def radialvelocity(self, rf='', v0='0m/s', off=None):
        """Defines a radialvelocity measure. It has to specify a reference
        code, radialvelocity quantity value (see introduction for the action
        on a scalar quantity with either a vector or scalar value, and when
        a vector of quantities is given), and optionally it can specify an
        offset, which in itself has to be a radialvelocity.


        :param rf: reference code string; Allowable reference
                   codes are: *LSRK LSRD BARY GEO TOPO GALACTO*
                   Note that additional ones may become available. Check with::

                       dm.list_codes(dm.radialvelocity())

        :param v0: longitude or x as quantity or string
        :param off: an optional offset measure of same type

        """
        loc = {'type': "radialvelocity",
               'refer': rf,
               'm0': dq.quantity(v0)}
        if is_measure(off):
            if not off['type'] == "radialvelocity":
                raise TypeError('Illegal offset type specified.')
            loc["offset"] = off
        return self.measure(loc, rf)
Defines a radialvelocity measure. It has to specify a reference
        code, radialvelocity quantity value (see introduction for the action
        on a scalar quantity with either a vector or scalar value, and when
        a vector of quantities is given), and optionally it can specify an
        offset, which in itself has to be a radialvelocity.


        :param rf: reference code string; Allowable reference
                   codes are: *LSRK LSRD BARY GEO TOPO GALACTO*
                   Note that additional ones may become available. Check with::

                       dm.list_codes(dm.radialvelocity())

        :param v0: longitude or x as quantity or string
        :param off: an optional offset measure of same type