I am trying to plot the groundtrack of a satellite through a combination of packages, animate the satellite movement, mark a field of view from the subsatellite point (which is just arbitrary circles in this code) and then export the file as a video of some kind. So far, I have been able to do all of this except that when I try to export the video, the Nightshade feature doesn't animate so much as overlay and eventually blacks out most of the screen. Is there something I'm missing on how to properly animate the Nightshade feature? I know that I'm essentially recreating a new feature inside the update function everytime it runs a frame but I could not figure out how to update it as I do the scatter plots.
I've included my sample code below.
import pandas as pd
from sgp4.api import WGS72
from sgp4.api import Satrec
from skyfield.api import EarthSatellite, load, N, W, wgs84
import datetime
import numpy as np
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import matplotlib.animation as animation
from cartopy.feature.nightshade import Nightshade
# CREATE THE SATELLITE DATA
epoch = datetime.date(1949, 12, 31)
sat = Satrec()
sat.sgp4init(
WGS72, # gravity model
'i', # 'a' = old AFSPC mode, 'i' = improved mode
5, # satnum: Satellite number
(datetime.date.today() - epoch).days, # epoch: days since 1949 December 31 00:00 UT
0, # bstar: drag coefficient (1/earth radii)
6.969196665e-13, # ndot (NOT USED): ballistic coefficient (revs/day)
0.0, # nddot (NOT USED): mean motion 2nd derivative (revs/day^3)
0.1, # ecco: eccentricity
280 * np.pi / 180, # argpo: argument of perigee (radians)
50 * np.pi / 180, # inclo: inclination (radians)
275 * np.pi / 180, # mo: mean anomaly (radians)
0.0472294454407, # no_kozai: mean motion (radians/minute)
50 * np.pi / 180, # nodeo: right ascension of ascending node (radians)
)
# DEFINE A FEW BASIC PARAMETERS FOR THE PROGRAM
P = sat.mo / sat.no_kozai # min, period of orbit. LEOs orbit between 84-127 minutes
ts = load.timescale()
sat1 = EarthSatellite.from_satrec(sat, ts)
hours = np.arange(0, 6, 0.05)
time = ts.utc(2021, 6, 31, hours)
pos = sat1.at(time).position.km
pos_ec = sat1.at(time).ecliptic_position().km
sp = wgs84.subpoint(sat1.at(time))
latitude = sp.latitude
longitude = sp.longitude
elev = sp.elevation
# CREATE A DATAFRAME OF THE DATA FOR REVIEW LATER IF NEEDED
df = pd.DataFrame([time.utc_datetime(), latitude.degrees, longitude.degrees, elev.km],
index=['DTS', 'lat', 'lon', 'elev']).T
df.lat = df.lat.astype('float32')
df.lon = df.lon.astype('float32')
df.elev = df.elev.astype('float32')
df.set_index('DTS', inplace=True)
# ASSIGN RELEVANT DATA FOR THE SUBSATELLITE POINT
ssp = np.transpose(np.array([longitude.degrees, latitude.degrees]))
line = ssp.copy()
pos = np.where(np.diff(np.abs(line[:, 0] >= 0)))[0]
line[pos, :] = np.nan
# CREATE DATE TIME RANGES FOR USE WITH THE NIGHTSHADE FEATURE
base = datetime.datetime(2000, 1, 1)
dates = np.array([base + datetime.timedelta(hours=i) for i in range(len(hours))])
shades = [Nightshade(date, alpha=0.2) for date in dates]
### CREATE FIGURE AND IMAGE
fig = plt.figure(figsize=(16, 8))
ax = fig.add_subplot(1, 1, 1, projection=ccrs.PlateCarree())
ax.stock_img()
# plot lines that will show the ground track that will be animated
ax.plot(line[:, 0], line[:, 1], '--k')
# create a blank scatter to start
scatter = ax.scatter(None, None, color='r', s=30)
# initiate the circles around the scatter point
circle1 = plt.Circle((longitude.degrees[0], latitude.degrees[0]), radius=30, color='blue', alpha=0.3)
circle2 = plt.Circle((longitude.degrees[0], latitude.degrees[0]), radius=40, color='yellow', alpha=0.3)
# add the circles to the axis
ax.add_patch(circle1)
ax.add_patch(circle2)
# Add the nightshade feature (but set it to be invisible so it doesn't stay through the whole animation)
ns = ax.add_feature(Nightshade(base, alpha=0.0))
# Create all the updates for the animation
def update(i):
lon = ssp[i, 0]
lat = ssp[i, 1]
scatter.set_offsets(np.c_[lon, lat])
# add a feature for the next Nightshade feature
ns = ax.add_feature(shades[i], alpha=0.2)
circle1.center = (lon, lat)
circle2.center = (lon, lat)
return scatter, circle1, circle2, ns
# Run the animation
anim = animation.FuncAnimation(plt.gcf(), update, frames=df.shape[0],init_func=None, interval=250, blit=True)
plt.show()
# WRITE THE VIDEO
Writer = animation.writers['ffmpeg']
writer = Writer(fps=10, metadata=dict(artist='Me'), bitrate=1800)
anim.save('gt.mp4', writer=writer)