# Research Progress on Synergistic Technologies of Agricultural Multi-Robots
## 1 Introduction  
With the constant progression of urbanization and industrialization, the mobility of rural young and middle-aged laborers has intensified [1]. The sustainable transfer of non-agricultural labor has led to a decline in agricultural labor, and the problem of aging of the agricultural labor force has become more serious [2]. For example, the proportion of agricultural production and management personnel aged 55 and over is as high as 33.6% according to the main data of the Third Agricultural Census Bulletin of China in 2017. Furthermore, as aging continues, the physical health of the elderly labor force continues to decline, which results in a significant reduction in the supply of effective rural labor and an adverse effect on agricultural output [3, 4]. Most agricultural production tasks are labor-intensive and seasonally oriented projects that exacerbate the constraint of seasonal labor shortages [3, 4, 5] and increase the cost of agricultural labor. For example, according to a survey conducted by [PERSON] et al. [6], during the rural busy season, the labor cost of agricultural planting increased from 80 CNY per person per day in 2015 to 90 CNY in 2016 to 100 CNY per person per day in 2017, and the labor cost of technical agricultural labor is even higher [7]. Some statistics show that the agricultural unit labor cost in developedcountries such as Germany, Japan, and the United States decreased by 30.17%, 44.22%, and 23.44%, respectively, while agricultural labor productivity increased by 64.78%, 81.22%, and 34.83%, respectively, from 2005 to 2014. On the contrary, China's agricultural unit labor cost increased by 45.17%, and agricultural labor productivity, which is much smaller than that of developed countries, increased by 50.28% [8]. Therefore, the increasing cost of labor will lead to an increase in the cost of agriculture, which will result in a relative decrease in agricultural productivity and international competitiveness [9]. It is urgent to enhance the innovation of agricultural science and technology and replace extensive and expensive repetitive manual operations with intelligent agricultural machines or robots [10]. Due to the growing maturity of computer technology, sensor technology, and control theory, different types of agricultural robots have been developed based on characteristics of agronomy, such as fruit- or vegetable-picking robots, spraying robots, and harvesting robots. The agricultural robot can replace traditional human efforts to engage in all kinds of labor-intensive and complicated agricultural production activities and reduce the decline of output caused by improper human operation, negligence, inaccurate operation, and other reasons, as well as major physical injuries and even casualties of operators [11]. However, the operation efficiency of a single agricultural robot is too low and cannot meet the operation demand in busy seasons without coordination and cooperation by artificial auxiliary resources or other robots [12]. As early as 2009 and 2012, [PERSON] et al. [13], [PERSON] et al. [14] and others in the United States replaced a single robot with a group of agricultural mobile robots to complete mud moss harvesting and orchard spraying successively with appropriate cooperative operation mechanism, which can reduce production costs and improve operational efficiency [15]. Therefore, to adapt to the increasing scale of production, meet the needs of social development, and narrow the gap with other international world powers with advanced scientific capabilities, it is necessary to research the relevant technology of agricultural multiple robot systems. This work focuses on the research progress of the cooperative operation, one of the key technologies of agricultural multiple robot systems. There are many types of agricultural multi-robots, and this article mainly focuses on unmanned aerial vehicles (UAVs), unmanned ground vehicles (UGVs), and unmanned underwater vehicles (UUVs).