GNSS Evolutions for Maritime an Incremental Approach Istock Photo by Ivan Cholakov Ivan by Photo Istock

WORKING PAPERS GNSS Evolutions for Maritime An Incremental Approach iStock photo by Ivan Cholakov Ivan by photo iStock

At the end of 2013, the European Space Agency (ESA) started analyzing is defined as the “harmonised collec- possible GNSS evolutions within the maritime sector. Various techniques tion, integration, exchange, presentation and analysis of marine information on have been investigated in close collaboration with the maritime board and ashore by electronic means to community. The use of satellite-based augmentation system (SBAS) enhance berth to berth navigation and corrections in maritime receivers is currently unregulated. This article related services for safety and security at sea and protection of the marine envi- discusses two possible strategies (mid-term and long-term) for the ronment” (IMO 2008). development of a dedicated standard for combining use of SBAS with IMO and the International Associa- differential GNSS services to provide a resilient positioning, navigation, tion for Lighthouse Authorities (IALA) identify several technological enablers and timing capability for maritime applications, including e-Navigation. that support the e-Navigation concept. These include, amongst others: accurate rends for marine accidents show electronic navigation charts (ENCs), a rising numbers and costs that are robust electronic position, navigation mainly associated with collisions and timing (PNT) system (with redun- Tand groundings. Research indicates that dancy), and an agreed communications about 60 percent of these accidents are infrastructure to link ship and shore. caused by human error. The majority of Further, robust PNT information them could have been avoided by pro- requires three complementary com- viding suitable input to the navigation ponents: a core GNSS, augmentation decision-making process, according to systems, such as differential GNSS a 2008 report by the International Mari- (DGNSS) or satellite-based augmenta- time Organization (IMO) Marine Safety tion system (SBAS), to ensure that the Committee. (See IMO 2008 in Addition- GNSS performance is able to meet the al Resources section near the end of this application requirements, and an ade- MARCO PORRETTA, DAVID JIMENEZ BANOS, article.). quate backup in the event of a GNSS MASSIMO CRISCI, GIORGIO SOLARI, AND In order to improve safety of naviga- system failure. ALESSANDRA FIUMARA tion and to reduce errors, IMO is pro- The backup system is based on moting the e-Navigation concept. This employing several independent terres-

54 InsideGNSS MAY/JUNE 2016 www.insidegnss.com trial schemes that could be made avail- Do not use the system! able to users. They include, for example, enhanced Loran (eLoran), ranging mode (R-Mode) navigation using signals independent of GNSS, or enhanced radar positioning techniques. Used in conjunction with GNSS, such independent, Navigation Signals User redundant systems significantly increase System Integrity Information the resilience of the on-board PNT sys- Generic Radio Navigation System (RNS) tem against possible “feared events” (Not necessarily satellite-based) (FEs) affecting a single component of FIGURE 1 Integrity at system level the system. This positioning approach is also known as “resilient PNT” and is duces the requirements for a “future,” tor the quality of the navigation service currently being supported by the IMO not yet identified, GNSS. provided (the “system integrity monitor- in the development of the e-Navigation The two documents express the asso- ing” task). System integrity information concept. ciated requirements using performance is then broadcast to the user together One of the most immediate benefits parameters that have a different scope with navigation signals. of resilient PNT is an increased robust- and, therefore, a different approach. As Figure 1 presents a schematic of ness of the GNSS component against a result, the two resolutions demand this type of integrity service, where local error sources. These include, but different methods to monitor the navi- the “Generic RNS” block indicates the are not limited to, multipath events, gation performance and to verify the WWRNS component. If a failure is unintentional interference, and deliber- level of compliance with the associated detected in one of the RNS modules, ate interference attacks such as jamming requirements. This verification is a key the user is promptly warned that the or spoofing. element for safety-of-life (SoL) applica- system (or the information provided In this context, possible roles for tions. by that specific module) should not be SBAS have been investigated and dis- IMO A.1046(27). The feasibility of used. cussed with the maritime community. a WWRNS has been investigated since This approach for integrity monitor- This article discusses preliminary results 1983. These studies produced an amend- ing considers only the “system-related of an analysis of these roles. In it, we will ment of Chapter V of the 1974 SOLAS failures” that might prevent the RNS first discuss the operational require- convention that includes “a require- from broadcasting accurate naviga- ments for radio navigation systems ment for ships to carry means of receiv- tion signals. This concept is normally (RNSs) to be used in maritime applica- ing transmissions from suitable radio referred to as “integrity at system level” tions and then outline the current sta- navigation systems throughout their and is not able to promptly warn the user tus of GNSS for maritime applications. intended voyage.” The global nature of in case of performance degradations due Next, we will present the main features the WWRNS requires the overall system to local error sources (i.e., multipath or of DGNSS and SBAS as well as the pos- to be formed by different sub-systems (or interference). sibility of offering SBAS as a possible components), possibly including GNSS If the user requires additional pro- augmentation system complementing together with the associated augmen- tection from local error sources, further DGNSS. Finally, we propose two strate- tation systems (e.g., DGNSS or SBAS), checks — for example, receiver autono- gies (“mid-term” and “long-term,”) for other regional satellite navigation sys- mous integrity monitoring (RAIM) the use of SBAS in maritime applica- tems, and other (not necessarily satellite- techniques — must be autonomously tions. based) radio navigation systems, such performed by the user receiver, thereby as eLoran. WWRNS specifically allows locally verifying the consistency of the GNSS Performance Requirements for implementation of the resilient PNT navigation signals provided by the sys- for Maritime concept, provided that independent tem. Note, however, that these checks are Two IMO resolutions outline the oper- RNSs are recognized as a component of recommended but not mandatory under ational requirements for the use of the overall system. IMO A.1046(27). RNSs in maritime applications. IMO Procedures and responsibilities IMO specifies operational require- A.1046(27) specifies the requirements concerning the recognition of an RNS ments in more detail using four param- of an RNS as a generic component of a as a component of the WWRNS are eters: accuracy, system integrity, signal World Wide Radio Navigation System detailed in the IMO A.1046(27) resolu- availability, and service continuity. In (WWRNS). Here, a WWRNS com- tion. In particular, a generic component particular, these requirements define ponent can be either a satellite-based (not necessarily satellite-based) of the service continuity as the capability of (GNSS) or a terrestrial-based system. WWRNS has two main tasks. It must the system to provide its service (navi- In contrast, IMO A.915(22) focuses on first provide the users with navigation gation signals and broadcast of system GNSS as a stand-alone system and intro- signals and must also constantly moni- integrity information), without interrup- www.insidegnss.com MAY/JUNE 2016 InsideGNSS 55 WORKING PAPERS

Requirement/ Harbor entrance, harbor Once these parameters are set, the Navigation Phase Ocean Waters approaches and coastal waters integrity at the user level is monitored by Accuracy (95% 100 meters 10 meters calculating, at the user receiver, a maxi- horizontal navigation mum error bound (protection level, PL) system error) for the navigation system error (NSE). Integrity: “An This estimation is done at each epoch integrity warning of “… as soon as practicable and normally uses navigation signals system malfunction, by Maritime Safety “… within 10s” (observables), system integrity informa- non-availability or Information (MSI) discontinuity should be systems” tion, and suitable models for the effects provided to users …” due to local error sources. The definition of these models requires a detailed char- Signal Availability: “Signal availability should “… 99.8%” “… 99.8%” acterization of the operational environ- exceed…” ment, including, for example, an analy- “The system shall be considered sis of the expected levels for multipath available when it provides the and interference. required integrity for the given Based on the input data provided Service Continuity N/A accuracy level. When the system by the three variables used to estimate is available, the service continuity user-level integrity, the resulting PL cal- should be ≥99.97% over a period of 15 minutes” culation takes into account both system- related failures and possible effects due Table 1 Operational requirements for a component of the WWRNS to local error sources. The PL calculation can be completed either completely Do not use the system! autonomously (e.g., via RAIM tech- HAL niques) or based on additional information provided by the system (e.g., S when SBAS is used for aviation applications per the RTCA standards). Figure 2 represents the concept of integrity at Ge neric RN Navigation Signals and the user level for a generic RNS that, in System Integrity Information HPL this case, is a future GNSS system. If the User user wants to perform a given operation, the current value of the protection FIGURE 2 Integrity at the user level (HPL and HAL denote the horizontal protection level and the horizontal alert limit, respectively, while the actual, unknown, horizontal position error is level is compared with the AL require- indicated by a green dot) ment associated with that operation. If, because of a failure, the PL exceeds tions, throughout the duration of a given system include both the GNSS service the AL specification, the receiver will operation. (i.e., the properties of the signal in space, promptly warn the user that the position Based on these parameters, IMO or SIS, provided by both the space and estimation cannot be trusted. requirements address two navigation the ground segments of the GNSS) and In Figure 2, the actual position error phases: (1) ocean waters and (2) harbor the user receiver. (PE), which is normally unknown, is entrance, harbor approaches, and coast- This is a fundamental difference indicated by a green dot. The PL pro- al waters. Table 1 presents the parame- with the operational requirements vides a conservative estimation of the ters of these operational requirements. needed for a single component of PE. In case the PL exceeds the AL However, the IMO requirements do not the WWRNS as described in IMO specification, a “Do not use the sys- specify the actual navigation perfor- A.1046(27), which only considers system” warning is communicated to the mance at the user level. tem-related aspects. In particular, the end-user. IMO A.915(22). This IMO resolution underlying integrity concept is now The protection level calculation expresses operational requirements for at the user level and includes, for each should always over-bound the position future GNSS based on the actual perfor- maritime operation, the specification error, thus avoiding critical situations mance at the user level. The parameters of three parameters, i.e., the alert limit where the PE exceeds the alert limit, include accuracy, integrity, continu- (AL), the time to alert (TTA), and the while the estimated PL is below the AL ity, and availability as defined for civil integrity risk (IR) (See IMO, Revised (PL < AL < PE). These situations are aviation applications by the RTCA Inc. Maritime Policy and Requirements for referred to as “hazardously misleading (see RTCA DO-229D in Additional a Future Global Navigation Satellite information” (HMI) events, and their Resources). In other words, according System, GNSS,” in Additional Resourc- probability of occurrence must be below to the RTCA, the properties of a GNSS es section for further details.) the integrity risk requirement.

