2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/35649Omitting the motivations and historical connections, and also the detailed calculations, I state succinctly the principles that determine the relativistic idealization of a GPS system. These determine the results that Ashby presents in his tutorial.LaTeX, 5 pages plus one uuencoded ps figure; notes of remarks given at a workshop on GPS (Global Positioning System)General Relativity and Quantum Cosmologytext