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Worldwide Availability of Maritime Medium-Frequency Radio Infrastructure for R-Mode-Supported Navigation

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Worldwide Availability of Maritime Medium-Frequency Radio Infrastructure for R-Mode-Supported Navigation Journal of Marine Science and Engineering Article Worldwide Availability of Maritime Medium-Frequency Radio Infrastructure for R-Mode-Supported Navigation Paul Koch * and Stefan Gewies German Aerospace Center (DLR), Institute of Communications and Navigation, Kalkhorstweg 53, 17235 Neustrelitz, Germany; [email protected] * Correspondence: [email protected] Received: 10 January 2020; Accepted: 11 March 2020; Published: 18 March 2020 Abstract: The Ranging Mode (R-Mode), a maritime terrestrial navigation system under development, is a promising approach to increase the resilient provision of position, navigation and timing (PNT) information for bridge instruments, which rely on Global Navigation Satellite Systems (GNSS). The R-Mode utilizes existing maritime radio infrastructure such as marine radio beacons, which support maritime traffic with more reliable and accurate PNT data in areas with challenging conditions. This paper analyzes the potential service, which the R-Mode could provide to the mariner if worldwide radio beacons were upgraded to broadcast R-Mode signals. The authors assumed for this study that the R-Mode is available in the service area of the 357 operational radio beacons. The comparison with the maritime traffic, which was generated from a one-day worldwide Automatic Identification System (AIS) Class A dataset, showed that on average, 67% of ships would operate in a global R-Mode service area, 40% of ships would see at least three and 25% of ships would see at least four radio beacons at a time. This means that R-Mode would support 25% to 40% of all ships with position and 67% of all ships with PNT integrity information. The relatively high number of supported ships compared to the total radio beacon coverage of about 9% of the earth’s surface is caused by the good coverage of busy ports and areas such as the coast of China, North Sea and Baltic Sea. These numbers emphasize the importance of marine radio beacons for the R-Mode system. Keywords: R-Mode; backup system; radio-beacon coverage; maritime traffic; AIS; medium frequency 1. Introduction Maritime traffic today is, with respect to navigation, tightly coupled on one or several Global Navigation Satellite Systems (GNSS). Many bridge systems use position, navigation and timing (PNT) information provided by GNSS receivers for their data processing, synchronization or information to the mariner. Unfortunately, GNSS signals are very weak when they arrive at the user so they are prone to interferences [1]. Furthermore, the ionosphere degrades the GNSS performance [2]. In order to provide reliable PNT information, additional sensors have to be included [1,3]. One approach is to use, in addition to satellite-based absolute positioning, terrestrial sources for positioning. Here, the low-frequency navigation systems Loran-C/eLoran and Chayka were developed in the past to provide positioning over wide areas of the earth’s surface. Since several countries have started decommissioning Loran-C sites, it is very unlikely that it will again become a global system in the future. An alternative terrestrial approach is the R-Mode (Ranging Mode) [4]. Here, an existing maritime radio infrastructure will be synchronized with 10 ns accuracy with an external R-Mode reference time [5]. The radio infrastructure is modified so that it is able to broadcast R-Mode signals or messages, J. Mar. Sci. Eng. 2020, 8, 209; doi:10.3390/jmse8030209 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 209 2 of 14 which are carrying the R-Mode time information, together with the communication signal or messages of the legacy service. The user measures either the time of arrival of the R-Mode broadcast or the time difference of the arrival of the broadcast from two R-Mode stations and can from this calculate the distance or distance difference and finally its own position with a trilateration or hyperbolic lateration approach [6–8]. Transmitter sites considered so far are marine radio beacons [6,9] and base stations of the Automatic Identification System (AIS) [6,7,10] or the Very high frequency Data Exchange System (VDES) [11]. These systems share the common factor that they are well distributed along the shore of waters with high traffic density and/or challenging navigation conditions. The R-Mode system is designed as a backup for GNSS. It extends the positioning capabilities of the mariner with additional navigation signals, which are transmitted in different frequency bands and using different propagation paths as compared to GNSS. Furthermore, the R-Mode signals are received with higher signal strength by the user. They are therefore unaffected by the sources of GNSS signal disturbances. Additionally, the R-Mode