Satellites (pycraf.satellite)#
Introduction#
Often the compatibility of space-based radio communications with other
services has to be studied. Then, it may happen, that one has to determine the
position of one or more satellites with respect to an observer on Earth’s
surface. The satellite sub-package uses the sgp4 package to calculate Geocentric cartesian
inertial (ECI) position for a given time and adds the transformations to the (
local) horizontal coordinates (azimuth, elevation, distance) of an observer.
Using pycraf.satellite#
The first step to calculate a satellite’s position is to obtain a so-called Two-line element set (TLE).
Note
The TLEs are usually published once a day, because the contained parameters quickly change; drag forces cause rapid changes in the orbits of almost all satellites.
On most websites that offer TLEs (e.g., Celestrak) in fact three-line strings are used, where the first line just contains the name of the satellite:
ISS (ZARYA)
1 25544U 98067A 13165.59097222 .00004759 00000-0 88814-4 0 47
2 25544 51.6478 121.2152 0011003 68.5125 263.9959 15.50783143834295
The pycraf package follows this format. To calculate the horizontal coordinates of the ISS at a given time, download the current TLE and do:
>>> import datetime
>>> import numpy as np
>>> from astropy.coordinates import EarthLocation
>>> from astropy import time
>>> from pycraf import satellite
>>> tle_string = '''ISS (ZARYA)
... 1 25544U 98067A 13165.59097222 .00004759 00000-0 88814-4 0 47
... 2 25544 51.6478 121.2152 0011003 68.5125 263.9959 15.50783143834295'''
>>> # define observer location
>>> location = EarthLocation(6.88375, 50.525, 366.)
>>> # create a SatelliteObserver instance
>>> sat_obs = satellite.SatelliteObserver(location)
>>> dt = datetime.datetime(2010, 7, 22, 8, 38, 57)
>>> obstime = time.Time(dt)
>>> az, el, dist = sat_obs.azel_from_sat(tle_string, obstime)
>>> print('az, el, dist: {:.1f}, {:.1f}, {:.0f}'.format(az, el, dist))
az, el, dist: -165.5 deg, 7.4 deg, 1713 km
>>> # can also use arrays of obstime
>>> mjd = 55123. + np.array([0.123, 0.99, 50.3])
>>> obstime2 = time.Time(mjd, format='mjd')
>>> az, el, dist = sat_obs.azel_from_sat(tle_string, obstime2)
>>> np.set_printoptions(precision=3)
>>> az
<Quantity [ 28.397, -143.170, -6.974] deg>
>>> el
<Quantity [-43.169, -21.468, -36.773] deg>
>>> dist
<Quantity [ 9388.292, 5675.039, 8361.284] km>
The azel_from_sat method also accepts
a sgp4.io.Satellite object. This could be created by using the
sgp4 package directly. pycraf also offers a convenience routine, get_sat, which does the TLE
parsing for your and returns the satellite name and Satellite object. It also
uses caching to save some computing time, if many different satellites are to
be processed:
>>> from pycraf import satellite
>>> tle_string = '''ISS (ZARYA)
... 1 25544U 98067A 13165.59097222 .00004759 00000-0 88814-4 0 47
... 2 25544 51.6478 121.2152 0011003 68.5125 263.9959 15.50783143834295'''
>>> satname, sat = satellite.get_sat(tle_string)
>>> satname
'ISS (ZARYA)'
>>> sat.satnum
25544
# using sgp4 directly, to get position and velocity in ECI coordinates
>>> from sgp4.api import jday
>>> jd, fr = jday(2017, 6, 29, 12, 50, 19)
>>> err_code, position, velocity = sat.sgp4(jd, fr)
>>> position # km
(3289.302521216188, 3816.880925531413, 4443.175627001508)
>>> velocity # km/s
(-2.958995807371371, 6.335950621185331, -3.241778555003016)
See Also#
Astropy Units and Quantities package, which is used extensively in pycraf.
Reference/API#
pycraf.satellite Package#
This sub-package provides a helper class to get position of a satellite in horizontal system (i.e, azimuth/elevation) from a two-line element string (TLE).
All the complicated calculations are done with the help of the sgp4 Python package.
Functions#
|
Construct a satellite instance ( |
Classes#
|
Calculate position of a satellite relative to an observer on Earth. |