an Earth-pointing platform and so the microsatellite is maintained to within 0.3⁰ of nadir by employing a combination of gravity-gradient stabilization (using a 6 meter boom) and closed-loop active damping using electromagnets operated by the on-board computer. Attitude determination is provided by Sun, geomagnetic field sensors, and star field cameras, whilst orbital position is determined autonomously to with -+50 meters by an onboard GPS receiver. Electrical power is generated by four body-mounted GaAs solar array panels, each generating ~35W, and is stored in a 7Ah NiCd rechargeable battery.

Communications are supported by VHF uplinks and UHF downlinks, using fully error-protected AX.25 packet link protocols operating at 9.6 to 76.8 kbps, and are capable of transferring several hundred kBytes of data to brief-case sized communications 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, which runs a 500 kbyte real-time multi-tasking operating system with a 128 MByte solid-state CMOS RAMDISK. In addition, there is a secondary 80C186 OBC with 16 MByte SRAM, two 20 MHz T805 Transputers with 4 MBytes SRAM, and a dozen other microcontrollers. A primary feature of the OBDH philosophy is that all the software on-board 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 OBC-386 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-386 and either transmitted immediately and/or stored in the RAMDISK whilst the satellite is out of range of the Control Station. The OBC’s 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 satellites systems and payloads.

The latest SSTL microsatellite platforms have enhanced sub-systems supporting the following major new features for greatly increased performance and payload capacity. The first version of the enhanced microsatellite platform was used on the FASat-A mission for Chile launched into LEO by a Ukrainian TSYKLON launcher in 1995 and has since been further developed and used on the new FASatBravo, TMSAT, TiungSat-1, PICOSat and now Tsinghua-1 missions providing:

• distributed telemetry/ telecommand for easy expansion;
• 5 MIPS (386) OBC + 256 MB RAMDISK;
• 1 Mbps on-board LAN;
• autonomous GPS navigation (50m);
• attitude determination (0.001⁰);
• 0.25⁰ rms nadir pointing pitch/roll axes, 2⁰ yaw axis;
• 128 kbps BPSK downlinks.

1. APPLICATIONS OF NFICRO/MINISATELLITES

The early UOSAT missions demonstrated the potential capabilities of microsatellites and generated considerable interest in applications such as digital store-&-forward file transfer and in-orbit technology verification, however wider applications such as space science and Earth observation were slow to develop. Emerging space nations, however, were quick to recognize the benefits of entering the space-faring community with an affordable, low-risk ‘first step’ via an extremely inexpensive yet realistic microsatellite programme.

The need to handle this growing interest, to catalyze wider industrial & commercial applications, and to generate regular income to sustain a research activity in satellite engineering at the University of Surrey without dependence on government funding, stimulated the formation in 1985 of a University company – Surrey Satellite Technology Ltd (SSTL). SSTL provides 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.

Since UoSAT-5, all the Surrey microsatellites have been design and built for individual commercial customers. The income generated by SSTL is then re-invested to support the academic activities of the Surrey Space Center, which has now become the largest European ‘centre of excellence’ in satellite engineering research, teaching and applications.

The range of applications of microsatellites can be demonstrated by reviewing recent examples of payloads carried by the UoSAT/SSTL microsatellite platform.

LEO Communications

Various constellations of small satellites in LEO have been proposed to provide world-wide communications using only hand-held portable terminals; these broadly fall into two main categories:

• real-time voice/data services (e.g. Iridium, Globalstar);
• non-real-time data transfer (e.g. Orbcom, HealthNet).

The close proximity of the satellites in LEO to the user and the consequent reduction in transmission loss and delay time appear attractive when compared to traditional communications satellites in a distant geostationary orbit holding out the promise of less expensive ground terminals and regional frequency reuse. The communications characteristics associated with a LEO constellation pose, however, quite different and demanding problems, such as varying communications path & links, high Doppler shifts, and hand-over from satellite to satellite.