56 InsideGNSS MAY/JUNE 2016 www.insidegnss.com HMI events might be due to unex- Current Status of GNSS position and the pseudorange domains. pected local threats whose effects are for Maritime If a pseudorange or a position alarm is not covered by the error models used At the moment, only GPS, GLONASS, generated, the user is promptly warned for PL calculation. Essentially, this may and BeiDou are recognized as part of via a “do not use that SV” or “do not use happen for two reasons: the local threat the WWRNS. The European Commis- the system” flag, respectively, which is was not considered as a potential feared sion initiated the recognition process included in the DGNSS message. event when designing the algorithm for for Galileo, which is currently on-going. The rationale underlying this IM PL computation, or the threat was actu- Although already accepted as a com- concept is that the IM receiver will expe- ally taken into account but the adopted ponent of the WWRNS, these systems rience the same level of performance as error model is shown not to be adequate need proper augmentation systems to each user receiver in the RS coverage in that specific situation. perform the most critical operations, area. Therefore, such a concept is not Therefore, even if integrity at the i.e., navigation in harbor entrances, able to protect the user from possible user level is effectively provided by the approaches, and restricted waters. Now- local error sources. This is an inherent PL in nominal conditions and for all adays, the most used augmentation sys- limitation of all differential systems. the foreseen FEs, additional barriers tem is DGNSS. The possible use of SBAS The effects due to local error sources should be put in place against unex- is also being investigated in a number of can be detected by RAIM algorithms, pected situations. These barriers are a different research projects (e.g., see the which are able to identify possible outli- key element when a given operation is article by Kvam et alia). ers and to estimate the expected accura- not performed in a “controlled” envi- DGNSS. Historically, the predomi- cy (error ellipse) of the resulting position ronment (e.g., an airport), where the nant use of DGNSS arose from the solution. In general, these techniques actual levels of multipath or interfer- need for augmentation during the most can be used with both augmented and ence are kept below acceptable thresh- critical maritime operations, especially un-augmented pseudorange observa- olds. A harbor environment is typically when selective availability (SA) was tions and do not depend on the particu- not controlled; so, additional protection still in place for GPS. DGNSS provides lar augmentation system in use. schemes might be required. One of the the user with differential corrections In order to monitor the performance most effective barriers is the concur- (improved accuracy) and system integ- of a specific DGNSS service, IALA pro- rent use of location information from rity information. Differential correc- vides some recommendations, but the independent positioning schemes, as tions, together with the associated user scope of these guidelines is limited to actually recommended by the resilient differential range error (UDRE) indica- service provision. For example, signal PNT concept. tors, are evaluated at a reference station availability, A, is evaluated as: The IMO A.915(22) resolution indi- (RS), which is placed at a known loca- cates the operational requirements for tion. The corrections, together with the A = MTBO/(MTBO + MTSR) (1) a number of navigation phases and associated UDRE, are then broadcast to maritime applications. However, several the users in the RS coverage area using where MTBO and MTSR indicate the discussions with maritime community a dedicated medium frequency (MF) mean time between outages and the representatives confirmed that none of radio data link. mean time to service restoration, respec- the existing GNSS systems is designed The nominal accuracy of IALA tively. This recommendation covers both to fulfil this resolution and that no DGNSS beacons is typically between scheduled outages (e.g., due to planned maritime operations currently require three and five meters. The larger the maintenance activities) and unsched- compliance with it. The basis for the distance is between the user and the RS, uled outages (e.g., due to unexpected requirements indicated in the resolu- the larger the expected accuracy degra- failures). tion is, therefore, unclear. For example, dation (0.67 meters per 100 kilometers Similarly, service continuity, C, is one of the amendments under discus- from the RS, according to the IALA calculated as: sion is the operation duration, which recommendation on DGNSS services). may be reduced from three hours to 15 As mentioned, the DGNSS integrity C = e(−CTI/MTBF) (2) minutes to gain consistency with IMO concept is applied at the system level A.1046(27). only and is based on the assessment where CTI is the continuity time inter- As a result, the specifications cur- of the quality of corrections. Integrity val (i.e., the operation duration of 15 rently indicated in IMO A.915(22) can monitoring (IM) is performed with the minutes), while MTBF is the mean time only be viewed as a starting point for use of a separate DGNSS receiver, which between (unexpected) failures. future evolutions. These are expected is also placed at a known location, nor- If a user is aware of the lack of to target the harmonization with IMO mally a few hundred meters from the DGNSS service or that a scheduled out- A.1046(27) and the consolidation of the reference station to avoid possible cor- age is going to take place, the operation GNSS role in a resilient PNT system. relations of multipath events. will not be executed using DGNSS. For The integrity monitoring evaluates that reason, only unexpected failures the quality of corrections in both the are assumed to affect service continuity www.insidegnss.com MAY/JUNE 2016 InsideGNSS 57 WORKING PAPERS

