PNT for Defense

Abstract

Space-Based Position, Navigation, and Timing (PNT) services were conceived, designed, and developed in response to specific defense capability needs. PNT services are recognized as critical enablers for defense operations and as such must be available with the greatest possible robustness and dependability. Operational advantage of having access to such services is so evident that not only the military but also civilians started to exploit them up to the point that nowadays space-based PNT have become a commodity significantly affecting not only defense operations but also the global economy. Threats and attacks to such services are for these reasons increasing, putting at risk the continuous availability required for military operations.

The contents reported in this chapter reflect the opinions of the authors and do not necessarily reflect the opinions of the respective Agency/Institutions.

Introduction

Today, the secure service of the global positioning service (i.e., GPS) provided by the US government is an indispensable part of modern warfare for NATO allies, from strategic decision making or operational planning to the conduct of military operations. Since its conception, the main driver of space-based radio (or Satellite) navigation (nowadays addressed as a whole as Global Navigation Satellite Systems, GNSS) was to ensure that the service they provide could support the widest possible spectrum of military operations on a global scale (Fig. 1).

Fig. 1

Prototype of 3D positioning system for soldier. (© European Defence Agency)

The general dependence of military tasks from such space services increases the number and typologies of attacks to space systems. From an operational point of view, such ubiquitous GNSS dependency, regardless the service considered (e.g., GPS, Galileo, or others), creates new weaknesses that adversaries could exploit easily and effectively. Indeed, despite their potential and strategic dimension, GNSS signals are vulnerable to several factors:

• Natural effects, such as multipath or ionospheric scintillations

• Signal deformations (GPS ringing phenomenon) and data corruptions, e.g., orbital and clock errors

• Nonintentional interference caused by radio transmitters, mobile communication networks, airborne navigation instruments, etc.

• Intentional interferences such as jamming and spoofing

Within the current scenarios, intentional interference and threats (as well as technologies available to adversaries) are evolving in an extraordinary manner and EU Member States’ Armed Forces need to face unprecedented challenges, whether stemming from modern high-tech warfare or more hybrid threats. As threats will continue to evolve quickly, so must PNT-dependent systems and platforms in response. Being able to rely on more secure and resilient PNT services for future operations in congested and contested scenarios is therefore a prerequisite which needs to be fully taken into account in the conception, design, development, and implementation of any PNT solutions for defense users. This has both political and operational consequences, rendering much more critical the identification and assessment of the vulnerabilities that adversaries could exploit.

As a matter of fact, space services and in particular GNSS could be considered, following Gen. von Clausewitz’s theories (von Clausewitz 1832), as a center of gravity in the future fifth- or sixth-generation scenarios, where the global scale of future wars could be matched by the global coverage on Earth that only satellite-based services can provide. These considerations lead to multiple consequences and were probably also the basis of recent public political declarations from the EU Commissioner related to the need to start thinking on an EU Space Force (Teffer 2019) in parallel with the development of EU space capabilities such as Galileo, echoing the decision of the Trump Presidency to set up a plan for the creation of an independent US Space Force (Wall 2019).

Finally, it is important to underline how, after the transformation of satellite navigation in a commodity, a set of innovative technologies has emerged to improve and complement GNSS in any possible environment and overcome known weaknesses and vulnerabilities. These have to be fully considered and analyzed for the provision of highly robust and dependable PNT services for military forces. Even if GPS was in the driving position (and is going to remain there for a long time to come), PNT is today much more than GPS.

History of Space-Based PNT for Defense

Satellite navigation has its origins in the launch of the first artificial satellite by the Russians, the Sputnik, in 1957. After its launch, scientists in the US Johns Hopkins University discovered in 1958 that, due to the Doppler effect, the radio signals broadcasted by the satellite could be used to localize the satellite. This was used to reverse the problem and exploited to locate an object on the ground based on the knowledge of satellite position.

Based on this idea, in 1959 the U.S. Navy started the development of TRANSIT, the first navigation system to rely on satellites which became fully operational in 1964. Its main scope was to provide position information to the U.S. submarine ballistic missile force. It was not as accurate as today’s satellite navigation systems (more than 20 m 2D accuracy, and performance greatly degrading as the speed of the platform increased), but introduced a set of innovations which are the basis of modern GNSS (Fig. 2).

Fig. 2

The transit concept. (© The Johns Hopkins University Applied Physics Laboratory (Danchik and Lee 1990))

In parallel to TRANSIT development, a study was performed by the Aerospace Corporation for the U.S. Military to analyze tactical applications and utility of improved positioning accuracy. The study concluded with a proposal, in 1966, of satellites relying on highly stable (atomic) clocks to broadcast ranging signals continuously to receivers able to locate moving vehicles anywhere on Earth and in the air on a 24/7 basis. This was the beginning of the satellite navigation system as we know it today. After a series of other technical studies (among which some led to the identification of spread spectrum communication as the best way to transmit the ranging signals), the first four satellites constellation was developed leading to first demonstrations in 1974.

