diff --git a/v2/Cargo.lock b/v2/Cargo.lock
index d9648e583..b36c93f14 100644
--- a/v2/Cargo.lock
+++ b/v2/Cargo.lock
@@ -9573,6 +9573,15 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "ruview-survey"
+version = "0.3.1"
+dependencies = [
+ "serde",
+ "serde_json",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "ruview-swarm"
 version = "0.1.0"
diff --git a/v2/Cargo.toml b/v2/Cargo.toml
index 284a32bf7..6a3ad9e16 100644
--- a/v2/Cargo.toml
+++ b/v2/Cargo.toml
@@ -120,6 +120,7 @@ members = [
     "crates/ruview-counterfactual",# ADR-313 counterfactual spatial inference
     "crates/ruview-infogain",      # ADR-314 information-gain scheduler
     "crates/ruview-active",        # ADR-309 active sensing control
+    "crates/ruview-survey",        # ADR-381 node self-survey (check mode)
     # ADR-324 — clean-room Kooima off-axis (head-coupled perspective) projection.
     # Dependency-free native core; wasm-bindgen surface only on wasm32.
     "crates/ruview-offaxis",
diff --git a/v2/crates/ruview-survey/Cargo.toml b/v2/crates/ruview-survey/Cargo.toml
new file mode 100644
index 000000000..8a39275e1
--- /dev/null
+++ b/v2/crates/ruview-survey/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "ruview-survey"
+version.workspace = true
+edition.workspace = true
+authors.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Node self-survey check mode: compare pairwise node ranges with configured node positions (ADR-381)"
+
+[dependencies]
+thiserror.workspace = true
+serde = { workspace = true, features = ["derive"] }
+
+[dev-dependencies]
+serde_json.workspace = true
diff --git a/v2/crates/ruview-survey/examples/check_ranges.rs b/v2/crates/ruview-survey/examples/check_ranges.rs
new file mode 100644
index 000000000..236044d2e
--- /dev/null
+++ b/v2/crates/ruview-survey/examples/check_ranges.rs
@@ -0,0 +1,78 @@
+//! Run check mode on a JSONL file of range observations.
+//!
+//! ```text
+//! cargo run -p ruview-survey --example check_ranges -- \
+//!     --node-positions "1:0,0,1.5;2:4.2,0,1.5;3:4.2,3.6,1.5" \
+//!     --observations ranges.jsonl [--profile ftm|uwb]
+//! ```
+//!
+//! Each line of `ranges.jsonl` is one `RangeObservation`, for example
+//! `{"initiator":1,"responder":2,"range_m":4.6,"sigma_m":0.3,"method":"ftm"}`.
+//! Prints the report as JSON. Exit status 0 when consistent, 1 on warnings
+//! or insufficient data, 2 on bad input. Never writes anything.
+
+use std::process::ExitCode;
+
+use ruview_survey::{check, parse_node_positions, CheckParams, RangeObservation, Verdict};
+
+/// Largest observations file read, in bytes.
+const MAX_FILE_BYTES: u64 = 4 * 1024 * 1024;
+
+fn run() -> Result<Verdict, String> {
+    let mut positions = None;
+    let mut path = None;
+    let mut profile = "ftm".to_string();
+    let mut args = std::env::args().skip(1);
+    while let Some(arg) = args.next() {
+        let mut value = || args.next().ok_or(format!("{arg} needs a value"));
+        match arg.as_str() {
+            "--node-positions" => positions = Some(value()?),
+            "--observations" => path = Some(value()?),
+            "--profile" => profile = value()?,
+            other => return Err(format!("unknown argument {other}")),
+        }
+    }
+    let positions = parse_node_positions(&positions.ok_or("--node-positions is required")?)
+        .map_err(|e| e.to_string())?;
+    let path = path.ok_or("--observations is required")?;
+    let params = match profile.as_str() {
+        "ftm" => CheckParams::ftm_uncalibrated(),
+        "uwb" => CheckParams::uwb(),
+        other => return Err(format!("unknown profile {other} (ftm|uwb)")),
+    };
+
+    let meta = std::fs::metadata(&path).map_err(|e| format!("{path}: {e}"))?;
+    if !meta.is_file() || meta.len() > MAX_FILE_BYTES {
+        return Err(format!(
+            "{path}: not a regular file under {MAX_FILE_BYTES} bytes"
+        ));
+    }
+    let text = std::fs::read_to_string(&path).map_err(|e| format!("{path}: {e}"))?;
+    let observations = text
+        .lines()
+        .enumerate()
+        .filter(|(_, l)| !l.trim().is_empty())
+        .map(|(i, l)| {
+            serde_json::from_str::<RangeObservation>(l)
+                .map_err(|e| format!("{path}:{}: {e}", i + 1))
+        })
+        .collect::<Result<Vec<_>, _>>()?;
+
+    let report = check(&positions, &observations, &params).map_err(|e| e.to_string())?;
+    println!(
+        "{}",
+        serde_json::to_string_pretty(&report).map_err(|e| e.to_string())?
+    );
+    Ok(report.verdict)
+}
+
+fn main() -> ExitCode {
+    match run() {
+        Ok(Verdict::Consistent) => ExitCode::SUCCESS,
+        Ok(_) => ExitCode::from(1),
+        Err(e) => {
+            eprintln!("check_ranges: {e}");
+            ExitCode::from(2)
+        }
+    }
+}
diff --git a/v2/crates/ruview-survey/src/calibration.rs b/v2/crates/ruview-survey/src/calibration.rs
new file mode 100644
index 000000000..30e1b0bd6
--- /dev/null
+++ b/v2/crates/ruview-survey/src/calibration.rs
@@ -0,0 +1,154 @@
+//! Per-boot range calibration.
+//!
+//! On the 2026-10-04 FTM bench the spread inside one responder start was
+//! ±0.2 to 0.4 m with no drift beyond that over two minutes (40 sessions,
+//! half medians 3.30 and 3.00 m), but the bias changed with
+//! every responder start (−0.5 to +1.9 m at 1.14 m). So an offset is only
+//! trusted for the link and responder boot it was measured on. An
+//! observation from any other boot passes through uncalibrated.
+//!
+//! The reference must be a known non-trivial distance. Zero-separation
+//! calibration (ESP-IDF's `cm0`) is suspect on this hardware because
+//! readings clamped to 0 below about 0.8 m.
+
+use std::collections::BTreeMap;
+
+use crate::{median, RangeObservation, SurveyError};
+
+/// Shortest reference distance accepted, in metres. Below this the bench
+/// readings clamped to zero, so an offset measured there is meaningless.
+pub const MIN_REFERENCE_M: f64 = 1.0;
+
+/// Offsets keyed by `(initiator, responder, responder_boot)`.
+#[derive(Debug, Clone, Default, PartialEq)]
+pub struct CalibrationTable {
+    offsets: BTreeMap<(u8, u8, u32), f64>,
+}
+
+impl CalibrationTable {
+    /// An empty table.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Record an offset from sessions taken at a known reference distance.
+    ///
+    /// Every observation must share initiator, responder and responder boot.
+    /// The offset is `median(range_m) - reference_m` and is returned.
+    pub fn record_reference(
+        &mut self,
+        observations: &[RangeObservation],
+        reference_m: f64,
+    ) -> Result<f64, SurveyError> {
+        if !reference_m.is_finite() || reference_m < MIN_REFERENCE_M {
+            return Err(SurveyError::Calibration(format!(
+                "reference {reference_m} m is below {MIN_REFERENCE_M} m"
+            )));
+        }
+        let first = observations
+            .first()
+            .ok_or_else(|| SurveyError::Calibration("no reference observations".into()))?;
+        let boot = first.responder_boot.ok_or_else(|| {
+            SurveyError::Calibration("reference observations carry no responder boot".into())
+        })?;
+        let key = (first.initiator, first.responder, boot);
+        let mut ranges = Vec::with_capacity(observations.len());
+        for o in observations {
+            o.validate()?;
+            if (o.initiator, o.responder, o.responder_boot) != (key.0, key.1, Some(key.2)) {
+                return Err(SurveyError::Calibration(
+                    "reference observations mix links or responder boots".into(),
+                ));
+            }
+            ranges.push(o.range_m);
+        }
+        let offset = median(&ranges).unwrap_or(0.0) - reference_m;
+        self.offsets.insert(key, offset);
+        Ok(offset)
+    }
+
+    /// The offset for an observation's link and boot, if one was recorded.
+    pub fn offset_for(&self, obs: &RangeObservation) -> Option<f64> {
+        let boot = obs.responder_boot?;
+        self.offsets
+            .get(&(obs.initiator, obs.responder, boot))
+            .copied()
+    }
+
+    /// Return a calibrated copy when an offset matches, else an unchanged copy.
+    pub fn apply(&self, obs: &RangeObservation) -> RangeObservation {
+        let mut out = obs.clone();
+        if obs.calibrated {
+            return out;
+        }
+        if let Some(offset) = self.offset_for(obs) {
+            out.range_m = (obs.range_m - offset).max(0.0);
+            out.calibrated = true;
+        }
+        out
+    }
+
+    /// Number of recorded offsets.
