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Satellite-Based Communications Security: A Survey of Threats, Solutions, and Research Challenges

Satellite-based Communication systems are gaining renewed momentum in Industry and Academia, thanks to innovative services introduced by leading tech companies and the promising impact they can deliver towards the global connectivity objective tackled by early 6G initiatives. On the one hand, the emergence of new manufacturing processes and radio technologies promises to reduce service costs while guaranteeing outstanding communication latency, available bandwidth, flexibility, and coverage range. On the other hand, cybersecurity techniques and solutions applied in SATCOM links should be updated to reflect the substantial advancements in attacker capabilities characterizing the last two decades. However, business urgency and opportunities are leading operators towards challenging system trade-offs, resulting in an increased attack surface and a general relaxation of the available security services. In this paper, we tackle the cited problems and present a comprehensive survey on the link-layer security threats, solutions, and challenges faced when deploying and operating SATCOM systems.Specifically, we classify the literature on security for SATCOM systems into two main branches, i.e., physical-layer security and cryptography schemes.Then, we further identify specific research domains for each of the identified branches, focusing on dedicated security issues, including, e.g., physical-layer confidentiality, anti-jamming schemes, anti-spoofing strategies, and quantum-based key distribution schemes. For each of the above domains, we highlight the most essential techniques, peculiarities, advantages, disadvantages, lessons learned, and future directions.Finally, we also identify emerging research topics whose additional investigation by Academia and Industry could further attract researchers and investors, ultimately unleashing the full potential behind ubiquitous satellite communications.


Introduction
Satellite-based Communications (SATCOMs) play a vital role in the global telecommunication systems, having found applications in a plethora of domains throughout the last 50 years, including radio broadcasting, weather forecast, maritime communications, assisted navigation, and military operations, to name a few [1]. While the attention of Academia and Industry in the last years was mainly focused on ground communication systems, recent business initiatives launched by leading tech companies such as SpaceX, Facebook, and Amazon generated new renewed interest in satellite-based systems [2,3]. In particular, satellites are being deployed to provide services in a variety of new application domains, e.g., to reach remote locations providing unmatched connectivity (as per bandwidth and cost), or to support low-power constrained Internet of Things (IoT) devices [4]. As a result, recent commercial and standardization activities clearly indicate SATCOMs as one of the most important enabling technologies for supporting the development of the upcoming sixth-generation (6G) networks [5]. In addition, the business driving factors also seem to indicate a bright future for SATCOMs. Indeed, according to a dedicated research report Despite the promising applications and forecasting, the adoption of satellite links generally widens the threat surface of a system, introducing new vulnerabilities. Indeed, the ease to eavesdrop, tamper, disrupt, and reroute the satellite traffic provides the attacker with an extensive portfolio of opportunities to launch cyber-attacks at scale and affect the operations of such systems in different ways [6]. To complete the scenario, there is also the fact that the military satellite backbone is subject to the same (if not bigger) issues [7].
The severity of the cited threats could be even higher when considering that most of the current satellite systems either do not integrate security at all or run outdated security techniques, unable to face the complex attacks launched today [8]. As a result, security solutions for SATCOM should be revisited and coupled with the unique features of satellite-based systems.
3 [11], [12], [14], [18], [15], and [16]. However, they only preliminarily identified the Global Navigation Satellite System (GNSS) spoofing and GNSS jamming attacks and presented the adopted solutions and mitigation techniques available from the literature. Moreover, the study by the authors in [10] only summarize the schemes targeting data secrecy at the physical layer, with a focus on information-theoretic schemes only. Other surveys, such as the one in [19], focused specifically on some research areas, such as quantum computing, missing the contextualization of such areas to other research in the SATCOM domain. It is also worth mentioning the recent contribution by [20], highlighting the threats affecting specific application areas where SATCOM links play a role. However, such a survey focuses on the review of the literature, rather than on the identification of the research areas where such contributions are provided. Moreover, although many contributions are available on secure routing, e.g. [21] and [22], the focus of our investigation is link-layer security in satellite communications, and secure routing is therefore out of of scope. As a result, we notice that the current literature is still missing a comprehensive survey, presenting and exploring all the facets of the threat surface to be considered when deploying SATCOM systems at the link-layer, as well as related countermeasures.
Contribution. In this paper, we fill the afore-described gaps by providing a comprehensive survey on security threats, solutions, mitigation strategies, and research challenges faced when designing and deploying secure SATCOM systems. In detail, we classify the security solutions related to the link-layer of SATCOM systems available in the literature into two main research domains, i.e., physical-layer approaches and cryptography techniques. Next, we delve into each area, looking at the offered security services and how such schemes guarantee the desired security objectives. For each area, as a novel contribution, we describe the threat models, assumptions, system requirements, and operational strategy, and we cross-compare the most important proposals along their characterizing features (see Figure 1 for a graphical overview). Finally, within each research domain, we also identify novel future research directions and additional research challenges. Roadmap. The rest of this paper is organized as follows. Section 2 introduces the basics of SATCOMs; Section 3 introduces information-theoretic security strategies and solutions to enhance data secrecy; Section 4 analyzes the applications of cryptography schemes in SATCOMs; Section 5 outlines emerging research domains; and, finally, in Section 6 we tighten some conclusions. Refer to Table 2 for the acronyms list.