In 1978, the United States Department of Defense (DoD) started the launch of the first operational satellites (even if first Block I satellite with the first on-board atomic clock was launched in 1980) of the Navstar Global Position System, more commonly known as GPS, with the primary purpose to provide Position, Navigation, and Timing (PNT) information to defense users. DoD’s primary purpose in developing GPS was to use it in precision weapon delivery, answering to the objectives of the US DoD second-offset strategy. As a second-order objective, such a space-based, all weather, and worldwide available and accurate PNT capability could address the needs of a broad spectrum of applications and would limit the proliferation of specialized PNT equipment supporting specific mission requirements reducing interoperability burdens, hence its almost immediate success.

Despite its early developments, it was only during the 1990s Gulf War (just after the first handheld GPS device for civilian applications, the Magellan NAV1000, was developed) that GPS demonstrated its full potential in operations. GPS navigation proved to be a crucial force multiplier for desert warfare. GPS satellites, even without a fully functional constellation (in 1991, there were only 19 GPS satellites in orbit, https://www.af.mil/News/Article-Display/Article/703894/evolution-of-gps-from-desert-storm-to-todays-users/), enabled forces to navigate, maneuver, and fire with unprecedented accuracy in the desert almost 24 h a day despite difficult conditions – sandstorms, no maps, no vegetative cover, few natural landmarks. GPS’s fully operational capability was achieved in 1995 with the last of the first 27 operational satellites (including the spares) was launched. In the same year, the US DoD, fearing that adversaries could take advantage of the service, decided to decrease the accuracy of the openly available service through the activation of the selective availability.

The importance of this capability has been soon recognized worldwide also for civilian applications, as demonstrated by the development and deployment of other Global Navigation Satellite Systems (GNSS) especially after the end of the Selective Availability in 2000. Since then, GPS-based PNT has deeply changed the way many military operations are conducted by providing (an almost) continuous and ubiquitous precise positioning and timing for a vast variety of platforms at a reduced cost. The trend in the GPS/GNSS device market is not expected to decrease. Instead, it is expected to increase up to US$2.8 billion by 2027, at a CAGR of more than 2.7% (Market Research 2017). GNSS devices intended for munitions, soldiers, and ground platforms constitute the bulk of defense applications market. These are being acquired either as stand-alone devices or as part of soldier modernization programs. One of the most important factors driving the increase of the market is the fact that new constellations are becoming available and new technological advances are being integrated into PNT systems to increase their robustness by augmenting and complementing space-based capabilities.

GNSS PNT for Defense Users

Almost the totality of todays’ PNT services rely, either directly or indirectly, on GNSS. Such services are key enabling capabilities in military operations contributing to all the military tasks, fundamental for the freedom of movement and acting as force multipliers. As such, PNT solutions must be secure and resilient. They have to be designed to withstand potential malfunctions and degradations and need to comprise adequate mitigation measures against complex attacks. This is achieved through the concept of PNT superiority against adversaries.

One of the key concepts of PNT superiority is commonly known as NAVWAR (Navigation Warfare). NAVWAR is defined as “the deliberate defensive and offensive action to assure and prevent positioning, navigation and timing information through coordinated employment of space, cyberspace, and electronic warfare. Desired effects are generated through the coordinated employment of components within information operations, space operations, and cyberspace operations, including electronic warfare, space control, space force enhancement, and computer network operations” (US FNP 2017). The underlying benefit of the NAVWAR doctrine for the military is to ensure military operations PNT superiority and advantage in the area of conflict without disrupting allied forces outside the theater of operations. This is substantially implemented by: “protecting authorized use of GPS; preventing the hostile use of GPS, its augmentations, or any other PNT service; and preserving peaceful civil GPS use outside an area of military operations.”

Since its origin, GPS has become the “gold standard” by which other PNT solutions were (and still are) benchmarked. However, in order to improve the performance of navigation systems in cases of poor satellite coverage and low availability, new GNSS systems are being developed (notably EU is implementing the Galileo program, which is expected to reach full operational apability in 2020). Yet there are still a lot of concerns as space-based services can be denied or degraded in tactical environment; therefore, PNT superiority cannot be limited to NAVWAR. Given the reliance on PNT for operations, the challenge is to maintain a high degree of resilience and the highest possible confidence in any operational scenario even when no external aids to navigation and localization are available.

For the majority of defense forces’ operations, the availability of a globally accurate, precise, and real-time location and timing information can provide a crucial advantage over adversaries. GPS first and other GNSS today are able to provide this capability in many operational conditions allowing, e.g., the effective engagement of opposing forces through accurate targeting, enhanced navigation, and maneuvering activities, and it helps preventing or minimizing collateral damage. This capability is used at several and different military levels such as a strategic analysis, which can take advantage of reliable and global positioning and timing information, or tactical operations, enabling the engagement of high-accuracy weapons’s guidance. This needs to be clearly kept in mind, especially with regard to military rule of engagements that can effectively guide armed forces’ actions and activities.