+    pub fn len(&self) -> usize {
+        self.offsets.len()
+    }
+
+    /// True when no offsets are recorded.
+    pub fn is_empty(&self) -> bool {
+        self.offsets.is_empty()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn obs(range: f64, boot: Option<u32>) -> RangeObservation {
+        RangeObservation {
+            responder_boot: boot,
+            ..RangeObservation::ftm(1, 2, range, 0.3)
+        }
+    }
+
+    #[test]
+    fn offset_applies_only_to_the_same_boot() {
+        let mut t = CalibrationTable::new();
+        let off = t
+            .record_reference(
+                &[obs(3.0, Some(7)), obs(3.1, Some(7)), obs(3.2, Some(7))],
+                1.143,
+            )
+            .unwrap();
+        assert!((off - (3.1 - 1.143)).abs() < 1e-9);
+
+        let same = t.apply(&obs(5.0, Some(7)));
+        assert!(same.calibrated);
+        assert!((same.range_m - (5.0 - off)).abs() < 1e-9);
+
+        let other_boot = t.apply(&obs(5.0, Some(8)));
+        assert!(!other_boot.calibrated);
+        assert_eq!(other_boot.range_m, 5.0);
+
+        let no_boot = t.apply(&obs(5.0, None));
+        assert!(!no_boot.calibrated);
+    }
+
+    #[test]
+    fn rejects_short_reference_and_mixed_links() {
+        let mut t = CalibrationTable::new();
+        assert!(t.record_reference(&[obs(0.0, Some(1))], 0.5).is_err());
+        assert!(t.record_reference(&[obs(1.0, None)], 1.2).is_err());
+        assert!(t
+            .record_reference(&[obs(1.0, Some(1)), obs(1.0, Some(2))], 1.2)
+            .is_err());
+        assert!(t.record_reference(&[], 1.2).is_err());
+        assert!(t.is_empty());
+    }
+
+    #[test]
+    fn calibrated_range_never_goes_negative() {
+        let mut t = CalibrationTable::new();
+        t.record_reference(&[obs(3.0, Some(1))], 1.0).unwrap();
+        assert_eq!(t.apply(&obs(0.5, Some(1))).range_m, 0.0);
+    }
+}
diff --git a/v2/crates/ruview-survey/src/check.rs b/v2/crates/ruview-survey/src/check.rs
new file mode 100644
index 000000000..bb3bfbdb4
--- /dev/null
+++ b/v2/crates/ruview-survey/src/check.rs
@@ -0,0 +1,394 @@
+//! Check mode: compare measured node ranges with configured node positions.
+//!
+//! Warn-only. [`check`] returns a [`CheckReport`]; nothing in this module
+//! produces positions to write back.
+
+use std::collections::{BTreeMap, BTreeSet};
+
+use crate::positions::distance;
+use crate::report::{
+    CheckParams, CheckReport, Finding, LinkResidual, LinkStatus, ScaleFit, Verdict,
+};
+use crate::{median, NodePositions, RangeObservation, SurveyError, MAX_OBSERVATIONS};
+
+/// Largest node set the id-swap search permutes (8! = 40 320 assignments).
+pub const MAX_SWAP_NODES: usize = 8;
+
+/// Unit factors tried by the scale check: configured value × factor = metres.
+const UNIT_FACTORS: [(&str, f64); 4] = [
+    ("feet", 0.3048),
+    ("inches", 0.0254),
+    ("centimetres", 0.01),
+    ("millimetres", 0.001),
+];
+
+/// Cap on one link's contribution to the assignment cost, in squared
+/// tolerances, so one bad link cannot dominate the id-swap search.
+const LINK_COST_CAP: f64 = 4.0;
+
+/// Compare observations with configured positions. Never mutates either.
+pub fn check(
+    positions: &NodePositions,
+    observations: &[RangeObservation],
+    params: &CheckParams,
+) -> Result<CheckReport, SurveyError> {
+    if observations.len() > MAX_OBSERVATIONS {
+        return Err(SurveyError::TooMany {
+            what: "observations",
+            got: observations.len(),
+            max: MAX_OBSERVATIONS,
+        });
+    }
+    let mut findings = Vec::new();
+    let mut unknown: BTreeMap<u8, usize> = BTreeMap::new();
+    let mut by_pair: BTreeMap<(u8, u8), Vec<&RangeObservation>> = BTreeMap::new();
+    for o in observations {
+        o.validate()?;
+        let mut known = true;
+        for node in [o.initiator, o.responder] {
+            if !positions.contains_key(&node) {
+                *unknown.entry(node).or_default() += 1;
+                known = false;
+            }
+        }
+        if known {
+            by_pair.entry(o.pair()).or_default().push(o);
+        }
+    }
+    for (node, n) in unknown {
+        findings.push(Finding::UnknownNode {
+            node,
+            observations: n,
+        });
+    }
+
+    let links: Vec<LinkResidual> = by_pair
+        .iter()
+        .map(|(&(a, b), obs)| link_residual(a, b, obs, positions, params))
+        .collect();
+    let scored: Vec<&LinkResidual> = links
+        .iter()
+        .filter(|l| matches!(l.status, LinkStatus::Ok | LinkStatus::Outlier))
+        .collect();
+
+    let ranged: BTreeSet<u8> = scored.iter().flat_map(|l| [l.a, l.b]).collect();
+    if !scored.is_empty() {
+        for node in positions.keys().filter(|n| !ranged.contains(n)) {
+            findings.push(Finding::UnrangedNode { node: *node });
+        }
+    }
+    if scored.is_empty() {
+        return Ok(CheckReport {
+            links,
+            scale: None,
+            findings,
+            verdict: Verdict::Insufficient,
+        });
+    }
+
+    let scale = fit_scale(&scored);
+    let unit = scale
+        .as_ref()
+        .and_then(|s| unit_mismatch(s.scale, &scored, params));
+    let explained = unit.is_some();
+    findings.extend(unit);
+
+    let mut swapped = false;
+    if !explained {
+        if ranged.len() > MAX_SWAP_NODES {
+            findings.push(Finding::SwapSearchSkipped {
+                nodes: ranged.len(),
+            });
+        } else if ranged.len() >= 3 {
+            if let Some(f) = swap_search(&ranged, &scored, positions, params) {
+                swapped = matches!(f, Finding::IdSwapSuggested { .. });
+                findings.push(f);
+            }
+        }
+    }
+
+    if !explained && !swapped {
+        let moved = moved_nodes(&ranged, &scored);
+        for l in scored.iter().filter(|l| l.status == LinkStatus::Outlier) {
+            if !moved.contains(&l.a) && !moved.contains(&l.b) {
+                findings.push(Finding::LinkOutlier {
+                    a: l.a,
+                    b: l.b,
+                    residual_m: l.residual_m,
+                });
+            }
+        }
+        for node in &moved {
+            let touching: Vec<&&LinkResidual> = scored
+                .iter()
+                .filter(|l| l.a == *node || l.b == *node)
+                .collect();
+            findings.push(Finding::NodeMoved {
+                node: *node,
+                flagged_links: touching
+                    .iter()
+                    .filter(|l| l.status == LinkStatus::Outlier)
+                    .count(),
+                links: touching.len(),
+            });
+        }
+    }
+
+    let verdict = if findings.iter().any(Finding::is_warning) {
+        Verdict::Warnings
+    } else {
+        Verdict::Consistent
+    };
+    Ok(CheckReport {
+        links,
+        scale,
+        findings,
+        verdict,
+    })
+}
+
+fn link_residual(
+    a: u8,
+    b: u8,
+    obs: &[&RangeObservation],
+    positions: &NodePositions,
+    params: &CheckParams,
+) -> LinkResidual {
+    let usable: Vec<&&RangeObservation> = obs
+        .iter()
+        .filter(|o| o.rssi_dbm.is_none_or(|r| r >= params.min_rssi_dbm))
+        .collect();
+    let pool: Vec<&&RangeObservation> = if usable.is_empty() {
+        obs.iter().collect()
+    } else {
+        usable.clone()
+    };
+    let ranges: Vec<f64> = pool.iter().map(|o| o.range_m).collect();
+    let sigmas: Vec<f64> = pool.iter().map(|o| o.sigma_m).collect();
+    let measured = median(&ranges).unwrap_or(0.0);
+    let session_sigma = median(&sigmas).unwrap_or(0.0);
+    let spread = if ranges.len() >= 3 {
+        let mean = ranges.iter().sum::<f64>() / ranges.len() as f64;
+        (ranges.iter().map(|r| (r - mean).powi(2)).sum::<f64>() / ranges.len() as f64).sqrt()
+    } else {
+        0.0
+    };
+    let sigma = session_sigma.max(spread);
+    let calibrated = pool.iter().all(|o| o.calibrated);
+    let expected = distance(&positions[&a], &positions[&b]);
+    let floor = if calibrated {
+        params.calibrated_floor_m
+    } else {
+        params.floor_m
+    };
+    let tolerance = floor + params.sigma_k * sigma;
