Aerospace and Electronic Systems Magazine September 2017 - 28

Feature Article:

DOI. No. 10.1109/MAES.2017.160167

RCS Measurements and ISAR Images of Small UAVs
Massimiliano Pieraccini, Lapo Miccinesi, Neda Rojhani, University of Florence,
Firenze, Italy

INTRODUCTION
Currently small unmanned aerial vehicles (UAV) pose a serious
threat for the safety of flights. The Aviation Authorities are dealing with this issue worldwide. Recently (October 2015), the U.S.
Federal Aviation Administration gave permission to test antidrone
technology that would counter rogue drones flying within a fivemile radius of selected airports [1]. Airport safety is only one of
the problems that the increasing number of UAVs can pose. A
critical issue is to prevent UAVs being used for terrorist attacks,
espionage, or other malicious activities against sites with critical
infrastructure. Last but not least, UAVs flying in private area pose
privacy concerns [2].
Radar could be the technology of choice for detecting them,
but standard air defense is ill-prepared for UAV detection: UAVs
are low-velocity aircraft with a very weak radar signature. Despite
this, the scientific literature lacks detailed experimental studies on
the radar cross section (RCS) of small UAVs [3], [4], [5], especially
for quadcopters that are the most popular civil UAVs. Therefore, the
first aim of this article is to carry out RCS measurements of small
drones, in particular of a toy drone and a professional quadcopter.
The RCS measurements give global information on a target, but
they do not provide information on which features are mainly responsible for the radar response. Inverse Synthetic Aperture Radar
(ISAR) [6], [7], [8] processing provides just this kind of information.

THE MEASUREMENT EQUIPMENT
A sketch of the measurement equipment is shown in Figure 1. A
vector network analyzer (HP 8720A) operates as Continuous Wave
Step Frequency transceiver. It is linked through microwave cables
to a radar front-end held on a tripod.
The front-end is provided with a pair of single-pole doublethrow switches that provides a direct path (through a −40 dB attenuator) between the transmitter and the receiver in order to perform calibrated measurements. The antennas are two equal horns
linearly polarized, with a rectangular aperture 5.5 cm × 7.5 cm,
Authors' address: Department of Information Engineering
(DINFO), University of Florence, Via Santa Marta, 3, Firenze,
Italy, 50139, E-mail: (massimiliano.pieraccini@unifi.it).
Manuscript received August 4, 2016, revised November 23,
2016, and ready for publication February 6, 2017.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 IEEE
28

designed for operating in the 8-12 GHz band. Their measured efficiency has been η = 0.446 ± 0.040.
As shown in Figure 1, the target under test was positioned on a
platform that can be rotated step-by-step. For each k angular position, the equipment carried out a sweep of Nf frequencies between
8 GHz and 12 GHz with the switches connected to antennas and a
second sweep with the switches connected to the attenuator (−40
dB). The ratio, frequency by frequency, gives a calibrated measurement. A complete acquisition is a matrix of complex numbers
Ei,k , with i index relative to the frequency and k index relative to
the angular position. After the radar acquisition, the target under
test was removed and a single frequency sweep was performed.
This later acquisition (called empty room) was subtracted to each
column of matrix Ei,k (background removal).
Before the measurements session of the targets under test, the
equipment was calibrated using three known targets positioned at
17 m in front of the antennas: a metallic sphere of 0.45 m diameter,
a corner reflector of 0.30 m side, and a second corner reflector of
0.50 m side. From the radar equation [9], the RCS (σ) can be obtained from the following equation:

σk =

γ
4π

 4π FR02λ 
uk 

 αη A


(1)

where A is the physical area of RX antenna, η is the antenna efficiency, λ wavelength, uk is peak amplitude of the inverse fast
Fourier transform (IFFT) (along the i-index) of Ei,k , γ power attenuation of the calibration path (−40 dB), and F padding factor
of IFFT. The α factor takes into account the decreasing of the peak

Figure 1.

Sketch of the measurement equipment.

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2017

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