The revised EDA 2018 Capability Development Plan (CDP 2018) within the 11 identified EU capability priorities clearly reflects the indispensability of space-based communication and information services as an enabler for the defense systems, with a special emphasis on unmanned and autonomous systems (EDM 2018). Unmanned maritime high-end platforms, for instance, which have just been identified as a European priority to achieve maritime surface superiority through long endurance at sea, are only one example where support from space-based applications has become critical. If such systems do not have access to strong and resilient PNT support provided by satellites, they cannot be considered fully operational.

GNSS devices for defense applications are widespread across all operational domains and platforms: soldiers, vehicles, aircrafts, vessels, communications systems, and munitions are routinely equipped with GNSS systems to provide any combination on PNT information. Secure/encrypted GPS receivers for navigation and guidance solutions are available from a handful of manufacturers. Receivers are available either as stand-alone navigation devices or as embedded devices to be operated within a larger mechanical or electronic system.

Space-based PNT services used by European Union Member States and NATO Allies’ armed forces are the US encrypted Precise Positioning System (P(Y) code). Today the de facto user equipment standard is the Defense Advanced GPS Receiver (DAGR) used to provide precision guidance capabilities for vehicular, hand-held, sensor, and gun-laying applications. The latest generation being termed SAASM (Selective Availability Anti-spoof Module) with a quite small and low-weight hand-held form factor for war fighters, with an easy-to-use interface (Graphical User Interface and moving maps). Today there are models resembling conventional smartphone functionalities (including MP3 and camera), but there are several other form factor receivers matching different platform requirements (Fig. 3).

Fig. 3

Left: Hand-held micro Defense Advanced GPS Receiver (DAGR). (© Collins Aerospace). Right: TOPSTAR M for avionics platforms. (Photo Thales © E. Raz)

The availability of new GNSS could increase the robustness of the PNT services available to the defense users for the implementation of national or multinational operations. The defense sector should seek to maintain the right level of PNT capability in light of programmatic and technological opportunities and increasing threats to PNT information assurance. Key principles for the usage of space-based PNT solutions (mainly GNSS) as defense-enabling capability can be summarized as follows (ERNP 2018):

• The performance of PNT services in terms of accuracy, continuity, and integrity shall be commensurate to operational needs as defined by Member States in the Capability Development Plan.

• The delivery of PNT services must be subject to agreed governance arrangements and must be under full European control or dependably provided by an allied defense partner.

• Due to the worldwide extent of the EU’s area of strategic interests, PNT services must ensure global coverage. Therefore, EU Member States shall have the right to unlimited and uninterrupted access to secured space-based PNT services worldwide.

• The PNT services for military use must ensure a high degree of resilience against all threats and risks; this should explicitly include all aspects of cyber warfare.

• The highest levels of PNT services availability should be sought. Space-based PNT should thus be highly resistant to disruption, denial, deception, and degradation.

• The use of PNT services must be accessible in contested and congested environments.

• It must be possible to deny the exploitation of the secure PNT services by adversary forces.

• Protection and robustness of the equipment must be adapted to the operational environments (e.g., physical security measures, antitampering).

• The interoperability of the PNT services must be ensured in areas of common interest negotiated with GNSS providers.

• The PNT services must be available with a high level of reliability in all operational environments (in particular urban).

• Augmentation systems, regional or local, may be considered in order to enhance available GNSS services.

• PNT services shall be workable from strategic to tactical level, and from the most complex weapon system down to, e.g., dismounted soldiers’ equipment.

Space-Based PNT Systems Landscape for Defense Users

Until now, we have been focusing on GNSS as the source of PNT information for military forces. However, as a consequence of the increasing dependency and operational advantages associated to the mastering and control of GNSS systems, several techniques and devices have been developed that can severely degrade the performance of GNSS services. Such degradation, nevertheless, not always causes a complete denial of the service, but most often it causes misleading PNT information (spoofing). Such threats to GNSS might adversely affect various military tasks. In this context, PNT sources and systems can be identified to allow implementing diverse PNT architectures delivering different levels of performance according to the operational scenarios, threats, and missions.

GNSS Services and Systems

Broadly speaking, two main groups of satellite navigation services can be identified:

• GNSS open services: Provide positioning, velocity, and timing information that can be accessed free of direct user charge. The civilian services are accessible to any user equipped with a receiver, with no authorization required.

• GNSS-regulated and access-controlled services: These are robust services for the provision of PVT (positioning, velocity, and timing) information to authorized users. The regulated signals are typically designed with a focus on the robustness of its signal, which protects it against spoofing and makes it more resistant to jamming.

The only satellite navigation system currently used by military users in EU and allied forces is the US GPS system. However, in addition to GPS, there are three additional satellite navigation constellations in Medium Earth Orbit (MEO): one fully operational (the Russian Glonass), and the other two (the European Galileo and the Chinese BeiDou (BDS) navigation systems) under deployment. The four constellations transmit navigation signals in five different frequency bands featuring both open and regulated services according to the grouping below (Fig. 4).