Functionality DGNSS SBAS where the U.S. Coast Guard is currently Differential Corrections Yes: corrections are evaluated Yes: corrections are evaluated authorizing the Coast Guard vessels to (Accuracy) locally at each Reference at a central processing facility use the Wide Area Augmentation Sys- Station (RS) based on the monitoring tem (WAAS) as a complement to DGPS. station measurements. For that reason, SBASs might be effec- Quality of Corrections: Yes: UDREs for each Space Yes: UDREs (ephemeris and tively used to complement DGNSS, UDRE (System Vehicle (SV) are evaluated at clock corrections only) for thus increasing service continuity. Integrity) the RS and broadcast to the each SV and Grid Ionospheric SBAS. WAAS and the European users Vertical Errors (GIVEs) for each Ionospheric Grid Point (IGP) Geostationary Navigation Overlay Ser- are evaluated and checked at vice (EGNOS) are examples of SBASs. the central processing facility; These systems are able to provide, over a these are then broadcast to wide (or regional) area, the same type of the users. information offered by a DGNSS service, Quality of Corrections: Yes: additional checks in Yes: additional checks in the i.e., differential corrections and system check in the pseudo- the pseudo-range domain pseudo-range domain are integrity information, through the use range domain (System are done at the Integrity done at the central processing of additional, satellite-transmitted mes- Integrity) Monitoring (IM) station; if a facility and, locally, at each problem is detected for an SV, monitoring site; if a problem sages (SBAS messages). the user is warned not to use is detected for a SV, the user is SBASs are based on a network of that SV. warned not to use that SV. ground monitoring stations located at Quality of corrections: Yes: additional checks in the Yes: additional checks in the accurately surveyed points that monitor check in the position position domain are done at position domain are done at the signals of GNSS satellites. The sys- domain (System the IM station; if a problem is the central processing facility tem integrity is monitored by checking Integrity) detected, the user is warned and, locally, at each monitoring the quality of differential corrections in not to use the system. site; if a problem is detected, the user is warned not to use both the pseudorange and the position the system (e.g. see W. Werner domains. The measurements are col- et alia). lected and processed at a central facility Quality of corrections: Yes: it can be done at the Yes: it can be done at the user where the SBAS messages are created. autonomous user level based on RAIM level based on augmented These data are then uplinked to one consistency check at algorithms which verify the GNSS observations; for or more satellites (e.g., geostationary, the user level consistency of augmented example, the same RAIM GEO, satellites) and finally broadcast GNSS observations (e.g. algorithm, (UKOOA), UKOOA in Additional recommended for the DGPS to the end users in the SBAS coverage Resources); the result is an case can be used; only the area. error ellipse which describes corrections are different. Differential corrections and sys- the confidence levels in the tem integrity information provided by position estimation. WAAS, EGNOS, and other SBASs can, Table 2 A comparison of DGNSS and SBAS functionalities in principle, always be used to increase the position estimation accuracy and to and are, therefore, considered in the service provision will not be interrupted. protect the user from possible system- vice continuity equation. Note also that, Although service continuity might related failures. Therefore, depending according to the IALA recommenda- be a limitation in those areas where the on the application requirements, SBASs tion, all the mean times (MTBO, MTSR, user is able to receive differential correc- can be used to support a range of opera- and MTBF) shall be evaluated based on tions from one RS only (single coverage tions in various transportation domains, a two-year averaging period. areas), DGNSS is, at the moment, the including maritime. DGNSS is currently not formally incumbent GNSS solution for maritime recognized by IMO as a component applications. SBAS as a Complement of the WWRNS, primarily because of Some DGNSS Service Providers of DGNSS not being able to meet the service con- (SPs), from both public and private sec- Table 2 presents a summary comparison tinuity requirement. Service continu- tors, are already anticipating its long- of SBAS and DGNSS functionalities. ity performance might be improved by term goal of making the most of dif- Because both systems provide users with increasing the number of reference sta- ferent multi-frequency GNSSs. At the similar information, SBAS can be used tions, thus enlarging the areas (multiple same time, in order to reduce the sig- to effectively complement DGNSS. This coverage areas) where a user can receive nificant maintenance costs of a DGNSS option might be particularly attractive differential corrections from more than infrastructure, some SPs are also pro- in the maritime sector, where DGNSS is one RS. If a failure is affecting one RS, moting the use of complementary aug- currently the most used augmentation the user will still be able to receive the mentation systems. This is happen- system. As a matter of fact, SBASs are corrections from another; so, the ser- ing, for example, in the United States, potentially able to fill possible gaps in the

