Ships nowadays greatly rely on Global Navigation Satellite Systems (GNSSs) in order to deter-

mine their position. Since GNSS outages or jamming events do occur, there are efforts to reduce

the dependency on GNSS for maritime navigation. One such effort is called R-Mode (Ranging

Mode), and focuses on complementing maritime communication systems by a ranging compo-

nent to enable a vessel to determine its position. One of the systems to be extended by R-Mode

is the VHF Data Exchange System (VDES). The VDES communication system is currently in

standardization and offers 100 kHz of bandwidth in the maritime VHF band. It utilizes sin-

gle carrier modulation with pi/4-QPSK. The proposed R-Mode extension works by sending a

precisely timed known data sequence, so that time of arrival estimation allows determination

of the range. Using software defined radios (SDR), we implemented a test setup for VDES R-Mode with three base stations on land and one receiver located on a vessel. Using this setup,

we performed the first VDES R-Mode positioning trials on the Lake Ammer in Germany. By

determining the time of the arrival as well as the Doppler shift of the received signals we tracked

the vessels position with an Unscented Kalman Filter. The positioning accuracy performance

ranged to up to 22 m under favourable conditions. Crucial was the consideration of the Doppler

measurements to enhance tracking performance considerably

Dammann, Armin

Raulefs, Ronald

Wirsing, Markus

Institute of Transport Research:Publications

On the Positioning Performance ofVDES R-ModeMarkus Wirsing1 and Armin Dammann1 and Ronald Raulefs11German Aerospace Center (DLR)(E-mail: markus.wirsing@dlr.de)Ships nowadays greatly rely on Global Navigation Satellite Systems (GNSSs) in order to deter-mine their position. Since GNSS outages or jamming events do occur, there are efforts to reducethe dependency on GNSS for maritime navigation. One such effort is called R-Mode (RangingMode), and focuses on complementing maritime communication systems by a ranging compo-nent to enable a vessel to determine its position. One of the systems to be extended by R-Modeis the VHF Data Exchange System (VDES). The VDES communication system is currently instandardization and offers 100 kHz of bandwidth in the maritime VHF band. It utilizes sin-gle carrier modulation with π/4-QPSK. The proposed R-Mode extension works by sending aprecisely timed known data sequence, so that time of arrival estimation allows determinationof the range. Using software defined radios (SDR), we implemented a test setup for VDES R-Mode with three base stations on land and one receiver located on a vessel. Using this setup,we performed the first VDES R-Mode positioning trials on the Lake Ammer in Germany. Bydetermining the time of the arrival as well as the Doppler shift of the received signals we trackedthe vessels position with an Unscented Kalman Filter. The positioning accuracy performanceranged to up to 22m under favourable conditions. Crucial was the consideration of the Dopplermeasurements to enhance tracking performance considerably.KEYWORDSVDES. R-Mode. Positioning.1. INTRODUCTION. As maritime navigation nowadays strongly relies on Global Nav-igation Satellite Systems (GNSS), and alternatives such as LORAN are progressively beingdismantled, there are concerns about being dependent on a single system, which providesaccurate position information but is not free of failures (Sadlier et al., 2017). Thus, there isa need for a backup system that can be used in case of failure or jamming of the satellitesystems. One such effort to provide a backup system is called R-Mode (Ranging Mode).R-Mode consists of two parts: One part that relies on Medium Frequency (MF) signal radi-ated by the IALA beacon system (Johnson and Swaszek, 2014), and another part that relieson the VHF Data Exchange System (VDES) that is currently in standardization (IALA,2020). In this paper, we will consider the VDES part of R-Mode.In Wirsing et al. (2020), we investigated the design of a suitable signal for VDES R-Mode, and in Wirsing et al. (2021), we presented an approach for the position