Polish scientists are developing an alternative navigation system that could help ships in the Baltic Sea determine their position when satellite signals are disrupted or deliberately falsified. Researchers at Poland’s National Institute of Telecommunications have reported significant improvements in the accuracy of experimental radio-based measurements, bringing the technology closer to its next major test at sea.
The system, known as R-Mode, uses radio signals transmitted from coastal stations rather than relying on satellites. Its purpose is to provide vessels with an independent source of positioning information when conventional navigation systems cannot be trusted.
The institute published its latest findings on 23 October, following a scientific seminar three days earlier. The research brings together several years of experimental work, including trials aboard Baltic ferries and detailed measurements between stations on Poland’s northern coast.
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One of the most notable results concerns the effect of weather on the accuracy of radio measurements. By developing corrections for atmospheric conditions, the Polish team reduced the measured error by approximately 37% in one experimental dataset.
Researchers stress, however, that the results were obtained under controlled conditions and do not yet demonstrate equivalent positioning accuracy aboard moving vessels.
Why ships need an alternative to GPS
Satellite navigation has become fundamental to modern shipping.
Vessels use signals from systems such as the American GPS and Europe’s Galileo to determine their position, speed and time. This information can also feed electronic charts, automatic identification systems and other onboard equipment.
The weakness is that signals arriving from satellites are relatively faint and can be disrupted by stronger radio transmissions originating closer to Earth.
There are two main threats.
Jamming interferes with satellite signals, preventing receivers from obtaining reliable positioning information.
Spoofing involves transmitting deceptive signals that can cause receivers to calculate an incorrect position while apparently continuing to function normally.
The second threat is particularly dangerous because navigational displays may show a convincing but false location.
The Baltic Sea has experienced significant problems with satellite-navigation interference in recent years.
Poland’s National Institute of Telecommunications reports that monitoring undertaken by its Gdańsk research team has identified deliberate disruption and falsification of satellite-navigation signals in the region.
The concern extends beyond Poland. In March 2025, the International Maritime Organization and two other United Nations agencies warned about the growing danger of interference with satellite-based navigation.
Such disruption can undermine maritime safety, particularly in busy shipping lanes and around ports.
How the Polish navigation system works
R-Mode, short for Ranging Mode, is based on a relatively straightforward principle: a ship can calculate its position by measuring radio signals from transmitters at known locations on land.
A receiver measures the time taken for a signal to arrive and uses that information to estimate the distance from the transmitting station.
With measurements from several appropriately positioned stations, the vessel can calculate its location without relying directly on satellite positioning.
Even information from a single coastal station could provide a useful independent check on a position reported by GPS.
Polish researchers are concentrating on a version of R-Mode using the VHF Data Exchange System (VDES), a digital maritime communications technology.
The approach builds on existing maritime radio infrastructure, potentially allowing coastal communications stations to perform an additional navigation function.
This is not an entirely new concept. The institute has participated in international Baltic R-Mode research since 2017, working through successive projects and cooperating with overseas research organisations.
The International Organization for Marine Aids to Navigation is also developing international technical guidance for VDES R-Mode as a potential satellite-independent positioning service.
The Polish contribution includes improving radio signals, refining the algorithms used to calculate distances and investigating the environmental conditions that affect measurement accuracy.
Researchers cut measurement errors during Baltic experiments
One of the team’s most important experimental installations connects Gdynia and Jastarnia, two locations on Poland’s Baltic coast.
Researchers established a radio link across approximately 20 kilometres of water while using around 40 kilometres of fibre-optic cable to synchronise the timing systems at the two stations.
Extremely precise synchronisation is essential because small timing errors can produce substantial errors in distance calculations.
The fibre-optic connection uses a technology known as White Rabbit, which enables highly accurate distribution of time and frequency information.
With the stations synchronised, researchers could investigate how radio signals behave as they travel across the sea.
They discovered that atmospheric conditions, particularly rainfall, affected the measured distances even though the antennas remained stationary.
The team consequently developed an experimental correction model using weather measurements, including rainfall intensity and atmospheric water vapour.
Their results showed:
| Experimental dataset | RMS error before correction | After correction |
|---|---|---|
| Selected rainy-day measurements | 7.62 m | 4.83 m |
| Separate validation measurements | 7.23 m | 6.11 m |
The first result represents an improvement of approximately 37%, while the second represents around 15%.
The independent validation is important because the correction was tested against measurements that had not been used to fit the original model.
The team analysed approximately 3.12 million files containing radio measurements and more than 100,000 meteorological records collected during a year-long monitoring programme.
The institute’s Gdańsk research director, Dr Krzysztof Bronk, explained that the work was intended to demonstrate the potential for modelling weather-related errors in the maritime VHF radio band.
The findings represent progress towards more dependable radio-distance measurements, rather than proof that ships can already navigate throughout the Baltic with five-metre accuracy.
Ferry tests and the next stage of development
The Polish team has already carried out experimental work aboard moving vessels.
Its first maritime tests took place in June 2019 using a Stena Line ferry sailing between Gdynia and Karlskrona in Sweden.
Further campaigns followed in November 2019 and June 2020, using the newer VDES-based approach.
During one trial, researchers received a signal approximately 150 kilometres from the transmitting station, although measurement accuracy deteriorated considerably when direct radio visibility was lost.
The latest research has produced a demonstrator capable of receiving signals, calculating estimated distances and determining positions in real time.
However, some earlier tests used simulated multiple-station arrangements rather than a fully operational network of separate coastal transmitters.
The next major step is planned for spring 2027, when researchers intend to conduct another measurement campaign aboard a Unity Line ferry operating between Świnoujście in Poland and Ystad in Sweden.
The planned trial will use signals from multiple VDES R-Mode stations and examine how the system performs during an actual voyage.
It will be an important test of whether the promising results obtained in fixed installations can be translated into reliable positioning aboard a moving vessel.
Could Baltic ships eventually navigate without GPS?
The system is intended to complement existing navigation technology rather than replace satellite positioning entirely.
To operate reliably across a wider maritime area, R-Mode would require a coordinated network of transmitters, suitable onboard receivers, accurate timing infrastructure and internationally compatible technical standards.
The Polish researchers are investigating how existing coastal radio stations and telecommunications infrastructure might contribute to such a network.
Crucially, any genuine satellite-independent backup would also need to maintain reliable timing when satellite timing signals are unavailable.
There is currently no confirmed timetable for commercial deployment of the technology across the Baltic Sea.
Nevertheless, the latest findings show measurable technical progress towards an important objective: giving ships another way to check their location when satellite navigation cannot be trusted.
For vessels operating in one of Europe’s busiest and increasingly vulnerable maritime regions, an independent positioning source could eventually become a valuable additional safeguard.
