# Autonomous Balloon Controls for Protection against Projectiles with Known Destinations
## 1 Introduction  
Consider the problem of protection against projectiles with known destinations. We introduce a defense strategy against an enemy projectile (EP) trying to reach its destination, such as a military base, which is known a priori. We further assume that the location of the platform that launches the EP is known a priori. Since both the platform and the EP's destination are known a priori, we can predict the trajectory of the EP before the EP is launched. The proposed defense strategy is to deploy multiple balloons on the EP's viable trajectories so that they block the incoming EP effectively. Each balloon has GPS sensors for locating itself and IR sensors to detect an incoming projectile. Once the EP is sufficiently close to a balloon, the balloon explodes to destroy the EP. Since the EP's purpose is reaching its destination, the balloons can effectively intercept the EP utilizing this blocking strategy. In the past, a fast interceptor has been widely used to destroy a fast-moving EP [1]. However, an interceptor needs to be controlled precisely, in order to hit a fast-moving EP. For this interception, the target's real-time position must be measured precisely using expensive radar systems. Moreover, an interceptor must move faster than a fast-moving EP, in order to capture the EP. Thus, an interceptor is much more expensive than an EP. Since an interceptor is destroyed after the engagement, we argue that this traditional interceptor system is not suitable for protection against cheap EPs. Since balloons move much slower than an EP, we can consider a balloon as a static interceptor capturing an EP. In other words, an EP can be considered as a target, and our goal is to destroy the target utilizing a static interceptor (balloon). The proposed defense strategy is useful when many cheap EPs are launched towards their destination simultaneously. We can deploy balloons considering the position of each EP, and the balloons can block the path of each EP effectively. The balloons provide cheap and effective defense systems against the attack of cheap EPs. The References [2; 3; 4] utilized motion camouflage to develop the motion control of an interceptor. Here, we say that the interceptor is in the motion camouflage state if the interceptor approaches a target while appearing stationary at a focal point. References [5; 6] addressed a 2D motion camouflage control with respect to a static focal point. In [6], a neural network architecture was utilized to achieve simulated motion camouflage. Proportional Navigation Guidance (PNG) laws were widely utilized to let an interceptor capture a target [1; 7]. PNG laws were addressed considering an interceptor, which can measure the bearing of the target utilizing on-board sensors. Command guidance is a control, such that a base station relays signals to a guided interceptor via radio control. The principle of command to line-of-sight (CLOS) guidance [8; 9] is to make the interceptor fly as near as possible along the LOS line. With the advancement of anti-missile technology, cooperative multi-missile attack and defense is attracting increasing attention. The authors of [10] proposed a guidance law with controllable attack time and angle-of-attack constraint and applied it to the salvo attack of anti-ship missiles. With progress in the consensus of multi-agent systems, researchers have begun to use the consensus theory to study the cooperative guidance and control of multi-interceptors. Using the coordination strategy under the cooperative guidance framework, Reference [11] adjusted missile trajectories so that the coordination variable of each missile can approach the expected coordination variable for realizing cooperative guidance. Reference [2] considered a team of aerial robots towing the boundary of a capture net so that the net intercepts a fast unmanned aerial vehicle (UAV) with variable velocity. The robot team tows the net boundary so that the net captures the targeted UAV as fast as possible.