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Coordinate Frame Transforms

The satkit.frametransform module provides functions for transforming between various coordinate frames used in satellite tracking and orbit determination. These include multiple variations of "inertial" coordinate frames, and multiple versions of "Earth-fixed" coordinate frames.

Some notes:

  • Most of the algorithms in this module are from the book "Fundamentals of Astrodynamics and Applications" by David Vallado.
  • The frame transforms are defined as arbitrary rotations in a 3-dimensional space. The rotations are a function of time, and are represented as quaternions.
  • The rotation from the Geocentric Celestial Reference Frame (GCRF) to the Earth-Centered Inertial (ECI) frame is defined by the International Astronomical Union (IAU), available at https://www.iers.org/. See IERS Technical Note 36 for the latest values.

frametransform

Transformations between coordinate frames, and associated utility functions

Coordinate frame transforms are mostly pulled from Vallado: https://www.google.com/books/edition/Fundamentals_of_Astrodynamics_and_Applic/PJLlWzMBKjkC?hl=en&gbpv=0

or the IERS: https://www.iers.org/

earth_orientation_params(time)

Get Earth Orientation Parameters at given instant

Parameters:

Name Type Description Default
time time

Instant at which to query parameters

 required

Returns:

Type Description
tuple[float, float, float, float, float, float]

(float, float, float, float, float, float) | None: Tuple with following elements: 0 : (UT1 - UTC) in seconds 1 : X polar motion in arcsecs 2 : Y polar motion in arcsecs 3 : LOD: instantaneous rate of change in (UT1-UTC), msec/day 4 : dX wrt IAU-2000A nutation, milli-arcsecs 5 : dY wrt IAU-2000A nutation, milli-arcsecs
Notes
  • Returns None if the time is outside the range of available Earth Orientation Parameters (EOP)
  • EOP are only available from 1962 to current, and predict to current + ~ 4 months
  • If EOP are not available, all values will be set to zero, and a warning will be printed to stderr
  • See: https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html
Example 
t = satkit.time(2024, 1, 1)
eop = satkit.frametransform.earth_orientation_params(t)
if eop is not None:
    ut1_utc, xp, yp, lod, dx, dy = eop
    print(f"UT1-UTC: {ut1_utc:.6f} s")

disable_eop_time_warning()

Disable the warning printed to stderr when Earth Orientation Parameters (EOP) are not available for a given time.

Notes
  • This function is used to disable the warning printed when EOP are not available for a given time.
  • If not disabled, warning will be shown only once per library load,