# Compact Bistatic Iterative Passive Radar Based on Terrestrial Digital Video Broadcasting Signals
## 1 Introduction  
Nowadays, everything is connected, and knowing devices' exact positions is desirable. Knowledge of devices' locations is very important for a variety of applications such as fleet management, operation and maintenance, and performance enhancement. When devices are connected to a communication network, especially mobile networks such as 4G or 5G [1], their signals can be exploited for those purposes [2]. However, if the device is not connected or is connected to a different network (particularly if localization needs to be avoided), the situation becomes more complicated, and traditional radar methods need to be augmented. Traditional (active) radar involves sending a specifically designed signal in a particular direction and waiting for the echo of this signal, using it to then estimate the position of the target. The advantages of these systems are their strength and accuracy [3]. Recent proposals have made use of re-configurable surfaces to improve performance [4]. Besides, in order to solve the problem of small-sized low-flying objects like drones, some new approaches are being proposed to leverage the background radar and reflections on the Earth [5]. However, they have also exhibited some significant drawbacks; they are easily located and destroyed (if necessary), and countermeasures can be designed to skip (i.e., avoid or even cheat) these signals, leading to stealth aircraft. Moreover, they are expensive to deploy and maintain. In efforts to mitigate these drawbacks without compromisingaccuracy, passive radars have become very popular nowadays [6]. Indeed, passive radar are intrinsically related to one of the current hot topics, namely integrated sensing ad communications (ISACs) which jointly use communication signals for sensing [7]. Their central concept involves using another signals already in the environment such as digital video broadcasting (DVB-T) [8; 9; 10], frequency modulation(FM) [11], cellular [12] or a mixture (fusion) of all of them [13] including satellite transmissions [14] as primary signals, and evaluate their reflection into the targets to estimate the position. Within this method, no new specifically designed signal is transmitted, so the radar becomes invisible to the enemy; indeed, they cannot know whether there is a radar. Moreover, countermeasures are complex because they need to damage legacy signals; as a result, any interference can be detected rapidly. Furthermore, since no extra bandwidth or licensing is required, the system is cheaper to operate. This method's advantages make it an attractive prospect [15]. However, those methods also present some drawbacks. The main one is that they are less accurate than their active counterparts because they use signal which are not well suited or designed for positioning. Another challenge is that the signal is usually broadcast and it is not intended to be in the sky but more close to the Earth [6]. DVB-T passive radar has a great number of potential applications. Since the coverage of this signal is extremely broad (covering cities, countryside, roads, wide open areas, etc. ), the implementation of a passive radar based on this signal is an extensive undertaking. Moreover, DVB-T passive radar equipment is cheap, and it can be implemented cheaply to improve the surveillance capabilities of other systems. For example, in [16], a counter and tracking drone system was proposed to increase privacy. Its main disadvantage was its accuracy, because the signals used were not specifically designed for this purpose; such signals were broadcast and not sent in a particular direction [17]. To solve this, several transmitters or receivers can be used in a multi-static configuration. These passive radars align with recent trends in integrated sensing and communications (ISAC) philosophy in 6G [18].