A survey of small satellites domain: challenges, applications and communications key issues

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I. Introduction

Traditional satellite missions are extremely expensive to design, build, launch and operate. Consequently, both the aerospace industry and the research community have started directing their attention to missions involving many, small, distributed and inexpensive satellites. Furthermore, many space projects in universities laboratories are focused on the development of micro-, nano- and pico-satellites for both scientific and educational purposes.

New concepts arise as small satellite domain imposes itself as a particular field. Therefore, concepts such as, constellation, cluster, and swarm became popular because of their potential to perform coordinated measurements for remote control missions and its capacity of long-term mission.

The first part of this paper discusses the complementary aspects of conventional satellites and small satellites.

The reminder of the paper is organized as follows. Section III presents small satellite challenges while Section IV reveals the numerous applications of small satellites. Section V describes formation flying concept and outlines the advantages and disadvantages of satellite distributed systems. Finally, routing approaches for nanosatellite networks are discussed in Section VI.

II. Complementary aspects of large and small satellites

The small satellite technology has opened a new era of satellite engineering by decreasing space mission cost, without reducing the performance.

Small satellite missions are supported by four contemporary trends:

• Advances in electronic miniaturization, the progress in data manipulation, storage, imaging technology, autonomous intelligence and associated performance capability;
• The appearance of new small launchers on the market (e.g., modified long-range and intercontinental military missiles, special structure for auxiliary payloads which allows simultaneous launching of several small satellites);
• The possibility of ‘independence’ in space (small satellites can provide an affordable way for many countries to achieve Earth Observation and/or defense capability, without relying on inputs from the major space-faring nations) [1];
• Ongoing reduction in mission complexity and management costs.

The new technologies such as, formation flying algorithms, constellation self-reconfiguration, accurate precision algorithms, developed for small satellites are often later used on major missions, involving large spacecrafts (e.g., space telescope missions: ESA’s Darwin, NASA’s Constellation-X).

Some applications can be better solved by using distributed systems (e.g. employing constellations of micro-satellites or nanosatellites optimally configured to achieve global cover). Yet, other space missions need centralized systems (e.g., employing large optical instruments, using high power, direct broadcast, communications systems etc.).

On the other hand, certain mission with stringent requirements could imply an equal or even greater cost of developing a small satellite constellation than a large satellite.

Through the eyes of NASA philosophy “faster, better, cheaper”, we tend to think that small satellites is the best way for reducing space mission cost. Generally, space missions are design so as to obtain the lowest cost design consistent with the mission requirements and constraints. Certainly, not all missions have been low-cost and nowadays, the main aim of most space agencies and organizations over the world, in long term, is to reduce the cost of space missions without reducing the performance

Considering this context, some questions rise such as:

Can we get the same product, with the same performance for less money?

Can we achieve reliability throughout simple designing?

Can we meet the mission objectives with a low-cost, small satellite with respect to a traditional mission?

Can a small satellites constellation accomplish more complex mission than a conventional satellite?

It is a known fact that, by dramatically reducing mission cost, the resulting system will be fundamentally different in at least some features. In the literature, there are many methods for reducing the cost of space missions. It is important to note that there is no single, broad method for reducing mission cost. Each low-cost program uses a specific approach to fill its particular requirements. Paper [1] presents some cost reduction methods which are selectively used by the builders of low-cost missions.

Considering all these aspects, we cannot state that, in the future, small satellites will replace large satellites. Certainly, every type of satellite (i.e., large or small), has its own advantages and limitations. Table 1 summarizes some advantages and limitations of small satellites over traditional satellites.

As mentioned in IAA study [1], the main advantages of small satellite missions are:

• more frequent mission opportunities and therefore faster return of science and for application data;
• larger variety of missions and therefore also greater diversification of potential users;
• more rapid expansion of the technical and/or scientific knowledge base;
• greater involvement of local and small industry.

Therefore, the choice between small satellites and traditional satellites will mainly depend on the goals and objectives of the mission, its requirements and design. For the purpose of this study, we consider large satellite missions and small satellite missions being complementary rather than competitive.