Currently, there is only one small LEO satellite service in full operation (HealthNet) which employs a ‘constellation’ of just two microsatellites, HealthSat-1 & 2, both built by SSTL for the network operator, SatelLife (USA). HealthNet, and the services waiting to be implemented (e.g. TemiSat, OrbComm, VITASat, GEMStar) use narrow-band VHF/UHF frequencies recently allocated to the ‘little-LEO’ services to provide digital data store-&-forward "email" capabilities for use with small, low-power user ground terminals that can be located in remote regions where existing the telecommunications infrastructure is inadequate or non-existent [3].

These VHF/UHF frequency allocations exhibit such deleterious effects as multi-path propagation and, in particular, man-made co-channel interference which can very significantly reduce the performance that can be achieved in practice by the satellite communications system compared to that expected from a simple theoretical model. A thorough understanding of the real LEO communications environment at VHF & UHF is therefore necessary in order to select optimum modulation and coding schemes.

The KITSAT-1 & PoSAT-1 microsatellites carry a Digital Signal Processing Experiment (DSPE) which has been designed to provide a sophisticated in-orbit test bed for research into optimising communications links with satellites in LEO. The DSPE comprises a TMS320C25 and a TMS320C30 with supporting PROM, RAM and data interfaces to the spacecraft communications sub-systems enabling it to replace the hardware onboard modems with an in-orbit, re-programmable modem. The DSPE is being used in a research programme to evaluate adaptive communications links-continuously optimizing modulation / demodulation techniques, data rates and coding schemes in response to traffic characteristics during the microsatellite’s transit of the ground station.

The interference characteristics of the VHF LEO frequency allocations have been measured using an experimental communications payloads on the S801T and HealthSat-2 microsatellite missions. In conjunction with a mobile groundstation, S801T has measured the VHF spectrum ‘noise’ and interfering signals to evaluate the use of VHF frequencies for a’ full-scale LEO communications service (S80). S801T was completed by SSTL for the Center National d’etudes Spatiales (France), from proposal to launch, within 12 months!

Space Science

Microsatellites can offer a very quick turn-around and 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. SSTL missions have demonstrated that it is possible to progress from concept through to launch and orbital operation within 12 months and within a budget of £2-3M. 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 large-scale missions) and to be able to initiate a programme of research, propose and build an instrument, and retrieve orbital data for analysis and presentation for a thesis within a normal period of post-graduate study.

UoSAT-3 & 5, KITSAT-1 & 2 & PoSAT-1 provide examples of the use of a microsatellite platform for collaborative space science research between the University of Surrey, the UK Defense Research Agency, UK AEA, the UK Science & Engineering Research Council, KAIST and Portugal.

A ‘Cosmic Ray Effects & Dosimetry’ (CREDO) payload monitors the near-Earth radiation environment and provides an important opportunity to validate groundbased numerical models with flight data yielding simultaneous measurements of the radiation environment and its effects upon on-board systems (especially SEUs in VLSI devices) [4].

Earth Observation

Conventional Earth observation and remote sensing satellite missions are extremely costly - typically in excess of £150M each - and thus there are relatively few such missions and the resulting data, whilst impressive, is correspondingly expensive. The development of high-density two-dimensional semi-conductor Charge-Coupled Device optical detectors, coupled with low-power consumption yet computationally powerful microprocessors, presents a new opportunity for remote sensing using inexpensive small satellites.

UoSAT-1 & 2 both carried the first experimental 2D-CCD Earth imaging cameras which lead to the development of the CCD Earth Imaging System on-board UoSAT-5, intended to demonstrate the potential of inexpensive, rapid-response microsatellite missions to support remote sensing applications. Clearly, the limited mass, volume, attitude stability and optics that can be achieved with a tiny microsatellite means that a different approach must be taken to produce worthwhile Earth observation. For these reasons, SSTL employs electronic cameras with 2-dimensional CCD arrays to gather imagery from its microsatellites. Because cameras capture whole images in a single snap-shot, they preserve scene geometry and are therefore immune to the residual attitude drift experienced on microsatellites.

The Earth Imaging System (EIS) on-board the UoSAT-5, KITSat & POSAT microsatellites comprise an EEV (UK) 576x578 pixel area CCD digitized to 256 levels of gray. The digitized data is stored within a 2 Mbyte CMOS RAM which is accessed by two Transputers to allow the image data to be processed to enhance quality and compressed to reduce storage and transmission requirements.

The EIS data is transferred via the microsatellite’s local area network to the 80C186/386 on-board computers and stored as files within the 32-128 Mbyte RAM DISK for later transmission to ground - some 60 images can be stored within the RAM DISK at any one time. The EIS is commanded to collect an image of a particular area of the Earth’s surface by the on-board computer which operates a multi-tasking, real-time operating system responsible for the automatic (and in some cases, autonomous) operation of the microsatellite mission.

Ground controllers select a sequence of images of areas of interest anywhere on the Earth’s surface and, checking the time and position of the microsatellite using an on-board GPS receiver and orbital model, instruct the on-board computer to collect the images according to a ‘diary’ that is loaded periodically in advance to the microsatellite. The PoSAT-1 microsatellite carries two independent cameras providing a wide-field ground resolutions of 2 km for meteorological imaging and a narrow-field ground resolution of 200 meters for environmental monitoring, with 650 nm (± 40 nm) optical filters providing good separation of arid/vegetation and land/sea boundaries and disaster warning.

The latest generation of small Earth observation satellites using the SSTL platform, such as TMSAT support EIS cameras yielding better than 90-meter ground sampling distance with 3 spectral bands providing multispectral image data in LANDSAT-compatible bands.

The TiungSAT-1 microsatellite will provide 75-meters resolution in 4 spectral bands and the latest SSTL microsatellites for launch in 1999 will provide 50m 4-band multi-spectral imaging with the capability of ±15⁰ (± 200 km) off-nadir imaging coverage upon demand.

In-Orbit Technology Verification

Microsatellites also provide an attractive and low-cost means demonstrating, verifying and evaluating new technologies or services rapidly in a realistic orbital environment and within acceptable risks prior to a commitment to a full-scale, expensive mission.

UoSAT-based microsatellites have supported a wide range of such in-orbit technology demonstrations, covering:

• New solar cell technologies;
• Modern VLSI devices in space radiation;
• Demonstration of advanced communications;
• ‘Pilot’ demonstrations of new communications.

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. Knowledge of the long-term behavior of different types of cells in the radiation environment experienced in orbit is, therefore, essential. The continuing development of solar cell technology, based upon a variety of materials and different process techniques, yields a range of candidate cells potentially suitable for satellite missions.

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 (SCTE) designed to evaluate the performance of a range of 27 samples of GaAs, Si and InP solar cells and from a variety of manufacturers. When the sun passes directly overhead of the panel mounted on the body of the microsatellite, the monitoring electronics are triggered automatically and measure typically 100 current/voltage points for each cell sample. These data are then sent in a burst to the microsatellite’s on-board computer, together with associated temperature and radiation dose data, for storage prior to transmission later to ground. SCTE measurements are taken repeatedly immediately after launch, when the radiation degradation is most rapid, and then at increasing intervals thereafter.

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 deplorable solar panels has been developed to support various military payloads. The main differences between the ‘commercial’ and ‘military’ versions of the platform is in the specification 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 to the ‘commercial’ procurement of the platform.

The first use of the SSTL military microsatellite platform was on the CERISE mission designed and built for the French MoD and launched 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 (rocket fragment) which severed its stabilization boom. However, due to the flexibility of the microsatellite systems, SSTL engineers were able to re-stabilize Cerise by unloading new attitude control algorithms and return it to operations.

