diff --git a/docs/api/tools.md b/docs/api/tools.md
index a7156268..9f77acd9 100644
--- a/docs/api/tools.md
+++ b/docs/api/tools.md
@@ -37,6 +37,7 @@ functions and classes with no BART command behind them.
 | {obj}`~bartorch.tools.upat` | Regular undersampling pattern with a fully sampled centre |
 | {obj}`~bartorch.tools.raga` | Indices of a RAGA (rational approximation of the golden angle) radial ordering |
 | {obj}`~bartorch.tools.grog` | Radial data gridded onto a Cartesian grid by GROG |
+| {obj}`~bartorch.tools.igrog` | Non-Cartesian samples moved onto the grid by closed-form GRAPPA kernels (iGROG) |
 | {obj}`~bartorch.tools.psf` | Point spread function of a trajectory |
 | {obj}`~bartorch.tools.wavepsf` | Wave-CAIPI point spread function in hybrid space |
 | {obj}`~bartorch.tools.estdims` | Image dimensions implied by a non-Cartesian trajectory |
diff --git a/src/bartorch/tools/__init__.py b/src/bartorch/tools/__init__.py
index 41f74618..1fbeda78 100644
--- a/src/bartorch/tools/__init__.py
+++ b/src/bartorch/tools/__init__.py
@@ -16,6 +16,7 @@
 from bartorch.tools import (
     _calib,
     _correct,
+    _igrog,
     _lowrank,
     _motion,
     _process,
@@ -24,6 +25,7 @@
 )
 from bartorch.tools._calib import *  # noqa: F401,F403
 from bartorch.tools._correct import *  # noqa: F401,F403
+from bartorch.tools._igrog import *  # noqa: F401,F403
 from bartorch.tools._lowrank import *  # noqa: F401,F403
 from bartorch.tools._motion import *  # noqa: F401,F403
 from bartorch.tools._process import *  # noqa: F401,F403
@@ -31,7 +33,7 @@
 from bartorch.tools._simulate import *  # noqa: F401,F403
 
 __all__ = sorted(
-    {*_simulate.__all__, *_sampling.__all__, *_calib.__all__, *_process.__all__}
+    {*_simulate.__all__, *_sampling.__all__, *_igrog.__all__, *_calib.__all__, *_process.__all__}
     | {*_lowrank.__all__, *_correct.__all__, *_motion.__all__}
 )
 
diff --git a/src/bartorch/tools/_igrog.py b/src/bartorch/tools/_igrog.py
new file mode 100644
index 00000000..03107128
--- /dev/null
+++ b/src/bartorch/tools/_igrog.py
@@ -0,0 +1,266 @@
+"""iGROG: non-Cartesian samples moved onto the Cartesian grid by closed-form GRAPPA kernels."""
+
+from __future__ import annotations
+
+import math
+from dataclasses import dataclass
+
+import torch
+
+__all__ = ["igrog"]
+
+
+@dataclass
+class _Correlation:
+    """Coil cross-correlation ``Q`` on displacements ``-reach..reach``, ``1 / upsample`` apart.
+
+    ``table`` is ``(C, C, *[2 reach upsample + 1] * d)``, displacement axes in trajectory order.
+    """
+
+    table: torch.Tensor
+    reach: int
+    upsample: int
+
+
+@dataclass
+class _Gridded:
+    values: torch.Tensor
+    index: torch.Tensor
+    shot: torch.Tensor
+    sample: torch.Tensor
+    shape: tuple
+    noise: torch.Tensor
+
+    def weights(self) -> torch.Tensor:
+        """``1 / sqrt(noise)`` at unit mean square: the weights that whiten the gridded samples."""
+        w = self.noise.clamp(min=1e-12).rsqrt()
+        return w / w.square().mean().sqrt()
+
+
+def _correlation(calib: torch.Tensor, reach: int, upsample: int) -> _Correlation:
+    """Tabulate ``Q`` from Cartesian calibration k-space ``(C, [z,] y, x)``, centre ``n // 2``."""