Fig. 4

MEO satellite navigation signals (red: regulated signals/services; blue: open signals/services). (Inside GNSS 2013)

In all the aforementioned satellite navigation systems with the exception of the European Galileo, access to regulated (i.e., encrypted) signals is reserved to users authorized by the Ministry of Defense, and therefore mainly targeting military uses and applications. However, albeit being considered and developed as a civil system under civilian control, Galileo will deliver the Public Regulated Service (PRS) which will offer strong and encrypted navigation signals. This secure service is restricted to governmental authorized users and is therefore suitable for services where robustness and complete reliability must be ensured, such as, but not limited to, military operations.

The access to PRS is regulated by Decision No 1104/2011/EU of 25 October 2011. According to it, the PRS is a service which is restricted exclusively to Member States, the Council, the Commission, and the European External Action Service (“EEAS”) and, where appropriate, duly authorized European Union Agencies. It should also be possible for certain third countries and international organizations to become PRS participants through separate agreements. Decision No 1104/2011/EU mentions also that the PRS provide unlimited and uninterrupted service worldwide to PRS participants.

As soon as the Galileo PRS service will become operational, and after the availability of the associated defense user equipment, it could be expected that PRS will become an additional primary source of PNT for EU and allied forces. Indeed, EGNSS could increase resilience, availability, integrity of PNT information, and services for EU CSDP and MS operations.

Alternative Space-Based PNT

The military use of other satellite navigation services, like space-based augmentation (e.g., WAAS and EGNOS) and their benefit in terms of increased robustness in a military context is nowadays considered limited. The same applies to civil Differential Global Positioning Systems, RTK, and Post-processing Positioning Services (PPS), especially with respect to the robustness of a military system when relying on such civil augmentation systems. Currently, the main benefit of augmentation in a military context is for aviation applications, with dual-use of the civil system components, like SBAS receivers, and in parallel with military GNSS receivers.

GNSS (GPS, Galileo, Glonass, and Beidou) employ satellites in medium Earth orbit (MEO) broadcasting signals transmitted at an altitude of >20,000 km. This implies about 30 dB more path loss than signals transmitted from LEO satellites (at an altitude of less than 1000 km, like Iridium (https://en.wikipedia.org/wiki/Iridium_satellite_constellation)). Consequently, with similar transmit power available at the satellite, LEO signals are received at Earth’s surface with a significantly higher power level than GNSS signals. For GPS and Galileo, the observed carrier to noise density ratio (C/N0) in good reception condition is around 50 dBHz, while, e.g., for Iridium, C/N0 values up to 80 dBHz are observed. The higher signal power levels lead to a much stronger jamming resistance of a LEO-based GNSS. Additionally, this allows the LEO signals to penetrate into difficult attenuation environments like deep indoors, where the reception of GPS or Galileo signals is not possible.

Another consequence of the lower altitude of the LEO satellites is a smaller antenna footprint on the Earth’s surface, as illustrated in Fig. 5. In order to have one satellite in view at all times, only around ten MEO satellites are required, while at LEO rather one hundred satellites would be needed. On the other hand, while significantly more LEO satellites are required to provide the same coverage as MEO satellites, launching an LEO satellite is less expensive than launching an MEO satellite.

Fig. 5

LEO and MEO antenna footprints on Earth’s surface. (© Reid et al. 2016)

The lower altitude also leads to a shorter orbital period: for GPS and Galileo, the orbital periods are 12 and 14 h, respectively, while in LEO, the orbital period is around 100 min. Therefore, the LEO satellite geometry, as observed by a user on the Earth, changes much faster than for an MEO GNSS. This has several advantages: the user is observing larger Doppler shifts, which is beneficial for Doppler positioning (the same principle used in TRANSIT, see section “History of Space Based PNT for Defense”). Another positive impact of the fast changing geometry is that it whitens the multipath, which then averages out faster.

Currently, the only LEO constellation with global coverage that offers navigation capabilities is Iridium. Iridium/IridiumNEXT consists of 66 satellites at an altitude of 780 km and is used mainly for communication purposes and is available on a commercial basis. Since May 2016, Iridium offers a Satellite Time and Location (STL) service. Iridium uses overlapping spot beams and randomized broadcasts, which provides a mechanism for location-based authentication that is extremely difficult to spoof (Lawrence et al. 2017). Together with the high signal power levels providing jamming robustness and coverage in deep indoor environments, this antispoofing capability makes Iridium a very robust PNT source. However, Iridium does not yet achieve the accuracy offered by GPS or Galileo. After a convergence time of 10 min, the following performance has been observed in field tests: positioning accuracies between 20 and 35 m (1σ) and timing accuracy of 0.5 ms.

Even if the performance of Iridium is not comparable with those of the current GNSS, LEO-based PNT is a very dynamic environment, which has the potential for disruptive innovations in the near future that can complement classical GNSS considerably. In the near future, several other LEO-based constellations with possible navigation capabilities can be expected: OneWeb consists of a constellation of 648 satellites for broadband internet provision; the initial operational capability is planned for 2019 (Reid et al. 2016). SpaceX recently got approval from FCC to deploy its satellite internet constellation called Starlink, which adds up to more than 4000 spacecrafts (and up to 12,000) to be operational by the mid-2020s (Ralph 2019). Samsung proposed a LEO constellation of 4600 satellites, and Boeing published plans for a LEO constellation of 3000 satellites (Reid et al. 2018).

Additionally, the U.S. Defense Advanced Research Projects Agency (DARPA) already started to analyze whether military constellations (payloads: global surveillance, tactical communications, and PNT) in low Earth orbit are cheaper and nimbler alternatives to traditional military satellites. In 2017, DARPA launched the project known as Blackjack with the final goal to develop a low Earth orbit constellation to provide global persistent coverage for military operations (Erwin 2018). The project will aim to demonstrate an architecture showing the utility of a global LEO constellation for a wide variety of military payloads and missions.

In a further effort to enhance the PNT capabilities provided by MEO constellations (specifically GPS), the Air Force Research Laboratory (AFRL) in the US will launch in 2022 an experimental satellite in Geostationary orbit (AFRL 2017) called NTS-3 (Navigation Technology Satellite-3) as it follows NTS-1 and NTS-2 launched in 1974 and 1977, respectively, to test initial GPS functionalities). The experimental PNT satellite is intended to test new technologies and hardware to improve robustness and resilience of MEO-based constellations through a supporting layer of geosynchronous Earth orbit satellites. Technologies that will be tested include advanced antenna options, reprogrammable hardware, advanced clock technologies, and new signal structures. NTS 3 will also investigate secured-design technologies to enhance cyber-resilience, as well as modern cyber risks management approaches.

Modernization of Space-Based Secured PNT Services

In parallel to the completion of the deployment of new GNSS systems and the studies on possible alternative space-based PNT, possibly the most important aspect to be emphasized is the current modernization effort ongoing from US DoD to develop a new secure signal to improve the security features and jamming resistance properties of military navigation using GNSS, the M-code.

The motivations for such modernization can be found in the need to improve NAVWAR performance under the assumption that the threats against the military user (mainly driven by the hybridization of the warfare) are continuously evolving, resulting in an increasingly complex, more congested, and contested environment.

The M-code signal will provide better jamming resistance through much higher power transmission (up to +20 dB above current level, with the possibility through spot-beam transmission to direct the signal toward a specific area of interest) without degrading C/A-code or P(Y)-code reception and in openly available signals on L1 and L2 bands (Fig. 6).

Fig. 6

M-code signal compared to legacy GPS signal around the L1 (1575.42 Mhz) frequency. (© GPSWorld)

The M-code design also features a more robust signal acquisition, more flexibility in its configuration and better security in terms of exclusivity, authentication, and confidentiality (hence better spoofing resistance), along with a simplified key distribution (Barker 2000). In particular, the signal is able to support NAVWAR activities: the energy signal is split into two lobes separated from the center frequency, enabling selective jamming of the open GPS (C/A code) without impacting military signal reception (“blue force jamming”). Currently, such blue force jamming is not possible with P(Y) code receivers, without also degrading the friendly force’s receiver.

The M-code is designed to be autonomous, and so authorized users will be able to calculate their positions without requiring the use of other signals (e.g., the C/A-code in the case of the P(Y)), providing at least comparable performance to the P(Y)-code.

To the best of the authors’ knowledge (GMV 2011), also within the Galileo program, in the frame of the design of its second generation (G2G), services are expected to deliver improved performance and features such as reliability, maintainability, availability, continuity, accuracy, and integrity. It is therefore auspicable that PRS performance will be improved in addition, likely to reflect the evolving threat landscape in which defense (a recognized user community of Galileo) will operate.

Technologies for Future Defense PNT Solutions

Multi-constellation Defense GNSS Receivers

Several techniques and technologies could be used to improve the robustness of GNSS receivers. Based on an analysis of publicly available information (F-DEPNAT), an overview of possible technologies and techniques for use in a military grade GNSS receiver is presented in Table 1.

Table 1 Technologies and Techniques for Military grade GNSS Receiver

In addition to the usage of multi-frequency (which is the de facto standard for military grade receivers), the use of multiple constellations allows for robust GNSS through improved availability of satellites with good geometry. Use of multiple constellations also mitigates against failure modes associated with a single constellation and supports diversity of signals across constellations and frequencies.

In a military context, the multi-constellation and multi-frequency receivers could make use of different secure, encrypted signals. In addition, a diversity of open signals may be used alongside advanced interference and spoofing mitigations to provide the highest levels of PNT security and robustness for users. For countries in NATO, the use of multiple secure signals will most likely mean GPS-PPS + Galileo PRS. The clear benefits of dual constellation GPS and Galileo receivers for employing encrypted signal services from different constellations are likely the key driver of the requests from United States (Gibbons et al. 2017) and Norway (De Selding 2015) governments to access the Galileo PRS.

