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1 INTRODUCTION
INMARSAT (GEO) is the most widely used maritime
communication system between 70° south and 70°
north latitude. GEO satellites do not provide coverage
beyond these latitudes. Global communication beyond
this range may be supported in the future by LEO
satellite constellations such as Iridium, Starlink, or
OneWeb; however, these are currently commercial
systems with limited emergency applications. Under
adverse weather conditions, satellite channel
attenuation caused by the Earth's atmosphere can
result in the complete loss of satellite communication,
regardless of satellite orbit. Such attenuation may
exceed 100 dB. Therefore, in emergencies involving
severe atmospheric conditions, terrestrial
communication in the VHF, MF, and HF bands is
employed. The VHF band enables communication over
several tens of kilometers, MF during daytime up to
approximately 150 km, while nighttime MF and the HF
band support continuous long-range communication
when ships are beyond the range of VHF and daytime
MF coverage.
Communication below 30 MHz (MF/HF) is
particularly vulnerable to ambient noise at the receiver
antenna site. Improper selection of a GMDSS station
location may result in the inability to receive
emergency signals in the MF and HF bands due to
insufficient signal-to-noise ratio (SNR). Analyzing the
noise power at a proposed GMDSS station site
increases the likelihood of successful reception of
distress alerts. Periodic verification of the surrounding
infrastructure is recommended due to potential
increases in man-made noise resulting from
technological development. Variations in noise power
between sites can exceed 20 dB, which corresponds to
more than a hundredfold increase in transmitter
The Influence of Environmental Noise Power
on the Probability of Receiving GMDSS Alarm Signals
B. Uljasz
Military University of Technology, Warsaw, Poland
ABSTRACT: The article briefly presents the Global Maritime Distress and Safety System (GMDSS), which
provides global emergency communication capabilities for maritime navigation through the use of satellite
technologies (INMARSAT) and terrestrial VHF, MF, and HF frequency bands. In GMDSS-defined sea areas A3
and A4, particularly under conditions of severe satellite channel attenuation (greater than 100 dB), HF
communication remains crucial. However, its effectiveness strongly depends on the signal-to-noise ratio (SNR),
which may be significantly degraded by electromagnetic interference. The dominant noise source in the MF/HF
range is man-made noise (MMN). ITU-R Recommendations and ITU-R Reports define standardized
methodologies for noise measurement, including spectral analysis. Before installing a GMDSS station, and
periodically during its operation, the interference environment must be assessed to ensure reliable reception of
both voice and Digital Selective Calling alerts. The article presents a procedure for verifying changes in noise
levels at the GMDSS antenna site.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 19
Number 3
September 2025
DOI: 10.12716/1001.19.03.33
986
power. This article presents a verification procedure
for evaluating GMDSS station locations in terms of
noise interference.
The Global Maritime Distress and Safety System
(GMDSS) [1–5] is designed for transmitting maritime
safety and distress alerts over large bodies of water
using high-frequency (HF) communication. The speed
and effectiveness of rescue operations and,
consequently, the ability to save human lives or
mitigate environmental damage caused by maritime
disasters often depend on the error-free exchange of
information.
Locating GMDSS coastal stations in areas with high
noise levels reduces the probability of successful
distress signal reception. Therefore, the proper
selection of station locations and their periodic
verification with respect to ambient noise background
becomes a critical factor in maintaining system
reliability.
Noise and interference in radio communications
have a detrimental effect on the ability to correctly
receive desired signals.
In high-frequency (HF) communications, the
dominant noise sources are typically man-made.
Human infrastructure has a significant impact on the
ambient noise level in this frequency range [6].
However, the level of man-made noise is less
dependent on population density and more closely
related to the technological sophistication of local
infrastructure. Power supplies of certain lighting
systems, electric motor startups, generators, switch-
mode power supplies, and large-scale data centers can
substantially elevate local noise levels. As a result,
during peak operational hours, noise levels may
significantly exceed acceptable limits defined by
international standards.
Due to the continuous development of
infrastructure and the increasing technological
complexity, even in previously low-noise areas,
periodic noise assessments at GMDSS station sites are
justified and necessary.
