193 lines
6.3 KiB
Python
193 lines
6.3 KiB
Python
"""Compute things from a NASA SPICE binary PCK kernel file.
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ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/pck.html
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"""
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from numpy import array, empty, empty_like, flip, 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 PCK(object):
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"""A JPL binary PCK (extension ``.bcp``) kernel.
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You can load a binary PCK file by specifying its filename::
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kernel = BinaryPCK.open('moon_pa_de421_1900-2050.bpc')
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Run ``print(kernel)`` see which segments are inside and iterate
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across ``kernel.segments`` to access them each in turn.
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To see the text comments, call ``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, source, descriptor)
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for source, descriptor in self.daf.summaries()]
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@classmethod
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def open(cls, path):
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"""Open the file at `path` and return a binary PCK instance."""
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return cls(DAF(open(path, 'rb')))
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def close(self):
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"""Close this 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 # TODO: explicitly close each memory map
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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 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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class Segment(object):
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"""A single segment of a binary PCK file.
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There are several items of information about each segment that are
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loaded from the underlying PCK 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.initial_second - initial epoch, as seconds from J2000
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segment.final_second - final epoch, as seconds from J2000
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segment.body - integer body 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.initial_second, self.final_second, self.body, self.frame,
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self.data_type, self.start_i, self.end_i) = descriptor
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self.initial_jd = jd(self.initial_second)
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self.final_jd = jd(self.final_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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body = titlecase(target_names.get(self.body, 'Unknown body'))
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text = ('{0.initial_jd:.2f}..{0.final_jd:.2f} frame={0.frame}'
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' {1} ({0.body})'.format(self, body))
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if verbose:
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text += ('\n 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 _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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else:
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raise ValueError('only binary PCK data type 2 is supported')
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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 = flip(coefficients, 0)
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return init, intlen, coefficients
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def compute(self, tdb, tdb2, derivative=True):
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"""Generate angles and derivatives for time `tdb` plus `tdb2`.
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If ``derivative`` is true, return a tuple containing both the
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angle and its derivative; otherwise simply return the angles.
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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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init, intlen, coefficients = data
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coefficient_count, component_count, n = coefficients.shape
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seconds = (tdb - T0) * S_PER_DAY - init + tdb2 * S_PER_DAY
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index, offset = divmod(seconds, intlen)
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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 = init + intlen * n
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raise ValueError('segment only covers dates %.1f through %.1f'
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% (init, final_epoch))
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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. We accumulate results starting with the
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# final coefficient to retain accuracy for as long as possible.
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jmax = coefficients.shape[0]
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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 = dw0 = dw1 = 0.0
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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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if derivative:
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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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components = coefficients[-1] + (s * w0 - w1)
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if scalar:
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components = components[:,0]
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if not derivative:
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return components
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# Chebyshev differentiation.
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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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if scalar:
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rates = rates[:,0]
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return components, rates
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def titlecase(name):
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"""Title-case body `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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