__all__ = (
"create_constellation",
"starlink_tles_v1",
"starlink_tles_v2",
"oneweb_tles",
"Constellation",
"sun_alt_limits",
)
import numpy as np
from astropy import constants as const
from astropy import units as u
from rubin_scheduler.utils import Site, gnomonic_project_toxy, point_to_line_distance, survey_start_mjd
from shapely.geometry import LineString, Point
from skyfield.api import EarthSatellite, load, wgs84
MJDOFFSET = 2400000.5
mjd0 = survey_start_mjd()
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def sun_alt_limits():
"""Return sun altitude limits (degrees) at which zero illuminated
satellites above 20 degrees altitude result.
Different constellations have different limits at which zero illumination
above 20 degrees occurs.
Returns
-------
sun_alt_limits : `dict` [`str`: `float`]
Dict with satellite constellation name keys,
altitude limits values (degrees).
"""
# Estimated in sun_alts_limits.ipynb
result = {"slv1": -36.0, "slv2": -36.0, "oneweb": -53.0}
return result
def satellite_mean_motion(altitude, mu=const.GM_earth, r_earth=const.R_earth):
"""Calculate mean motion of satellites at a given altitude in Earth's
gravitational field.
See https://en.wikipedia.org/wiki/Mean_motion#Formulae
Parameters
----------
altitude : `float`
Altitude of the satellite.
Should be a float with astropy units attached.
Returns
-------
mean_motion : `float`
"""
no = np.sqrt(4.0 * np.pi**2 * (altitude + r_earth) ** 3 / mu).to(u.day)
return 1 / no
def tle_from_orbital_parameters(sat_name, sat_nr, epoch, inclination, raan, mean_anomaly, mean_motion):
"""Generate TLE strings from orbital parameters.
Parameters
----------
sat_name : `str`
Satellite name
sat_nr : `float`
Satellite nr
epoch : `float`
Epoch
inclination : `float`
Inclination, probably with astropy units attached
raan : `float`
RA of the ascending node. Assumes astropy units.
mean_anomaly : `float`
Mean anomaly.
mean_motion : `float`
Mean motion.
Returns
-------
tle : `str`
Notes
-----
epoch has the format: first two digits are the year,
next three digits are the day from beginning of year,
then fraction of a day is given, e.g.
20180.25 would be 2020, day 180, 6 hours (UT?)
"""
# Note: RAAN = right ascention (or longitude) of ascending node
# I suspect this is filling in 0 eccentricity everywhere.
def checksum(line):
s = 0
for c in line[:-1]:
if c.isdigit():
s += int(c)
if c == "-":
s += 1
return "{:s}{:1d}".format(line[:-1], s % 10)
tle0 = sat_name
tle1 = checksum(
"1 {:05d}U 20001A {:14.8f} .00000000 00000-0 50000-4 " "0 0X".format(sat_nr, epoch)
)
tle2 = checksum(
"2 {:05d} {:8.4f} {:8.4f} 0001000 0.0000 {:8.4f} "
"{:11.8f} 0X".format(
sat_nr,
inclination.to_value(u.deg),
raan.to_value(u.deg),
mean_anomaly.to_value(u.deg),
mean_motion.to_value(1 / u.day),
)
)
return "\n".join([tle0, tle1, tle2])
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def create_constellation(
altitudes,
inclinations,
nplanes,
sats_per_plane,
epoch=23274.0,
name="Test",
seed=42,
):
"""Create a set of orbital elements for a satellite constellation then
convert them to TLEs.
Parameters
----------
altitudes : `np.ndarray`, (N,)
Altitudes (degrees).
inclinations : `np.ndarray`, (N,)
Inclinations (degrees).
nplanes : `np.ndarray`, (N,)
Number of satellite planes.
sats_per_plane : `np.ndarray`, (N,)
Number of satellites per orbital plane.
epoch : `float`
Epoch.
name : `str`
Satellite name.
seed : `float`
Random number seed.