A second microsatellite for the French MoD is now being completed (Clementine) for launch into LEO in 1999 and a microsatellite (PICOsat) is being built for the USAIF FCT programme.

The Geostationary Transfer Orbit (GTO) provides a good opportunity to study the effects of a severe radiation environment on satellite components - especially solar cells and VLSI components. Surrey has provided platform subsystems and payloads to the UK Defense Research Agency (DRA - Farnborough) for their two Space Technology Research Vehicle (STRV-1) microsatellites which were launched into GTO in early 1994. STRV-la & b carry a range of in-orbit technology demonstration payloads particularly to study the effects of the space radiation environment on military satellite components

Education & Training Using Microsatellites

Although microsatellites are physically very small, they are nevertheless complex and exhibit virtually all the characteristics of a large satellite - but in a microcosm. This makes them particularly suitable as a focus for the education and training of scientists and engineers by providing a means for direct, hands-on experience of all stages and aspects (both technical and managerial) of a real satellite mission - from design, construction, test and launch through to orbital operation.

The very low cost, rapid timescale and manageable proportions makes this approach very attractive to emerging space nations who wish to develop and establish a national expertise in space technology through an affordable small satellite

Each technology transfer and training (TTT) 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 (MSc, PhD degrees);
• Technology Training (seconded to SSTL);
• Groundstation (installed in country);
• Microsatellites (1^st at SSTL, 2 ^nd in country);
• Technology Transfer (satellite design license)

Six highly successful international technology transfer programmes have been completed by Surrey & SSTL with two programmes under way and a new programme with PR China just starting:

Country	Dates	Satellites
Pakistan	1985-89	BADR-1
South Africa	1989-91	UoSAT-3/4/5
South Korea	1990-94	KITSat- ½
Portugal	1993-94	PoSAT-1
Chile	1995-97	FASat-Alfa/Bravo
Thailand	1995-98	TMSAT-1
Singapore	1995-99	Merlion payload
Malaysia	1996-98	TiungSAT-1
China	1998-99	Tsinghua-1

SSTL technology transfer & training programmes

A total of 70 engineers have been trained through these in-depth TTT programmes - a further 320 students from countries world wide have graduated from the MSc course in Satellite Communications Engineering unrelated to these M programmes.

1. "SURREY SPACE CLUB"

As each of these international technology transfer & training programmes has been based around an SSTL microsatellite, the participating organizations share a common experience with Surrey; a common spacecraft design heritage; common spacecraft-groundstations; and common communications protocols. After returning to their home country, each organization has commenced its own national space activities and research projects. In order to be able to share new ideas, discuss common interests, and work together to achieve common space goals, the "Surrey Space Club" has been formed as a regular forum for this ‘commonwealth’ of developing space nations to meet and learn from each other - thus to:

Share satellite recourses - there are 8 Surrey-designed microsatellites currently in operation in orbit from partners;

Gain greater access time to spacecraft in orbit through coordinated and shared access to the network of 9 Surrey-based groundstations around the world;

Exchange new ideas on small satellites research and development;

Build small satellite constellations together (e.g. the Disaster Network of EO Microsatellites);

Co-ordinate low cost launch opportunities for members;

Help new members maintain the momentum of a long term national space programme;

Contribute to human planetary exploration through international co-operation (e.g. Surrey’s Lunar Mission 2001).

It is believed that all the members will derive great benefit through closer collaboration via the "Surrey Space Club" whilst at the same time, importantly, maintaining their own clear independence.



The first forum of the "Surrey Space Club" will be held at Surrey during the visit of Her Majesty The Queen to Surrey on 4^th December 1998 when she will open the new Surrey Space Center building.

All past (and future!) Surrey technology transfer partners are welcome to join this unique and imaginative opportunity for international co-operation.

2. MICROSATELLITE GROUNDSTATIONS

Compact and low-cost mission control groundstations have been developed by SSTL to operate the microsatellites once in orbit. These groundstations are based on PCs and are highly automated - interacting autonomously with the microsatellite in orbit to reduce manpower requirements and to increase reliability.

