Altimeter¶

The Seaglider's altimeter is an acoustic bottom detector: it pings downward during the dive so the glider can start its apogee maneuver a safe margin off the seafloor instead of trusting $D_TGT or a bathymetry map. The same board is also a transponder/pinger, usable with ship tracking systems for relocation. Unlike a dedicated downward altimeter, though, it is effectively omnidirectional — a fact that drives most of its quirks.

Source

Paraphrased from the APL-UW IOP office-hours material ("How to properly use the altimeter", April 2026), the APL-UW SGX Documentation, and the Applied Acoustic Engineering LF Transponder/Altimeter Board Set Operation Manual (SPC-5414). Check parameter definitions against the Parameter Reference Manual for your firmware.

First question: do you actually need it?¶

IOP's advice starts a step earlier than configuration — the altimeter is not always the right tool:

It uses power that could go to science or endurance.
Bathymetry maps are often more consistent: a good bathymap on the compact flash lets the glider choose dive depth without pinging at all, and $D_OFFGRID (the depth assumed when off the map) is another option.
False positives cost energy: every phantom bottom means a short dive, and short dives spend proportionally more energy per distance.
Missed detections cost more: a non-responding altimeter on an altimeter-dependent mission ends with the glider on the bottom.
It is omnidirectional — the nearest big reflector becomes the range, whether that is the seabed, a wall, or the surface (see the $ALTIM_PING_DEPTH notes at seaglider.pub/parms for how the surface return is ignored).

Bottom-detection modes¶

One ping per dive¶

Set $ALTIM_BOTTOM_PING_RANGE (with $ALTIM_BOTTOM_TURN_MARGIN): the glider pings once, at the given range above the presumed apogee depth, and adjusts the turn depth from that single measurement. Cheapest and simplest — fine when the bathymetry is gentle and roughly known.

Repeated pings with a linear fit¶

For real bottom-following, the glider pings continuously and fits a line to the returns to decide whether the bottom is genuinely approaching:

Parameter	Meaning
`$ALTIM_PING_DEPTH`	Depth at which pinging starts
`$ALTIM_PING_DELTA`	Ping every this many metres of descent
`$ALTIM_PING_FIT`	Packed three-digit control of the fit (below)

$ALTIM_PING_FIT packs three settings into its decimal digits:

Ones — number of pings used in the fit (one every $ALTIM_PING_DELTA m).
Tens — the R² threshold the fit must exceed (default 0.8).
Hundreds — slope threshold: |fitted slope| must be greater than this value and less than its reciprocal.

A good starting value is 873. The fit output appears in the $PING lines of the GC table: $PING, depth, x1, x2, m, rsq — current depth, first and most recent fitted bottom depths, fitted slope (negative = bottom approaching), and fitted R². Current firmware has real limitations here — notably no separate up/down control of the ping schedule.

Tuning the detector¶

Parameter	Range [default]	Effect
`$ALTIM_FREQUENCY`	10–25 kHz [15]	Ping frequency; also the escape hatch when another acoustic source is interfering
`$ALTIM_SENSITIVITY`	0–5 [1]	Envelope-detector threshold; 0 disables the envelope detector and triggers on any return at the right frequency
`$ALTIM_PULSE`	1–9 ms [5]	Transmitted pulse width; a valid return must exceed the sensitivity voltage for a full pulse width

The defaults are good. The field heuristics:

False hits vs. missed bottom

False altimeter hits (phantom bottoms, short dives): alternately increase $ALTIM_PULSE and $ALTIM_SENSITIVITY one unit at a time.
Unable to detect the bottom: alternately decrease them, incrementally.

The hardware¶

The board set is an Applied Acoustic Engineering LF transponder / pinger / altimeter (SPC-5414 family), mounted with the transducer in the vehicle and run from the glider's 24 V (transmit) and 10 V (receive) supplies. Points that matter operationally:

It defaults to transponder mode at power-up and quietly reverts to transponder mode after ~2 minutes without serial activity; the glider wakes it over a 9600-baud serial link when it wants altimetry.
The receiver uses a variable-gain amplifier (compensating attenuation with range) feeding an envelope detector — this is what $ALTIM_SENSITIVITY and $ALTIM_PULSE are tuning.
Maximum altimeter range is 500 m (1000 m round trip, ~667 ms).
As a transponder it supports well over a hundred channel combinations for Simrad HPR/ORE Trackpoint-class tracking systems — useful for relocating a glider from a ship.
Never operate transponder or altimeter with the transducer disconnected.

For bench checks, the glider menu tree has an altim section (ping to fire a test ping, config to upload configuration, direct for pass-through serial comms with the board). Verify altimeter behavior on the first dives of a mission before relying on it: watch the $PING records and the apogee depths against known bathymetry.

When it goes wrong at sea¶

Two abort/recovery codes tie back to this system: BOTTOM_OBSTACLE_DETECTED (bottom or obstacle seen when the altimeter is in use for bottom detection) and SURFACE_OBSTACLE_DETECTED (surface detection — relevant under ice). Repeated short dives with plausible-looking $PING returns usually mean the sensitivity/pulse combination is too eager — see the tuning heuristics above — or that another acoustic instrument on board is stepping on $ALTIM_FREQUENCY.