Acoustic Littoral Engagement Response to Threats – ALERT A System to Provide Advanced Warning of an Anti-Ship Missile Attack

Naval warfare is shifting from an almost exclusive deep water, open ocean engagement to a littoral environment where high-value surface ships are forced to operate in close proximity to land, exposing those ships to a new, significant threat the high speed, sea skimming missile launched from coastal defenses, mobile land launchers, and patrol craft and maritime aircraft operating close to shore. This is a formidable threat. The anti-ship missile is a low radar signature target, making it difficult to detect in any environment. As a sea-skimmer, the missile trajectory is low on the radar's scan area, and in a high clutter region, adding to the detection difficulty. However, in this case, the clutter is magnified by the land background. With missile speeds in excess of 3⁄4 Mach, there is precious little time between detection and missile impact in which to engage this threat. The Georgia Tech Research Institute (GTRI) at the Georgia Institute of Technology has investigated a concept to provide early detection of these sea-skimming missiles and add additional response time for the surface ship. The Acoustic Littoral Engagement Response to Threats (ALERT) system employs an underwater vertical transducer array to detect airborne acoustic noise from the missile that penetrates the air-water boundary. The system capitalizes on the 6 dB signal enhancement due to the pressure-doubling effect at the air-water boundary to improve detectability of the signal, plus the factor of five increase in sound speed in water versus air to reduce detection time. This paper describes the operational ALERT concept, system description, and analytical assessment of system performance. Signal processing of the information provided by the ALERT sensor can also provide a bearing to the missile and an indication of missile speed, enhancing the deployment of defensive systems. To evaluate the feasibility of the ALERT concept, GTRI conducted an internal R&D acoustic measurement program in a lake close to Georgia Tech to measure the detectability of airborne acoustic signals with an underwater transducer array. Results of these tests are presented. The paper also presents potential problems and issues associated with the operational implementation of the concept. 1.0 INTRODUCTION For more than a decade, naval forces throughout the world have experienced significant increased importance of missions in the littoral environment, which has resulted in major cultural, tactical. operational, and systems changes for both naval powers such as the US, who were primarily oriented towards blue-water operations and for those navies whose primary missions were homeland security and defense. The littoral is a harsh, high-threat environment. Shallow water depths limit the speed and maneuverability of large ships; reduce the effectiveness of acoustic, RF, optical, and infrared sensors for detection and navigation; and restrict the accessibility of key operating regions. And, the threat of mines in these operating areas is ever-present. Add to this already harsh environment the increased threat from high speed, sea skimming, anti-ship missiles, launched from coastal defenses, mobile land launchers, high-speed surface patrol craft, maritime aircraft (both fixed-wing and rotary-wing), shoulder-fired manpads, or disguised civilian and RTO-MP-SET-079 19 1 Paper presented at the RTO SET Symposium on “Capabilities of Acoustics in Air-Ground and Maritime Reconnaissance, Target Classification and Identification”, held in Lerici, Italy, 26-28 April 2004, and published in RTO-MP-SET-079. UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED