## 1 Introduction  
The search for injured people in outdoor environments has always been a hot topic in the field of social public safety and emergency rescue medicine, and mainly include two types [1]. The first search scenario is about the trapped survivors under ruins in an abnormal post-disaster environment, such as natural or sudden disasters (earthquakes, building collapses, landslides, etc.). The detection challenge under this scenario is how to penetrate the ruins to reliably detect weak physiological movements of surviving human beings in the post-disaster site with relatively limited area. To address this problem, a new _bio-radar_ detection technology, which is a combination of biomedical engineering technology and radar technology, was firstly proposed by our group in the academic field. Bio-radar emits electromagnetic waves to detect survivors' physiological activities (breathing and heartbeat) through the ruins and corresponding vital signs would be acquired by demodulating the radar echo. Leveraging the bio-radar and various signal processing technologies [2; 3; 4], a series of bio-radar equipment was developed, and corresponding functions are gradually enriched so that we can not only detect the vital signs [5; 6; 7; 8] but also localized information. Specifically, with our proposed equipment and algorithms, we can even detect multiple (Max. 3) survivors simultaneously [9; 10]. Currently, our latest technologies are aiming to distinguish between human and animals (non-human targets) under ruins [11; 12], and even recognize human activities [13; 14]. In practical applications, typical equipment we have developed was also successfully used in many search-and-rescue operations (such as \"SJ-3000\" and \"SJ-6000\" search-and-rescue UWB bio-radar used in the Wenchuan earthquake in 2008, Yushu earthquake in 2010, Ludian earthquake in 2014, and so on in China) and made a great contribution. Another widespread and frequently occurring search scenario is about the injured person in a normal outdoor environment, namely a natural environment, such as the lost donkeys, crashed parachute jumpers, and wilderness travelers almost submerged in the vast and diverse natural environment. For example, during a distressing disaster at a sporting event, a large number of athletes suffered a safety accident due to the sudden change of weather in the 2021 4 th Yellow River Shilin Mountain Marathon 100 km cross-country race in China [15]. Many athletes experienced the severe hypothermia phenomenon and were trapped in the mountains, making it urgent but difficult to locate and detect them quickly. However, due to the lack of air-to-ground rapid search and location technology, the information of the trapped people in the outdoor environment was not sent to the rear security center in time, resulting in a number of deaths that could not be treated in time. In general, human target search technology in an outdoor natural environment can be divided into constrained and unconstrained modes. For constrained methods, humans need pre-wear auxiliary positioning devices, such as portable radio stations, wearable GPS personal terminals, or wireless search devices with vital signs monitoring functions. Nevertheless, these methods suffer from some inevitable deficiencies including increasing body load, inconvenient operation, and high costs [16; 17; 18]. Moreover, in the above scenario, our searching targets are often located in some extreme mountain forest environments or carry-on devices might have been seriously damaged, making it hard or even incapable to receive the information. Instead of wearing the auxiliary device, the unconstrained search technology based on unmanned detection technology could effectively avoid the above problems [19]. Currently, there is indeed some unconstrainedly unmanned aerial vehicle (UAV)-based air-to-ground detection technologies for human target search in ideal background environments.