# Using Ground-Based Passive Reflectors for Improving UAV Landing
## 1 Introduction
This problem can be solved in several ways [18; 19]; however, to use any of them, at least the following conditions must be met:  
* Landing must be carried out without the pilot (operator) performing additional operations to control the aircraft [20; 21];
* The algorithms for the operation of the automatic control system (ACS) for landing must remain standard (the same as using course-glide systems);
* The landing information support should be carried out by existing sensors [22];
* The developed algorithms of the automatic landing system should ensure its interface with other onboard systems, without their significant modification, including radar [23];
* The UAV control point display system should provide control of the entire landing process [24; 25; 26], including the landing and ground roll on the runway (RWY), during the instrument flight meteorological conditions, taking into account the fulfillment of all landing restrictions;
* Using an operator, the system must provide the possibility of landing in manual and director modes and in the absence of an operator, in an automatic mode;
* The system should not interfere with the landing of the aircraft using ground-based radio landing equipment during their operation, and should ensure further autonomous landing and mileage [27]. The system should have high mobility and provide deployment (folding) for a very short time (compared to ground-based radio and lighting equipment) in any area of the surface suitable for landing, as well as be very economical, not requiring large material and human costs. It should be mentioned that the constantly improving indicators of the air-borne computer system [28] and information sensors allow us to solve the problem of creating such a simplified version of the automatic landing system [29]. Section 2 describes the possible ways to implement an autonomous landing [30] on an airfield that is not equipped with radio engineering, based on the use of an onboard radar in the Earth survey mode, with processing signals coming from corner reflectors or responder beacons located in a special way near the runway. The more developed options of UAV landing supporting systems can be based on onboard imaging systems. Such systems may function in different ranges of electromagnetic waves, leading to varying technical solutions, yet sharing a similar theoretical framework. The system exploiting the millimeter wave radar, which provides a spatial resolution of 2.5 m, is given in [31]. A good example of the onboard imaging system based on an infrared camera is presented in [32], where infrared lamps are assumed to be placed in particular points within a runway, while the onboard receiving camera is equipped with a color filter, making it sensitive to the near-infrared radiation. Both above-mentioned systems require a significant computational recourse to perform the data processing onboard. Alternatively, theses [33] describes the low-cost prototype of an optic system based on a monocular video camera onboard a slowly moving quadrotor. However, the main processing in that system is performed on a ground-based computer to where the video stream is transferred. The systems based on deep image recognition can be considered as a viable modern alternative to assisting the navigation system of the UAV during its landing. A practical realization of such a system is proposed in [34] and described in greater details in [35]. Thatsystem includes the network consisting of optic stereo cameras placed on pan-tilt units and the specialized data-processing workstation performing all required computation. The system supports flexible configurability and demonstrates high performance in many final approach scenarios. However, it requires the developed on-ground facilities, having to be deployed prior and properly maintained during its life cycle. This can turn out to be rather costly in regions with a harsh climate, especially during cold seasons, as well as for airfields deployed in areas distant from settlements or naturally occurring inhabitants.