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2021/06/22
A New Approach to the Design and Development of Global Navigation Satellite Systems
By Daniele Gianni, Marco Lisi, Pierluigi De Simone, Andrea D’Ambrogio, and Michele Luglio
INNOVATION INSIGHTS by Richard Langley
MY FIRST DEGREE is in applied physics from the University of Waterloo. Founded in 1957, Waterloo was one of the first universities to introduce co-operative education. Co-operative education (or “co-op” as it is commonly known) is a program that uses both classroom study and temporary jobs to provide students with practical experience. Applied Physics was a co-op program and I worked in both industry and research environments including stints at Philips Electronics and the Atomic Energy of Canada Limited’s Chalk River Laboratories.
Both on campus and on the job, I met fellow co-op students from a variety of disciplines including mathematics (computer science) and various branches of engineering. One of those was systems design engineering or systems engineering for short. At that time, I really didn’t know much about systems engineering except that it was an all-encompassing branch of engineering and the most challenging of all of the engineering programs at Waterloo — at least according to the students in the program.
Systems engineering is an interdisciplinary field of engineering focusing on the design and management of complex engineering projects. According to the International Council on Systems Engineering, systems engineers establish processes “to ensure that the customer and stakeholder’s needs are satisfied in a high quality, trustworthy, cost efficient and schedule compliant manner throughout a system’s entire life cycle. This process is usually comprised of the following seven tasks: State the problem, Investigate alternatives, Model the system, Integrate, Launch the system, Assess performance, and Re-evaluate [or, SIMILAR, for short].”
Central to the systems engineering process and the end-product design is the generation of models. Many types of system models are used, including physical analogs, analytical equations, state machines, block diagrams, functional flow diagrams, object-oriented models, computer simulations, and even mental models. (If you want to learn a bit about mental and other kinds of models, including how to fix radios by thinking, you could do no better than to look at some of Richard Feynman’s writings including the eminently readable “Surely You’re Joking, Mr. Feynman!”: Adventures of a Curious Character.)
As aids to the modeling process, systems engineers have developed specialized modeling languages including the Unified Modeling Language (UML) and the Systems Modeling Language (SysML). These are graphical-based languages that can be used to express information or knowledge about systems in a structure that is defined by a consistent set of rules. Both UML and SysML are widely used in systems engineering. However, both are limited when it comes to representing the signal-in-space (SIS) interfaces for global navigation satellite systems.
In this month’s column, a team of authors affiliated with the Galileo project discusses the Interface Communication Modeling Language, an extension of UML that allows engineers to clearly represent SIS interfaces, critical for the design of GNSS receivers.
“Innovation” is a regular feature that discusses advances in GPS technology andits applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering, University of New Brunswick. He welcomes comments and topic ideas. To contact him, see the “Contributing Editors” section on page 4.
In this article, we present the results of ongoing research on the use of a modeling language, namely Interface Communication Modeling Language (ICML), for signal-in-space (SIS) interface specification of global navigation satellite systems (GNSS). Specifications based on modeling languages (also known as model-based specifications) have proven to offer a wide range of benefits to systems engineering activities, for supporting system interoperability, reducing design risk, automating software development, and so on. We argue that similar benefits can be obtained for satellite navigation systems and receivers, if a model-based approach is used for defining and expressing the SIS interface specification. In particular, we outline how a model-based SIS interface specification can support the identification of solutions to two key issues: GNSS interoperability and the design of GNSS receivers, particularly Galileo receivers. Both issues are becoming increasingly central to the Galileo program since it entered the In-Orbit-Validation (IOV) phase and is steadily approaching the 2014 milestone, when the first early services — the Open Service (OS) and the Search and Rescue Service — will be provided to users.
GNSS interoperability concerns the integration of different GNSS with the purpose of being used together, along with regional positioning systems, to provide a seamless navigation capability and improved services in terms of availability, continuity, accuracy, and integrity, for example. GNSS interoperability should be addressed in terms of intra-GNSS interoperability and GNSS-receiver interoperability. The intra-GNSS interoperability concerns the data exchanged among the GNSS, including coordination to guarantee data coherence and consistency over time. For example, GNSS may need to share terrestrial reference frames and constantly synchronize their global time references. On the other hand, GNSS-receiver interoperability concerns the capability of the receiver to use independent GNSS signals for the computation of positions globally. This capability implicitly requires that the receiver computations are decoupled from the SIS interface of any particular GNSS. A key condition to achieve this decoupling is that the SIS interface specification is available in a consistent, unambiguous, and possibly standard format, which can support engineers to more effectively design interoperable receivers. A model-based SIS interface specification would considerably facilitate this as it enables designers to use the processing capabilities of a computer system for the verification of the specification consistency and completeness, for example. Moreover, a model-based SIS interface specification would ease the visual and electronic inspection of the data messages, therefore facilitating the automatic identification of different data representations for the same orbital and temporal parameters.
