R-Mode: the radio beacon we thought was obsolete could once again become GPS’s backup

From Pulse by James Crawford

What I learned watching DGPS stations go dark

I started navigating during the GPS boom, when radio direction finding was already part of the recent past.

For decades, mariners had tuned into coastal radio beacons to take bearings on reference points and establish their position, but in the mid-1990s the Chilean Navy began dismantling those stations because of their high maintenance cost and low accuracy compared with satellite constellations.

For my generation, the satellite was simply how you navigated.

Radio Navigation Development Plan for Participating Commonwealth of Independent States

for 2019 to 2024

 

Years later, during the 11 years I represented the State of Chile on the IALA Council, I watched history repeat itself with DGPS stations.
One after another, those medium-frequency antennas next to lighthouses and headlands stopped transmitting, and we experienced it with a mix of logic and nostalgia.
The logic was clear, because the service was born to correct an intentionally degraded GPS, and that problem no longer existed.
The nostalgia was real, because those stations had been a quiet part of the aids to navigation system for decades.

The decisions made sense for their time.
The United States switched off its last stations in June 2020, and the UK, Ireland and Canada followed in 2022.
The reasoning was sound, since undegraded GPS, supported by satellite-based augmentation, met navigational accuracy needs.

Yet today’s problem is no longer GNSS accuracy, but its availability and integrity.
In June 2025, IMO circulated a communication from several governments warning of increasing GNSS disturbances in European waters, particularly in the Baltic, and that same month the Swedish Maritime Administration issued a notice to mariners on recurring interference across the region.
A very weak signal, transmitted from roughly 20,000 km above us, remains the position and time reference for ships, pilots, VTS and port systems.
That is the paradox, because the terrestrial infrastructure we watched being dismantled as unnecessary is precisely what could now give us resilience.

The key idea is called R-Mode (Ranging Mode).
Instead of building a new system, it reuses the same medium-frequency beacon, synchronises it precisely and adds two continuous-wave carriers on either side of its signal.
The shipborne receiver measures the phase of each station, estimates its range and, with three or more stations, computes a position without satellites.
For an AtoN manager, what matters is that existing DGNSS receivers keep getting their corrections as before, and that medium-frequency propagation allows wide coastal coverage with few stations.
 

The concrete example is the Baltic.
Germany, Sweden and Poland modified existing beacons to build a testbed, and in 2021 medium-frequency R-Mode delivered positioning at sea for the first time, with horizontal accuracy better than 12 metres (95%) in the central area between three transmitters.
For reference, IALA Recommendation R-129 calls for 100 metres in coastal navigation and 10 metres in port approaches.
Moreover, this is no longer just research, as IALA published Guideline G1187 in December 2024, standardising the signal and navigation message, and in June 2026 IMO’s NCSR 13 finalised draft performance standards for shipborne R-Mode receivers, which go to MSC 112 this December.

That said, it would be a mistake to present R-Mode as a cost-free solution.
At night, sky-wave propagation degrades performance and, in the Baltic trials, forced stations to be excluded from the position solution.
Transmitters themselves can show phase instabilities that require monitoring and corrections, and many States, Chile among them, have already dismantled their radio beacon infrastructure, so reinstalling or upgrading stations carries a real cost.
On top of that, receiver standards are still awaiting approval and the system is conceived as a voluntary backup.

There is also a factor the Baltic does not face and that is decisive in our reality: geometry.
Mariners who used radio direction finding knew it well, because a good fix depended on crossing lines from radio beacons at favourable angles, and the same principle governs R-Mode today.
The system computes position from ranges to several stations, and its accuracy improves when those stations surround the ship.
In an enclosed sea like the Baltic this happens naturally, but along a long, narrow coastline such as Chile’s the stations would be almost aligned, which would weaken the solution precisely in the direction perpendicular to the coast.
This is why no technology should be assessed in isolation.
South Korea, for example, already operates three eLoran stations covering the Yellow Sea as a terrestrial backup, and VHF R-Mode over VDES shore stations emerges as another possible complement.

For the same reason, the relevant question for Chile is not whether to adopt R-Mode, but who assesses whether it makes sense, where, and with which complements.
Answering it requires analysing coverage, geometry, risk, integration with other services and operational continuity, a task for certified professionals in marine Aids to Navigation and telecommunications, able to, among others, apply IALA standards to the reality of each coastline and each port.
Looking back, the lesson I take from that period is that when we assess decommissioning a station, we tend to judge it by the function it was built for, but the site, the mast, the frequency and the timing hold value that can resurface through another service.

 

Links :

• Inside GNSS : eLoran Surging as Protection for GNSS and Users 
• GPS world : 10 answers about eLoran / South Korea partners with broadcaster on eLoran and 10-cm GPS  / eLoran: Part of the solution to GNSS vulnerability  / China expanding Loran as GNSS backup / Russia-CIS navigation plan emphasizes GNSS, Loran, inertial 
• SplashTech : Shipping builds a new navigation stack for the spoofing age 
• GeoGarage blog : Loran leap: China's boost to maritime navigation accuracy / GPS back-up: World War Two technology employed / eLoran: Part of the solution to GNSS vulnerability  /Britain urges ships to prepare for life without reliable GPS / Cyber threats prompt return of radio for ship navigation / Navigational backup to aid ships in Dover straits /  GPS jamming and spoofing: when good signals go bad

Banque Populaire XI : a trimaran taking flight at 50 knots

Aboard the Maxi Banque Populaire XI. At the helm, Armel Le Cléac’h steers his giant trimaran through the waves at 50 knots—about 90 km/h. 

Spectacular footage offering a glimpse into Ultim-class sailing in the context of the 2026 Route du Rhum.
The bow lifts, spray sweeps across the deck, and the boat accelerates.

Carried by its foils, the Maxi Banque Populaire XI lifts its hulls out of the water and appears to glide over the waves.

But behind this impression of lightness, every movement reflects the considerable forces the trimaran is subjected to.
At this speed, the landscape flies by, and the impacts serve as a reminder that flight is a delicate balance that must be maintained.

The boat rises, drops, and regains its trim.

These moments offer a tangible sense of what a 50-knot top speed means on an offshore racing yacht: noise, spray, and constant vigilance.
Armel Le Cléac’h at the helm, heading for the Route du Rhum
At the helm, Armel Le Cléac’h responds to the boat’s movements.

His gaze shifts from the sea to the sails, while his actions seek to maintain course and stability. For the skipper, harnessing the power of Banque Populaire XI also means preserving the vessel.
With an eye toward the 2026 Route du Rhum, these images bring home the full scale of the Ultim challenge: crossing the Atlantic solo, from Saint-Malo to Guadeloupe, at the helm of multihulls capable of such high speeds.