341 lines
12 KiB
Python
341 lines
12 KiB
Python
"""Compute positions from a NASA SPICE SPK ephemeris kernel file.
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http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/req/spk.html
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"""
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from numpy import array, interp, rollaxis
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from .calendar import compute_calendar_date
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from .daf import DAF
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from .descriptorlib import reify
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from .exceptions import OutOfRangeError
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from .names import target_names
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T0 = 2451545.0
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S_PER_DAY = 86400.0
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def _jd(seconds):
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"""Convert a number of seconds since J2000 to a Julian Date."""
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return T0 + seconds / S_PER_DAY
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class SPK(object):
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"""A JPL SPK ephemeris kernel for computing positions and velocities.
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You can load an SPK by specifying its filename::
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kernel = SPK.open('de431.bsp')
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Run ``print(kernel)`` to list the ephemeris segments. You can also
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loop across all of the segments in the list ``kernel.segments`` or,
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as a convenience, you can select a particular segment by providing a
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center and target integer in square brackets. So ``kernel[3,399]``
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will select the segment that computes the distance between the
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Earth-Moon barycenter (3) and the Earth itself (399).
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To extract the text comments from the SPK use ``kernel.comments()``.
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"""
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def __init__(self, daf):
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self.daf = daf
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self.segments = [
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build_segment(self.daf, source, descriptor)
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for source, descriptor in self.daf.summaries()
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]
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self.pairs = dict(((s.center, s.target), s) for s in self.segments)
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@classmethod
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def open(cls, path):
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"""Open the file at `path` and return an SPK instance."""
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f = open(path, 'rb')
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try:
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return cls(DAF(f))
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except Exception:
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f.close()
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raise
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def close(self):
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"""Close this SPK file."""
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self.daf.file.close()
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for segment in self.segments:
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if '_data' in segment.__dict__:
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del segment._data
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self.daf._array = None
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self.daf._map = None
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def __str__(self):
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daf = self.daf
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d = lambda b: b.decode('latin-1')
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lines = [
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'File type {0} and format {1} with {2} segments:'
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.format(d(daf.locidw), d(daf.locfmt), len(self.segments))
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]
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lines.extend(str(segment) for segment in self.segments)
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return '\n'.join(lines)
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def __getitem__(self, key):
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"""Given (center, target) integers, return the last matching segment."""
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return self.pairs[key]
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def comments(self):
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"""Return the file comments, as a string."""
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return self.daf.comments()
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def __enter__(self):
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return self
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def __exit__(self, exc_type, exc_val, exc_tb):
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self.close()
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def build_segment(daf, source, descriptor):
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data_type = descriptor[5]
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cls = _segment_classes.get(data_type, BaseSegment)
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return cls(daf, source, descriptor)
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class BaseSegment(object):
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"""A single segment of an SPK file.
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There are several items of information about each segment that are
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loaded from the underlying SPK file, and made available as object
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attributes:
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segment.source - official ephemeris name, like 'DE-0430LE-0430'
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segment.start_second - initial epoch, as seconds from J2000
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segment.end_second - final epoch, as seconds from J2000
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segment.start_jd - start_second, converted to a Julian Date
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segment.end_jd - end_second, converted to a Julian Date
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segment.center - integer center identifier
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segment.target - integer target identifier
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segment.frame - integer frame identifier
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segment.data_type - integer data type identifier
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segment.start_i - index where segment starts
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segment.end_i - index where segment ends
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"""
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_data = None
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def __init__(self, daf, source, descriptor):
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self.daf = daf
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self.source = source
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(self.start_second, self.end_second, self.target, self.center,
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self.frame, self.data_type, self.start_i, self.end_i) = descriptor
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self.start_jd = _jd(self.start_second)
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self.end_jd = _jd(self.end_second)
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def __str__(self):
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return self.describe(verbose=False)
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def describe(self, verbose=True):
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"""Return a textual description of the segment."""
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start = '%d-%02d-%02d' % compute_calendar_date(self.start_jd + 0.5)
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end = '%d-%02d-%02d' % compute_calendar_date(self.end_jd + 0.5)
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center = titlecase(target_names.get(self.center, 'Unknown center'))
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target = titlecase(target_names.get(self.target, 'Unknown target'))
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text = ('{1}..{2} Type {0.data_type}'
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' {3} ({0.center}) -> {4} ({0.target})'
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.format(self, start, end, center, target))
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if verbose:
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text += ('\n frame={0.frame} source={1}'
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.format(self, self.source.decode('ascii')))
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return text
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def compute(self, tdb, tdb2=0.0):
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"""Compute the component values for the time `tdb` plus `tdb2`."""
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raise ValueError(
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'jplephem has not yet learned how to compute positions'
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' from an ephemeris segment with data type {0}'
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.format(self.data_type)
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)
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def compute_and_differentiate(self, tdb, tdb2=0.0):
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"""Compute components and differentials for time `tdb` plus `tdb2`."""
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raise ValueError(
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'jplephem has not yet learned how to compute positions and'
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' velocities from an ephemeris segment with data type {0}'
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.format(self.data_type)
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)
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class Segment(BaseSegment):
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# Type 2 or type 3 segment.
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def compute(self, tdb, tdb2=0.0):
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"""Compute the component values for the time `tdb` plus `tdb2`."""
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for position in self.generate(tdb, tdb2):
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return position
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def compute_and_differentiate(self, tdb, tdb2=0.0):
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"""Compute components and differentials for time `tdb` plus `tdb2`."""
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return tuple(self.generate(tdb, tdb2))
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@reify
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def _data(self):
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"""Map the coefficients into memory using a NumPy array.