The design of GNSS receivers, and particularly those for Galileo, is increasingly of interest, and a model-based SIS interface specification can similarly support the definition of future solutions. For Galileo, specifically, the receiver design is critical to support the marketing strategies that are promoting the use of Galileo services. Key issues underlying any marketing strategy concern the Galileo receiver market appealing from a cost-to-performance ratio point of view. As Galileo receivers may require new design and adaptation of existing software (SW) or hardware (HW), as well as new production chains, higher costs — in particular non-recurring ones — are likely to occur for the production of the Galileo receivers with respect to the well-established GPS receivers. As a consequence, limitations may be experienced in market penetration and in the growth velocity of Galileo receivers’ share of the receiver market. In turn, this may hinder the estimated economic return for the Galileo project.
Preventing and counteracting this possibility is therefore a critical issue if we aim to achieve the widest possible success of the Galileo project. Market barriers inherently originate from the following needs:
Designing new SW and HW solutions for Galileo receivers;
Reusing existing SW and HW for GPS receivers;
Converting existing production chains to the new Galileo-specific SW and HW solutions.
GNSS receivers often use established mathematical models that can determine the receiver position from a fundamental set of parameters, such as satellite orbit and system time. As a consequence, the intrinsic representation of the parameter set is a major factor in the adaptation of the existing design and implementation of SW and HW solutions.
To reduce the impact of the above needs, a model-based SIS interface specification may play a pivotal role in several ways, such as:
reducing ambiguities in the Galileo SIS interface specification;
enhancing the communication with the involved stakeholders;
linking the SIS interface specification to the design schemas of GNSS receivers — particularly Galileo ones — for tracing the interface elements onto the receiver functional and physical schema, thereby supporting the reuse and adaptation of existing HW and SW solutions;
supporting the model-based design of security solutions for blocking, jamming, and spoofing.
Galileo Project
In October 2012, the final two IOV satellites were launched into orbit, completing the designed configuration for the Galileo IOV phase — the initial stage of the Galileo constellation development. In this phase, preliminary validation tests will be performed and the initial navigation message will be broadcast to the Galileo ground segment for further validation. Shortly after the conclusion of this phase, a series of launches will take place to gradually deploy the remaining 26 satellites that will form the Galileo Full Operational Capability (FOC) configuration. Currently, the Galileo Early Open Service (EOS) is expected to be available by the end of 2014. The EOS will provide ranging capabilities and will enable receiver manufacturers to begin to design and test their technological solutions for Galileo receivers and Galileo overlay services, such as search and rescue.
In the meantime, the European GNSS Agency has been established and assigned the governance of the Galileo sub-systems, including activities such as:
initiating and monitoring the implementation of security procedures and performing system security audits;
system infrastructure management, maintenance, improvement, certification, and standardization, and service provision;
development and deployment of activities for the evolution and future generations of the systems, including procurement activities;
contributing to the exploitation of the systems, including the marketing and promotion of applications and services, including market analysis.
With the now-rapid development of the Galileo project, it becomes increasingly important to support the receiver manufacturers in the design and implementation of global navigation solutions based on the Galileo services. This is necessary to guarantee the widespread use of the Galileo services, particularly in an increasingly crowded GNSS panorama.
Model-Based Systems Engineering
Model-based systems engineering (MBSE) is predicated on the notion that a system is developed by use of a set of system models that evolve throughout the development lifecycle, from abstract models at the early stages down to the operational system. A visual presentation is provided by FIGURE 1, which shows the roles of MBSE approaches within the systems engineering V-shaped process. Specifically, the MBSE approaches enable the designer to effectively trace the requirements and design alternatives on the descending branch of the “V.” For the same characteristics, MBSE facilitates the verification through a model repository that interconnects not only the design products, but also the stakeholders involved in the entire process. In addition, MBSE approaches support the automatic generation of the documentation and of other artifacts, particularly software. All of these capabilities eventually enable the validation of the implementation activities on the ascending branch of the V-process. Also, in this case, MBSE and the model repository play a major role in connecting design to implementation, and users and designers to developers.