Returns
-------
my_sat_tles : `list` of `str`
"""
rng = np.random.default_rng(seed)
my_sat_tles = []
sat_nr = 8000
for alt, inc, n, s in zip(altitudes, inclinations, nplanes, sats_per_plane):
if s == 1:
# random placement for lower orbits
mas = rng.uniform(0, 360, n) * u.deg
raans = rng.uniform(0, 360, n) * u.deg
else:
mas = np.linspace(0.0, 360.0, s, endpoint=False) * u.deg
mas += rng.uniform(0, 360, 1) * u.deg
raans = np.linspace(0.0, 360.0, n, endpoint=False) * u.deg
mas, raans = np.meshgrid(mas, raans)
mas, raans = mas.flatten(), raans.flatten()
mm = satellite_mean_motion(alt)
for ma, raan in zip(mas, raans):
my_sat_tles.append(
tle_from_orbital_parameters(name + " {:d}".format(sat_nr), sat_nr, epoch, inc, raan, ma, mm)
)
sat_nr += 1
return my_sat_tles
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def starlink_tles_v1():
"""Create a list of satellite TLE's, appropriate for
Starlink v1 (as of July 2022). Should create 4,408 orbits
Returns
-------
my_sat_tles : `list` [`str`]
"""
altitudes = np.array([550, 540, 570, 560, 560]) * u.km
inclinations = np.array([53, 53.2, 70, 97.6, 97.6]) * u.deg
nplanes = np.array([72, 72, 36, 6, 4])
sats_per_plane = np.array([22, 22, 20, 58, 43])
my_sat_tles = create_constellation(altitudes, inclinations, nplanes, sats_per_plane, name="starV1")
return my_sat_tles
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def starlink_tles_v2():
"""Create a list of satellite TLE's appropriate for
Starlink v2 (as of July 2022). Should create 29,988 orbits
Returns
-------
my_sat_tles : `list` [`str`]
"""
altitudes = np.array([340, 345, 350, 360, 525, 530, 535, 604, 614]) * u.km
inclinations = np.array([53, 46, 38, 96.9, 53, 43, 33, 148, 115.7]) * u.deg
nplanes = np.array([48, 48, 48, 30, 28, 28, 28, 12, 18])
sats_per_plane = np.array([110, 110, 110, 120, 120, 120, 120, 12, 18])
my_sat_tles = create_constellation(altitudes, inclinations, nplanes, sats_per_plane, name="starV2")
return my_sat_tles
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def oneweb_tles():
"""Create a list of satellite TLE's appropriate
for OneWeb plans (as of July 2022). Should create 6,372 orbits
Returns
-------
my_sat_tles : `list` [`str`]
"""
altitudes = np.array([1200, 1200, 1200]) * u.km
inclinations = np.array([87.9, 40, 55]) * u.deg
nplanes = np.array([49, 72, 72])
sats_per_plane = np.array([36, 32, 32])
my_sat_tles = create_constellation(altitudes, inclinations, nplanes, sats_per_plane, name="oneWe")
return my_sat_tles
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class Constellation:
"""Holds the constellation TLEs and calculates their appearance
in a series of observations.
Parameters
----------
sat_tle_list : `list` [`str`]
A list of satellite TLEs to be used
alt_limit : `float`
Altitude limit below which satellites can be ignored (degrees)
fov : `float`
The field of view diameter (degrees)
"""
def __init__(self, sat_tle_list, alt_limit=20.0, fov=3.5):
self.alt_limit_rad = np.radians(alt_limit)
self.fov_radius_rad = np.radians(fov / 2.0)
# Load ephemeris for sun position
self.eph = load("de421.bsp")
self.sat_list = []
self.ts = load.timescale()
for tle in sat_tle_list:
name, line1, line2 = tle.split("\n")
self.sat_list.append(EarthSatellite(line1, line2, name, self.ts))
self._make_location()
def _make_location(self):
telescope = Site(name="LSST")
self.observatory_site = wgs84.latlon(telescope.latitude, telescope.longitude, telescope.height)
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def update_mjd(self, mjd):
"""Calculate and record the alt/az position and illumination status
for all the satellites at a given time.
