Tutorial Inmarsat C

Tutorial: Inmarsat-C

What it is. Inmarsat-C is a maritime satellite messaging system for ships. The ground station (NCS) broadcasts a continuous common channel, and in it the Enhanced Group Call (EGC) and SafetyNET services send safety information to ships: navigational warnings, weather and search-and-rescue messages. OnAir decodes this broadcast channel. You get public safety bulletins and system information, not private ship traffic.

What you need

• Radio. A radio that tunes about 1525 to 1559 MHz. The mode asks for 2 Msps (lowest working rate 250 ksps; the channel is only 5 kHz wide). An RTL-SDR does not reach this band by itself. See Supported-radios.
• Antenna. A right-hand-circular L-band patch or helix with an LNA and filter, pointing at the geostationary satellite. Antenna power: bias-tee or injector. A small dish improves the signal.
• Frequencies from the repository's frequency database and the mode's list. Inmarsat-C NCS TDM carriers at 1537.100 MHz (Indian Ocean) and 1541.450 MHz (channels differ per region, check current lists). Satellites in the list: Alphasat 25E (Europe, Africa, Indian Ocean) 1537.10 MHz; Inmarsat-4 F3 98W (Americas) 1537.70 MHz (AOR-W); Inmarsat-3 F5 54W (Atlantic East) 1541.45 MHz (AOR-E); Inmarsat-4 F1 143.5E (Pacific, Asia) 1541.45 MHz (a community value, not confirmed from Inmarsat's own list).

Before you start

• Pick the satellite you can see from your location (elevation above the horizon).
• Gain: moderate with an LNA. The carrier is 1200 baud BPSK, narrow.
• Patience: a frame takes 8.64 s and the first packets can take a minute.

Step by step

1. Mode list, group Satellite & navigation, entry EGC, L-band.
2. Choose the Satellite from the list; it tunes the NCS common channel (you can still tune by hand).
3. Optional: find more channels ("Look for other Inmarsat-C channels in the captured band and decode up to three of them at the same time"). Raise the rate for more channels.
4. Press Start.
5. Open the tabs Messages, Channel and System.

What you should see

After a minute or two the first frames; on Messages, rows with Time, Prio, Satellite and a text; the panel says "no message yet: a frame takes 8.64 s and a long message needs several frames" until then.

Reading the screen

• Messages. Time UTC, Priority, Service, Pkts; click a message to read it all. Long messages are cut for the older ones to save memory ("the rest of this older message was cut to keep the list small"). A filter hides messages that do not match.
• Channel. Per channel: kHz, State, Frames, Msgs, and the packets good / failed.
• System. The NCS frame number and time, Station, Packets by type (Descriptor, Meaning, Good, Failed), and the services the NCS advertises ("waiting for a bulletin board packet" until it is heard).

Options and settings

Satellite, find more channels, and a test signal ("simulated NCS", "inverted").

Troubleshooting

• No message yet. Wait: EGC messages are infrequent. Check the dish pointing.
• Frames but no messages. Long messages need several frames.
• Wrong satellite. Pick the one above your horizon.
• Weak signal. Add an LNA and aim carefully.

Tips

Start with the satellite whose region you are in: IOR for Europe/Africa, AOR-W for the Americas.

Legal and privacy notes

SafetyNET and EGC are public safety broadcasts. Individual ship messages travel on other channels and are not decoded here. Receiving is legal in many countries; check yours. See Legal.

Under the hood

NCS TDM common channel: 1200 baud BPSK, frames of 8.64 s with a bulletin board and packets (EGC, SafetyNET, FleetNET). Message formats follow the Inmarsat-C system definition (IMO resolutions on SafetyNET).

Related

Mode-Inmarsat-C, Tutorial-Inmarsat-Aero, Tutorial-NAVTEX, Legal