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1 INTRODUCTION

Global Navigation Satellite Systems (GNSS), today

the primary source for position, navigation and time

(PNT) information for collision avoidance, navigation

and communication systems on-board of a vessel, are

highly vulnerable to unintentional and intentional

interference, e.g. jamming and spoofing. A backup

system is needed to overcome this threat and to

enable new applications with an increased need for

resilient PNT information.

Today only regional terrestrial navigation systems

like eLORAN, LORAN-C or Chayka are available. An

extension of them on a global scale is unlikely because

important countries or regions like USA and Europe

stopped the service a few years ago. Therefore,

another system is needed.

One option is to use so called Signals of

Opportunity (SoOP) broadcasted from existing

maritime radio infrastructure. Under considerations

are the signals of maritime radio beacons and land

based stations of the Automatic Identification System

(AIS) (Oltmann & Hoppe 2008). Both have in common

their good distribution of stations along the coastline

near the main maritime traffic routes.

The ranging mode (R-Mode) system makes use of

these SoOP. By transmitting synchronized ranging

signals from modified maritime radio beacons

(Johnson & Swaszek 2014a, Johnson et al. 2017) or AIS

base stations (Johnson & Swaszek 2014b, Hu et al.

2015) it could be shown that range and position

estimation with this approach is feasible.

It is generally assumed that the implementation of

additional ranging signal components on the legacy

SoOP is a cost effective way to setup a terrestrial

navigation system. However, fundamental research,

development, validation and standardization

activities are necessary to establish R-Mode as a

Availability of Maritime Radio Beacon Signals for R-

Mode in the Southern Baltic Sea

S. Gewies, L. Grundhöfer & N. Hehenkamp

German Aerospace Center (DLR), Neustrelitz, Germany

ABSTRACT: This paper presents an overview of the development of a terrestrial positioning system called

Ranging Mode (R-Mode) in the Southern Baltic Sea region which utilizes already existing maritime radio

infrastructure. Here, an R-Mode testbed is planned to be set up until 2020 that meets maritime user needs for

resilient PNT. First measurements of radio beacon signals on-board a vessel sailing in the Southern Baltic Sea

show the good availability of beacon signals in this region. A comparison of received signals with a coverage

prediction based on the nominal range of radio beacons shows the shortcoming of this approach and

emphasizes the need for more elaborated coverage predictions which consider all effects of medium frequency

wave propagation at day and night. In the measurements results the skywave has a major impact on the beacon

signal stability in the night. The time stability of the signal amplitude seems to be a good indicator for disturbed

reception conditions.

http://www.transnav.eu

the

International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 14

Number 1

March 2020

DOI:

10.12716/1001.14.01.21

174

recognized maritime backup system to GNSS (IALA

2016).

This paper presents the ongoing process of the

implementation of an R-Mode testbed in the Baltic Sea

and shows results of a signal strength measurement

campaign in the destination area.

2 R-MODE TESTBED IN THE SOUTHERN BALTIC

SEA

2.1 An area with high demand on resilient PNT

The Southern Baltic Sea is the region between

Germany, Denmark, Sweden and Poland which

exhibits a very high density of maritime traffic and on

the other hand challenging conditions with respect to

the risk of collisions and groundings. Tankers,

container freighters and bulk carriers cross on their

way along the main traffic route from east to west the

way of ferries sailing from north to south.

Furthermore, vessels have to stay in their lane to

prevent collisions with oncoming vessels on the

neighboring lane in traffic separation schemes.

Additional static infrastructure such as wind farms

reduces the maneuvering space nowadays and

generates the need for new navigation applications.

The Baltic Sea is generally a shallow water.

Especially the navigation in coastal areas, port

approaches and ports requires high quality of

navigational support (IMO 2002). To prevent any

traffic disturbance and environmental harm for this

ecological sensitive region resilient PNT information

is necessary.

2.2 Precondition of the region

Maritime administrations around the Baltic Sea are

aware of the challenges and implemented a dense

network of maritime radio beacons to support

differential GNSS (DGNSS) for accurate positioning

(better than 10 m) with integrity and continuity for

operation from coastal region to the berth (IALA

2015). Furthermore, a dense network of AIS base

stations allows sea traffic monitoring along the

coastline with a range of up to a few 10 kilometers

and can support navigation by provision of safety

relevant information from ashore.

Figure 1. Southern Baltic Sea surrounded by Poland,

Germany, Denmark and Sweden. Radio beacons are

indicated by dots and AIS base stations by triangles.

The operational DGNSS and AIS stations today are

shown in Figure 1. Due to the different signal main

propagation paths for the two systems, groundwave

for DGNSS and line of sight for AIS, the range of the

two station types differs and the AIS network is

denser to compensate for its smaller station service

area.

A precondition for the use of the SoOP for ranging

and positioning is the visibility of the signals. For this

reason the signal coverage was calculated based on

the given nominal range of radio beacons which could

be found on the IALA website, and for the AIS base

stations the calculation considers the height of a base

station antenna and a height of a ship antenna of 10 m

above sea level. The predictions are shown in Figure 2

and Figure 3.

Obviously the density of available SoOP depends

on the location in the Baltic Sea. The main east west

traffic route, which is in the middle between the

Swedish and German / Polish coast, is well covered

with overlapping radio beacon service areas of these

three countries and one Danish beacon. Instead the

coastline especially at the border of Germany and

Poland has highest density of AIS signals.

Figure 2. Number of available radio beacon signals

calculated with nominal range of beacons.

Figure 3. Number of available AIS base station signals

calculated with base station antenna height.

Depending on the R-Mode positioning approach

three or four SoOP have to be visible at the same time.

This number could not be reached by just one SoOP in

the entire Southern Baltic Sea. Both SoOP have to be

analyzed in more detail. Therefore, the project R-

Mode Baltic was initiated that will implement a

testbed with both SoOP in the Baltic Sea.

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2.3 Project R-Mode Baltic

Considering the results of former R-Mode activities

which were summarized by Hoppe (Hoppe 2018), in

2017 a European research and development project of

industry, national maritime administrations and

research institutions was started. It has the goal to

demonstrate that R-Mode is able to meet the maritime

user PNT requirement for a backup system in the

Southern Baltic Sea. An R-Mode testbed will be

implemented until 2020 (Gewies et al. 2018).

The R-Mode technology is in an early

development phase. First feasibility studies with

different demonstrators are performed but it is still

open which is the best way of implementing the

ranging signal into the legacy signal of radio beacons

and AIS base stations. The project team of R-Mode

Baltic will investigate in

− R-Mode signal development for radio beacons

which operate in the medium frequency (MF)

band and reduction of main error sources,

− R-Mode signal development for AIS base stations

which operate in the very high frequency (VHF)

band,

− the analysis of the impact of different error

components on the position,

− time synchronization methods for the R-Mode

transmitter sites,

− the maritime user requirement.

Furthermore, the project team will develop R-

Mode MF and VHF receivers and a VHF R-Mode

transmitter. To show the benefit of having R-Mode

signals available, an existing PNT data processing

unit will be extended by additional R-Mode

processing channels. This allows for automatically

switching to R-Mode based positioning in case of

unavailability of GNSS. A portable pilot unit will be

adapted so that it continuously provides a position

and warn the pilot about the reduced accuracy in case

of showing an R-Mode based position.

The findings and developments of the project will

be implemented in the R-Mode testbed. Up to six

radio beacons and four AIS base stations will be

upgraded so that they transmit synchronized R-Mode

signals. An R-Mode receiver and a PNT data

processing unit will be used for static and dynamic

validation of R-Mode.

In the following the focus is on a specific part of R-

Mode.

3 MF R-MODE

The MF R-Mode system uses maritime radio beacons

as a source for SoOP. The radio beacons operate in

Europe in the maritime band between 283.5 kHz and

315 kHz. Here, each radio beacon uses a channel of

500 Hz bandwidth and transmits data via MSK

modulated RCTM messages with a data rate of 100 or

200 bits per second.

To enable ranging using the MF signal, two

continuous wave (CW) signals are added to the

transmission, one 225 Hz below and one 225 Hz

above the carrier frequency. Thus, the original

capability of legacy receivers to use the beacon is not

impeded since the MSK DGNSS messages can still be

received.

Through phase estimation of each CW signal, the

time of arrival (TOA) can be determined. Due to the

relatively small wavelength of approximately 1 km in

comparison with the intended transmission range of

300 km, ambiguities in the phase estimate have to be

resolved. This can be achieved by using the beat

frequency of both CW signals. On top of that, the

MSK data transmissions can be used to assist

ambiguity resolution and error correction.

The ranging through phase estimation requires a

high time and oscillator stability as well as

synchronization of all R-Mode transmitters to

determine the TOA precisely. Thus, the transmitter

hardware has to be updated by using a precise source

of UTC.

The intended ranging accuracy of the R-Mode

system lies below 10 m. To achieve that, various

errors have to be mitigated.

Aside from the error introduced by clock

instabilities, the largest source of error is caused by

skywave propagation of the MF signal. The MF signal

propagates both as groundwave and skywave which

is reflected in a height of about 100 km at the E layer

of the ionosphere. Since the skywave takes a different

path on its way to the receiver it interferes with the

groundwave with a different phase at the receiver

antenna. This causes incorrect phase and thus

distance estimation. The effect is especially present at

night due to a weaker attenuation of the skywave by

lower layers of the ionosphere. The mitigation of

skywave interference is one of the main challenges on

the way to the implementation of MF R-Mode.

4 RADIO BEACON SIGNAL AVAILABILITY IN

THE SOUTHERN BALTIC SEA

A measurement campaign was performed to analyze

the availability of SoOP in the Southern Baltic Sea.

While sailing in the area between Poland, Germany,

Denmark and Sweden, measurements of the field

strength of all radio beacon signals in the maritime

band reserved for the DGNSS service were conducted

using the setup shown in Figure 4.

Figure 4. Used setup for field strength measurement.

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For the measurements an R&S HE010E monitor

antenna, which was fed with 24 V using a bias tee,

and an R&S FS1000 spectrum analyzer were used. In

order to enable a direct field strength measurement,

corrections for the bias and a conversion factor

(antenna factor) were stored in the FS1000. Each

minute the GNSS position and a spectrum, which was

calculated as the average over five spectra, were

recorded. In post-processing, signal peaks were

detected in the spectrum and assigned to known MF

radio beacons. The number of received beacons is

assigned to each measurement.

Figure 5 shows the track derived from the

measured position data of the polish research vessel

Nawigator XXI for four days in September. Here, the

track itself is drawn in grayscale with the intensity

indicating the number of received beacons from 0

(black) to 19 (light gray) at that location.

Figure 5. Track of Nawigator XXI drawn in grayscale, which

indicate the number of available radio beacons signals at

that position.

Figure 5 indicates that most of the time more than

four transmitters are visible, which is the minimum

value of required signals when using the TOA

positioning approach. Furthermore, the comparison

with Figure 2 reveals that the number of identified

beacons in the received spectrum deviates from the

number in the coverage prediction. Especially in the

western part of the track when the vessel was sailing

along the German coastline at daytime, the number of

received beacon signals is lower than the predicted

number. This can be explained with the stronger

attenuation of the signal caused by the lower ground

conductivity of the surrounding mainland and islands

which is not sufficiently reflected in the given

nominal range of the radio beacons.

Furthermore, Figure 5 shows a section with a

significantly increased number of received signals

several times in the track. This coincides with times

between sunset and sunrise. This can be explained

with the impact of the skywave. During the day only

those radio beacons were visible which were received

as groundwave within the range of the beacon.

During nighttime signals from radio beacons were

additionally received which are outside the

groundwave range but within the range of a reflection

at the ionosphere (skywave). This effect is not

considered in the coverage prediction of Figure 2.

Another way to look at the data is to plot the

distance and measured field strength over time for the

signal of one radio beacon. Figure 6 shows this in (a)

and (b) for the German transmitter Groß Mohrdorf.

The dotted lines indicate sunset and sunrise for these

days.

The effect of stronger attenuation of the signal is

clearly visible for larger distances. Furthermore, there

is an increase of the variance in the field strength

during nighttime. This is caused by the second

effective propagation path during the night, the

skywave. Because the ionosphere is stable for only a

few minutes, the phase and amplitude of the skywave

change frequently which cause a rapidly changing

superimposed signal at the receiver site.

Figure 6. Distance over time (a) and field strength over time

(b) for the German transmitter station Groß Mohrdorf. The

dotted lines indicate times of sunrise or sunset.

For times when the vessel was near the radio

beacon, the skywave has lesser impact as can be seen

in Figure 6 (a) and (b) for the first night. This could be

explained with the higher amplitude of the

groundwave and the unfavorable characteristic of the

signal propagation through the sky with respect to the

transmitter and receiver antenna pattern.

Figure 7. Field strength over distance for the German

transmitter station Groß Mohrdorf.

Figure 7 shows the field strength over the distance

for Groß Mohrdorf. As expected it has a linear

behavior for the field strength on a logarithmic scale.

For smaller distances, the variance is higher since

177

more measurements have been conducted at shorter

distances but at different locations. Here, different

propagation paths cause different attenuation of the

groundwave as described comprehensively by ITU-R

(ITU-R 2007).

Figure 8 shows the results for the Swedish radio

beacon Holmsjö. Again the subplot (a) shows the

distance from the beacon to the vessel and (b) shows

the field strength over time. Times at which no signal

was received appear as a gap in Figure 8 (a) and (b).

Figure 9 presents the relation between field strength

and distance.

Figure 8. Distance over time (a) and field strength over time

(b) for the Swedish transmitter station Holmsjö. The dotted

lines indicate times of sunrise or sunset.

The main difference between the German and

Swedish radio beacon is, that the Swedish is mostly at

a larger distance to the vessel. It is clearly visible in

Figure 8 that the signal can be received during day

only at distances below 250 km. But at nighttime the

visibility increases up to beyond 400 km which is

caused by the skywave (first and second night).

In the morning after the first night a drop in field

strength can be observed when the sky wave

vanishes. As Figure 7 already indicated, the skywave

effect becomes more relevant at larger distances. This

is also shown in Figure 9 where a large variation in

field strength appears when the field strength is

plotted over the distance. For lower distances up to

250 km there is continuous reception. For larger

distances the field strength does not follow the linear

behavior shown in Figure 7. This confirms the static

measurements presented by Hoppe (Hoppe 2018).

What does this mean for the R-Mode system? Due

to the fact that a CW superimposes with a delayed

copy of itself to a CW with the same frequency but

different phase the occurrence of the skywave is

difficult to determine. But these measurements can

help to develop an indicator for skywave disturbance.

As shown in Figure 6 and Figure 8 the scatter of field

strength in undisturbed conditions at the day is

significantly smaller than 10 dB whereas between

sunset and sunrise it is typically in the order of 10 dB.

The measurements were conducted with the legacy

MSK signal. They have to be repeated when complete

R-Mode signals with two CW are available in the

Southern Baltic Sea.

Figure 9. Field strength over distance for the Swedish

transmitter station Holmsjö.

5 CONCLUSIONS

This paper introduces R-Mode as a terrestrial backup

system for GNSS which can support mariners with

resilient PNT information for applications which

require continuous PNT data provision. The Southern

Baltic Sea is a predestinated area to implement R-

Mode on existing maritime radio infrastructure of

radio beacons and AIS base stations. Until 2020 a

testbed will be available that enables the

demonstration of the R-Mode system.

A measurement campaign was performed to

analyze the availability of radio beacon signals in the

testbed area. The comparison of measurement results

with the coverage prediction based on the nominal

range reveals deviation between the number of visible

radio beacons and the predicted number at day time.

This is a clear indicator that this approach for the

coverage prediction is insufficient. All different

ground types have to be considered for adequate

calculation of attenuation of the groundwave on the

way to the mariner and thus the signal availability.

During the night the skywave as additional

propagation path of the transmission disturbs the

measurement of the groundwave. Clearly visible is

the much higher number of available radio beacon

signals compared to the coverage prediction and an

increased scatter of the measured field strength.

Monitoring the scatter can help to identify time

periods of disturbed receiving conditions. One can

assume that at those times MF R-Mode has a reduced

performance.

ACKNOWLEDGEMENT

We thank the Maritime University of Szczecin for letting us

use the Nawigator XXI for the signal strength

measurements in the Baltic Sea.

The R-Mode Baltic project is co-financed by the European

Union through European Regional Development Fund

within the Interreg Baltic Sea Region Programme.

178

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