estimationusing a Kalman filter, as well as first results from our experiments on Lake Ammer. In thispaper we will present more detailed results from the experiments.2 M. WIRSING, A. DAMMANN, R. RAULEFSThe paper is structured as follows: In Section 2 we give an overview of the VDES R-Mode system. We describe our experiments on Lake Ammer in Section 3, and discuss theresults in Section 4. In Section 5 we summarize our findings.2. THE VDES R-MODE SYSTEM. VDES is a new digital maritime communicationsystem that comprises the existing Automatic Identification System (AIS), a satellite com-ponent, and a terrestrial component (IALA, 2017). The terrestrial component is intended tocover the same areas that are currently covered by AIS.Terrestrial VDES will utilize a linear modulation with a bandwidth of 100 kHz (IALAGuideline G1139: THE TECHNICAL SPECIFICATION OF VDES, ed. 3, 2019). In com-parison to that, AIS uses Gaussian Minimum Shift Keying (GMSK) modulation, and abandwidth of 25 kHz. The higher bandwidth gives VDES a significant advantage over AISwhen considering its suitability for ranging purposes (Safar et al., 2020; Wirsing et al.,2019).The concept of VDES R-Mode is to extend the VDES communication signal by a rang-ing component. For this ranging component, the VDES base stations will regularly transmita precisely timed ranging signal that consists of a pre-defined data sequence. Apart from notutilizing channel coding, the ranging signal should conform to the physical layer specifica-tion of the VDES communication system. Maritime vessels can obtain an estimate on theirposition by estimating multiple ranges to shore based R-Mode transmitters, and performingmultilateration.The data sequence used for ranging in VDES R-Mode consists of two concatenatedparts: An alternating sequence, and a pseudo-noise sequence based on a Gold code (IALA,2020). This design was chosen due to considerations about the achievable ranging perfor-mance in different Signal to Noise Ratio (SNR) ranges (Wirsing et al., 2020). A parameterγrepresents the fraction of the alternating sequence of the total sequence, and the remain-der of the sequence uses the Gold code. The advantage of the alternating sequence is thenarrower main peak of the autocorrelation, but comes with a the drawback of stronger side-lobes. Therefore, depending on the SNR the joint sequence can be adapted to the localconditions.3. R-MODE EXPERIMENTS ON LAKE AMMER. In order to experimentally deter-mine the achievable performance of a VDES R-Mode implementation, we performedmeasurements on the Lake Ammer. Three transmitter stations were placed on the shoreof Lake Ammer, and a receiver was placed on a boat of the Wasserwacht (water rescue).The transmitters were mounted on portable masts, with a height of approximately 10m.A transmitter station is shown in Figure 2a The mounting of the receive antenna is shownin Figure 2b. The location of the transmitters was chosen, so that a favourable geometrywould be achieved in the northern part of Lake Ammer. Specifically, the transmitters werelocated in the villages Utting, Schondorf and Buch.The transmitter hardware consisted of a USRP B210 Software Defined Radio (SDR),a power amplifier, and a laptop to control the SDR. The SDR at each station was alsoconnected to a GNSS receiver that provided an accurate timing signal. Notably, the receiverstation on the boat was also equipped with a GNSS receiver that was used as a timing sourcefor the SDR as well as for providing a position reference. This means that, differing fromVDES R-MODE POSITIONING PERFORMANCE 3Figure 1.: Transmit schedule of the VDES base stations.