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Fix EarthSatellite.epoch float-day truncation #1143

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Fix EarthSatellite.epoch float-day truncation #1143

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8 changes: 5 additions & 3 deletions skyfield/sgp4lib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -103,7 +103,9 @@ def __init__(self, line1, line2, name=None, ts=None):
else:
year = two_digit_year + 1900

self.epoch = ts.utc(year, 1, satrec.epochdays)
whole_days = int(satrec.epochdays)
seconds = (satrec.epochdays - whole_days) * DAY_S
self.epoch = ts.utc(year, 1, whole_days, 0, 0, seconds)

self._setup(satrec)

Expand Down Expand Up @@ -131,8 +133,8 @@ def from_satrec(cls, satrec, ts):
# be unified to always use epochyr and epochdays.
whole, fraction = divmod(satrec.jdsatepoch, 1.0)
year, month, day = compute_calendar_date(whole)
day += 0.5 # convert integer Julian day into Julian date float
self.epoch = ts.utc(year, month, day + fraction + satrec.jdsatepochF)
seconds = (0.5 + fraction + satrec.jdsatepochF) * DAY_S
self.epoch = ts.utc(year, month, day, 0, 0, seconds)

self._setup(satrec)
return self
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17 changes: 17 additions & 0 deletions skyfield/tests/test_earth_satellites.py
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Expand Up @@ -207,3 +207,20 @@ def _at(self, t):
True, True, True, False,
True, True, True, False, # Last two fake sats can see each other.
]

def test_tle_epoch_matches_sgp4_library():
from numpy import asarray
from sgp4.api import Satrec
from skyfield.constants import DAY_S
line1 = '1 55910U 23035C 23076.01808579 -.00000195 00000-0 00000+0 0 9994'
line2 = '2 55910 44.0037 312.8770 0004210 247.2472 260.1626 14.90311335 06'
ts = api.load.timescale(builtin=True)
satrec = Satrec.twoline2rv(line1, line2)
sat = EarthSatellite(line1, line2, ts=ts)
fraction = sat.epoch.tai_fraction - sat.epoch._leap_seconds() / DAY_S
offset_s = ((sat.epoch.whole - satrec.jdsatepoch)
+ (fraction - satrec.jdsatepochF)) * DAY_S
assert abs(offset_s) < 1e-9
_, r_library, _ = satrec.sgp4(satrec.jdsatepoch, satrec.jdsatepochF)
r_epoch, _, _ = sat._position_and_velocity_TEME_km(sat.epoch)
assert abs(asarray(r_epoch) - r_library).max() < 1e-12
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