V1 record — describes retired software; superseded for V2 by
srd.md, constitution 2.0.0 and ADR-0027.
ADR-0002: Decorrelation timescale is a field, authored per feature
Status: Accepted Date: 26 August 2026 Requirement: SRD FR-05 (v0.3); consumed by FR-08, FR-32, FR-34 Supersedes: the provisional per-feature position taken in SRD v0.2
Context
Something must govern how fast a location loses memory of a measurement. That quantity — the decorrelation timescale, tau — drives the revisit cadence during loiter (FR-08) and is an input to every cell the planner scores (FR-32, FR-34).
SRD v0.2 left the question open and started with tau as a property of each seeded feature. That was recorded as an open question rather than a decision, and it did not survive contact with the requirements that consume it.
Three formulations were available.
Per-feature. Each of the four seeded features of FR-03 carries its own timescale. Simple to author, simple to record in the ground-truth manifest, and it matches the intuition that an eddy decorrelates faster than open water. But it leaves the background water with no timescale at all — and FR-08 explicitly requires that quiet water be left alone, which is a statement about the background, not about the features. The planner would have to invent a value for every cell outside a feature, and an invented value is not ground truth and cannot be scored.
Per-region. A static map over the domain, giving every location a timescale including the background. This fixes the gap, and it is defensible physically. But FR-03 requires a feature of known drift velocity, and a static map cannot follow it: the fast-decorrelating patch would stay put while the feature it describes moves away, so the revisit pattern of FR-08 would track the wrong water.
A field, authored per feature. tau(latitude, longitude, depth, time), evaluated at every location, where the value is a domain-wide background blended with the contribution of any feature overlapping that location, and a moving feature's timescale advects with the feature.
Decision
The decorrelation timescale is a field, tau(latitude, longitude, depth, time). It is authored per feature over a domain-wide background value, and evaluated per location. A moving feature's timescale advects with it. Both the background value and the per-feature timescales are ground truth and are recorded in the FR-04 manifest.
Authoring stays per feature, so a scenario is still written the way a person thinks about it: a background, and four features that disturb it. Evaluation is per location, so every consumer gets a defined answer everywhere.
Consequences
- The environment generator (C-02) gains a fourth derived field alongside temperature, salinity and pressure. It must be advected in step with the feature that authored it.
- The manifest gains a background timescale and per-feature timescales, both scorable. Recovery of tau becomes a measurable claim rather than an assumption, consistent with Constitution IX.
- The planner may assume a defined tau at every H3 cell and depth layer it evaluates, including open background water. No special case for cells outside a feature.
- The blending rule between background and overlapping features is a modelling choice that this record does not fix. It must be stated explicitly in the generator's documentation and recorded in the manifest, since two features may overlap.
- Authoring and evaluation are now different shapes. The generator must not leak the authored per-feature representation to consumers, who see only the evaluated field.