Pointing frame code

imap_processing/spice/pointing_frame_handler.py

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+"""
+Generate Pointing Frame.
+
+Notes
+-----
+Kernels that are required to run this code:
+1. imap_science_0001.tf - pointing frame kernel
+2. imap_sclk_0000.tsc - spacecraft clock kernel
+3. imap_wkcp.tf - spacecraft frame kernel
+4. naif0012.tls - standard NAIF leapsecond kernel
+5. imap_spin.bc - test attitude kernel generated by simulate_imap_attitude.m.
+These need to be placed in tests/pointing_frame/test_data.
+
+References
+----------
+https://spiceypy.readthedocs.io/en/main/documentation.html.
+"""
+
+import logging
+import os
+from pathlib import Path
+
+import numpy as np
+import spiceypy as spice
+from numpy.typing import NDArray
+
+logger = logging.getLogger(__name__)
+logger.setLevel(logging.INFO)
+
+# TODO : Add multiple pointings to the pointing frame.
+
+
+def get_et_times(ck_kernel: str) -> tuple[float, float, np.ndarray]:
+    """
+    Create the pointing frame.
+
+    Parameters
+    ----------
+    ck_kernel : str
+        Path of ck_kernel used to create the pointing frame.
+
+    Returns
+    -------
+    et_start : float
+        Start time of ck_kernel.
+    et_end : float
+        End time of ck_kernel.
+    et_times : numpy.ndarray
+        Array of times between et_start and et_end.
+    """
+    # Get the spacecraft ID.
+    # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.gipool
+    id_imap_spacecraft = spice.gipool("FRAME_IMAP_SPACECRAFT", 0, 1)
+
+    # TODO: Queried pointing start and stop times here.
+
+    # Get the coverage window
+    # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.ckcov
+    cover = spice.ckcov(ck_kernel, int(id_imap_spacecraft), True, "SEGMENT", 0, "TDB")
+    # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.wnfetd
+    et_start, et_end = spice.wnfetd(cover, 0)
+    # Assumes a pointing is ~ 1 day.
+    # 1 spin/15 seconds * 86400 seconds/pointing * 10 quaternions / spin
+    et_times = np.linspace(et_start, et_end, 57600)
+
+    return et_start, et_end, et_times
+
+
+def average_quaternions(et_times: np.ndarray) -> NDArray:
+    """
+    Average the quaternions.
+
+    Parameters
+    ----------
+    et_times : numpy.ndarray
+        Array of times between et_start and et_end.
+
+    Returns
+    -------
+    q_avg : np.ndarray
+        Average quaternion.
+    """
+    z_eclip_time = []
+    aggregate = np.zeros((4, 4))
+
+    for tdb in et_times:
+        # we use a quick and dirty method here for grabbing the quaternions
+        # from the attitude kernel.  Depending on how well the kernel input
+        # data is built and sampled, there may or may not be aliasing with this
+        # approach.  If it turns out that we need to pull the quaternions
+        # directly from the CK there are several routines that exist to do this
+        # but it's not straight forward.  We'll revisit this if needed.
+
+        # Rotation matrix from IMAP spacecraft frame to ECLIPJ2000.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.pxform
+        body_rots = spice.pxform("IMAP_SPACECRAFT", "ECLIPJ2000", tdb)
+        # Convert rotation matrix to quaternion.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.m2q
+        body_quat = spice.m2q(body_rots)
+        # z-axis of the ECLIPJ2000 frame.
+        z_eclip_time.append(body_rots[:, 2])
+
+        # Standardize the quaternion so that they may be compared.
+        body_quat = body_quat * np.sign(body_quat[0])
+
+        # Aggregate quaternions into a single matrix.
+        aggregate += np.outer(body_quat, body_quat)
+
+    # Reference: "On Averaging Rotations"
+    # Link: https://link.springer.com/content/pdf/10.1023/A:1011129215388.pdf
+    aggregate /= len(et_times)
+
+    # Compute eigen values and vectors of the matrix A
+    # Eigenvalues tell you how much "influence" each
+    # direction (eigenvector) has.
+    # The largest eigenvalue corresponds to the direction
+    # that has the most influence.
+    # The eigenvector corresponding to the largest
+    # eigenvalue points in the direction that has the most
+    # combined rotation influence.
+    eigvals, eigvecs = np.linalg.eig(aggregate)
+    # q0: The scalar part of the quaternion.
