# python 3.7 suggested

import datetime
import time
from skyfield.api import load, Topos, EarthSatellite

TLE_FILE = "https://celestrak.com/NORAD/elements/active.txt"
ISS_NAME = "ISS (ZARYA)"
LONGITUDE = -73.9940702
LATITUDE = 40.7546283

class SatelliteObserver:

def __init__(self, where: Topos, what: EarthSatellite):
self.where = where
self.sat = what
self.sat_name = what.name
self.ts = load.timescale(builtin=True)

@classmethod
def from_strings(cls, longitude: str or float, latitude: str or float, sat_name: str, tle_file: str) -> 'SatelliteObserver':
place = Topos(latitude, longitude)
satellites = load.tle(tle_file)
print("loaded {} sats from {}".format(len(satellites), tle_file))
_sats_by_name = {sat.name : sat for sat in satellites.values()}
satellite = _sats_by_name[sat_name]
return cls(place, satellite)

def altAzDist_at(self, at: float) -> (float, float, float):
"""
:param at: Unix time GMT (timestamp)
:return: (altitude, azimuth, distance)
"""
current_gmt = datetime.datetime.utcfromtimestamp(at)
current_ts = self.ts.utc(current_gmt.year, current_gmt.month, current_gmt.day, current_gmt.hour,
current_gmt.minute, current_gmt.second + current_gmt.microsecond / 1000000.0)
difference = self.sat - self.where
observer_to_sat = difference.at(current_ts)
altitude, azimuth, distance = observer_to_sat.altaz()
return (altitude.degrees, azimuth.degrees, distance.km)

def current_altAzDist(self) -> (float, float, float):
return self.altAzDist_at(time.mktime(time.gmtime()))

def above_horizon(self, at: float) -> bool:
"""
:param at: Unix time GMT
:return:
"""
(alt, az, dist) = self.altAzDist_at(at)
return alt > 0


def main():
iss = SatelliteObserver.from_strings(LONGITUDE, LATITUDE, ISS_NAME, TLE_FILE)
elevation, azimuth, distance = iss.current_altAzDist()
visible = "visible!" if elevation > 0 else "not visible =/"
print("ISS from latitude {}, longitude {}: azimuth {}, elevation {} ({})".format(LATITUDE, LONGITUDE, azimuth, elevation, visible))

if __name__ == "__main__":
main()


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