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Some useful information about CubeSats

History

The CubeSat origin lies with Prof. Twiggs of Stanford University and was proposed as a vehicle to support hands-on university-level space education and opportunities for low-cost space access [REF1]. At its most fundamental level, the CubeSat can be defined as a discrete but scalable 1 kg1 100 x 100 x 100 mm cuboid spacecraft unit; this is now commonly referred to as a 1U(nit) CubeSat. Figure 1 shows a Pumpkin CubeSat Structure, with Clyde Space Solar Panels attached. The CubeSat definition is scalable, and so CubeSat units may be combined to produce larger mass and volume systems (up to 3 U CubeSats have been demonstrated in-orbit, and 6 U systems are proposed).

cube figure 1

Figure 1: Pumpkin 1U CubeSat Structure with Clyde Space Solar Panels.

Central to the CubeSat concept is the standardisation of the interface between the launch vehicle and the spacecraft, which allows developers to pool together for launch and so reduce costs and increase opportunities. As a university-led initiative, CubeSat developers have advocated many cost-saving mechanisms, namely

  • A reduction in project management and quality assurance roles
  • Use of student labour with expert oversight to design, build and test key subsystems
  • Reliance on non-space rated Commercial-Off-The-Shelf (COTS) components
  • Limited or no built-in redundancy (often compensated for by the parallel development of CubeSats)
  • Access to launch opportunities through standardised launch interfaces
  • Use of amateur communication frequency bands and support from amateur ground stations
  • Simplicity in design, architecture and objective

The approach has since been adopted by numerous universities and organisations, and to date has been used as the basis of 40 missions (as at the end of October 2008) which have been launched since 2003, with an estimated 60+ active projects in development; Figure 1 shows a typical CubeSat, whilst Figure 2 gives the growth in development since 2003.

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Figure 2: Growth in CubeSat launches since 2003

Since the initial proposal of the concept, further efforts have been made to define internal and external interfaces made by various developers of CubeSat subsystems, products and services that have defined the CubeSat 'standard' as it is today2. A core strength of the CubeSat is its recognition of the need for flexibility in the definition of standards, and since conception the standard has evolved to ensure that these design rules are as open as possible. The most significant of these further advances in definition have been for the POD systems (in order to meet launch requirements) and the modularisation of the internal electronics.

Whilst the form requirement of a 100 x 100 x 100 mm unit is likely to remain for launch interface reasons3, strict adherence to the 1 kg mass requirement is already becoming redundant, allowing developers a greater freedom to create innovative payload packages. With regard to modularisation, an internal electronic card structure simplifies the subsystem level interfaces and facilitates the inclusion of 3rd party modules off-the-shelf4, so enhancing quality assurance and lowering cost to the developer. Figure 3 shows a typical module, the Electric Power System, as supplied by Clyde Space.

EPS

Figure 3: Clyde Space EPS

Past missions

The in-orbit success rate of university-led CubeSat projects (not withstanding launch failures) is around 50%; this is an understandable result of using the CubeSat as an education tool, where development itself is a learning process and in-orbit failure is a disappointment but should not be considered the primary focus. For projects involving significant participation of companies with experience in satellite development, all but one were a success and demonstrated the strength of the CubeSat for non-educational applications. It is estimated that 12 CubeSat missions could be considered to have demonstrated significant successful in-orbit operations for a sustained period (see Table 1 for list).

Date

Organisation

Name

Current Status

2003

Tokyo Institute of Technology

CUTE-1

Active (post-mission)

 

University of Tokyo

XI-IV

Active (post-mission)

 

Stanford University & QuakeSat LLC

QuakeSat

Inactive

 

University of Würzburg

UWE-1

Inactive

2005

University of Tokyo

XI-V

Active (mission)

2006

Consortium led by NASA Ames

GeneSat-1

Active (post-mission)

2007

The Boeing Company

CSTB-1

Unknown

 

University of Sergio Arboleda

Libertad-1

Inactive

2008

Ålborg University

AAUSat-2

Active (mission)

 

Fachhochschule Aachen

Compass One

Active (mission)

 

Tokyo Institute of Technology

CUTE-1.7+APDII

Active (mission)

 

Delft Institute of Technology

Delfi-C3

Active (mission)

Table 1: CubeSats successfully operated in orbit and current status

All Cubesats missions to date may be considered to have had technological objectives to some degree, be it the demonstration of devices and system architectures developed in-house, or demonstration of Non-Space-Rated (NSR) Commercial-Off-The-Shelf (COTS) component performance. Some CubeSats have also attempted to fulfil other mission objectives, although categorising these accurately can be difficult (e.g. does a radiation sensor imply the mission is scientific for assessing the low-Earth orbital environment, or should it be considered a supporting sensor for validating technology demonstration components). Table 2 attempts to categorise the previous 40 missions into four objective categories beyond technology demonstration: Earth imaging, novel communication, science and utility.

Earth imaging is a commonly quoted objective for a CubeSat mission, typically achieved using a CMOS camera and without any complex lensing systems. As a critical impediment to the development of a highly capable platform for mission operations, the testing and evaluation of novel approaches for increasing downlink data rate and reliability is also a common objective. Whilst less common than Earth imaging, real science objectives are becoming increasingly popular as recognition (primarily by NASA) of CubeSat capabilities increase and collaborations between engineering and science groups emerge. Utility covers objectives not covered by the other categories and developed to handle a particular non-scientific demand.

