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# ITRS

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

  Bases: `BaseCoordinateFrame`

  A coordinate or frame in the International Terrestrial Reference System (ITRS). This is approximately a geocentric system, although strictly it is defined by a series of reference locations near the surface of the Earth (the ITRF). For more background on the ITRS, see the references provided in the See Also section of the documentation.

  This frame also includes frames that are defined relative to the center of the Earth, but that are offset (in both position and velocity) from the center of the Earth. You may see such non-geocentric coordinates referred to as “topocentric”.

  Topocentric ITRS frames are convenient for observations of near Earth objects where stellar aberration is not included. One can merely subtract the observing site’s EarthLocation geocentric ITRS coordinates from the object’s geocentric ITRS coordinates, put the resulting vector into a topocentric ITRS frame and then transform to `AltAz` or `HADec`. The other way around is to transform an observed `AltAz` or `HADec` position to a topocentric ITRS frame and add the observing site’s EarthLocation geocentric ITRS coordinates to yield the object’s geocentric ITRS coordinates.

  On the other hand, using `transform_to` to transform geocentric ITRS coordinates to topocentric ITRS, observed `AltAz`, or observed `HADec` coordinates includes the difference between stellar aberration from the point of view of an observer at the geocenter and stellar aberration from the point of view of an observer on the surface of the Earth. If the geocentric ITRS coordinates of the object include stellar aberration at the geocenter (e.g. certain ILRS ephemerides), then this is the way to go.

  Note to ILRS ephemeris users: Astropy does not currently consider relativistic effects of the Earth’s gravatational field. Nor do the `AltAz` or `HADec` refraction corrections compute the change in the range due to the curved path of light through the atmosphere, so Astropy is no substitute for the ILRS software in these respects.

  Parameters:
    data `BaseRepresentation` subclass instance

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

    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 and its precession.

    location `EarthLocation`

      The location on the Earth. This can be specified either as an `EarthLocation` object or as anything that can be transformed to an `ITRS` frame. The default is the centre of the Earth.

  Attributes Summary

  `default_differential` | Default representation for differential data (e.g., velocity)
  `default_representation` | Default representation for position data
  `earth_location` | The data in this frame as an `EarthLocation` class.
  `frame_attributes` |
  `frame_specific_representation_info` | Mapping for frame-specific component names
  `location` | The location on Earth of the observer
  `name` |
  `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

  earth_location

    The data in this frame as an `EarthLocation` class.

  frame_attributes = {'location': <astropy.coordinates.attributes.EarthLocationAttribute object>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object>}
  frame_specific_representation_info

    Mapping for frame-specific component names

  location

    The location on Earth of the observer

    Default: (0.0, 0.0, 0.0) km

  name = 'itrs'
  obstime

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

    Default: J2000.000