Slocum Altimeter One-Pager¶

Hardware¶

The transducer is a custom AIRMAR design built for Teledyne Webb — it is not a catalogue part you can source directly from AIRMAR. The externally mounted transducer is the same across board generations; what changed over time is the control board inside the forward section (see below).

Handling¶

Installation¶

If an altimeter reads nothing at all, suspect the wiring before the sensor: remove the nose cone and confirm the altimeter wire harnesses are fully seated, then re-run the Functional Checkout procedure.

Analog vs. digital control board (altimeter vs altimeter_232)¶

Two control boards exist, and knowing which one your glider has is essential — the wrong device name in autoexec.mi is a common reason an altimeter "doesn't work."

• The two boards are not interchangeable by name: if the device listed in autoexec.mi doesn't match the installed board, the glider may not recognise the altimeter or will fail to find the bottom. (A real case: a glider that couldn't find bottom was listed as altimeter_232 but actually had the analog board — switching the device to altimeter fixed it.)
• The switchover happened around certain serial numbers but is not strictly chronological — some later gliders shipped with the analog board, so verify per vehicle with the use command rather than assuming by age.
• The digital altimeter_232 board is not compatible with older Persistor processors (e.g. OS 8.5). A nose-section swap between a Persistor glider and a newer-processor glider requires changing the control board, which involves de-soldering/re-soldering (doable and reversible, but take lots of photos).
• Newer altimeter_232 units often need more tuning to get consistent bottom detection and inflections at the target altitude.
• TWR publishes a Slocum Glider RS-232 Altimeter Guide describing the digital board — request it from Glider Support if you operate altimeter_232 vehicles.

Running an altimeter_232 (digital RS-232 board)¶

When a glider carries the digital board, several things change from the analog board's defaults and habits:

autoexec.mi:

• List altimeter_232 under installed devices and remove altimeter (the two cannot both be installed).
• TWR recommends adding two sensors to override the masterdata defaults for the digital board:

sensor: f_airmar_altimeter_time_until_good_reading(s) 12   # longer boot time of the 232 (default 4)
sensor: u_max_altimeter(m)                            200  # larger expected range of the 232 (default 100)

Behaviour and quirks:

• In a mission the altimeter_232 should behave the same as the analog board (c_alt_time 0 = ping as fast as possible, c_alt_time -1 = off). Keep u_alt_reduced_usage_mode 1 in most cases.
• m_altimeter_voltage produces no output on the digital board — the analog "2.5 V = no echo" diagnostic does not apply (see the 232 functional test below).
• adtest altimeter is obsolete; use talk altimeter instead.
• The 232 has greater range — TWR has operationally observed up to ~130 m (vs ~100 m on the analog board), and more may be possible.
• The clicking cadence sounds slightly different from the analog board.

Glider modularity (mixing forward and aft sections):

• The aft-section/science-bay connector 305912-NFC (Rev. C) was modified to carry the extra altimeter line to the SOM carrier board. It is backwards compatible with the analog board.
• To run an altimeter_232 forward section against an aft section not built for it, install the Rev. C 305912-NFC connector and make the software changes above.
• To run an analog forward section against an aft section built for the 232, the Rev. C connector works as-is — but revert the software: remove altimeter_232, install altimeter, and restore the masterdata defaults for f_airmar_altimeter_time_until_good_reading (4 s) and u_max_altimeter (100 m) by commenting those lines out of autoexec.mi.

How to Test¶

In the lab:

1. Remove the sonar dome.
2. Type put c_alt_time 0 to set the altimeter to update as fast as possible.
3. Type report ++ m_altimeter_voltage m_raw_altitude.
4. Place your ear against the face of the altimeter — listen for a faint, regular tick no louder than a watch.
5. Type report clearall.

Testing the digital altimeter_232 board (the voltage/click method above does not work — m_altimeter_voltage never updates):

1. put c_alt_time 0 — power the altimeter on.
2. put u_alt_debug 1 — verbose altimeter debug output.
3. Confirm the transducer is clicking.
4. Confirm the debug stream shows altimeter_232_measure, received '$SDDPT…' and altimeter_232_measure() RX valid message '$SDDPT…' lines (the board reports depth over RS-232 as the $SDDPT NMEA sentence).
5. put c_alt_time -1 — power off; wait for altimeter_232_power_off(), power removed, uart closed and confirm the clicking stops.
6. put u_alt_debug 0 — turn verbose debug back off.

Key Settings¶

Valid reading window (u_min_altimeter / u_max_altimeter)

• A m_raw_altitude reading is only accepted if it falls between these two values (inclusive); anything outside is rejected.
• On G3S gliders the autoexec defaults are typically u_min_altimeter 2 m and u_max_altimeter 200 m. Adjust to suit your terrain (see false-hit mitigation below).

Altimeter min depth (u_alt_min_depth)

• In deep water, set >100 m to avoid false returns from the surface or thermocline.
• If the bottom will always be deeper than the glider's operational dive depth, consider disabling the altimeter entirely to save energy.
• The altimeter will kick the glider out of low-power mode. In stable-depth water, raise u_alt_min_depth so the glider spends more time in low-power mode.

Reduced usage mode (u_alt_reduced_usage_mode)

• Default: ON (1). After a good hit, the glider calculates the halfway point to the bottom and powers the altimeter off until then — saving energy.
• Disable with put u_alt_reduced_usage_mode 0 when operating in areas with unknown or highly variable depth. Note: this increases energy use.

Water depth lifetime (u_max_water_depth_lifetime)

• Sets how long the glider keeps using the last known m_water_depth when no new altimeter fix is acquired. In newer firmware: if m_water_depth is not updated within u_max_water_depth_lifetime × m_avg_yo_time seconds, it is set to -1 (marked unusable).
• If set to 1 and a false hit is recorded, the next dive will turn around at the false depth — but by dive 3 the value is discarded and normal dive behaviour resumes.

Reducing false bottom hits¶

Premature inflections from false returns — off the thermocline, scattering layers, or zooplankton — are a common operational problem, especially over complex bathymetry. (False returns are often worse in daylight, when zooplankton are concentrated, and improve at night as the layer disperses.)

Mitigations operators have used:

• Increase u_alt_reqd_good_in_a_row — how many good readings in a row are required before the glider acts. Default is 3; operators have raised it to 10 or even 25 to suppress false layers.
• Raise u_min_altimeter (e.g. to ~20 m) so intermediate scattering layers close to the vehicle don't trigger a turn.
• Lower u_max_altimeter — in one case dropping the max range from 100 to 65 eliminated periodic full-range false hits late in a long mission, behaving as if the detection threshold had drifted with sensitivity.
• Median filter (u_alt_filter_enabled 1) helps smooth noisy readings.

m_altimeter_status codes¶

m_altimeter_status reports why a reading was or wasn't used:

Acoustic signature (for passive-acoustic users)¶

For passive-acoustic monitoring, the Slocum altimeter pings at 170 kHz on the downcast only, roughly every 4 seconds. Because the pulse is very sharp, it smears across the full spectrum of a spectrogram rather than appearing only at 170 kHz, and the contamination is worse the closer the glider is to the bottom. (For contrast, a Seaglider altimeter pings near 12 kHz.)

Relevant Sensors¶