Tutorial Inmarsat Aero

Tutorial: Inmarsat Aero

What it is. Inmarsat's classic Aero service is the satellite data link between ground stations and aircraft in L-band. OnAir decodes the P channels, the packet-mode channels from the ground station to the aircraft, at 600, 1200 and 10500 bit/s. These carry ACARS-style messages, log-on information and signalling. It is a niche mode for people with a small L-band dish or patch antenna and a bit of patience.

What you need

• Radio. A radio that tunes about 1525 to 1559 MHz. The mode asks for 4 Msps (lowest working rate 240 ksps). Raise the rate so the 600/1200 bit/s and 10500 bit/s groups both sit in the capture; a 4 Msps radio covers them together. 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 band-pass filter, pointing at the geostationary satellite you chose (a small dish for more signal). Antenna power: bias-tee or injector.
• Frequencies from the repository's frequency database. Inmarsat Aero P-channel group 1545.000 MHz and the app default 1545.570 MHz. The mode's satellite list gives the channels: Alphasat 25E (Europe, Africa): P channels 1545.115 to 1545.130 MHz (600 and 1200 bit/s, Fucino), 10500 bit/s at 1546.0125 and 1546.0275 MHz; Inmarsat-4 F3 98W (Americas): P channels 1545.020 to 1545.205 MHz (the busiest group), 10500 bit/s at 1546.005 to 1546.0775 MHz; Inmarsat-3 F5 54W (Atlantic, South America): 1545.025 to 1545.040 MHz (600 bit/s); Inmarsat-6 F1 83.5E (Indian Ocean, Asia-Pacific): 1545.160 to 1545.225 MHz; Inmarsat-4 F1 143.5E (Pacific, Asia): no published list, the receiver searches about 1 MHz either side.

Before you start

• You need a satellite that is above your horizon; look up its longitude and elevation from your position.
• Gain: moderate to high with a dish and LNA; avoid saturating near cellular signals.
• Not decoded: C channel voice. R and T channels (aircraft to ground) are not started.

Step by step

1. Mode list, group Satellite & navigation, entry Aero, L-band.
2. Choose your Satellite from the list (or "Not chosen (tune by hand)"). The tip says "pick your satellite above so the receiver starts on its channels".
3. Set the rate to 4 Msps so both channel groups are in view.
4. Press Start.
5. Open the tabs Messages, Aircraft map, Aircraft, Log-ons, Channels and Signal units.

What you should see

On Channels, one row per P channel: MHz, State, bit/s, Level dB, Frames, UW miss, SUs ok / bad, Raw BER. On Messages, rows with Time UTC, AES, Reg, Flight, Label and a decoded meaning; click one to read all of it. Aircraft messages are rare: a busy channel gives a few a minute.

Reading the screen

• Messages. Filter by label ("H1", "A6"); a checkbox also lists messages whose ACARS check or parity failed. Decoded and Text show the content of the message; Meaning explains the label.
• Aircraft map and Aircraft. Aircraft positions from their own reports (ADS-C, text positions), every few minutes, not live like ADS-B. On a P channel the ground stations talk to the aircraft, so a real P channel gives few or none; the test signal sends some. Blue marks an ADS-C report, amber a text report, grey older than 30 minutes. The table has AES address, Registration, Messages, Last label, Last heard, Position, Alt ft.
• Log-ons. Time, AES, GES, Channel, Event.
• Signal units. Counts of signal units (12 bytes) by type, the ones with a failed check counted separately.

Options and settings

SNR sound plays a tone that rises with the strongest channel's Eb/N0 to help you aim the antenna. Test signal: simulated P channels, drift, CFO.

Troubleshooting

• No carrier seen yet. The band search needs a few seconds ("no carrier seen yet").
• Wrong satellite. Pick the one you can see; the channel list differs per satellite.
• Nothing decoded. Aim at the right satellite; check the LNA power.
• No positions. Expected on P channels.
• Low rate. Both groups need about 4 Msps.

Tips

Start with the 98W or the 25E group, which are the busiest.

Legal and privacy notes

Aircraft messages can contain private information. In many countries it is illegal to disclose it; keep what you decode to yourself and follow local rules. See Legal. Voice is not decoded.

Under the hood

L-band packet channels (P): R/T-style signalling in signal units (SUs) of 12 bytes with a unique word; rates 600, 1200 and 10500 bit/s found automatically. Channel tables follow Inmarsat's published L-band frequency list as used by community decoders.

Related

Mode-Inmarsat-Aero, Tutorial-Inmarsat-C, Tutorial-ACARS, Tutorial-Iridium, Legal