III. Small satellite challenges

The proliferation of low-cost, “micro-“, “nano-“ and “pico-satellite” missions in low-earth orbit has presented new challenges to the research community.

The small mass limits the amount of energy provided by solar arrays as well; therefore, power availability is a constraint on both the spacecraft processor and the communications systems. RF power available on low-cost, small satellites ranges from 0.5 watts on nanosatellites to a few dozen of watts on microsatellites. In addition to this, small satellites use various communications systems, including VHF, UHF and microwave links ranging from 300 baud to 1Mbaud. Nanosatellites mostly use omni antennas without gain because no tracking is needed for these antennas.

Depending on the orbit, small satellites can be out of contact with the ground station for over a week. The limited communications opportunities and limited bandwidth impose constraints on data handling, fault detection and correction and instrument commanding in general. Because of this, the spacecraft need to operate autonomously and handle any anomalies that occur. This autonomy must be accomplished within a processing capability that is less powerful than on a conventional satellite. Redundancy requires selection Because of the limited resources, small satellite designers add a minimum level of redundancy to back up only critical sub-systems.

Making small satellites more cost-effective demands new technologies that must be certified for spaceflight. Certainly, a key issue in small satellite domain is managing risk. Since no complex system can be designed and tested against all failure modes, the first fly experience is often the best and only way to perform flight verification. Considering all these aspects, small satellites can be effective platforms to raise the Technology Readiness Level or TRL of various sub-systems that can be used in future, more complex, space missions.

As mentioned in paper [1], IAA Committee considers that the main challenge faced by the small satellite community is to gain a broader acceptance of the notion that TRLs can be raised as an integral part of a mission rather than by implementing a dedicated mission.

According to IAA Commission [1], the biggest long-term challenge for the small satellite community is that of developing a robust commercial market that supports the infrastructure that has been developed to produce small satellites. To achieve this objective, the manufacturers must remain relevant and cost-effective.

Nowadays, we cannot speak of the existence of a robust commercial market for small satellites. The government continues to be the main financial support of the small satellite community. This community is still linked to education and research activities – activities that rely on government support. This situation will remain in force until some economies of scale can be achieved. Two notable examples of commercial ventures are SSTL [5] and RapidEye [6]. In the future, we could expect many small satellite commercial vendors to come to live.

IV. Small satellites applications

A. Telecommunications

Telecommunications activity potentially involves many applications ranging from mobile communications (including messaging, email and localization) to telemedicine – the transmission of information obtained by cheap, simple sensors sited in remote areas to complex processing units in large medical centers. All these applications could be established using small satellites placed on LEO orbit. This solution can be attractive to users in remote areas/regions lacking communications infrastructure.

In the last two decades, various constellations of small satellites in LEO have been proposed to provide worldwide communications using only portable terminals for real-time voice/data services (e.g., Iridium, Globalstar) and non-real-time data transfer (e.g., Orbcom, VITASat, GEMStar, E-SAT).

Two notable exemples of telemedicine projects worth to be mentioned:

• the HEALTHNET project, which employs a 60 kg micro-satellite (HealthSat) flown in LEO to relay medical data recorded in a number of African countries to North America.
• the Colombian Space Agency’s telemedicine pilot project dedicated to ECG signals transmission and reception. The main objective is to evaluate the performance of a tele-cardiology system on Internet. The principal interest is to bring medical services to isolated communities through small satellite networks.

Mobile satellite communications can play an important role in large natural disasters, by providing rescue teams with important logistical support. An example is Disaster Monitoring Constellation [7], a network of seven small satellites, which provides emergency Earth imaging for disaster situations..

B. Earth Observations

Earth Observation applications cover activities related to data collection and to imagery. Remote sensing using low-cost small satellites which allow direct data downlink to various, small, ground stations. Also, in the disaster prevention domain, there a many demands for earthquake forecasts, storms early detection and predictions of volcanic activity.

