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Argos

Argos is the Slocum's one-way satellite position beacon — the backstop that lets you find a glider when the two-way links (Iridium and Freewave) can't be reached. The glider carries a Seimac X-Cat PTT (Platform Transmitter Terminal) whose 401 MHz antenna lives in the tail fin alongside the GPS/Iridium and Freewave antennas. It transmits last-known GPS positions and the Argos network also derives periodic surface fixes accurate to ~100 m from the bursts themselves.

Unlike Iridium, Argos is transmit-only: you cannot command the glider over it, and you cannot send data down to it. It exists for one job — buying you a search area when everything else is silent.

Source

Paraphrased from the Slocum G3 Glider Operators Manual (Rev. 1, "Argos Satellite Platform Transmitter Terminal", "Communicating with Argos", and Appendix H), the Slocum G3 Maintenance Manual, the UG2 community Slack, and the Teledyne Webb Research user forum. This is a condensed field reference — always defer to the official Teledyne documentation, and contact glidersupport@teledyne.com (and Service Argos) before changing any Argos configuration.

How Argos is activated

This is the question that trips people up: Argos transmits only while the glider is powered up at the surface — never underwater. 401 MHz radio doesn't propagate through seawater, so there is nothing to gain (and nothing emitted) while diving. The PTT keys up only when the glider is sitting on top with its antenna in the air.

Behaviour is set by a single sensor, c_argos_on:

c_argos_on Behaviour
< 0 PTT always off, even at the surface
0 (default) PTT powered off underwater, but auto-enabled at the surface
> 0 PTT powered on and transmitting (1 = no diagnostics; 2 = transmitted chars to MLOG/TERM; 3 = transmitted and received chars logged)
  • With the default c_argos_on 0, the glider brings the PTT up on every surfacing and shuts it off when it dives — so in normal operation it is pinging at the surface, just not all the time and not at depth.
  • During an abort, the flight code turns on all communication and location devices (GPS, Argos, etc.) so you have every possible way to locate the vehicle.
  • The emergency circuit independently powers the Argos PTT (along with the air pump and the jettison-weight burn wire). So even after the emergency battery takes over, Argos keeps transmitting — by design, because that is exactly the scenario where you need to find the glider. Note the emergency batteries are not monitored, so don't assume they're at full capacity after a deployment.

Taking Argos out of service to save power

Argos itself is low-power, so it's rarely worth disabling. One operator only removed it (use - argos) out of desperation months from a recovery window — it is not a meaningful power lever compared with the science bay or the Freewave console. Leave it on unless you have a specific reason not to.

The ~90-second ping is also a source of electrical noise

Argos doesn't sit quietly. The standard repetition rate is ~90 seconds (the value you register with Service Argos), and each burst is ~1 W at ~401 MHz. That periodic, fairly strong RF transmission can couple electrically onto other sensor lines that share an A/D, showing up as a periodic spike in the data.

  • The classic symptom is voltage spikes on leakdetect_voltage / leakdetect_voltage_forward — identical in timing and amplitude (a tell-tale that the cause is electrical, not a real leak). When an operator reported this, TWR's first suspect was coupling to Argos transmissions; the next suspect was another device keying on the same A/D. These spikes appear at the surface, which lines up with Argos only transmitting up top.
  • This is the same family of problem as the false digifin leak detect aborts caused by Iridium line noise — an RF transmitter injecting noise into the leak-detect path. If you see periodic leak-detect dips that line up with comms, rule out RF coupling before you suspect water.

90 s vs. \"every two minutes\"

Operators often remember the cadence as roughly every couple of minutes. The documented standard repetition rate is 90 s; the exact spacing you observe can drift a bit and isn't perfectly regular, so don't be surprised if it looks closer to ~2 minutes in a given record. The takeaway is the periodicity, not the exact number.

Service, IDs, and the antenna

  • Service Argos / CLS administers the network. Submit a program application form to the Argos User Office (North America useroffice@argosinc.com, Australia/NZ clsargos@bom.gov.au, all others useroffice@cls.fr, http://www.argosinc.com); they return an acknowledgment, user manual, and IDs.
  • You must provide the glider's ID in both decimal and hexadecimal. The standard repetition rate is 90 seconds. The ID lives in the glider's autoexec.mi.
  • Antenna integrity matters for recovery. TWR specifically warns against reflective tape on the fin — testing showed it significantly degrades the antenna VSWR and can cause very poor Argos/Freewave/Iridium performance, the worst case being trouble locating a glider in distress. If you need to mark the glider, the suggested spot is the aft cowling, not the fin.

