# Shape Optimization of an Integrated Mast for RCS Reduction of a Stealth Naval Vessel
## 1 Introduction  
The rapid development of radar technology has made it possible to detect the presence of enemy vessels early but it has become difficult to operate or hide without notifying the enemy of the presence of friendly naval ships. In other words, the survivability of friendly naval ships has also been threatened [1]. Thus, it is essential to apply low observable technologies to naval ships to avoid detection by enemy radar. Due to the fact that the smaller a radar cross section is (RCS), the better the naval ships can evade radar detection, RCS reduction is a very important factor in stealth technology [1, 2]. Electromagnetic (EM) stealth technology focuses on reducing the RCS and it has become a vital design goal of tactical naval vessels [2, 3, 4]. The Zumwalt destroyer, a state-of-the-art United States Navy destroyer, is designed to have a remarkably low RCS. Although the Zumwalt destroyer is much larger than any other active destroyer or cruiser such as an Arleigh Burke-class destroyer its RCS is similar to a fishing boat. For this extreme stealth-oriented design, an integrated mast module system must be developed [5]. The dozens of radars, sensors and antennas installed on conventional naval vessels severely increase the RCS of naval vessels due to the influence of multiple reflections whereas the integrated mast consists of a housing that accommodates all major radars, sensors and antennas of naval vessels, which can dramatically reduce the RCS of naval vessels. As enemy radars in distant areas may only detect the integrated mast due to the curvature of the earth, the RCS of the naval ship and also the RCS of the integrated mast itself are very important in the stealth ship. Therefore, it is necessary to design the integrated mast with low observable characteristics. The most practical techniques for designing the integrated mast with a low RCS include shaping techniques and radar absorbing materials (RAM)/radar absorbing structures (RAS). Extensive studies have been conducted to develop RAM/RAS for RCS reduction [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Composite radar absorbers have been fabricated and measured based on the composite material [6; 7; 8], the metasurface [9; 10] and the reactive Salisbury screen [11]. Furthermore, the RCS of naval ships covered with RAM was numerically analyzed to evaluate its stealth performance [12; 13; 14; 15; 16] and the RCS of an integrated mast coated with multi-layered RAM was also analyzed [17]. However, they usually have a narrow absorption bandwidth and are too bulky and fragile in operational environments, which incurs astronomical costs for its ongoing maintenance. Furthermore, the surface of the integrated mast that can be coated with RAM/RAS is very limited due to the dozens of radars, sensors and antennas installed on the integrated mast. Therefore, RAM/RAS techniques are rarely applied to operational naval ships. On the other hand, shaping techniques must be used in stealth design because it is the most effective technique that can be used to reduce the RCS [2]. Shaping techniques are generally first used to have low RCS properties in the primary threat sectors before RAM/RAS techniques are applied. As the shape of the integrated mast is one of the most important factors and must be considered in the early stages of the concept/design, it is necessary to know in advance the characteristics of the RCS of the integrated mast and the naval vessel equipped with the integrated mast. To investigate the RCS characteristics of naval vessels, there have been many studies [13; 18; 19; 20; 21; 22]. The RCS of naval vessels has been investigated [18; 19] and its properties have been analyzed according to the side slope of the superstructure [13] and exterior equipment arrangement such as the sensor and radar [20]. However, related studies on the RCS of the integrated mast itself seem to be lacking.