feat: add Unscented Kalman Filter (UKF)

src/dynaris/__init__.py

@@ -18,7 +18,14 @@
     Regression,
     Seasonal,
 )
-from dynaris.filters import ExtendedKalmanFilter, KalmanFilter, ekf_filter, kalman_filter
+from dynaris.filters import (
+    ExtendedKalmanFilter,
+    KalmanFilter,
+    UnscentedKalmanFilter,
+    ekf_filter,
+    kalman_filter,
+    ukf_filter,
+)
 from dynaris.smoothers import RTSSmoother, rts_smooth
 
 __version__ = "0.1.0"
@@ -41,8 +48,10 @@
     "SmootherProtocol",
     "SmootherResult",
     "StateSpaceModel",
+    "UnscentedKalmanFilter",
     "__version__",
     "ekf_filter",
     "kalman_filter",
     "rts_smooth",
+    "ukf_filter",
 ]

src/dynaris/filters/__init__.py

@@ -2,10 +2,13 @@
 
 from dynaris.filters.ekf import ExtendedKalmanFilter, ekf_filter
 from dynaris.filters.kalman import KalmanFilter, kalman_filter
+from dynaris.filters.ukf import UnscentedKalmanFilter, ukf_filter
 
 __all__ = [
     "ExtendedKalmanFilter",
     "KalmanFilter",
+    "UnscentedKalmanFilter",
     "ekf_filter",
     "kalman_filter",
+    "ukf_filter",
 ]

