Aerospace and Electronic Systems Magazine September 2017 - 30

RCS Measurements and ISAR Images of Small UAVs

Figure 6.

2D-ISAR image of SYMA quadcopter.

condition is always respected, But the latter condition is not respected in the measurements with the bigger UAV (AIRVISION
NT4CONTRAS). Fortunately this is not a serious issue, as suitable
algorithms have been developed for transforming near-field data
in far-field data [10], [11]. In particular, we used the well-known
algorithm by Mensa and Vaccaro as described in [11].

Figure 4.

RCS angular pattern of SYMA quadcopter.

ISAR PROCESSING
The ISAR processing allows to obtain a two-dimensional (2D) (or
even a three-dimensional (3D)) image of the RCS distribution. It is
particularly useful to identify the features of the target that contribute most to the radar signal. The basic idea of ISAR is to exploit
the spatial diversity of data acquired for focusing a high-resolution
image.
Using the above-mentioned equipment, the result of a measurement session is a matrix Nf × Np of complex numbers:
Ei , k = I i , k + jQi , k

(2)

where Ii,k and Qi,k are the in-phase and the quadrature components
acquired at ith frequency (1 < i < Nf) and at the kth angular position (1 < k < Np). The basic formula for focusing in a generic point
identified by the coordinate (x, y, z) is [6]:
j

4π
f i Rk ( x , y )
c

Figure 5.

I ( x, y , z ) =  Ei , k e

Table 1.

where Rk (x, y, z) is the distance between the image-point (x, y, z)
and the kth angular position of the antenna. Equation (8) is computationally very heavy, but several faster algorithms have been
developed [6], [7], [8].

Measured RCS
Vertical
Polarization

Horizontal
Polarization

SYMA X5SC-1

0.0312 m

0.0229 m

AIRVISION NT4CONTRAS

0.271 m

0.276 m2

30

(3)

i,k

RCS distribution of AirVision quadcopter.

2

2

2

2D ISAR IMAGES
The SYMA quadcopter was placed on the platform as shown in
Figure 1. The antennas operated in vertical polarization. The distance between the antennas and the rotation center was 202 cm.
The angular step was 1 deg. The bandwidth was B = 4GHz; that

IEEE A&E SYSTEMS MAGAZINE

SEPTEMBER 2017



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