Module 6 — Inmarsat C and EGC
Inmarsat C satellite terminal: login, position update, messaging priorities, distress alerts and MSI reception via EGC / SafetyNET.
Inmarsat C is a common GMDSS ship earth station for vessels covering A3 with that recognized service. The student must understand the satellite workflow even if the vessel uses another approved RMSS.
Basic operation
Distress alert
Dedicated DISTRESS button. The terminal sends MMSI, position, time and nature to the RCC associated with the LES.
Enhanced Group Call (EGC) and SafetyNET
EGC automatically receives MSI (maritime safety information) addressed to the vessel's area: NAVAREA messages, weather warnings, SAR alerts.
Common errors
Failing to log in again after a power cut, stale position, confusing an EGC inbox message with a personal inbox message.
From Inmarsat to RMSS (recognized satellite services)
Section 3.3 of the 2024 manual no longer anchors satellite on a single brand. The services recognized by IMO for use in the GMDSS are Inmarsat-C (in all four ocean regions AOR-E, POR, IOR, AOR-W), Inmarsat Fleet Safety and Iridium GMDSS services (resolution MSC.451(99), fully operational since 2020 on a 66-satellite LEO constellation in the 1.6 GHz L-band). The concept is learned in the plural: recognized mobile-satellite services.
Inmarsat C architecture
Inmarsat C is a low-speed store-and-forward data terminal for messages, EGC (SafetyNET), distress alerting and data reporting. It uses a compact omnidirectional antenna, which suits any ship. The network relies on LES (Land Earth Stations) and a 24/7 NOC in the United Kingdom with backup in the Netherlands.
Distress alert via Inmarsat C
The alert is started by pressing and holding the dedicated button for at least 5 seconds. The message carries MMSI, position and UTC time (from an integrated or external GNSS) and nature of distress if pre-selected. It reaches the LES associated with the ship, which routes it to the responsible RCC. Follow-on distress traffic is carried by telex or e-mail on the same terminal.
EGC: SafetyNET and SafetyCast
International SafetyNET (over Inmarsat) and Iridium SafetyCast broadcast MSI and SAR information across their respective coverage. The operator must configure the terminal with the current NAVAREA / MetArea and the correct position; bad filters can silently drop relevant messages. Section 4.4 of the manual describes EGC in detail.
Testing and discipline
Testing the Inmarsat C distress button must use only the built-in self-test function; a live alert is never transmitted as a test. An accidental alert is cancelled immediately by contacting the LES and the RCC to record the cancellation.
Message terminal: inbox, address book and LES
Inmarsat C runs an EGC inbox (SafetyNET) and a directed-message inbox; the operator composes messages in the terminal's editor, addresses them through the address book or by entering the recipient identity, and selects the LES through which the message will leave the ship. Logging in marks the terminal as available on the network and allows it to receive EGC; logging out releases network resources before a long shutdown and must be done deliberately. The terminal identity is the IMN (Inmarsat Mobile Number, 9 digits starting with 4, or equivalent format), distinct from the MMSI; each LES has a two-digit access code that must be combined with the destination address.
Priority, destination and acknowledgement
When composing a message the operator selects one of four priorities: distress, urgency, safety or routine; the first three get prioritized handling on the network. The same terminal carries telex, e-mail (via the LES gateway), SMS and fax depending on LES-supported services. If the recipient does not answer or the address is wrong, the network returns an NDN (Non-Delivery Notification); an unread NDN means the message was not delivered, and the operator must resend by another route or correct the address.
Exact Inmarsat C alert sequence
- 1Lift the cover or break the seal on the dedicated DISTRESS button.
- 2Press and hold ≥5 s; a countdown is shown on the terminal.
- 3Position and UTC time are auto-filled from the internal or external GNSS.
- 4If time allows, pre-select nature from the menu (fire, flooding, collision, grounding, listing, sinking, disabled, abandoning, piracy, man overboard, undesignated).
- 5The terminal transmits to the default associated LES, normally configured to a LES co-located with the RCC responsible for the area.
- 6The LES forwards the alert to the MRCC.
- 7Follow-on traffic by telex or e-mail on the same terminal, with 'distress' priority flagged on every message. Iridium SafetyCast: equivalent sequence with a dedicated button held ≥3-5 s depending on maker; the LEO constellation at ~780 km also covers the poles and removes the classic A4 gap of geostationary Inmarsat.