C. Scientific Research

As mentioned by Prof. Martin Sweeting in the lecture [8], nanosatellites 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. Therefore, scientists could have more opportunities to gain 'real-life' experience of satellite and payload engineering and to be able to initiate a research program.

Many ongoing co-operative scientific programs in the area of solar and space-plasma physics illustrate the advantage of using a coordinated group of satellites to obtain multi-point measurements of various phenomena. A particular case is provided by the International Solar-Terrestrial Physics program [9], which involves the co-ordination of data from the Solar and Heliospheric Observatory (SOHO) spacecraft of ESA, the WIND and POLAR spacecraft of NASA and the Geotail spacecraft of ISAS.

D. Technology Demonstrations

Nanosatellites can provide an attractive and low-cost means of testing, verifying and evaluating new technologies or services on a real orbital environment and within acceptable risks prior to a commitment to a full-scale, expensive mission.

Such technology demonstrations were mounted on Japan’s Hypersat Class spacecraft and on ESA’s Project for On-Board Autonomy (PROBA) [10].

E. Military Applications

A military version of the SSTL microsatellite platform has been developed to support various military payloads. The main differences between the ‘commercial’ and ‘military’ versions relies on are in the components’ specification and in the amount of paperwork for hardware and procedures. This involves also an increase factor for cost and time of 1.5 with respect to 'commercial' version.

F. Academic Training

Small satellites programs are a mean to enhance the industrial domain and to provide education and training of students, scientists and engineers in space related skills, by allowing them direct, hands-on, experience at all stages (technical and managerial) of a particular space mission (including design, production, test, launch and orbital operations). At present, many universities and schools of engineering in several countries in Europe, Japan, U.S.A, etc, had already developed, launched and operated their own small satellites.

G. Low Cost Launches

There are two opportunities for small satellites to access space: launch on a dedicated, expendable launch vehicle or secondary payload launching (piggyback). Choosing between different launch opportunities involves weighing up the requirements of a desired mission against the capabilities, costs and constraints characterizing a particular option.

Over the past decades, many countries have developed indigenous launch capabilities. The small class of expendable launch vehicles can deliver payloads weighing between 25 kg and 1500 kg to LEO. Long-range and intercontinental missiles from military arsenals are, in addition, presently available for civilian space launches.

Table 2 illustrates the launching cost/kg for various launching vehicles.

Table 2. Launching cost for various small launchers (Source: Futron, 2002)

H. Ground Segment

The complexity of the ground station depends on the mission type. Generally, the ground station for small satellite mission can be based on a simple, very high frequency (VHF), antenna (e.g., University of Surrey’s UoSAT satellite series). Contrary, an Earth Observation mission can require more complex support for collecting a large volume of data.

The small satellites missions launched until present rely on on-board autonomy and safe modes. Thus, the continuous ground monitoring is no needed, which simplifies and reduces the overall cost of the ground segment.

Certainly, there are some recommendations for small satellite ground stations designing that need to be followed:

• the system reliability should remain sufficient to ensure that satellite passes/data transmissions are not missed;
• the system should offer a fast return of critical data, as well as a rapid response to critical commanding.

GENSO (Global Educational Network for Satellite Operations) [11] is an ambitious project launched and coordinated by ESA’s Education Office. The main aim is to increase the return from educational space missions by forming a worldwide network of ground stations and spacecraft which can interact via a software standard.

I. Economic Benefits

IAA Committee mentions in paper [1] that the main benefits within a country from using small satellites include:

• Improvement of agricultural and animal productivity in medium to large-size farms due to better weather predictions, identification of soil characteristics, improvements in communications and transportation;
• Reducing transportation costs, by optimizing truck, bus and ship routing, location and early robbery detection, with favorable impact on the price of goods;
• Communication provision for the basic needs of rural settlements in remote areas;
• Improvements in natural disaster detection, by using systems that integrate scientific communications and remote-sensing satellite networks;
• Educational programs for populations in remote areas.

