Source code for skplatform.satellite.satellitesgp4
from typing import Tuple
from math import pi
import numpy as np
from datetime import datetime, timedelta
import sktimeutils
from sgp4.earth_gravity import wgs84
from sgp4.io import twoline2rv
from .satellitebase import SatelliteBase
from .satellitekepler import SatelliteKepler
# -----------------------------------------------------------------------------
# class SatelliteKepler
# -----------------------------------------------------------------------------
[docs]
class SatelliteSGP4(SatelliteBase):
"""
Implements the SGP4 satellite propagator. This is just a shallow wrapper for the `SGP4 python package <https://pypi.org/project/sgp4/>`_
from PyPi
"""
[docs]
def __init__(self, twolines: Tuple[str, str] = None):
"""
Creates and initializes the sgp4 orbit predictor using two line ali_elements provided by the user. The lines must be in the
promper two line format. It is possible to leave the lines as None and initialize the predictor after creation.
Parameters:
twolines: Tuple( str, str )
Two strings that contain the two line ali_elements used to initialize the sgp4 orbit predictor. This paremeter
may be None, in which case, the object can be configured by calling :meth:`~.from_kepler_orbit` or :meth:`~.from_twoline_elements`
"""
super().__init__()
self.m_sgp4 = None
if (twolines is not None):
self.from_twoline_elements(twolines[0], twolines[1])
# -----------------------------------------------------------------------------
# from_kepler_orbit
# -----------------------------------------------------------------------------
[docs]
def from_kepler_orbit(self, kepler: SatelliteKepler, bstar=0.0):
"""
Initializes the sgp4 orbit predictor with two line ali_elements extracted from a kepler orbit.
A simple algorithm that extracts two line ali_elements from a kepler orbit and feeds them
into the SGP4 model. Note that the kepler orbit does not correct coordinates to the true equator mean equinox (TEME)
coordinates used by sgp4 so there will be minor issues for people seeking precision. On the other hand it is a
simple way to fire up the SGP4 from a Kepler orbit, which is quite useful when modelling simple classes of orbit for
study work.
Parameters:
kepler : SatelliteKepler
The Kepler ali_elements for the satellite. The two line ali_elements are generated for the current time and eciposition
stored in the kep[ler satellite.
bstar:float
The B* drag term used by SGP4. This is not part of the Kepler ali_elements and must be manually included by
the user. We recommend you leave this as default 0.0 unless you know what you are doing as the drag term
used by SGP4 is tricky to get right (it can change by one or two orders of magnitude for example)
"""
twolines = kepler.make_two_line_elements(bstar=bstar)
self.from_twoline_elements(twolines[0], twolines[1])
# -----------------------------------------------------------------------------
# from_twoline_elements
# -----------------------------------------------------------------------------
[docs]
def from_twoline_elements(self, line1: str, line2: str):
"""
Initializes the sgp4 orbit predictor with classic two line ali_elements.
Parameters:
line1:str
The first line of the two line ali_elements
line2:str
The second line of the two line ali_elements
"""
self.m_sgp4 = twoline2rv(line1, line2, wgs84)
orbit_number = int(line2[63:63 + 5]) # Get the orbit number of this orbit.
self.set_orbit_number_from_last_equator_crossing(orbit_number, self.m_sgp4.epoch)
self.update_eci_position(self.m_sgp4.epoch)
# ---------------------------------------------------------------------------
# SatelliteKepler::orbital_period
# ---------------------------------------------------------------------------
[docs]
def period(self) -> timedelta:
"""
Return the orbital period of this orbit using the mean motion
Returns:
float
The orbital period in seconds
"""
mean_motion = self.m_sgp4.no / 60.0 # Convert sgp4 mean motion from radians/minute to radians per second
p = timedelta(seconds=2.0 * pi / mean_motion)
return p
# -----------------------------------------------------------------------------
# eccentricity
# -----------------------------------------------------------------------------
[docs]
def eccentricity(self) -> float:
return self.m_sgp4.ecco
# --------------------------------------------------------------------------
# SatelliteKepler::update_eci_position
# --------------------------------------------------------------------------
[docs]
def update_eci_position(self, autc: datetime):
utc = sktimeutils.ut_to_datetime(autc)
if (self._m_time is None) or (utc != self._m_time): # Do we need to update eciposition or is it already the cached value
r, v = self.m_sgp4.propagate(year=utc.year, month=utc.month, day=utc.day, hour=utc.hour, minute=utc.minute, second=(utc.second + utc.microsecond / 1.0E6))
pos = np.array(r) * 1000.0
vel = np.array(v) * 1000.0
self._set_current_state(pos, vel, utc)