FIGURE 1. Systems engineering V-process supported by an model-based systems engineering with model repository (courtesy of the INCOSE Survey).
Main Concepts. MBSE approaches are gaining increasing popularity with the widespread adoption of standard modeling languages, such as Unified Modeling Language (UML) and Systems Modeling Language (SysML).
UML is a formally defined general-purpose graphical language and is mainly used in the context of software systems development. It has been developed and is being managed by the Object Management Group and is the core standard of the Model Driven Architecture (MDA) effort, which provides a set of standards to shift from code-centric to model-driven software development. By use of an MDA-based approach, a software system is built by specifying and executing a set of automated model transformations.
SysML is defined as an extension of UML and provides a general-purpose modeling language for systems engineering applications (See FIGURE 2). SysML supports MBSE approaches in the development of complex systems that include hardware, software, information, processes, personnel, and facilities.
FIGURE 2. UML-SysML relationships. (UML 2 is the second generation version of UML introduced in 2005.)
Advantages. With respect to the conventional document-based approaches, MBSE approaches present the following advantages:
Conformance to standard specifications and availability of development tools;
Increased level of automation due to the formal specification and execution of model transformations that take as input a model at a given level of abstraction and yield as output a refined model at a lower level of abstraction;
Better understanding of the system in its operational context;
Support for simulation activities at different levels of detail and at different development stages, from concept exploration to dynamic system optimization;
Support for the coherent extension of standard modeling languages to adapt them to a specific target or domain.
These capabilities have motivated and have been sustaining an increasing trend of moving from document-centric to model-centric systems engineering.
ICML Language
UML and SysML are widely used languages for MBSE. A plethora of tools and technologies are available to compose models, transform models into documents, derive software products from models, and share and reuse models by means of repositories. However, neither of these languages offers capabilities for the representation of SIS interfaces, which are the critical interfaces for the design of Galileo receivers. For this reason, we have introduced ICML: a modeling language that can enable a full MBSE approach for the design of Galileo receivers. Moreover, ICML extends UML, and therefore it can integrate with system specifications based on compliant technologies as well as be used within standard tools.
Layout of Interface Specification. The typical layout of ICML-based interface specification is shown in FIGURE 3. The specification covers the definition of both the message structure and conversion processes. The message structure consists of five abstraction levels, and describes how the data is structured within the message. The conversion processes describe how the data values are transformed between adjacent levels of the message specification.
FIGURE 3. Layout of ICML-based interface specifications.
The message structure is defined at five levels: Data Definition, (Logical) Binary Coding, Logical Binary Structure, Physical Binary Coding, and Physical Signal, each covering specific aspects of the SIS interface specification.
For example, the Data Definition level covers the specification of the logical data structure, which includes the data items composing the message information. A data item is either of application or control type. An application data item represents a domain-specific concept that conveys the information expected by the message recipient. On the other hand, a control data item represents a domain-independent concept that can support the correctness and integrity verification of the associated application data items. A data item can also be associated with semantic and pragmatic definitions. The former specifies the meaning of the data item and the latter specifies the contextual interpretation for the semantic definition.
Analogously, the Binary Coding level covers the specification of the binary coding for each of the data items defined at the above level. For a data item, the binary coding is represented as a binary sequence and it includes at least a sequence identifier, the semantic definition, and the pragmatic definitions. Similarly to the above level, the semantic and pragmatic definitions enrich the interface specification, conveying an accurate representation of the binary coding.
The conversion processes describe the activities to be performed for deriving message values between adjacent levels of the above structural specification. As shown in Figure 3, eight processes should be defined to specify all the conversions between adjacent levels. For example, the DataDefinition2BinaryCoding process defines the activities to be performed for the derivation of the logical binary sequences representing data values. Similarly, the LogicalBinary2PhysicalBinary process defines the activities for the implementation of convolution or encryption algorithms on the logical binary sequence. However, these processes do not always need to be explicitly defined. In particular, if the implementation of a process is trivial or standard, a textual note referring to an external document may suffice for the specification purposes.