+    let residual = measured - expected;
+    let status = if usable.is_empty() {
+        LinkStatus::Weak
+    } else if expected < params.min_expected_m {
+        LinkStatus::TooShort
+    } else if sigma > params.max_sigma_m {
+        LinkStatus::Noisy
+    } else if residual.abs() > tolerance {
+        LinkStatus::Outlier
+    } else {
+        LinkStatus::Ok
+    };
+    LinkResidual {
+        a,
+        b,
+        measured_m: measured,
+        expected_m: expected,
+        residual_m: residual,
+        tolerance_m: tolerance,
+        sigma_m: sigma,
+        observations: pool.len(),
+        calibrated,
+        status,
+    }
+}
+
+fn fit_scale(scored: &[&LinkResidual]) -> Option<ScaleFit> {
+    let usable: Vec<&&LinkResidual> = scored.iter().filter(|l| l.expected_m > 0.0).collect();
+    if usable.len() < 2 {
+        return None;
+    }
+    let num: f64 = usable.iter().map(|l| l.measured_m * l.expected_m).sum();
+    let den: f64 = usable.iter().map(|l| l.expected_m * l.expected_m).sum();
+    (den > 0.0).then(|| ScaleFit {
+        scale: num / den,
+        links: usable.len(),
+    })
+}
+
+fn unit_mismatch(scale: f64, scored: &[&LinkResidual], params: &CheckParams) -> Option<Finding> {
+    let outliers = scored
+        .iter()
+        .filter(|l| l.status == LinkStatus::Outlier)
+        .count();
+    if outliers == 0 {
+        return None;
+    }
+    UNIT_FACTORS.iter().find_map(|&(unit, factor)| {
+        if ((scale / factor) - 1.0).abs() > params.unit_band {
+            return None;
+        }
+        let rescaled = scored
+            .iter()
+            .filter(|l| (l.measured_m - l.expected_m * factor).abs() > l.tolerance_m)
+            .count();
+        (rescaled < outliers).then(|| Finding::UnitMismatch {
+            unit: unit.to_string(),
+            factor,
+            fitted_scale: scale,
+        })
+    })
+}
+
+fn swap_search(
+    ranged: &BTreeSet<u8>,
+    scored: &[&LinkResidual],
+    positions: &NodePositions,
+    params: &CheckParams,
+) -> Option<Finding> {
+    let nodes: Vec<u8> = ranged.iter().copied().collect();
+    let index: BTreeMap<u8, usize> = nodes.iter().enumerate().map(|(i, n)| (*n, i)).collect();
+    let pos: Vec<[f64; 3]> = nodes.iter().map(|n| positions[n]).collect();
+    let links: Vec<(usize, usize, f64, f64)> = scored
+        .iter()
+        .map(|l| (index[&l.a], index[&l.b], l.measured_m, l.tolerance_m))
+        .collect();
+    let cost = |perm: &[usize]| -> f64 {
+        links
+            .iter()
+            .map(|&(i, j, m, tol)| {
+                let r = (m - distance(&pos[perm[i]], &pos[perm[j]])) / tol;
+                (r * r).min(LINK_COST_CAP)
+            })
+            .sum()
+    };
+
+    let n = nodes.len();
+    let identity: Vec<usize> = (0..n).collect();
+    let configured_cost = cost(&identity);
+    let mut all: Vec<(f64, Vec<usize>)> = Vec::new();
+    for_each_permutation(n, |perm| all.push((cost(perm), perm.to_vec())));
+    let best = all.iter().map(|(c, _)| *c).fold(f64::INFINITY, f64::min);
+    let tie = best + 0.1 * best + 0.05;
+    if configured_cost <= tie {
+        return None;
+    }
+    if configured_cost < params.min_swap_cost || best > params.swap_gain * configured_cost {
+        return None;
+    }
+    // Several assignments can fit equally well: ranges cannot tell a layout
+    // from its reflections, and corner-mounted nodes in a rectangular room
+    // are nearly symmetric. Prefer the assignment that changes the fewest
+    // ids (an operator typo moves few ids), and give up only when that
+    // minimum is itself tied.
+    let changed = |perm: &[usize]| perm.iter().enumerate().filter(|(i, p)| i != *p).count();
+    let winners: Vec<&(f64, Vec<usize>)> = all.iter().filter(|(c, _)| *c <= tie).collect();
+    let fewest = winners.iter().map(|(_, p)| changed(p)).min()?;
+    let minimal: Vec<&&(f64, Vec<usize>)> = winners
+        .iter()
+        .filter(|(_, p)| changed(p) == fewest)
+        .collect();
+    if minimal.len() > 1 {
+        return Some(Finding::IdSwapAmbiguous {
+            equivalent_assignments: winners.len(),
+        });
+    }
+    let (best_cost, perm) = minimal[0];
+    let assignment = perm
+        .iter()
+        .enumerate()
+        .filter(|(i, p)| *i != **p)
+        .map(|(i, p)| (nodes[i], nodes[*p]))
+        .collect();
+    Some(Finding::IdSwapSuggested {
+        assignment,
+        cost_configured: configured_cost,
+        cost_suggested: *best_cost,
+        equivalent_assignments: winners.len(),
+    })
+}
+
+/// Visit every permutation of `0..n` (Heap's algorithm, iterative).
+fn for_each_permutation(n: usize, mut visit: impl FnMut(&[usize])) {
+    let mut perm: Vec<usize> = (0..n).collect();
+    let mut c = vec![0usize; n];
+    visit(&perm);
+    let mut i = 1;
+    while i < n {
+        if c[i] < i {
+            if i % 2 == 0 {
+                perm.swap(0, i);
+            } else {
+                perm.swap(c[i], i);
+            }
+            visit(&perm);
+            c[i] += 1;
+            i = 1;
+        } else {
+            c[i] = 0;
+            i += 1;
+        }
+    }
+}
+
+fn moved_nodes(ranged: &BTreeSet<u8>, scored: &[&LinkResidual]) -> BTreeSet<u8> {
+    ranged
+        .iter()
+        .copied()
+        .filter(|node| {
+            let touching: Vec<&&LinkResidual> = scored
+                .iter()
+                .filter(|l| l.a == *node || l.b == *node)
+                .collect();
+            let flagged = touching
+                .iter()
+                .filter(|l| l.status == LinkStatus::Outlier)
+                .count();
+            flagged >= 2 && 2 * flagged > touching.len()
+        })
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn heap_visits_every_permutation_once() {
+        for n in 0..=5 {
+            let mut seen = BTreeSet::new();
+            let mut count = 0;
+            for_each_permutation(n, |p| {
+                seen.insert(p.to_vec());
+                count += 1;
+            });
+            let expected: usize = (1..=n).product();
+            assert_eq!(count, expected.max(1));
+            assert_eq!(seen.len(), expected.max(1));
+        }
+    }
+
+    #[test]
+    fn no_scored_links_is_insufficient() {
+        let positions = crate::parse_node_positions("1:0,0,0;2:0.5,0,0").unwrap();
+        let obs = [RangeObservation::ftm(1, 2, 0.0, 0.1)];
+        let r = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+        assert_eq!(r.verdict, Verdict::Insufficient);
+        assert_eq!(r.links[0].status, LinkStatus::TooShort);
+    }
+
+    #[test]
+    fn rejects_too_many_observations() {
+        let positions = crate::parse_node_positions("1:0,0,0;2:3,0,0").unwrap();
+        let obs = vec![RangeObservation::ftm(1, 2, 3.0, 0.1); MAX_OBSERVATIONS + 1];
+        assert!(matches!(
+            check(&positions, &obs, &CheckParams::ftm_uncalibrated()),
+            Err(SurveyError::TooMany { .. })
+        ));
+    }
+}
diff --git a/v2/crates/ruview-survey/src/fill.rs b/v2/crates/ruview-survey/src/fill.rs
new file mode 100644
index 000000000..b38f5167b
--- /dev/null
+++ b/v2/crates/ruview-survey/src/fill.rs
@@ -0,0 +1,116 @@
+//! Fill-mode gate.
+//!
+//! Fill mode would write solved positions to a file the operator then passes
+//! to `--node-positions`. It is not implemented here. This gate records the
+//! ADR-381 rule so that whatever implements it later cannot be enabled on a
+//! source that has not earned it: MEASURED evidence, enough links, and a p90
+//! link error of at most [`FILL_MAX_P90_M`].
+
+use serde::{Deserialize, Serialize};
+
+use crate::RangeMethod;
+
+/// Largest p90 link error, in metres, at which fill mode may be enabled.
+pub const FILL_MAX_P90_M: f64 = 0.3;
+
+/// Fewest ground-truth links behind the p90 figure.
+pub const FILL_MIN_LINKS: usize = 6;
+
+/// Evidence tag of a source accuracy figure (repository rule).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "SCREAMING_SNAKE_CASE")]
+pub enum EvidenceTag {
+    /// Our own bench, with a reproducer.
+    Measured,
+    /// Vendor or paper figure we have not reproduced.
+    Claimed,
+    /// Simulation.
