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:

dataBaseRepresentation subclass instance

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

representation_typeBaseRepresentation 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_typeBaseDifferential 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.

copybool, optional

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

Other Parameters:

obstimeTime

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

locationEarthLocation

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
name
obstime

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 = <EarthLocation (0., 0., 0.) km>

name = 'itrs'

obstime = <Time object: scale='tt' format='jyear_str' value=J2000.000>