The common error: treating sea areas as geographic zones on a chart
ETOs sometimes speak of sea areas as fixed bands drawn at set distances from shore — a mental model borrowed from radio propagation rules of thumb. The practical error this causes is assuming a vessel's equipment fit is determined solely by how far offshore it operates, rather than by the actual coverage provided by specific shore infrastructure at the planned trading area.
GMDSS sea areas are defined by the capabilities of coast station networks, not by distance or latitude lines.
A1 — VHF DSC coverage
A1 is the area within range of at least one VHF coast station providing continuous DSC alerting on channel 70. Because VHF propagation is essentially line-of-sight, A1 coverage is typically coastal, but its precise boundary depends entirely on which stations exist and where. Two ports in different countries may have very different A1 footprints.
A2 — MF DSC coverage beyond A1
A2 extends MF coast station DSC coverage into sea areas not reached by VHF. MF propagates further than VHF, so A2 stretches further offshore. Critically, A2 is defined as beyond A1: any area already covered by A qualifying VHF coast station is A1, not A2, even if MF coverage also reaches it.
A3 — Recognised satellite service coverage
A3 covers ocean areas within the footprint of a recognised satellite service, beyond A1 and A2. The exact boundary of A3 is not a fixed line; it depends on which satellite service or services are designated for GMDSS use on any given route. This is an area of active change as the GMDSS modernisation programme expands the list of recognised mobile satellite services — the specific definition of A3 coverage is therefore subject to ongoing regulatory development and should be confirmed against current flag state and IMO guidance for each voyage.
A4 — Everywhere else
A4 is the residual area: anywhere that falls outside A1, A2 and A3. In practice, this means polar regions beyond satellite and MF coast station coverage. Equipment requirements for A4 are the most demanding of the four categories.
Why the distinction matters for carriage
A vessel's required GMDSS fit is determined by the sea areas in which it operates. A yacht trading exclusively within A1 has a lighter statutory equipment burden than one passing through A3 or A4. When a vessel changes trading pattern — a refit, a repositioning passage, a new charter ground — the ETO must confirm whether the existing fit remains compliant for the new sea area. The sea area designations are the entry point to every carriage requirement decision.