Satellite GMDSS: the recognized services (Inmarsat C, Inmarsat Fleet Safety, Iridium) and legacy radiotelex

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.

6Exam keys & interpretation

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.

Two globes: on the left three geostationary satellites on a ring around the equator cover the middle latitudes while the grey polar caps stay uncovered; on the right many low-orbit satellites on polar orbits cover the whole Earth, poles included. — 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
This is why A3 depends on the service your ship carries. Not to scale.

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.

References: SOLAS IV as amended by MSC.496(105)

GOC exam check

Compared with a geostationary system such as Inmarsat, a LEO system such as Iridium offers:
1. Higher orbit, fixed satellites and polar coverage
2. Lower orbit, moving satellites and polar coverage
3. Lower orbit, fixed satellites and no polar coverage
4. Higher orbit, lower delay and no handover
Show the answer
B. Lower orbit, moving satellites and polar coverage
Why: LEO satellites fly under 1,000 km, move across the sky with network handover and can cover the poles. GEO is about 36,000 km up with poor high-latitude geometry.
Which best describes Inmarsat C?
1. A store-and-forward text and data service
2. A live voice circuit for distress calls
3. An HF data link through coast stations
4. A broadband service for video and voice
Show the answer
A. A store-and-forward text and data service
Why: Inmarsat C is low-rate store-and-forward messaging with distress alerts and EGC. It is not a voice system.
A crew member has a deep cut. You want medical advice, not an evacuation, using an Inmarsat C special access code. Which code?
1. 38
2. 39
3. 41
4. 32
Show the answer
D. 32
Why: Code 32 is medical advice. Code 38 is medical assistance, meaning evacuation, and 39 is maritime assistance.
After you press DISTRESS on the Inmarsat C terminal it shows that the alert was sent. What does that tell you?
1. The RCC has started coordinating the rescue
2. The alert left the terminal or reached the network
3. An RCC acknowledgement will follow automatically
4. You can stop the MF/HF and VHF alerting
Show the answer
B. The alert left the terminal or reached the network
Why: Sent is not rescue coordinated. Read the status messages, watch for the RCC's acknowledgement and keep other means ready.
An Inmarsat C distress alert is routed with priority to:
1. The RCC associated with the selected LES
2. The nearest coast station on VHF
3. The MCC that serves the ocean region
4. All ships in the same ocean region
Show the answer
A. The RCC associated with the selected LES
Why: Traffic goes through a land earth station, and distress is routed to the RCC associated with that LES. An MCC belongs to the Cospas-Sarsat chain.
An EGC message arrives with distress priority. How does the receiver treat it?
1. It stores it silently for later reading
2. It alarms unless the category is filtered out
3. It alarms only for messages for your own area
4. It sounds an alarm that cannot be suppressed
Show the answer
D. It sounds an alarm that cannot be suppressed
Why: Distress and urgency EGC messages sound an alarm and cannot be suppressed. Filtering cannot silence them.
On an approved Iridium GMDSS terminal, MSI is received through:
1. SafetyNET
2. Safety Voice
3. SafetyCast
4. NAVTEX
Show the answer
C. SafetyCast
Why: SafetyCast is Iridium's MSI service; SafetyNET is the Inmarsat EGC equivalent. Safety Voice is the voice service, not MSI.
Two ships lie side by side in polar waters. One carries an approved Iridium GMDSS terminal, the other only Inmarsat C. Which statement is correct?
1. Both are in A4, because A3 ends at 70° latitude
2. Both are in A3, because the position is identical
3. Both are in A3, because Inmarsat C works at the poles
4. One may be in A3 and the other in A4
Show the answer
D. One may be in A3 and the other in A4
Why: A3 depends on the RMSS supported by the terminal on board, so the same position can be A3 for one ship and A4 for the other. GEO geometry is poor at high latitudes.
MV ALTAIR is 1,100 NM off West Africa in A3 at 0400 UTC with an Inmarsat C terminal. A fire is spreading in the cargo hold; you press DISTRESS and the terminal shows that the alert was sent. How do you give the RCC the details the short alert cannot carry?
1. Send a second alert with the nature of distress changed
2. Send the details with special access code 39
3. Send a distress-priority text message to the RCC
4. Broadcast the details to nearby ships by EGC
Show the answer
C. Send a distress-priority text message to the RCC
Why: The distress button sends a short pre-formatted alert; when time allows you follow it with a distress-priority text message giving the details to the RCC. Code 39 is a real service for maritime assistance, but it is not the distress channel, and ships cannot transmit EGC broadcasts.
Same scenario: an hour later the fixed system has put the fire out, nobody is hurt and the master decides no assistance is needed. What do you do on the terminal?
1. Message the RCC that help is no longer needed
2. Log out of the terminal so the alert stops
3. Press DISTRESS again to send a cancellation
4. Send code 39 to cancel the maritime assistance
Show the answer
A. Message the RCC that help is no longer needed
Why: The RCC is coordinating the response, so you tell it through the terminal, or any other means, that assistance is no longer required. Logging out only stops traffic being queued for you; it does not tell the RCC anything, and pressing DISTRESS again sends another alert.

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

IMO GMDSS · SOLAS IV/10

Schematic two-way communications between a ship, a recognized mobile satellite service, its ground network and a rescue coordination centre. Arrows show communication directions, not guaranteed routing through the same individual satellite or a worldwide coverage promise.