R12 (tick 5) was a NEGATIVE result: naive SVD-spectrum cosine distance
detected structure changes at 0.69x the natural drift floor (= undetectable).
R12 explicitly identified the revision: 'PABS over Fresnel basis'.
R6.1 (tick 18) shipped the multi-scatterer Fresnel forward operator.
This tick implements PABS on top of it.
PABS = ||y_observed - y_predicted||^2 / ||y_observed||^2
Benchmark (5 m link, 2.4 GHz, subject + 4 wall reflectors expected):
| Scenario | PABS / drift | SVD (R12) / drift |
|--------------------------------|---------------:|------------------:|
| Empty room (subject missing) | 7,362x | 65x |
| Subject as expected (sanity) | 0x | 0x |
| +1 new furniture | 84x | 11x |
| +1 unexpected human | 1,161x | 11x |
| Subject moved 10 cm | 21,966x | 90x |
| Natural drift (5% wall shift) | 1x | 1x |
PABS detects unexpected human at 1161x natural drift; R12 SVD detected
at 11x. ~100x lift purely from physics-grounded prediction vs naive
statistical eigenshift.
R12 NEGATIVE -> POSITIVE. The meta-lesson: a research loop that catalogues
NEGATIVE results creates a backlog of revisitable work that pays off
when later tools become available. R12 -> R12 PABS is the worked example.
R13 cannot be similarly revisited -- its 5 dB shortfall is a hard
physics floor, not a missing model.
The subject-moved-10cm caveat: PABS detects ANY mismatch between
expected and observed scene. Real production PABS needs a pose-aware
forward model that updates from pose_tracker.rs in real-time. The
actual detection signal is PABS-after-pose-update. ~50-100 LOC Rust
glue, catalogued as R12.1 follow-up.
Composes:
- R6.1 unblocked this implementation
- R7 gets precise per-link consistency: residual small on all links =
no structure; spike on one = local structure OR compromised link;
mincut disambiguates
- R11 enables maritime container-tamper / hatch-seal apps
- R14 gets V0 security feature (intruder detection w/o biometric storage)
- ADR-029 needs to reference PABS as structure-detection primitive
- R10 PABS-vs-canopy works if forest modelled or learned
Honest scope:
- Pose-PABS closed loop not yet built
- Synthetic data only; real-world drift floor needs measurement
- Population-prior body; per-subject would tighten residual
- Single time-frame; real pipeline needs temporal averaging
Coordination: ticks/tick-19.md, no PROGRESS.md edit.
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