(a) Transmitter station with VDES trans-mitter antenna(b) VDES R-Mode receive antenna and GNSS receiver antenna on theboatFigure 2.: Transmitter and receiver setup at Lake Ammer.what is planned in the R-Mode standard, our receiver also had access to a stable timingsource.Each transmitter station transmitted R-Mode ranging sequences as described in Wirsinget al. (2020), with γ = 13 . Each of the stations transmitted once per second, during anassigned timeslot. Even though VDES uses a transmit power of 12W, the transmit powerin our experiment was limited to 1W due to regulatory constraints. The transmit scheduleis shown in Figure 1.We chose a maximum likelihood approach to evaluate the received signal. The time ofarrival estimate τ̂ Doppler shift estimate ∆f̂ were obtained by:(τ̂ ,∆f̂) = argmaxτ,∆f∑kx[k] · s∗(kTs − τ) ej2πkTs∆f (1)with x[k] being the received samples from the software defined radio receiver, and s(t)being the transmitted signal. A numerical optimization method was used to evaluate theargmax operation. From the estimated time of arrival, a range r was estimated as r̂ = c0 ·(τ̂ − τoffset). With c0 being the speed of light, and τoffset being an experimentally determinedoffset parameter.4 M. WIRSING, A. DAMMANN, R. RAULEFS0 2 4 6 8 10 12 14 16 18 20 22 24 2601,0002,0003,0004,000Time [minutes]Range[m]BuchSchondorfUttingGNSS reference(a) Ranges0 2 4 6 8 10 12 14 16 18 20 22 24 26−20−15−10−5051015Time [minutes]Radialvelocity[m/s]BuchSchondorfUttingGNSS reference(b) Radial velocitiesFigure 3.: The range and radial velocity measurements for the scenario with good line ofsight and geometry. The associated position estimation is shown in Figure 4.After determining the ranges and Doppler shifts, an Unscented Kalman Filter (UKF) wasused to track the boats position. The UKF filter was initialized with the position and velocityfrom the GNSS reference. This approach allowed us to utilize the observed Doppler shiftin the tracking (Wirsing et al., 2021). In order to assess the difference in achievable rangingperformance, the evaluation was performed once with the Doppler measurements takeninto account, and once with the Doppler measurements disregarded. The results are shownin Figures 4,6, and 9.A number of scenarios were considered for the experiments:1. A line of sight scenario with favourable geometry.2. A scenario with the signal being shadowed by land areas.3. A scenario with a passenger ship blocking the line of sight to one of the transmitters.Figure 7 shows the passenger ship next to the boat with our receiver.For the first scenario, we chose a path that generally stayed within or close to the trianglespanned by the three transmitter stations. The path of the boat and the result of the trackingin that scenario are shown in Figure 4a. The path starts south of Utting and continuestowards the Buch station. Notably, the path of the boat also included a number of rathertight circles, which are shown in detail in Figure 4b. The associated ranges between the boatand the receiver are shown in Figure 3a, and the radial velocities in Figure 3b. It can be seenthat the measurements for the Utting station show significant outliers at the beginning ofthe scenario. This is likely due to non line-of-sight conditions caused by buildings. Anotherinteresting aspect of this scenario is the inclusion of tight turns, such as those shown inFigure 4b. It can be seen that they could only be resolved when the Doppler measurementswere included in the evaluation.The second scenario we considered, took us further away from the base stations and intothe bay near Herrsching on the eastern part of Lake Ammer. Figure 5 shows the measuredrange and Doppler values for this scenario. The scenario consisted of a route that startedeast of Schondorf and continued southwards and eastwards towards the bay. When enteringthe bay, the line-of-sight contact to Buch was lost. This can be seen in Figure 5a at aboutVDES R-MODE POSITIONING PERFORMANCE 511.100 11.110 11.120 11.130 11.14048.01548.02048.02548.03048.03548.04048.04548.05048.055BuchSchondorfUttingLongitude [°]Lattitude[°]Lake Ammer OutlineTracked R-Mode position(ranges only)Tracked R-Mode position(Ranges and Doppler)GNSS reference trackR-Mode base stations(a) Tracked positon11.109 11.110 11.111 11.112 11.11348.02648.02748.02848.02948.030Longitude [°]Lattitude[°](b) Position detailFigure 4.: The tracked position of the boat in the scenario with generally good line of sighttowards the station and with favourable geometry. In (b), it can be seen that the circles inthe boat’s path could only be resolved when the Doppler measurements were utilized.0 2 4 6 8051015Time [minutes]Range[km]BuchSchondorfUttingGNSS