Swift Spacecraft Frame (gdt.missions.swift.frame)

The Swift spacecraft frame, SwiftFrame, is the frame that is aligned with the Swift spacecraft coordinate frame, and is represented by a quaternion that defines the rotation from spacecraft coordinates to the ICRS coordinate frame. This frame takes advantage of the Astropy coordinate frame design, so we can use the SwiftFrame to convert Astropy SkyCoord objects between the SwiftFrame and any celestial frame.

While the SwiftFrame is typically initialized when reading from a mission position history file (e.g. SwiftSao) instead of manually by a user, we can manually define the frame with a Quaternion:

>>> from gdt.core.coords import Quaternion
>>> from gdt.missions.swift.frame import *
>>> quat = Quaternion([-0.218,  0.009,  0.652, -0.726], scalar_first=False)
>>> swift_frame = SwiftFrame(quaternion=quat)
>>> swift_frame
<SwiftFrame: 1 frames;
 obstime=[J2000.000]
 obsgeoloc=[(0., 0., 0.) m]
 obsgeovel=[(0., 0., 0.) m / s]
 quaternion=[(x, y, z, w) [-0.218,  0.009,  0.652, -0.726]]>

Notice that we can also define the frame with an obstime, which is useful for transforming between the SwiftFrame and a non-inertial time-dependent frame; an obsgeoloc, which can define the spacecraft location in orbit; and obsgeovel, which defines the spacecraft orbital velocity.

Now let us define a SkyCoord in RA and Dec:

>>> from astropy.coordinates import SkyCoord
>>> coord = SkyCoord(100.0, -30.0, unit='deg')

And we can simply rotate this into the Swift frame with the following:

>>> swift_coord = coord.transform_to(swift_frame)
>>> (swift_coord.az, swift_coord.el)
(<Longitude [200.39733555] deg>, <Latitude [-41.88750942] deg>)

We can also transform from the Swift frame to other frames. For example, we define a coordinate in the Swift frame this way:

>>> swift_coord = SkyCoord(50.0, 25.0, frame=swift_frame, unit='deg')

Now we can tranform to ICRS coordinates:

>>> swift_coord.icrs
<SkyCoord (ICRS): (ra, dec) in deg
    [(313.69000519, 26.89158349)]>

or Galactic coordinates:

>>> swift_coord.galactic
<SkyCoord (Galactic): (l, b) in deg
    [(71.5141302, -11.56931006)]>

or any other coordinate frames provided by Astropy.

Reference/API

gdt.missions.swift.frame Module

Functions

swift_to_icrs(swift_frame, icrs_frame)	Convert from the Swift frame to the ICRS frame.
icrs_to_swift(icrs_frame, swift_frame)	Convert from the ICRS frame to the Swift frame.

Classes

SwiftFrame(*args[, copy, ...])

The Swift spacecraft frame in azimuth and elevation.

Class Inheritance Diagram