The first prototypal version of ICML has been implemented and can be used within the open source TopCased tool. The prototypal version is available under the GNU General Public License (GPL) v3.0 from the ICML project website. We applied the profile and developed the example ICML-based specification given below.
Galileo-Like Specification. An ICML-based specification of a Galileo-like OS interface, concerning only the above-defined level 3, would display as shown in FIGURE 4. This figure specifically details a part of a reduced F/NAV (the freely accessible navigation message provided by the E5a signal for the Galileo OS) structure consisting of one data frame made up of two F/NAV subframes.
FIGURE 4. Example of ICML-based specification of an F/NAV-like message structure at the Logical Binary Coding level.
Benefits. ICML can bring the above-mentioned MBSE benefits to support GNSS interoperability and to GNSS and Galileo receiver design. For example, ICML can:
provide a reference guideline for structuring the specification data and thus facilitating the communication between the Galileo SIS designers and the receiver producers;
ease visual inspection of the specification for verification purposes and for the identification of data incompatibilities of two GNSS systems;
convey the data semantics as well as the measurement units, to guarantee that the binary data from different GNSS are correctly decoded and interpreted;
support syntactical model validation using existing tools;
provide support for future advance exploitation by means of a machine-readable data format.
In particular, the availability of a machine-readable format is also the basis for advanced use cases that can exploit the capabilities of modern computer technologies.
Advanced Future Use Cases. In line with the above-mentioned MBSE model exploitations, we foresee a number of possible exploitation cases:
Automatic generation of the interface specification documents;
Collaborative development of the interface specification;
Automatic completeness and consistency checking of the interface specification;
Integration of SIS specifications with model-driven simulation engineering approaches for the simulation of single- and multi-GNSS receivers;
Integration of SIS specifications with receiver design models in SysML, for requirements traceability and reuse of existing GNSS solutions.
The automatic generation of interface specification documents can be an important capability during the lifecycle of a specification. For example, the specification may be updated several times during the interface design, and the textual documentation may need to be produced several times. Using a model-based approach, it is possible to automate the error-prone activities related to the document writing as well as other important functions such as specification versioning.
Complex system specifications are often the product of collaborating teams, which may occasionally be geographically dispersed. Using a model-based approach, the interface specification can be stored within a version control system that can be concurrently accessed by team members.
Completeness and consistency checking is also a manual activity, which demands a high degree of mental attention, and it is consequently highly error prone. Once the specification is available in a machine-readable format, the checking can be easily automated by specifying the verification rules that the interface model must satisfy.
Existing technologies support the simulation of single- and multi-GNSS receivers. As the SIS specification has a major impact on the internal structure of the receiver, the interface specification is a key input for developing GNSS simulators as well as for determining the boundary properties of the input signal into the receiver, including the admitted analog signal and the format of the digital data.
Moreover, the model-based interface specification can be integrated with a receiver design schema in SysML. This would be important to provide traceability between the interface requirements and the receiver’s functional and physical components. In the following section, we provide an outline for a preliminary integration between the interface specification and the receiver design.
Designing Galileo Receivers
Model-based interface specifications can support the design of Galileo receivers in several ways. For example, a specification can provide a link between Galileo requirements down to the Galileo receiver specifications, as shown in FIGURE 5.
FIGURE 5. Links between ICML and SysML specifications.
This capability may be useful in several scenarios. In particular, we have identified three scenarios. Scenario 1 consists of the identification of the receiver requirements that are introduced or modified by the Galileo OS SIS, with respect to existing GPS receivers. Scenario 2 concerns the linking between the ICML specification and the receiver functional schema to identify how a Galileo receiver will differ from existing GPS solutions. Scenario 3 is a development of Scenario 1 and Scenario 2, in which the physical schema definition and the physical components identification (HW and SW) may further exploit the ICML-based approach for supporting the reuse of existing GPS components.
Below, we detail Scenario 2, introducing a simplified receiver functional schema in SysML and linking the above ICML example to the schema.
Example Functional Schema. In this section, we illustrate a preliminary SysML representation for a simplified GNSS receiver. However, the figures are meant for exemplification purposes only and are not to be considered fully realistic and detailed for real GNSS receivers. Nevertheless, the SysML hierarchical modeling capabilities can be used to further refine the model, up to a potentially infinitesimal level of detail.