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"""
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if self.data_type == 2:
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component_count = 3
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elif self.data_type == 3:
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component_count = 6
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else:
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raise ValueError('this class only supports SPK data types 2 and 3')
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init, intlen, rsize, n = self.daf.read_array(self.end_i - 3, self.end_i)
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coefficient_count = int(rsize - 2) // component_count
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coefficients = self.daf.map_array(self.start_i, self.end_i - 4)
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coefficients.shape = (int(n), int(rsize))
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coefficients = coefficients[:,2:] # ignore MID and RADIUS elements
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coefficients.shape = (int(n), component_count, coefficient_count)
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coefficients = rollaxis(coefficients, 1)
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coefficients = rollaxis(coefficients, 2)
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coefficients = coefficients[::-1]
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return init, intlen, coefficients
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def load_array(self):
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init, intlen, coefficients = self._data
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initial_epoch = _jd(init)
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interval_length = intlen / S_PER_DAY
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coefficients = coefficients[::-1]
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coefficients = rollaxis(coefficients, 2)
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coefficients = rollaxis(coefficients, 2)
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return initial_epoch, interval_length, coefficients
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def generate(self, tdb, tdb2):
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"""Generate components and differentials for time `tdb` plus `tdb2`.
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Most uses will simply want to call the `compute()` method or the
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`compute_differentials()` method, for convenience. But in those
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cases (see Skyfield) where you want to compute a position and
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examine it before deciding whether to proceed with the velocity,
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but without losing all of the work that it took to get to that
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point, this generator lets you get them as two separate steps.
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"""
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scalar = not getattr(tdb, 'shape', 0) and not getattr(tdb2, 'shape', 0)
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if scalar:
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tdb = array((tdb,))
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init, intlen, coefficients = self._data
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coefficient_count, component_count, n = coefficients.shape
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# Keeping fractions strictly separate from whole numbers
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# maintains the highest possible precision.
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index1, offset1 = divmod((tdb - T0) * S_PER_DAY - init, intlen)
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index2, offset2 = divmod(tdb2 * S_PER_DAY, intlen)
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index3, offset = divmod(offset1 + offset2, intlen)
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index = (index1 + index2 + index3).astype(int)
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if (index < 0).any() or (index > n).any():
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raise OutOfRangeError(
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'segment only covers dates %d-%02d-%02d through %d-%02d-%02d'
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% (compute_calendar_date(self.start_jd + 0.5) +
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compute_calendar_date(self.end_jd + 0.5)),
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out_of_range_times=(index < 0) | (index > n),
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)
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omegas = (index == n)
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index[omegas] -= 1
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offset[omegas] += intlen
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coefficients = coefficients[:,:,index]
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# Chebyshev polynomial.
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s = 2.0 * offset / intlen - 1.0
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s2 = 2.0 * s
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w0 = w1 = 0.0
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wlist = []
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for coefficient in coefficients[:-1]:
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w2 = w1
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w1 = w0
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w0 = coefficient + (s2 * w1 - w2)
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wlist.append(w1)
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components = coefficients[-1] + (s * w0 - w1)
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if scalar:
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components = components[:,0]
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yield components
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# Chebyshev differentiation.
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dw0 = dw1 = 0.0
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for coefficient, w1 in zip(coefficients[:-1], wlist):
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dw2 = dw1
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dw1 = dw0
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dw0 = 2.0 * w1 + dw1 * s2 - dw2
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rates = w0 + s * dw0 - dw1
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rates /= intlen
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rates *= 2.0
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rates *= S_PER_DAY
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if scalar:
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rates = rates[:,0]
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yield rates
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class Type9Segment(BaseSegment):
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"""Lagrange Interpolation - Unequal Time Steps"""
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def map_arrays(self):
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"""Raw coefficients and epochs as memory-mapped NumPy arrays."""
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i = self.end_i
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polynomial_degree, number_of_states = self.daf.read_array(i - 1, i)
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if polynomial_degree != 1:
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raise ValueError('jplephem does not yet support Type 9 segments'
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' with a polynomial degree of {0}'
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.format(polynomial_degree))
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number_of_states = int(number_of_states)
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i = self.start_i
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j = i + 6 * number_of_states - 1
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coefficients = self.daf.map_array(i, j)
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coefficients.shape = number_of_states, 6
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coefficients = coefficients.T
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epochs = self.daf.map_array(j + 1, j + number_of_states)
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return coefficients, epochs
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@reify
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def _data(self):
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"""Cached arrays that are ready for interpolation."""
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coefficients, epochs = self.map_arrays()
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# Make iteration faster by pre-creating tuples of separate arrays.
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positions = tuple(coefficients[:3])
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and_velocities = tuple(coefficients)
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epochs = _jd(epochs)
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return positions, and_velocities, epochs
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def compute(self, tdb, tdb2=0.0):
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"""Interpolate [x y z] at time `tdb` plus `tdb2`.
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A standard JPL Type 9 ephemerides will return kilometers.
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"""
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positions, and_velocities, epochs = self._data
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return array([interp(tdb, epochs, c) for c in positions])
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def compute_and_differentiate(self, tdb, tdb2=0.0):
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"""Interpolate [x y z dx dy dz] at time `tdb` plus `tdb2`.
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A standard JPL Type 9 ephemerides will return kilometers and
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kilometers per second.
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"""
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positions, and_velocities, epochs = self._data
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return array([interp(tdb, epochs, c) for c in and_velocities])
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def titlecase(name):
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"""Title-case target `name` if it looks safe to do so."""
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return name if name.startswith(('1', 'C/', 'DSS-')) else name.title()
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_segment_classes = {
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2: Segment,
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3: Segment,
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9: Type9Segment,
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}
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