+    Synthetic,
+}
+
+/// Accuracy evidence for one ranging source.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SourceEvidence {
+    /// The source.
+    pub method: RangeMethod,
+    /// p90 of |measured − ground truth| over the links below, in metres.
+    pub p90_link_error_m: f64,
+    /// Number of ground-truth links behind the figure.
+    pub links: usize,
+    /// Evidence tag.
+    pub tag: EvidenceTag,
+}
+
+/// Why fill mode stays off.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case", tag = "reason")]
+pub enum FillBlocked {
+    /// No evidence was supplied for the source.
+    NoEvidence,
+    /// The evidence is not MEASURED.
+    NotMeasured {
+        /// The tag that was supplied.
+        tag: EvidenceTag,
+    },
+    /// Too few ground-truth links.
+    TooFewLinks {
+        /// Links supplied.
+        links: usize,
+    },
+    /// p90 link error above [`FILL_MAX_P90_M`].
+    TooInaccurate {
+        /// p90 supplied.
+        p90_link_error_m: f64,
+    },
+}
+
+/// Decide whether fill mode may run for a source.
+pub fn fill_gate(evidence: Option<&SourceEvidence>) -> Result<(), FillBlocked> {
+    let e = evidence.ok_or(FillBlocked::NoEvidence)?;
+    if e.tag != EvidenceTag::Measured {
+        return Err(FillBlocked::NotMeasured { tag: e.tag });
+    }
+    if e.links < FILL_MIN_LINKS {
+        return Err(FillBlocked::TooFewLinks { links: e.links });
+    }
+    if !e.p90_link_error_m.is_finite() || e.p90_link_error_m > FILL_MAX_P90_M {
+        return Err(FillBlocked::TooInaccurate {
+            p90_link_error_m: e.p90_link_error_m,
+        });
+    }
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn ev(p90: f64, links: usize, tag: EvidenceTag) -> SourceEvidence {
+        SourceEvidence {
+            method: RangeMethod::Uwb,
+            p90_link_error_m: p90,
+            links,
+            tag,
+        }
+    }
+
+    #[test]
+    fn gate_blocks_until_measured_and_accurate() {
+        assert_eq!(fill_gate(None), Err(FillBlocked::NoEvidence));
+        assert!(matches!(
+            fill_gate(Some(&ev(0.1, 10, EvidenceTag::Claimed))),
+            Err(FillBlocked::NotMeasured { .. })
+        ));
+        assert!(matches!(
+            fill_gate(Some(&ev(0.1, 3, EvidenceTag::Measured))),
+            Err(FillBlocked::TooFewLinks { .. })
+        ));
+        // FTM on the 2026-10-04 bench: errors of metres.
+        assert!(matches!(
+            fill_gate(Some(&ev(4.1, 8, EvidenceTag::Measured))),
+            Err(FillBlocked::TooInaccurate { .. })
+        ));
+        assert!(fill_gate(Some(&ev(f64::NAN, 8, EvidenceTag::Measured))).is_err());
+        assert_eq!(fill_gate(Some(&ev(0.3, 6, EvidenceTag::Measured))), Ok(()));
+    }
+}
diff --git a/v2/crates/ruview-survey/src/lib.rs b/v2/crates/ruview-survey/src/lib.rs
new file mode 100644
index 000000000..1c7d00faa
--- /dev/null
+++ b/v2/crates/ruview-survey/src/lib.rs
@@ -0,0 +1,122 @@
+//! # `ruview-survey` — node self-survey, check mode (ADR-381)
+//!
+//! **Check mode only. Warn-only. This crate never writes node positions.**
+//!
+//! `--node-positions` is typed by hand and keyed by node id (#1804, #1866).
+//! A swapped id, positions entered in feet, or a node moved after setup all
+//! degrade multistatic fusion without any error. This crate takes pairwise
+//! node-to-node ranges ([`RangeObservation`], from Wi-Fi FTM today and UWB
+//! later) and the configured positions, and reports:
+//!
+//! - a per-link residual (measured minus configured distance) with a
+//!   tolerance and a status ([`LinkResidual`]);
+//! - a fitted scale, and a unit-mismatch finding when the scale sits near a
+//!   feet/inches/centimetre factor ([`Finding::UnitMismatch`]);
+//! - a suggested id swap when a different assignment of the configured
+//!   positions to node ids fits much better ([`Finding::IdSwapSuggested`]),
+//!   or an explicit ambiguity when the layout is symmetric;
+//! - a node whose links disagree as a group ([`Finding::NodeMoved`]).
+//!
+//! Ranges see only distances, so any rotation, reflection or translation of
+//! the layout fits equally well. Check mode compares distances, not
+//! coordinates, and reports symmetric ambiguity rather than guessing.
+//!
+//! ## Evidence
+//!
+//! Default tolerances for FTM ([`CheckParams::ftm_uncalibrated`]) come from a
+//! MEASURED two-board ESP32-S3 bench on 2026-10-04 (20 MHz, channel 1, 32
+//! frames per session). Within one responder start the spread was ±0.2 to
+//! 0.4 m, but the bias moved from −0.5 to +1.9 m between responder starts at
+//! 1.14 m, a 3.71 m link read 7.8 m, and readings clamp to 0 below about
+//! 0.8 m. That is good enough to catch gross configuration mistakes and not
+//! good enough to place nodes. Fill mode (writing solved positions) is gated
+//! off by [`fill_gate`] until a source has a MEASURED p90 link error of at
+//! most [`FILL_MAX_P90_M`].
+//!
+//! ## Determinism and bounds
+//!
+//! No I/O, no clock, no randomness. Inputs are bounded ([`MAX_NODES`],
+//! [`MAX_OBSERVATIONS`], [`MAX_RANGE_M`]); the id-swap search runs only up to
+//! [`MAX_SWAP_NODES`] nodes. Malformed input yields a typed [`SurveyError`],
+//! never a panic.
+//!
+//! ```
+//! use ruview_survey::*;
+//!
+//! let positions = parse_node_positions("1:0,0,1;2:4,0,1;3:0,3,1").unwrap();
+//! let obs = [
+//!     RangeObservation::ftm(1, 2, 4.2, 0.3),
+//!     RangeObservation::ftm(1, 3, 3.1, 0.3),
+//!     RangeObservation::ftm(2, 3, 5.4, 0.3),
+//! ];
+//! let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+//! assert_eq!(report.verdict, Verdict::Consistent);
+//! ```
+
+#![forbid(unsafe_code)]
+#![warn(missing_docs)]
+
+mod calibration;
+mod check;
+mod fill;
+mod observation;
+mod positions;
+mod report;
+
+pub use calibration::CalibrationTable;
+pub use check::{check, MAX_SWAP_NODES};
+pub use fill::{fill_gate, EvidenceTag, FillBlocked, SourceEvidence, FILL_MAX_P90_M};
+pub use observation::{RangeMethod, RangeObservation, MAX_OBSERVATIONS, MAX_RANGE_M};
+pub use positions::{parse_node_positions, NodePositions, MAX_NODES};
+pub use report::{CheckParams, CheckReport, Finding, LinkResidual, LinkStatus, ScaleFit, Verdict};
+
+/// Errors from malformed survey input.
+#[derive(Debug, Clone, PartialEq, thiserror::Error)]
+pub enum SurveyError {
+    /// A range observation failed validation.
+    #[error("invalid range observation: {0}")]
+    InvalidObservation(String),
+    /// The node position string failed to parse.
+    #[error("invalid node positions: {0}")]
+    InvalidPositions(String),
+    /// An input exceeded its bound.
+    #[error("too many {what}: {got} > {max}")]
+    TooMany {
+        /// What overflowed.
+        what: &'static str,
+        /// How many were supplied.
+        got: usize,
+        /// The bound.
+        max: usize,
+    },
+    /// A calibration reference was unusable.
+    #[error("calibration: {0}")]
+    Calibration(String),
+}
+
+/// Median of a non-empty slice. Returns `None` for an empty slice.
+pub(crate) fn median(values: &[f64]) -> Option<f64> {
+    if values.is_empty() {
+        return None;
+    }
+    let mut v = values.to_vec();
+    v.sort_by(|a, b| a.total_cmp(b));
+    let mid = v.len() / 2;
+    Some(if v.len() % 2 == 0 {
+        (v[mid - 1] + v[mid]) / 2.0
+    } else {
+        v[mid]
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::median;
+
+    #[test]
+    fn median_handles_odd_even_and_empty() {
+        assert_eq!(median(&[]), None);
+        assert_eq!(median(&[3.0, 1.0, 2.0]), Some(2.0));
+        assert_eq!(median(&[4.0, 1.0, 2.0, 3.0]), Some(2.5));
+    }
+}
diff --git a/v2/crates/ruview-survey/src/observation.rs b/v2/crates/ruview-survey/src/observation.rs
new file mode 100644
index 000000000..8330afb92
--- /dev/null
+++ b/v2/crates/ruview-survey/src/observation.rs
@@ -0,0 +1,154 @@
+//! [`RangeObservation`]: one measured distance between two fixed nodes.