Figure 7 illustrates a proposal for a dual mode receiver architecture employing GPS-PPS and Galileo-PRS. This architecture is based on separate security modules to process the encrypted PPS and PRS signals but maximizing the commonality of the remainder of the receiver (e.g., use of common frequency reference subsystem, radio front end). In the figure, a separate navigation processor is shown, but it is potentially feasible for this to be incorporated within one of the security modules.

Fig. 7

Example of GPS/Galileo dual constellation receiver. (Courtesy of Nigel Davies © QinetiQ)

According to Davies et al. (2016), future challenges to be solved for dual constellation receivers include:

• Technical: Even if the development of operational PRS receivers is ongoing in Europe and subject to a number of initiatives, the development of the next generation of GPS-PPS, namely M-code, is more advanced.

• Security: There are separate US and European security rules and sensitivities. Therefore, security constraints arise for the key management and handling.

• Legal/political: The GPS-PPS security modules can be developed and approved by US contractors under the control of the US DOD and US National Security Agency (NSA). Similarly in Europe, the PRS access rules legislation requires that PRS security modules are developed and manufactured by authorized European manufacturers. Furthermore, use of GPS-PPS outside the US is limited to authorized nations for the purposes of defense under bilateral agreements with the United States.

These points, in addition to the agreement on which military uses could mostly benefit from a combined use of GPS- and Galileo-secured signals and the establishment of the relevant agreements, should be addressed adequately in order to ensure that the defense forces (in addition to the mass and automotive market which are already benefiting from open-service multi-constellation) would be able to fully exploit the benefit of multi-constellation performance.

Developments in the United States

The Military GPS User Equipment (MGUE) program started the development of M-code capable GPS receivers in 2013. According to the MGUE program schedule (Wilson 2015), GB-GRAM-M receiver kits are now available for integration in receiver housing and into military platforms, as shown in Fig. 8. In (Menschner 2018) the GPS enterprise, roadmap is shown for the full system, including also the MGUE Integration and test phases for the various user platforms which are expected to last until 2021.

Fig. 8

MGUE Increment 1 M-code receiver development. (© GPS.GOV, Wilson 2015)

Developments in Europe

The deployment of the Galileo Public Regulated Service user segment has been steered in recent years actively by the European GNSS Agency (GSA). One essential prerequisite for the future adoption of PRS by multiple user communities is the availability of receivers for different applications. Thereto, GSA has initiated the EU-funded projects P3RS-2 (awarded to an Italian/German consortium, (Leonardo 2014) and PRISMA ((GSA2015, which, based on publicly available information seems being awarded at least to French and German companies) for the development of the first generation of preoperational receivers and first prototypes. In addition to the prototype and proof of concepts developments driven by the GSA, several national development activities are running under the control of national defense agencies and competent PRS authorities. However, information on the expected availability date of the testing and integration of fieldable products are not publicly available.

Other PNT Sources

Considering the strategic nature of space-based PNT infrastructures and in order to reap the most benefit from it for critical defense applications, its military potential needs to be recognized and fully understood. On this basis, resilience should be integrated to grant reinforced reliability and dependability to match the growing operational expectations following the evolution of modern warfare. For this reason, and despite not linked with space security, it is important to mention the fact that defense users are currently looking for alternative and autonomous (i.e., not dependent from GNSS or other PNT aids external to the concerned system) PNT sources to assure its continuity under all conditions.

Even if the widespread and worldwide adoption of GNSS in applications which were not even imaginable at its conception have implied the assumption that GNSS PNT is taken for granted, and the GNSS modernization in well underway with the progressive introduction of new constellations and signals, PNT and GNSS are not to be considered synonyms (a quite common mistake nowadays). With a host of technological evolutions on the horizon, PNT is much more than GNSS these days. Other technologies and techniques to augment and complement GNSS need to be studied to ensure PNT superiority to defense forces when GNSS is degraded (or simply not available for physical limitations). This is why several communities (laboratories, research organizations, civilian industry, and others) are being looked at and challenged by defense organizations to propose and explore novel and disruptive solutions. This is of utmost importance also considering the pace at which commercial innovation, without the constraints of traditional defense R&D, are progressing.

In particular, both hybrid and autonomous systems and components would need to be considered. As an example, it will be crucial to develop further Inertial Navigation Systems (INS), traditionally the optimal GNSS complement to guarantee and improve robustness of PNT information in GNSS denied or degraded environments. Work is ongoing to exploit advanced fiber optics (i.e., hollow core), quantum, or micro-PNT technologies (under development under multiple DARPA programs, McCaney 2015) to develop high-performing autonomous (i.e., not depending on external inputs) navigation sensors. In the area of quantum devices, Europe is also progressing with the development of the first commercially available inertial sensors (a quantum gravimeter, Fig. 9). However, current technological status is limited to quite big and static apparatuses with limited dynamic range and high sensitivity to environmental effects. Significant work is needed to develop solutions suitable for compact and dynamic environments and devices able to withstand defense operational and environmental conditions. Integration into defense platforms cannot be expected before 10–20 years.