58 InsideGNSS MAY/JUNE 2016 www.insidegnss.com coverage area of a DGNSS service, thus Based on these considerations, two system integrity information, as well as providing the user with a “seamless,” possible strategies may be identified to navigation performance comparable to augmented, navigation solution. promote the use of SBAS in maritime. that of DGNSS. In particular, SBAS nominal per- The “mid-term” strategy consists of pro- Based on these considerations, SBAS formance has the potential to meet the moting the use of SBAS as a complement could be immediately promoted as a IMO requirements for a component or a back-up to DGNSS. In this case, the complement or a back-up to DGNSS. As of the WWRNS, as shown in Table 3, underlying integrity concept is at the indicated in Table 3, the interoperability where the operational requirements for system level only. The “long-term” strat- between SBAS and DGNSS is expected the most critical operations (navigation egy is, on the other hand, based on the to be beneficial especially for criti- in harbor entrances, harbor approaches, development of a novel integrity concept cal navigation phases of the WWRNS and coastal waters) are considered. designed in close cooperation with the (harbor entrance, harbor approach, and As mentioned, the opportunity of maritime community that will closely coastal waters). using SBAS as a complement for DGNSS follow the evolution of the resilient PNT The use of SBAS should be encour- has been already taken by the USCG, concept. The next two sections discuss aged through the development of stan- which has acknowledged the use of these strategies. dards that are specifically tailored for WAAS as an official augmentation sys- maritime. Table 4 describes some of the tem for GPS receivers. The USCG clearly ‘As Is’: The Mid-Term Strategy major issues associated with such an states that the “National Differential The mid-term strategy would use SBAS effort. For each aspect, the table lists the GPS (NDGPS) system and the WAAS “as is,” i.e., with no modifications to underlying problem (open issue) and are the only GPS corrections currently the SBAS message structure or to the outlines a possible solution. These (and authorized for Coast Guard vessel use in ground segment infrastructure. The other) issues will need to be investigated high-risk (e.g., restricted waters) naviga- strategy is based on SBAS capability of in close cooperation with the maritime tional zones/areas.” providing differential corrections and community. The objective is to make the

Operational Requirement for a Component of the WWRNS (IMO 2011) for the most stringent operation (Navigation in harbour entrances, harbour approaches and coastal waters) Performance Requirement SBAS Parameter “Where a radio-navigation system is used to assist in the navigation of ships in such Accuracy waters, the system should Yes: SBAS nominal accuracy is typically between 3–5 meters. provide positional information with an error not greater than 10m with a probability of 95%.” “An integrity warning of system malfunction, non-availability Yes: System integrity information is included in SBAS messages and provided in System Integrity or discontinuity should be accordance with a TTA requirement of 10 seconds. provided to users within 10s.” Yes: The signal availability is actually related to the provision of SBAS corrections. - This exactly matches the availability of at least one GEO satellite broadcasting the SBAS messages in the SBAS coverage area. “Signal availability should - Signal availability, A, can be calculated based on the relevant Signal Availability exceed 99.8%” (No requirement recommendations for DGPS (Equation 1), and normally exceeds the 99.8% about system availability) requirement. - For example, assuming an MTBO (over two years) of 103 h, the requirement is met if the corresponding MTSR (over two years) is below 1.67 h 1.5 h (Equation 1). Yes: The service continuity is actually related to the uninterrupted provision of SBAS corrections throughout the operation duration (15 minutes). - As for signal availability, service continuity, C, can be calculated based on the relevant recommendations for DGPS (Equation 2). “When the system is available, - In this case, the MTBF value is the mean time between two subsequent Service Continuity the service continuity should be failures, which completely prevents the system from SBAS messages ≥99.97% over a period of 15min.” broadcasting. These failures are so rare that the 99.97% requirement is normally met. - In particular, with CTI=15mi, the requirement is met if the MTBF (over two years) is larger than 833 h 103 h (Equation 2). Table 3 SBAS nominal performance versus the WWRNS operational requirements. www.insidegnss.com MAY/JUNE 2016 InsideGNSS 59 WORKING PAPERS

