Iridium¶

Iridium is the Slocum's satellite link and the primary way you talk to a glider once it is deployed and out of Freewave range. The glider carries an Iridium 9522b bidirectional satellite modem on the lower electronics tray; a low-noise-amplifier (LNA) switch board lets the modem share its quad-helix antenna with the GPS (Iridium ~1626 MHz, GPS 1575 MHz), so the glider can fix its position and then call home through the same antenna in the tail.

Where Freewave is fast, free, and line-of-sight, Iridium is slow and metered but global: when the glider surfaces anywhere in the world with a clear view of the sky, it can reach your Dockserver.

How the call reaches shore: RUDICS vs. dial-up (PSTN)¶

When the glider dials in, energy leaves the tail antenna, bounces satellite-to- satellite across the Iridium constellation, and comes down at a ground station. What happens after the ground station is the difference between the two transports:

• RUDICS is the modern primary. Today the glider's primary number is a RUDICS connection, with a dial-up modem kept as the backup in case the RUDICS host goes down.
• The carrier matters for dial-up: operators have had data calls that worked on one phone carrier simply stop passing data on another — if you run a PSTN backup, test the actual data call, not just the ring.

Primary and alternate numbers¶

The primary and alternate phone numbers the glider dials live in its autoexec.mi (in the config directory). You can override one temporarily with put to test, but the persistent values belong in autoexec.mi.

• The alternate exists as a backup so a glider isn't cut off if the primary host fails (the feature was added after an operator lost a full day of comms to a shoreside power outage).
• The backup logic is more nuanced than a simple "try A, then B" — confirm how your firmware and Dockserver are configured rather than assuming, and test both numbers before relying on the fallback.

The surface comms sequence¶

Every time a surface behavior brings the glider up, it runs the same sequence — worth knowing because it explains when and how you get a console:

1. GPS fix first — the glider tries to fix position for up to 300 s (5 min). Older/slow GPS modules are why some groups raise this.
2. Hand off to comms — once it has a fix it establishes a console: it prefers Freewave if a carrier is present, otherwise it dials Iridium.
3. Surface dialog — it prints the surface dialog and waits ~5 minutes for you to act (end mission, send data, send in new .ma files, retask waypoints or behaviors, etc.). Every option is reminded to you in the dialog itself.
4. No input → continue — if nobody (or no script) interacts within the wait window, the glider simply resumes the mission as written.

Iridium vs. Freewave during a mission¶

• In GliderDOS, Iridium is the primary route and can run alongside a Freewave connection at the same time.
• Mid-mission, the glider picks one: it uses Freewave if a carrier is present and only calls Iridium when Freewave is absent. Data transfers the same way — over Iridium only if Freewave isn't available, unless you force it.
• Force a transfer over Iridium with the -f=irid flag, e.g. S -f=irid -num=3 *.sbd *.tbd.

A neat side effect of two-way Iridium: you can use the glider as a (very expensive) text relay to talk to a field/deployment crew — passing command of the vehicle back and forth between shore and boat through the glider.

SIM card and service¶

• The Iridium phone and SIM card are configured at the factory (talk iridium in PicoDOS). Depinning a SIM is normally a factory step — only needed if you install your own card or change services, and best done with Teledyne's help.
• Buy a data-only commercial Iridium SIM — no extra equipment is needed. Several resellers serve glider users (e.g. NAL Research, JouBeh, CLS America, others). The card must be activated ~30 days before shipment, and Teledyne needs the card and unlock PIN during manufacturing.

Testing Iridium¶

Place the glider outside with a clear view of the sky (buildings, hills, and cars degrade the link far more than open ocean does), then use callback:

• 30 minutes is the maximum callback interval.
• Watch m_iridium_signal_strength (0 = no signal, 5 = max).
• Connection time varies with cloud cover and satellite availability; gpredict can confirm coverage for a test window.

Data transfer over Iridium is slow — plan for it¶

Iridium bandwidth is low, so surface time is dominated by how much science data you ship. Practical guidance:

• Aim for ~10–20 minutes of surface time per call. Much beyond that and you accumulate surface drift (the glider is pushed by wind/current the whole time it sits up top) — and you're paying by the minute.
• If you routinely exceed 20 minutes, you're probably sending too much — trim sbdlist/tbdlist, decimate, or sample on the downcast only.
• Simulate a real dive's worth of data in the lab and try the transfer before deploying; one second of simulation = one second of real time, so you'll quickly see if your data volume is unrealistic.
• The G3's high-speed wired comms port transfers roughly 5× faster than Iridium/Freewave for bench data offload — use it in the lab rather than waiting on the radio. (G1/G2 had no speed advantage from a wire.)

Argos — the one-way emergency position beacon¶

The glider also carries Argos, a one-way 401 MHz satellite beacon (the Seimac X-Cat PTT) used to locate a glider when Iridium and Freewave can't be reached. It is a separate system from the two-way Iridium link covered here — it sends position only, you can't command the glider over it, and its periodic ~90 s transmissions can even couple electrical noise onto other sensor lines.

➡️ See the dedicated Argos page for activation (c_argos_on), the noise behaviour, service/IDs, message decoding, and testing.

Field troubleshooting¶