+//!
+//! The record is source-agnostic. FTM, UWB, BLE Channel Sounding and a tape
+//! measure all produce the same shape; [`RangeMethod`] says which one did.
+
+use serde::{Deserialize, Serialize};
+
+use crate::SurveyError;
+
+/// Largest range accepted, in metres. Anything longer is a parse or unit
+/// error for an indoor deployment.
+pub const MAX_RANGE_M: f64 = 100.0;
+
+/// Most observations accepted by one [`crate::check`] call.
+pub const MAX_OBSERVATIONS: usize = 10_000;
+
+/// Which ranging source produced an observation.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum RangeMethod {
+    /// Wi-Fi Fine Timing Measurement (802.11mc RTT), ESP32 STA to SoftAP.
+    Ftm,
+    /// UWB two-way ranging (ADR-144 hardware family).
+    Uwb,
+    /// Bluetooth Channel Sounding, through the BLE CS adapter boundary.
+    BleCs,
+    /// Operator-entered distance (tape or laser).
+    Manual,
+}
+
+/// One measured node-to-node distance.
+///
+/// For FTM, `initiator` is the station and `responder` is the SoftAP. The
+/// pair is unordered for geometry, but the roles matter for calibration
+/// because the measured bias moved with each responder start.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct RangeObservation {
+    /// Node id that initiated the exchange (`node_id` in `--node-positions`).
+    pub initiator: u8,
+    /// Node id that responded.
+    pub responder: u8,
+    /// Reported distance in metres (session median for FTM).
+    pub range_m: f64,
+    /// One-sigma spread of the session in metres. Use 0 when unknown.
+    #[serde(default)]
+    pub sigma_m: f64,
+    /// Ranging method.
+    pub method: RangeMethod,
+    /// Number of valid frames or exchanges behind `range_m`.
+    #[serde(default)]
+    pub samples: u16,
+    /// Median RSSI of the session, if the source reports one.
+    #[serde(default)]
+    pub rssi_dbm: Option<i16>,
+    /// Channel bandwidth in MHz, if relevant (FTM: 20 or 40).
+    #[serde(default)]
+    pub bandwidth_mhz: Option<u16>,
+    /// Host receive time in microseconds since the Unix epoch.
+    #[serde(default)]
+    pub at_us: u64,
+    /// Boot counter or boot nonce of the responder. Calibration is only
+    /// valid within one responder boot.
+    #[serde(default)]
+    pub responder_boot: Option<u32>,
+    /// True once a per-boot calibration offset has been applied.
+    #[serde(default)]
+    pub calibrated: bool,
+}
+
+impl RangeObservation {
+    /// A minimal FTM observation, mostly for tests and examples.
+    pub fn ftm(initiator: u8, responder: u8, range_m: f64, sigma_m: f64) -> Self {
+        Self {
+            initiator,
+            responder,
+            range_m,
+            sigma_m,
+            method: RangeMethod::Ftm,
+            samples: 0,
+            rssi_dbm: None,
+            bandwidth_mhz: None,
+            at_us: 0,
+            responder_boot: None,
+            calibrated: false,
+        }
+    }
+
+    /// The unordered node pair, smaller id first.
+    pub fn pair(&self) -> (u8, u8) {
+        if self.initiator <= self.responder {
+            (self.initiator, self.responder)
+        } else {
+            (self.responder, self.initiator)
+        }
+    }
+
+    /// Reject self links, non-finite values and out-of-range distances.
+    pub fn validate(&self) -> Result<(), SurveyError> {
+        if self.initiator == self.responder {
+            return Err(SurveyError::InvalidObservation(format!(
+                "node {} ranged to itself",
+                self.initiator
+            )));
+        }
+        if !self.range_m.is_finite() || !(0.0..=MAX_RANGE_M).contains(&self.range_m) {
+            return Err(SurveyError::InvalidObservation(format!(
+                "range {} m for {}-{} is outside 0..={MAX_RANGE_M}",
+                self.range_m, self.initiator, self.responder
+            )));
+        }
+        if !self.sigma_m.is_finite() || !(0.0..=MAX_RANGE_M).contains(&self.sigma_m) {
+            return Err(SurveyError::InvalidObservation(format!(
+                "sigma {} m for {}-{} is not a finite non-negative value",
+                self.sigma_m, self.initiator, self.responder
+            )));
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn pair_is_unordered() {
+        assert_eq!(RangeObservation::ftm(5, 2, 1.0, 0.1).pair(), (2, 5));
+        assert_eq!(RangeObservation::ftm(2, 5, 1.0, 0.1).pair(), (2, 5));
+    }
+
+    #[test]
+    fn validate_rejects_bad_values() {
+        assert!(RangeObservation::ftm(1, 1, 1.0, 0.1).validate().is_err());
+        assert!(RangeObservation::ftm(1, 2, -0.1, 0.1).validate().is_err());
+        assert!(RangeObservation::ftm(1, 2, f64::NAN, 0.1)
+            .validate()
+            .is_err());
+        assert!(RangeObservation::ftm(1, 2, 101.0, 0.1).validate().is_err());
+        assert!(RangeObservation::ftm(1, 2, 1.0, f64::INFINITY)
+            .validate()
+            .is_err());
+        assert!(RangeObservation::ftm(1, 2, 0.0, 0.0).validate().is_ok());
+    }
+
+    #[test]
+    fn deserializes_with_optional_fields_missing() {
+        let o: RangeObservation =
+            serde_json::from_str(r#"{"initiator":1,"responder":2,"range_m":3.5,"method":"ftm"}"#)
+                .unwrap();
+        assert_eq!(o.sigma_m, 0.0);
+        assert_eq!(o.responder_boot, None);
+        assert!(!o.calibrated);
+    }
+}
diff --git a/v2/crates/ruview-survey/src/positions.rs b/v2/crates/ruview-survey/src/positions.rs
new file mode 100644
index 000000000..49b31003d
--- /dev/null
+++ b/v2/crates/ruview-survey/src/positions.rs
@@ -0,0 +1,123 @@
+//! Parse the sensing server's `--node-positions` string.
+//!
+//! Same grammar as `field_bridge::parse_node_position_entries`:
+//! `node_id:x,y,z;node_id:x,y,z` or, without the prefix, `x,y,z;...` keyed by
+//! list index from 0. The server skips bad entries with a warning; a checker
+//! should not, so this parser returns an error instead.
+
+use std::collections::BTreeMap;
+
+use crate::SurveyError;
+
+/// Most node positions accepted.
+pub const MAX_NODES: usize = 64;
+
+/// Configured positions keyed by node id, in metres.
+pub type NodePositions = BTreeMap<u8, [f64; 3]>;
+
+/// Parse a `--node-positions` value into a keyed map.
+pub fn parse_node_positions(input: &str) -> Result<NodePositions, SurveyError> {
+    let entries: Vec<&str> = input
+        .split(';')
+        .map(str::trim)
+        .filter(|e| !e.is_empty())
+        .collect();
+    if entries.len() > MAX_NODES {
+        return Err(SurveyError::TooMany {
+            what: "node positions",
+            got: entries.len(),
+            max: MAX_NODES,
+        });
+    }
+    let mut map = NodePositions::new();
+    for (idx, entry) in entries.iter().enumerate() {
+        let (id, triplet) = match entry.split_once(':') {
+            Some((id, rest)) => {
+                let id = id.trim().parse::<u8>().map_err(|_| {
+                    SurveyError::InvalidPositions(format!(
+                        "entry {idx} '{entry}': node id must be 0..=255"
+                    ))
+                })?;
+                (id, rest)
+            }
+            None => (
+                u8::try_from(idx).map_err(|_| {
+                    SurveyError::InvalidPositions(format!("entry {idx}: index exceeds 255"))
+                })?,
+                *entry,
+            ),
+        };
+        let parts: Vec<&str> = triplet.split(',').map(str::trim).collect();
+        if parts.len() != 3 {
+            return Err(SurveyError::InvalidPositions(format!(
+                "entry {idx} '{entry}': expected x,y,z"
+            )));
+        }
+        let mut xyz = [0.0f64; 3];
+        for (slot, part) in xyz.iter_mut().zip(&parts) {
+            let v = part.parse::<f64>().map_err(|_| {
+                SurveyError::InvalidPositions(format!(
+                    "entry {idx} '{entry}': '{part}' is not a number"
+                ))
+            })?;
+            if !v.is_finite() {
+                return Err(SurveyError::InvalidPositions(format!(
+                    "entry {idx} '{entry}': non-finite coordinate"
+                )));
+            }
+            *slot = v;
+        }
+        if map.insert(id, xyz).is_some() {
+            return Err(SurveyError::InvalidPositions(format!(
+                "node {id} is given twice"
+            )));
+        }
+    }
+    Ok(map)
+}
+
+/// Euclidean distance between two positions.