Background
This section introduces the main notions related to SATCOM systems used within our manuscript, including the satellites constellations, architectures, and the involved protocols. Overall, this section aims to provide the reader with the needed background on SATCOM technologies and their main features, that will be used in the sequel of the paper.  Table 3.
For instance, Inmarsat [28] is a provider of SATCOM services that adopt GEO satellites to provide telephone and data services to users worldwide. Companies like SpaceX [29] and Iridium [30,31] are planning to launch in orbit thousands of LEO satellites to provide low latency, broadband internet systems, voice, and data services anywhere on Earth.
7  The space segment of a SATCOM architecture is one of the three main components of a SATCOM system. This segment comprises GEO satellites to support business in navigation, data, mobile television, and radio broadcasting systems. At the same time, MEO satellites are deployed to deliver low-latency and high-bandwidth data connectivity to service providers, agencies and industries, and to support the network connectivity in the avionic/maritime domain.
LEO satellite constellations are also adopted for several applications such as imaging, and low-bandwidth telecommunications and broadband internet. Each of these satellites is placed in orbit by a launch vehicle. The space segment also and air-to-ground communications-illustrating use-cases, scenarios, channels to be used, and modulation formats, to name a few [33].
Finally, from the security perspective, note that attacks can be launched in any of the identified segments. Thus, the ground segment should be appropriately secured, as well as any communication originating from the satellite should be protected, independently from its target destination (ground or space). More details on the specific threats will be provided in the following sections.

Physical Layer Security Schemes for SATCOM
In this section, we introduce, review, and classify approaches investigating security issues of SATCOM technologies at the physical layer. Specifically, Sec- Indeed, attacks on SATCOM channels was conceived by the operators as hard to achieve, and overall, security was thought as a slow-down factor rather than an enabler. Thus, many satellites do not implement any security protection, and updating them today would require high costs [34].
Taking into account the above exposed issues, in the last years many contributions proposed to provide confidentiality to SATCOM scenarios by applying information-theoretic security schemes. The rationale supporting such schemes is the following: information-theoretic security approaches leverage the inherent randomness and noise of SATCOM communication channels to minimize the amount of information that can be extracted at the PHY layer by an unauthorized receiver [35]. This is typically achieved by guaranteeing that the quality of the channel, expressed in terms of Signal-to-Noise Ratio (SNR), exceeds a given bound only at the authorized receiver's location while remaining below the set threshold in other locations, so that an eavesdropper cannot decode the received message. Moreover, such schemes typically do not assume any constraints for the eavesdropper in terms of processing capabilities or network parameter knowledge, and the resulting security features can be quantified analytically via dedicated channel-level metrics. The afore-mentioned security features are achieved without resorting to cryptography materials (e.g. shared key, certificates) or crypto related computations, hence resulting in a very efficient solution (from the computational, storage, and bandwidth point of view).
We present a comprehensive classification of the scientific contributions that achieve confidentiality via information-theoretic schemes in SATCOM scenarios in Table 4. In the following, we summarize the most important features identified throughout our analysis.
Performance Metrics. Many performance metrics can be used to evaluate the effectiveness of a scheme proposed to ensure confidentiality at the PHY-layer via information-theoretic schemes, including, e.g., the secrecy capacity/secrecy rate, average secrecy rate, and secrecy outage probability, to name a few. We hereby present the definition of the most important ones, leading to the definition of all the others.
We assume that a malicious user E is interested in eavesdropping the traffic exchanged between two legitimate network entities and correctly decoding the information. The Secrecy Rate of the communication link is defined as the difference between the capacity of the eavesdropper E and the capacity of the legitimate user [56], where the capacity is the maximum transmission rate at which an eavesdropper is unable to decode any information. The secrecy rate of the legitimate source-to-destination communication link at the physical layer is defined as in the following Eq. 1: where C L is the capacity of the legitimate channel and C E is the capacity of the channel source-eavesdropper.
Another important metric is the Secrecy Outage Probability (SOP), defined as the probability that the instantaneous secrecy capacity drops below a specific threshold value, representing a target secrecy rate [57]. The other metrics listed