reference(a) Ranges0 2 4 6 8−1001020Time [minutes]Radialvelocity[m/s](b) Radial velocitiesFigure 5.: The range and radial velocity measurements for the scenario where the line ofsight is blocked by land. The associated position estimation is shown in Figure 6.7 minutes of elapsed time. In Figure 6, it can be seen that the R-Mode position estimatestarted to diverge from the true position when the line-of-sight was interrupted.The third scenario included a passenger ship that is serving a scheduled service on LakeAmmer. The goal of the scenario was to determine whether the presence of the passengership within the line-of-sight path has a noticeable impact on the positioning performance.Figure 7 shows the passenger ship and the boat with our receiver next to each other. For thismeasurement scenario, we accompanied the passenger ship on its scheduled route betweenHerrsching and Stegen at the northern end of the lake. The ranges and relative velocities areshown in Figure 8. It can already be seen here that the measurements show several outliers.6 M. WIRSING, A. DAMMANN, R. RAULEFS11.090 11.100 11.110 11.120 11.130 11.140 11.150 11.160 11.17047.98047.99048.00048.01048.02048.03048.04048.050 BuchSchondorfUttingLongitude [°]Lattitude[°]Lake Ammer OutlineTracked R-Mode position(ranges only)Tracked R-Mode position(Ranges and Doppler)GNSS reference trackR-Mode base stationsFigure 6.: The position estimate for the scenario with shadowing. The boat travels south-wards. It can be seen that the position estimate becomes unreliable once the line of sighttowards Buch is interrupted. Especially the estimates relying on the range only diverged tooutside the scope of this figure. The associated range measurements can be seen in Figure 5.Figure 9 illustrates this even further. On several occasions, the estimated track of the boatsignificantly deviates from the reference track obtained from the GNSS receiver.To compare the performances achieved in the different scenarios, with and without theDoppler measurements, Figure 10 shows a cumulative distribution function of the position-ing error that was achieved during our trials. For the first scenario, the beginning of the data,where non line-of-sight conditions where observed, was disregarded for the calculation ofthe CDF, so that the first scenario represents line-of-sight conditions. The other two scenar-ios are shown in full, so that the effect of the non line-of-sight conditions are included. Inthe LOS scenario, a 95th percentile error of 22m was achieved when the Doppler measure-ments were utilized. When only the range measurements were used, 144m were achieved.In case of the scenario with the passenger ship, the error was below 84m 95% of the timewith the Doppler data, and below 443m without the Doppler data.4. DISCUSSION. Our measurement results show that it is possible to achieve a goodpositioning accuracy using VDES R-Mode under line-of-sight conditions when utilizingthe Doppler effect measurements. In our first measurement scenario, line-of-sight con-ditions were present most of the time. When only the line-of-sight part of the data wasconsidered, a 95th percentile positioning error of 22m was achieved.The two scenarios with obstructions in the line-of-sight path showed that environmentalfactors such as other ships need to be considered when implementing an R-Mode system.VDES R-MODE POSITIONING PERFORMANCE 7Figure 7.: The boat with our receiver, next to a larger passenger ship. The ship served asan obstacle in the line of sight path between the boat and the land stations.0 2 4 6 8 10 12 14 16 18 20 22 24 2601,0002,0003,0004,0005,0006,0007,0008,000Time [minutes]Range[m]BuchSchondorfUttingGNSS reference(a) Ranges0 2 4 6 8 10 12 14 16 18 20 22 24 26−20−1001020Time [minutes]Radialvelocity[m/s]BuchSchondorfUttingGNSS reference(b) Radial velocitiesFigure 8.: The range and radial velocity measurements for the scenario where the line ofsight is blocked by another ship. The associated position estimation is shown in Figure 9.This seems especially relevant in environments close to harbours, where an accurate posi-tion information is vital, and at the same time many