Huntsville is located in northern Alabama, near the Tennessee River. Long a trade and distribution center for agricultural products, Huntsville developed into a major aerospace and military research, development, and manufacturing center after 1950. Facilities in Huntsville and the Redstone Arsenal, situated just outside the city, include the United States Army Missile Command; the U.S. Army Space and Strategic Defense Command; the Army Corps of Engineers; and the George C. Marshall Space Flight Center. A 2006 real estate survey has estimated the population of Huntsville, Alabama at 168,132.