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# ADR-287: `wifi-densepose-sar` — coherent wideband RF tomography research crate
| Field | Value |
|-------|-------|
| **Status** | Accepted — implemented (P1), **published** |
| **Date** | 2026-07-30 |
| **Parent** | ADR-278 (radar inverse rendering research program), ADR-282 (mandatory L0–L5 evidence ladder) |
| **Relates to** | ADR-273/274 (`ruview-unified`'s `FmcwRadarCube` adapter, the eventual integration point), ADR-275 (`GaussianMap`, ditto), ADR-286 (`wifi-densepose-sar-harness`, the MetaHarness minted for this crate) |
| **Published** | [`wifi-densepose-sar` v0.3.1](https://crates.io/crates/wifi-densepose-sar) on crates.io (2026-07-31) |
## 0. PROOF discipline
Every accuracy number this crate produces is **SYNTHETIC / evidence level L0** (ADR-282): generated by the crate's own forward simulator (`measurement::simulate_measurement`), scored against its own known ground truth (`ScatteringTarget` positions). Nothing here has been validated against real wideband RF hardware, and the crate contains no such hardware integration.
## 1. Context
A YC-backed company, Applied Electrodynamics ("WaveSight"), publicly launched a handheld "camera that can see through walls" using undisclosed radio-imaging technology. Comparing it against this repo's capabilities surfaced a real gap: `wifi-densepose-signal::ruvsense::tomography` implements *radio tomographic imaging* (Wilson & Patwari 2010) — RSS-based shadowing attenuation on a fixed-link topology, no coherent phase, no multi-frequency stepping, no synthetic aperture. It is a different technique from what a SAR-style through-wall imager needs: coherent, wideband, multi-position backprojection.
`ruview-unified`'s `FmcwRadarCube` adapter (ADR-274) already normalizes wideband radar cubes into range profiles per position, and ADR-278 already names a radar-cube-output extension of the ADR-276 synthetic world generator as the intended sandbox for any future radar-inverse research. Neither, before this ADR, contained an actual backprojection reconstruction kernel — the primitive every candidate technique (matched-filter SAR, GPR imaging, RISE/DiffRadar-style inversion) is built on.
## 2. Decision
Ship `wifi-densepose-sar` as a standalone leaf crate (the `nvsim` pattern: pure Rust, deterministic ChaCha20 seeding, zero coupling to `wifi-densepose-hardware` or any real ingestion path) implementing:
1. **Forward measurement model** (`measurement.rs`): simulates the complex, stepped-frequency returns a monostatic synthetic-aperture radar would record from known point scatterers — `y_{m,k} = Σ_j σ_j/R_{m,j}² · exp(-i·4π·f_k·R_{m,j}/c) + noise`.
2. **Backprojection reconstruction** (`reconstruct.rs`): the matched-filter inverse of (1) onto a 3D voxel grid, parallelized over voxels (rayon).
3. **Point-cloud extraction** (`pointcloud.rs`): threshold + local-maximum extraction from the dense voxel image.
4. **Closed-form resolution/coherence formulas** (`resolution.rs`): `ΔR = c/2B` (range resolution), `δ_CR ≈ λR/2L` (cross-range/synthetic-aperture resolution), `Δp ≤ λ/8` (antenna-pose coherence budget, derived from a quarter-wavelength round-trip-path tolerance) — checked against the reconstruction's actual behavior in `tests/physics_validation.rs`, not merely documented.
This is deliberately scoped **one level below** ADR-278's RISE/DiffRadar/GeRaF reproduction program: it is the bare measurement-model + backprojection primitive, not a reproduction of any specific published system, and not a claim about Applied Electrodynamics' undisclosed product (their waveform, antenna count, bandwidth, and algorithm are unknown; this crate applies the same well-established SAR/GPR physics — see Skolnik, *Radar Handbook* — to synthetic data).
## 3. What this explicitly is NOT
- Not a hardware driver. No VNA/SDR/wideband-RF-frontend code exists anywhere in this crate or was added to `wifi-densepose-hardware`.
- Not wired into `ruview-unified`'s `FmcwRadarCube` adapter or `GaussianMap`. That integration is real future work (§5), deliberately deferred so the reconstruction physics validates in isolation first — the same staging ADR-278 §2.3 already prescribes ("sandbox-first... before hardware").
- Not a reproduction of RISE, DiffRadar, or GeRaF. ADR-278's gates (G1–G4) are untouched by this ADR.
