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.
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(Credit: Sir Francis Canker Photography)
Seamounts are vital oases in the open sea. 