303

1 INTRODUCTION

El Maritime transport depends inter alia on navigation

systems and communications equipment for safe

navigation. However, due to the technological

advances that are coming to ships in these sectors [13],

students in the grades or masters of this maritime

sector must have good training.

In all areas of the maritime sector, port

management, maritime transport... require a qualified

staff[3].Therefore, the training must offer a number of

qualities and skills to obtain great professionals in the

sector. All this with the existence of a solid foundation

in the formation.

Therefore, when theoretical classes are a bit

complicated to understand, we must put forward

different proposals to solve the problem.

Consequently, students do not understand complex

concepts as the process of moving from working with

independent teams to doing it together, this project

was proposed. Where the exchange of information on

a navigation bridge is carried out in a continuous

manner.

This proposal, which follows the previous study

[14], presents some practices on theory, through the

use of low-cost devices available on the market. With

only a small investment of the order of 20 or 30 € and

the use of a PC, which in this case can be the student's

own personal computer.

In order to carry out the practice we need a receiver

capable of tuning the AIS frequencies, the PC, and

Shareware or Freeware software.

Visualization, Vulnerability and Manipulation in

Internal Communications Systems on the Navigation

Bridges

R.E. Rey-Charlo & F.J Visglerio-Varo

University of Cádiz, Cádiz, Spain

ABSTRACT: This paper is the continuation of a project previously presented for the training of students in both

the Bachelor's and Master's degrees in Radio Engineering. The proposal makes it possible to visualise and

understand how navigation equipment shares information on board a ship, using low-cost devices. Through an

SDR receiver, free software (SDA, ShipPlotter, OpenCPN) and minimal investment, students can receive AIS

transmissions, decode them, get their NMEA frames to view over electronic letters. This practice recreates an

ECDIS environment in a PC, promoting self-learning, technical curiosity and innovation. In addition, it explores

the possibility of generating false NMEA frames to demonstrate the vulnerability of the system, reinforcing the

importance of information integrity in navigation. The project demonstrates how, without costly equipment, it is

possible to assimilate complex concepts into communication and navigation systems, reinforcing the motivation

and active learning of students.

http://www.transnav.eu

the International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 20

Number 2

June 2026

DOI: 10.12716/1001.20.02.06

304

2 DESCRIPTION

This project focuses particularly on understanding

how navigation equipment (GPS, AIS, ECDIS, ETC)

share information. This requires knowledge of the

NMEA standard, as well as IEC 61162-1 [18] and IEC

61162-2 [19] standards, which define technical aspects

such as wiring and data format.

An economical practice is proposed that uses a low-

cost receiver capable of tuning marine VHF channels

87B and 88B, along with an appropriate PC and

software, to receive real-time AIS information. The

signal received, initially in audio format, will be

digitally processed using free or test software to obtain

NMEA frames with static, dynamic and navigation

data from ships, coastal stations and digital aids.

This information can be viewed on electronic charts,

offering a complete view of the maritime environment.

However, it is noted that reliability depends on the

veracity of the data issued, as there is no control over

its origin.

Finally, students are sought to understand how to

share these NMEA frames between on-board

computers, either via USB converters to RS422/RS485

[16] or within a single PC, simulating a basic ECDIS

system and encouraging practical learning of internal

communications on a government bridge.

3 MATERIALS USED IN PRACTICE

3.1 Receptor software o SDR

For reception in the channels, 87B and 88B in VHF

assigned to the AIS system we will use a low cost radio

receiver capable of receiving the channels of maritime

communications assigned to the maritime mobile

service. The receiver to be used is a receiver initially

designed as a dongle for receiving digital terrestrial

television on a PC. Nowadays they are popularly

known as RTL-SDR, and allow changing their

performance only by installing a different driver to the

one needed to demodulate transmissions of digital

terrestrial television or TDT. In this simple way, they

can be transformed into a Radio Device Designed by

Software or SDR [9]. The installation of free radio

software also offers the possibility of receiving

transmissions ranging from the OM band to UHF, and

all this thanks to the joint work of two integrated

circuits well known by radio engineering students, the

popular CI tuner R820T/2/R860 [11] among others

together with the IC demodulator RTL2832U [12].

Both circuits in combination and constant

communication constitute a radio receiver in full rule

and completely break with the concept of the

traditional hardware superheterodyne receiver where

functionality depended on physical elements such as

filters, amplifiers, demodulators etc.

On the contrary, the SDR receiver works basically

and in a very small way converting the received RF

signal to a lower frequency, called FI, which allows its

sampling in a digital analog converter, ADC, with its

own limitations imposed according to Nyquist

sampling theorem. In this way the received signal

passes from analog to digital once it has been

quantified and encoded.

Figure 1. Block Diagram RTL-SDR

Part of this work is carried out in the first stage of

the device, and more specifically in the tuner IC, where

the received signal is amplified, filtered and mixed

with the signal generated by the local oscillator in order

to obtain the FI. Once the FI is amplified and the

unwanted components are removed, it is driven to the

demodulator circuit to be scanned.

The FI signal already in the demodulator is first

sampled and quantified before passing through a

square modulator [6] decomposing it into two new

signals, known as phase I signal and Q-square signal.

Later these two signals are decimated into a

downstream digital converter [21], DDC, to move the

digital signal of interest or FI to a lower frequency by

sampling at a lower frequency than required according

to Nyquist's theorem. In this way, the initial FI signal is

broken down into two new baseband signals for

further treatment.

Figure 2. Circuit RTL-SDR

After obtaining I and Q signals, and how to expect,

only the digital signal processor or DSP [5, 10] analysis

remains. Processing this data using specific software

allows reverse engineering operations by being able to

emulate filters, demodulators, detectors etc. without

the need for specific hardware.

Figure 3. Physical device SDR

305

As this type of device is initially designed to operate

with digital terrestrial television and not beyond the

frequencies used for this purpose, it is therefore

completely necessary to change its initial performance.

Thus converting them into radio frequency receivers,

which is possible by switching the original driver to a

free driver known as Zadig.

3.2 Driver Zadig, software SDRConsole y audio VB-

CABLE Virtual

Installing this driver is relatively simple and simply

connect the USB device, cancel the Windows

installation itself, and manually install the driver

Zadig[22].

Figure 4. Software Zadig 1

Figure 5. Software Zadig 2

After the installation of the Zadig driver, the USB

device will function as a radio receiver capable of

tuning stations from 500 kHz. up to 1.7 GHz.

approximately. At this point, there is therefore a radio

receiver capable of replacing a commercial AIS

receiver, if the appropriate antenna is used at the

frequency to be received.

The control of this physical device must be carried

out by installing software capable of controlling on the

one hand the receiving zone composed of the tuner

R820T/2/R860, and on the other hand, the digital signal

processor composed of the demodulator RTL2832U.

Although there is a wide variety of free software

capable of managing the receiver proposed for this

practice, we will decline by SDR Console (v3) [15] due

to its pleasant and complete graphical interface that

allows to visualize both the receiving stage and the

signal processing simultaneously.

Figure 6. Receiver

Figure 7. Mode

Figure 8. Filter

Figure 9.Received Signal

At this point in practice, it is already possible to hear

any audio broadcast that our receiver is able to tune,

always limited by the antenna used. In the case of VHF

band frequencies, we will use for practice a small

telescopic antenna, although we could use one

specifically calculated for the reception of the channels

of our interest 87B and 88B, that is, the frequencies

161,975 MHz and 162,025 MHz respectively.

By tuning the frequencies mentioned above, it is

already possible to receive transmissions from nearby

ships from the AIS, which will be perceived as

repetitive sound pulses impossible to understand by

the human ear, although carriers of digital information.

To extract this information it is necessary to use a

system capable of analyzing that sound signal and

transforming its frequency variations into "0" and "1"

logical.

This work will be carried out by internally injecting

the audio obtained from the AIS channels into some

software capable of carrying out this arduous work.

To inject the audio internally and avoid the

annoyance of hearing it we will choose to install a

virtual audio output. It will be done by installing the

free VB-CABLE Virtual Audio Device utility [20],

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which gives the SDA/Onsole receiver a new audio

output from the existing ones on the PC, and allows it

to be directed internally without the need for a physical

connection.

3.3 Software shareware ShipPlotter

The extraction of the digital information of interest will

be done by installing one of the many software

available on the network.

We have opted for ShipPlotter [4] test software for

this work. The function therefore is to analyze the

audio signal that arrives, the detection of the frequency

changes present in it, the transformation of these

changes into binary states and the grouping of the "0"

and "1" into frames that comply with the norm

NMEA0183

Figure 10. Software shareware Ship Plotter

In addition, the program also allows the

dissemination of digitized information to other devices

through different types of communication ports.

In our practice we will opt for a UDP port as an

output port for the dissemination of the obtained

NMEA information, as it is intended to reduce the

entire practice to the use of a single PC.

For a better understanding of the information

ShipPlotter sends to other devices, it is convenient and

advisable to know how AIS information frames are

structured, so it is recommended to read ITU-R M.1371

[7]

Figure 11. Input/output

3.4 Software shareware OpenCPN

ShipPlotter software, in addition to extracting NMEA

statements through audio input, representing them on

an electronic letter, is also able to act as a talker in an

NMEA network, which is why the practice makes use

of a second program that acts as a NMEA listener and

represents the information received.

In order to visually represent the information

received and decoded will be made use of OpenCPN

free software [17], designed as a navigation planner

capable of displaying nautical charts on which the

information that said software reads through different

input pathways overlaps, be it TCP network

connections, UDP GPSD, SIGNAL K or serial type.

Figure 12. Connections

OpenCPN acts as a receiver or listener and allows

monitoring the NMEA frames that reach you through

the different ports and represent the information of

them on an electronic nautical chart for interpretation

by the pilot.

The NMEA frame debugging window allows the

radio engineering student to observe some of the 26

different types of messages that are transmitted

according to the ITU-R M.1371.

Figure 13. Sentences monitored by OpenCPN

As an example, an NMEA debug screen has been

captured in the listener, OpenCPN, where information

from the talker, ShipPlotter, is observed.

Although it is difficult to read and understand an

AIS-type NMEA statement in plain view, the utility

allows the student to copy one of them and extract their

information on one of the many free websites that do

so. Besides being able to employ specific software that

marine equipment manufacturers provide, as is the

case with Actisense NMEA Reader [1]. As an example,

we have chosen to copy one of the many frames

received and after decoding it we observe the different

parameters that make it up and that fit the

aforementioned ITU-R M.1371

307

By looking at it, we can know that the statement

corresponds to a message of type AIS of an external

station (AIVDM). It contains a total number of one

single judgment (AIVDM, 1) transmitted to include the

information, that the judgment in question is the first

of a total of 1 (AIVDM, 1, 1) and finally that was

received in the channel AIS A (AIVDM, 1, 1, A). The

rest of the included data is much more complicated to

read in plain sight, as unlike the usual NMEA frames

that employ ASCII encoding [2], the ones used in the

AIS system employ a 6-bit binary encoding because of

the large data content they must include. This is why it

is necessary and convenient to use the specific software

mentioned above.

Figure 14. Sentence NMEA

This particular NMEA plot or judgment

corresponds to a programmed position report from the

Sailing Aid (AtoN) San Sebastián Castle with MMSI

992242123 and placed in the nautical chart at 36º

31.6999 'N and 6º 18.9700' W. etc

Figure 15. Castillo de San Sebastián

4 ASSEMBLING THE PRACTICE

It may seem that the main objective of the practice is

the transformation of a personal computer into a

complete ECDIS system, where we can see in real time

the fleet that sails near our coast. However, the real

goal is to motivate radio learners by innovation and

self-learning as they can achieve the same result in a

particular way and outside the laboratory with

minimal investment.

Figure 16. Assembling the practice

First, in the laboratory we will use a personal

computer that has previously installed the necessary

software mentioned above, ShipPlotter and OpenCPN.

In this way, we transform the computer into a complete

ECDIS system capable of showing the information it

receives through the VHF radio receiver associated

with it.

Our VHF radio receiver will be responsible for

capturing signals transmitted from ships, base stations,

radio stations etc. and providing them to ShipPlotter

software in the form of audio, which in turn will extract

the digital information they contain to be sent later to

OpenCPN in the form of NMEA frames that will be

presented on an electronic letter.

This way ShipPlotter will act as a talker while

OpenCPN will act as a listener.

Finally, it remains to equip our practice with the

radio receiver necessary to receive the channels 87B

and 88B what we achieved by connecting our SDR

device to one of the many USB ports available on the

PC.

To share information the NMEA between different

real equipment of a navigation bridge, it is necessary a

wired connection, however, our practice will dispense

with the connected one as the information will be

shared via a UDP port common to both talker and

listener since both are software type. In addition, UDP

connections allow you to send data quickly even in

multicast mode, supporting the delivery of data

packets, even if they are not complete etc.

Table 1. Physical connections

At the point we only need to turn on the VHF, SDR,

on any of the AIS channels (87B or 88B), which we will

do by tuning 161,975 MHz and/or 162,025 MHz on the

receiver software SDHonsole.

We will patiently wait a few seconds and, if near

our receiver there are AIS transmissions, we will be

able to observe how on the electronic letter shown by

OpenCPN the information is appearing. As is the static,

dynamic and crossing data of the ships around us, of

the stations based on land and of course of the aids to

navigation.

Here are some interesting screenshots showing the

good results of the practice.

308

Figure 17. Audio, input output

We see how the audio signal injected into the talker

is analyzed and decoded, transmitting it to the listener

via UDP port, but no longer as audio but as NMEA

frame. The listener monitors the talker´s UDP port and

therefore the information received is interpreted as

incoming information NMEA0183 and will be used

according to the function programmed in the listener.

Figure 18. Debugging window

The NMEA information that the listener receives

can be monitored in real time thanks to a debug

window. This utility allows to extract certain sentences,

or all of them, for further study and understanding and

even to register in a file that may later be used for other

purposes.

Information frames can be represented in different

ways, one of them being a radar screen simulation, but

always remembering that it is simulation and not a real

radar, since the principle of operation of a real radar is

completely different.

Figure 19. AIS Radar view

Especially the information received is represented

on an electronic letter where data on, ship, base

stations, etc. transmitting AIS information will be

observed.

As these are repetitive transmissions, the

representation on an electronic letter also allows

showing the defeats maintained by the ships in route,

as well as static data etc. that ultimately provides very

useful visual information to the pilots and centers of

marine traffic coastal.

Figure 20. Ship UCADIZ

We will show below several screenshots carried out

in different days and hours, being able to observe the

situation of maritime traffic in the bay of Cadiz in each

of those moments.

Figure 21. AIS target list

Figure 22. AIS target list, chart and radar simulation screen

Figure 23. Radar simulation screen

309

Figure 24. Chart and NMEA sentence

Figure 25. Chart and tracking

The work presented here reveals the feasibility of

our practice to show the actual information decoded

and presented on an electronic letter. But in addition, it

allows to propose to the students challenges such as the

creation of false NMEA frames that are mixed with real

ones intentionally and that can carry out in personal

way because it is not necessary a great economic

investment.

To further encourage the interest of the students, we

have generated a FALSE LIGHTHOUSE near the Rio

San Pedro along with an AIRCRAFT in the same area,

all while we can visualize real information about ships

docked and en route.

Figure 26. False Lighthouse

It should be noted that at no time has it been

transmitted in the channels assigned to the AIS system,

only the "false" NMEA plots have been elaborated to

inject them into our network. Needless to say, any plot

injected into any VHF transmission system capable of

digitally modulating by Gaussian minimum

displacement, GMSK [7], the channel 87B or 88B,

would undoubtedly appear on those AIS receivers

located in the transmitter coverage area, compromising

the safety of navigation.

To make the falsity of the information presented

more evident, another capture of AIS targets is shown

in which two SAR aircraft are observed whose MMSI

and assigned data are hugely striking. Specifically, the

first MMSI 111111111 is reserved for aircraft group

identity, and the second MMSI 000111111 does not

comply with Rec at all. ITU-R M.585-9 [8] on

Assignment and use of maritime mobile service

identities, as 00 is reserved for coastal stations. The

reader will also be able to observe the existence of a

third SAR aircraft with MMSI 11125896 listed as

"dedicated to fishing."

Everything seen in this last part of the practice aims

to make it clear that it is possible to introduce false

information into the real system, which is why we have

used absurd information on whites.

In our case it has been experimented without

transmitting in the AIS channels, but by injecting

frames of false information about the NMEA data

network

Figure 27. AIS targets

5 CONCLUSION

The study presented demonstrates the feasibility of

integrating innovative and low-cost practices to

facilitate the understanding of complex concepts in the

field of electronic navigation. By using accessible

technologies such as SDR receivers, free software, and

simulation tools, students can experience in a practical

way how AIS data is shared and visualized on a

navigation bridge. This approach encourages self-

learning, technical curiosity and the acquisition of real

competencies in the use of NMEA and ECDIS systems.

It also makes it possible to understand in a tangible

way the importance of interoperability between

equipment and the structure of the data frames used in

the maritime environment.

The proposed assembly converts a conventional PC

into an ECDIS system, capable of receiving, decoding

and visualizing AIS frames in real time, thus

facilitating training outside the laboratory and with

minimal investment. In addition, the final exercise on

the creation of false frames allows us to reflect on the

reliability of navigation systems and cybersecurity in

the maritime field. In short, an effective, innovative

and economically viable pedagogical practice

strengthens the technical capacities of future maritime

professionals.

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