solar_system.py

# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""
This module contains convenience functions for retrieving solar system
ephemerides from jplephem.
"""

import os.path
import re
from urllib.parse import urlparse

import erfa
import numpy as np

from astropy import units as u
from astropy.constants import c as speed_of_light
from astropy.utils import indent
from astropy.utils.data import download_file
from astropy.utils.decorators import classproperty, deprecated
from astropy.utils.state import ScienceState

from .builtin_frames import GCRS, ICRS, ITRS, TETE
from .builtin_frames.utils import get_jd12
from .representation import CartesianDifferential, CartesianRepresentation
from .sky_coordinate import SkyCoord

__all__ = [
    "get_body",
    "get_moon",
    "get_body_barycentric",
    "get_body_barycentric_posvel",
    "solar_system_ephemeris",
]


DEFAULT_JPL_EPHEMERIS = "de430"

"""List of kernel pairs needed to calculate positions of a given object."""
BODY_NAME_TO_KERNEL_SPEC = {
    "sun": [(0, 10)],
    "mercury": [(0, 1), (1, 199)],
    "venus": [(0, 2), (2, 299)],
    "earth-moon-barycenter": [(0, 3)],
    "earth": [(0, 3), (3, 399)],
    "moon": [(0, 3), (3, 301)],
    "mars": [(0, 4)],
    "jupiter": [(0, 5)],
    "saturn": [(0, 6)],
    "uranus": [(0, 7)],
    "neptune": [(0, 8)],
    "pluto": [(0, 9)],
}

"""Indices to the plan94 routine for the given object."""
PLAN94_BODY_NAME_TO_PLANET_INDEX = {
    "mercury": 1,
    "venus": 2,
    "earth-moon-barycenter": 3,
    "mars": 4,
    "jupiter": 5,
    "saturn": 6,
    "uranus": 7,
    "neptune": 8,
}

_EPHEMERIS_NOTE = """
You can either give an explicit ephemeris or use a default, which is normally
a built-in ephemeris that does not require ephemeris files.  To change
the default to be the JPL ephemeris::

    >>> from astropy.coordinates import solar_system_ephemeris
    >>> solar_system_ephemeris.set('jpl')  # doctest: +SKIP

Use of any JPL ephemeris requires the jplephem package
(https://pypi.org/project/jplephem/).
If needed, the ephemeris file will be downloaded (and cached).

One can check which bodies are covered by a given ephemeris using::

    >>> solar_system_ephemeris.bodies
    ('earth', 'sun', 'moon', 'mercury', 'venus', 'earth-moon-barycenter', 'mars', 'jupiter', 'saturn', 'uranus', 'neptune')
"""[
    1:-1
]


class solar_system_ephemeris(ScienceState):
    """Default ephemerides for calculating positions of Solar-System bodies.

    This can be one of the following:

    - 'builtin': polynomial approximations to the orbital elements.
    - 'dexxx[s]', for a JPL dynamical model, where xxx is the three digit
      version number (e.g. de430), and the 's' is optional to specify the
      'small' version of a kernel. The version number must correspond to an
      ephemeris file available at:
      https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/
    - 'jpl': Alias for the default JPL ephemeris (currently, 'de430').
    - URL: (str) The url to a SPK ephemeris in SPICE binary (.bsp) format.
    - PATH: (str) File path to a SPK ephemeris in SPICE binary (.bsp) format.
    - `None`: Ensure an Exception is raised without an explicit ephemeris.

    The default is 'builtin', which uses the ``epv00`` and ``plan94``
    routines from the ``erfa`` implementation of the Standards Of Fundamental
    Astronomy library.

    Notes
    -----
    Any file required will be downloaded (and cached) when the state is set.
    The default Satellite Planet Kernel (SPK) file from NASA JPL (de430) is
    ~120MB, and covers years ~1550-2650 CE [1]_.  The smaller de432s file is
    ~10MB, and covers years 1950-2050 [2]_ (and similarly for the newer de440
    and de440s).  Older versions of the JPL ephemerides (such as the widely
    used de200) can be used via their URL [3]_.

