Aerospace and Electronic Systems Magazine May 2017 - 5

4. The azimuth angular resolution should be high enough to distinguish catenary poles near the track from another bullet train
or an obstacle on the track.

designed to be conformal antennas so as not to increase the aerodynamic drag of the high-speed bullet train. A MUSIC algorithm
based on conformal SNLA is also proposed for the angular superresolution estimation.

A few CA radar systems have been designed for ordinary
trains. One of them was proposed in [12] and is installed along
the railway rather than on the train platform, which may suffer the
CA RADAR SYSTEM DESIGN AND IMPLEMENTATION
same signal failure as the railway communication system previThe CA radar system for the bullet train has been designed to
ously introduced. The millimeter wave CA radar system proposed
obtain the range (from 100 m to 3 km), the radial velocity, and
in [13] claimed sufficiently high range and azimuth resolution in
the azimuth information of an obstacle on the same track, espethe single target scenario; however, the necessary velocity esticially another bullet train traveling ahead in the same direction.
mations of the obstacles are not provided, so the operator cannot
The use of a high-frequency Ku band signal makes the radar
predict a coming collision or maneuver separate targets with difsystem sensitive even to small barriers. The implementation of a
ferent speeds to avoid a collision. In addition, to reduce the cost
LFMCW signal with a bandwidth of 50 MHz provides high resoof the whole system to make it attract for cost-sensitive civilian
lution up to 3 m in the range dimension, and the receive array
application, 16 receiving channels are used in the receiver in [12]
with a large aperture of 2.44 m promises possible high-enough
to form a sparse uniform linear array (SULA). Although the gratangular resolution.
ing lobes that will cause serious angular ambiguity are suppressed
The overall block diagram of the CA radar is shown in
by placing the fixed null of the pattern of the transmit antenna on
Figure 2. There are three main functional components in this systhe grating lobes of the receive array, it leaves less freedom in the
tem: the antenna arrays, the radar analog front end, and the digital
receive beampattern design of the receive array. In addition, the
section. The antenna arrays consist of a transmit antenna and a
conventional direction-finding method with the digital beamformreceive array, which are used to transmit the radio frequency (RF)
ing (DBF) technique used in [12] only applies to a single target
signal into the space and receive the echoes reflected off targets,
scenario, and it fails to separate close targets at the same range because the angle
resolution of the receive array is strictly
limited by the width of the aperture, which
is related to the width of the train.
In this article, a coherent linear frequency modulated continuous wave
(LFMCW) with wide bandwidth is used
to achieve high range resolution, as well
as to obtain the Doppler estimation. Only
eight receiving elements are adopted in the
CA radar to reduce the cost while adequate
performance is kept. To achieve this goal,
a sparse non-uniform linear array (SNLA)
and the multiple signal classification (MUSIC) algorithm [14]-[16] is proposed. The
SNLA is utilized to overcome the angle
ambiguity, and the MUSIC algorithm is
utilized to provide high angle resolution.
Figure 2.
In particular, the antennas in the SNLA are Overall structure of the CAR system for the bullet train.
MAY 2017

IEEE A&E SYSTEMS MAGAZINE

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine May 2017