+pub(crate) fn distance(a: &[f64; 3], b: &[f64; 3]) -> f64 {
+    ((a[0] - b[0]).powi(2) + (a[1] - b[1]).powi(2) + (a[2] - b[2]).powi(2)).sqrt()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_keyed_and_indexed_forms() {
+        let keyed = parse_node_positions("3:0,0,1.5; 7:4.5,0,1.5").unwrap();
+        assert_eq!(keyed.len(), 2);
+        assert_eq!(keyed[&7], [4.5, 0.0, 1.5]);
+
+        let indexed = parse_node_positions("0,0,1;2,0,1").unwrap();
+        assert_eq!(indexed[&1], [2.0, 0.0, 1.0]);
+    }
+
+    #[test]
+    fn rejects_malformed_entries() {
+        assert!(parse_node_positions("1:0,0").is_err());
+        assert!(parse_node_positions("x:0,0,0").is_err());
+        assert!(parse_node_positions("1:0,a,0").is_err());
+        assert!(parse_node_positions("1:0,0,inf").is_err());
+        assert!(parse_node_positions("1:0,0,0;1:1,1,1").is_err());
+    }
+
+    #[test]
+    fn empty_input_is_empty_map() {
+        assert!(parse_node_positions("").unwrap().is_empty());
+    }
+
+    #[test]
+    fn rejects_too_many_entries() {
+        let s = (0..=MAX_NODES)
+            .map(|i| format!("{i}:0,0,0"))
+            .collect::<Vec<_>>()
+            .join(";");
+        assert!(matches!(
+            parse_node_positions(&s),
+            Err(SurveyError::TooMany { .. })
+        ));
+    }
+}
diff --git a/v2/crates/ruview-survey/src/report.rs b/v2/crates/ruview-survey/src/report.rs
new file mode 100644
index 000000000..835063453
--- /dev/null
+++ b/v2/crates/ruview-survey/src/report.rs
@@ -0,0 +1,243 @@
+//! Check-mode parameters and report types.
+
+use serde::Serialize;
+
+/// Tolerances and gates for [`crate::check`].
+///
+/// A link's sigma is the larger of the median per-session sigma and, with
+/// three or more sessions, the spread of the session ranges. Its tolerance
+/// is `floor + sigma_k × sigma`, where `floor` is
+/// [`CheckParams::calibrated_floor_m`] when every observation on the link was
+/// calibrated for the current responder boot and [`CheckParams::floor_m`]
+/// otherwise. A link whose sigma exceeds [`CheckParams::max_sigma_m`] is not
+/// scored: a wide tolerance would otherwise accept anything.
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct CheckParams {
+    /// Tolerance floor for uncalibrated links, in metres.
+    pub floor_m: f64,
+    /// Tolerance floor for calibrated links, in metres.
+    pub calibrated_floor_m: f64,
+    /// Multiplier on the per-link sigma.
+    pub sigma_k: f64,
+    /// Links whose configured length is below this are not scored.
+    pub min_expected_m: f64,
+    /// Observations below this RSSI are dropped.
+    pub min_rssi_dbm: i16,
+    /// Links with a sigma above this are not scored.
+    pub max_sigma_m: f64,
+    /// A swap is suggested only if it cuts the assignment cost to at most
+    /// this fraction of the configured assignment's cost.
+    pub swap_gain: f64,
+    /// A swap is suggested only if the configured assignment's cost is at
+    /// least this (in squared tolerances).
+    pub min_swap_cost: f64,
+    /// Relative band around a unit factor that counts as a unit match.
+    pub unit_band: f64,
+}
+
+impl CheckParams {
+    /// Wi-Fi FTM, ESP32-S3, 20 MHz, from the 2026-10-04 bench (MEASURED).
+    ///
+    /// - `floor_m = 2.0`: bias between responder starts at 1.14 m ranged
+    ///   from −0.5 to +1.9 m. One start at 3.71 m read +4.1 m and will be
+    ///   flagged as a link outlier; that is the honest state of uncalibrated
+    ///   FTM, so a single-link outlier is reported as such and not as a
+    ///   configuration error.
+    /// - `calibrated_floor_m = 0.6`: spread within one start was ±0.2 to
+    ///   0.4 m with no drift beyond that over two minutes. Longer stability is untested
+    ///   (ESTIMATE).
+    /// - `min_expected_m = 1.0`: readings clamped to 0 below about 0.8 m.
+    /// - `min_rssi_dbm = -70`: Espressif's guidance. The bench's 7.7 m
+    ///   placement outlier had a median of −68 dBm and passed this gate.
+    /// - `max_sigma_m = 1.0`: that outlier's session spread was 1.92 m; every
+    ///   other run's spread was 0.49 m or less.
+    pub fn ftm_uncalibrated() -> Self {
+        Self {
+            floor_m: 2.0,
+            calibrated_floor_m: 0.6,
+            sigma_k: 3.0,
+            min_expected_m: 1.0,
+            min_rssi_dbm: -70,
+            max_sigma_m: 1.0,
+            swap_gain: 0.5,
+            min_swap_cost: 1.0,
+            unit_band: 0.2,
+        }
+    }
+
+    /// UWB two-way ranging. ESTIMATE pending our own bench: published
+    /// line-of-sight accuracy is about 0.1 m (CLAIMED).
+    pub fn uwb() -> Self {
+        Self {
+            floor_m: 0.3,
+            calibrated_floor_m: 0.3,
+            sigma_k: 3.0,
+            min_expected_m: 0.0,
+            min_rssi_dbm: i16::MIN,
+            max_sigma_m: 0.3,
+            swap_gain: 0.5,
+            min_swap_cost: 1.0,
+            unit_band: 0.1,
+        }
+    }
+}
+
+/// Status of one link.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum LinkStatus {
+    /// Residual within tolerance.
+    Ok,
+    /// Residual beyond tolerance.
+    Outlier,
+    /// Configured length below `min_expected_m`; not scored.
+    TooShort,
+    /// Every observation was below `min_rssi_dbm`; not scored.
+    Weak,
+    /// Sigma above `max_sigma_m`; not scored.
+    Noisy,
+}
+
+/// Measured versus configured length of one node pair.
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct LinkResidual {
+    /// Smaller node id.
+    pub a: u8,
+    /// Larger node id.
+    pub b: u8,
+    /// Median measured range, metres.
+    pub measured_m: f64,
+    /// Distance between the configured positions, metres.
+    pub expected_m: f64,
+    /// `measured_m − expected_m`.
+    pub residual_m: f64,
+    /// Tolerance applied.
+    pub tolerance_m: f64,
+    /// Link sigma (see [`CheckParams`]).
+    pub sigma_m: f64,
+    /// Observations behind the median.
+    pub observations: usize,
+    /// True if every observation used was calibrated.
+    pub calibrated: bool,
+    /// Status.
+    pub status: LinkStatus,
+}
+
+/// Least-squares scale `s` in `measured ≈ s × expected`.
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct ScaleFit {
+    /// Fitted scale.
+    pub scale: f64,
+    /// Links used.
+    pub links: usize,
+}
+
+/// A warning or note from [`crate::check`].
+#[derive(Debug, Clone, PartialEq, Serialize)]
+#[serde(rename_all = "snake_case", tag = "kind")]
+pub enum Finding {
+    /// Observations name a node that has no configured position.
+    UnknownNode {
+        /// Node id.
+        node: u8,
+        /// Observations dropped.
+        observations: usize,
+    },
+    /// A configured node has no scored link, so it was not checked.
+    UnrangedNode {
+        /// Node id.
+        node: u8,
+    },
+    /// The fitted scale matches a unit factor: positions were probably
+    /// entered in that unit while the server reads metres.
+    UnitMismatch {
+        /// Unit name.
+        unit: String,
+        /// Factor from that unit to metres.
+        factor: f64,
+        /// Fitted scale.
+        fitted_scale: f64,
+    },
+    /// Another assignment of the configured positions fits much better.
+    /// Each pair is `(node, position_of)`: the node's ranges match the
+    /// position configured for `position_of`. When several assignments fit
+    /// equally well, this is the one that changes the fewest ids.
+    IdSwapSuggested {
+        /// Changed assignments only.
+        assignment: Vec<(u8, u8)>,
+        /// Cost of the configured assignment.
+        cost_configured: f64,
+        /// Cost of the suggested assignment.
+        cost_suggested: f64,
+        /// Assignments that fit as well as the suggestion, itself included.
+        /// Above 1, the layout is (nearly) symmetric and the suggestion
+        /// rests on the fewest-changes rule.
+        equivalent_assignments: usize,
+    },
+    /// The configured assignment fits badly and the best-fitting
+    /// assignments that change the fewest ids are tied (symmetric layout);
+    /// no single swap can be named.
+    IdSwapAmbiguous {
+        /// Number of equally good alternative assignments.
+        equivalent_assignments: usize,
+    },
+    /// Most of a node's links disagree: it was probably moved.
+    NodeMoved {
+        /// Node id.
+        node: u8,
+        /// Outlier links touching it.
+        flagged_links: usize,
+        /// Scored links touching it.
+        links: usize,
+    },
+    /// One link is out of tolerance with no configuration explanation
+    /// (bias, a body or wall in the path, or a bad session).