2 GMDSS
The Global Maritime Distress and Safety System
(GMDSS) entered into force on February 1, 1992, and
reached full operational capability on February 1, 1999
[1-5].
The primary objective of GMDSS is to organize all
communications related to maritime safety and
distress into a unified and structured framework. The
system was initiated by the International Maritime
Organization (IMO), in cooperation with the
International Hydrographic Organization (IHO), the
World Meteorological Organization (WMO), and the
International Maritime Satellite Organization
(INMARSAT).
Initially, GMDSS aimed to develop effective
system-level, organizational, and legal solutions to
ensure proper maritime safety, with particular
emphasis on reliable and timely distress alerting
methods. This, in turn, was intended to enhance the
responsiveness and efficiency of search and rescue
operations. Based on years of observations, experience
from numerous rescue missions, and analysis of
distress alert procedures, a set of operational
assumptions was developed. These assumptions now
form the basis for the functioning of the modern
GMDSS.
The core assumptions of the system include:
− widespread adoption and use of satellite-based
positioning and localization systems,
− high-frequency (HF) radio communication based
on the Single Side Band (SSB) system,
− radioteletype (RTTY) communication adapted for
error detection and correction,
− data exchange using Digital Selective Calling (DSC)
in terrestrial communications across the Very High
Frequency (VHF), High Frequency (HF), and
Medium Frequency (MF) bands.
For operational purposes, the GMDSS divides
maritime waters into four sea areas. This division is
based on the effective communication range of various
radio technologies and equipment installed on vessels.
Accordingly, the GMDSS defines sea areas A1, A2, A3,
and A4.
Sea area A1 is defined as the area within coverage
of at least one VHF coast station that provides
continuous and reliable distress alerting using a VHF
radiotelephone equipped with a Digital Selective
Calling (DSC) controller, typically within a range of up
to 30 nautical miles.
Sea area A2 is the area outside sea area A1, but
within the coverage of at least one MF coast station
capable of distress alerting using DSC on the frequency
2187.5 kHz, typically extending up to 150 -200 nautical
miles from the coast station. Vessels operating in this
area are also required to carry the same equipment as
for sea area A1.
Sea area A3 encompasses the regions between 70
degrees north and 70 degrees south latitude. Within
this area, satellite communication coverage via the
INMARSAT system is available. Vessels operating in
this region are required to be equipped with either an
HF radio with a DSC controller or an INMARSAT
terminal. Additionally, they must carry the same
communication equipment as required for sea areas A1
and A2. Furthermore, vessels must be equipped to
receive Maritime Safety Information (MSI) relevant to
sea area A3, either via Enhanced Group Call (EGC)
through INMARSAT or via HF radio telex.
Sea area A4 includes all maritime regions outside
sea areas A1, A2, and A3. Due to the lack of
INMARSAT satellite coverage in these high-latitude
regions (beyond the coverage of geostationary
satellites), vessels operating in sea area A4 must be
equipped with an HF radio fitted with a DSC
controller. They are also required to carry all
equipment specified for sea areas A1, A2, and A3.
Regardless of the sea area, every maritime vessel
must be equipped with a 406 MHz Emergency Position
Indicating Radio Beacon (EPIRB) and a Search and
Rescue Radar Transponder (SART) for localization and
guidance to the distress site.
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Digital Selective Calling (DSC) distress alerts in sea
areas A3 or A4, transmitted in the MF/HF bands, may
be sent on one to six designated distress frequencies.
Table 1. Distress Frequencies in the MF/HF Bands
No.
Band
1
MF
2
HF
3
4
5
6
Figure 1 illustrates the transmission method of
distress alerts as a sequence of up to six consecutive call
attempts, each transmitted on a different designated
distress frequency, one in the MF band and up to five
in the HF bands.
Figure 1. GMDSS DCS Multi-frequency call attempts [3]
black – Basic distress alert message; blue – Expansion
message (Rec. ITU-R M.821) (Message Initial transmission
from one station and identical Automatic retransmission
message from two stations)
Distress alerts in the VHF band may be transmitted
simultaneously with alerts in the MF/HF bands.
Stations transmitting multi-frequency distress call
attempts in the MF/HF bands must be capable of
continuously monitoring for acknowledgments on all
frequencies used except for the one currently
transmitting, or must be able to terminate the call
attempt within 1 minute.