Fig. 9

First commercially available quantum inertial sensor. (Absolute Quantum Gravimeter, © Muquans, 2019)

GNSS evolutions will naturally benefit from related technological advances, some of which will make their signals less susceptible to interference and spoofing (e.g., through the usage of quantum cryptography). It is in fact important not to forget that, thanks to its worldwide availability and attainable cost/performance ratio, it is unlikely that GNSS will be replaced by another technology anytime soon.

Today’s trend is in the provision of assured PNT relying on the integration of traditional PNT technology with nontraditional and emerging technology to improve the robustness and dependability of mission-critical applications in all the military operational domains. Fusing different sensor modalities to create a combined navigation solution is anything but a new idea. The benefits of combining GPS with an inertial sensor were recognized a long time ago, and this classic pairing continues to be the subject of research today. In particular, the deep integration produces an increasing of robustness to GPS jamming, compared with tightly coupled systems. However, technologies available to make PNT systems resilient are evolving and nowadays (and in the foreseeable future) a wide range of solutions to match different operational conditions and environments could be imagined.

By combining GNSS-based PNT equipment with detection and mitigation systems, we will continue to rely on trusted GNSS as the main source for PNT services. The combination of several PNT sources can generate different ways to deliver and to use the service. For example, based on the military mission or task, different PNT sources could be selected as main PNT provider, and standard operational procedures could be developed to support the use of different PNT source in case of NAVWAR environment. Such procedures could also be implemented in technical solutions, but in any case need to be flexible and adaptable to the operational tasks and the external threats.

PNT Superiority Impact on Military Tasks

The Generic Military Task List (GMTL) is a taxonomy agreed between EU Member States in the framework of the European Defence Agency to categorize the full spectrum of military activities divided into six domains: command, inform, deploy, engage, protect and sustain. Usually military systems are designed and developed to generate capabilities able to provide services in a subset of the above-mentioned domains.

Several systems can be operated to support one or more of these generic tasks; nevertheless, there are cross-cutting capabilities enabling a very large or, sometimes, the entire spectrum of GMTL tasks. Among those, PNT is one of the most critical and it gives a justification to why PNT superiority needs to be ensured. In Table 2, some examples of how PNT technologies can support the full spectrum of GMTL domains are reported.

Table 2 Examples of PNT support to military tasks

Timing sources are crucial for the synchronization and functioning of all networks and digital platforms at strategic, operational, and tactical levels. At tactical level, synchronization enables fundamental capabilities such as effective interoperability, and joint and combined operations that only a globally available service, such as GNSS, can automatically enable for an unlimited amount of users in different locations.

The dependence from PNT services is particularly critical for high precision engagement and missile defense, while a high level of dependence can be assessed for air defense and C4ISTAR services (ItAF 2014). As far as air high precision engagement capabilities, navigation, and guidance of weapons are highly impacted by a PNT-denied environments. As demonstrated by a NATO study (Schmidt 2013) through dedicated simulations, performance of precision-guided munitions can be degraded by a wideband GPS jammer close to the target up to a positioning error 10 times higher than in a noncontested environment. This can lead to catastrophic collateral effects due to the increased circular error probability (CEP), affected, at the same time, also by potential weapons’ guidance systems’ additional errors due to possible GNSS interferences. For these reason, in some circumstances, due to the rules of engagement, missions might be inexecutable in PNT-denied environments. In order to minimize such risks, air-launched precision-guided weapons, which rely on GPS during the navigation phase, can be complemented by laser inertial gyros to provide a precise guidance close to the target. Furthermore, once the weapon is very close to the target, an infrared receiver on the weapon compare the acquired imagery with the one memorized, providing high-level accuracy guidance in the last phase of the guidance (ItAF 2014).

There are air-launched unguided bombs that can be converted into precision-guided munition through an integrated GPS-aided INS guidance kit to improve the laser seeker and infrared technologies. The bomb could be used also with the laser seeker only; nevertheless, the accuracy degradation when GNSS is not available produces an increase of the target location error which in turn increases the probability of collateral damage (ItAF 2014). Also, as far as missile defense capabilities, there is a strong vulnerability of RADAR systems from GPS positioning information in the C2 management system (ItAF 2014) (Fig. 10).

Fig. 10

Pit drop trials during Joint Direct Attack Munitions (JDAM) integration on Tornado

Examples of these effects have been observed during the Iraq War in 2003, when the Iraqi Army used GNSS jammer to disrupt US GPS-guided missiles (Miles 2004). Several additional reports of GNSS jamming have been published about activities in Iraq and Afghanistan that have undermined military operations since then. Although in many cases such incidents can be quickly controlled in a military environment (they usually rely on a high power jammer which can be easily localized with modern equipment), it underlines the criticality of GNSS and the need not only to exploit space services but also to protect them.