+    LinkOutlier {
+        /// Smaller node id.
+        a: u8,
+        /// Larger node id.
+        b: u8,
+        /// Residual, metres.
+        residual_m: f64,
+    },
+    /// Too many nodes for the exhaustive id-swap search.
+    SwapSearchSkipped {
+        /// Nodes with scored links.
+        nodes: usize,
+    },
+}
+
+impl Finding {
+    /// True for findings that should raise the verdict to warnings.
+    pub fn is_warning(&self) -> bool {
+        !matches!(
+            self,
+            Finding::UnrangedNode { .. } | Finding::SwapSearchSkipped { .. }
+        )
+    }
+}
+
+/// Overall result.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
+#[serde(rename_all = "snake_case")]
+pub enum Verdict {
+    /// Every scored link agrees with the configuration.
+    Consistent,
+    /// At least one warning finding.
+    Warnings,
+    /// No link could be scored.
+    Insufficient,
+}
+
+/// Result of [`crate::check`].
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub struct CheckReport {
+    /// One entry per observed pair whose nodes are both configured.
+    pub links: Vec<LinkResidual>,
+    /// Scale fit over scored links, when at least two exist.
+    pub scale: Option<ScaleFit>,
+    /// Findings, in the order they were derived.
+    pub findings: Vec<Finding>,
+    /// Verdict.
+    pub verdict: Verdict,
+}
diff --git a/v2/crates/ruview-survey/tests/check_synthetic.rs b/v2/crates/ruview-survey/tests/check_synthetic.rs
new file mode 100644
index 000000000..63c110b6f
--- /dev/null
+++ b/v2/crates/ruview-survey/tests/check_synthetic.rs
@@ -0,0 +1,369 @@
+//! SYNTHETIC check-mode scenarios. Ranges are generated from a known layout
+//! with FTM-like bias and noise taken from the 2026-10-04 bench envelope
+//! (bias −0.5 to +1.9 m per link, spread ±0.3 m). No hardware is involved.
+
+use ruview_survey::*;
+
+/// Asymmetric five-node layout in a 10 × 6 m space, metres.
+const TRUTH: [(u8, [f64; 3]); 5] = [
+    (1, [0.3, 0.2, 1.0]),
+    (2, [9.6, 0.5, 1.2]),
+    (3, [8.8, 5.7, 1.0]),
+    (4, [1.1, 5.2, 1.4]),
+    (5, [4.4, 2.1, 0.8]),
+];
+
+fn positions_string(layout: &[(u8, [f64; 3])], scale: f64) -> String {
+    layout
+        .iter()
+        .map(|(id, p)| format!("{id}:{},{},{}", p[0] * scale, p[1] * scale, p[2] * scale))
+        .collect::<Vec<_>>()
+        .join(";")
+}
+
+fn dist(a: &[f64; 3], b: &[f64; 3]) -> f64 {
+    ((a[0] - b[0]).powi(2) + (a[1] - b[1]).powi(2) + (a[2] - b[2]).powi(2)).sqrt()
+}
+
+/// Deterministic pseudo-random value in [0, 1) from a seed.
+fn unit(seed: u64) -> f64 {
+    let x = seed
+        .wrapping_mul(6_364_136_223_846_793_005)
+        .wrapping_add(1_442_695_040_888_963_407);
+    ((x >> 11) as f64) / ((1u64 << 53) as f64)
+}
+
+/// Ranges for every pair of `layout`: per-link bias in [bias_lo, bias_hi]
+/// plus ±`noise` m session noise (reported as sigma), `sessions` per link.
+fn ranges_with(
+    layout: &[(u8, [f64; 3])],
+    bias_lo: f64,
+    bias_hi: f64,
+    noise: f64,
+    sessions: u64,
+) -> Vec<RangeObservation> {
+    let mut out = Vec::new();
+    for (i, (a, pa)) in layout.iter().enumerate() {
+        for (b, pb) in layout.iter().skip(i + 1) {
+            let link_seed = u64::from(*a) * 131 + u64::from(*b);
+            let bias = bias_lo + (bias_hi - bias_lo) * unit(link_seed);
+            for s in 0..sessions {
+                let jitter = noise * (2.0 * unit(link_seed * 977 + s) - 1.0);
+                let range = (dist(pa, pb) + bias + jitter).max(0.0);
+                out.push(RangeObservation {
+                    rssi_dbm: Some(-55),
+                    ..RangeObservation::ftm(*a, *b, range, noise)
+                });
+            }
+        }
+    }
+    out
+}
+
+/// FTM-like ranges: ±0.3 m session noise, as on the bench.
+fn ranges(
+    layout: &[(u8, [f64; 3])],
+    bias_lo: f64,
+    bias_hi: f64,
+    sessions: u64,
+) -> Vec<RangeObservation> {
+    ranges_with(layout, bias_lo, bias_hi, 0.3, sessions)
+}
+
+#[test]
+fn correct_config_with_ftm_bias_is_consistent() {
+    let positions = parse_node_positions(&positions_string(&TRUTH, 1.0)).unwrap();
+    let obs = ranges(&TRUTH, -0.5, 1.9, 5);
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert_eq!(report.verdict, Verdict::Consistent, "{report:#?}");
+    assert_eq!(report.links.len(), 10);
+    assert!(report.links.iter().all(|l| l.status == LinkStatus::Ok));
+}
+
+#[test]
+fn positions_entered_in_feet_are_flagged_as_unit_mismatch() {
+    // Operator measured in feet and typed the numbers as if they were metres.
+    let feet = 1.0 / 0.3048;
+    let positions = parse_node_positions(&positions_string(&TRUTH, feet)).unwrap();
+    let obs = ranges(&TRUTH, -0.5, 1.9, 5);
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert_eq!(report.verdict, Verdict::Warnings);
+    let unit = report
+        .findings
+        .iter()
+        .find_map(|f| match f {
+            Finding::UnitMismatch {
+                unit, fitted_scale, ..
+            } => Some((unit.clone(), *fitted_scale)),
+            _ => None,
+        })
+        .unwrap_or_else(|| panic!("no unit finding: {report:#?}"));
+    assert_eq!(unit.0, "feet");
+    assert!((0.25..0.37).contains(&unit.1), "scale {}", unit.1);
+    // The unit finding explains the residuals; no swap or per-link noise.
+    assert!(!report.findings.iter().any(|f| matches!(
+        f,
+        Finding::IdSwapSuggested { .. } | Finding::LinkOutlier { .. } | Finding::NodeMoved { .. }
+    )));
+}
+
+#[test]
+fn positions_in_centimetres_are_flagged() {
+    let positions = parse_node_positions(&positions_string(&TRUTH, 100.0)).unwrap();
+    let obs = ranges(&TRUTH, -0.5, 1.9, 3);
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert!(report
+        .findings
+        .iter()
+        .any(|f| matches!(f, Finding::UnitMismatch { unit, .. } if unit == "centimetres")));
+}
+
+#[test]
+fn swapped_ids_are_suggested_back() {
+    // The operator typed node 2's position under id 4 and vice versa.
+    let mut configured = TRUTH;
+    configured[1].1 = TRUTH[3].1;
+    configured[3].1 = TRUTH[1].1;
+    let positions = parse_node_positions(&positions_string(&configured, 1.0)).unwrap();
+    let obs = ranges(&TRUTH, -0.5, 1.9, 5);
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert_eq!(report.verdict, Verdict::Warnings);
+    let (assignment, equivalent) = report
+        .findings
+        .iter()
+        .find_map(|f| match f {
+            Finding::IdSwapSuggested {
+                assignment,
+                equivalent_assignments,
+                ..
+            } => Some((assignment.clone(), *equivalent_assignments)),
+            _ => None,
+        })
+        .unwrap_or_else(|| panic!("no swap finding: {report:#?}"));
+    assert_eq!(assignment, vec![(2, 4), (4, 2)]);
+    // Nodes 1-4 sit near the corners of a rectangle, so at FTM tolerances a
+    // four-id rotation fits about as well. The fewest-changes rule picks
+    // the two-id swap and the tie is reported.
+    assert!(equivalent >= 2, "{report:#?}");
+}
+
+#[test]
+fn three_way_rotation_is_suggested_back() {
+    // Ids 1, 3, 5 rotated: 1 got 3's position, 3 got 5's, 5 got 1's.
+    let mut configured = TRUTH;
+    configured[0].1 = TRUTH[2].1;
+    configured[2].1 = TRUTH[4].1;
+    configured[4].1 = TRUTH[0].1;
+    let positions = parse_node_positions(&positions_string(&configured, 1.0)).unwrap();
+    let obs = ranges(&TRUTH, -0.5, 1.9, 5);
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    let assignment = report
+        .findings
+        .iter()
+        .find_map(|f| match f {
+            Finding::IdSwapSuggested { assignment, .. } => Some(assignment.clone()),
+            _ => None,
+        })
+        .unwrap_or_else(|| panic!("no swap finding: {report:#?}"));
+    // Node 1's ranges match the position configured for node 5, and so on.