Aspect Open Issue Possible Solution Receiver Design No standard is currently available to unambiguously Especially for critical (e.g., SoL) maritime applications, a (SBAS message define SBAS message processing for maritime receivers. unique methodology should be standardized, where processing) As a result, receiver manufacturers are now developing the recommended processing is able to ensure the maritime products based on their own adaptation of provision of integrity at the system level. the processing recommended for aviation operations (RTCA). SBAS/DGNSS Nowadays, many commercial receivers for maritime A suitable method should be standardized to interoperability automatically switch from DGNSS to SBAS (and vice- unambiguously select (assuming both DGNSS and versa) without even telling the user what the actual SBAS are available) the most convenient augmentation augmentation system in use is. In both cases, the system. solution is simply marked in the receiver log file as a This will require a number of trade-off analyses. For differential position fix, with no further indication about example, the selection might be based on the distance the source of the associated differential corrections. between the user and the closest available DGNSS This uncertainty might lead to serious liability problems reference station and/or the number of SBAS ground in case of a legal controversy, as DGNSS and SBAS monitoring stations surrounding the user. are normally operated and maintained by different The standard should also identify how the user is authorities, which act as SPs. notified about the actual augmentation system in use. Receiver installation, Performance requirements, methods of testing and A dedicated standard, similar to that of the IEC, should antenna siting and required test results for ship-borne DGNSS equipment be developed for SBAS. Test Case Procedures are currently standardized by the International (TCPs). Electrotechnical Commission (IEC). The available standard provides recommendations for a number of aspects, including receiver installation, antenna siting, or mitigation techniques for noise and interference. Each of these aspects is complemented by the relevant TCP. Log files produced by a DGNSS receiver which is installed, tested, and operated according to the IEC standard can be used as evidence in a legal controversy. A similar standard is currently not available for SBAS. Information about Both scheduled maintenance activities and An SBAS normally includes a “Notice To Air Men” both scheduled unscheduled outages may affect the provision of a (NOTAM) service which provides aeronautical users with and unscheduled DGNSS service. Information about these outages should prompt alerts about both scheduled and unscheduled outages. be promptly communicated to the maritime users outages in the system. using suitable systems, such as coastal radio stations or The service is compliant with International Civil the Vessel Traffic Service (VTS). Aviation Organization (ICAO) recommendations for the According to IALA, the date and the expected Aeronautical Information Services (AIS) (see ICAO in downtime of scheduled maintenance shall be Additional Resources), and can be potentially re-used communicated at least one week in advance (one for maritime applications as well. month is recommended), while the expected However, proper communication links must be downtime for an unscheduled outage shall be established to broadcast these alerts to maritime users. communicated as soon as possible and, in any case, not more than one hour after the occurrence. A similar service should be put in place for SBAS when it is used to complement DGNSS. Table 4 Open issues for the use of SBAS as a complement of DGNSS (Mid-Term Strategy) best use of the available SBAS perfor- for various applications, the concept In a resilient PNT scheme, the use mance in compliance with the WWRNS might eventually allow for protection- of differential systems, such as SBAS operational requirements. level calculations at the user receiver, or DGNSS, is expected to significantly thereby offering integrity at the user increase the overall resilience level. These Maritime Integrity: level. systems are actually able to improve the The Long-Term Strategy This integrity concept is currently quality of GNSS observations and to The objective of the long-term strategy is expected to be developed in the frame- provide system integrity information. the development of an integrity concept work of a resilient PNT system, where For that reason, they immediately offer precisely tailored for maritime applica- (augmented) GNSS observations are a “certified” protection against possible tions. This concept may benefit from the used in conjunction with additional system-related failures. use of SBAS corrections, together with measurements available from other If augmented GNSS observations the relevant system integrity informa- terrestrial-based RNSs and/or from on- are then combined with additional, tion. Depending on the specific needs board inertial sensors (e.g., inertial mea- non-GNSS measurements, a much expressed by the maritime community surements units, or IMUs). more robust navigation solution can