13
in Table 4 can be derived from the ones previously introduced. Additional details can be found in surveys dedicated to the topic, such as [35].
Link. The largest part of the analyzed works considered the Satellite-to-Ground (SG) link, applying information-theoretic approaches to secure the communications from the satellite to the ground receivers. To the best of our knowledge, only four works considered the Ground-to-Satellite link, i.e., [39], [42], [46], and [54], while other links are never considered. State Information (CSI)). The vast majority of the works assume the perfect knowledge of the channel quality at the eavesdropper side. This is the most secure approach to analyze the problem from an information-theoretic perspective, as it allows to maximize in analytical terms the difference between the legitimate channel and the (potentially) eavesdropped one, i.e. the secrecy rate.
In practice, the cited objective is guaranteed by either reducing the probability of correct signal decoding by the adversary, or maximizing the secrecy rate of the main communication links. Few other approaches, instead, assume to know either partially (Imperfect) or completely the channel experienced by potential eavesdroppers. Such adversary model is often referred to as an active eavesdropper, as it is a legitimate network node, interacting with the network at times (so, its CSI parameter is known), but also equipped with eavesdropping capabilities on other communications. A few contributions, such as [36,39,41,45,51,52,54] evaluated the impact of the aforementioned assumption on the related security metrics.
Adversary. In line with related work, we consider two types of adversaries: the internal and the external one. In particular, we define an internal adversary as an attacker playing the role of a legitimate network entity, actively participating in the network activities by transmitting and receiving information, such as an active eavesdropper [58]. We define an external adversary as an attacker who is not part of a legitimate SATCOM, also striving to remain not detectedhidden. This latter category also comprises the external eavesdropper, needing a simple receiving antenna tuned on the same frequency of the legitimate communication channel to receive packets successfully.
Adversarial Receiving Antennas. The simplest adversary model, considered by most of the analyzed contributions, is a single eavesdropper, not sharing any information with other receivers [36,37,39,41,42,43,45,46,47,48,49,50,52,53,54]. We remark that this is the easiest to analyze from the security perspective, as the previously-mentioned performance metrics only have to consider a single adversary. Just a few works, i.e., [51] and [44], considered multiple antennas, even if no collusion between them is considered, thus reducing the adversary model to the same of multi single-antenna adversaries.
Adversarial Antenna Type. All the analyzed contributions consider adversaries equipped with omnidirectional antennas, i.e., antennas capturing the information independently from the source location and (partially) radio environment. While such a model could appear the strongest, it does not consider a realistic SATCOM scenario, where obstacles at the ground could alter the profile of the received signal, as well as the previously-mentioned performance metrics.