The same study [1] shows that investments in the space sector result in a very high multiplier effect on the gross national product (by a factor of the order of seven). Developing small and micro-satellite systems provides a powerful means to acquire national expertise in space domain.

V. Formation flying

Across the Formation Flying research community there are a wide range of definitions for formation flying and related terms. We will consider some representative definitions that are generally consistent with most elements of the community.

From the perspective of engineering definition, according to Jesse Leitner [13], “formation flying involves the control of relative distances or geometric configuration between spacecraft.”

Another definition is presented by F. Bauer et Al. in paper [14] as “the on-orbit position maintenance of multiple spacecraft relative to measured separation errors”.

Also, Nicholas M. Short mentions in [15] a more detailed definition: &ldquot;groupings of duplicate or similar satellites, having sensors in common or are complementary (related), that talk to each other and share data processing (onboard and/or by means of utilizing comparable ground stations and facilities), payloads, and mission functions.&rdquot;

The concept of formation flying mission is to replace a large satellite with a &ldquot;virtual satellite&rdquot; – a cluster of smaller satellites, flying in very precise relative positions. Rather than using a single, large, expensive satellite to perform a given mission, many small, inexpensive satellites can be flown in a constellation more effectively.

Some applications for satellite formations flying are:

• Large sensor apertures in order to obtain an increased resolution;
• Servicing, by replacing failed formation elements individually;
• Upgrade and Maintenance: working on individual components without removing whole mission;
• Change formation geometry: evolving mission sensing requirements.

There are different terms used to describe spacecraft formations. Those encountered during the literature survey are listed in Table 3. The formation types are illustrated in Figure 1.

Figure 1. Spacecraft Formations: (a) Constellation (b) Cluster (c) Leader-Follower [18].

Replacing a single satellite with a formation flying system could be beneficial for some missions, but uneconomical for others. Many opinions exist in scientific research community, and some of the advantages and disadvantages of multiple-satellite systems are summarized in Table 4.

An example of formation flying mission is e-CORCE (e-Constellation d’Observation REcurrente Cellulaire) [16], [17] project of CNES, the French Space Agency. It is an innovative satellite remote-sensing system, capable of generating a high-resolution picture of Earth on the Web, refreshed every week.

The solution proposed by CNES scientists consists of:

• using simple, inexpensive satellites with telescopes-based viewing system weighting only 40 kg;
• using image compression – ocean imagery is compressed more than images of cities – on board the satellites, a “psycho-visual” compression that is transparent to Web users; images covering a swath of 28 km will be compressed up to 50 times;
• dispatching data to 50 centers connected to the Internet all around the globe, by using new methods of receiving and processing information distributed across the planet.

Google Earth works very well but its resolution varies greatly from one region to another and its imagery is updated randomly. Despite Google Earth, e-CORCE constellation will be capable to photograph all continents in color at a resolution of 1 meter, every week and will disseminate the pictures directly over the Internet.

e-CORCE will consist of 13 Earth-orbiting microsatellites at 600 km altitude and 50 ground stations. The system could be operational by 2014 at an estimated cost of Є400 million.

VI. Routing approaches for nanosatellite networks

New emerging protocols might deliver new and interesting ways for interconnecting nanosatellites networks and sensor/or Ad hoc networks. But several different problems are usually encountered on these networks that require us to revise communication protocol design, network management, and to consider novel routing mechanisms to accomplish “more with less”. For instance, common problems of nanosatellite networks are onboard resources, limited communications opportunities, limited bandwidth, scalability, redundancy, power availability, high-speed node mobility, the type of communication among satellites, assigning or not a separate communication channel for positioning, timing and synchronization issues. Finally, a lot of new services via small satellite will come into service once small satellite operator comes to live.

In order to identify candidate protocols and network topologies that can be used or adapted for small satellite networks, we conducted a study of routing mechanisms in traditional satellite network, Ad Hoc network and sensor networks. This study is part of PERSEUS (Projet Etudiant de Recherche Spatiale Européen Universitaire et Scientifique) program [19, 20], launched by CNES (Centre National d'Etudes Spatiales) in June 2005.

