1The lesson in plain words
In one sentence…
Inmarsat-C exchanges data, sends distress alerts and receives SafetyNET without voice.
What is it for?
Inmarsat-C exchanges short messages, supports distress alerting and receives MSI through EGC. It does not provide ordinary voice, so it cannot replace a voice conversation with the rescue authority.
What is EGC?
It is a shore-to-ship data broadcast for MSI, including SafetyNET. Configure the voyage area and read incoming warnings.
What do you check first?
Check the network, identity, position and UTC time; confirm compatibility, actual coverage and availability because actual coverage matters for the service.
What do you do after an accidental alert?
Notify the RCC promptly through the system or another effective means and record the cancellation; switching off does not cancel it.
2Step by step
Inmarsat-C distress alert
- 1
Confirm power, terminal or antenna status and the registered identity.
You ensure the equipment and its identity are available.
- 2
Check the position and UTC time shown by the terminal.
The alert should use the terminal's current data.
- 3
Confirm that the network is available and the service covers the voyage.
Actual coverage and availability determine whether the service works.
- 4
For practice, use a manufacturer-supported test or simulation; do not press the live control.
Practice must not send a distress alert.
- 5
For an emergency, send the alert using the terminal's dedicated procedure.
It is the approved way to alert through Inmarsat-C.
- 6
Follow the supported rescue path and, if accidental, notify the RCC and record cancellation.
Switching off the terminal does not replace notice or recorded cancellation.
3Key data
- Main function
- Data, alerting and SafetyNET
- Voice
- No ordinary voice radiotelephony
- EGC
- Shore-to-ship MSI broadcast
- Voyage MSI
- Configure area and read messages
- Checks
- Approval, compatibility, coverage, availability
- Consumer equipment
- Does not prove SOLAS compliance
4Key points
- 1.
Inmarsat-C = data, alerting and SafetyNET; no voice.
- 2.
EGC broadcasts shore-to-ship MSI; it is not DSC.
- 3.
Check network, identity, position and UTC time.
- 4.
Accidental alert: notify the RCC; switching off does not cancel it.
5Frequent mistakes
Wrong: "Inmarsat-C lets me call the RCC by voice."
Right: Use the supported distress service; Inmarsat-C does not provide ordinary voice.
Wrong: "Receiving EGC is the same as sending a DSC alert."
Right: EGC receives shore-to-ship MSI; use the dedicated procedure to alert.
Wrong: "A consumer satellite messenger proves SOLAS compliance."
Right: Approval, compatibility, coverage and availability must be checked separately.
6Practice case & exam keys
Situation
A vessel receives a SafetyNET warning about a drifting container 15 NM ahead, and the operator proposes replying to the broadcast through the terminal as if it were a voice call to the RCC.
What is the correct sequence?
Show the answer →
Plot the warning against the route, brief the officer of the watch, adjust the passage if needed and log it; a reply is neither possible nor needed.
Why: EGC is a one-way broadcast with no return path. The warning calls for a navigational decision, not a radio reply. If the ship later has its own emergency, the terminal’s distress button and distress-priority message are used, followed by a voice path to the RCC.
Exam keys
If you see "voice" with Inmarsat-C, be cautious: the service does not provide ordinary voice radiotelephony.
"EGC/SafetyNET" means shore-to-ship MSI reception, not a DSC alert.
"Switch off" or "log out" after a false alert is insufficient: notify the RCC promptly.
How to read it
Think of Inmarsat-C as a data mailbox with alerting: it handles messages and SafetyNET, but voice conversation needs another supported path.
When you read "coverage", separate recognized service, fitted terminal, available network and actual coverage; none replaces those checks.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
- 1. Geostationary satellite, about 36,000 km up over the equator (e.g. Inmarsat)
- 2. Polar cap outside its coverage
- 3. Low-orbit satellite, under 1,000 km, handing calls to the next one (e.g. Iridium)
- 4. Polar region covered by the low-orbit constellation
From "Inmarsat" to RMSS
Modern GMDSS speaks of a Recognized Mobile Satellite Service (RMSS). IMO assesses and recognizes services that meet requirements for availability, coverage, distress priority, routing to an RCC, MSI broadcasting and oversight. IMSO (International Mobile Satellite Organization) oversees the operating providers through public-service agreements, monitoring and contingency exercises.