Anti-Spoofing Schemes
Without loss of generality, spoofing refers to disguising a communication from an unknown source as being from a known, trusted source [18]. and Meteor [59].
Given the considerable threat surface, the design of anti-spoofing techniques for SATCOM scenarios has been largely focused on GNSS technologies due to their widespread use and increasing importance in the modern connected society [60]. Today, thanks to the sensitive advances in the design of Software Defined Radios (SDRs), performing GNSS spoofing attacks is surprisingly easy.
An attacker needs an SDR and an omnidirectional transmitting antenna; by downloading freely-available tools such as gps-sdr-sim [61], the attacker just needs to run a script to emulate a complete satellite constellation and move the target wherever in the world, also for a significant period of time [62]. Considering that GNSS satellites are also used for time synchronization in several IoT deployments, detecting spoofing attacks with lightweight and effective techniques is of paramount importance. Indeed, such modifications woul require temporarily stopping the operation of the GNSS satellite, causing significant and unmanageable costs and effort.
Usage of Multiple Receiving Antennas. Simplest GNSS spoofing attacks assume that the adversary transmits a fake GNSS signal using only a single antenna. Signal cross-correlation detection methods identify such attacks by discriminating the injected signal from the expected one, as it is transmitted using different devices than the ones the target satellite is equipped with.
Comparing the correlation of the received signals using more than one antenna  [31] IRIDIUM Ring Alert at the receiver side will result in a significant difference in signal characteristics, such as the carrier phase and amplitude. This is just an example of a technique employing multiple receiving antennas to detect GNSS spoofing, and other examples include the contributions by the authors in [65,59,67,68,70,71,76].
Although being relatively cheap to deploy, such solutions usually require the deployment of multiple antennas and their connection to a single system, i.e., the hardware modification of the receiving device. Often, such operations might be too expensive or impractical to be applied due to application-specific limitations.
Usage of PHY-layer information. GNSS messages transmitted by satellites go through different phases of signal processing before being converted into a digital form at the receiver side. The anomalies in the characteristics of signals during any of these processing phases could be used to detect counterfeit GNSS signals [91]. Anomalies can be found in signals features such as received power, Carrier-to-Noise Ratio (CNR), quality, correlation, Automatic Gain Control (AGC), clock bias, and angle of arrival, to name a few. Such features are used in many approaches, e.g., [64,65,66,74], to detect GNSS spoofing attacks. On the one hand, such techniques could be compelling and reliable. On the other hand, they require the receiving devices to access such information, which is not always possible. Indeed, many modern chipsets do not provide PHY-layer information to the devices they are connected to or integrated into, preventing the application of such approaches.  [86]. Similarly to previous approaches using multiple antennas, such techniques might be very efficient. However, they require the adoption of compatible hardware, which sometimes does not represent a viable solution.

Anti-Jamming Strategies
In this section, we discuss the most important contributions dealing with anti-jamming methods in SATCOM scenarios. Without loss of generality, jamming is defined as the injection of intentional interference into the wireless channel in a way to disrupt the operations of a legitimate communication channel [92]. Several classes of jammers have been proposed in the scientific literature throughout the last years [93]. With reference to the portion of time where they are active, jammers can be constant, alternate, proactive (if they choose a channel in advance, and jam it) [94], or reactive (if they jam a specific channel only when RF activity is detected on that channel). Considering the number of frequencies jammed at the same time, jammers can be spot (a single jammed frequency) [95], sweep (multiple frequencies, at different times) [96], or barrage (multiple frequencies at the same time) [97]. Finally, based on the type of signal injected to cause interference, we can have noise-jammers (if noise of a different type, e.g., Additive White Gaussian Noise (AWGN) is injected on the channel), or deceptive-jammers (if they inject a signal similar to the legitimate ones). The effectiveness of a jammer strictly depends on the communication parameters [98] set by both the transmitter and the receiver [99]. Overall, to maximize the performance of the jammer, the adversary should carefully analyze the radio link and then design and deploy the appropriate type of jammer.
Jamming is particularly relevant in the context of SATCOM links, and several contributions highlight the inefficacy of currently-deployed communication schemes, e.g., Code Division Multiple Access (CDMA), when powerful jammers target the communication links [100]. Table 6 reviews the most important contributions providing anti-jamming techniques in the SATCOM context and cross-compares them across reference features.
In the following, we summarize the most important considerations emerging from our analysis.
Link. Similarly to previous physical-layer solutions, also anti-jamming strategies mostly considered the Satellite-to-Ground communication link, focusing on increasing the availability of satellite services on the ground. A few works, such as [107], [111], and [119], considered also the Ground-to-Satellite link, while only one work, i.e, [119], discussed anti-jamming solutions for the Ground-to- Type of Jammer. Characterizing and profiling the type of jamming affecting the communication link is the first step towards the deployment of an showing the effectiveness of their solutions through via practical experiments or real-world data.

Lessons Learned
In the following, we summarize the main lessons learned from the investigation and cross-comparison of the approaches working on improving the security of SATCOM deployments via physical-layer solutions.
No Satellites Hardware Update. All the analyzed approaches do not propose the modification of the transmitted signals or the transmitting chain.
Indeed, modifying a satellite is assumed to be too expensive to be performed, both from the financial and the operational perspective. Thus, assuming that the authenticity/availability received signal cannot be fully guaranteed, the studied proposals come up with solutions able to minimize the impact of different security attacks. This is a crucial difference from approaches providing confidentiality using informationtheoretic schemes that prevent the attacker from gaining information. Such difference is due to the different adversarial models (active in the first two mentioned cases, passive in the second one), that lead to different countermeasures.

Significant Receivers
Combining approaches for multiple security objectives, e.g., anti-jamming and confidentiality, might require new feasibility studies and solutions.

Future Directions
We semi-directional antennas focus the radiated power in narrow beams, particularly in one direction only [122]. From the security perspective, this is an additional powerful feature for an adversary, as it can help to reduce the interference caused by other radio activities, improving the adversary's expected performance. This is an interesting scenario to be investigated for satellite links' security, which has still not been fully explored by Industry and Academia. receiving signals [125]. A few recent works investigated the theoretical performance of secure communications associated with intelligent reflecting surfaces and the deployment of intelligent reflecting surfaces on UAVs and satellites, e.g., [126], [127] and [128], just to name a few. Therefore, in line with the current trend, we expect a surge of scientific contributions in the upcoming period focusing on the application of IRS for SATCOM scenarios. On the one hand, such works will explore the validity of previous results on PHY-layer security for terrestrial links when deployed to satellite links. On the other hand, when IRSs are deployed, new security threats might arise, specific to the new technology.
Just to provide an example in this direction, an adversary could use a drone flying at significant altitude to boost its SNR when receiving a signal from an IRS deployed on a satellite, or to move within an area with a better coverage without being noticed by terrestrial receivers.
GNSS Spoofing Detection via Artificial Intelligence. In line with a worldwide scientific trend, many recent proposals applied Artificial Intelligence (AI) algorithms to solving GNSS issues, including the detection of spoofing attacks [129]. Specifically, AI-based solutions can be used to detect anomalies in the received signals, so as to identify the presence of the attacker. For instance, the recent proposal by the authors in [130] use cross-correlation of the received signals to detect anomalous messages, indicating the presence of an attacker.
Still, there are several challenges that are to be solved, including the discrimination of legitimate and malicious interference, mobility of the attacker, and use of publicly-available data of satellites (e.g., ephemeris) to imitate satellites movement. In this context, we also notice that several real-world dataset are available and released as open-source, such as [131] and [132]. Thus, we the are seems ripe to experience a renewed interest, likely ignited by researchers and industry with expertise in the application of AI algorithms to different domains. Link. Most of the analyzed works considered the User to Ground Station/Network Control Center through Satellite link depicted in Figure 2, applying standard cryptography techniques to secure the channel. Many schemes, i.e., [141,142,143,144,147] and [148], considered the protection of the GPS/GNSS communication link, while other links are rarely taken into account.
Key Sharing Technique. Many different key sharing techniques are used for equipping the entities with the crypto material necessary to run authentication protocols. The authors in [133,136,149] proposed a public key cryptographic scheme based on Elliptic Curve Cryptography (ECC) to provide peer entity authentication. Conversely, other proposals assume a pre-shared key, statically known by one or more users. Some other approaches, i.e., [141, 142,  Security Analysis. The schemes proposed by the authors in [133,139,143,149] are formally proved as secure with reference to certain formal specifications or properties. Some tools that help to prove these properties are ProVerif [151], CryptoVerif [152], AVISPA [153], and Tamarin [154], to name a few. Note that only the security of the cryptographic scheme can be verified through this mechanism, while its integration in the reference system architecture could widen the threat surface.
Assessment Methodology. Similarly to the works dealing with physicallayer security, most of the analyzed schemes use simulation-based evaluation.