Firstly, we surveyed sensor networks and we made an analogy between sensors and nanosatellites. A sensor network is constituted of small, low-power, and low-cost devices with limited computational and wireless communication capabilities. Table 5 presents the common features of sensor networks and nanosatellite networks.

By studying sensor network routing, we identified the following approaches for nanosatellites:

• Sensor networks could be integrated with nanosatellite networks for space and Earth monitoring missions.
• Self-reconfiguration network after initial deployment. Once placed in the orbit, the nanosatellite constellation could reconfigure itself in order to compensate for a lost nanosatellite without loss of the mission. For doing this, a certain level of artificial intelligence is integrated on every spacecraft. Self-reconfiguration is also a way of maximizing system’s flexibility.
• A certain level of redundancy is needed in order to achieve a higher level of efficiency. Generally, small satellites have only their critical sub-system backed up.
• One of the main challenges of sensor network is traffic minimization. This principal applies also to nanosatellite networks. Traffic should be minimized so that the network won’t be overloaded with unnecessary information.
• Using MAC routing protocols:
  • TRAMA (Traffic-Adaptive Medium Access) is greatly reducing the energy loss caused by packet collisions;
  • STEM (Sparse Topology and Energy Management) allows nodes activation only when traffic is generated, thus allowing an energy efficient routing mechanism.
• Using routing-based on resources. Two types of routing strategies have been identified: energy-aware routing and fidelity-aware routing.
• Using data-centric protocols. Query-based protocols depend on the naming of desired data, eliminating this way many redundant transmissions.
• Using location-based protocols. By using position information, the data is relayed to the desired regions rather than to the whole network;
• Other sensor network concepts that might be interesting for nanosatellite networks are: Intelligent Sensor and Web Sensor.

We have also turned our attention to Ad hoc network domain to find routing approaches that can be used in nanosatellites networks. We identified common challenges for both Ad hoc and nanosatellite networks, such as:

• scalability issues due to large number of nodes;
• dynamic topology;
• time-varying network links;
• severely limited resources in terms of power, storage capacity, memory, which implies no complex and energy intensive computation;
• time-varying achievable channel bit rate;
• topology maintenance: updating information of dynamic links among mobile nodes.

By studying the three types of Ad Hoc routing (i.e., proactive routing, reactive routing and hybrid routing), we can state that reactive routing approach is preferred for small satellite networks with restrictive resources.

The main benefits of adapting reactive routing to small satellites networks are:

• Proactive calculation of nanosatellites position reduces delay and control overhead.
• Passive listening allows listening to neighbor’s routing packages in a passively manner and updating their local routing table. Thus, broken links can be detected easily.
• Suspend mode allows to a node to suspend and notify its neighbors not to communicate with it even though its communication channels are in good condition. This mode is a good way of energy saving, a critical aspect in nanosatellite networks.

Lastly, we interested on traditional satellites networks to investigate if various protocols specific to large satellites can be used or adapted for small satellites. According to our study, XSTP (eXtended Satellite Transport Protocol) could be a candidate for small satellites communication links, considering that it is specifically optimized for asymmetrical satellite networks, characterized by high BER and variable RTTs. Paper [21] shows that XSTP attained higher effective throughput, much lower overhead, and better channel efficiency as compare to TCP clones, in case of high BER conditions.

VII. Conclusion

In this paper, we have presented a state-of-the-art of small satellite domain. This paper identifies and examines challenging issues of small satellites and explains formation flying concept. In addition, the main advantages and limitations of formation flying systems are presented. We have also addressed how such miniaturized systems will take advantage of technological advancement in small spacecraft and emerging information technologies. Small satellites applications are briefly presented as well.

The main question this work tries to answer is: can we find useful concepts that can be transposed to nanosatellite networks, by surveying other domains (i.e., sensor networks, Ad hoc networks and satellite networks). Therefore, we have highlighted important ongoing and future research directions for small satellites routing.

References

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