A GNSS receiver functional schema has been derived from A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach (see Further Reading) and its equivalent SysML internal block diagram (IBD) is shown in FIGURE 6.
FIGURE 6. High-level receiver internal block diagram (functional schema).
In particular, the IBD illustrates the functional blocks (instances and types) and connections among these blocks that define the GNSS receiver. In particular, each of these block types is also described in other diagrams, in which the designers can specify the operations performed by the block, the attributes of the block, the referred properties, and the defined values, for example.
In this short article, we have particularly focused on the navigation data decoder. The data decoder is defined by a Block Definition Diagram (BDD) and an IBD, which are shown in FIGURES 7 and 8, respectively.
FIGURE 7. Navigation data decoder block definition diagram.
FIGURE 8. Navigation data decoder internal block diagram.
In particular, the BDD indicates that the navigation data decoder is composed of four types of blocks: shift buffer, parity checker, binary adder, and data item retriever. The shift buffer receives the incoming physical sequence of bits, which is subsequently verified by the parity checker. The verified sequence is then processed to retrieve the standard binary format from the SIS-specific logical coding for the data item. This function is guided by the data item retriever, which stores the defined properties of each incoming data item, in the form of a physical sequence of bits (level 1). As a consequence, the navigation data decoder is involved with data defined at several of the above-defined ICML levels. From this description, it is also possible to sketch the preliminary IBD diagram of Figure 8.
Using a model-based approach, it becomes easier to establish links between interface elements and the functional blocks in the receiver schema.
Moreover, these links can also be decorated with a number of properties that can be used to further describe the type of the relationship between the interface element and the functional block. The link identification is important to the receiver design in several ways. For example, linking the interface elements to the receiver functional blocks, it becomes easier to identify which functional blocks are affected by each element of the SIS interface. Moreover, the tracing can be transitively extended to the physical schema, enabling the receiver designers to more immediately identify which physical components can be reused and which ones must be replaced in existing GNSS solutions.
We exemplify the tracing of interface elements on the above data decoding functional schema in FIGURE 9. This figure shows the navigation data decoder’s BDD in conjunction with ICML level 3 elements (with a white background). As in Figure 7, the relationships are drawn in red, including a richer set of relationship qualifiers. For example, the > qualifier indicates that the originating block uses the data specified in the connected ICML element. Similarly, the > qualifier indicates that the originating block takes in input instances of the ICML element. ICML level 4 elements are also relevant to this BDD; however, they are not shown for the sake of conciseness.
FIGURE 9. Linking level 3 elements to the navigation data decoder block definition.
Conclusions
Galileo receivers may face market barriers that are inherently raised by the costs linked with the introduction of new technologies with respect to the existing GPS ones. In this article, we have advocated that a model-based SIS interface specification can help mitigate possible extra costs in several ways. For example, the model-based interface specification can ease the communication among stakeholders, promote the reuse and adaptation of existing GPS software and chipsets, and support the implementation of receiver-side multi-GNSS interoperability. With the objective of supporting model-based interface specifications, we have designed ICML, which has been provided with a UML profile implementation in an open-source modeling tool. We have also shown an excerpt of a possible model-based specification for a simplified Galileo OS interface. Moreover, we have outlined how the model-based specification can integrate with SysML models of GNSS receivers and support the reuse and adaptation of existing solutions. A preliminary identification of potential exploitations and further benefits is also included. Further research is ongoing to generalize the existing ICML language to more complex types of SIS interfaces.
Acknowledgments
The authors would like to thank the students Serena Annarilli and Carlo Di Bartolomei (University of Rome Tor Vergata) for implementing the first prototype version of the ICML profile. The authors would also like to thank Marco Porretta, European Space Agency (ESA) / European Space Research and Technology Centre (ESTEC), for the suggestions of the GNSS example. The ICML project has been partially sponsored by the ESA Summer of Code in Space Initiative, edition 2012. No endorsement is made for the use of ICML for the official Galileo SIS interface specification.
DANIELE GIANNI is currently a requirement engineering consultant at EUMETSAT in Germany. EUMETSAT is the European operational satellite agency for monitoring weather, climate and the environment. Gianni received a Ph.D. in computer and control engineering from University of Rome Tor Vergata (Italy), in the field of modeling and simulation, in 2007. He has previously held research appointments at ESA, Imperial College, and Oxford University.
