Small Satellites – Bringing space within reach

Professor Sir Martin Sweeting OBE FRS FREng

Small Satellites – Bringing space within reach

Earth orbiting satellites come in a wide range of shapes and sizes to meet a diverse variety of uses and applications. Large satellites with masses over 1000kg support high resolution remote sensing of the Earth, high bandwidth communications services and world-class scientific studies but take lengthy developments and are costly to build and launch. The advent of commercially available, high-volume and hence low-cost microelectronics has enabled a different approach through miniaturisation to produce physically far smaller satellites that dramatically reduces timescales and costs and that are able to provide operational and commercially viable services. These developments have catalysed a new approach to access to and exploitation of space.

I.               INTRODUCTION

In the early decades of the space era, space was the preserve of the wealthiest and most technically advanced nations who enjoyed the advantage and benefits brought from the high vantage-point of Earth orbit, primarily for observation and communications.

The use of space is expanding. It is now an essential infrastructure supporting national economies, their security and population well-being. Space provides global communications, pin-point positioning and navigation aids, the ability to optimise agriculture and the use of increasingly scarce land and water resources, and enables the monitoring and mitigation of natural and man-made disasters.

Earth orbiting satellites come in a wide range of shapes and sizes to meet a diverse variety of uses and applications. Large satellites with masses over 1000kg support high resolution remote sensing of the Earth, high bandwidth communications services and world-class scientific studies but take lengthy developments and are costly to build and launch.

The advent of commercially available, high-volume and hence low-cost microelectronics has enabled a different approach through miniaturisation to produce physically far smaller satellites that dramatically reduces timescales and costs and that are able to provide operational and commercially viable services. The University of Surrey (UK) pioneered the development of ‘microsatellites’ in the 1980s by taking advantage of the capabilities of ‘commercial off-the-shelf’ components to achieve a high performance to cost ratio.

These developments have catalysed a new approach to access to and exploitation of space and, by 2018, these small yet sophisticated and capable satellites have enabled everyone to have direct access to space – whether it be a developing economy, small companies, universities and even high schools.


Class

Mass (kg)

Large satellite

>1000

Small satellite

500 to 1000

Mini-satellite

100 to 500

Micro-satellite

10 to 100

Nano-satellite

1 to 10

Pico-satellite

0.1 to 1

Femto-satellite

<0.1

Table: General classification of femto-pico-nano-micro-mini-small-large satellites.

Following the launch of Surrey’s first microsatellites (UoSAT-1 & 2) in 1981 and 1984, the University formed a spin-out company, Surrey Satellite Technology Ltd (SSTL), to transfer the results of their academic research in small satellites across to industry for commercial exploitation. Over the following decades, Surrey has built and launched some 60 small satellites using 10 different launch vehicles working with international partners from 22 countries. Eighteen of these programmes involved know-how training alongside the design and construction of the satellites in order to assist developing space nations gain knowledge and build indigenous capacity for space activities.

II.             Small Satellite programmes

The early microsatellite programmes focussed on providing digital store-&-forward communications between remote regions – before the advent of the internet infrastructure. Pilot projects provided daily email connectivity between health workers in Africa and Southeast Asia with medical centres in Europe and the US, and email communications with scientific outposts in the Antarctic where geostationary communications satellites were inaccessible due to being below the local polar horizon.

However, Earth observation became the prime application for small satellites – starting with fairly modest capabilities but improving rapidly from mission-to-mission through the use of increasingly capable COTS components. The combination of affordable satellite cost and corresponding launch opportunities as secondary payloads made it possible, for the first time, to launch a constellation of remote sensing microsatellites to achieve daily revisit over any specific area of the Earth’s surface at least once each day – indeed more frequently at higher latitudes.

Taking advantage of this, 6 nations collaborated with Surrey in 2003 to construct the international Disaster Monitoring Constellation comprising 7 microsatellites carrying multispectral Earth observation cameras and solid-state on-board memories. Each of the participating nations funded a satellite, several including capacity-building training programmes, at Surrey and once launched into low Earth orbit the image data collected by the constellation was shared between the participants. Thus, by funding a single affordable satellite, the participants were able to benefit from data from a constellation of 7 satellites, increasing both data quantity and timeliness.

The DMC responded to many major natural disasters, such as forest fires, flooding, snow storms and earthquakes – particularly responding to the Indian Ocean tsunami and Chinese Wenchuan earthquake disasters and the hurricane Katrina inundation in the USA. The DMC became a member of the International Charter on Space and Major Disasters, contributing regular data on a ‘no cost’ basis in response to formally-declared major incidents.

In addition to providing rapid response imagery in response to disasters worldwide, the DMC partners provided excess data onto the commercial Earth Observation market for agricultural, mapping and water management, thus recouping some of their initial investment. The DMC collected forest cover information in the Amazon on a 6-monthly basis to assist in the assessment of the rates of de-forestation.