The SSTL Mission Control Center at Surrey operates eleven microsatellites in LEO with just a single operator.





3. SSTL MINISATELLITES

In response to growing payload demands for power, volume & mass - but still within a small-scale financial budget - UOSAT & SSTL are developing an enhanced, modular, multi-mission minisatellite platform capable of supporting missions up to 400 kg.

Once developing space nations have mastered microsatellite technology, the minisatellite provides a logical next step in the development of an increasingly capable national space infrastructure.

The minisatellite platform has been designed to meet a variety of mission objectives and capable of operating in different orbits, its primary features are:

• 400 kg total mass;
• 150 kg payload capacity;
• 1.2 m diameter, 1 m height;
• 3-axis, 0.1 degree, attitude control;
• GPS autonomous orbit & attitude determination;
• 1 Mbps L/S-band communications links;
• On-board propulsion for orbit maneuvers a cold gas thrusters for attitude control;
• 300 watts orbit average power, 1 kW peak power.

The SSTL minisatellite platform has been designed according to similar cost-effective principles that have proved so successful on the UoSAT/SSTL microsatellites resulting in a basic platform cost of £4-5M and compatible with a range of affordable launch options - on Ariane, CIS, Chinese & US (Pegasus) launchers.

UoSAT-12 Minisatellite

The UoSAT-12 minisatellite carries 35-meter resolution multispectral and 8-m panchromatic CCD Earth cameras.

Sophisticated frequency-agile VHF/UHF and L/S-band DSP regenerative transponders will provide both real-time and store-&-forward communications to small terminals.

Three-axis control is provided by a combination of magnetorquers, momentum wheels and cold gas N2) thrusters - whilst an experimental electric H₂O ‘resisto-jet’ thruster will provide orbit trimming and maintenance demonstrations for future network constellations. UoSAT-12 be launched by a converted SS18 ICBM (Dnepr) launcher into LEO in April 1999.

1. New SSTL ‘Enhanced microsatellite’

A new SSTL 100 kg ‘enhanced microsatellite’, based on the proven flight heritage of the 50 kg microsatellites, supports missions requiring larger payloads at low cost. The 600 x 600 x 500 mm bus structure accommodates larger solar panels, and provides flexible internal and external payload accommodation. Nevertheless, this bus fits comfortably on low-cost secondary payload carriers such as the new Ariane-V ASAP.

Four body-mounted GaAs solar panels provide 38 W average power. A full 3-axis attitude control system allows payloads to be pointed at any terrestrial or celestial target with an accuracy better than 0.2⁰. A network of on-board computers and embedded controllers automates all telemetry and telecommand, communications, payload control and data management functions. Communications links in the VHF and UHF bands use standard packet protocols to provide connection to the mission control center and to any remote communications or data collection terminals. A 1 Mbit/s S-band downlink provides high-speed communications for the remote sensing payload, using international standard CCSDS ground/space link techniques.



The SSTL enhanced-microsatellite can carry 5 CCD remote sensing cameras providing 3-band multi-spectral imaging with 50 metres ground resolution and panchromatic imaging with 20 meters ground resolution; and a panchromatic or color-mosaic wide area meteorological imager with 1 km ground resolution.

A clear payload volume measuring approximately 300 x 300 x 200 mm is available on the Earth-pointing facet of the satellite, and additional volume is available within the space frame. Payloads up to 30 kg mass can be accommodated in the Enhanced Small Satellite.

2. NANOSATELLITES !



A tiny, highly integrated, 2 kg ‘nanosatellite’ SNAP-1 is being built as a research project at Surrey for launch in 1999 and intended as an ‘inspector’ satellite to image the ‘mother’ minisatellite and launch vehicle. Future applications for the nanosatellite are for remote inspection of satellites, the international space station and monitoring of deployments systems in orbit, and carrying small space science instruments requiring measurements with spatial diversity.

