How do ships communicate at sea?

For ships covered by the SOLAS convention, the core radio equipment is set by international rules before any business need is considered. The International Maritime Organization (IMO) describes the Global Maritime Distress and Safety System (GMDSS) as an integrated communications system using satellite and terrestrial radio, fully implemented on 1 February 1999. Under it, all passenger ships and cargo ships over 300 gross tonnage on international voyages must carry specified equipment for sending and receiving distress alerts and maritime safety information, as well as for general communications. The rules sit in chapter IV of the SOLAS convention, and the IMO adopted amendments to modernise the system; its page gives 1 January 2024 as their entry-into-force date.

The US Coast Guard explains that the GMDSS changed distress communications from being primarily ship-to-ship to primarily ship-to-shore, through rescue coordination centres.

Under the GMDSS, what a ship carries depends on where it sails. Governments define the sea areas, and the US Coast Guard summarises them:

The Coast Guard lists Inmarsat-C and Iridium as the recognised satellite services as of 2023, and notes that for a ship using a service with global coverage there is no Sea Area A4. For purely geostationary systems, A4 is the polar regions.

Every GMDSS ship also carries a satellite EPIRB (an emergency beacon), a NAVTEX receiver for safety broadcasts where NAVTEX is available (or an Inmarsat-C SafetyNET receiver where it is not), a DSC-equipped VHF radio, two or more VHF handhelds and a search and rescue radar transponder.

The automatic identification system (AIS) is an example of ships’ equipment talking directly to each other. The IMO requires it on ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 gross tonnage and upwards not on international voyages, and all passenger ships. AIS sends a ship’s identity, type, position, course, speed and navigational status automatically to other ships, shore stations and aircraft, and receives the same from them. The technical characteristics are in ITU-R Recommendation M.1371, which defines AIS as a VHF system using time division multiple access.

AIS needs no shore network to work between two ships in range. It carries a fixed set of navigation data, though, not the records a crew produces.

For business and crew traffic, ships buy commercial satellite services. Inmarsat, for example, presents Inmarsat C as its GMDSS safety service and markets crew connectivity as a separate service. Because these are services bought for the vessel, its link to shore is a resource to manage rather than a network every app can assume is there.

Some of a crew’s work happens between people on the same vessel: engine room rounds, safety inspections, permits to work, handovers between watches, cargo checks. None of it needs a satellite hop to reach the next person, yet an app that saves only to a shore server makes it wait for one.

Software that lets devices on board exchange records directly keeps this traffic local. Phones and tablets near each other can pass a finding to the officer of the watch, or keep a shared checklist as a replicated document that merges when devices reach each other again. With store-and-forward delivery, records waiting for shore stay queued on board until the ship’s link, or a port connection, is available, and a single device acting as a gateway can forward them in a batch.

Two design choices matter. First, decide which records only need to reach the next person on board and which must reach the company’s shore systems. Second, decide what counts as done: a record handed to another device on board is not the same as one the shore backend has accepted, and an app should show the difference, as described in delivered, accepted, committed.

All of this sits alongside the GMDSS, not inside it. Distress and safety communication stays on the approved equipment the regulations require.