    .. [1] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/aareadme_de430-de431.txt
    .. [2] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/aareadme_de432s.txt
    .. [3] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/a_old_versions/
    """

    _value = "builtin"
    _kernel = None

    @classmethod
    def validate(cls, value):
        # make no changes if value is None
        if value is None:
            return cls._value
        # Set up Kernel; if the file is not in cache, this will download it.
        cls.get_kernel(value)
        return value

    @classmethod
    def get_kernel(cls, value):
        # ScienceState only ensures the `_value` attribute is up to date,
        # so we need to be sure any kernel returned is consistent.
        if cls._kernel is None or cls._kernel.origin != value:
            if cls._kernel is not None:
                cls._kernel.daf.file.close()
                cls._kernel = None
            kernel = _get_kernel(value)
            if kernel is not None:
                kernel.origin = value
            cls._kernel = kernel
        return cls._kernel

    @classproperty
    def kernel(cls):
        return cls.get_kernel(cls._value)

    @classproperty
    def bodies(cls):
        if cls._value is None:
            return None
        if cls._value.lower() == "builtin":
            return ("earth", "sun", "moon") + tuple(
                PLAN94_BODY_NAME_TO_PLANET_INDEX.keys()
            )
        else:
            return tuple(BODY_NAME_TO_KERNEL_SPEC.keys())


def _get_kernel(value):
    """
    Try importing jplephem, download/retrieve from cache the Satellite Planet
    Kernel corresponding to the given ephemeris.
    """
    if value is None or value.lower() == "builtin":
        return None

    try:
        from jplephem.spk import SPK
    except ImportError:
        raise ImportError(
            "Solar system JPL ephemeris calculations require the jplephem package "
            "(https://pypi.org/project/jplephem/)"
        )

    if value.lower() == "jpl":
        # Get the default JPL ephemeris URL
        value = DEFAULT_JPL_EPHEMERIS

    if re.compile(r"de[0-9][0-9][0-9]s?").match(value.lower()):
        value = (
            "https://naif.jpl.nasa.gov/pub/naif/generic_kernels"
            f"/spk/planets/{value.lower():s}.bsp"
        )

    elif os.path.isfile(value):
        return SPK.open(value)

    else:
        try:
            urlparse(value)
        except Exception:
            raise ValueError(
                f"{value} was not one of the standard strings and "
                "could not be parsed as a file path or URL"
            )

    return SPK.open(download_file(value, cache=True))


def _get_body_barycentric_posvel(body, time, ephemeris=None, get_velocity=True):
    """Calculate the barycentric position (and velocity) of a solar system body.

    Parameters
    ----------
    body : str or other
        The solar system body for which to calculate positions.  Can also be a
        kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
        kernel.
    time : `~astropy.time.Time`
        Time of observation.
    ephemeris : str, optional
        Ephemeris to use.  By default, use the one set with
        ``astropy.coordinates.solar_system_ephemeris.set``
    get_velocity : bool, optional
        Whether or not to calculate the velocity as well as the position.

    Returns
    -------
    position : `~astropy.coordinates.CartesianRepresentation` or tuple
        Barycentric (ICRS) position or tuple of position and velocity.

    Notes
    -----
    Whether or not velocities are calculated makes little difference for the
    built-in ephemerides, but for most JPL ephemeris files, the execution time
    roughly doubles.
    """
    # If the ephemeris is to be taken from solar_system_ephemeris, or the one
    # it already contains, use the kernel there.  Otherwise, open the ephemeris,
    # possibly downloading it, but make sure the file is closed at the end.
    default_kernel = ephemeris is None or ephemeris is solar_system_ephemeris._value
    kernel = None
    try:
        if default_kernel:
            if solar_system_ephemeris.get() is None:
                raise ValueError(_EPHEMERIS_NOTE)
            kernel = solar_system_ephemeris.kernel
        else:
            kernel = _get_kernel(ephemeris)

