Shipping’s GNSS-independent backup reaches a draft standard. Its accuracy and its spectrum are both unsettled

The standard exists. The performance of one carrier does not match the requirement

The IMO’s Sub-Committee on Navigation, Communications and Search and Rescue finalised a draft performance standard for shipborne R-mode receiver equipment at its thirteenth session, held in London from 22 to 26 June 2026 ^[1][4]. The draft resolution has been forwarded to the Maritime Safety Committee, which meets from 7 to 11 December 2026 ^[2][3][5]. Several other NCSR 13 drafts travel to the same session ^[3]. R-mode is a terrestrial ranging service: it derives position from radio signals designed to carry data rather than to be measured, and it is framed as a backup supplementary to GNSS rather than a replacement ^[2].

This is the first time the maritime equipment standards system has specified a shipborne receiver for a positioning source that does not depend on satellites. The sub-committee’s outcome is reported as a “terrestrial, GNSS-independent backup system in response to increasing GNSS vulnerabilities” ^[1]. Two things deserve attention before it reaches MSC 112. On published evidence, one of its two carriers sits well outside the accuracy requirement the backup is meant to satisfy. And the spectrum the service depends on is not yet allocated.

Two carriers, two very different results

R-mode has two carriers: medium-frequency broadcasts and the VHF Data Exchange System ^[2]. IALA has maintained a guideline for the VDES variant since 10 December 2020, now Edition 2.0, last revised 13 December 2024 ^[6]. VDES rests on Recommendation ITU-R M.2092-1, and IALA describes R-mode as the means by which vessels can “enhance position integrity and determine their position and accurate time when GNSS performance is degraded” ^[7].

The requirement a maritime positioning source has to meet is set by IMO Resolution A.915(22): 10 m horizontal accuracy at 95 per cent for ocean, coastal and port-approach phases with a 25 m alert limit, and 1 m with a 2.5 m alert limit for port operations ^[10].

Medium-frequency R-mode is close to that. Baltic Sea trials published in NAVIGATION reported 95 per cent horizontal accuracy better than 12 m in the centre of a three-transmitter configuration: mean error 3.7 m with a 95 per cent bound of 8.4 m in quasi-static and dynamic conditions, and 5.2 m and 12.0 m at higher speed ^[8]. The authors assessed the result as in line with the requirement for a coastal navigational backup and as nearly meeting the port-approach requirement ^[8].

VDES R-mode is not close. DLR trials on Lake Ammer with three shore-based transmitters and a vessel-mounted receiver recorded a 95th-percentile error of 22 m where Doppler measurements were used, and 144 m where they were not ^[9]. With line of sight obstructed by a passenger ship, the same configuration produced errors below 84 m for 95 per cent of measurements with Doppler, and below 443 m without ^[9]. The Doppler dependency is not marginal: the authors put the improvement at a factor of five to six across all scenarios tested ^[9].

A 22 m best case against a 10 m requirement is not a rounding difference, and an 84 m obstructed case is a different order of capability. It matters operationally because both carriers sit under one receiver standard. A shipowner reads the standard; it does not read the trial literature.

The protection is an ITU decision, not an IMO one

NCSR 13 also finalised the IMO position for ITU negotiations, directed at protecting GMDSS and maritime radio services and at securing new VHF band allocations for R-mode implementation ^[2]. Final positioning was deferred to NCSR 14 in April 2027 ^[1][2]. The allocation itself falls to WRC-27.

That is the sequencing problem. A receiver standard may be approved in December 2026, but the spectrum is not settled until the conference, type-approved equipment reaches ships after that, and shore transmitters have to exist within range before any of it yields a position. No R-mode transmitter infrastructure has been announced for the Malaysian or wider APAC coastline; the trials cited above were conducted in the Baltic and in southern Germany ^[8][9].

The enforcement picture for the bands R-mode will occupy is the one that already failed GNSS. The ITU framework rests on Article 45 of the Constitution and on Radio Regulations Nos. 4.10, 15.1, 15.35 and 15.37: safety services are entitled to “special measures to ensure their freedom from harmful interference”, administrations must promptly investigate interference from their stations, and cases are reported to the Radiocommunication Bureau ^[12]. The ITU records grave concern at rising case numbers affecting civil aviation and maritime operations across the Middle East, the Baltic Sea and the Korean Peninsula, and publishes no case count ^[12]. IMO, ICAO and ITU jointly urged member states on 25 March 2025 to protect the radio navigation satellite service, strengthen resilience, maintain conventional navigation infrastructure as contingency backup, and report interference ^[11].

Every instrument in that list is an obligation on an administration. None is a control a master can apply on the bridge. R-mode does not change that, because R-mode is itself a radio service in MF and VHF bands ^[7], inside the same machinery. GNSS-independent is not interference-independent, and the draft standard should not be read as though it were.

What the environment looks like while the standard is in process

The Secure World Foundation’s Global Counterspace Capabilities 2026, published 8 April 2026, characterises GNSS interference as a persistent feature of conflict zones on multiple continents rather than an episodic disruption ^[13]. Its maritime figures are the relevant ones. During the June 2025 Iran-Israel conflict, 970 ships a day experienced GPS jamming in the Strait of Hormuz, and traffic through the strait fell by 20 per cent as vessels restricted transits to daylight hours ^[13]. Poland’s recorded interference cases rose from 1,908 in October 2024 to 2,732 in January 2025, and Estonia reported over EUR 500,000 in damages within a three-month period ^[13].

Flag administrations are not waiting for MSC 112. The Republic of the Marshall Islands issued guidance on 7 October 2025 telling operators to deploy inertial, celestial and radar positioning to cross-verify GNSS, fit anti-jamming receivers with controlled reception pattern antennas, drill crews on recognising spoofing as distinct from jamming, plan routes away from high-threat areas, and consider eLoran or R-mode where available ^[14]. It also requires incidents to be reported to the administration, UKMTO, the NATO Shipping Centre and the US Coast Guard Navigation Center, with position, time, duration and screenshots ^[14].

What to do

• Treat the December MSC 112 outcome as a procurement signal, not a delivered capability. Approving a receiver standard does not place a transmitter within range of a regional coastal route ^[2][5].
• Ask which carrier any R-mode claim rests on. The published gap between medium-frequency and VDES results decides whether equipment can satisfy A.915(22) at all ^[8][9][10].
• Keep the non-RF layer of the backup. Inertial navigation, radar fixing and celestial practice do not depend on an allocation decision in 2027 ^[14].
• Report interference every time, to the addresses the flag administration names. The ITU case system runs on administration reports ^[12][14].
• Record the degraded-navigation drill, not only the policy. Recognising spoofing as distinct from jamming is a training record, not a fitting ^[14].
• Track NCSR 14 in April 2027, when the IMO’s WRC-27 spectrum position is finalised ^[1][2].

Domain close

The maritime sector has done something the aviation sector has not: written a positioning source that does not depend on satellites into its shipborne equipment standards. That is the right direction, and a draft standard is a real outcome rather than a statement of intent. But it arrives with one of its two carriers an order of magnitude short of the accuracy requirement in published trials, with its spectrum unresolved until 2027, and into an electromagnetic environment whose enforcement mechanism is an obligation on the administration doing the transmitting. For the next several years the operational answer at sea is unchanged: cross-check position against something that is not a satellite, and write down what you saw.