diff --git a/fuse/fuse_trajectory.py b/fuse/fuse_trajectory.py
new file mode 100644
index 0000000..ece37ce
--- /dev/null
+++ b/fuse/fuse_trajectory.py
@@ -0,0 +1,183 @@
+# fuse/fuse_trajectory.py
+"""
+Umeyama-based trajectory fusion.
+Aligns lingbot local poses to world coordinates using USBL/GPS absolute fixes.
+Gracefully handles missing lingbot poses or no absolute AUV fixes.
+"""
+import sys
+from pathlib import Path
+import numpy as np
+import h5py
+
+
+def umeyama(src: np.ndarray, dst: np.ndarray,
+            weights: np.ndarray | None = None) -> tuple[float, np.ndarray, np.ndarray]:
+    """
+    Weighted Umeyama: find scale, R, t such that dst ≈ scale * R @ src + t
+    src, dst : (N, 3) float64
+    weights  : (N,) non-negative
+    Raises ValueError if N < 3.
+    """
+    N = len(src)
+    if N < 3:
+        raise ValueError(f"Umeyama requires at least 3 point pairs, got {N}")
+    if weights is None:
+        weights = np.ones(N, dtype=np.float64)
+    w = weights / weights.sum()
+    mu_s = (w[:, None] * src).sum(0)
+    mu_d = (w[:, None] * dst).sum(0)
+    src_c, dst_c = src - mu_s, dst - mu_d
+    cov = (dst_c * w[:, None]).T @ src_c
+    U, D, Vt = np.linalg.svd(cov)
+    S = np.eye(3)
+    if np.linalg.det(U) * np.linalg.det(Vt) < 0:
+        S[2, 2] = -1
+    R = U @ S @ Vt
+    var_s = (w * np.sum(src_c ** 2, axis=1)).sum()
+    scale = float(np.sum(D * np.diag(S)) / var_s) if var_s > 0 else 1.0
+    t = mu_d - scale * R @ mu_s
+    return scale, R, t
+
+
+def fuse(fixes_h5: str, poses_npz: str, out_h5: str,
+         outlier_sigma: float = 2.0) -> None:
+
+    # 1. Load fixes
+    with h5py.File(fixes_h5, "r") as f:
+        usv_e    = f["usv_gps/easting"][:]
+        usv_n    = f["usv_gps/northing"][:]
+        usv_t    = f["usv_gps/t_ns"][:]
+        utm_zone = f["usv_gps"].attrs.get("utm_zone", "31T")
+
+        auv_lat  = f["auv_mcap/lat"][:]
+        auv_lon  = f["auv_mcap/lon"][:]
+        auv_dep  = f["auv_mcap/depth_m"][:]
+        auv_t    = f["auv_mcap/t_ns"][:]
+
+        usbl_n = f["usbl_fixes/north_m"][:] if "usbl_fixes" in f else np.array([])
+        usbl_e = f["usbl_fixes/east_m"][:]  if "usbl_fixes" in f else np.array([])
+        usbl_d = f["usbl_fixes/depth_m"][:] if "usbl_fixes" in f else np.array([])
+        usbl_t = f["usbl_fixes/t_ns"][:]    if "usbl_fixes" in f else np.array([], dtype=np.int64)
+
+    # 2. Check for lingbot poses
+    poses_path = Path(poses_npz)
+    if not poses_path.exists():
+        print(f"WARNING: lingbot poses not found at {poses_npz}")
+        print("Saving sources only — re-run after Plan 2 generates lingbot_poses.npz")
+        with h5py.File(out_h5, "w") as f:
+            f.attrs["status"] = "no_lingbot_poses"
+            f.attrs["utm_zone"] = utm_zone
+        return
+
+    # 3. Load lingbot poses
+    data = np.load(poses_npz)
+    poses_34 = data["poses"]          # (N, 3, 4)
+    pose_t   = data["timestamps_ns"]  # (N,) int64
+    ling_xyz = poses_34[:, :3, 3]     # camera positions in local frame
+
+    # 4. Build absolute AUV reference points
+    has_usbl_fixes = (len(usbl_n) > 0 and
+                      not np.all(usbl_n == 0) and
+                      not np.all(usbl_e == 0))
+
+    has_auv_gps = not np.all(auv_lat == 0.0)
+
+    if not has_usbl_fixes and not has_auv_gps:
+        print("WARNING: No absolute AUV position fixes available.")
+        print("Cannot perform Umeyama alignment — saving local lingbot trajectory only.")
