Tutorial Iridium

Tutorial: Iridium

What it is. Iridium is a constellation of 66 low-orbit satellites that carries satellite phones, pagers and data. Each satellite moves across the sky in about ten minutes, so the signals come in short bursts with Doppler shift. OnAir listens to the L-band downlink and shows the network information that is broadcast in the clear: satellites and their beams on a map, the Iridium network time, pager messages and ring alerts (the page that tells a handset to wake up). It does not decode voice or private calls.

What you need

• Radio. A radio that tunes about 1610 to 1630 MHz and runs at a high rate. The mode asks for 10 Msps; the lowest rate that works is 2 Msps, with a channel width of 10.5 MHz. At 2 Msps you see only part of the band. A HackRF, Airspy, bladeRF, USRP, SDRplay RSPdx or similar is needed; an RTL-SDR does not tune this high. See Supported-radios.
• Antenna. An active L-band patch antenna (or a helical antenna) with a built-in LNA, pointed at the sky from outdoors or a window. A band-pass filter for 1616 to 1626.5 MHz keeps strong cellular and Wi-Fi signals out of the radio. The antenna needs power: the radio's bias-tee or a separate injector.
• Frequencies from the repository's frequency database. Iridium L-band downlink 1626.000 MHz (the band is 1616 to 1626.5 MHz with 1 MHz-wide simplex channels), ring alert / paging 1626.270 MHz, band centre 1622.000 MHz (the app default). The tuning range the mode table describes is 1610 to 1630 MHz.

Before you start

• Gain: raise it until the noise rises, but not into overload. With an active antenna, a modest radio gain is enough.
• Keep the capture centred near 1622 MHz so the simplex channels and the ring-alert channel are all inside.
• Satellites are in view all the time somewhere; your sky view decides how many bursts you hear.

Step by step

1. Mode list, group Satellite & navigation, entry Satellites, 1622 MHz.
2. Check frequency (1622 MHz) and rate (10 Msps, or the highest your radio allows).
3. Set your position so the sky geometry works; satellite positions come from the ring alerts and TLEs.
4. Press Start. Until the receiver runs, the panel says "start the receiver to see satellites".
5. Open the tabs Map, Messages, Ring alerts and Bursts.
6. Wait several minutes. Iridium bursts are frequent in the downlink, but positions arrive only with ring alerts ("no satellite positions yet (they come in ring alerts)").

What you should see

On the Map, satellites with a position and their beam centres ("n of n satellites with a position; dots: beam centres"). On Messages, pager messages if any are sent. On Ring alerts, a list of pages. On Bursts, counts of bursts, demodulated bursts and duplicates, with blocks ok and bad.

Reading the screen

• Map. Satellites, beams (shown as dots), amber lines for simplex channels. Click a satellite for its details: Sat, Frames, Beams, Offset kHz, Position, Heard. fit shows all. The Iridium time (IBC) is shown when heard.
• Messages. Pager messages (MSG) with RIC, Seq, Text; select one to read it in full ("Newest pager message"). The identities (TMSIs) paged are not kept.
• Ring alerts. Columns Time, Beam, Alt km, Paged, MHz. They show that a handset was paged on a beam, not who.
• Bursts. Technical counters: downlink and uplink bursts, blocks ok and bad, duplicates. Voice frames are counted only, never decoded.
• Save for iridium-toolkit. A setting writes every demodulated burst as gr-iridium RAW lines (a format name and file name in the recordings folder) so you can process it with the toolkit's parser. OnAir never decodes voice.

Options and settings

• Threshold: burst level over the noise floor needed to demodulate a burst. Lower finds weaker bursts but costs more CPU on noise.
• The built-in test signal lets you try the screens (satellite count, seed, "no traffic").

Troubleshooting

• Nothing seen. Rate too low (below about 2 Msps the band is cut), antenna not powered, or no sky view.
• Bursts but no map. Positions come only in ring alerts; wait longer.
• High CPU. Lower the threshold only if needed; noise bursts cost CPU.
• Overload with strong cell signals. Add a 1.6 GHz band-pass filter.
• Only bursts at the edge. Retune so 1622 MHz is centred.

Tips

Run for an hour and watch how many satellites get a position. A good sky view matters more than gain.

Legal and privacy notes

Satellite phone traffic is private communication. In many countries it is illegal to listen to, record or disclose the content of private calls or messages; the rules differ, so check yours. OnAir decodes only the network information (time, satellites, ring alerts and pager text) and counts voice frames without decoding them. See Legal.

Under the hood

Iridium L-band: 1616 to 1626.5 MHz, TDMA/TDD frames of 90 ms, simplex and duplex channels, DE-QPSK, with the ring-alert channel at 1626.27 MHz. The framework follows the open gr-iridium and iridium-toolkit work (see Mode-Iridium for credits).

Related

Mode-Iridium, Tutorial-Orbcomm, Tutorial-Globalstar, Tutorial-Inmarsat-Aero, Tutorial-Inmarsat-C, Legal, Troubleshooting