http://arxiv.org/abs/1110.0741 http://arxiv.org/abs/1110.0741 http://www.thorium.at/?page_id=4 http://www.thorium.at/?page_id=4 I could not find many referenc

http://arxiv.org/abs/1110.0741 http://arxiv.org/abs/1110.0741 http://www.thorium.at/?page_id=4 http://www.thorium.at/?page_id=4 I could not find many references, but I think clocks mainly make the instrument cheaper, or possible in the case of long-baseline satellite instruments.