is a cost efficient way to implement a terrestrial navigation system by reusing existing maritime radio infrastructure. Marine radio beacons are providing a differential GNSS (DGNSS) correction service with Medium Frequency (MF) broadcasts within a coverage of up to 550 km. Using this correction service enables the mariner to perform positioning with an accuracy better than 10 m [12], which is needed for sailing in coastal and restricted waters, harbor entrances, harbor approaches and inland waterways [13,14]. Any adaptation of marine radio beacons on the R-Mode service has to be done without disturbing the legacy service and equipment. One approach that makes use of the entire channel of the 500 Hz–1000 Hz bandwidth, which is dedicated to each radio beacon, is to add two continuous wave signals near the channel boundaries [15,16]. By estimating the phases of the two continuous wave signal components at full seconds, it is possible to measure the distance [15]. AIS is a communication system for exchanging relevant information between ships and base stations operating in the Very High Frequency (VHF) radio band around 162 MHz [17]. It uses a time-division multiple access method to separate the broadcasts of ships and ashore base stations. Each data slot has a length of about 27 ms. Depending on the antenna heights of the transmitter and receiver, the signals can usually be received up to a distance of several 10 km. R-Mode can be implemented on AIS using customized AIS messages, which could use up to five AIS slots [7]. AIS works with a 9.6 kbit/s data rate on two channels with a bandwidth of 25 kHz each. VDES is the successor of AIS. It includes AIS frequencies and services, and has additional channels of up to a 100 kHz bandwidth available for the communication [18]. It also usesthe time-division multiple access method with 2250 slots per minute. With VDES, data rates of 307.2 kbits/s are possible. Using VDES for R-Mode transmissions would significantly increases the R-Mode performance as compared to AIS [11,19]. Therefore, currently the VHF R-Mode activities concentrate on the usage of VDES. This paper focuses on the potential availability of marine radio-beacon signals for R-Mode positioning and support of the integrity evaluation of PNT information. It presumes that all marine radio beacons are upgraded to broadcast MF R-Mode signals that enable ranging [15]. Depending on the number of received R-Mode signals, the mariner’s multi-system ship borne radio navigation receiver (MSR) [20] is able to perform position and timing estimation based on R-Mode or even perform integrity checks of the PNT information. For the position and timing estimation, the signals of four R-Mode stations are necessary [5]. If additional information such as bearing, rough position or timing is available, three R-Mode signals might be sufficient. Any number of R-Mode signals can be used in the MSR for integrity checks of the PNT information. Due to the limited range of marine radio beacons of 50–550 km [21], the R-Mode will not cover the entire world. It is restricted to waters near the shore where higher requirements were defined by the International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) for a backup system [22]. The paper shows which services could be offered based on the areas analyzed to J. Mar. Sci. Eng. 2020, 8, 209 3 of 14 increase the safety of maritime transport with integrity checks, positioning with additional information and full R-Mode-based positioning. The density of maritime traffic in a region depends on many factors, e.g., geographical conditions and nearby important ports. The traffic is therefore far from evenly distributed. With AIS, a system exists that is mandatory for medium and large ships (e.g., merchant and passenger ships). These ships have to implement the International Convention for the Safety of Life at Sea (SOLAS), which means they are obliged to have an AIS transceiver on board [23]. Their transmission of dynamic and static ship data can be used to track single ships. Unfortunately, the range of the AIS transmission is limited to the line of sight. Therefore, only a combination of the received AIS messages from many AIS base stations ashore and AIS satellites enable the generation of a worldwide maritime traffic picture. Within this paper, a combined AIS dataset from shore and satellite AIS receivers is used to estimate the worldwide maritime traffic density and to combine it with the potential availability of the R-Mode service. This paper discusses only the marine radio-beacon component of the R-Mode system. A complete system with VDES R-Mode transmitter sites would increase the availability of the R-Mode further [6,8]. 2. Worldwide Potential Service Area for the Medium-Frequency R-Mode Due to the high demand in position and timing backup on a global scope, it is necessary to provide an overview of the possible service area of the R-Mode system using a worldwide coverage prediction.
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