def sun_synchronous(cls,
*,
alt_km=None,
ecc=None,
inc_deg=None,
ltan_h=12,
date=None,
ta_deg=0):
"""Creates Sun synchronous predictor instance.
Parameters
----------
alt_km : float, optional
Altitude, in km.
ecc : float, optional
Eccentricity.
inc_deg : float, optional
Inclination, in degrees.
ltan_h : int, optional
Local Time of the Ascending Node, in hours (default to noon).
date : datetime.date, optional
Reference date for the orbit, (default to today).
ta_deg : float
Increment or decrement of true anomaly, will adjust the epoch
accordingly.
Notes
-----
See Vallado "Fundamentals of Astrodynamics and Applications", 4th ed (2013)
section 11.4.1.
"""
if date is None:
date = dt.datetime.today().date()
try:
with np.errstate(invalid="raise"):
if alt_km is not None and ecc is not None:
# Normal case, solve for inclination
sma = R_E_KM + alt_km
inc_deg = degrees(
np.arccos(
(-2 * sma**(7 / 2) * OMEGA * (1 - ecc**2)**2) /
(3 * R_E_KM**2 * J2 * np.sqrt(MU_E))))
elif alt_km is not None and inc_deg is not None:
# Not so normal case, solve for eccentricity
sma = R_E_KM + alt_km
ecc = np.sqrt(
1 - np.sqrt((-3 * R_E_KM**2 * J2 * np.sqrt(MU_E) *
np.cos(radians(inc_deg))) /
(2 * OMEGA * sma**(7 / 2))))
elif ecc is not None and inc_deg is not None:
# Rare case, solve for altitude
sma = (-np.cos(radians(inc_deg)) *
(3 * R_E_KM**2 * J2 * np.sqrt(MU_E)) /
(2 * OMEGA * (1 - ecc**2)**2))**(2 / 7)
else:
raise ValueError(
"Exactly two of altitude, eccentricity and inclination must be given"
)
except FloatingPointError as e:
raise InvalidOrbitError(
"Cannot find Sun-synchronous orbit with given parameters"
) from e
# TODO: Allow change in time or location
# Right the epoch is fixed given the LTAN, as well as the sub-satellite point
epoch = dt.datetime(date.year, date.month, date.day,
*float_to_hms(ltan_h))
raan = raan_from_ltan(epoch, ltan_h)
return cls(sma, ecc, inc_deg, raan, 0, ta_deg, epoch)
def test_raan_from_ltan(when_utc, raan, ltan):
assert pytest.approx(raan_from_ltan(when_utc, ltan), abs=1 / 3600) == raan
def sun_synchronous(cls,
*,
alt_km=None,
ecc=None,
inc_deg=None,
ltan_h=12,
date=None):
"""Creates Sun synchronous predictor instance.
Parameters
----------
alt_km : float, optional
Altitude, in km.
ecc : float, optional
Eccentricity.
inc_deg : float, optional
Inclination, in degrees.
ltan_h : int, optional
Local Time of the Ascending Node, in hours (default to noon).
date : datetime.date, optional
Reference date for the orbit, (default to today).
"""
if date is None:
date = dt.datetime.today().date()
# TODO: Allow change in time or location
epoch = dt.datetime(date.year,
date.month,
date.day,
*float_to_hms(ltan_h),
tzinfo=dt.timezone.utc)
raan = raan_from_ltan(epoch, ltan_h)
try:
with np.errstate(invalid="raise"):
if alt_km is not None and ecc is not None:
# Normal case, solve for inclination
sma = R_E_KM + alt_km
inc_deg = degrees(
np.arccos(
(-2 * sma**(7 / 2) * OMEGA * (1 - ecc**2)**2) /
(3 * R_E_KM**2 * J2 * np.sqrt(MU_E))))
elif alt_km is not None and inc_deg is not None:
# Not so normal case, solve for eccentricity
sma = R_E_KM + alt_km
ecc = np.sqrt(
1 - np.sqrt((-3 * R_E_KM**2 * J2 * np.sqrt(MU_E) *
np.cos(radians(inc_deg))) /
(2 * OMEGA * sma**(7 / 2))))
elif ecc is not None and inc_deg is not None:
# Rare case, solve for altitude
sma = (-np.cos(radians(inc_deg)) *
(3 * R_E_KM**2 * J2 * np.sqrt(MU_E)) /
(2 * OMEGA * (1 - ecc**2)**2))**(2 / 7)
else:
raise ValueError(
"Exactly two of altitude, eccentricity and inclination must be given"
)
except FloatingPointError:
raise InvalidOrbitError(
"Cannot find Sun-synchronous orbit with given parameters")
return cls(sma, ecc, inc_deg, raan, 0, 0, epoch)