## 1 Introduction  
Passive Coherent Location radars, shortly called \"passive radars\", are highly advanced radiolocation systems dedicated for the detection of ground and air moving targets. The main idea of such a radar is based on the reception of a group of signals emitted by the other transmitters called transmitters of opportunity. These signals may be of different kind, e.g., frequency-modulated (FM) radio, Digital Video Broadcasting (DVB-T) or The Global System for Mobile Communications (GSM) [1].  
The PCL system receives the opportunity signal in two ways: first is the signal directly from a transmitter, second is the signal reflected from an object of interest (target), as shown in Figure 1. Then, the comparison and specialized analysis of these two signals as a result allows the detection of an object's presence and its distance and velocity [2, 3, 4]. The main part of this receiver is an antenna and a low-noise amplifier. This kind of system does not emit a signal to illuminate a target. It relies exclusively on the signals emitted from the transmitters of opportunity. Therefore, theoretically, such a system is completely undetectable due to lack of any EM emission. This feature is frequently underlined by specialists, that the passive radar is regarded as impossible to be located [5]. Moreover, this feature causes that a passive radar cannot be attacked by an anti-radar missile. It seems to be the perfect sensor in the meaning of tactical application: inexpensive, easy to deploy, and undetectable for other sensors [6]. The advanced methods of signal processing developed nowadays allow to perform a higher-order analysis, e.g., target tracking or synthetic aperture imaging [7, 8] with such a system. There are also concepts and experiments when a PCL uses other non-cooperating active radars as the illuminators of opportunity [9].  
That is why there is an urgent need to develop a new kind of method of detection and localization of hidden PCL systems. In the paper, they are further called as Counter-PCL techniques, shortly C-PCL.  
There is a lack of open literature concerning detection of PCLs. The topic of countermeasures against PCLs has become more popular recently. There is the approach of passive radar jamming, with the use of various active methods [10, 11, 12].  
In the following analysis, further in the paper, it is assumed that a PCL system emits no signals; moreover, it is perfectly shielded in the EMC meaning. Then, the only way to penetrate the system hardware is through the receiving antenna.  
The paper concerns the analysis of a method of the remote detection of a PCL system. This method assumes the use of sounding signals emitted towards a PCL. The detection process relies on the reception of signals emitted back from a PCL input antenna, which are generated due to nonlinear effects in the PCL front-end caused by the excitation with the C-PCL sounding signals.  
The aim of the work is to analyze and validate the proposed method by theoretical calculations of the requirements for a system structure and measurements of the low-noise amplifier nonlinear response.  
In further sections of the paper, a basic idea of the method is presented. The issues related to the practical implementation are discussed. The input intermodulation effect is measured with the use of an example of a low-noise amplifier (LNA) and a special measurement setup. The intermodulation generation efficiency is derived from the measurement results.  
With the use of measured results, the analytical calculations of a model of the C-PCL system are performed. The relationships of transmitted power, detection range and antenna gains are calculated and presented. The possibility of the remote detection of a PCL system is evaluated.