## 5 Discussion and Conclusions  
In this paper, the concept of applying the reassignment technique in passive FSR applications has been proposed. The main purpose of using this method was to enhance the readability of the energy distribution in the TF domain, which improved the result of the Hough transform and finally the precision of the Doppler rate estimation in the passive FSR system. In the considered case, the passive FSR system used a DVB-T VHF signal as a source of illumination, and the cooperating target (Cessna aircraft) crossed the baseline between the signal transmitter and the passive radar demonstrator. The investigation showed that the low-frequency illuminating signal allows detection in the specific geometry to be carried out and, additionally, by using the concept of energy concentration the signal processing pipeline may be improved. Such an improved energy representation can be utilized in further processing for different purposes, such as the estimation of the maneuvering target trajectory for the passive FSR configuration where the range information is ambiguous and the only data describing the target is its Doppler rate related to the velocity and the trajectory. The concept of using passive FSR systems may be implemented in various real-life systems known from the literature as well as in completely new applications. The authors consider realization of the technique in such applications:  
* Border surveillance--the system may prevent to illegal transport and emigration outside the border crossing. In such a case, the illuminator of opportunity can be located both, in the same region as the receiver and in a foreign terrain which expands the possibility of the systems. In addition, passive FSR with other radars with rotating antenna as a source of illumination can be used in such scenario to detect and localize slow moving targets, which can be an extension of work described by [PERSON] et al. (2018), where a cooperative transmitter was employed as an illuminator of opportunity. * Debris detection--a passive FSR system can detect and estimate movement parameters of debris which may be of particular importance for space applications. In that case, the baseline can be established between a stationary receiver and the transmitter in the orbit working with one of the popular radiocommunication systems (Starlink, GPS, etc.). Such configuration may allow fast and precise debris detection in order to prevent other satellites destruction ([PERSON] et al., 2018; [PERSON] et al., 2018). * Airport runway--the problem of airport runway security arises simultaneously with technology related to drones production and development. The passive FSR system may be deployed at the airport and detect non-cooperating objects appearing in the flight path of the aircraft. The system can be particularly useful for facilities not equipped with a transponder. In order not to employ another signal transmitter, an Airport Surveillance Radar with a rotating antenna may be used, which is routinely available at most airports ([PERSON] et al., 2018). * Ground aerial monitoring--where traffic is prohibited or where the velocity is strictly limited the passive FSR system and especially the algorithm proposed in this paper may be useful. The Doppler rate estimate may be an efficient solution for regions in which the velocity of cars has to be restricted and precisely monitored. * Intruder detection--another application of the system in question is its usage in security systems for restricted areas, e.g., military or governmental areas. Implementation of the passive radar receiver in such a terrain when surrounding commercial transmitters (e.g., DVB-T) may improve detection capabilities and increase security. In the future, the authors intend to work on the above applications and their implementation in passive FSR systems. An additional perspective is to investigate the possibility of applying real-time processing for the methods presented in this paper. Formal analysis, M.P. and K.A. ; investigation, M.P. and K.A.