+    calib = calib.to(torch.complex128)
+    cal = calib.shape[1:]
+    d = len(cal)
+    dims = tuple(range(1, d + 1))
+    img = torch.fft.fftshift(
+        torch.fft.ifftn(torch.fft.ifftshift(calib, dim=dims), dim=dims, norm="forward"), dim=dims
+    )
+    q = img[:, None] * img[None].conj()
+    m = torch.arange(-reach * upsample, reach * upsample + 1, dtype=torch.float64) / upsample
+    for ax in range(d):  # consumes the leading image axis, appends its displacement axis
+        n = cal[ax]
+        r = torch.arange(n, dtype=torch.float64) - n // 2
+        q = torch.tensordot(q, torch.exp(-2j * math.pi * torch.outer(m, r) / n), dims=([2], [1]))
+    q = q.permute(0, 1, *reversed(range(2, 2 + d))).contiguous()  # image order -> trajectory order
+    return _Correlation(q.to(torch.complex64), reach, upsample)
+
+
+def _lagrange4(t: torch.Tensor) -> torch.Tensor:
+    """Cubic Lagrange weights of the nodes -1, 0, 1, 2 at ``t``."""
+    return torch.stack(
+        [
+            -t * (t - 1) * (t - 2) / 6,
+            (t + 1) * (t - 1) * (t - 2) / 2,
+            -(t + 1) * t * (t - 2) / 2,
+            (t + 1) * t * (t - 1) / 6,
+        ],
+        -1,
+    )
+
+
+def _interpolate(cc: _Correlation, table: torch.Tensor, disp: torch.Tensor) -> torch.Tensor:
+    """``Q`` at displacements ``(..., d)``, as ``(..., C, C)``."""
+    d, U, R = disp.shape[-1], cc.upsample, cc.reach
+    u = (disp.double() + R) * U
+    base = torch.floor(u).long()
+    w = _lagrange4(u - base)
+    coils, size = table.shape[0], table.shape[2]
+    flat_table = table.reshape(coils, coils, -1)
+    shape = disp.shape[:-1]
+    out = torch.zeros(*shape, coils, coils, dtype=torch.complex128, device=disp.device)
+    for taps in torch.cartesian_prod(*[torch.arange(4)] * d).reshape(-1, d).tolist():
+        flat = torch.zeros(shape, dtype=torch.long, device=disp.device)
+        wt = torch.ones(shape, dtype=torch.float64, device=disp.device)
+        for a in range(d):
+            flat = flat * size + (base[..., a] + taps[a] - 1)
+            wt = wt * w[..., a, taps[a]]
+        out += wt[..., None, None] * flat_table[:, :, flat].movedim((0, 1), (-2, -1))
+    return out
+
+
+def _kernels(
+    cc: _Correlation, table: torch.Tensor, offsets: torch.Tensor, lam: float
+) -> torch.Tensor:
+    """Wiener kernels ``(M, C, D C)`` of sources at ``offsets`` ``(M, D, d)`` from their targets."""
+    M, D, _ = offsets.shape
+    C = table.shape[0]
+    disp = (offsets[:, :, None] - offsets[:, None]).reshape(M, D * D, -1)
+    Rss = (
+        _interpolate(cc, table, disp)
+        .reshape(M, D, D, C, C)
+        .permute(0, 1, 3, 2, 4)
+        .reshape(M, D * C, D * C)
+    )
+    Rss = 0.5 * (Rss + Rss.mH)
+    Rts = _interpolate(cc, table, -offsets).permute(0, 2, 1, 3).reshape(M, C, D * C)
+    tr = torch.diagonal(Rss, dim1=-2, dim2=-1).real.mean(-1)
+    Rss = Rss + (lam * tr)[:, None, None] * torch.eye(D * C, dtype=Rss.dtype, device=Rss.device)
+    return torch.cholesky_solve(Rts.mH, torch.linalg.cholesky(Rss)).mH
+
+
+def _sources(traj: torch.Tensor, shot: torch.Tensor, sample: torch.Tensor, sources: int, spacing):
+    """Sample indices ``(M, sources)`` of each target's sources along its readout."""
