# Development and Testing of a Two-UAV Communication Relay System
## 1 Introduction  
In recent years, as a result of decreased weights and sizes, reduced costs, and increased functionalities of different sensors and components, Unmanned Aerial Vehicles (UAVs) are becoming increasingly popular in a wide range of domestic applications (photography, surveillance, environment monitoring, search-and-rescue, etc.) [1; 2; 3]. As the market develops and the variety of applications expands, more requirements are now being imposed on extending the flight range and increasing the adaptability to the complex missions of UAV systems. In missions such are surveillance and search-and-rescue, UAV systems are usually required to operate in complicated environments while still being able to maintain uninterrupted communication with the ground control station for the purpose of reporting, monitoring, and control. For example, in the mountainous or high-density urban regions, the communication between UAVs and the ground control station can be easily interrupted, causing the ground control station to lose real-time data feedback from the UAVs, leading to mission failure. In addition, if the mission requires long flight range, the communication might also be interrupted by the distance.  
Commercially available yet economical UAV communication solutions, however, have a number of limitations that potentially hinder their widespread application. First, the communication range of major civil UAV communication solutions is comparatively short. Telemetry modules, such as 3 DRSik2, XBee, and other Wi-Fi modules, which are common options for civil UAVs, have communication ranges that are limited to a few kilometers. Second, it is difficult to establish a stable data link when obstacles such as trees, tall buildings, or mountains separate the UAVs and GSC.  
In situations discussed above, it can be difficult to complete the mission with only one UAV deployed, while keeping cost and system complexity requirements met [4; 5; 6; 7]. To harness the advantages of UAVs in these situations and minimize the drawbacks at the same time, an alternative solution is to deploy a multi-UAV system, which could utilize the inter-connectivity among multiple UAVs to maintain uninterrupted communication between every UAV and the ground control station. In this study, a UAV communication relay solution was developed, which uses relay and routing to extend the communication range and bypass obstacles at low cost. The objective was to develop and test a proof-of-concept two-UAV system that demonstrated the ability to relay radio communication. This system only consisted of two UAVs, but its design could accommodate the addition of more UAVs. The system uses one UAV as communication relay point and enables the other UAV to operate in areas where direct communication with ground control cannot be established. This configuration allows communication to be established across obstacles or over a distance exceeding the range of the onboard radio transceiver, both of which will be demonstrated in field tests later. The typical application scenario of this system is shown in Figure 1 schematically.  
The paper is organized as follows. Section 2 introduced the related works. Section 3 introduces the hardware configurations of the multiple UAVs communication relay system. Section 4 discusses the software aspects of the system. In Section 5, both the indoor and outdoor tests methods and results are presented. A conclusion of the work is given in Section 6.