src/dynaris/filters/ukf.py

@@ -0,0 +1,355 @@
+"""Unscented Kalman Filter for nonlinear state-space models.
+
+Propagates sigma points through nonlinear transition and observation functions
+to capture the posterior mean and covariance without linearization. Uses the
+scaled unscented transform with configurable alpha, beta, kappa parameters.
+
+References:
+    Julier, S.J. and Uhlmann, J.K. (2004). "Unscented Filtering and
+    Nonlinear Estimation." Proceedings of the IEEE, 92(3), 401-422.
+"""
+
+from __future__ import annotations
+
+from typing import NamedTuple
+
+import jax
+import jax.numpy as jnp
+from jax import Array
+
+from dynaris.core.nonlinear import NonlinearSSM
+from dynaris.core.results import FilterResult
+from dynaris.core.types import GaussianState
+
+# ---------------------------------------------------------------------------
+# Sigma-point weights
+# ---------------------------------------------------------------------------
+
+
+class SigmaWeights(NamedTuple):
+    """Weights for the unscented transform."""
+
+    wm: Array  # mean weights, shape (2n+1,)
+    wc: Array  # covariance weights, shape (2n+1,)
+    lam: Array  # scaling parameter lambda (scalar array)
+
+
+def compute_weights(
+    n: int,
+    alpha: float = 1e-3,
+    beta: float = 2.0,
+    kappa: float = 0.0,
+) -> SigmaWeights:
+    """Compute sigma-point weights for the scaled unscented transform.
+
+    Args:
+        n: State dimension.
+        alpha: Spread of sigma points around the mean (typically 1e-4 to 1).
+        beta: Prior knowledge of distribution (2.0 is optimal for Gaussian).
+        kappa: Secondary scaling parameter (typically 0 or 3-n).
+
+    Returns:
+        SigmaWeights with mean weights, covariance weights, and lambda.
+    """
+    lam = alpha**2 * (n + kappa) - n
+
+    wm = jnp.full(2 * n + 1, 1.0 / (2.0 * (n + lam)))
+    wm = wm.at[0].set(lam / (n + lam))
+
+    wc = jnp.full(2 * n + 1, 1.0 / (2.0 * (n + lam)))
+    wc = wc.at[0].set(lam / (n + lam) + (1.0 - alpha**2 + beta))
+
+    return SigmaWeights(wm=wm, wc=wc, lam=jnp.array(lam))
+
+
+# ---------------------------------------------------------------------------
+# Sigma-point generation
+# ---------------------------------------------------------------------------
+
+
+def sigma_points(state: GaussianState, lam: Array) -> Array:
+    """Generate 2n+1 sigma points from a Gaussian state.
+
+    Args:
+        state: Gaussian belief with mean (n,) and covariance (n, n).
+        lam: Scaling parameter lambda (scalar).
+
+    Returns:
+        Sigma points, shape (2n+1, n).
+    """
+    n = state.mean.shape[0]
+    scaled_cov = (n + lam) * state.cov
+    L = jnp.linalg.cholesky(scaled_cov)  # (n, n)
+
+    # Build sigma points: [mean, mean + L_i, mean - L_i]
+    offsets = jnp.concatenate([
+        jnp.zeros((1, n)),
+        L,    # rows of L as positive offsets
+        -L,   # rows of L as negative offsets
+    ], axis=0)  # (2n+1, n)
+
+    return state.mean[None, :] + offsets
+
+
+# ---------------------------------------------------------------------------
+# Internal scan carry
+# ---------------------------------------------------------------------------
+
+
+class _ScanCarry(NamedTuple):
+    filtered: GaussianState
+    log_likelihood: Array  # scalar
+
+
+class _ScanOutput(NamedTuple):
+    predicted_mean: Array
+    predicted_cov: Array
+    filtered_mean: Array
+    filtered_cov: Array
+
+
+# ---------------------------------------------------------------------------
+# Pure-function predict and update steps
+# ---------------------------------------------------------------------------
+
+
+def predict(
+    state: GaussianState,
+    model: NonlinearSSM,
+    weights: SigmaWeights,
+) -> GaussianState:
+    """UKF predict step (time update).
+
+    Generates sigma points, propagates them through the transition function,
+    and recovers the predicted mean and covariance.
+    """
+    pts = sigma_points(state, weights.lam)  # (2n+1, n)
+
+    # Propagate sigma points through transition function
+    pts_pred = jax.vmap(model.f)(pts)  # (2n+1, n)
+
+    # Recover predicted mean
+    mean = jnp.sum(weights.wm[:, None] * pts_pred, axis=0)  # (n,)
+
+    # Recover predicted covariance
+    diff = pts_pred - mean[None, :]  # (2n+1, n)
+    cov = jnp.sum(weights.wc[:, None, None] * (diff[:, :, None] * diff[:, None, :]), axis=0)
+    cov = cov + model.Q
+
+    return GaussianState(mean=mean, cov=cov)
+
+
+def update(
+    predicted: GaussianState,
+    observation: Array,
+    model: NonlinearSSM,
+    weights: SigmaWeights,
+) -> tuple[GaussianState, Array]:
+    """UKF update step (measurement update).
+
+    Generates sigma points from the predicted state, propagates through the
+    observation function, and computes the Kalman gain.
+
+    Returns the filtered state and the log-likelihood contribution.
+    Handles missing observations (NaN) by skipping the update.
+    """
+    y = observation
+    pts = sigma_points(predicted, weights.lam)  # (2n+1, n)
+
+    # Propagate through observation function
+    pts_obs = jax.vmap(model.h)(pts)  # (2n+1, m)
+
+    # Predicted observation mean
+    y_pred = jnp.sum(weights.wm[:, None] * pts_obs, axis=0)  # (m,)
+
+    # Innovation covariance S = sum wc * (y_diff)(y_diff)' + R
+    y_diff = pts_obs - y_pred[None, :]  # (2n+1, m)
+    S = jnp.sum(weights.wc[:, None, None] * (y_diff[:, :, None] * y_diff[:, None, :]), axis=0)
+    S = S + model.R  # (m, m)
+
+    # Cross-covariance P_xy = sum wc * (x_diff)(y_diff)'
+    x_diff = pts - predicted.mean[None, :]  # (2n+1, n)
+    P_xy = jnp.sum(weights.wc[:, None, None] * (x_diff[:, :, None] * y_diff[:, None, :]), axis=0)
+    # (n, m)
+
+    # Kalman gain K = P_xy @ S^{-1}
+    K = jnp.linalg.solve(S.T, P_xy.T).T  # (n, m)
+
+    # Innovation
+    e = y - y_pred  # (m,)
+
