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"""SGP4 propagator. This is a wrapper around PyPI SGP4 propagator.
It constructs an artificial TLE and passes it to the PyPI module."""
import time
import numpy as np
from sgp4.earth_gravity import wgs72
from sgp4.io import twoline2rv
avg_bstar = 0.21109E-4
def propagate(kep,init_time,final_time,bstar=avg_bstar):
"""Propagates a set of keplerian elements.
Args:
kep(1x6 numpy array): osculating keplerian elements at epoch
init_time(float): initial time (epoch)
final_time(float): final time
bstar(float): bstar drag coefficient
Returns:
pos(1x3 numpy array): the position at tf
vel(1x3 numpy array): the velocity at tf
"""
t0 = time.gmtime(init_time)
t0 = ((t0.tm_year%100)*1000 +
t0.tm_yday+
t0.tm_hour/24 + t0.tm_min/1440 + t0.tm_sec/86400)
t0 = "{:14.8f}".format(t0)
tf = time.gmtime(final_time)
mu = 398600.4405
n = 86400/2/np.pi * (mu/kep[0]**3)**0.5
tanom = np.radians(kep[5])
e = kep[1]
ecc = np.arctan2((1-e**2)**0.5*np.sin(tanom),e+np.cos(tanom))
ecc = ecc%(2*np.pi)
mean = ecc - e*np.sin(ecc)
mean = np.degrees(mean)
inc = "{:8.4f}".format(kep[2])
raan = "{:8.4f}".format(kep[4])
e = "{:.7f}".format(e)[2:]
argp = "{:8.4f}".format(kep[3])
mean = "{:8.4f}".format(mean)
n = "{:11.8f}".format(n)
bexp = np.floor(np.log10(abs(bstar)))+1
bstar = "{:+5}{:+.0f}".format(int(bstar*10**(-bexp+5)),bexp)
line1 = ('1 00000U 000000 '+t0+' '
'.00000000 00000-0 '+bstar+' 0 0000')
line2 = ('2 00000 '+inc+' '+raan+' '+e+' '+argp+
' '+mean+' '+n+'000000')
satellite = twoline2rv(line1, line2, wgs72)
position, velocity = satellite.propagate(
tf.tm_year, tf.tm_mon, tf.tm_mday, tf.tm_hour, tf.tm_min, tf.tm_sec)
return position,velocity
if __name__ == "__main__":
t0 = 1526927274
tf = 1526932833
kep = np.array([6782.96, 0.0004084, 51.6402, 108.2140, 150.4026, 238.0528])
pos, vel = propagate(kep,t0,tf)
print(pos)
print(vel)
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