## 2 Architecture of the Tethered Hexacopter System
### Navigational Equipment for Landing Pad Position Tracking  
This subsection provides a description of the navigational equipment required to implement precision UAV navigation. This equipment included a landing pad navigation station, an onboard computer, and an autopilot. * Landing pad navigation station  
The landing pad navigation station (LNS) served as a command, measurement, and communication apparatus. Its primary functions included measuring the navigation variables of the landing pad, managing the unmanned aerial vehicle (UAV)'s flight, and exchanging data with the UAV's onboard computer through a radio link. The navigation variables measured by the LNS included the landing pad's current position in the WGS-84 reference system, velocity, and heading. The LNS was responsible for transmitting take-off and landing directives to the UAV and exchanging navigation data with the UAV's onboard computer. The landing pad navigation station is presented in Figure 5. The station comprised a main computer (see 1 in Figure 5), two GNSS modules with antennas (primary 2 and secondary 3), an electronic compass 4, and a radio module 6 for communication between the LNS and the UAV. The station also featured a Wi-Fi router 5 to establish a service or supervisory connection with the main computer. The structural framework of the LNS was constructed using aluminum profiles connected by mounting brackets and captive nuts. Figure 5 presents the key dimensions of the station. All measuring devices within the LNS were linked to the computing unit located inside the main computer 1. The hardware platform of the computing unit was an embedded Kontron computer [27]. The navigation system was controlled by a dedicated application that ran on the computing unit. This application acted as the UAV's ground controll system (GCS) software, responsible for collecting measurement data from the station and managing the entire flight of the aircraft. The primary GNSS module determined the position and velocity of the landing pad in the WGS-84 reference system. Meanwhile, the secondary GNSS module calculated the heading of the landing pad by analyzing the positions of both the primary and secondary GNSS module antennas. Spherical trigonometric relationships were used to compute the heading accurately within approximately \(\pm 1^{\circ}\). To achieve this accuracy, the secondary GNSS operated in RTK (real-time kinematic) moving-base mode. The GNSS modules were interconnected via UART--the primary GNSS transmitted RTCMs to the second GNSS. Figure 4: Structure of the power cord unwinder: 1—frame, 2—winding roll, 3—guiding trolley with tension sensor (potentiometer), 4—AC asynchronous triple-phase motor, 5—belt transmissions, 6—damper, 7—50 m of power cord. On the right-hand side, there is a picture that presents the concept of tension measurement by the potentiometer. Furthermore, the separation distance between the GNSS antennas had to exceed 1 m [28]. The RTK functionality was delivered by GNSS U-blox modules [28]. To ensure system reliability when the RTK mode was unavailable, such as during adverse weather conditions, the navigation station included an electronic compass [29]. * Onboard computer  
The onboard computer served as a crucial component, enabling the execution of the positioning and navigation of the UAV. It was tasked with obtaining and filtering accurate relative positioning data based on the UAV's and base station's GNSS coordinates. This estimate was further enhanced by including position information from the local positioning system (the UWB system). In order to provide this functionality, the onboard computer serviced the UWB tag mounted on the airframe. The computer operated an ROS-based framework that used a Mavlink bridge to forward the messages between the autopilot and the ground control station. The Odroid onboard computer and the Pixhawk autopilot communicated via the UART protocol. To establish a downstream connection with the ground station, a radio link was employed, utilizing the serial protocol.