        jd1, jd2 = get_jd12(time, "tdb")
        if kernel is None:
            body = body.lower()
            earth_pv_helio, earth_pv_bary = erfa.epv00(jd1, jd2)
            if body == "earth":
                body_pv_bary = earth_pv_bary

            elif body == "moon":
                # The moon98 documentation notes that it takes TT, but that TDB leads
                # to errors smaller than the uncertainties in the algorithm.
                # moon98 returns the astrometric position relative to the Earth.
                moon_pv_geo = erfa.moon98(jd1, jd2)
                body_pv_bary = erfa.pvppv(moon_pv_geo, earth_pv_bary)
            else:
                sun_pv_bary = erfa.pvmpv(earth_pv_bary, earth_pv_helio)
                if body == "sun":
                    body_pv_bary = sun_pv_bary
                else:
                    try:
                        body_index = PLAN94_BODY_NAME_TO_PLANET_INDEX[body]
                    except KeyError:
                        raise KeyError(
                            f"{body}'s position and velocity cannot be "
                            f"calculated with the '{ephemeris}' ephemeris."
                        )
                    body_pv_helio = erfa.plan94(jd1, jd2, body_index)
                    body_pv_bary = erfa.pvppv(body_pv_helio, sun_pv_bary)

            body_pos_bary = CartesianRepresentation(
                body_pv_bary["p"], unit=u.au, xyz_axis=-1, copy=False
            )
            if get_velocity:
                body_vel_bary = CartesianRepresentation(
                    body_pv_bary["v"], unit=u.au / u.day, xyz_axis=-1, copy=False
                )

        else:
            if isinstance(body, str):
                # Look up kernel chain for JPL ephemeris, based on name
                try:
                    kernel_spec = BODY_NAME_TO_KERNEL_SPEC[body.lower()]
                except KeyError:
                    raise KeyError(
                        f"{body}'s position cannot be calculated with "
                        f"the {ephemeris} ephemeris."
                    )
            else:
                # otherwise, assume the user knows what their doing and intentionally
                # passed in a kernel chain
                kernel_spec = body

            # jplephem cannot handle multi-D arrays, so convert to 1D here.
            jd1_shape = getattr(jd1, "shape", ())
            if len(jd1_shape) > 1:
                jd1, jd2 = jd1.ravel(), jd2.ravel()
                # Note that we use the new jd1.shape here to create a 1D result array.
                # It is reshaped below.
            body_posvel_bary = np.zeros(
                (2 if get_velocity else 1, 3) + getattr(jd1, "shape", ())
            )
            for pair in kernel_spec:
                spk = kernel[pair]
                if spk.data_type == 3:
                    # Type 3 kernels contain both position and velocity.
                    posvel = spk.compute(jd1, jd2)
                    if get_velocity:
                        body_posvel_bary += posvel.reshape(body_posvel_bary.shape)
                    else:
                        body_posvel_bary[0] += posvel[:4]
                else:
                    # spk.generate first yields the position and then the
                    # derivative. If no velocities are desired, body_posvel_bary
                    # has only one element and thus the loop ends after a single
                    # iteration, avoiding the velocity calculation.
                    for body_p_or_v, p_or_v in zip(
                        body_posvel_bary, spk.generate(jd1, jd2)
                    ):
                        body_p_or_v += p_or_v

            body_posvel_bary.shape = body_posvel_bary.shape[:2] + jd1_shape
            body_pos_bary = CartesianRepresentation(
                body_posvel_bary[0], unit=u.km, copy=False
            )
            if get_velocity:
                body_vel_bary = CartesianRepresentation(
                    body_posvel_bary[1], unit=u.km / u.day, copy=False
                )

        return (body_pos_bary, body_vel_bary) if get_velocity else body_pos_bary

    finally:
        if not default_kernel and kernel is not None:
            kernel.daf.file.close()


def get_body_barycentric_posvel(body, time, ephemeris=None):
    """Calculate the barycentric position and velocity of a solar system body.

