1- Chong, Dr. Albert, 1987. “The Steps to PGS”. The professional Surveyor, Volume 7, # 6, pp.24-27, Falls Church, VA 22046.
2- Collins, Dr. James, 1987, “Global Positioning System.
3- The Economics”, P.O.B. Seminars, 87, P.O.B. Publishing Company, Wayne, KI. 48184.