Parameters
----------
mjd : `float`
New MJD.
"""
jd = mjd + MJDOFFSET
t = self.ts.ut1_jd(jd)
self.altitudes_rad = []
self.azimuth_rad = []
self.illum = []
for sat in self.sat_list:
current_sat = sat.at(t)
illum = current_sat.is_sunlit(self.eph)
self.illum.append(illum.copy())
if illum:
topo = current_sat - self.observatory_site.at(t)
# this returns an Angle object
alt, az, dist = topo.altaz()
self.altitudes_rad.append(alt.radians + 0)
self.azimuth_rad.append(az.radians + 0)
else:
self.altitudes_rad.append(np.nan)
self.azimuth_rad.append(np.nan)
self.altitudes_rad = np.array(self.altitudes_rad)
self.azimuth_rad = np.array(self.azimuth_rad)
self.illum = np.array(self.illum)
# Keep track of the ones that are up and illuminated
self.visible = np.where((self.altitudes_rad >= self.alt_limit_rad) & (self.illum == True))[0]
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def paths_array(self, mjds):
"""Calculate and return the RA/Dec/Alt and illumination status
for all the satellites at an array of times.
Parameters
----------
mjds : `np.ndarray`, (N,)
Modified Julian Dates.
Returns
-------
ras : `np.ndarray`, (N,)
RAs at each MJD
decs : `np.ndarray`, (N,)
Decs at each MJD
alts : `np.ndarray`, (N,)
Altitudes at each MJD
illums : `np.ndarray`, (N,)
Array of bools for if satellite is illuminated
"""
jd = mjds + MJDOFFSET
t = self.ts.ut1_jd(jd)
ras = []
decs = []
alts = []
illums = []
for sat in self.sat_list:
current_sat = sat.at(t)
illum = current_sat.is_sunlit(self.eph)
illums.append(illum.copy())
topo = current_sat - self.observatory_site.at(t)
ra, dec, distance = topo.radec()
alt, az, dist = topo.altaz()
ras.append(ra.radians)
decs.append(dec.radians)
alts.append(alt.radians)
return np.vstack(ras), np.vstack(decs), np.vstack(alts), np.vstack(illums)
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def check_pointings(
self,
pointing_ras,
pointing_decs,
mjds,
visit_time,
fov_radius=1.75,
test_radius=10.0,
dt=2.0,
):
"""Calculate streak length and number of streaks in a set of visits.
Parameters
----------
pointing_ras : `np.ndarray`, (N,)
The RA for each pointing (degrees).
pointing_decs : `np.ndarray`, (N,)
The dec for each pointing (degrees).
mjds : `np.ndarray`, (N,)
The MJD for the (start) of each pointing (days).
visit_time : `np.ndarray`, (N,)
The start to end time for a visit (seconds).
fov_radius : `float`
The radius of the science field of view (degrees)
test_radius : `float`
The radius to use to see if a streak gets close (degrees).
Should be large, because satellites can be moving at ~1 deg/s.
dt : `float`
The timestep to use for high resolution checking
if a satellite crossed (seconds).
Returns
-------
streak length : `float`
The total length of satellite streaks in the FoV (degrees)
n_streak : `int`
The number of streaks that were in the FoV.