Key Agreement
Key agreement protocols (a.k.a. key establishment protocols) are used to allow two (or possibly more) entities that could not have anything in common to agree on a shared key to be used to secure further mutual communications [155].
Nowadays, key agreement protocols via public key cryptography or pre-shared keys are used in a range of different security protocols. Although several works proposed lightweight key establishment solutions integrating well-known cryptographic approaches in a variety of application domains, key establishment mechanisms in SATCOM have received only reduced attention. Table 8 provides a comprehensive classification of the scientific contributions dealing with key agreement in SATCOM scenarios, considering reference system requirements and features.
Link. Most of the presented approaches focus on the key establishment in SG and SS links. It is also worth noting that most of the solutions require a software update that can be done via radio link or, in particular situations, by intervening offline on the satellite.
Cryptography Technique. Many different techniques are used to allow the entities to derive a shared secret. Overall, the same considerations introduced for the key sharing techniques in Section 4.1 are still valid for key agreement protocols adopted in SATCOM links. For completeness, we introduce the pro and cons also for the Identity Based Cryptography and the Chaotic Maps.
In Identity Based Encryption (IBE) schemes, each communicating entity owns a unique identifier, adopted to compute the correspondent public key. With such an approach, no certificates are needed and no pre-enrollment is required.
However, the architecture requires a key generation center, which might be vulnerable to key-escrow attacks, and therefore, exposed to a risk of information disclosure [164]. Unlike public-key cryptography systems, chaotic maps do not require modular arithmetic, being therefore very fast for both encryption and digital signature. Moreover, algorithms based on chaotic maps might not require that large private keys, being computationally efficient. On the downside of such a technique, chaotic maps can produce ciphertext that are bigger than the plain-text [165].
Target Security Service. Most of the presented schemes focus on the balanced protection of the Confidentiality, Integrity, and Availability of data (CIA) by providing, at the same time, identity verification.
Adversary Model. The Canetti-Krawczyk (CK) and the extended Canetti-Krawczyk (eCK) security models [166], are widely used to verify and provide the aforementioned security properties for key agreement protocols. These models have been developed to build secure protocols that guarantee peer entity authentication and message authenticity during the key exchange procedure. Indeed, the main aim of these models is to provide a method so that the security protocols proposed in the literature can have a match between the implementation and the adoption in a real environment by taking into consideration also possible attacks of an active adversary [167].
Assessment Methodology. In order to evaluate the performance of a key agreement protocol, authors can evaluate the offered security properties by using the formal analysis or discuss the solution. Further, they can estimate the efficiency via simulation tools, like [156,158] and [160], or by performing a complexity analysis such as [162,163]. The only contributions providing performance on a real system, i.e., [156,159], worked on the ground link due to the viability of the approach and its reduced costs.

Quantum Key Distribution
Although the soundness of the adopted encryption techniques typically relies on traditional mathematics proofs, quantum architectures are coming out of laboratories to be used in many contexts, based on different assumptions. Overall, Quantum key distribution (QKD) allows two remote parties to securely nego- This aspect is leading to an acceleration of the adoption of countermeasures (e.g. post-quantum encryption algorithms), especially to protect data and critical infrastructures such as SATCOMs [170,171]. Table 9 provides an overview of the most important QKD techniques proposed in the context of SATCOM, and cross-compares them across reference system features and requirements.  as [172,173,175] and [183] adopt the well known BB84 quantum key distribution scheme, developed by Charles Bennett and Gilles Brassard in 1984. BB84 was originally described using photon polarization states where no quantum entanglement was required. Conversely, the authors in [174], and [184] propose  [188].
When it comes to QKD, while the high-level protocols are well understood and are taught in university courses, and commercial products are available on the market [189], the underlying phenomena that make them possible are rooted on the evolving frontiers of physics [190]. As such, a detailed discussion of the cited protocols is beyond the scope set for this survey. However, interested readers can resort to recent authoritative surveys on QKD available in the literature, such as [191,192,193], to cite a few.
Main Contribution. A large variety of contributions used to distribute the cryptographic key via QKD. Approaches use either free space QKD ( [172,180]), or perform feasibility and efficiency analysis with reference to some specific schemes ( [175,174,177,183,179,186]). All these techniques share the basic consideration that the quantum technology cannot be compromised due its intrinsic properties.
Assessment Methodology. Differently from approaches based on classical cryptography, most of the contributions focusing on QKD carry out also an experimental assessment, allowing the authors to demonstrate the feasibility of satellite-based quantum communications by experimentally analyzing their efficiency, error tolerance, and security properties.

Lessons Learned
In the following, we summarize the most important lessons learned from Quantum Key Agreement for SATCOM. Due to its promising level of security, quantum computing strategies show appealing advantages for SATCOM links, and researchers have already started to evaluate its feasibility due to the large involved distances [188]. We expect to see many contributions to come on this topic in the following years. Quantum Channel Security Assessment Tools. At the time of this writing, no works are available that evaluate the physical-layer security of QKD signal generation tools. In principle, passive side-channel attacks can be conducted to extract meaningful information from a QKD channel, without affecting and compromising the robustness provided by QKD strategies. It is essential to explore this research area to provide methods and tools to mitigate this open issue [194].