environmental factors such as otherships are present. In case of a harbour located in a bay, receiving VDES stations locatedelsewhere can become challenging if buildings are located in between the transmitter andthe receiver.8 M. WIRSING, A. DAMMANN, R. RAULEFS11.120 11.125 11.130 11.135 11.14048.03048.03548.04048.04548.05048.05548.06048.06548.07048.07548.080BuchSchondorfUttingLongitude [°]Lattitude[°]Lake Ammer OutlineTracked R-Mode position(ranges only)Tracked R-Mode position(Ranges and Doppler)GNSS reference trackTrack of passenger shipR-Mode base stationsFigure 9.: The position estimate for the scenario with shadowing by a passenger ship andthe track of the passenger ship. The boat travels northwards. The associated range measure-ments can be seen in Figure 8.10−1 100 101 102 103 10400.20.40.60.81Positioning error [m]CDFWith DopplerWithout DopplerScenario 1Scenario 2Scenario 3Figure 10.: Error CDF for the different scenariosThe influence of the Doppler measurements on the achievable accuracy was consider-able in all of the considered measurement scenarios. The difference was especially high inthe third considered scenario with a larger ship in the vicinity of the receiver.VDES R-MODE POSITIONING PERFORMANCE 9Our measurements still differed from how a finished R-Mode system would be imple-mented. One important difference was that our receiver was time synchronized to thetransmitter stations by means of GNSS. Due to the nature of a GNSS backup system, aproduction R-Mode receiver will not have that option. When the synchronization is notavailable, the time offset of the local clock will need to be estimated from the received sig-nal. Another difference was that our transmit power was limited to 1W, while VDES canuse 12W according to the standard. This means that the performance could be improvedby utilizing the full transmit power.5. CONCLUSION. We performed experiments on Lake Ammer to practically evaluatethe performance of VDES R-Mode, using range and Doppler measurement data, We foundthat utilizing the Doppler measurements can improve the performance by a factor of 5 to6. In our experiments, we were able to achieve a 95th percentile accuracy of 22m underline-of-sight conditions. In the more challenging scenario with the passenger ship close tothe receiver, 84m were achieved.Acknowledgment We would like to thank the staff of the Wasserwacht Utting for theirvaluable support during the experiment at Lake Ammer.REFERENCESIALA (2017), ‘IALA guideline g1117: VHF data exchange system (VDES) overview ed.2.0’.URL: https://www.iala-aism.org/product/vhd-data-exchange-system-vdes-overview-1117/IALA (2020), ‘Iala guideline g1158: VDES R-Mode ed. 1’.URL: https://www.iala-aism.org/product/g1158-vdes-r-mode/IALA Guideline G1139: THE TECHNICAL SPECIFICATION OF VDES, ed. 3 (2019).URL: https://www.iala-aism.org/product/g1139-technical-specification-vdes/Johnson, G. and Swaszek, P. (2014), Feasibility study of R-Mode using MF DGPS transmissions, Technicalreport, Accseas Project.URL: https://www.iala-aism.org/content/uploads/2016/08/accseas_r_mode_feasibility_study_mf_dgps_trans-missions.pdfSadlier, G., Flytkjær, R., Sabri, F. and Herr, D. (2017), The economic impact on the UK of a disruption toGNSS, Technical report, Innovate UK.URL: https://www.gov.uk/government/publications/the-economic-impact-on-the-uk-of-a-disruption-to-gnssSafar, J., Grant, A., Williams, P. and Ward, N. (2020), ‘Performance bounds for VDES r-mode’, Journal ofNavigation 73(1), 92114.Wirsing, M., Dammann, A. and Raulefs, R. (2019), Investigating R-Mode signals for the VDE system, in‘OCEANS 2019 MTS/IEEE SEATTLE’, pp. 1–5.Wirsing, M., Dammann, A. and Raulefs, R. (2020), Designing a ranging signal for use with VDE R-Mode,Proceedings IEEE/ION Position, Location and Navigation Symposium (PLANS), pp. 822–826.Wirsing, M., Dammann, A. and Raulefs, R. (2021), VDES R-Mode Performance Analysis and ExperimentalResults.URL: https://doi.org/10.1002/sat.1424

On the Positioning Performance of VDES R-Mode

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On the Positioning Performance of VDES R-Mode

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