Objective

Launched

Successful

Elements of previous missions

Technology

40
(100%)

12
(50%)

COTS component integration and radiation hardness, experimental sensors, system architectures, radiation and fault tolerance, solar array performance, tethered systems, deployable systems, wireless links, power management

Earth imaging

13
(33%)

5
(21%)

COTS CMOS camera, dedicated processor, attitude determination algorithms

Novel communication

6
(18%)

1
(4%)

Non-AX.25 protocols, active grid and patch antennae, redundant links, advanced modulation techniques

Science

10
(25%)

3
(13%)

Charged particles, solar sailing, earthquakes, airglow, animal tracking, DNA denaturing, gamma-ray bursts, atmospheric GPS scintillation, atomic oxygen, radiation

Other utility

6
(15%)

1
(4%)

Ship AIS monitoring and data relay, risk reduction for future missions or technology demonstration testbed

Table 2: breakdown of mission objectives for CubeSats launched (out of 40) and successful (out of 24)

Additional capabilities of proposed future missions either in planning or in development include: space weather monitoring, inflatable de-obit devices, Earth imaging with optical lens, cosmic ray showers, shape memory alloys, star mapping, data relay, reprogrammable computing, nanometeorid dust, plasma probe, and multi-spectral remote sensing.

Cost reduction in these projects has been achieved through a number of mechanisms, some of which are unavailable to the conventional space industry. The lowest cost yet successful mission is reported to be estimated as under $100,000 USD (although the mission was not fitted with solar arrays). A typical cost for a university project varies considerably but a very approximate estimation might be from $50,000 to $150,000 USD for launch and $100,000 USD parts cost per unit. In some instances piggyback launches have been offered for free to CubeSats by launch vehicle operators and space agencies, so negating the majority of launch costs.

A Clyde Space Perspective

Clyde Space was one of the first companies to recognise the enormous potential of the CubeSat both on a product level and beyond education for servicing previously unfeasible mission concepts and applications. As an experienced company in developing spacecraft systems, Clyde Space brings its commercial satellite knowledge and techniques to the CubeSat, producing high levels of quality assurance. Clyde Space is able to provide a full range of CubeSat products and services to all organisations from a team experienced in over 20 commercial missions.

Under its SCOTSAT development program, Clyde Space and the University of Strathclyde are working to commercialise a full CubeSat multi-mission platform and subsystem and so enable a vast range of cost-effective and responsive mission applications and opportunities.

SCOTSAT

Figure 4: outline of the SCOTSAT conceptual architecture

SCOTSAT will consist of a number of spacecraft bus and payload modules incorporating NSR COTS components. It is recognised that even utilising cutting-edge NSR COTS devices not available to the rest of the space industry for reasons of risk, CubeSats are unable to compete on a direct spacecraft for spacecraft capability with other small satellites. Although a complex problem, this essentially reflects current and predicted future technology levels and its rate of miniaturisation to fit within the Cubesat form constraint. Ultimately, this will become limited by the laws of physics. Against this, is the use of the available capability in the identification and exploitation of mission opportunities using the SCOTSAT platform and then deriving requirements from these applications to drive development. There are three fields of application in which it is considered that SCOTSAT can contribute: social, commercial, and scientific missions.

The problem of technology miniaturisation could be solved by deviating from the CubeSat standard. Although possible in the future, deviation from the standard would separate a developer from the community, and the potential market this represents. One of the core CubeSats strengths in the competitive space market is that of developer and spacecraft number, and it is therefore an unacceptably risky strategy to be pursued in the near term for SCOTSAT. Such an approach would also diminish or destroy the core strengths of open COTS modularity, cost effective and timely life cycles, and simplified access to launch opportunities. Rather, encouragement and facilitation of the CubeSat standard to organically evolve within the community to complement SCOTSAT is the preferred option, in order to compensate for the inherent limitations found when following any fixed set of design rules.

In parallel to this process two further strategies have been identified in order to help mitigate the disadvantages experienced by CubeSats in relation to other small satellites: those of innovation and specialisation. In these terms innovation and specialisation are defined as follows:

Innovation - the introduction of a new idea, method, or device not conventionally applied to space missions in order to raise the capability of the CubeSat platform (Figure 4a).

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Table 4a: examples of innovation for CubeSats

Specialisation - adaptation of the CubeSat platform for a specific use, generally into new, niche or previously unfeasible applications and markets not serviced by conventional space missions (Figure 4b).

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Figure 4b: examples of specialisation for CubeSats

As has been previously indicated, it is recognised that a core strength of the CubeSat standard and hence SCOTSAT is the development and availability of COTS subsystem level modules. In order for these modules to have the potential utility on as wide a range of missions as possible, a consistent reconfigurable plug 'n' play approach to the platform architecture is required. This approach must embrace modularity on a number of levels, in order that it allows reconfiguration of the platform to: (1) the mission and payload; (2) the bus functionality; and (3) the bus subsystem capability. These three requirements may be loosely mapped onto the suggested architecture of the CubeSat platform, facilitating different levels or of modularity and therefore allowing multiple classes of device to be incorporated into the resultant system design (in this case a clear distinction should be noted between modularity at multiple levels of the CubeSat architecture, and the CubeSat modules which represent a single level of modularity). This is illustrated in Figure 5.

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Figure 5: SCOTSAT approach to encompassing multi-level modularity

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Figure 6: accelerated development cycle in SCOTSAT program

Another important and related aspect in the design approach is that of modularity in a complete and integrated SCOTSAT CubeSat life cycle, effectively representing a modular system of systems. The accelerated life cycle demonstrated consistently by small satellites, and harnessed by many CubeSat developers, can be further enhanced by the application of modularity to the complete life cycle. Before this can be achieved however, a clear vision as to how the life cycle should be arranged needs to be addressed and should be defined so as to be a driving factor, but as far as possible indistinct with the open philosophy of the CubeSat community. The concept for this cycle proposed for SCOTSAT is depicted in Figure 6.