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Based on work of Derrick, Coll, and Morales, we define a `symmetric' nullcoframe with {\it four real null covectors}. We show that this coframe isclosely related to the GPS type coordinates recently introduced by Rovelli.Comment: Latex script, 9 pages, 4 figures; references added to work of  Derrick, Coll, and Morales, 1 new figur

Real null coframes in general relativity and GPS type coordinates