232 lines
8.0 KiB
Python
232 lines
8.0 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, empty, empty_like, rollaxis
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from .daf import DAF
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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)`` see which segments are inside. 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 = [Segment(self.daf, *t) for t in self.daf.summaries()]
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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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return cls(DAF(open(path, 'rb')))
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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 hasattr(segment, '_data'):
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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 = (str(segment) for segment in self.segments)
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return 'File type {0} and format {1} with {2} segments:\n{3}'.format(
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d(daf.locidw), d(daf.locfmt), len(self.segments), '\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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class Segment(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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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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self._data = None
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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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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 = ('{0.start_jd:.2f}..{0.end_jd:.2f} {1} ({0.center})'
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' -> {2} ({0.target})'.format(self, center, target))
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if verbose:
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text += ('\n frame={0.frame} data_type={0.data_type} 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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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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def _load(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('only SPK data types 2 and 3 are supported')
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init, intlen, rsize, n = self.daf.read_array(self.end_i - 3, self.end_i)
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initial_epoch = jd(init)
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interval_length = intlen / S_PER_DAY
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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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return initial_epoch, interval_length, coefficients
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def load_array(self):
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data = self._data
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if data is None:
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self._data = data = self._load()
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return data
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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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data = self._data
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if data is None:
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self._data = data = self._load()
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initial_epoch, interval_length, coefficients = data
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component_count, n, coefficient_count = coefficients.shape
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# Subtracting tdb before adding tdb2 affords greater precision.
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index, offset = divmod((tdb - initial_epoch) + tdb2, interval_length)
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index = index.astype(int)
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if (index < 0).any() or (index > n).any():
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final_epoch = initial_epoch + interval_length * n
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raise ValueError('segment only covers dates %.1f through %.1f'
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% (initial_epoch, final_epoch))
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omegas = (index == n)
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index[omegas] -= 1
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offset[omegas] += interval_length
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coefficients = coefficients[:,index]
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# Chebyshev polynomial.
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T = empty((coefficient_count, len(index)))
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T[0] = 1.0
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T[1] = t1 = 2.0 * offset / interval_length - 1.0
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twot1 = t1 + t1
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for i in range(2, coefficient_count):
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T[i] = twot1 * T[i-1] - T[i-2]
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components = (T.T * coefficients).sum(axis=2)
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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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dT = empty_like(T)
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dT[0] = 0.0
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dT[1] = 1.0
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if coefficient_count > 2:
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dT[2] = twot1 + twot1
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for i in range(3, coefficient_count):
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dT[i] = twot1 * dT[i-1] - dT[i-2] + T[i-1] + T[i-1]
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dT *= 2.0
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dT /= interval_length
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rates = (dT.T * coefficients).sum(axis=2)
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if scalar:
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rates = rates[:,0]
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yield rates
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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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