+    assert_eq!(assignment, vec![(1, 5), (3, 1), (5, 3)]);
+}
+
+/// A 6 × 4 m rectangle with a node in each corner.
+const RECT: [(u8, [f64; 3]); 4] = [
+    (1, [0.0, 0.0, 1.0]),
+    (2, [6.0, 0.0, 1.0]),
+    (3, [6.0, 4.0, 1.0]),
+    (4, [0.0, 4.0, 1.0]),
+];
+
+#[test]
+fn relabelling_that_is_a_symmetry_is_invisible() {
+    // Ids rotated 180 degrees (1<->3, 2<->4). Every pairwise distance is
+    // unchanged, so ranges cannot see it and the check must not invent a
+    // swap. Check mode cannot catch this class of mistake.
+    let mut configured = RECT;
+    configured[0].1 = RECT[2].1;
+    configured[2].1 = RECT[0].1;
+    configured[1].1 = RECT[3].1;
+    configured[3].1 = RECT[1].1;
+    let positions = parse_node_positions(&positions_string(&configured, 1.0)).unwrap();
+    let obs = ranges_with(&RECT, 0.0, 0.05, 0.05, 3);
+    let report = check(&positions, &obs, &CheckParams::uwb()).unwrap();
+    assert_eq!(report.verdict, Verdict::Consistent, "{report:#?}");
+}
+
+#[test]
+fn swap_along_a_wall_of_a_rectangle_is_ambiguous() {
+    // Ids 1 and 2 swapped along the long wall. Mirroring the rectangle
+    // turns that into a 3<->4 swap with identical distances, so both
+    // two-id fixes fit equally and the check must say it cannot choose.
+    let mut configured = RECT;
+    configured[0].1 = RECT[1].1;
+    configured[1].1 = RECT[0].1;
+    let positions = parse_node_positions(&positions_string(&configured, 1.0)).unwrap();
+    let obs = ranges_with(&RECT, 0.0, 0.05, 0.05, 3);
+    let report = check(&positions, &obs, &CheckParams::uwb()).unwrap();
+    assert_eq!(report.verdict, Verdict::Warnings);
+    assert!(
+        report
+            .findings
+            .iter()
+            .any(|f| matches!(f, Finding::IdSwapAmbiguous { equivalent_assignments } if *equivalent_assignments >= 2)),
+        "{report:#?}"
+    );
+}
+
+#[test]
+fn moved_node_is_flagged_without_blaming_others() {
+    // Node 5 was moved 2 m after setup; its configured position is stale.
+    // UWB-grade ranges: uncalibrated FTM tolerances hide moves this size.
+    let mut actual = TRUTH;
+    actual[4].1 = [2.6, 1.2, 0.8];
+    let positions = parse_node_positions(&positions_string(&TRUTH, 1.0)).unwrap();
+    let obs = ranges_with(&actual, 0.0, 0.1, 0.05, 5);
+    let report = check(&positions, &obs, &CheckParams::uwb()).unwrap();
+    assert_eq!(report.verdict, Verdict::Warnings);
+    let moved: Vec<u8> = report
+        .findings
+        .iter()
+        .filter_map(|f| match f {
+            Finding::NodeMoved { node, .. } => Some(*node),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(moved, vec![5], "{report:#?}");
+    assert!(!report
+        .findings
+        .iter()
+        .any(|f| matches!(f, Finding::IdSwapSuggested { .. })));
+}
+
+#[test]
+fn bench_style_single_link_outlier_is_reported_as_link_outlier() {
+    // One link over-reads by +4.1 m, as the 3.71 m bench start did.
+    let positions = parse_node_positions(&positions_string(&TRUTH, 1.0)).unwrap();
+    let mut obs = ranges(&TRUTH, 0.0, 0.5, 3);
+    for o in obs.iter_mut().filter(|o| o.pair() == (1, 5)) {
+        o.range_m += 4.1;
+    }
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert_eq!(report.verdict, Verdict::Warnings);
+    assert!(report
+        .findings
+        .iter()
+        .any(|f| matches!(f, Finding::LinkOutlier { a: 1, b: 5, .. })));
+    assert!(!report.findings.iter().any(|f| matches!(
+        f,
+        Finding::IdSwapSuggested { .. } | Finding::NodeMoved { .. } | Finding::UnitMismatch { .. }
+    )));
+}
+
+#[test]
+fn per_boot_calibration_tightens_tolerance() {
+    // Same responder boot for every session; one reference link at a taped
+    // 3.0 m recovers a +0.7 m bias, so a 1.5 m misplacement that
+    // uncalibrated FTM cannot see becomes visible.
+    let positions = parse_node_positions("1:0,0,1;2:5,0,1").unwrap();
+    let boot = Some(42);
+    let reference: Vec<RangeObservation> = (0..5)
+        .map(|_| RangeObservation {
+            responder_boot: boot,
+            ..RangeObservation::ftm(1, 2, 3.0 + 0.7, 0.3)
+        })
+        .collect();
+    let mut table = CalibrationTable::new();
+    let offset = table.record_reference(&reference, 3.0).unwrap();
+    assert!((offset - 0.7).abs() < 1e-9);
+
+    // Node 2 actually sits at 6.5 m, not the configured 5.0 m.
+    let live = RangeObservation {
+        responder_boot: boot,
+        ..RangeObservation::ftm(1, 2, 6.5 + 0.7, 0.1)
+    };
+    let raw = check(
+        &positions,
+        std::slice::from_ref(&live),
+        &CheckParams::ftm_uncalibrated(),
+    )
+    .unwrap();
+    assert_eq!(raw.links[0].status, LinkStatus::Ok);
+
+    let calibrated = table.apply(&live);
+    let report = check(&positions, &[calibrated], &CheckParams::ftm_uncalibrated()).unwrap();
+    assert!(report.links[0].calibrated);
+    assert_eq!(report.links[0].status, LinkStatus::Outlier, "{report:#?}");
+
+    // After a responder reboot the offset no longer applies.
+    let rebooted = RangeObservation {
+        responder_boot: Some(43),
+        ..live
+    };
+    assert!(!table.apply(&rebooted).calibrated);
+}
+
+#[test]
+fn unknown_and_unranged_nodes_are_reported() {
+    let positions = parse_node_positions("1:0,0,1;2:4,0,1;3:0,3,1;9:2,2,1").unwrap();
+    let obs = [
+        RangeObservation::ftm(1, 2, 4.0, 0.2),
+        RangeObservation::ftm(1, 3, 3.0, 0.2),
+        RangeObservation::ftm(2, 3, 5.0, 0.2),
+        RangeObservation::ftm(1, 7, 2.0, 0.2),
+    ];
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert!(report.findings.contains(&Finding::UnknownNode {
+        node: 7,
+        observations: 1
+    }));
+    assert!(report.findings.contains(&Finding::UnrangedNode { node: 9 }));
+    assert_eq!(report.verdict, Verdict::Warnings);
+}
+
+#[test]
+fn weak_sessions_are_not_scored() {
+    let positions = parse_node_positions("1:0,0,1;2:4,0,1").unwrap();
+    let obs = [RangeObservation {
+        rssi_dbm: Some(-80),
+        ..RangeObservation::ftm(1, 2, 12.0, 0.2)
+    }];
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    assert_eq!(report.links[0].status, LinkStatus::Weak);
+    assert_eq!(report.verdict, Verdict::Insufficient);
+}
+
+#[test]
+fn noisy_link_is_not_scored() {
+    // Shaped like the bench's 7.7 m run at 1.14 m: −68 dBm passes the RSSI
+    // gate, but the session ranges spread 5.25 to 12.3 m.
+    let positions = parse_node_positions("1:0,0,1;2:4,0,1;3:0,3,1").unwrap();
+    let mut obs = vec![
+        RangeObservation::ftm(1, 3, 3.2, 0.3),
+        RangeObservation::ftm(2, 3, 5.3, 0.3),
+    ];
+    for r in [5.25, 6.9, 7.7, 7.75, 9.6, 12.3] {
+        obs.push(RangeObservation {
+            rssi_dbm: Some(-68),
+            ..RangeObservation::ftm(1, 2, r, 0.3)
+        });
+    }
+    let report = check(&positions, &obs, &CheckParams::ftm_uncalibrated()).unwrap();
+    let link = report.links.iter().find(|l| (l.a, l.b) == (1, 2)).unwrap();
+    assert_eq!(link.status, LinkStatus::Noisy, "{report:#?}");
+    assert!(link.sigma_m > 1.0);
+    assert_eq!(report.verdict, Verdict::Consistent);
+}
+
+#[test]
+fn report_serializes_to_json() {
+    let positions = parse_node_positions(&positions_string(&TRUTH, 1.0)).unwrap();
+    let report = check(
+        &positions,
+        &ranges(&TRUTH, 0.0, 0.5, 2),
+        &CheckParams::ftm_uncalibrated(),
+    )
+    .unwrap();
+    let json = serde_json::to_value(&report).unwrap();
+    assert_eq!(json["verdict"], "consistent");
+    assert_eq!(json["links"].as_array().unwrap().len(), 10);
+}