+    S, P, _ = traj.shape
+    win = torch.clamp(sample - sources // 2, 0, P - sources)[:, None] + torch.arange(sources)
+    if spacing is None:
+        return win
+    step = torch.linalg.vector_norm(traj[:, 1:] - traj[:, :-1], dim=-1).double()
+    arc = torch.cat([torch.zeros(S, 1, dtype=step.dtype), step.cumsum(1)], 1)
+    big = float(arc[:, -1].max()) + 2 * spacing * sources + 1
+    flat = (arc + big * torch.arange(S, dtype=step.dtype)[:, None]).ravel()
+    o = (torch.arange(sources, dtype=step.dtype) - (sources - 1) / 2) * spacing
+    want = arc[shot, sample][:, None] + o + big * shot[:, None].to(step.dtype)
+    hi = torch.searchsorted(flat, want.contiguous()).clamp(max=S * P - 1)
+    lo = (hi - 1).clamp(min=0)
+    pick = torch.where((flat[hi] - want).abs() < (flat[lo] - want).abs(), hi, lo)
+    first = shot[:, None] * P
+    pick = torch.minimum(torch.maximum(pick, first), first + P - 1) - first
+    srt = pick.sort(1).values
+    return torch.where((srt[:, 1:] == srt[:, :-1]).any(1)[:, None], win, pick)
+
+
+def igrog(
+    kspace: torch.Tensor,
+    traj: torch.Tensor,
+    shape: tuple,
+    calib,
+    *,
+    group: torch.Tensor | None = None,
+    sources: int = 3,
+    lam: float = 1e-3,
+    spacing: float | None = 1.0,
+    reach: int = 4,
+    upsample: int = 8,
+    batch: int = 65536,
+    device=None,
+):
+    r"""Non-Cartesian samples moved to their nearest grid point by closed-form GRAPPA (iGROG).
+
+    Each sample on the grid's extent is moved to its nearest Cartesian point by
+    a kernel over ``sources`` samples of its readout and every coil.  iGROG
+    (Abraham et al. [1]_) learns that kernel from the source offsets; here it
+    is the Wiener (linear minimum mean square error) interpolator under the
+    coil cross-correlation of the calibration k-space,
+
+    .. math::
+
+        W = R_{ts} (R_{ss} + \lambda \bar r I)^{-1}, \qquad
+        Q_{cc'}(\delta) = \sum_r m_c(r) \overline{m_{c'}(r)} e^{-2\pi i \delta \cdot r / n},
+
+    with :math:`R_{ss}` and :math:`R_{ts}` the correlation :math:`Q` between
+    the sources and from them to the target, :math:`m_c` the calibration's
+    coil images and :math:`\bar r` the mean signal power.  :math:`Q` is
+    tabulated once on a fine grid of displacements and interpolated, so there
+    is no fit and no seed.  The gridded samples can then be encoded by
+    :func:`bartorch.linop.CartesianSense` with ``positions=``, whose normal is
+    FFTs on the image grid: no NUFFT and no oversampled grid.
+
+    Parameters
+    ----------
+    kspace : torch.Tensor
+        ``(coils, *shots, samples)`` samples.
+    traj : torch.Tensor
+        ``(*shots, samples, ndim)`` in grid units, ``kx, ky[, kz]``, as for
+        :class:`bartorch.linop.NUFFT`.
+    shape : tuple
+        The image grid, ``([z,] y, x)``.
+    calib : torch.Tensor or sequence of torch.Tensor
+        Cartesian calibration k-space ``(coils, [z,] y, x)`` centred at
+        ``n // 2``, on the grid's k-space spacing; a sequence of them with
+        ``group`` when the contrast changes over the scan.
+    group : torch.Tensor, default=None
+        ``(*shots,)`` index into ``calib`` of each shot's calibration.
+    sources : int, default=3
+        Readout samples per kernel.
+    lam : float, default=1e-3
+        Tikhonov weight relative to the mean signal power.
+    spacing : float, default=1.0
+        Arc length between sources along the readout, in grid units; ``None``
+        takes consecutive samples.
+    reach : int, default=4
+        Displacements the correlation is tabulated to, in cells.
+    upsample : int, default=8
+        Table steps per cell.
+    batch : int, default=65536
+        Kernels solved at once.
+    device : torch.device, default=None
+        Where the kernels are solved (a card is much faster); the result is on
+        the host.  ``None`` uses the samples' device.
