scoords

The scoords library defines an astropy custom coordinate frame. This allows to specify coordinates relative to the local spacecraft coordinates and transform them to any other system using astropy's machinery.

SkyCoord objects are instantiated by passing a location with specified units and a coordinate frame. Use SpacecraftFrame to specify a coordinate in the reference system attached to the spacecraft:

import astropy.units as u
from astropy.coordinates import SkyCoord
from scoords import SpacecraftFrame

c = SkyCoord(lon = 45*u.deg, lat = 10*u.deg, frame = SpacecraftFrame())

This allows you to know the reference frame of the coordinate, e.g.

c.frame

<SpacecraftFrame Coordinate (attitude=None, obstime=None, location=None): (lon, lat) in deg
    (45., 10.)>

However, in order to transform it into other coordinate system you need to specify the orientation of the spacecraft with respect to an inertial reference frame --i.e. the attitude

from scoords import Attitude

attitude = Attitude.from_rotvec(45*u.deg*[0,0,1], frame = 'icrs')

c = SkyCoord(lon = 0*u.deg, lat = 0*u.deg, frame = SpacecraftFrame(attitude = attitude))

There are class methods to specify the orientation in any of the following formats:

• A rotation matrx
• A vector co-directional to the axis of rotation
• A quaternion
• A scipy's Rotation object
• The direction the spacecraft coordinates axes point to

Once the attitude is specified, we can transform from/to any other frame supported by astropy

c.transform_to('icrs')

<SkyCoord (ICRS): (ra, dec) in deg
    (45., 0.)>

c.transform_to('galactic')

<SkyCoord (Galactic): (l, b) in deg
    (176.96129126, -48.90264434)>

Although it does not play a role in this particular coordinates transformation, the observation time and location can also be specified in case it is needed by any other algorithm:

from astropy.time import Time
from astropy.coordinates import EarthLocation

frame = SpacecraftFrame(attitude = attitude,
                        obstime = Time('2026-01-01T00:00:00'),
                        location = EarthLocation(lon = 10*u.deg, lat = 46*u.deg, height = 400*u.km))

c = SkyCoord(lon = 45*u.deg, lat = 10*u.deg, frame = frame)

c.frame.obstime

<Time object: scale='utc' format='isot' value=2026-01-01T00:00:00.000>

c.frame.location.geodetic

GeodeticLocation(lon=<Longitude 10. deg>, lat=<Latitude 46. deg>, height=<Quantity 400. km>)