def setup_orbital_elements(acquisition, tle_path):
"""
Given an ephemeral object and a datetime object, calculate the
satellite orbital paramaters used for calculating the angle grids.
:param acquisition:
An `Acquisition` object.
:param tle_path:
A `str` to the directory containing the Two Line Element data.
:return:
A floating point np array of 3 elements containing the
satellite ephemeral bodies orbital paramaters.
* Index 0 contains the obrital inclination in degrees.
* Index 1 contains the semi major raidus in metres.
* Index 2 contains the angular velocity in radians/sec^1.
Also a np dataset of the following datatype:
* dtype = [('orbital_inclination', 'float64'),
('semi_major_radius', 'float64'),
('angular_velocity', 'float64')]
"""
dtype = np.dtype([('orbital_inclination', 'float64'),
('semi_major_radius', 'float64'),
('angular_velocity', 'float64')])
dset = np.zeros(1, dtype=dtype)
ephemeral = load_tle(acquisition, tle_path)
# If we have None, then no suitable TLE was found, so use values gathered
# by the acquisition object
if ephemeral is None:
# orbital inclination (degrees)
dset['orbital_inclination'] = math.degrees(acquisition.inclination)
# semi_major radius (m)
dset['semi_major_radius'] = acquisition.semi_major_axis
# angular velocity (rad sec-1)
dset['angular_velocity'] = acquisition.omega
else:
ephemeral.compute(acquisition.acquisition_datetime)
pi = np.pi
n = ephemeral._n # number or orbits per day
s = 24 * 60 * 60 # Seconds in a day
mu = 398600441800000.0 # Earth Gravitational parameter m^3s^-2
# orbital inclination (degrees)
dset['orbital_inclination'] = np.rad2deg(ephemeral._inc)
# semi_major radius (m)
# http://smallsats.org/2012/12/06/two-line-element-set-tle/
dset['semi_major_radius'] = (mu / (2 * pi * n / s)**2)**(1 / 3)
# angular velocity (rad sec-1)
dset['angular_velocity'] = (2 * pi * n) / s
return np.array(dset.tolist()).squeeze(), dset
def test_load_tle_l8_mtl1(self):
acq = acquisitions(LS8_SCENE1).get_acquisitions(group='RES-GROUP-1')[0]
data = load_tle(acq, TLE_DIR)
self.assertIsInstance(data, ephem.EarthSatellite)
def test_load_tle_l5_mtl1(self):
acq = acquisitions(LS5_SCENE1).get_acquisitions()[0]
data = load_tle(acq, TLE_DIR)
self.assertIsInstance(data, ephem.EarthSatellite)