Based on this information, it is clear that the improvement of GNSS services play a fundamental role. For example, the improvement of antijam capability of GNSS services is a clear requirement for military users. With the future generation of GPS Block III satellites, the accuracy of GPS service will increase from 3 m to 30–15 cm and in addition the new M-code in replacement of the current Y-mode will produce higher spoofing resilience. Antijamming capability will be increased with the increasing of the signal power, +20 dB above current level, with the possibility to direct the signal toward the area of higher interest.

Several techniques and devices have been developed and new ones will be certainly developed in the future that might severely degrade the performance of GNSS services, ultimately resulting in a complete denial of service, or even worse causing misleading PNT information (deception and spoofing). In particular, GNSS spoofing events have been regularly observed (C4ADS 2019) in the last few years in the Russian Federation, Crimea, and Syria, demonstrating how the military use and development of GNSS threats is growing to pursue tactical and strategic advantages both at home and in foreign territories.

All such threats to GNSS adversely impact various military tasks, such as those described above as some examples. For this reason, PNT superiority integration into military operations and systems is more and more relevant in view of ensuring force superiority in the battlefield of the future. This should be related to the understanding of the relevance of space domain and in particular the PNT sources in military operations. The threat scenario “A day without space,” in fact, reveals a clear dependency (of current and future military operations) on space. At the moment, however, such a threat can be considered as an expression of the soft power (Nye 2005), even if it is clear that in a future strategic prospective the situation will evolve. As defined in “On War” (Von Clausewitz 1832) a center of gravity is “the source of power that provides moral or physical strength, freedom of action, or will to act,” and in this sense, the space domain is already moving toward becoming a center of gravity for fifth-generation warfare.

A direct consequence of this qualitative consideration is based on the space threats proliferation that will generate the need to defend the space as a critical infrastructure, strategic nodes, or a military capability.

The real issue is that we are facing threats that were not planned to be dealt with, for technological but also mainly political reasons. Indeed, not only defense equipment is being targeted, but also the entire space segment as witnessed by several developments in this area. A recent report from the office of the Secretary of Defense (USDEF 2019) underlines how China is improving its counterspace capabilities. In addition to directed energy weapons, its antisatellite missile systems (following the tests in 2014) are being further developed. Even if the Chinese government has not acknowledged any specific antisatellite program, there are several publications from defense-funded academies that stress the necessity of “destroying, damaging, and interfering with enemy’s reconnaissance…and communication satellites” suggesting that such systems (and the navigation ones) could be among the targets for attacks. The very recent Indian tests of antisatellite weapons (Foust 2019) as well as the decision to constitute a new military service dedicated to Space Force by U.S. President D. Trump (Wall 2019) are a clear signal toward this direction.

The peaceful use of space (UN 2008) would possibly become a right to defend and not an acquired status relying on international agreements. Space will follow the same path of the sea and air domains, started as a research and development environment when men were not able to navigate or to fly, and then transformed into capability domains to defend from external threats for the benefits of all the civil population. Rather sooner than later, the space domain will evolve to a sphere to be reached before the others, to a source full of enabling services. The above-mentioned recent anti-satellite’s strike capabilities performed as test demonstrators are clear evidence that we need to start preparing to defend space from such kinds of threats.

In this view, the recent declaration of European Commission during the last European Space Policy conference emphasizing the need of start thinking to an EU Space Force (Teffer 2019) in parallel with the development of EU space capabilities, such as Galileo, looks as the product of a defense-oriented Strengths, Weaknesses, Opportunities, and Threats (SWOT) analysis performed in a process of strategic thinking. Indeed, as remarked in the EU Global Strategy, performant and robust PNT services are a key factor to enhance the responsiveness, the credibility, and the responsibility of the EU. In the end, denial or disruption of satellite navigation signals may heavily impact the effectiveness and the capabilities of EU military forces.

Conclusion

The assumption that space-based PNT services will be always and in any conditions guaranteed to EU Member States is debatable and the last EU Space Global Strategy clearly refers to this as a need of strategic autonomy. Such awareness in EU Member States is not homogenously spread. It could be erroneously considered that GNSS services are a given-capability, even without the standard requirements’ definition process.

Recent events are confirming that space domain is already a congested and competitive domain, where several and new forms of threats will appear, operate, and evolve. PNT services will be affected, and considering the high level of military capability dependency from GNSS, a broader and comprehensive approach needs to be put in place supporting the development of dependable PNT services and sources capable of meeting European defense operational requirements in any of the envisaged scenarios.

All this, coupled with the almost ubiquitous dependency on space-based PNT services, require that defense planners and leaders understand that building more resilient PNT capabilities needs careful thinking and the implementation of architectures that transcend individual PNT-enabled systems, taking into account the need to protect not only the delivery of PNT services to users but also to render the space assets more robust and resilient to new types of threats.

A common definition and agreement of PNT requirements and related concepts of operations aimed to identify primary, alternate, and back PNT sources for defined military scenarios and platforms is indeed a crucial step to guarantee a holistic and robust PNT service to armed forces during military operations.