60 InsideGNSS MAY/JUNE 2016 www.insidegnss.com be designed in which the user is also be unambiguously solved through stan- 2. IALA Guideline No. 1112 on Performance and protected from the effects of local error dardization activities, the use of SBAS as Monitoring of DGNSS Services in the Frequency Band 283.5 – 325 kHz, Edition 1, Saint Germain sources. As noted, these effects cannot a complementary augmentation system en Laye, France, May 2015 be mitigated by differential systems could be promoted in the medium term. 3. IALA “World Wide Radio Navigation Plan,” Edi- alone. Operational requirements for tion 2, Saint Germain en Laye, France, December At the moment, the high-level archi- future GNSS are indicated in the IMO 2012 tecture of a resilient PNT system is A.915(22) resolution. These require- 4. ICAO, “Aeronautical Information Services,” still being defined. However, it already ments call for the provision of integ- Annex 15 to the Convention on International appears clear that the development of rity at the user level as defined in this Civil Aviation, 14th Edition, Montreal, Quebec, a resilient PNT system will take into article. However, no GNSS system is Canada, July 2013 account future evolutions of SBAS and, designed to fulfil IMO A.915(22) in its 5. International Electrotechnical Commission, “Maritime Navigation and Radio-communication in general, of GNSS. Several aspects current version, and there are no mari- Equipment and Systems – Global Navigation Sat- (and trade-offs) need to be carefully time operations that require compli- ellite Systems (GNSS) – Part 4: Ship-borne DGPS evaluated. These will include, for exam- ance with it. and DGLONASS Maritime Radio Beacon Receiver ple, the use of dual-frequency multi- Depending on the actual needs Equipment – Performance Requirements, Meth- constellation (DFMC) measurements expressed by the maritime community ods of Testing and Required Test Results,” Refer- ence number IEC 61108-4:2004(E), First Edition, (together with the relevant corrections, for different applications, this integ- Geneva, Switzerland, July 2004 if available) and the method (i.e., loosely, rity concept might be developed in the 6. IMO, “Recognition of Galileo as a component tightly, or deeply coupled) used to com- framework of a resilient PNT receiver, of the WWRNS - Galileo GNSS provision of initial bine (augmented) GNSS observations where (augmented) GNSS observations services,” sub-committee on Navigation, Com- and inertial measurements. will be used in combination with other munications and Search and Rescue, 3rd Session, measurements available from terrestri- Agenda Item 5, NCSR 3/5, London, UK, December Conclusions and Way Forward al-based RNSs and IMUs. In particular, 10, 2015 The use of SBAS corrections in maritime this approach is expected to provide an 7. IMO, “Revised Maritime Policy and Require- ments for a Future Global Navigation Satellite receivers is currently not regulated. This increased robustness against the effects System (GNSS),” Resolution A.915(22), London, article discussed two possible strategies due to local error sources, which cannot United Kingdom, January 22, 2002 for the development of a dedicated stan- be tackled by differential systems. 8. IMO, “Report of the Maritime Safety Committee dard. These are based on the operational The resilient PNT strategy is one on its eighty-fifth session (MSC 85/26),” London, requirements specified by the IMO for of the technological enablers for the United Kingdom, December 19, 2008 a component of the WWRNS and for e-Navigation concept, which is currently 9. IMO, “Worldwide Radio-navigation System,” future GNSS, respectively. being promoted by the IMO to increase Resolution A.1046(27), London, United Kingdom, Operational requirements for a com- the safety of maritime navigation. The December 20, 2011 ponent of the WWRNS are indicated in evolution of this strategy is expected to 10. International Convention for the Safety of Life the IMO A.1046(27) resolution. These give due consideration to future develop- at Sea (SOLAS), London, United Kingdom, 1974 requirements call for the provision of ments of SBAS and, in general, of GNSS. 11. Kvam, P. E. and Jeannot, M., “The Arctic Test Bed – Providing GNSS Services in the Arctic integrity at the system level only and do For that reason, the European Space Region,” Proceedings of the 26th International not consider possible performance deg- Agency, in close collaboration with EC Technical Meeting of the Satellite Division of The radation due to local error sources. At and GSA, is also analyzing potential Institute of Navigation (ION GNSS+ 2013), Nash- the moment, only GPS, GLONASS, and benefits associated with new techniques ville, Tennessee USA, September 2013 BeiDou are recognized as components such as (augmented) DFMC schemes or 12. RTCA, “Minimum Operational Performance of the WWRNS. However, these sys- the horizontal-advanced receiver auton- Standards for Global Positioning System/Wide tems need augmentation to perform the omous integrity monitoring (H-ARA- Area Augmentation System Airborne Equip- ment,” RTCA DO-229D, Washington, D.C., Decem- most critical operations, i.e., navigation IM) method. ber 13, 2006 in harbor entrances, harbor approaches, 13. UK Offshore Operators Association (UKOOA), and coastal waters. Disclaimer Surveying and Positioning Committee, “Guide- DGNSS is the most used solution The views expressed in this article are lines for the Use of DGPS in Offshore Surveying,” for augmentation. It can provide both solely the opinions of the authors and do Issue 1, London, United Kingdom, September differential corrections and immediate not reflect those of the European Space 1994 alerts in the event of a system-related Agency. 14. USCG, “Coast Guard Navigation Standards failure. An SBAS is able to effectively Manual,” COMDTINST M3530.2E, Washington, D.C., March 2016 provide the same type of information. Additional Resources 15. Werner, W., Rossbach, U., and Wolf, R., “Algo- It could be, therefore, immediately pro- 1. Grant, A., Williams, P., Hargreaves, C., and Brans- rithms and Performance of the EGNOS CPF Inde- moted to complement DGNSS in all by, M., “Demonstrating the Benefits of Resilient pendent Check Set,” Proceedings of ION GPS 2000, the situations where augmentation is PNT,” Proceedings of the 26th International Techni- Salt Lake City, Utah USA, September 2000 cal Meeting of the Satellite Division of The Institute needed. Although some open issues (e.g., of Navigation (ION GNSS+ 2013), Nashville, Ten- SBAS/DGNSS interoperability) should nessee USA, pp. 598-604, September 2013 www.insidegnss.com MAY/JUNE 2016 InsideGNSS 61 WORKING PAPERS

Authors Marco Porretta received both has been managing GNSS activities for the Agen- the telecommunications cy since their inception in the early 1990s and Engineering M.Sc. and the continued his career with different functions at Ph.D. in information engi- various ESA sites in Europe Noordwijk, Nether- neering from the Univer- lands, Brussels and Paris. Amongst others, he sity of Pisa, Italy. He has organized the first EGNOS Trials at Sea in Genoa more than 10 years of in February 2000. professional experience in telecommunications Alessandra Fiumara is the liai- and navigation systems. This experience includes son officer on GNSS Evolu- the development of high-frequency methods tion Program for the Euro- for electromagnetic propagation analysis, deci- pean Space Agency. She sion support tools (DST) for air traffic manage- has been working in the ment (ATM), and integrity algorithms for Global space sector for 25 years, Navigation Satellite Systems (GNSS). Porretta both in public and private joined ESA ESTEC in September 2011, where he contexts, and has acquired a deep knowledge of is currently involved in projects and studies on the related political, strategic and financial GNSS, with particular attention to the EGNOS aspects. As electronic engineer, she started with and the Galileo programs. technical and scientific activities in the radar David Jiménez-Baños has application domain, then moved to strategic M.Sc. degrees in telecom- planning and financial control responsibilities as munications engineering well as international relations. At the end of 2009, and information and com- she moved from Rome to Paris to join the Navi- munication technologies gation and Galileo-related Activities Directorate from the Universitat at ESA. She is involved in the analysis on the pos- Politècnica de Catalunya. sible role of current and future GNSS systems in Since 2006 he has been working at the Radio transport safety domains (maritime and rail- Navigation Systems and Techniques section of ways). ESA/ESTEC, The Netherlands. His main areas of Prof.-Dr. Günter Hein serves as work have been on GNSS receivers, signal pro- the editor of the Working cessing for indoor applications, SBAS systems, Papers column. He served and GNSS simulation tools for receiver perfor- as the head of the EGNOS mance evaluation, and, more recently, GNSS and GNSS Evolution Pro- signal processing techniques for attitude and gram Department of the orbit control system (AOCS) applications in geo- European Space Agency stationary and higher orbits. and continues to advise on scientific aspects of Massimo Crisci is the head of the Navigation Directorate as well as being a Radio Navigation Systems member the ESA Overall High Level Science and Techniques Section at Advisory Board. Previously, he was a full profes- ESA. He is the domain sor and director of the Institute of Geodesy and responsible for the field of Navigation at the Universität der Bundeswehr radio-navigation. This München (UniBW), where he is now an ““Emeri- encompasses radio-navi- tus of Excellence.” In 2002, he received the gation systems for satellite, aeronautical, mari- Johannes Kepler Award from the U.S. Institute of time, land-mobile users (including indoor) appli- Navigation (ION). He is one of the inventors of cations, equipment/techniques/receivers for the CBOC signal. (hybrid satellite/terrestrial) navigation/localiza- tion for ground and space applications, signal- in-space design, end-to-end performance analysis for current and future systems. He is the head of an engineering team providing expert support to the various ESA programs (EGNOS, Gali- leo and their evolutions). He holds a Ph.D. in automatics from the University of Bologna and a Master’s degree in electronics from the Univer- sity of Ferrara. Giorgio Solari is currently serving as Head of GNSS/ Safety of Life Innovation & Evolution Strategy and External Coordination Office at the European Space Agency HQ in Paris. He has been working in the space sector for more than 30 years, serving ESA since 1988. He

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