+
+    Returns
+    -------
+    gridded
+        With ``values`` ``(coils, M)`` complex64, their flat grid ``index``
+        ``(M,)``, the ``shot`` (flat index of ``*shots``) and ``sample`` each
+        came from, ``noise`` ``(M,)`` (each value's noise variance per coil for
+        unit white noise on the samples) and ``weights()``, the weights of unit
+        mean square that whiten the values.
+
+    References
+    ----------
+    .. [1] Abraham D, Blumenthal M, Uecker M, Ong F, et al. Implicit GROG for
+       non-Cartesian MRI (iGROG). arXiv:2310.10823.
+
+    Examples
+    --------
+    >>> g = igrog(kspace, traj, (256, 256, 256), calib)
+    >>> values = g.values * g.weights()
+    """
+    d = traj.shape[-1]
+    P = traj.shape[-2]
+    tr = traj.reshape(-1, P, d).cpu()
+    y = kspace.reshape(kspace.shape[0], -1, P)
+    card = torch.device(device) if device is not None else y.device
+    if sources > P:
+        raise ValueError("more sources than samples per readout")
+    calibs = [calib] if isinstance(calib, torch.Tensor) else list(calib)
+    if len(calibs) > 1 and group is None:
+        raise ValueError("several calibrations need the group of each shot")
+    tables = [_correlation(c.cpu(), reach, upsample) for c in calibs]
+
+    # Targets: every sample on the grid, with its nearest grid point.
+    S = tr.shape[0]
+    t = torch.round(tr)
+    n = torch.tensor(list(reversed(shape)), dtype=tr.dtype)
+    inside = ((t >= -(n // 2)) & (t <= n - n // 2 - 1)).all(-1)
+    lin = torch.zeros(S, P, dtype=torch.long)
+    for a in reversed(range(d)):  # flat index in image order ([z,] y, x)
+        lin = lin * int(n[a]) + (t[..., a].long() + int(n[a]) // 2)
+    shot = torch.arange(S)[:, None].expand(S, P)[inside]
+    sample = torch.arange(P)[None].expand(S, P)[inside]
+    lin = lin[inside]
+
+    src = _sources(tr, shot, sample, sources, spacing)
+    off = tr[shot[:, None], src] - torch.round(tr[shot, sample])[:, None]
+    far = max(float((off[:, :, None] - off[:, None]).abs().amax()), float(off.abs().amax()))
+    lim = reach - 2.0 / upsample
+    if far > lim:
+        raise ValueError(f"sources reach {far:.2f} cells apart; the table reaches {lim:.2f}")
+
+    out = torch.empty(y.shape[0], shot.numel(), dtype=torch.complex64)
+    noise = torch.empty(shot.numel(), dtype=torch.float32)
+    which = torch.zeros_like(shot) if group is None else group.reshape(-1).cpu()[shot]
+    for gi, cc in enumerate(tables):
+        table = cc.table.to(card)
+        idx = torch.nonzero(which == gi).squeeze(1)
+        for b in range(0, idx.numel(), batch):
+            ib = idx[b : b + batch]
+            s, sb = shot[ib], src[ib]
+            W = _kernels(cc, table, off[ib].to(card), lam)  # (B, C, D C)
+            v = y[:, s[:, None], sb].permute(1, 2, 0).reshape(len(s), -1)  # (B, D C), source-major
+            out[:, ib] = torch.einsum("bcj,bj->cb", W, v.to(card, W.dtype)).to(
+                "cpu", torch.complex64
+            )
+            noise[ib] = (W.abs().square().sum((1, 2)) / W.shape[1]).float().cpu()
+    return _Gridded(out, lin, shot, sample, tuple(shape), noise)
diff --git a/tests/test_igrog.py b/tests/test_igrog.py
new file mode 100644
index 00000000..9da44d21
--- /dev/null
+++ b/tests/test_igrog.py
@@ -0,0 +1,77 @@
+"""iGROG: samples moved by closed-form GRAPPA kernels land on the grid's own k-space."""
+
+import math
+
+import torch
+
+from bartorch import tools
+
+
+def _object(n=32, coils=6, seed=0):
+    """A smooth 2-D object under smooth coil maps, and its exact k-space at any position."""