| Service | Architecture | Teaching status (2026) |
|---|---|---|
| Inmarsat C with SafetyNET | Geostationary satellites plus terrestrial network | Operational and widely installed; distress messaging and EGC/MSI; store-and-forward |
| Inmarsat Fleet Safety | Modern service on the Inmarsat network | Recognized; availability depends on the service and approved terminals |
| Iridium GMDSS (Safety Services) | Interconnected low-earth-orbit constellation | Operational; distress alert, Safety Voice and SafetyCast MSI; global coverage including polar regions |
| BDMSS (BeiDou Message Service System) | Regional BeiDou service | Recognized by IMO with regional coverage; check IMSO for its current implementation status before relying on it |
GEO versus LEO
| Aspect | GEO (e.g. Inmarsat) | LEO (e.g. Iridium) |
|---|---|---|
| Altitude | High geostationary orbit, about 36,000 km | Low earth orbit, under 1,000 km |
| Apparent motion | Almost fixed in the sky | Satellites move; the network hands calls over |
| Polar coverage | Unfavourable geometry at high latitudes | Possible, including the poles |
| Signal delay | Higher | Lower |
| Antenna | Can point at one orbital region | Tracking/handover managed by the system |
Inmarsat C, as an operator sees it
Inmarsat C is a low-rate digital messaging service based on store-and-forward: your message goes into the network and is delivered onward, not over a live circuit. A GMDSS terminal can send priority distress alerts to an associated MRCC, send and receive text and data, and receive EGC. It is not a voice system. Its simplicity, robustness and installed base explain why it has lasted.
| Field on the terminal | What to check |
|---|---|
| STATUS / LOGGED IN | Terminal registered and available |
| POSITION / UTC | Current GNSS data, not a frozen value |
| EGC | MSI and SAR messages waiting, and their priority |
| INBOX / OUTBOX | What has arrived, what is sent and what is still pending |
| DISTRESS | Nature, position, time, priority; never delay the alert to perfect them |
You do not select AM, FM or SSB on a satellite terminal: it manages the radio layer. You operate services: login, store-and-forward messages, EGC/MSI, position, distress and logs.
Inmarsat C operating detail
- Region and LES. The terminal logs in to one ocean region at a time and sends traffic through a land earth station (LES) that you select or that is preset. A distress alert or distress message is routed with priority to the RCC associated with that LES.
- Distress alert versus distress message. The distress button sends a short, pre-formatted alert (identity, position, time, nature). When time allows, follow it with a distress priority text message giving the details, addressed to the RCC.
- Two-digit special access codes send short messages free of charge to the right service through the LES:
| Code | Service |
|---|---|
| 32 | Medical advice |
| 38 | Medical assistance (evacuation) |
| 39 | Maritime assistance |
| 41 | Meteorological reports from the ship |
| 42 | Navigational hazards and warnings reported by the ship |
| 43 | Ship position reports (e.g. to AMVER or national systems) |
- EGC priorities. EGC messages carry a priority (distress, urgency, safety or routine). Distress and urgency messages sound an alarm and cannot be suppressed.
| Operation | What it does | When |
|---|---|---|
| Login | Registers the terminal with the ocean region's network coordination station (NCS) so it can receive traffic and EGC | After switching on or changing region |
| Logout | Tells the network the terminal is going off air, so traffic is not queued for it | Before switching off for a long period |
| PVT (performance verification test) | Network-run test of the terminal's send and receive paths, including a simulated distress alert that is not passed to an RCC | After installation or when the manual says |
| EGC area by position | The receiver accepts area messages by its current position, so keep the position updated | Always; check after GNSS faults |
| Distress message editor | Pre-filled distress alert (identity, position, time, nature) plus a free-text message sent with distress priority | Only in a real distress |
| DNID data reporting and polling | Shore systems can ask the terminal for automatic position reports, used for fleet tracking and LRIT | Configured by the company or Administration |
Fleet Safety and Iridium GMDSS terminals
Newer RMSS terminals work differently from Inmarsat C, but the operator's jobs are the same.
| Job | Inmarsat Fleet Safety | Iridium GMDSS |
|---|---|---|
| Distress alert | Guarded distress button: alert with identity and position to the associated RCC | Guarded distress button: alert with identity and position to the associated RCC |
| Talking to the RCC | Distress priority voice and data through the terminal | Safety Voice: a priority voice call to the RCC |
| MSI | SafetyNET II, by the ship's position and selected areas | SafetyCast, by the ship's position and selected areas |
| Cancelling a false alert | Use the terminal's cancel function, then confirm with the RCC by voice or message | Use the terminal's cancel function, then confirm with the RCC by Safety Voice |
| Daily check | Terminal status, position, MSI reception | Terminal status, position, MSI reception |
Iridium GMDSS
Iridium uses a constellation of 66 operational interconnected low-orbit satellites, giving global coverage including the polar regions. Its GMDSS services include Distress Alert, Safety Voice and MSI through SafetyCast, depending on the approved terminal.
EGC: Enhanced Group Call
EGC is a selective satellite broadcast that distributes MSI and SAR information to groups of ships by area, service or other criteria. SafetyNET is Inmarsat's EGC service for MSI; SafetyCast fulfils the equivalent role on Iridium. The ship's receiver filters and presents messages according to its position, configuration and message category.
GOC exam check
Compared with a geostationary system such as Inmarsat, a LEO system such as Iridium offers:
8Full lesson & sources
Open the full lesson (diagrams and official references)
Inmarsat-C provides data messaging, distress alerting and SafetyNET reception; it does not provide voice. Other approved satellite equipment may support different services, including Iridium GMDSS with SafetyCast. Approval, installed terminal compatibility, actual coverage and service availability all matter. A consumer satellite messenger does not establish SOLAS GMDSS compliance.
Check the terminal's network status, identity, position and UTC time. Send a real distress alert using its dedicated procedure and follow up with the rescue authority through the supported distress communication service. Configure MSI reception for the voyage and check messages. If a false alert is sent, promptly notify the RCC through the system or another effective means; logging out or turning off the terminal does not cancel it.
Inmarsat C as a data and alerting service
Inmarsat C moves short text messages by satellite on the store-and-forward principle. You write a message, the terminal sends it to a land earth station (LES) through a geostationary satellite about 36,000 km up, and the LES delivers it onward, much as email does. A routine message may take a few minutes to arrive. It does not provide ordinary voice radiotelephony, so you cannot talk to the RCC through it.
What it does provide is three GMDSS functions in one box:
- Distress alerting. The distress button sends a short alert with identity, position, time and nature of distress, and the LES routes it with priority to its associated RCC.
- Distress messages. When time allows, a follow-up text with distress priority gives the details the button cannot.
- EGC reception. The receiver picks up MSI and SAR broadcasts addressed to its area.
| You need to | On Inmarsat C | Not on Inmarsat C |
|---|---|---|
| Alert an RCC by satellite | Distress button, then a distress-priority message | — |
| Talk to the RCC | — | VHF, MF/HF voice, or a satellite voice service such as Iridium Safety Voice |
| Read navigational and weather warnings | EGC SafetyNET | — |
| Report a medical case for free | Special access code 32 for medical advice | — |
EGC and SafetyNET
EGC is a shore-to-ship broadcast facility. A NAVAREA coordinator or RCC sends one message, and every terminal in the addressed area receives it: a whole NAVAREA, a circle around a position, or a rectangle. ITU’s handbook description describes EGC as a data broadcast used for MSI, including SafetyNET. Nothing goes back to shore, so you cannot answer a SafetyNET message by replying to it.
That is why the terminal's position matters. If the GNSS feed freezes at a position 200 NM astern, the terminal throws away a circular-area warning for the water you are actually in, because it believes you are elsewhere. The printer stays quiet and the warning is lost without anyone noticing.
A deliberate terminal workflow
Run this check at the start of each watch, or after any power interruption:
- 1Status shows LOGGED IN to the right ocean region.
- 2Position and UTC on the display match the GNSS, and the time is current.
- 3The EGC receiver is enabled and set to the right NAVAREA/METAREA.
- 4The outbox has nothing stuck as pending.
- 5The distress message editor is filled in with name, call sign and MMSI, ready for use.
In a real distress, press the distress button first, then send the distress-priority message with details, then use a voice path for coordination. If you send an alert by accident, send a cancellation to the RCC at once through the terminal or another means, and log it.
The IMO GMDSS page and SOLAS IV amendments set out the context: a satellite service counts for GMDSS only if IMO has recognized it and your terminal is approved for it.
Worked insight: one terminal, two functions
At 1040 UTC a SafetyNET navigational warning reports a drifting container 15 NM ahead of your track. At 1120 your own engine-room fire becomes uncontrollable. The same terminal handles both events, but the actions are different. For the warning you plot it, brief the officer of the watch, alter course if needed and log it. For the fire you press the distress button, send a distress-priority message and then speak to the RCC on a voice path. Receiving the first does not help with the second.
Inmarsat C is not the only recognized service: Iridium's low-orbit constellation also offers voice and covers the poles. A3 is defined by the coverage of the recognized service your terminal supports, not by a fixed latitude.
The mobile satellite services recognized by IMO for the GMDSS are Inmarsat, including Inmarsat C; Iridium, recognized by MSC.451(99), with SafetyCast for MSI; and BDMSS, recognized by MSC.529(106), with regional coverage only. Each is a different approved service. Avoid latitude shortcuts: A3 is defined by recognized service coverage and the carried ship earth station, not by a universal north-south boundary.
Official references · 2026-09-09
9Test your knowledge
What should you do to send a real Inmarsat-C alert?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: