0060 — Auv/CTD lag states open at zero, not at the field values: the environment is unresolved in InitialConditionSetup

ID	0060
Class	BUG
Severity	2
Status	ready
Models	Auv/CTD
Found	2026-09-25, while working FISH-0051
Decision needed	Model owner: wait for FHSIM-0025 (shared resources published before InitialConditionSetup, which fixes this with no change here), or work around it locally now (for example seed the lag states in FinalSetup, which is not allowed to set initial conditions, or leave them unset until a later pass, which the host does not offer once the environment is missing). Recommendation: wait for FHSIM-0025 and correct the misleading comment now.

Evidence

SensorCtd::InitialConditionSetup (src/auv/SensorCtd.cpp:238-263) resolves the environment and seeds the three lag states from the field at the sensor:

// The environment is a shared resource, so it cannot be fetched in the constructor,
// and FinalSetup runs after this method rather than before it. Resolving here as well
// is what lets the lag states open at the field values instead of at zero.
ResolveEnvironment(creator);                                        // :247
...
const FieldValues field = FieldValuesAt(dT, position);             // :259
SetInitialConditionIfUnset(adCurrentIC, adUpdatedIC, m_IStateLagT, field.temperature);

FhSim publishes shared resources only after every InitialConditionSetup has run (FHSIM-0025: fhsim/src/simobject/SimObjectOrganizer.cpp:47-54, called at fhsim/src/engine/model/ModelAssemblyService.cpp:103), so GetSharedResource("Environment") returns null here. FieldValuesAt then returns the defaults of FieldValues (src/auv/SensorCtd.h:387-391), 0 degC and 0 PSU, and those become the initial conditions of LagT and LagS. Checked at HEAD 2026-09-25: the log of Auv_CtdStatic says "CtdOrigin tried to access a shared resource Environment which is unset or set to a null pointer", and its CtdOrigin.LagT_0 and LagS_0 are 0 at every output time while Temperature_0 is 8.975 degC and Salinity_0 34.34 PSU.

Effect

With a non-zero time constant (the defaults are 0.07 s, 0.06 s and 0.02 s) the measured temperature and salinity start at 0 and relax towards the field over a few time constants, so the first samples of every CTD record are wrong: the default thermistor reads 0 degC at t = 0 and is still about 1 % short after 0.3 s. The regression fixtures hide it: Auv_CtdStatic uses zero time constants, where the lag state is not read, and the field of Auv_CtdRamp is 0 at the start position. LagD is seeded from the position, not the environment, and is right.

Possible fix

1. Upstream: FHSIM-0025 publishes shared resources before InitialConditionSetup; the existing code then works as its comment says, with no change here.
2. Until then, correct the comment at :244-246, which states the opposite of what happens.

Test that would prove it

In tests/AuvCtd_Test.cpp, a fixture with the default time constants and a field that is not zero at the start position: at t = 0, LagT_0 and Temperature_0 must equal the field there. Fails today (both 0).

Risk

None for option 2. Option 1 changes the first seconds of every CTD result with a non-zero time constant; baselines that record them move.

Owner decision 2026-09-25

Karl-Johan Reite: wait for FHSIM-0025 for the real fix; correct the misleading code comment in SensorCtd now, and record in this issue that it stays open until FHSIM-0025 lands. Do not resolve it.

Status 2026-09-25

The comment in SensorCtd::InitialConditionSetup (src/auv/SensorCtd.cpp:244-249) is corrected: it now says that GetSharedResource returns null there until FHSIM-0025 lands, so LagT and LagS open at 0. Comment only; no behaviour change. The issue stays open until FHSIM-0025 lands.