+    # q1, q2, q3: The vector part of the quaternion.
+    q_avg = eigvecs[:, np.argmax(eigvals)]
+
+    return q_avg
+
+
+def create_rotation_matrix(et_times: np.ndarray) -> NDArray:
+    """
+    Create a rotation matrix.
+
+    Parameters
+    ----------
+    et_times : numpy.ndarray
+        Array of times between et_start and et_end.
+
+    Returns
+    -------
+    rotation_matrix : np.ndarray
+        Rotation matrix.
+    """
+    # Averaged quaternions.
+    q_avg = average_quaternions(et_times)
+
+    # Converts the averaged quaternion (q_avg) into a rotation matrix
+    # and get inertial z axis.
+    # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.q2m
+    z_avg = spice.q2m(list(q_avg))[:, 2]
+    # y_avg is perpendicular to both z_avg and the standard Z-axis.
+    y_avg = np.cross(z_avg, [0, 0, 1])
+    # x_avg is perpendicular to y_avg and z_avg.
+    x_avg = np.cross(y_avg, z_avg)
+
+    # Construct the rotation matrix from x_avg, y_avg, z_avg
+    rotation_matrix = np.asarray([x_avg, y_avg, z_avg])
+
+    return rotation_matrix
+
+
+def create_pointing_frame() -> Path:
+    """
+    Create the pointing frame.
+
+    Returns
+    -------
+    path_to_pointing_frame : Path
+        Path to dps frame.
+    """
+    # TODO: this part will change with ensure_spice decorator.
+    # Mount path to EFS.
+    mount_path = Path(os.getenv("EFS_MOUNT_PATH", ""))
+
+    # TODO: this part will change with ensure_spice decorator.
+    kernels = [str(file) for file in mount_path.iterdir()]
+    ck_kernel = [str(file) for file in mount_path.iterdir() if file.suffix == ".bc"]
+
+    # Furnish the kernels.
+    with spice.KernelPool(kernels):
+        # Get timerange for the pointing frame kernel.
+        et_start, et_end, et_times = get_et_times(str(ck_kernel[0]))
+        # Create a rotation matrix
+        rotation_matrix = create_rotation_matrix(et_times)
+
+        # Convert the rotation matrix to a quaternion.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.m2q
+        q_avg = spice.m2q(rotation_matrix)
+
+        # TODO: come up with naming convention.
+        path_to_pointing_frame = mount_path / "imap_dps.bc"
+
+        # Open a new CK file, returning the handle of the opened file.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.ckopn
+        handle = spice.ckopn(str(path_to_pointing_frame), "CK", 0)
+        # Get the SCLK ID.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.gipool
+        id_imap_sclk = spice.gipool("CK_-43000_SCLK", 0, 1)
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.sce2c
+        # Convert start and end times to SCLK.
+        sclk_begtim = spice.sce2c(int(id_imap_sclk), et_start)
+        sclk_endtim = spice.sce2c(int(id_imap_sclk), et_end)
+
+        # Get the pointing frame ID.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.gipool
+        id_imap_dps = spice.gipool("FRAME_IMAP_DPS", 0, 1)
+
+        # Create the pointing frame kernel.
+        # https://spiceypy.readthedocs.io/en/main/documentation.html#spiceypy.spiceypy.ckw02
+        spice.ckw02(
+            # Handle of an open CK file.
+            handle,
+            # Start time of the segment.
+            sclk_begtim,
+            # End time of the segment.
+            sclk_endtim,
+            # Pointing frame ID.
+            int(id_imap_dps),
+            # Reference frame.
+            "ECLIPJ2000",  # Reference frame
+            # Identifier.
+            "IMAP_DPS",
+            # Number of pointing intervals.
+            1,
+            # Start times of individual pointing records within segment.
+            # Since there is only a single record this is equal to sclk_begtim.
+            np.array([sclk_begtim]),
+            # End times of individual pointing records within segment.
+            # Since there is only a single record this is equal to sclk_endtim.
+            np.array([sclk_endtim]),  # Single stop time
+            # Average quaternion.
+            q_avg,
+            # 0.0 Angular rotation terms.
+            np.array([0.0, 0.0, 0.0]),
+            # Rates (seconds per tick) at which the quaternion and
+            # angular velocity change.
+            np.array([1.0]),
+        )
+
+        # Close CK file.
+        spice.ckcls(handle)
+
+    return path_to_pointing_frame