Cancellation and LES selection
Cancellation: open the sent-messages log, generate a 'distress' priority message to the same LES with text 'Cancel my distress alert of [UTC time] sent via [LES name]. No distress, no distress. [Vessel name, MMSI, master name]'. Additionally, contact the LES and the RCC by voice (Inmarsat Fleet, Iridium voice or a normal telephone) to confirm cancellation. LES selection: for efficiency pick the LES nearest the responsible RCC; the list of LES and their two-digit access codes lives in the SafetyNET / SafetyCast manuals and in ITU List IV.
Store-and-forward vs real time
Inmarsat C is store-and-forward: it is not suitable for on-scene voice; each message sits in a buffer until a satellite window opens (typical latency 1-5 min). Iridium offers real-time voice through its LEO constellation, although Iridium GMDSS SafetyCast is used mainly for messaging and alerting; dial-up Iridium voice is not itself recognized GMDSS, but it can act as a secondary means to contact the RCC.
RCC routing and recognized-RMSS verification
(1) Inmarsat-C distress: ship terminal → NCS (Network Coordinating Station) of the selected ocean region → associated LES → responsible RCC (typically the MRCC of the country where the LES is located, per routing table).
(2) Iridium Safety Voice and SafetyCast: LEO constellation at ~780 km → Iridium gateway → designated RCC per the service routing table.
(3) BDMSS (BeiDou Maritime Safety Service) and other regional RMSS are recognized by resolution A.1001(25) as revised, but their regional availability and operational readiness at the time must be verified against the GMDSS Master Plan (GISIS) before sailing — global GMDSS equivalence cannot be assumed for every satellite service.
(4) If the terminal fails to log in to the network, there is no distress routing: verify ocean region and NCS synchronisation before departure; a terminal without login is equivalent to a set switched off from the system's point of view.
(5) The EGC receiver filters by NAVAREA and by geographic rectangle: a stale position on the GNSS interface can silence critical alerts addressed to the current area — always verify the GNSS input to the EGC receiver.
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context. Do not let AIS or one isolated bearing replace systematic observation.
Safe speed is judged by what the ship can actually do in the moment: stop, turn, reduce exposure and avoid collision within the available sea room.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
If schedule, fuel economy or comfort appears in the options, it is not a Rule 6 factor.
Key Takeaways
Inmarsat C is a common satellite medium in A3 when that recognized service is installed on board.
EGC / SafetyNET automatically receives MSI for the vessel's area.
The terminal must always be logged in with a live position.
The RMSS recognized for GMDSS are Inmarsat-C, Inmarsat Fleet Safety and Iridium (MSC.451(99)).
Inmarsat C handles alerting, EGC (SafetyNET) and telex/e-mail with an omnidirectional antenna.
The distress button must be held at least 5 seconds; the alert routes via LES to the RCC.
SafetyNET and SafetyCast broadcast MSI/SAR and require correct NAVAREA and position setup.
The Inmarsat C terminal handles EGC and directed inboxes; addressing via the address book reduces errors.
Explicit login/logout marks availability and frees resources when leaving coverage.
Distress/urgency/safety/routine priority changes network handling; setting it correctly is part of the job.
IMN and LES access code are identifiers distinct from the MMSI; an NDN flags non-delivery.
Inmarsat C sequence: cover → button ≥5 s → auto position/time → nature → send via LES → confirm with RCC.
Iridium LEO at ~780 km covers the poles and closes the classic satellite A4 gap.
Cancellation: 'distress'-priority message to the same LES plus voice call to LES and RCC.
Common Mistakes
Terminal logged out after a power cut.
Static position left in the terminal for hours.
Treating an EGC safety warning like ordinary mail and not informing the bridge.
Reducing A3 to Inmarsat C without considering Iridium or the other recognized services.
Testing the distress button by live transmission instead of using the self-test.
Leaving Inmarsat C with a previous voyage's NAVAREA and missing warnings for the current area.
Assuming the terminal 'always has position': check the GNSS source as on a VHF-DSC set.
Composing a message and forgetting to set 'safety' priority when it applies.
Ignoring an NDN and assuming the message arrived.
Switching off the terminal without a logout and leaving queued messages the LES cannot deliver.
Confusing the IMN with the MMSI when giving the ship's identity over the radio.
Expecting real-time communication over Inmarsat C: it is store-and-forward and has latency.
Picking a random LES instead of the one nearest the responsible RCC and adding administrative latency.
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