**6. The internet of satellites paradigm**

Alternatively to these massive satellite constellations, a collaborative and distributed approach has been proposed in the last years. The concept of Federated Satellite Systems (FSS) was presented by Prof. Golkar in [10]. This new satellite system essentially consists of spacecraft networks in which satellites trade unused or inefficiently allocated resources commodities, such as data storage, data processing, downlink capacity, power supply, or instrument time. This concept is analogous to terrestrial applications, such as peer-to-peer file sharing, cloud computing, and electrical power grids. In this way, FSS tried to avoid the underutilization of expensive space assets in already existing missions. The establishment of cooperation frames beyond the common mission interactions based on ground postprocessing and merging of instrument data becomes more and more a necessity. Distinct-stakeholder satellite missions would leverage the establishment of in-orbit collaborations by improving current system performance or by achieving new goals.

The terminology presented in [10] allows understanding the nature of these collaborations. A *satellite federation* is composed of a group of satellites which decide

### *From Monolithic Satellites to the Internet of Satellites Paradigm: When Space, Air… DOI: http://dx.doi.org/10.5772/intechopen.97200*

to engage in a collaboration with each other during their mission. These federations allow the satellites to share or trade available *resources* which are the tangible and intangible assets that a spacecraft has (e.g. propellant, power, data processing, downlink capacity, etc.) Although the original definition encompasses generic assets, the later work investigated federations with data-centered resources, such as processing, downlink, and storage capacity.

The establishment of a satellite federation has a decision-making component not necessarily autonomous—with which a satellite *opportunistically* deems if the collaboration is beneficial, where the benefit is defined as either economic profit or generic value. Despite the opportunity refers to the federation profit, a temporal aspect also is integrated with these characteristics. Due to the mission lifecycle of a satellite, resources are not constantly available, enabling temporal windows of opportunity in which they can be traded. During this transaction, the satellite that supplies the corresponding resources are defined as satellite *suppliers* or *providers*. When instead a satellite is seeking to request the resources, it plays the role of a *customer* in the federation. These roles may be switched over satellite lifetime depending on their interest, being also possible that a satellite acts as both customer and supplier at the same time. In the end, the joint set of customers and suppliers makes a satellite federation.

The FSS concept also differs from the other approaches, because they can be conceived as virtual satellite systems. These systems represent a group of satellites that are part of a distinct physical system–like a constellation–and they decide to create a new one that is fictitious. Traditional applications offered to ground users can be achieved with these systems, but new ones proliferate with this virtual group of satellites. Machine-to-machine applications may be deployed among the different satellites that conform the virtual system, like trajectory applications (e.g. flight formation, collision avoidance), applications that require data fusion (e.g. cloud detection, different instruments), among others. In terms of communications, this can be represented as an autonomous satellite application that deploys some services through and for satellites. All these characteristics require solutions that are flexible, adaptable, and scalable that must be reflected in all the development levels, included in the inter-satellite communications ones.

For this reason, our work in the last years has been focused on conceiving and developing the Internet of Satellites (IoSat) paradigm [17]. This approach proposes an interconnected space segment that follows these premises from satellite federations. A custom satellite infrastructure that corresponds to a network backbone is not proposed in this paradigm. Instead, it promotes the establishment of networks using peer-to-peer architectures, in which interested satellites are part of the network. The IoSat paradigm cannot be understood without firstly observe the nature of satellite federations.

FSS encourage the establishment of sporadic and opportunistic collaborations to share unallocated resources among heterogeneous satellites. It is important to understand concept-by-concept what this statement means.

The **sporadic** term refers to the possibility to deploy this collaboration at any moment. This feature makes satellite federations unpredictable events that may occur without notice, and they cannot–normally–be estimated in advance. Despite this randomness, the need to deploy federations is related to the satellite resources and the potential benefit that a satellite can award.

This is related to the **opportunistic** term, which suggests that federations are only established if related satellites envision to garner some benefits (e.g. enhancement of mission performance, an extension of satellite capabilities, payment for resources shared). This opportunism also refers to the mandatory non-degradation of the original mission. Satellites that establish a federation are designed to perform a specific mission (e.g. observation of the soil moisture, relay data from ground terminals, observe the galaxy), which must remain as its main priority. Therefore, the federation cannot degrade the performance of this mission by the undesired depletion or allocation of resources.

If the satellite does not identify a potential benefit, it must be able to **decide** not establishing a federation. This decision-making capacity becomes crucial to deploy federations and entails the awarding of a certain level of autonomy to the satellites. Finally, the satellites that collaborate are equipped with different resources and capacities.

This **heterogeneous** configuration poses multiple challenges related to resource sharing, connectivity, among others.

Following these features, the paradigm suggests dynamic, sporadic, and opportunistic satellite networks that are temporally established depending on the necessity and choice to deploy federations. These temporal networks have been called Inter-Satellite Networks (ISN) following the traditional nomenclature originated in [33]. This kind of network is created by the decision to collaborate–not necessarily for free–of satellites, that become the intermediate nodes of the network. In particular, the creation of an ISN is achieved thanks to the combination of point-to-point federations among intermediate nodes that share the possibility to communicate. **Figure 5** illustrates the paradigm philosophy by showing three ISNs (*ISN*1, *ISN*2, and *ISN*3) which coexist simultaneously. These ISNs are created depending on the FSS requirements and they adapt themselves to manage network dynamism. Note also that some nodes can participate in multiple ISNs at the same time.

A satellite federation is established only when the transaction is required, after that the federation is no longer needed. This temporality is also reflected in the definition ISN. This corresponds that ISNs have three phases that characterize their lifetime: (1) the establishment phase, (2) the maintenance phase, and (3) the destruction phase.

The establishment of an ISN is the negotiation process in which intermediate federations are created to configure the network. During this phase, its members can decide to not accept this interaction due to their state or strategy interests. Moreover, the establishment phase ensures that the ISN can satisfy FSS requirements by providing the required services. For instance, if a security level is required, intermediate nodes should have secure mechanisms to provide it. This implies that during the ISN establishment, nodes shall indicate which services they can provide.

Once the ISN is established, the maintenance phase ensures that the network adapts to different events. In particular, as a satellite network is a dynamic

**Figure 5.** *IoSat space segment representation. Figure from [17].*

### *From Monolithic Satellites to the Internet of Satellites Paradigm: When Space, Air… DOI: http://dx.doi.org/10.5772/intechopen.97200*

environment in which nodes are in constant movement, this phase is responsible to update network connections when intermediate links are broken. Therefore, it should be able to replace old intermediate nodes by adding new ones. Moreover, some satellites could request to participate in an existing federation that would need to add more intermediate nodes to increase the current ISN. Thus, the ISN should be able to adhere new satellite nodes as per their request, or by the need to keep the topology stable.

Finally, in the destruction phase (once the ISN is no longer required) all the nodes that have participated in the network should perform the destruction process which cleans their internal state and recovers their usual activity. This is an important phase because the resources shall be released when they are no more needed.

There is a common need that should be respected in an ISN. Satellites are embedded systems with severe limitations in terms of energy, computation, and data storage resources, which means that additional inter-satellite communications capabilities could jeopardize the mission. This could appear because satellites are normally conceived to accomplish a specific mission, and the integration of these new capabilities could suppose an additional resource consumption that could deplete the satellite. In other words, the deployment of an ISN shall not impact the mission of intermediate satellites. Therefore, this network is deployed using a resource-aware strategy while trying to satisfy application requirements. Moreover, if a satellite decides that its participation in the network compromises the accomplishment of its mission, it can decide to leave the network. Therefore, satellites require a certain level of intelligence to autonomously take this decision. An ISN is a completely dynamic and constantly changing scenario, due to satellite mobility, node participation, and node resource state.

Our previous researches have addressed the multiple technology challenges associated with the IoSat paradigm. A predictive algorithm was developed in [34] to provide autonomous capabilities to satellites. In particular, this algorithm can estimate future satellite contacts and predict routes overtime in which federations can be established. Moreover, new protocols regarding the necessity to notify resources available (e.g. downlink opportunities) and the procedure to establish a federation were published in [35, 36]. These two protocols have been evaluated in an scenario with Earth Observation (EO) satellites that uses federations with a megaconstellation to download data. **Figure 6** presents the achieved results of these simulation, being able to duplicate the amount of bytes downloaded per day when

#### **Figure 6.**

*Downloaded data of saturated EO satellites per day according to the publisher satellites and ISL subsystems with differnet maximum range (left figure) and data rates dmax and different data rates Rb (right figure). Figure from [35].*

more satellites of the mega-constellation participates as providers of the downlink service.

Apart from these innovations, the paradigm still poses considerable challenges that must be tackled in future researches.