"""
test_radius_rad = np.radians(test_radius)
dt = dt / 3600 / 24 # to days
visit_time = visit_time / 3600.0 / 24.0
# Arrays to hold results
lengths_rad = np.zeros(pointing_ras.size, dtype=float)
n_streaks = np.zeros(pointing_ras.size, dtype=int)
input_id_indx_oned = np.arange(pointing_ras.size, dtype=int)
# Convert everything to radians for internal computations
pointing_ras_rad = np.radians(pointing_ras)
pointing_decs_rad = np.radians(pointing_decs)
fov_radius_rad = np.radians(fov_radius)
# Note self.paths_array should return an array that is
# N_sats x N_mjds in shape
# And all angles in radians.
sat_ra_1, sat_dec_1, sat_alt_1, sat_illum_1 = self.paths_array(mjds)
mjd_end = mjds + visit_time
sat_ra_2, sat_dec_2, sat_alt_2, sat_illum_2 = self.paths_array(mjd_end)
# broadcast the pointings to be the same shape as the satellite arrays.
pointing_ras_rad = np.broadcast_to(pointing_ras_rad, sat_ra_1.shape)
pointing_decs_rad = np.broadcast_to(pointing_decs_rad, sat_ra_1.shape)
input_id_indx = np.broadcast_to(input_id_indx_oned, sat_ra_1.shape)
# Which satellites are above the altitude limit and illuminated
# np.where confuses me when used on a 2d array.
above_illum_indx = np.where(
((sat_alt_1 > self.alt_limit_rad) | (sat_alt_2 > self.alt_limit_rad))
& ((sat_illum_1 == True) | (sat_illum_2 == True))
)
# point_to_line_distance can take arrays,
# but they all need to be the same shape,
# thus why we broadcast pointing ra and dec above.
distances = point_to_line_distance(
sat_ra_1[above_illum_indx],
sat_dec_1[above_illum_indx],
sat_ra_2[above_illum_indx],
sat_dec_2[above_illum_indx],
pointing_ras_rad[above_illum_indx],
pointing_decs_rad[above_illum_indx],
)
close = np.where(distances < test_radius_rad)[0]
# Numpy broadcasting is such a dark art
sat_indx = np.arange(len(self.sat_list), dtype=int)[np.newaxis]
sat_indx = np.broadcast_to(sat_indx.T, sat_ra_1.shape)
mjd_broad = np.broadcast_to(mjds, sat_ra_1.shape)[above_illum_indx][close]
visit_broad = np.broadcast_to(visit_time, sat_ra_1.shape)[above_illum_indx][close]
# ok, this is pretty ugly, but should get the job done
# Loop over all the potential collisions we have found
for p_ra, p_dec, ob_indx, mjd, vt, sat_in in zip(
pointing_ras_rad[above_illum_indx][close],
pointing_decs_rad[above_illum_indx][close],
input_id_indx[above_illum_indx][close],
mjd_broad,
visit_broad,
sat_indx[above_illum_indx][close],
):
mjd = np.linspace(mjd, mjd + vt, num=np.round(vt / dt).astype(int))
jd = mjd + MJDOFFSET
t = self.ts.ut1_jd(jd)
sat = self.sat_list[sat_in]
current_sat = sat.at(t)
topo = current_sat - self.observatory_site.at(t)
sat_ra, sat_dec, _distance = topo.radec()
length = _streak_length(sat_ra.radians, sat_dec.radians, p_ra, p_dec, fov_radius_rad)
if length > 0:
lengths_rad[ob_indx] += length
n_streaks[ob_indx] += 1
return np.degrees(lengths_rad), n_streaks
def _streak_length(sat_ras, sat_decs, pointing_ra, pointing_dec, radius):
"""Calculate streak lengths for satellites in a given (circular) pointing.
Parameters
----------
sat_ras : `np.ndarray`, (N,)
RA for each satellite (radians).
sat_decs : `np.ndarray`, (N,)
Decs for the satelltes (radians).
pointing_ra : `float`
RA of the pointing (radians).
pointing_dec : `float`
Dec of the pointing (radians).
radius : `float`
Radius of the field of view (radians).
Returns
-------
length : `float`
The length of the streak in radians
"""
# Hopefully this broadcasts properly
x, y = gnomonic_project_toxy(sat_ras, sat_decs, pointing_ra, pointing_dec)
ls = LineString(zip(x, y))
p = Point(0, 0)
circle_buffer = p.buffer(radius)
length = circle_buffer.intersection(ls).length
return length