# UAV Formation Trajectory Planning Algorithms: A Review
## 1 Introduction  
Since its outstanding performance in the Gulf War in 1991, drones have made good achievements in the Afghanistan War, the Iraq War, the fight against the Islamic State (ISIS) terrorist group, the \"Neptune Spear\" decapitation operation in 2011, and the Russia-Ukraine conflict in 2022. Their success has caused countries around the world to invest a large amount of manpower and financial resources in the research of UAV [1], as shown in Figure 1. After decades of development, UAVs have not only been applied in the military fields of reconnaissance, surveillance, communication relay, electronic countermeasures, combat assessment, harassment, decoy, anti-submarine, target attack, etc. At the same time, they have been widely used in agriculture [2], energy [3], civil [4] and other very important fields. However, there are some problems with a single drone performing its mission. For example, when a single UAV performs a reconnaissance mission, it may be limited by the observation angle and cannot observe the target area from multiple different orientations [5]; when faced with a large-scale search task, a single UAV cannot effectively cover the entire reconnaissance area [6]; during the attack, the combat range, killing radius, destruction capability and attack accuracy are limited, thus affecting the success rate of the entire combat mission [7]; if a single drone fails in the middle of a mission, it must immediately interrupt the mission and return, but in a war, it may delay the aircraft and destroy the entire operation plan. In order to improve combat effectiveness and make up for the deficiency of a single UAV, a multi-UAV cooperative formation (cluster) combat task is proposed. It refers to the formation, maintenance or reconstruction of a certain geometric formation during the execution of a task by multiple UAVs to adapt to the battlefield situation and task requirements. Unmanned aerial vehicle formation has incomparable advantages over a single unmanned aerial vehicle. When UAV formation is not possible in extreme weather, maintaining different formations can improve the aerodynamic efficiency of UAVs to varying degrees, thus reducing the overall flight resistance and saving fuel [8]; it can realize the all-round reconnaissance or observation of the target, such as the enemy target monitoring and reconnaissance, resource exploration and so on, and it can greatly improve the scope of target monitoring; 0069t can also realize simultaneous strikes on multiple mission targets, disrupt the enemy's combat command system, improve the lethality and hit rate of targets, and improve combat effectiveness, as shown in Figure 2. In the process of use, UAVs are equipped with intelligent devices, which can simulate the transport environment in real time, determine their own position, control their flight status, select effective trajectory points, and calculate safe trajectory. These are important guarantees for UAVs formation to reach the target point from the take-off point as well as important prerequisites for a UAV formation to complete tasks. Therefore, it is important to select a suitable algorithm for UAV formation trajectory planning. The purpose of trajectory planning of UAV formation is based on the specific tasks, terrain, weather and other environmental factors of each UAV as well as its own flight  
Figure 1: Statistics and forecast of global UAV market size from 2015 to 2024 (data source: Drone II): **(a)** Global UAV market 2015–2024 (blue column: global UAV investment; yellow line: upward trend); **(b)** Global UAV segment market share (blue column: proportion of consumer drones; yellow column: proportion of industrial drones). Figure 2: Unmanned aerial vehicles fly in formation to perform tasks. **(a)** UAV formation flight; **(b)** UAV formation electronic warfare; **(c)** Unmanned aerial vehicle (UAV) formation performs mission; **(d)** UAV formation communication relay; **(e)** UAV formation strikes target; **(f)** UAV formation reconnaissance. performance.