Real null coframes in general relativity and GPS type coordinates

Abstract

Based on work of Derrick, Coll, and Morales, we define a `symmetric' null coframe with {\it four real null covectors}. We show that this coframe is closely related to the GPS type coordinates recently introduced by Rovelli.

Latex script, 9 pages, 4 figures; references added to work of Derrick, Coll, and Morales, 1 new figure