MARCO LISI is currently GNSS services engineering manager at ESA’s Directorate of Galileo Programme and Navigation- Related Activities at ESTEC in Noordwijk, The Netherlands. He was previously responsible for system engineering, operations, and security activities in the Galileo project. He is also a special advisor to the European Commission on European space policies. Lisi has over thirty years of working experience in the aerospace and telecommunication sectors, holding management positions in R&D, and being directly involved in a number of major satellite programs, including Artemis, Meteosat Operational, Meteosat Second Generation, Globalstar, Cosmo-Skymed, and more recently Galileo.
PIERLUIGI DE SIMONE is currently working on system assembly, integration, and verification for the Galileo mission in ESA. He has worked on many software developments in the fields of graphics, safe mode software, and visual programming. He has worked on many space missions including Helios, Meteosat, Metop, Cosmo-Skymed, and Galileo. His main interests are in modeling paradigms and cryptography and he holds a master’s degree in physics from University of Rome Tor Vergata.
ANDREA D’AMBROGIO is associate professor of computer science at the University of Rome Tor Vergata. He has formerly been a research associate at the Concurrent Engineering Research Center of West Virginia University in Morgantown, West Virginia. His research interests are in the areas of engineering and validation of system performance and dependability, model-driven systems and software engineering, and distributed simulation.
MICHELE LUGLIO is associate professor of telecommunication at University of Rome Tor Vergata. He works on designing satellite systems for multimedia services both mobile and fixed. He received the Ph.D. degree in telecommunications in 1994.
FURTHER READING
• Interface Communication Modeling Language (ICML)
ICML project website.
“A Modeling Language to Support the Interoperability of Global Navigation Satellite Systems” by D. Gianni, J. Fuchs, P. De Simone, and M. Lisi in GPS Solutions, Vol. 17, No. 2, 2013, pp. 175–198, doi: 10.1007/s10291-012-0270-z.
• Use of ICML for GNSS Signal-in-Space Interface Specification
“A Model-based Signal-In-Space Interface Specification to Support the Design of Galileo Receivers” by D. Gianni, M. Lisi, P. De Simone, A. D’Ambrogio, and M. Luglio in Proceedings of the 6th ESA Workshop on Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing (NAVITEC), Noordwijk, The Netherlands, December 5–7, 2012, 8 pp., doi: 10.1109/NAVITEC.2012.6423066.
“A Model-Based Approach to Signal-in-Space Specifications for Designing GNSS Receivers” by D. Gianni, J. Fuchs, P. De Simone, and M. Lisi in Inside GNSS, Vol. 6, No. 1, January/February 2011, pp. 32–39.
• Related Modeling Languages
The Unified Modeling Language Reference Manual, 2nd edition, by G. Booch, J. Rumbaugh, and I. Jacobson, published by Addison-Wesley Professional, an imprint of Pearson Education, Inc., Upper Saddle River, New Jersey, 2005.
A Practical Guide to SysML: The Systems Modeling Language, 2nd edition, by S. Friedenthal, A. Moore, and R. Steiner, published by Morgan Kaufman and the Object Management Group Press, an imprint of Elsevier Inc., Waltham, Massachusetts, 2012.
• Systems Engineering
Systems Engineering: Principles and Practice, 2nd edition, by A. Kossiakoff, W.N. Sweet, S.J. Seymour, and S.M. Biemer, published by John Wiley & Sons, Inc., Hoboken, New Jersey, 2011.
Survey of Model-Based Systems Engineering (MBSE) Methodologies, INCOSE-TD-2007-003-02, published by Model Based Systems Engineering Initiative, International Council on Systems Engineering, Seattle, Washington, 2008.
• GNSS Receiver Operation
A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach by K. Borre, D.M. Akos, N. Bertelsen, P. Rinder, and S.H. Jensen, published by Birkhäuser Boston, Cambridge, Massachusetts, 2007.
• Galileo Status and Plans
“Status of Galileo” (Galileo System Workshop) by H. Tork in the Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, September 17–21, 2012, pp. 2474–2502.
“Galileo Integrated Approach to Services Provision” (Galileo System Workshop) by M. Lisi in the Proceedings of ION GNSS 2012, the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation, Nashville, Tennessee, September 17–21, 2012, pp. 2572–2596.
European GNSS (Galileo) Open Service Signal in Space Interface Control Document, Issue 1.1, European Union and European Space Agency, September 2012.
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