'Checks that the input is a Quantity with the necessary units (or the
special value ``0``).
Parameters
value : object
Input value to be converted.
Returns
out, converted : correctly-typed object, boolean
Tuple consisting of the correctly-typed object and a boolean which
indicates if conversion was actually performed.
Raises
ValueError
If the input is not valid for this attribute.'

def convert_input(self, value):

    if (value is None):
        return (None, False)
    elif isinstance(value, EarthLocation):
        return (value, False)
    else:
        from .builtin_frames import ITRS
        if (not hasattr(value, u'transform_to')):
            raise ValueError(u'"{0}"    was    passed    into    an    EarthLocationAttribute,    but    it    does    not    have    "transform_to"    method'.format(value))
        itrsobj = value.transform_to(ITRS)
        return (itrsobj.earth_location, True)
