# Using Ground-Based Passive Reflectors for Improving UAV Landing
## 3 Algorithm Synthesis  
### Concept Description of Using a Radar to Ensure the Aircraft's Landing  
In an autonomous landing system using an onboard radar [48; 49], the reference trajectory and its angular deviations can be calculated based on processing signals reflected from special reflectors [50] (passive repeaters). In the most general form, the possible corner placement relative to the runway is shown in Figure 4.  
Figure 3: The principle of the position determination in SLS.  
The reference trajectory in the horizontal plane can be formed by signals reflected from corner reflectors placed along the runway axis at some distance from its ends. The current deviations from the runway axis can be calculated from the aircraft position relative to the angles placed at a certain distance from the runway axis. In the absence of guidance errors, the distances from the aircraft to the corners placed on the side of the runway should be equal.  
Depending on the number of used reflectors and their locations, a large number of landing options can be implemented. It should be mentioned that, with an increase in the number of used reflectors due to averaging the measurement results, the methodological and fluctuation components of the measurement error can be reduced. At the same time, with an increase in the number of used reflectors, both the time for their installation and the time for preparing the landing place for receiving UAVs increase.  
In general, the mutual location of the aircraft and the characteristic points on the runway centerline during the guidance process on which the landing and further mileage are carried out can be implemented in various ways. Using signals reflected from reflectors with known coordinates, the location of the UAV can be determined on the basis of measuring the range to them (time-difference of arrival (TDOA) method) or on the basis of simultaneous measurements of the range to the corner reflectors and onboard bearings (angle of arrival (AOA) method). If the UAV position is estimated with a rather high accuracy using Doppler-inertial or inertial-satellite systems, then the knowledge of the exact coordinates of the reflector corners would not be required any longer.  
The choice of a specific method for determining the relative position of the UAV during its landing can be carried out during the simulation. A preliminary analysis has shown that the most acceptable are TDOA and AOA options, for determining the UAV location, using the course method of generating control signals with four corner reflectors. These methods for estimating the UAV location can also be implemented using active repeaters instead of corner reflectors that re-emit and amplify the input signal as shown in Figure 5. This approach allows reducing the required energy potential of the onboard radar during landing, but it requires the creation of power supply networks in the runway area.  
Let us further consider the issue of determining the UAV location in the process of it moving down and landing, using ground-based corner reflectors and a UAV radar.  
Figure 4: Landing system structure using the aircraft’s onboard radar.  
Figure 5: The system with active reflectors (repeaters).