Decoding Argos messages

Argos relays the glider's last-known GPS position (plus water-velocity estimates) as a packed hex string delivered by email or telnet from your Argos account.

  • The message format depends on the transmitter and ID width. Gliders with X-Cat PTTs and 28-bit IDs use the current format; older 20-bit IDs use the legacy format. The relevant sensor is f_argos_format. See Argos Data Format in Appendix H for the byte-by-byte layout.
  • A typical message carries an age/timestamp, a valid lat/lon, an invalid (lower-confidence) lat/lon, the last too-far fix, and water v_x / v_y (scaled by 0.05 before transmission — multiply by 0.05 to decode).
  • TWR publishes a decode utility (under …/glider/windoze/production/windoze-bin/), and the forum has a platform- independent open-source Python decoder that parses the hex string bitwise (two's-complement handling for negative lat/lon and water velocity).
  • If a glider is lost and you need fixes fast, contact Service Argos and ask them to enable ALP (All Location Processing) so you get the maximum number of derived positions. See the emergency-recovery notes.

Testing Argos

Argos satellites are in polar orbits, so coverage at any one spot comes and goes — give the test plenty of time.

  1. Put the glider outside with a clear view of the entire sky — not deployed, ideally on a bench power supply to save battery.
  2. Power up, ctrl-c to GliderDOS, then: 
    lab_mode on
use - iridium
    (taking Iridium out of service keeps it from competing while you let Argos run).
  3. Let it transmit for ~3 hours (1–3 h) so multiple passes get through.
  4. exit, bring the glider in, and power off.
  5. Log in to Service Argos / CLS America and confirm hits were received for the glider's ID (from autoexec.mi). You can also email Service Argos (useroffice@cls.fr) to request the data.

Confirming transmissions locally — no network needed

To verify the PTT is keying without waiting on the satellite network, use an RF detector/receiver sensitive to 401.650 MHz ±30 kHz. These range from a cheap passive chirper up to a handheld TSUR-400 Argos receiver, which shows the unit ID, the data bytes received, and some diagnostics. On the glider in PicoDOS, the legacy talk arg command keys the transmitter; every ping prints Transmitting / Send Data Now: — odd-looking, but correct.

PicoDOS / talk arg is lab-only

talk arg lives in PicoDOS — never enter PicoDOS on a deployed glider; it drops the flight code. Use it only on the bench.

Field troubleshooting

Symptom Things to check
Argos shows no hits in a test Needs 1–3 h of clear sky (polar-orbit coverage gaps); confirm c_argos_on ≥ 0 and the ID (decimal and hex) in autoexec.mi; confirm the account is active with Service Argos.
Periodic spikes on leakdetect_voltage (or false leak/digifin aborts) at the surface Suspect RF coupling from the Argos burst onto the shared A/D (identical-timing/amplitude spikes ⇒ electrical, not water). Same family as Iridium-line-noise false digifin leaks.
Glider in distress is hard to locate Confirm Argos is on; ask Service Argos to enable ALP; check the fin antenna isn't degraded (no reflective tape on the fin). Pair with a pinger and strobe for recovery.
Want to save power with Argos off Rarely worth it — Argos is low-power. Only use - argos as a last resort far from recovery; revisit the power-saving page first.

Argos is for finding, not flying

Argos sends position only — you cannot command the glider over it. It is the one-way backstop that turns a lost glider into a search area, which is why it (and a pinger and strobe) factor into every recovery plan. For two-way comms, see Iridium and Freewave.

Quick reference

Item Command / sensor
Enable / disable PTT c_argos_on ( <0 off · 0 auto-at-surface · >0 on )
Take Argos out of service (power) use - argos
Message format selector f_argos_format
Glider ID (decimal and hex) in autoexec.mi
Standard repetition rate 90 s
Transmit frequency / power ~401 MHz, ~1 W (RX detect 401.650 MHz ±30 kHz)
Local test receiver TSUR-400 / passive chirper
Manual key (lab/PicoDOS only) talk arg
Emergency position processing ask Service Argos to enable ALP