Emerging Research Challenges
The previous sections delved into the most active research branches related to the SATCOM domain and provided some appealing future research directions in those specific contexts. However, in addition to the identified research areas, our investigation highlighted further security-related SATCOM-based application domains that are receiving increasing attention from the scientific and business community. In the following, we discuss some of them, showing their key challenges and potential to attract additional interest in the years to come.
Cognitive Satellite Terrestrial Networks. Cognitive radio in the context of wireless communication systems is a research area that attracted lots of interest in the few last years. In a nutshell, cognitive radio systems allow the coexistence of primary users (using devices that own the license to use a specific frequency band) and secondary users (allowed to share resources with the primary network, but not in possession of the license) on the same network and spectrum, sharing the same radio resources. A few works applied the concept of cognitive radio networks in the context of SATCOM. For instance, the authors in [36,43,51,52] propose to secure the communication in cognitive satelliteterrestrial networks. They assume a scenario where the primary network is constituted by GEO, MEO, or LEO satellites, sending confidential messages to the fixed-satellite operator in the presence of eavesdroppers (secondary users) attempting to capture the satellite information signal. In their own (secondary) network, the network operator communicates with the user terminals. Still, doubts are there on the actual applicability of cognitive radio techniques in the context of satellite-terrestrial networks. This is mainly because of the extremely wide coverage of satellites, where the CR techniques might not work well. In line with a large amount of work done in the context of cognitive radio for terrestrial networks [195], we expect increased interest in this domain in the next years.
Drone-To-Satellite. Unmanned Aerial Vehicles (UAV), a.k.a. drones, have gained increased momentum in the last years, in both academia and Industry [196]. In the context of SATCOM, one of the most critical challenges consists of allowing secure communication between small/commercial UAVs and satellites. For instance, the authors in [197] proposed a physical layer security framework in space-air-ground (SAGIN) downlink multi-beam satellite-enabled vehicle communications, where the UAV is adopted as cooperative node, interacting with the legitimate user and acting as a source of artificial noise to mitigate eavesdropping. In the same network setup, the authors in [198] investigated the IoT computing offloading problem by proposing a reinforcement learning approach to allocate the resources of the UAV edge server efficiently. In the same domain, the authors in [199]  Software-defined satellites. The integration of Software Defined Networking (SDN) into SATCOM could improve the connectivity coverage and performance for using broadband communications by allowing the operators to reconfigure the satellites as needed [203,204,205]. However, SDN also come with their security issues, that are further specialized in SATCOM use-cases [206].
Additional research is needed in this context.
Network Slicing for the Internet of Space Things. The continuous development of nano-satellites is accelerating the deployment of low-cost satellite networks [207]. Emerging paradigms, such as the Internet of Space Things (IoST), require a network slicing framework to provide the support for the plethora of space-application scenarios. A network slice is a part of the network that is independent and logically separated from the rest. A specific slice has specific security policies, used to protect the slice while meeting specific system requirements. However, there are neither common strategies nor protocols suitable to design a network slice in the context of SATCOM. Despite initial studies in this context are available [208,209], major work is still to be done, and we expect increasing attention towards this topic.
Green Satellites. The design of environmentally-friendly satellites can help to reduce the environmental impact of a satellite, its production cost, and maintenance compared to traditional ones. However, reducing the cost and the impact of the satellite inevitably could affect the provided security services. This emerging research area, also suggested from the ESA [210], leads to a potential redesign of the existing procedures and technologies, also including the security domain. Definitively, a novel and interesting research topic.
Satellites Signals for Opportunistic Navigation. Specific satellites signals can be used to pinpoint a specific location on Earth, similar to the GPS.
A group of researchers developed a working solution based on the cited logic, leveraging signals broadcasted by Starlink internet service satellites [211]. The usage of additional satellite constellations could provide reliability and spoofing detection mechanisms for devices on Earth, and more research into the robustness of such solutions is needed.

