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# GCRS

class astropy.coordinates.GCRS(*args, copy=True, representation_type=None, differential_type=None, **kwargs)

  Bases: `BaseRADecFrame`

  A coordinate or frame in the Geocentric Celestial Reference System (GCRS).

  GCRS is distinct from ICRS mainly in that it is relative to the Earth’s center-of-mass rather than the solar system Barycenter. That means this frame includes the effects of aberration (unlike ICRS). For more background on the GCRS, see the references provided in the See Also section of the documentation. (Of particular note is Section 1.2 of USNO Circular 179)

  This frame also includes frames that are defined relative to the Earth, but that are offset (in both position and velocity) from the Earth.

  The frame attributes are listed under Other Parameters.

  Parameters:
    data `BaseRepresentation` subclass instance

      A representation object or `None` to have no data (or use the coordinate component arguments, see below).

    ra `Angle`, optional, keyword-only

      The RA for this object (`dec` must also be given and `representation` must be None).

    dec `Angle`, optional, keyword-only

      The Declination for this object (`ra` must also be given and `representation` must be None).

    distance `Quantity` [:ref: ‘length’], optional, keyword-only

      The Distance for this object along the line-of-sight. (`representation` must be None).

    pm_ra_cosdec `Quantity` [:ref: ‘angular speed’], optional, keyword-only

      The proper motion in Right Ascension (including the `cos(dec)` factor) for this object (`pm_dec` must also be given).

    pm_dec `Quantity` [:ref: ‘angular speed’], optional, keyword-only

      The proper motion in Declination for this object (`pm_ra_cosdec` must also be given).

    radial_velocity `Quantity` [:ref: ‘speed’], optional, keyword-only

      The radial velocity of this object.

    representation_type `BaseRepresentation` subclass, `python:str`, optional

      A representation class or string name of a representation class. This sets the expected input representation class, thereby changing the expected keyword arguments for the data passed in. For example, passing `representation_type='cartesian'` will make the classes expect position data with cartesian names, i.e. `x, y, z` in most cases unless overridden via `frame_specific_representation_info`. To see this frame’s names, check out `<this frame>().representation_info`.

    differential_type `BaseDifferential` subclass, `python:str`, `python:dict`, optional

      A differential class or dictionary of differential classes (currently only a velocity differential with key ‘s’ is supported). This sets the expected input differential class, thereby changing the expected keyword arguments of the data passed in. For example, passing `differential_type='cartesian'` will make the classes expect velocity data with the argument names `v_x, v_y, v_z` unless overridden via `frame_specific_representation_info`. To see this frame’s names, check out `<this frame>().representation_info`.

    copy bool, optional

      If `True` (default), make copies of the input coordinate arrays. Can only be passed in as a keyword argument.

  Other Parameters:
    obstime `Time`

      The time at which the observation is taken. Used for determining the position of the Earth.

    obsgeoloc `CartesianRepresentation`, `Quantity`

      The position of the observer relative to the center-of-mass of the Earth, oriented the same as BCRS/ICRS. Either [0, 0, 0], `CartesianRepresentation`, or proper input for one, i.e., a `Quantity` with shape (3, …) and length units. Defaults to [0, 0, 0], meaning “true” GCRS.

    obsgeovel `CartesianRepresentation`, `Quantity`

      The velocity of the observer relative to the center-of-mass of the Earth, oriented the same as BCRS/ICRS. Either [0, 0, 0], `CartesianRepresentation`, or proper input for one, i.e., a `Quantity` with shape (3, …) and velocity units. Defaults to [0, 0, 0], meaning “true” GCRS.

  Attributes Summary

  `default_differential` | Default representation for differential data (e.g., velocity)
  `default_representation` | Default representation for position data
  `frame_attributes` |
  `frame_specific_representation_info` | Mapping for frame-specific component names
  `name` |
  `obsgeoloc` | The observer location relative to Earth center
  `obsgeovel` | The observer velocity relative to Earth center
  `obstime` | The reference time (e.g., time of observation)

  Attributes Documentation

  default_differential

    Default representation for differential data (e.g., velocity)

  default_representation

    Default representation for position data

  frame_attributes = {'obsgeoloc': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obsgeovel': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object>}
  frame_specific_representation_info

    Mapping for frame-specific component names

  name = 'gcrs'
  obsgeoloc

    The observer location relative to Earth center

    Default: [0, 0, 0]

  obsgeovel

    The observer velocity relative to Earth center

    Default: [0, 0, 0]

  obstime

    The reference time (e.g., time of observation)

    Default: J2000.000