    Parameters
    ----------
    body : str or list of tuple
        The solar system body for which to calculate positions.  Can also be a
        kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
        kernel.
    time : `~astropy.time.Time`
        Time of observation.
    ephemeris : str, optional
        Ephemeris to use.  By default, use the one set with
        ``astropy.coordinates.solar_system_ephemeris.set``

    Returns
    -------
    position, velocity : tuple of `~astropy.coordinates.CartesianRepresentation`
        Tuple of barycentric (ICRS) position and velocity.

    See Also
    --------
    get_body_barycentric : to calculate position only.
        This is faster by about a factor two for JPL kernels, but has no
        speed advantage for the built-in ephemeris.

    Notes
    -----
    {_EPHEMERIS_NOTE}
    """
    return _get_body_barycentric_posvel(body, time, ephemeris)


def get_body_barycentric(body, time, ephemeris=None):
    """Calculate the barycentric position of a solar system body.

    Parameters
    ----------
    body : str or list of tuple
        The solar system body for which to calculate positions.  Can also be a
        kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
        kernel.
    time : `~astropy.time.Time`
        Time of observation.
    ephemeris : str, optional
        Ephemeris to use.  By default, use the one set with
        ``astropy.coordinates.solar_system_ephemeris.set``

    Returns
    -------
    position : `~astropy.coordinates.CartesianRepresentation`
        Barycentric (ICRS) position of the body in cartesian coordinates

    See Also
    --------
    get_body_barycentric_posvel : to calculate both position and velocity.

    Notes
    -----
    {_EPHEMERIS_NOTE}
    """
    return _get_body_barycentric_posvel(body, time, ephemeris, get_velocity=False)


def _get_apparent_body_position(body, time, ephemeris, obsgeoloc=None):
    """Calculate the apparent position of body ``body`` relative to Earth.

    This corrects for the light-travel time to the object.

    Parameters
    ----------
    body : str or other
        The solar system body for which to calculate positions.  Can also be a
        kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
        kernel.
    time : `~astropy.time.Time`
        Time of observation.
    ephemeris : str, optional
        Ephemeris to use.  By default, use the one set with
        ``~astropy.coordinates.solar_system_ephemeris.set``
    obsgeoloc : `~astropy.coordinates.CartesianRepresentation`, optional
        The GCRS position of the observer

    Returns
    -------
    cartesian_position : `~astropy.coordinates.CartesianRepresentation`
        Barycentric (ICRS) apparent position of the body in cartesian coordinates

    Notes
    -----
    {_EPHEMERIS_NOTE}
    """
    if ephemeris is None:
        ephemeris = solar_system_ephemeris.get()

    # Calculate position given approximate light travel time.
    delta_light_travel_time = 20.0 * u.s
    emitted_time = time
    light_travel_time = 0.0 * u.s
    earth_loc = get_body_barycentric("earth", time, ephemeris)
    if obsgeoloc is not None:
        earth_loc += obsgeoloc
    while np.any(np.fabs(delta_light_travel_time) > 1.0e-8 * u.s):
        body_loc = get_body_barycentric(body, emitted_time, ephemeris)
        earth_distance = (body_loc - earth_loc).norm()
        delta_light_travel_time = light_travel_time - earth_distance / speed_of_light
        light_travel_time = earth_distance / speed_of_light
        emitted_time = time - light_travel_time

    return get_body_barycentric(body, emitted_time, ephemeris)


def get_body(body, time, location=None, ephemeris=None):
    """
    Get a `~astropy.coordinates.SkyCoord` for a solar system body as observed
    from a location on Earth in the `~astropy.coordinates.GCRS` reference
    system.