Cybersecurity for Commercial Satellite Operations. The National
Institute of Standards and Technology (NIST) is seeking comments on the draft specification NISTIR 8270, which describes the security procedures and the concepts for commercial space operations. The draft considers the management aspects, risk management operations, and defines the requirements that "might coexist within space vehicle systems". The NIST is requiring feedback on the overall approach, the example use case, and the identified controls for the proposed use case [212]. We expect that several research contributions could come out due to the study and application of this (yet to come) recommendation to real use-cases.
Standardization of Security for Non-Terrestrial Networks. In the standardization community, and in particular, within the Third Generation Partnership Project (3GPP) committee, satellite communications are specifically considered in the design of the Non-Terrestrial Networks (NTNs), anticipating the upcoming tight integration between terrestrial, aerial and satellite networks [213]. Specifically, the standardization of Non-Terrestrial Networks (NTNs) has been launched by 3GPP in the 3GPP Release 16 [214]. At the same time, new security aspects have been recently defined by the 3GPP in the Release 17 [215], and new amendments are planned in the upcoming Release 18 [216].
In this context, none specifications edited by the 3GPP specifically took into account network security issues for NTNs. As a result, the current approach recommended by the 3GPP consists of a straightforward integration of the 5G security architecture and protocols into NTNs. Such an integration, however, comes with several challenges, in terms of communication overhead, software updates, and unreliability of the wireless links. Specifically, security issues in the operation of NTNs have been investigated by the Working Group on Satellite 5G, established within the IEEE Future Networks Initiative [217]. In one of the latest deliverables of the WG, i.e., [218], they drew a roadmap of the priorities to be addressed in this regard, highlighting that new security mechanisms might be needed for specific deployments, and that the emphasis should be put on the isolation of the end-users from the shared NTN network. The WG realized several security-related activities, i.e.,: (i) analyzed the state of the art about security for NTNs; (ii) provided a threat analysis for the NTN scenario; (iii) identified specific complications derived from using of 5G security solutions on 5G-NTN networks, by experimentally verifying them on prototyping platforms; and, finally, (iv) identified additional security concerns, mainly related to the integration of emerging technologies such as network slicing, edge computing, and multicasting over satellite networks (see Section 2.6.2 of [217].
The temporary recommendation proposed by the WP was to adopt the IPSec protocol suite to secure the communication link, but they also recommended further study into the issues at the 3GPP standardization level. However, the 3GPP refused to investigate further into the issue, at the time of this writing, still recommending a straightforward integration of 5G security into the NTN domain [218].
Nonetheless, due to the forecasted performance issues arising from such the integration of 5G-security into NTNs, we expect significant contributions by the research community in the years to come, potentially triggering dedicated and ad-hoc initiatives by the 3GPP.
Security and Privacy for 6G. 6G networks will accommodate satellites, UAVs, and undersea communications [219]. It is crucial that any security proposal framed in this context protect the communications while guaranteeing reliability, low latency, and secure and efficient transmission services. Physical-layer security is the first candidate defense for these new emerging technologies, but emerging cryptography-based solutions could also play a role if their integration is carefully systematized and orchestrated with the existing services.
In the context of 6G initiatives, the 3GPP claimed that for the next few years (2030s) additional research is needed into this application area. Adapting and integrating the security services on satellites with mobile terrestrial/sea systems while meeting the requirements of 6G communication services will indeed represent a complex and difficult challenge [220]. For the cited scenario, we expect the adoption of real-time security communication protocols and emerging architectural solutions, such as Zero Trust [221].

Conclusion
In this contribution, we have provided a survey of the most significant linklayer security issues, threats, and mitigation techniques adopted in the context of Satellite-based Communications systems. First, we presented general background on the SATCOM architecture, the most important constellations, and network parameters. Then, we divided the relevant literature on the topic into two major research areas, i.e., physical-layer security and cryptography, and we further identified dedicated topics in each macro-area, focusing on specific threats. For the physical-layer area, we discussed and cross-compared solutions based on the usage of information-theoretic security schemes, anti-jamming strategies, and anti-spoofing schemes. For the cryptography area, we specifically discussed approaches for authentication, key agreement, and key distribution based on the emerging quantum computing paradigm. We also identified lessons learned and specific future directions for each of the cited threats and research areas. Finally, we presented a few appealing emerging challenges in the SATCOM security domain, pointing out the main research challenges to be solved and the areas where new contributions from the scientific community might have major impact.
Overall, we believe that the exposed research challenges highlight that the design and testing of cybersecurity strategies for SATCOMs is still an active research domain. In particular, our contribution calls for collaboration between Industry and Academia to unlock new business opportunities and services, while enjoying the needed level of security for communications, applications, and infrastructures.