+    filtered_mean = predicted.mean + K @ e
+    filtered_cov = predicted.cov - K @ S @ K.T
+
+    # Log-likelihood: log N(e; 0, S)
+    m = observation.shape[-1]
+    log_det = jnp.linalg.slogdet(S)[1]
+    mahal = e @ jnp.linalg.solve(S, e)
+    ll = -0.5 * (m * jnp.log(2.0 * jnp.pi) + log_det + mahal)
+
+    # Handle missing observations
+    obs_valid = ~jnp.any(jnp.isnan(y))
+    filtered_mean = jnp.where(obs_valid, filtered_mean, predicted.mean)
+    filtered_cov = jnp.where(obs_valid, filtered_cov, predicted.cov)
+    ll = jnp.where(obs_valid, ll, 0.0)
+
+    filtered = GaussianState(mean=filtered_mean, cov=filtered_cov)
+    return filtered, ll
+
+
+# ---------------------------------------------------------------------------
+# Full forward pass via lax.scan
+# ---------------------------------------------------------------------------
+
+
+class UnscentedKalmanFilter:
+    """Unscented Kalman Filter for nonlinear state-space models.
+
+    Uses the scaled unscented transform to propagate sigma points through
+    nonlinear functions, avoiding the need for Jacobian computation.
+
+    Args:
+        alpha: Spread of sigma points (default 1e-3).
+        beta: Prior distribution parameter (default 2.0, optimal for Gaussian).
+        kappa: Secondary scaling parameter (default 0.0).
+    """
+
+    def __init__(
+        self,
+        alpha: float = 1e-3,
+        beta: float = 2.0,
+        kappa: float = 0.0,
+    ) -> None:
+        self.alpha = alpha
+        self.beta = beta
+        self.kappa = kappa
+
+    def predict(self, state: GaussianState, model: NonlinearSSM) -> GaussianState:
+        """UKF predict step (time update)."""
+        w = compute_weights(model.state_dim, self.alpha, self.beta, self.kappa)
+        return predict(state, model, w)
+
+    def update(
+        self,
+        predicted: GaussianState,
+        observation: Array,
+        model: NonlinearSSM,
+    ) -> GaussianState:
+        """UKF update step (measurement update)."""
+        w = compute_weights(model.state_dim, self.alpha, self.beta, self.kappa)
+        filtered, _ll = update(predicted, observation, model, w)
+        return filtered
+
+    def scan(
+        self,
+        model: NonlinearSSM,
+        observations: Array,
+        initial_state: GaussianState | None = None,
+    ) -> FilterResult:
+        """Run full forward UKF via jax.lax.scan."""
+        return _ukf_filter_impl(
+            model, observations, initial_state,
+            self.alpha, self.beta, self.kappa,
+        )
+
+
+def ukf_filter(
+    model: NonlinearSSM,
+    observations: Array,
+    initial_state: GaussianState | None = None,
+    *,
+    alpha: float = 1e-3,
+    beta: float = 2.0,
+    kappa: float = 0.0,
+) -> FilterResult:
+    """Unscented Kalman Filter forward pass.
+
+    Uses the scaled unscented transform with configurable parameters to
+    propagate sigma points through nonlinear transition and observation
+    functions.
+
+    Args:
+        model: Nonlinear state-space model with callable f and h.
+        observations: Observation sequence, shape (T, obs_dim).
+        initial_state: Initial state belief. Defaults to diffuse prior.
+        alpha: Spread of sigma points around the mean (default 1e-3).
+        beta: Prior distribution parameter (default 2.0, optimal for Gaussian).
+        kappa: Secondary scaling parameter (default 0.0).
+
+    Returns:
+        FilterResult with filtered/predicted states and log-likelihood.
+
+    Example::
+
+        import jax.numpy as jnp
+        from dynaris.core.nonlinear import NonlinearSSM
+        from dynaris.filters.ukf import ukf_filter
+
+        model = NonlinearSSM(
+            transition_fn=lambda x: x,
+            observation_fn=lambda x: x,
+            transition_cov=jnp.eye(1),
+            observation_cov=jnp.eye(1),
+            state_dim=1, obs_dim=1,
+        )
+        result = ukf_filter(model, observations)
+    """
+    return _ukf_filter_impl(model, observations, initial_state, alpha, beta, kappa)
+
+
+def _ukf_filter_impl(
+    model: NonlinearSSM,
+    observations: Array,
+    initial_state: GaussianState | None,
+    alpha: float,
+    beta: float,
+    kappa: float,
+) -> FilterResult:
+    """Internal implementation — weights computed before JIT boundary."""
+    if initial_state is None:
+        initial_state = model.initial_state()
+
+    weights = compute_weights(model.state_dim, alpha, beta, kappa)
+    return _ukf_scan(model, observations, initial_state, weights)
+
+
+@jax.jit
+def _ukf_scan(
+    model: NonlinearSSM,
+    observations: Array,
+    initial_state: GaussianState,
+    weights: SigmaWeights,
+) -> FilterResult:
+    """JIT-compiled scan loop for UKF."""
+    init_carry = _ScanCarry(
+        filtered=initial_state,
+        log_likelihood=jnp.array(0.0),
+    )
+
+    def _scan_step(
+        carry: _ScanCarry, obs: Array
+    ) -> tuple[_ScanCarry, _ScanOutput]:
+        predicted = predict(carry.filtered, model, weights)
+        filtered, ll = update(predicted, obs, model, weights)
+        new_carry = _ScanCarry(
+            filtered=filtered,
+            log_likelihood=carry.log_likelihood + ll,
+        )
+        output = _ScanOutput(
+            predicted_mean=predicted.mean,
+            predicted_cov=predicted.cov,
+            filtered_mean=filtered.mean,
+            filtered_cov=filtered.cov,
+        )
+        return new_carry, output
+
+    final_carry, outputs = jax.lax.scan(_scan_step, init_carry, observations)
+
+    return FilterResult(
+        filtered_states=outputs.filtered_mean,
+        filtered_covariances=outputs.filtered_cov,
+        predicted_states=outputs.predicted_mean,
+        predicted_covariances=outputs.predicted_cov,
+        log_likelihood=final_carry.log_likelihood,
+        observations=observations,
+    )