+        _save_local_only(out_h5, pose_t, poses_34, ling_xyz, utm_zone)
+        return
+
+    # 5. Build world reference: interpolate AUV absolute pos at lingbot timestamps
+    if has_auv_gps:
+        from pyproj import Transformer
+        zone_num = int("".join(c for c in utm_zone if c.isdigit()))
+        hemi = "north" if utm_zone[-1].upper() >= "N" else "south"
+        proj = f"+proj=utm +zone={zone_num} +{hemi} +ellps=WGS84"
+        tr = Transformer.from_crs("EPSG:4326", proj, always_xy=True)
+        auv_x, auv_y = tr.transform(auv_lon, auv_lat)
+        auv_xyz_world = np.column_stack([auv_x, auv_y, -auv_dep])
+        ref_t = auv_t
+    else:
+        # USBL relative offsets + USV GPS → AUV absolute
+        usv_e_i = np.interp(usbl_t.astype(float), usv_t.astype(float), usv_e)
+        usv_n_i = np.interp(usbl_t.astype(float), usv_t.astype(float), usv_n)
+        auv_xyz_world = np.column_stack([usv_e_i + usbl_e, usv_n_i + usbl_n, -usbl_d])
+        ref_t = usbl_t
+
+    # 6. Match lingbot timestamps → world reference points
+    src_pts, dst_pts, weights = [], [], []
+    for i, pt in enumerate(pose_t):
+        idx = np.argmin(np.abs(ref_t - pt))
+        if np.abs(ref_t[idx] - pt) > 5e9:  # > 5s gap → skip
+            continue
+        src_pts.append(ling_xyz[i])
+        dst_pts.append(auv_xyz_world[idx])
+        weights.append(1.0)
+
+    if len(src_pts) < 3:
+        print(f"WARNING: Only {len(src_pts)} correspondences (need ≥3). Saving local only.")
+        _save_local_only(out_h5, pose_t, poses_34, ling_xyz, utm_zone)
+        return
+
+    src = np.array(src_pts)
+    dst = np.array(dst_pts)
+    w   = np.array(weights)
+
+    # 7. Umeyama with outlier rejection
+    scale, R, t = umeyama(src, dst, w)
+    residuals = np.linalg.norm((scale * (R @ src.T).T + t) - dst, axis=1)
+    sigma = residuals.std()
+    mask = residuals < outlier_sigma * sigma if sigma > 0 else np.ones(len(src), dtype=bool)
+    print(f"Correspondences: {len(src)} total, {mask.sum()} after outlier rejection (σ={sigma:.3f}m)")
+    scale, R, t = umeyama(src[mask], dst[mask], w[mask])
+
+    # 8. Transform all lingbot poses to world frame
+    N = len(pose_t)
+    poses_world = np.zeros((N, 4, 4))
+    poses_world[:, 3, 3] = 1.0
+    for i in range(N):
+        R_local = poses_34[i, :3, :3]
+        t_local = poses_34[i, :3, 3]
+        poses_world[i, :3, :3] = R @ R_local
+        poses_world[i, :3, 3]  = scale * R @ t_local + t
+
+    xyz_world = poses_world[:, :3, 3]
+
+    # 9. Write trajectory_world.h5
+    with h5py.File(out_h5, "w") as f:
+        f.attrs["status"]   = "aligned"
+        f.attrs["utm_zone"] = utm_zone
+
+        al = f.create_group("alignment")
+        al.attrs["scale"]             = scale
+        al.attrs["rmse_m"]            = float(residuals[mask].mean())
+        al.attrs["n_correspondences"] = int(mask.sum())
+        al.create_dataset("R", data=R)
+        al.create_dataset("t", data=t)
+
+        pw = f.create_group("poses_world")
+        pw.create_dataset("t_ns",  data=pose_t,         compression="gzip")
+        pw.create_dataset("x_m",   data=xyz_world[:, 0], compression="gzip")
+        pw.create_dataset("y_m",   data=xyz_world[:, 1], compression="gzip")
+        pw.create_dataset("z_m",   data=xyz_world[:, 2], compression="gzip")
+        pw.create_dataset("T_4x4", data=poses_world,    compression="gzip")
+
+    print(f"Fusion OK: scale={scale:.4f} RMSE={residuals[mask].mean():.3f}m → {out_h5}")
+
+
+def _save_local_only(out_h5: str, pose_t, poses_34, ling_xyz, utm_zone):
+    with h5py.File(out_h5, "w") as f:
+        f.attrs["status"]   = "local_only"
+        f.attrs["utm_zone"] = utm_zone
+        pw = f.create_group("poses_world")
+        pw.create_dataset("t_ns", data=pose_t,        compression="gzip")
+        pw.create_dataset("x_m",  data=ling_xyz[:, 0], compression="gzip")
+        pw.create_dataset("y_m",  data=ling_xyz[:, 1], compression="gzip")
+        pw.create_dataset("z_m",  data=ling_xyz[:, 2], compression="gzip")
+        pw.create_dataset("T_4x4", data=poses_34,     compression="gzip")
+    print(f"Saved local lingbot trajectory (no world alignment) → {out_h5}")
+
+
+if __name__ == "__main__":
+    fuse(sys.argv[1], sys.argv[2], sys.argv[3])
diff --git a/tests/test_fuse_trajectory.py b/tests/test_fuse_trajectory.py
new file mode 100644
index 0000000..9f8d7d7
--- /dev/null
+++ b/tests/test_fuse_trajectory.py
@@ -0,0 +1,28 @@
+import tempfile, os
+import numpy as np
+import h5py
+
+def test_fuse_creates_output_without_lingbot():
+    """When lingbot_poses.npz doesn't exist, fuse creates HDF5 with sources only."""
