## 3 Implementation and Testing of an Indoor GPS System  
The Marvelmind navigation system is an off-the-shelf indoor navigation system designed to provide precise location data (\(\pm\)2 cm) for autonomous robots and vehicles (AGV--automated guided vehicles). It can also track moving objects through attached mobile beacons. This navigation system includes fixed ultrasonic beacons connected via radio, one or more mobile beacons on objects being tracked, and a router providing access to a computer or peripheral device for monitoring. The mobile beacon's location is calculated based on the ultrasonic time of flight (TOF) delay between stationary and mobile beacons using a trilateration algorithm. The system operates by measuring the distance between beacons and mobile devices using ultrasonic signals and trilateration algorithms. Marvelmind systems can cover distances up to 50 m between beacons, depending on environmental conditions such as obstacles or interference. The total coverage area can be extended by adding more beacons to create a larger grid. This scalability makes it suitable for warehouses, robotics, and industrial applications, ensuring reliable and accurate indoor tracking. The system uses ultrasonic signals combined with trilateration to calculate positions with centimeter-level accuracy. To cover larger areas, additional beacons can be deployed, forming a scalable grid system that significantly extends the overall coverage area. By strategically placing beacons, Marvelmind can efficiently track objects or robots across extensive indoor spaces such as warehouses, factories, and office buildings. The system is composed of stationary and mobile beacons as well as a modem needed to communicate both with the beacons and with an appropriate application. Regarding the stationary beacons (Figure 2), they are usually mounted on walls or ceilings above the robot, with ultrasonic sensors facing downwards to achieve optimal ultrasonic signal coverage. For automatic landing and indoor navigation of copters, it is recommended to install a mobile beacon on the bottom of the flying system, oriented downwards. The placement and orientation of the beacons should be configured to ensure maximum coverage of the ultrasonic signal. The system's effectiveness largely depends on the quality of the ultrasonic signal received by the stationary beacons. During the initial mapping configuration, stationary beacons emit and receive ultrasonic signals. Switching between roles (stationary and mobile) is accomplished via the application software and does not require and hardware modifications. Figure 2: Stationary/mobile beacon. As stated before, an important part of the system is the router, which acts as the central data aggregation point for the system. It must remain powered while the navigation system is operational. The router also enables configuration, monitoring, and dashboard interaction. It can be placed anywhere within radio coverage, typically up to 100 m with antennas from the starter kit. The router is represented in Figure 3.  
The indoor positioning system is accompanied by an application for the initial programming and configuration of the beacons. The application's front-end dashboard is shown in Figure 4.  
To obtain preliminary results and perform a general functionality test, the system was set up in a laboratory, where initial measurements were conducted. Figure 5 shows the equipment setup used for configuring the workspace, consisting of five beacons and a modem. Figure 4: Marvelmind dashboard. Figure 3: Router. For the initial testing of the Marvelmind indoor positioning system, a configuration with two fixed and three mobile beacons (2D setup) was chosen. In this configuration, two stationary beacons were mounted at a height of 1 m from the perimeter of the test area.