+    g = torch.Generator().manual_seed(seed)
+    r = torch.arange(n, dtype=torch.float64) - n // 2
+    yy, xx = torch.meshgrid(r, r, indexing="ij")
+    obj = torch.exp(-((xx / 7) ** 2 + (yy / 9) ** 2)) * (1 + 0.3 * torch.cos(xx / 3))
+    centres = torch.rand(coils, 2, generator=g, dtype=torch.float64) * n - n / 2
+    maps = torch.stack(
+        [torch.exp(-((xx - cx) ** 2 + (yy - cy) ** 2) / (2 * 12.0**2)) for cx, cy in centres]
+    )
+    phase = torch.exp(1j * torch.rand(coils, 1, 1, generator=g, dtype=torch.float64) * 2 * math.pi)
+    images = (maps * phase * obj).to(torch.complex128)  # (C, y, x)
+
+    def kspace(k):  # k (..., 2) = (kx, ky) in grid units -> (C, ...)
+        e = torch.exp(-2j * math.pi * (k[..., 0, None, None] * xx + k[..., 1, None, None] * yy) / n)
+        return torch.einsum("cyx,...yx->c...", images, e)
+
+    return images, kspace
+
+
+def test_gridded_samples_match_the_grid_k_space():
+    n = 32
+    _, kspace = _object(n)
+    # Radial-ish readouts through the centre at assorted angles, half a cell apart.
+    angles = torch.linspace(0, math.pi, 24, dtype=torch.float64)[:-1] + 0.1
+    s = torch.arange(-10, 10.5, 0.5, dtype=torch.float64)
+    traj = torch.stack(
+        [s[None] * torch.cos(angles)[:, None], s[None] * torch.sin(angles)[:, None]], -1
+    )
+    y = kspace(traj).to(torch.complex64)  # (C, shots, samples)
+    c = torch.arange(-8, 8, dtype=torch.float64)
+    ky, kx = torch.meshgrid(c, c, indexing="ij")
+    calib = kspace(torch.stack([kx, ky], -1)).to(torch.complex64)  # (C, 16, 16), centre at 8
+
+    g = tools.igrog(y, traj.float(), (n, n), calib, lam=1e-6)
+    assert (
+        g.values.shape == (6, g.index.numel())
+        and g.index.numel() == g.shot.numel() == g.sample.numel()
+    )
+    # The exact k-space at each target's grid point.
+    iy, ix = g.index // n - n // 2, g.index % n - n // 2
+    truth = kspace(torch.stack([ix, iy], -1).double())
+    err = (g.values.to(torch.complex128) - truth).norm() / truth.norm()
+    moved = (y[:, g.shot, g.sample].to(torch.complex128) - truth).norm() / truth.norm()
+    assert err < 0.02, err
+    assert err < moved / 5  # far closer than leaving each sample where it was
+    w = g.weights()
+    assert torch.isclose(w.square().mean(), torch.tensor(1.0)) and (w > 0).all()
+
+
+def test_groups_pick_their_own_calibration():
+    n = 32
+    _, kspace = _object(n)
+    s = torch.arange(-10, 10.5, 0.5, dtype=torch.float64)
+    angles = torch.tensor([0.3, 1.1, 2.0, 2.7], dtype=torch.float64)
+    traj = torch.stack(
+        [s[None] * torch.cos(angles)[:, None], s[None] * torch.sin(angles)[:, None]], -1
+    )
+    y = kspace(traj).to(torch.complex64)
+    c = torch.arange(-8, 8, dtype=torch.float64)
+    ky, kx = torch.meshgrid(c, c, indexing="ij")
+    calib = kspace(torch.stack([kx, ky], -1)).to(torch.complex64)
+    one = tools.igrog(y, traj.float(), (n, n), calib)
+    two = tools.igrog(y, traj.float(), (n, n), [calib, calib * 2], group=torch.tensor([0, 1, 0, 1]))
+    # Scaling a calibration leaves its Wiener kernel unchanged (lam is relative to its power).
+    assert torch.allclose(
+        one.values, two.values, rtol=1e-3, atol=1e-4 * float(one.values.abs().max())
+    )