+    from fuse.fuse_trajectory import fuse
+    with tempfile.TemporaryDirectory() as tmpdir:
+        out_h5 = os.path.join(tmpdir, "traj.h5")
+        # Create minimal sparse_fixes.h5
+        fixes_h5 = os.path.join(tmpdir, "fixes.h5")
+        with h5py.File(fixes_h5, "w") as f:
+            grp = f.create_group("usv_gps")
+            grp.create_dataset("t_ns",    data=np.array([1000, 2000], dtype=np.int64))
+            grp.create_dataset("easting", data=np.array([100.0, 101.0]))
+            grp.create_dataset("northing",data=np.array([200.0, 201.0]))
+            grp.create_dataset("rtk_status", data=np.array([0, 0], dtype=np.int8))
+            grp.attrs["utm_zone"] = "31T"
+            grp2 = f.create_group("auv_mcap")
+            grp2.create_dataset("t_ns",   data=np.array([1000, 2000], dtype=np.int64))
+            grp2.create_dataset("lat",    data=np.array([0.0, 0.0]))
+            grp2.create_dataset("lon",    data=np.array([0.0, 0.0]))
+            grp2.create_dataset("depth_m",data=np.array([5.0, 6.0]))
+
+        fuse(fixes_h5, "/nonexistent/lingbot.npz", out_h5)
+        assert os.path.exists(out_h5)
+        with h5py.File(out_h5, "r") as f:
+            assert "status" in f.attrs
diff --git a/tests/test_umeyama.py b/tests/test_umeyama.py
new file mode 100644
index 0000000..315eb5f
--- /dev/null
+++ b/tests/test_umeyama.py
@@ -0,0 +1,41 @@
+import numpy as np
+import pytest
+
+def test_umeyama_identity():
+    from fuse.fuse_trajectory import umeyama
+    src = np.random.default_rng(0).standard_normal((10, 3))
+    scale, R, t = umeyama(src, src)
+    assert abs(scale - 1.0) < 1e-5
+    assert np.allclose(R, np.eye(3), atol=1e-5)
+    assert np.allclose(t, np.zeros(3), atol=1e-5)
+
+def test_umeyama_known_transform():
+    from fuse.fuse_trajectory import umeyama
+    rng = np.random.default_rng(42)
+    src = rng.standard_normal((20, 3))
+    true_scale = 2.5
+    true_R = np.array([[0, -1, 0], [1, 0, 0], [0, 0, 1]], dtype=float)
+    true_t = np.array([1.0, 2.0, 3.0])
+    dst = true_scale * (src @ true_R.T) + true_t
+    scale, R, t = umeyama(src, dst)
+    assert abs(scale - true_scale) < 1e-4
+    assert np.allclose(R, true_R, atol=1e-4)
+    assert np.allclose(t, true_t, atol=1e-4)
+
+def test_umeyama_weighted():
+    from fuse.fuse_trajectory import umeyama
+    rng = np.random.default_rng(0)
+    src = rng.standard_normal((15, 3))
+    true_scale, true_t = 1.5, np.array([0.5, -0.5, 1.0])
+    dst = true_scale * src + true_t
+    weights = np.ones(15)
+    weights[0] = 0.0  # outlier with zero weight
+    scale, R, t = umeyama(src, dst, weights=weights)
+    assert abs(scale - true_scale) < 1e-3
+    assert np.allclose(t, true_t, atol=1e-3)
+
+def test_umeyama_raises_on_few_points():
+    from fuse.fuse_trajectory import umeyama
+    src = np.random.default_rng(0).standard_normal((2, 3))
+    with pytest.raises(ValueError, match="at least 3"):
+        umeyama(src, src)