3. LOW COST LAUNCHES

However, a sustained, commercial, low-cost small satellite programme must also be matched by correspondingly inexpensive and regular access to orbit through formal launch service contracts - as it makes little sense to construct sophisticated yet inexpensive microsatellites if the launch costs remain prohibitively high. Early microsatellites were launched virtually for free on a ‘favor’ basis by the US & USSR, but these launch opportunities were infrequent and unpredictable. The breakthrough came in 1988 when Arianespace developed the Ariane Structure for Auxiliary Payloads (ASAP) specifically to provide, for the first time, regular and affordable launch opportunities for 50 kg microsatellites into both LEO and GTO on a commercial basis

To date, some 18 microsatellites have been launched via the ASAP but, whilst it has been key to catalysing microsatellites world-wide, Ariane alone cannot now provide the number of launch opportunities into LEO needed to meet the burgeoning growth of small satellites. and so alternative, inexpensive launch options from the CIS (on Tsyklon, Zenit, & Cosmos) are now increasingly being used for micro/minisatellites. Within the last few years, the large stockpiles of ICBMs in the CIS have become available for use as small launchers through the demilitarization programme (e.g. SS-18/Dnepr; SS-19/Rockot; SS-25/START).





4. PROJECT MANAGEMENT

Solving the technical challenges associated with the design, construction, test and operation of a microsatellite is less than half of the story - in parallel with the technical considerations of the mission, effective project management is crucial to the realization of a successful low-cost, sophisticated small satellite project.

Affordable small satellites require a very different approach to management as well as technology if cost, performance and delivery targets are to be met. Several attempts at taking a traditional aerospace organization to produce such satellites have failed because of the rigidity of management structure bend ‘mind-set’. Small teams (25 persons), working in close proximity with good communications, with well-informed and responsive management, are essential.

These characteristics are best found in small companies or research teams rather than large aerospace organizations, who may find it difficult to adopt or modify procedures necessary to produce affordable small satellites using staff and structures that are designed for conventional aerospace projects. The main ingredients for a successful small satellite project can be summarized as:

• Highly innovative technical staff;
• Small, motivated teams;
• Personal responsibility for work rigor and quality;
• Good team communications, close physical proximity;
• Well-defined mission objectives and constraints;
• Knowledgeable use of volume, modern components;
• Layered, failure-resilient system architecture;
• Thorough burn-in;
• Technically-competent project management;
• Short timescale (prevents escalation of objectives!).

SUMMARY & CONCLUSIONS

The University of Surrey embarked upon the design of its first experimental microsatellite in 1978 and UoSAT-1 was launched by NASA in 1981 - since then a further eleven low cost yet highly sophisticated microsatellites have been built & launched into low Earth orbit. The UoSAT/SSTL missions have demonstrated that microsatellites can play a useful role in supporting specialized communications, Earth observation, small-scale space science, and in-orbit technology verification missions.

Whilst obviously limited in payload mass, volume and power, but with very real and attractive advantages in terms of cost and response time, microsatellites offer a complementary role to traditional ‘large’ satellites by providing an alternative ‘gap-filler’ for affordable quick response or exploratory missions for both civil and military objectives. Developing space nations have used rapid and inexpensive microsatellite projects to act as the focus for effective technology transfer and an affordable first step into orbit. A further 2 microsatellite missions were launched by SSTL in 1998 and 4 more microsatellites, a nanosatellite and a new minisatellite are scheduled in 1999.

Building upon its success with microsatellites, Surrey is now building an affordable, modular (400 kg) minisatellite capable supporting more demanding payloads - but still within a cost of ~£SM - in order to stimulate further use of small satellites to complement large space programmes.

Surrey has established itself firmly as the international center of excellence in academic research, teaching and commercial applications of small satellites.

Share on Facebook Share on Twitter Share on Google Plus

About marbun