PROGRESS IN SMALL SATELLITE TECHNOLOGY FOR EARTH OBSERVATION MISSIONS Alex da Silva Curiel, Andrew Cawthorne, Martin Sweeting Surrey Satellite Technology, Ltd Surrey Space Centre, Guildford, Surrey, GU2 7XH, UK http://www.sstl.co.uk
[email protected] INTRODUCTION The value of Small Earth Observation satellites can be measured directly by considering the lifecycle cost of the system, and the amount of valuable data delivered in its lifetime. In practice this needs to be validated by a science case or business plan, which assigns a market value to the data, and considers many of the underlying issues. These include the applications of the data, its quality, whether the data is perishable or retains its value, and whether there are alternative means of obtaining the data. Increasingly, small Earth Observation satellites operating within groups are considered to offer good value and utility, and are able to offer services that cannot be practically offered by deploying larger spacecraft. This is as a direct consequence of the growth in capability of small satellites in recent years. A constellation of small satellites has the benefit of increasing temporal resolution, and when coupled with significantly lower unit costs, can support niche markets that are otherwise economically unviable. It is anticipated that small satellites will make a significant input into the economic viability of commercial Earth Observation. Against a backdrop of benchmarking technology and performance trends in small Earth Observation satellites, this paper describes some of the advanced small satellite platform designs being developed at Surrey. TRENDS IN SMALL EO MISSIONS There has been significant debate in the small satellite community regarding the relationship between small satellites1 and larger satellites. Are small satellites a disruptive technology, and will they replace larger satellites eventually? Or are they complementary, and will be used to address applications not viable with larger spacecraft? There is unlikely to be an unambiguous answer to this, as it is highly dependent on the application and market factors. No doubt miniaturization will permit some space missions eventually to be accomplished with much smaller spacecraft than today, and consequently the cost-of-entry to execute space missions may reduce. Figure 1 provides some evidence that smaller spacecraft generally cost less then their larger counterparts. This figure uses published spacecraft and programme costs normalised to US$ (2003). The size of some Earth Observation missions will be still driven by the required physical apertures, and can only be accomplished with a larger spacecraft. Certainly in some cases, the overheads involved in deploying and operating a satellite may dictate that larger spacecraft provides a better economical solution if data return, or communication bandwidth are of primary concern. This is exemplified by the fact that 1
For the purpose of this paper small satellites are defined here as those with a wet mass of less than 500kg.
most Geostationary communications satellites have grown to be quite large in a commercial environment. Spacecraft mass and Programme cost $100,000,000
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Figure 1. Cost of spacecraft and complete mission plotted against spacecraft mass
As the value of a mission is largely dictated by the performance for a certain cost, capability of these small satellites must also be examined. If one examines the trends in capability of small Earth Observation satellites, it becomes apparent that this has advanced from early technology demonstrators, towards a point where missions increasingly provide operational services. Examples include the DLR BIRD satellite, the ESA PROBA satellite, and SSTL’s Disaster Monitoring Constellation (DMC). In Figure 2 it is demonstrated that alongside the steady progression and improvement in the Ground Sampling Distance for civil optical imaging missions, small satellites are catching up rapidly in this capability. This indicates that small satellite capabilities in instrument accommodation, miniaturisation of avionics, data return, power, and attitude control are all improving. As a result small satellites are starting to address operational applications traditionally carried out with larger satellites, and are encroaching on the larger markets for high-resolution imagery.
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Figure 2. Improvement of Ground Sampling Distance (a) All Satellites (b) Small Satellites
One of the areas where it is likely that small satellites will flourish is where there is a strong requirement and benefit in using groups of satellites; constellations, formations
and swarms. These become particularly attractive where launcher capacity can be used efficiently, for instance for deploying an entire plane of spacecraft simultaneously. It is often prohibitively expensive to deploy multiple large Earth Observation spacecraft for such applications. Groups of spacecraft offer unique capabilities that are often difficult or impossible to achieve through different means, for instance in enhancing temporal coverage. Individual spacecraft can improve their temporal resolution through off-pointing the instrument. This has an effect on the achievable Ground Sampling Distance at the extremities of the Field-Of-Regard for such repeat visits, as highlighted in Figure 3. In this case an instrument with a nominal GSD of 2.5m is assumed, leading to degradation in the GSD when pointing further off-track (but dependent on the terrain slope). Due to the resulting distortion, there is a fundamental limit to how far the instrument can be off-pointed to serve the target application. This off-pointing requirement can be relaxed by increasing the number of satellites, and it is readily shown that there is usually a near linear relationship between the temporal resolution and number of satellites deployed. Hence the use of constellations of imaging satellites does not only improve the revisit time, but also some aspects of the quality of such high temporal resolution data. off-pointing effect on GSD 11.00 10.00 9.00
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Daily coverage for any (illuminated) point on the globe with suitable spatial resolution can be achieved with four spacecraft dispersed in a 600km orbit. This is depicted in Figure 4, where each colour track represents the coverage of a single spacecraft. A single spacecraft with the same Field-Of-Regard can only achieve daily revisit on a regional basis, or global coverage with lower temporal resolution.
Figure 4. (a) One day coverage provided by four spacecraft at 600 km with 30 off-pointing capability compared with (b) single satellite capability.
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Other characteristics that can be well addressed by constellations of small low-cost satellites are freshness of the data, synopticity (the amount of data captured in one instance) and affordability of the data products. By examining the broad applications of spatial and temporal resolution (Figure 5), it becomes clear that small satellite capability has now matured to the point where a wide range of missions can be addressed. The capabilities of small spacecraft in constellation are already being exploited by emerging missions such as the operational DMC mission and planned RapidEye constellation. These missions demonstrate that new markets and business can be created. Surveillance / Tracking Disaster Management
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Figure 5. Applications map – Temporal and spatial resolution
State-of-the-art small satellites are currently being developed and deployed with GSD of 2- 5m, and technology developments will enable small satellites to still improve further. This permits small satellites to be employed across the broad range of traditional applications in optical Earth Observation, but their lower cost makes niche applications economically viable. In examining the applications for higher temporal resolution, it is clear that small low cost satellites in constellation can also enable new applications, competing with aerial surveys. SSTL, as a specialist in providing affordable small satellite solutions, has pioneered some of these Earth Observation applications using groups of small satellites, and is actively involved in the manufacture, design and development of highly capable Earth Observation spacecraft designed to operate cooperatively. In particular the recently deployed Disaster Monitoring Constellation highlights how small satellite solutions, coupled with novel ways of working, can be used to compete with more conventional Earth Observation systems. Furthermore, the use of such assets in high temporal resolution missions has been proposed to obtain revisit rates of just “a few hours”, alongside the concept of “dynamic constellations”, in which the assets are reconfigured to adapt to new applications.
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DMC and High resolution camera, 2.5m GSD. Hyperspectral and IR options
Designed to be part of constellation 5-7 year design life time DMC instrument and 4m GSD, 24km GSD panchromatic instrument.
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Table 1. SSTL high resolution mission products
REFERENCES 1.
“Rapid response high resolution imaging from space”, (ESA SP-571), Alex da Silva Curiel, Mazene Wazni, Lee Boland, Phil Davies, Stuart Eves, Wei Sun, Martin Sweeting, The 4S Symposium - Small Satellites Systems & Services, ESA/CNES, 20 – 24 September 2004, La Rochelle, France.