The DMC satellites also advanced several associated techniques for small satellites, notably communicating with their ground stations using the Internet Protocol for payload data transfer and command and control, so extending the Internet into space – including the first use of the ‘bundle’ protocol in space where Sensor data was successfully delivered from the satellite using this disruption- and delay-tolerant networking protocol designed for the Interplanetary Internet. The UK-DMC satellite included a GPS reflectometry experiment and on-board Internet router.

The success of the DMC stimulated the first fully commercial operational EO satellite constellation, RapidEye, comprising five 150kg microsatellites with 5-band multispectral imager (RGB, red edge, and near IR bands) with 6.5m GSD to generate land information products downlinked at 80 Mbit/s at X-band.

The demand for higher resolution and improved spectral fidelity for agricultural monitoring, precision farming and smart city management led to the need for more capable small satellite platforms with increased power, data handing and pointing requirements to carry larger aperture telescopes.

Three 450kg ‘minisatellites’ were built by SSTL and launched in 2015 to provide an optical imaging service with a ground resolution of 1-metre and were followed in 2018 by a fourth satellite and a synthetic aperture radar (SAR) minisatellite for all-weather, day/night monitoring for storms, deforestation and maritime surveillance.

Whilst higher fidelity minisatellites were launched, technology demonstrator missions showing the potential for 100kg microsatellites to provide high resolution (1-metre) full real-time video were launched – Carbonite-1 & 2.

III.            The space environment and space debris

Unfortunately, debris from spent rockets and defunct satellites are littering space to an increasing degree – especially in the popular low Earth orbits below 2000 km. At altitudes below ~500 km, the natural decay into the Earth’s atmosphere results in an ‘auto-cleaning’ function that reduces the lifetime of debris to manageable levels, however above that altitude the debris lifetimes can be measured in centuries and poses a serious threat to operational satellite systems. The density of debris is close to the threshold of the “Kessler Effect”, in which collisions lead to a runaway increase in numbers of pieces of debris.

The recent proposals for ‘mega constellations’ exacerbates this problem and mechanisms to ‘clean up space’ are being actively considered. It is impractical to remove the 500,000+ pieces of sizable debris in low Earth orbit, however potential chain reactions of debris generation can be reduced if even relatively few large objects or items that might potentially fragment could be removed.

Surrey has been developing means both to reduce the natural orbital lifetime of satellites after the end of their operational life by extending a drag sail (‘parachute’) for speed up re-entry and by active removal using nets or harpoons deployed from a mothership that then forces a controlled re-entry.

There are now several companies proposing commercial services to remove debris or defunct satellites – particularly relevant for the proposed mega-constellation – as debris would represent a significant risk to their operational business.

IV.           What next for small satellites?

The physical design and construction techniques for satellites have been dictated and constrained by the launcher volume under the fairing and ascent phase dynamics (vibration, noise, shock) necessary to survive the aggressive first 20 minutes or so of ascent to orbit. An effective means of constructing large apertures in space could be through robotic assembly in orbit of numbers of small satellites Lego-like to form physically larger structures that could be used for optical, radar or communications applications – for business, scientific or exploration objectives. The structures can be reconfigurable in orbit to meet changing mission objectives – such as spare apertures trading resolution against signal-to-noise. The small and relatively robust ‘lego-satellites’ can be launched in space-efficient stacks on a number of launchers meaning, in principle, an unlimited size of assembled structure in orbit. The challenges associated with precise autonomous robotic assembly in orbit are not trivial, especially if optical alignments are required. In order to demonstrate this concept, the “Autonomous Assembly of a Reconfigurable Space Telescope” (AAReST) mission has been developed by CalTech, JPL, Surrey, IIST.

The logical next step is to exploit terrestrial developments in additive (and subtractive) manufacturing techniques (so-called 3-D printing) to move the manufacturing of spacecraft into orbit where eventually raw materials alone are launched and then design software uploaded to manufacture the required functions on ‘gossamer’ spacecraft – thus completely bypassing the structural constraints of the launch phase and, possibly, also simplifying the demands on the launcher itself leading to lower launch costs.

V.             Conclusions

Small satellites have exploited advances in microelectronics to create a more affordable and timely access to space for a wider international community bringing space and its opportunities within the affordable grasp of every nation, commercial business and university. This has created new markets for space and its applications stimulating over 500 new start-up companies that, together with the proposed ‘mega-constellations’ have attracted some $25Bn investments. The resulting ‘NewSpace’ community is stimulating new approaches to space business and its applications to society. There is an opportunity to reduce the ‘digital divide’ and bring under-developed communities into the global economy. Persistent EO/RS will enable nations to have a more direct view and better manage their resources and the impact of human activity on their environment – reduce international tension.

New materials combined with robotics have given rise to new satellite/spacecraft manufacturing techniques that enhance small satellite capabilities and also further reduce cost and timescales. Robotic additive (and subtractive) manufacturing techniques now make possible product geometries that were previously physically impossible by human hands and digital manufacturing provides freedom of location and dramatically increased speed of the design evolution and the product innovation cycle.

Robotics and software-defined satellites manufactured in orbit will fundamentally change the space industry, access to and exploitation of space, and further increase our dependence on space – with an accompanying responsibility to manage properly that unique environment.