Multi-frequency call attempts may be repeated after
a random delay of between 3.5 and 4.5 minutes from
the start of the previous attempt.
For the transmission of digital DSC distress signals
in the HF and MF bands, F1B or J2B modulation is
used, with a frequency shift of 170 Hz and a symbol
rate of 100 baud [2]. In the case of J2B modulation, the
baseband signal is modulated onto a 1700 Hz
subcarrier.
According to Table 1 in Annex 1 of
Recommendation ITU-R F.339, for the above signal
parameters, the required signal-to-noise ratio (SNR) for
DSC transmission is 43 dB/Hz in a stable channel or 52
dB/Hz in a fading channel [2].
During adverse weather conditions, satellite
channel attenuation caused by the Earth's atmosphere
can result in the complete loss of satellite
communication, regardless of the satellite’s orbital
position. Such attenuation can exceed 100 dB.
Therefore, in emergencies, particularly in sea areas A3
and A4 under severe weather, ionospheric
communication in the HF band is utilized.
However, HF communication is not only affected
by ionospheric variability (primarily driven by solar
activity), but also by ambient noise at the receive
antenna location. Improper selection of a GMDSS
station site may lead to the inability to receive distress
signals in the HF band. The signal-to-noise ratio (SNR)
at such locations may fall below the threshold required
for error-free message reception.
Conducting noise power analysis at a proposed
GMDSS station site significantly increases the
probability of successful reception of Digital Selective
Calling (DSC) signals. Periodic verification of the
station’s surrounding infrastructure is also
recommended due to increasing urban noise levels.
Differences in noise power between locations may
exceed 20 dB, which corresponds to more than a
hundredfold increase in transmitter power.
The final part of the article presents a procedure for
evaluating GMDSS antenna locations with respect to
noise interference.
3 SOURCES OF RADIO NOISE
Radio noise may originate from both natural and man-
made sources. According to the definition provided in
ITU-R Recommendation P.372 [6], radio noise is the
aggregate of emissions from multiple sources that are
not intentional radiocommunication transmitters.
When no single dominant noise source is present at a
measurement location, the statistical amplitude
distribution of the radio noise often follows a normal
distribution and may be considered Gaussian white
noise.
However, due to the high density of noise-emitting
devices, especially in urban and residential areas, it is
practically impossible to find a location that is not at
least intermittently dominated by emissions from one
or more specific sources. These sources frequently
generate impulsive signals or single-carrier emissions.
Since radiocommunication equipment must operate
within such environments, excluding these
components from radio noise measurements may be
unrealistic.
According to [12], radio noise consists of three main
components: White Gaussian Noise (WGN), Impulsive
Noise (IN), and Single Carrier Noise (SCN).
White Gaussian Noise (WGN) is characterized by
the following properties:
− Uncorrelated electromagnetic field vectors
− Bandwidth equal to or greater than the receiver
bandwidth
− Spectral power level increases linearly with
bandwidth
Typical sources of this type of noise include:
computers, power line communication networks,
wired computer networks, and cosmic background
noise.
Impulsive Noise (IN) is characterized by the
following properties:
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− Correlated electromagnetic field vectors
− Bandwidth greater than the receiver bandwidth
− Spectral power level increases proportionally to the
square of the bandwidth
Typical sources of this type of noise include:
ignition sparks, lightning discharges, gas lamp starters,
computers, and ultra-wideband (UWB) devices.
Single Carrier Noise (SCN) is characterized by the
following properties:
− One or more distinct spectral lines
− Bandwidth narrower than the receiver bandwidth
− Spectral power level independent of the receiver
bandwidth
Typical sources of this type of noise include: wired
computer networks, computers, and switch-mode
power supplies.
While the WGN component can be sufficiently
characterized by its root mean square (RMS) value, this
is significantly more challenging for impulsive noise
(IN). Modern digital communication services almost
always employ error correction mechanisms, making
them more resilient, particularly to impulsive
disturbances. However, when impulse durations and
repetition rates exceed certain thresholds, IN can
severely degrade system performance.
Therefore, it is desirable to measure radio noise in a
manner that not only quantifies the level of IN but also
provides statistical information about the distribution
of impulse parameters.
Single Carrier Noise (SCN) is identified as such only
when it originates from a single source located near the
measurement site. When multiple single-carrier
sources are present, their emissions quickly combine
into a noise-like spectrum as their number increases.
ITU-R Recommendation P.372 [6] defines radio noise
as the aggregate of unintentional emissions from
various sources and explicitly excludes emissions from
single, identifiable sources.
Therefore, it is essential to select measurement
locations or frequencies that are not dominated by
emissions from such individual sources. As a result,
further consideration of SCN is not necessary in the
context of man-made noise (MMN) measurements.
Figure 2. Noise characteristics for the frequency range from
10 kHz to 100 MHz across different noise categories
This frequency range encompasses several noise
sources, each with distinct spectral characteristics.
According to ITU-R Recommendation P.372, the main
categories include:
A – Atmospheric noise (value exceeded 0.5% of the
time)
B – Atmospheric noise (value exceeded 99.5% of the
time)
C – Man-made noise (quiet receiving site)
D – Galactic noise
E – Median city area man-made noise
Additionally, the solid reference curve provided in
the ITU-R documentation represents the minimum
expected noise level under ideal receiving conditions.
These characteristic curves serve as a reference for
evaluating background noise levels across different
environments and are fundamental for assessing the
suitability of HF/MF receiving sites, particularly for
safety-critical systems such as GMDSS.
The solid line largely coincides with curve C, which
represents the minimum level of man-made noise
(MMN) at a quiet receiving site. In contrast, the
maximum noise levels observed most of the time are
described by curve E, corresponding to typical MMN
levels in urban environments.
This indicates that man-made noise (MMN) is the
dominant noise source in the MF/HF bands for the
majority of the time, particularly in populated and
industrialized areas. This conclusion is critical when
assessing receiver site suitability and ensuring reliable
radio communication in these frequency ranges.
Table 2. Significant Sources of Radio Noise by Frequency
Range [12]
Noise Source
Frequency Range
Atmospheric noise (lightning
discharges)
9 kHz - 30 MHz
Galactic (cosmic) noise
4 MHz - 100 MHz
Man-made noise
9 kHz - 1 GHz
These sources may overlap, and the dominant noise
component depends on the intensity and proximity of
the sources relative to the measurement location. In
most environments, man-made noise (MMN) is the
prevailing contributor in the MF/HF frequency bands.
Man-made noise may also consist of a combination
of the aforementioned categories. The dominance of a
specific noise type depends on the intensity of the
sources and their spatial proximity to the observation
point.
Man-made Additive White Gaussian Noise
(AWGN) typically occurs when numerous individual
sources of similar strength are located at a significant
distance from the receiver. In contrast, the presence of
Single Carrier Noise (SCN) indicates one nearby source
or a limited number of proximate sources. The former
scenario is more common in outdoor environments,
while the latter is typical of indoor settings.
Natural noise sources tend to remain relatively
stable over long periods. However, man-made noise
(MMN) can dominate specific portions of the radio
spectrum, and its intensity fluctuates in response to
increases in the number of electrical and electronic
devices, the introduction of new technologies, and
advancements in electromagnetic compatibility (EMC)
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practices. Therefore, in radio noise measurements used
to assess site suitability, particular attention must be
paid to man-made noise.
Median values of the AWGN noise factor for
various typical outdoor environments are shown in
Figure 3, which also includes the galactic noise curve.
Figure 3. Median values of man-made noise power for a short
vertical lossless grounded monopole antenna [6]
In all cases, the results conform to a linear variation
of the median noise factor, Fam, expressed in dB, as a
function of frequency f, in the following form [6]:
Fam = c – d log f (1)
where:
Fam - is the median external noise factor [dB],
f - is the frequency [MHz],
c and d - are constants dependent on the environmental
category.
When f is expressed in MHz, the constants c and d
take the values listed in Table 3.
Table 3. Values of the constants c and d for Different
Environmental Categories [6]
Environmental category
c
d
City (curve A)
76.8
27.7
Residential (curve B)
72.5
27.7
Rural (curve C)
67.2
27.7
Quiet rural (curve D)
53.6
28.6
Galactic noise (curve E)
52.0
23.0
It should be noted that Equation (1) is valid across
the entire MF/HF range for all environmental
categories except for curves D and E, as shown in the
figure.
Curve D represents man-made noise reflected by
the ionosphere and is observed below the
extraordinary critical frequency of the ionosphere
(fxF2), which varies depending on the time of day,
season, and the solar cycle.
Above fxF2, the ionosphere gradually becomes
transparent, and galactic noise becomes the dominant
component, as represented by curve E.
4 IMPACT OF INDUSTRIAL NOISE LEVELS
Industrial noise levels have a significant influence on
the reliability and availability of radiocommunication
links, particularly in the MF and HF bands used by
systems such as GMDSS. Elevated ambient noise,
typically present in industrial or urban environments,
directly reduces the signal-to-noise ratio (SNR), which
is a critical parameter for the correct reception of
Digital Selective Calling (DSC) and other safety-related
transmissions.
Increased electromagnetic emissions from
equipment such as motors, power converters,
industrial lighting, and data processing centers
contribute to higher man-made noise (MMN) levels.
This not only decreases the effective communication
range but also raises the probability of reception errors
or complete loss of signal detection.
As demonstrated in prediction models [7-10, 17]
and field measurements, even a moderate increase in
ambient noise, on the order of 10 to 20 dB, can
drastically reduce the time availability of links meeting
required SNR thresholds. Therefore, minimizing
industrial noise influence through proper site selection
and periodic monitoring is essential for maintaining
reliable GMDSS performance.
This section of the article presents sample
characteristics illustrating the impact of ambient noise
levels at the receiver antenna location on the
probability of successful DSC signal reception.
a)
b) c)
Figure 4. Time availability of SNR values exceeding the
required threshold and the LUF curve [17]: a) Remote
environment [6], b) Industrial environment [6], c) Scale
indicators used in Figures a and b
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Figures 4 through 6 show the results of a prediction
for Time Availability where SNR > Required SNR
(43 dB). The prediction was conducted for a
communication scenario in June 2025, with a sunspot
number (SSN) of 108, a Power transmitter of 100 W,
and a distance of 564 km between the transmitting and
receiving stations. The required SNR for reliable
reception was set to 43 dB/Hz.
These plots illustrate how different noise
environments affect the availability of sufficient signal-
to-noise ratio (SNR) for reliable reception, in
conjunction with the Lowest Usable Frequency (LUF)
curve. The comparison highlights the reduced time
availability in higher-noise (industrial) environments,
where elevated background noise levels limit effective
HF communication performance.
Based on the information presented in Figure 4, a
significant decrease in the probability of correct
reception of GMDSS DSC signals can be observed at a
receiving site with a higher environmental noise level
(a difference of 13.56 dB/Hz), despite identical
transmitter power and ionospheric conditions.
This emphasizes the critical influence of ambient
noise on HF communication reliability and the
necessity of careful site selection for GMDSS receiving
stations.
This scenario represents a low-noise environment,
typically corresponding to a remote or quiet rural area.
Under these conditions, the probability of establishing
and maintaining a reliable HF communication link at
8414 kHz is high, assuming standard ionospheric
propagation and system parameters. The low ambient
noise level significantly improves the signal-to-noise
ratio (SNR), increasing the likelihood of successful
reception of Digital Selective Calling (DSC) signals and
other critical communications.
This case illustrates communication performance in
a high-noise environment, such as an industrial or
urban area. The significantly elevated ambient noise
level (by approximately 23.6 dB compared to a quiet
site) leads to a substantial reduction in the signal-to-
noise ratio (SNR). Consequently, the probability of
successful communication at 8414 kHz decreases
markedly, increasing the risk of failed reception of
Digital Selective Calling (DSC) alerts and other safety-
critical transmissions. This highlights the critical
importance of proper site selection for GMDSS
receiving stations.
Figure 5. Probability of communication at 8414 kHz in an
environment with a noise level of 164 dBW/Hz
Figure 6. Probability of communication at 8414 kHz in an
environment with a noise level of 140.4 dBW/Hz
Figures 5 and 6 present the probability of
establishing communication, expressed as a
percentage. The figures were generated using a custom
application developed based on [7] and validated
against the results produced by publicly available tools
[8–10].
5 RADIO NOISE MEASUREMENT PROCEDURES
According to ITU-R guidelines SM.1753 [12], SM.2055
[13], SM.2155 [4], and SM.2157 [15], radio noise
measurement procedures consist of several key stages
that ensure the reliability and repeatability of results:
− Choosing an appropriate measurement location is
essential for obtaining representative results. In line
with ITU-R recommendations (particularly
SM.2155), the site should reflect the characteristics
of the investigated environment: urban, rural, or
remote. It is recommended that the location not be
near strong local interference sources (e.g., high-
voltage power lines, industrial machinery), which
could distort the recorded noise levels.
− Wideband receivers or spectrum analyzers
compliant with ITU-R SM.2055 specifications must
be used. Prior to recording, careful calibration of the
measurement system is required. Calibration
should be performed using known reference signal
sources according to SM.2055 procedures to ensure
data accuracy and repeatability.
− Antennas used for measurement must have
radiation patterns suitable for the observed
frequency range (e.g., HF: 3-30 MHz). As per
SM.2157, the antenna should be installed at a height
that minimizes reflections from the ground and
surrounding objects. The measurements must be
conducted under conditions that ensure signal
reception is stable and repeatable within the
designated band.
− The measurement procedure involves recording
noise levels at selected frequency points. According
to ITU-R SM.1753, the spacing between
measurement points on the frequency axis should
be no less than 0.05 decades to obtain a detailed
interference profile over the full measurement
range. Results are recorded for a defined detection
bandwidth, most commonly 9 kHz or 10 kHz.
Observation time at each frequency point should be
991
sufficient to average signal fluctuations, typically
ranging from several to a dozen minutes per point.
− Collected data is subjected to statistical analysis to
determine average, maximum, and minimum noise
levels for each frequency and location. Results
should be presented in graphical form, showing the
relationship between interference levels, frequency,
and measurement site, following SM.2155
guidelines. The measurement report must include a
description of the methodology used, equipment
configuration, environmental conditions, and
detailed numerical and graphical results to allow
comparison with other studies or international
standards.
− Recommendation SM.2157 also emphasizes the
need to document all factors that may influence
noise levels under specific measurement conditions,
such as weather, presence of atypical emission
sources, or environmental changes. The final report
should include both numerical data and detailed
descriptions of the measurement context.
− Adhering to the aforementioned ITU-R
recommendations and reports ensures the
reliability, comparability, and high quality of radio
noise measurement results, which are essential for
assessing the electromagnetic environment and
planning future radiocommunication infrastructure
deployments.
6 RADIO NOISE MEASUREMENT PROCEDURE
FOR THE GMDSS SYSTEM
The development of infrastructure in the vicinity of
GMDSS stations, particularly those operating in the
MF/HF bands, should be monitored by national and
international authorities. Before selecting an
installation site, a comprehensive assessment of the
noise characteristics at the proposed location must be
conducted. Periodic verification measurements should
also be performed, with particular attention to noise
originating from human activity.
The standard procedure for assessing site suitability
with respect to noise is described in Section 3.
However, considering the specific operational
characteristics of the Global Maritime Distress and
Safety System (GMDSS), this procedure may be
simplified to significantly reduce the time required for
measurements and subsequent data analysis.
A simplified measurement procedure is presented
below, based on guidelines from [12–16].
6.1 Procedure for Periodic Verification
To ensure the long-term reliability of GMDSS stations
operating in MF/HF bands, the following simplified
procedure is recommended for periodic noise level
verification at the station site:
− Conduct measurements at the GMDSS station
location.
Measurements must be performed directly at the
operational site of the GMDSS system to reflect the
actual electromagnetic environment affecting signal
reception.
− Use standard GMDSS receiving equipment.
The measurements should utilize the standard
receivers installed as part of the GMDSS setup,
ensuring relevance and operational
representativeness.
− Use the GMDSS receiving antennas.
Antennas normally used by the system should be
employed, maintaining consistent reception
characteristics and eliminating the influence of
differing antenna parameters.
− Record signals in the working frequency bands of
the GMDSS system.
Measurement should be performed in the vicinity
of operational frequencies to assess real-world
interference in channels actively used for
communication and distress alerts.
− Select interference-free radio frequencies
Prior to measurements, a spectral analysis must be
conducted around each GMDSS working frequency
(±50 kHz) to identify at least m noise-only
measurement frequencies, those free from
intentional emissions (e.g., broadcast,
communication, radar signals). These frequencies
should contain only natural and unintended man-
made noise.
− Measure electromagnetic field strength at each
selected frequency
Use RMS and AVG detectors to measure the electric
field strength. The RMS values are used to compute
absolute levels, while AVG values support
statistical deviation analysis. For each measurement
frequency: (2):
( ) ( ) ( ) ( )
RMS
E f U f AF f A f= + −
(2)
where:
E(f) – field strength at frequency f [dBµV/m]
URMS(f) – RMS level measured by the receiver
[dBµV]
AF(f) – antenna factor [dB/m]
A(f) – cable attenuation [dB].
− Calculate the external noise factor for each
frequency
( ) ( ) ( ) ( )
10 10
20 95.5 10Fa f E f log f log B= − + −
(3)
where:
Fa(f) – external noise factor [dB]
f – frequency [MHz]
B – receiver bandwidth [Hz]
− Determine the median noise factor and confidence
interval
For each measurement frequency, calculate the
median of the n measured Fa(f) values and
determine the confidence interval encompassing at
least 90% of observations. Present the results
graphically.
− Calculate the deviation of the signal level
( ) ( ) ( )
RMS AVG
Vd f U f U f=−
(4)
where:
Vd(f) – signal level deviation [dB]
URMS(f) – RMS value [dBµV]
UAVG(f) – average value [dBµV]
Repeat statistical analysis for Vd(f) across all n
values to determine the confidence interval and plot
the results.
− Evaluate site suitability
Based on Fa(f) and Vd(f) values:
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The site is suitable if Fa(f) does not exceed the
reference "quiet rural" level by more than 3–4 dB,
and Vd(f) during daytime does not exceed 3 dB.
− Document environmental factors
The final report must include all factors that could
influence the measurement (e.g., weather, solar
conditions, nearby emitters).
− Prepare comprehensive documentation including:
− General info (site, date, operator)
− Equipment setup (antenna, receiver, analyzer,
calibration)
− Measurement parameters (frequency range,
bandwidth, time)
− Environmental conditions (temperature,
humidity, power supply)
− Raw data and processed results
− Graphs, spectrograms, photos, and data files
This procedure provides a reliable and efficient
method for monitoring the electromagnetic noise
environment around GMDSS installations, ensuring
their ongoing readiness for distress alert reception.
7 CONCLUSIONS
The effectiveness of GMDSS communication in the MF
and HF bands depends significantly on the level of
local electromagnetic noise, which in urban
environments may reduce the signal-to-noise ratio
(SNR) below the threshold required for error-free
reception of DSC distress signals.
The dominant source of interference in the below
30 MHz band is man-made noise (MMN), the level of
which can vary by more than 20 dB depending on the
receiving station’s location. This corresponds to a
change in transmitter power of more than two orders
of magnitude.
Proper selection of GMDSS station sites should be
preceded by noise characterization measurements in
accordance with ITU-R recommendations (P.372,
SM.1753, SM.2055, SM.2155, SM.2157), using spectrum
analyzers, RMS/AVG detectors, and measurement
antennas with suitable directional characteristics.
Periodic verification of radio noise levels at the
GMDSS station site is essential due to ongoing
infrastructure development, which increases
impulsive noise (IN) and single carrier noise (SCN),
especially as urbanization and electromagnetic
emissions grow.
Applying a simplified site evaluation procedure
using the existing GMDSS infrastructure (antennas and
receivers) enables effective monitoring of changes in
radio noise without the need to expand the
measurement setup, while maintaining adequate
accuracy.
Measurement results should be analyzed
statistically, taking into account the median external
noise factor Fa(f) and signal level deviation Vd(f). This
allows for an objective evaluation of the
electromagnetic environment’s quality in the context of
distress communication reliability.
ACKNOWLEDGMENT
This work was financed by the Military University of
Technology under Research Project no. UGB/22-
059/2025/WAT on “Transmission Properties of Radio Wave
Propagation Environments in Military Applications”.
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