elevation angles are not believable

I am attempting to simulate visibility of a low-Earth-orbit satellite from a fixed ground station. The satellite elevation angles are all coming out around -17 degrees. This is not believable. As the satellite travels around the earth, the elevation angles should vary widely, and the satellite should at least occasionally be visible to the ground station (positive elevation angle). I’m assuming that I’m doing something wrong, but it is possible that there is a documentation issue here. My code follows.

# This script calculates visibility of a satellite to a single ground station.

# Section 1: Imports and constants.

from numpy import arange, empty, pi
from pdb import set_trace

import cartopy.crs as ccrs
from cartopy.feature.nightshade import Nightshade

from astropy import coordinates, time, units as u
from astropy.coordinates import AltAz, CartesianRepresentation, EarthLocation

from poliastro.bodies import Earth
from poliastro.twobody import Orbit, propagation
from poliastro.frames.fixed import ITRS
from poliastro.frames.equatorial import GCRS

RAD2DEG= 180 / pi


# Section 2: Simulate orbit.

a  = 7578.1 * u.km  # semi-major axis
ecc= 0      * u.one # eccentricity
inc= 63.4   * u.deg # inclination of the orbit plane
raan= 50    * u.deg # right ascension of the ascending node
argp= 270   * u.deg # argument of perigee
nu  = 0     * u.deg # true anomaly at epoch

T_sim_hrs= 24
T_step_min= 3

epoch= time.Time("2021-05-05 10:43")
orbit= Orbit.from_classical(Earth, a, ecc, inc, raan, argp, nu, epoch=epoch)

# Create time steps covering the duration of the simulation:
times= arange(0, 60*T_sim_hrs+1, T_step_min) * u.min

xyz= propagation.propagate(
  orbit,
  time.TimeDelta(times),
  method=propagation.cowell,
  rtol=1e-10,
)

epoch_times = epoch + times

# Calculate Earth-fixed (non-inertial) coordinates `itrs_xyz`:
gcrs_xyz = GCRS(xyz, obstime=epoch_times, representation_type=CartesianRepresentation)
itrs_xyz = gcrs_xyz.transform_to(ITRS(obstime=epoch_times))

# Convert satellite positions from Cartesian to spherical coordinates (radians and km):
sat_lat_lon= coordinates.cartesian_to_spherical(itrs_xyz.x, itrs_xyz.y, itrs_xyz.z)


# Section 3: Calculate satellite elevation angles.

# Reference: https://learn.astropy.org/rst-tutorials/Coordinates-Transform.html#transform-frames-to-get-to-altitude-azimuth-altaz

ground_station= EarthLocation.from_geodetic(
  lat   = 30 * u.deg,
  lon   = 30 * u.deg,
  height= 0  * u.km)

elevation= empty(len(sat_lat_lon[0]))

for i in range(len(sat_lat_lon[0])):
   satellite  = EarthLocation.from_geodetic(
     lat   =sat_lat_lon[1][i],
     lon   =sat_lat_lon[2][i],
     height=sat_lat_lon[0][i])

   # In the following call to `transform_to`, it is unclear why time is needed as an input.
   result= satellite.get_itrs().transform_to(AltAz(obstime=time.Time('J2010'),
     location=ground_station))

   elevation[i]= result.alt.value

set_trace()

[ayshih]

“test” --> “text”

[mkbrewer]

Done.