    Parameters
    ----------
    body : str or list of tuple
        The solar system body for which to calculate positions.  Can also be a
        kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
        kernel.
    time : `~astropy.time.Time`
        Time of observation.
    location : `~astropy.coordinates.EarthLocation`, optional
        Location of observer on the Earth.  If not given, will be taken from
        ``time`` (if not present, a geocentric observer will be assumed).
    ephemeris : str, optional
        Ephemeris to use.  If not given, use the one set with
        ``astropy.coordinates.solar_system_ephemeris.set`` (which is
        set to 'builtin' by default).

    Returns
    -------
    skycoord : `~astropy.coordinates.SkyCoord`
        GCRS Coordinate for the body

    Notes
    -----
    The coordinate returned is the apparent position, which is the position of
    the body at time *t* minus the light travel time from the *body* to the
    observing *location*.

    {_EPHEMERIS_NOTE}
    """
    if location is None:
        location = time.location

    if location is not None:
        obsgeoloc, obsgeovel = location.get_gcrs_posvel(time)
    else:
        obsgeoloc, obsgeovel = None, None

    cartrep = _get_apparent_body_position(body, time, ephemeris, obsgeoloc)
    icrs = ICRS(cartrep)
    gcrs = icrs.transform_to(
        GCRS(obstime=time, obsgeoloc=obsgeoloc, obsgeovel=obsgeovel)
    )

    return SkyCoord(gcrs)


@deprecated("5.3", alternative='get_body("moon")')
def get_moon(time, location=None, ephemeris=None):
    """
    Get a `~astropy.coordinates.SkyCoord` for the Earth's Moon as observed
    from a location on Earth in the `~astropy.coordinates.GCRS` reference
    system.

    Parameters
    ----------
    time : `~astropy.time.Time`
        Time of observation
    location : `~astropy.coordinates.EarthLocation`
        Location of observer on the Earth. If none is supplied, taken from
        ``time`` (if not present, a geocentric observer will be assumed).
    ephemeris : str, optional
        Ephemeris to use.  If not given, use the one set with
        ``astropy.coordinates.solar_system_ephemeris.set`` (which is
        set to 'builtin' by default).

    Returns
    -------
    skycoord : `~astropy.coordinates.SkyCoord`
        GCRS Coordinate for the Moon

    Notes
    -----
    The coordinate returned is the apparent position, which is the position of
    the moon at time *t* minus the light travel time from the moon to the
    observing *location*.

    {_EPHEMERIS_NOTE}
    """
    return get_body("moon", time, location=location, ephemeris=ephemeris)


# Add note about the ephemeris choices to the docstrings of relevant functions.
# Note: sadly, one cannot use f-strings for docstrings, so we format explicitly.
for f in [
    f
    for f in locals().values()
    if callable(f) and f.__doc__ is not None and "{_EPHEMERIS_NOTE}" in f.__doc__
]:
    f.__doc__ = f.__doc__.format(_EPHEMERIS_NOTE=indent(_EPHEMERIS_NOTE)[4:])


deprecation_msg = """
The use of _apparent_position_in_true_coordinates is deprecated because
astropy now implements a True Equator True Equinox Frame (TETE), which
should be used instead.
"""


@deprecated("4.2", deprecation_msg)
def _apparent_position_in_true_coordinates(skycoord):
    """
    Convert Skycoord in GCRS frame into one in which RA and Dec
    are defined w.r.t to the true equinox and poles of the Earth.
    """
    location = getattr(skycoord, "location", None)
    if location is None:
        gcrs_rep = skycoord.obsgeoloc.with_differentials(
            {"s": CartesianDifferential.from_cartesian(skycoord.obsgeovel)}
        )
        location = (
            GCRS(gcrs_rep, obstime=skycoord.obstime)
            .transform_to(ITRS(obstime=skycoord.obstime))
            .earth_location
        )
    tete_frame = TETE(obstime=skycoord.obstime, location=location)
    return skycoord.transform_to(tete_frame)