Item History: : In spring 1986, the United States accidentally hit the French Embassy in Tripoli, Libya, during a nighttime air strike aimed at Libyan military bases and suspected terrorist centers. As a result of this error, the Department of Defense contracted with the Boeing Company to modify the U.S. military’s Air Launched Cruise Missile (ALCM) with, among other changes, a navigation system that would improve the accuracy of weapons delivered under conditions of poor visibility. The Department of Defense asked for completion of the development, testing and fielding of these modifications within 12 months. The result would be the Conventional Air Launched Cruise Missile (CALCM), the Department of Defense’s first operational weapon with GPS navigation. The CALCM was initially fielded with an SPS SCR-100 receiver that could track the P-code that was being transmitted at the time. In 1989, the ALCMs were upgraded with the PPS SCR-200 receiver.
Rockwell Collins (then the Collins Government Avionics Division of Rockwell International) provided CALCM’s GPS single-channel aided receiver, the GPS SCR. To meet the aggressive schedule, the existing ALCM navigation software was adapted to replace TERCOM data with GPS data as the update source for the Inertial Navigation System. Additionally, the GPS SCR used inertial aiding to aid its tracking loops, enabling its single channel to acquire satellites and operate in a dynamic environment.
Three major restrictions steered the project into territory that was somewhat uncharted at the time, although well traveled today. First, the Department of Defense wanted to keep the GPS enhancements from being known and to reduce costs, so it permitted no changes to the B-52 aircraft that would launch the cruise missile. This meant the missile could have no observable GPS satellites before launch and no connection to aircraft antennae.
The GPS SCR was designed so it could acquire satellites immediately after launch, aided by velocity data from the missile’s aligned inertial navigation system.
Second, CALCMs needed to operate even after as much as 10 years in storage. The GPS receiver ordinarily would have a battery to maintain the almanac data it used during initial satellite acquisition, but the storage requirements made a battery undesirable. The solution: load fresh almanac data into the GPS SCR as part of the missile’s mission data, eliminating the need for a battery.
Third, since the GPS constellation consisted of only a small number of satellites at the time of the CALCM development, the GPS software was adapted to be able to provide a solution using a minimum of three satellites plus altitude.
The CALCM project was completed on schedule. After four years of operational testing, the military launched 35 CALCMs from seven B-52Gs that left Barksdale, Louisiana, for Iraq during the first day of the Gulf War in 1991. The military deemed the raid successful, achieving 85 to 91 percent of its objectives. CALCMs demonstrated that the U.S. military could maintain a global presence with weapons stationed in the United States that could reach targets anywhere in the world within hours.
J. T. Nielson, “The Untold Story of the CALCM: the Secret GPS Weapon Used in the Gulf War”, GPS World, Jan. 1995, pp 26-.32
J. T. Nielson, “GPS Conventional CALCM – The Untold Story of the Weapon Used to Start the Gulf War”, IEEE PLANS 1994, pp 474- 479.