The Missile Impact Location System or Missile Impact Locating System (MILS) is an ocean acoustic detection system designed to locate the impact position of test missile nose cones at the ocean's surface and then the position of the cone itself for recovery from the ocean bottom. The systems were installed in the missile test ranges managed by the U.S. Air Force. The systems were first installed in the Eastern Range, at the time the Atlantic Missile Range, and secondly in the Pacific, then known as the Pacific Missile Range. The Atlantic Missile Impact Location System and Pacific Missile Impact Location System were installed from 1958 through 1960. Design and development was by American Telephone and Telegraph Company (AT&T), with its Bell Laboratories research and Western Electric manufacturing elements and was to an extent based on the company's technology and experience developing and deploying the Navy's then-classified Sound Surveillance System (SOSUS). Early studies at Bell Laboratories' Underwater Systems Development Department examined the problem then the Bell System's other organizations began implementation. The company and Navy assets that had installed the first phase of SOSUS, starting in 1951, were engaged in MILS installation and activation. MILS took several forms, and each had a unique configuration based on purpose and local water column and bottom conditions. The target arrays were bottom-fixed hydrophones connected by cable to the shore stations. A variant, Sonobuoy MILS (SMILS), was composed of bottom mounted hydrophones augmented by air dropped sonobuoys when in use. The third covered wide ocean areas with fixed hydrophones at distant shore sites was termed broad ocean area (BOA) MILS. All systems exploited the SOFAR channel, also known as the deep sound channel, for long range sound propagation in the ocean.
Target arrays The target arrays received the acoustic effect of an object's impact with the ocean surface then by the effect of an explosive charge with location calculated by the difference in arrival times at the hydrophones arranged to form a rough pentagon with a sixth hydrophone at the center. A particular advantage of the pentagon configuration was that a rapid approximate position could be calculated on simple time sequence of the acoustic wave at the hydrophones, with detailed analysis producing a more exact location. The effectiveness depended on placement of the hydrophone in the deep sound channel. Since the downrange islands did not offer ocean bottom at that depth in the required configuration a system of suspended hydrophones was used. The difficulty of computing the calibration results for the Atlantic systems led to development of computer programs that became the standard for MILS operational data solutions. The distant placement of the systems revealed the limitations of the existing world geodetic system, with various datum systems based on the local geoid, something that would be solved by satellite systems that would develop the means to tie everything together. Target arrays were high-accuracy systems usually covering a target area of about 10 nmi (12 mi; 19 km) radius. The Atlantic MILS target arrays were located down range from Cape Canaveral about 700 nmi (810 mi; 1,300 km) at Grand Turk Island, 1,300 nmi (1,500 mi; 2,400 km) at Antigua and 4,400 nmi (5,100 mi; 8,100 km) at Ascension Island. The Pacific Missile Range (PMR), then Navy-managed as a complex of ranges, was one of the three national missile ranges. PMR began installation of a Pacific MILS to support Intermediate Range Ballistic Missile (IRBM) tests with impact areas northeast of Hawaii. That system terminated at the Marine Corps Air Station Kaneohe Bay. The IRBM array was operational November 1958. Tests of the Intercontinental Ballistic Missile (ICBM) required MILS monitoring impacts between Midway Island and Wake Island and between Wake Island and Eniwetok. The ICBM range was operational in May 1959 with two target arrays. One was located about 70 nmi (81 mi; 130 km) northeast of Wake and another in the corridor between Wake and Eniwetok. Shore facilities were at Kaneohe and each of the islands.
Broad ocean area (BOA MILS)
This system has less accuracy but extensive coverage area including whole ocean basins. It would cover test vehicles not making the target or other events not directly related to the accuracy tests. Accuracy was improved by pre test calibration by a ship precisely located by a fixed transponder field releasing SOFAR bombs. The BOA hydrophones were located near the deep sound channel axis and were located at Cape Hatteras, Bermuda, Eleuthera (Bahamas), Grand Turk, Puerto Rico, Antigua, Barbados and Ascension. In the Pacific a BOA system was installed to cover the Wake—Eniwetok—Midway impact area.
Experimental and other uses The BOA MILS sites were involved in events beyond missile testing. Those included both intentional experiments and acoustic incidents in which they were tasked after the fact to examine records. In some experiments MILS was a major participant while in others participation was mainly monitoring and contributing data. An example of that monitoring role is the nuclear shot "Sword Fish" in Operation Dominic in which both MILS and SOSUS operated normally simply making recordings and strip charts for a period before the detonation until several hours after. Data has also been provided to support research and support for the International Monitoring System monitoring for nuclear weapons tests. That effort also monitors earthquakes.
Acoustic propagation research
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