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  <front>
    <journal-meta>
      <journal-title-group>
        <journal-title>Workshop on Artificial Intelligence and Formal Verification, Logic, Automata, and Synthesis,
September</journal-title>
      </journal-title-group>
    </journal-meta>
    <article-meta>
      <title-group>
        <article-title>AI-guided optimal deployments of drone-intercepting systems in large critical areas</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Marco Esposito</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Computer Science Dept., Sapienza University of Rome</institution>
          ,
          <country country="IT">Italy</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2021</year>
      </pub-date>
      <volume>22</volume>
      <issue>2021</issue>
      <fpage>0000</fpage>
      <lpage>0003</lpage>
      <abstract>
        <p>The problem of designing efective systems to prevent risks and hazards caused by drones in critical areas has gained significant attention in recent years. In this short paper, we introduce the idea of computing optimal deployments of anti-drone sensors in a given region using simulation-based optimisation via heuristic-guided intelligent search and a geometric modelling of the problem, and show preliminary results in a real-world scenario.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;derivative-free optimisation</kwd>
        <kwd>symbolic modelling</kwd>
        <kwd>anti-drone systems</kwd>
        <kwd>optimal sensor placement</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
    </sec>
    <sec id="sec-2">
      <title>2. Modelling</title>
      <p>
        Large critical areas often present certain characteristics that make it dificult to compute an
optimal deployment of anti-drone sensors. In [
        <xref ref-type="bibr" rid="ref3 ref4">3, 4</xref>
        ], the RoI has been discretised in cells of
hundreds of square meters each, in order to compute, via Mixed Integer Linear Programming
(MILP), Pseudo-Boolean and Satisfiability/Optimisation Modulo Theory solvers an optimal
placement of relay nodes able to convey, in a fault-tolerant way, the network trafic from wisely
placed RF sensors to the gateway. Such a discretised problem formulation led to the generation
of millions of constraints.
      </p>
      <p>Unfortunately, following similar methods to compute an optimal placement of the RF sensors
themselves would be unviable, as, to achieve the desired accuracy in the computation of the
quality of the deployment, a much finer discretisation of the RoI would be needed. Also,
diferently from relay nodes, optimal positions of anti-drone RF sensors could easily belong to
the 3D space, as some sensors could be optimally placed on, e.g., , the roofs or walls of buildings.
All this would make a suitably-discretised version of the problem intractably large.</p>
      <p>We introduce a computational geometry–based approach to represent the environment and
the possible positions of the sensors, by defining the RoI and its obstacles (such as buildings
and terrain asperities) as well as RoI priority regions and admissible sensor positions in terms
of bounded convex polytopes. This allows us to achieve any desired level of accuracy, enables
the eficient computation of the coverage of a given deployment via computational geometry
techniques, and paves the way to the use of simulation-based optimisation approaches.</p>
    </sec>
    <sec id="sec-3">
      <title>3. Computing optimised deployments of sensors</title>
      <p>
        Our goal is to compute a deployment of a given set of sensors over a 3D region, optimising the
linear combination of two (possibly conflicting) performance metrics: (1) the RoI % not covered
by the sensors and (2) the economic cost of purchasing and physically deploying the sensors. The
coeficient of the linear combination are chosen so that the objective value defines an amount of
money, combining the expense for purchasing and placing the sensors and the implicit cost of
not covering part of the RoI. We implemented a simulator that takes as an input the geometric
description of the environment and a deployment of sensors and computes the associated value
of the objective function (objective value). This simulator uses the Parma Polyhedra Library
[
        <xref ref-type="bibr" rid="ref5">5</xref>
        ] to model the environment as sets of polytopes and to perform the necessary geometric
operations (along the lines of [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ]), and Monte Carlo estimation to compute an approximation of
the objective value, which is then used by our optimisation algorithm to guide the search.
      </p>
      <p>
        The latter combines derivative-free optimisation [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ] with domain-based heuristics, and
balances exploitation and exploration by alternating two diferent types of moves (as inspired
from [
        <xref ref-type="bibr" rid="ref8">8</xref>
        ]). At each iteration, a step based on the adaptive trust-region method [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] computes and
performs a greedy move that maximally improves the current deployment by moving a single
sensor. Whenever a local optimum is reached (i.e., a deployment such that moving any single
sensor does not yield any improvement), a heuristic-guided random move is performed, where
each sensor is randomly moved (in a new admissible position) by a distance randomly chosen
depending on the sensor’s contribution to the coverage of the RoI.
      </p>
    </sec>
    <sec id="sec-4">
      <title>4. A case study</title>
      <p>We evaluated the performance of our optimiser on a large, real-world scenario: the Leonardo da
Vinci International airport of Rome, Italy. We used sensors with range of 1 km, which could be
deployed both on the ground (but not on the runways, on the roads and on the taxiways) and
on top of the buildings. We varied the number of deployed sensors between 12 and 20. In each
experiment, we first sampled 100 random deployments and sorted them by their objective value
(to be minimised) in ascending order. Then, we executed the optimisation algorithm multiple
times using the sampled deployments as starting points, from the best to the worse. A time
budget of 24 hours was set and the optimisation algorithm was run until either the whole time
budget was consumed or all 100 runs were completed.</p>
      <p>In most of the cases, the algorithm was able to quickly improve the objective value and find
much better deployments than those found by random sampling. Among the best deployments
found with diferent number of sensors (Figure 1), the one that employed 16 sensors showed
the minimal objective value, i.e., was the one that best combined the coverage (73.1% of the RoI
volume covered) and the total expense for the deployment.</p>
    </sec>
    <sec id="sec-5">
      <title>5. Related work</title>
      <p>
        Many attempts have been proposed in the literature to solve large optimisation problems
defined via logic-, automata- or constraint-based formalisms ( e.g., [
        <xref ref-type="bibr" rid="ref10 ref11 ref12 ref13">10, 11, 12, 13</xref>
        ]). However,
such approaches cannot be applied when the problem model cannot be accurately defined within
such formalisms and is available, e.g., only as a black-box (e.g., [
        <xref ref-type="bibr" rid="ref14 ref15">14, 15</xref>
        ]). In these cases, various
kinds of intelligent black-box search or statistical model checking are often performed in the
search space and a simulator is used to assess the quality of the current parameter assignment
as well as to receive, when possible, some kind of gradient information to guide the search. Such
approaches have proved to scale well in diverse application domains such as, smart grids (e.g.,
[
        <xref ref-type="bibr" rid="ref16">16, 17, 18</xref>
        ]), system-level verification of cyber-physical systems ( e.g., [19, 20, 21, 22, 23, 25, 24]),
in silico medicine (e.g., [26, 27, 28, 29, 30]). As for the problem of optimally placing sensors in a
large region, existing methods difer depending on the types of sensors considered and on the
goal of the deployment.
      </p>
      <p>Most works focus on monitoring 2D regions, with or without obstacles (e.g., [31, 32, 33, 34,
35, 36]), hence are not easily adaptable to the localisation of flying objects. Approaches working
in the 3D space [31, 37] do exist. However they typically discretise the RoI and the possible
positions of sensors [38, 39], and thus do not scale over large scenarios when the targets must
be localised with high accuracy.</p>
    </sec>
    <sec id="sec-6">
      <title>6. Conclusions</title>
      <p>In this short paper we have shown how to compute optimal deployments of anti-drone sensors
in a given RoI through simulation-based optimisation via heuristic-guided intelligent search and
geometric problem modelling. We also presented preliminary results in a real-world scenario
(Leonardo da Vinci airport in Rome, Italy) which show our approach promising.
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    </sec>
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