- Not a real-world through-wall imaging performance claim. The forward model is free-space propagation only — no multipath, no per-material attenuation, no antenna gain pattern, no receiver noise figure. Real-world performance depends on all of these.
## 4. Simplifications (honesty boundary)
- **Monostatic, not MIMO.** A single antenna acts as both transmitter and receiver at each synthetic-aperture position (the standard stripmap-SAR simplification), not a multi-element MIMO array. Extending to bistatic/MIMO `(m, n)` transmitter/receiver pairs is straightforward given the existing measurement-model structure but not implemented.
- **Isotropic antenna, no gain pattern.** Every antenna position radiates/receives equally in all directions.
- **Free-space propagation only.** No multipath, no material transmission/reflection/attenuation (contrast `ruview-unified::synth::room`'s Fresnel material model, which is narrowband-CW and not yet extended to wideband — a natural follow-up, §5).
- **`1/R²` two-way amplitude falloff, no calibration.** Real receivers have finite dynamic range, noise figures, and require calibration against a known reference target; none of that is modeled.
## 5. Follow-up (not in this ADR's scope)
1. Extend `ruview-unified::synth::room`'s image-method ray tracer to emit wideband stepped-frequency multi-position cubes (per ADR-278 §2.3), and wire `wifi-densepose-sar::reconstruct` against that richer (multipath-aware) synthetic generator instead of the free-space-only model here.
2. A `ruview-unified` integration adapter converting `ReflectivityImage`/`PointCloudPoint` output into `RfGaussian`/`GaussianMap` primitives (ADR-278 §2.4's stated integration contract).
3. Bistatic/MIMO measurement model.
4. Any of ADR-278's actual gated reproductions (RISE first), if and when that program proceeds — this crate would be a component, not a substitute.
## 6. Consequences
- The workspace gains a real (if intentionally scoped-down) coherent-imaging primitive where before there was none — useful groundwork for ADR-278 if that research program proceeds, and a direct, honest answer to "could this repo build a WaveSight-like device" (no, not without the hardware program described in the motivating comparison; yes, this is the reconstruction-algorithm groundwork such a program would need).
- Zero risk to the existing `wifi-densepose-signal::ruvsense::tomography` (RSS-based RTI) code path or any production pipeline — this crate is not referenced by any of them.
- 25 tests (22 unit + 3 integration physics-validation), 0 failed, clippy-clean. Criterion bench: MEASURED 512/4096/32768-voxel backprojection reconstruction throughput (see crate README for the numbers as last recorded). The incremental-phasor-rotation optimization (§7) cut reconstruction time ~4.4-4.5x, proven equivalent to the direct per-frequency computation it replaced.
## 7. Follow-up optimization: incremental phasor rotation (2026-07-30, MEASURED)
`focus_at_point` originally called `Complex64::from_polar` (one `sin`/`cos` pair) per (pose, frequency) term. Since [`FrequencySweep::frequencies`](../../v2/crates/wifi-densepose-sar/src/measurement.rs) produces evenly-spaced frequencies by construction, the per-term phase is an arithmetic progression in the frequency index — so the phasor can be evaluated once per pose and advanced by a fixed complex-multiply step per frequency, replacing K trig evaluations with 2. `focus_at_point`'s signature changed from a raw `&[f64]` frequency slice to `&FrequencySweep`, making the evenly-spaced-frequencies precondition this optimization depends on a type-level invariant rather than a caller-observed one.
**MEASURED (criterion regression detection, p < 0.001): ~4.4-4.5x faster** across 512/4096/32768-voxel grids. **Proven equivalent**, not just faster: `reconstruct::tests::backprojection_incremental_rotation_matches_direct_per_frequency_computation` checks the optimized path against an independently reimplemented direct per-frequency reference, across four sweep sizes (including the `n_steps=1` degenerate case) and both on-target and off-target evaluation points, to <1e-9 relative error.
## 8. Published (2026-07-31)
`wifi-densepose-sar` v0.3.1 is live on [crates.io](https://crates.io/crates/wifi-densepose-sar) — `cargo add wifi-densepose-sar` resolves it from any Rust project. A MetaHarness minted for this crate (ADR-286, `wifi-densepose-sar-harness`) is published to npm alongside it. Publishing happened after this ADR's implementation and §7 optimization landed; no code changed as part of publishing itself.