Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 92

Sense and Avoid for Unmanned Aircraft Systems
Table I

Properties of Noncooperative Sensing Sources
Energy
Light

Method

Active
(requires electronic interrogation)
LIDAR

Optical (camera)

* Direct Energy

* Visible light

* Coherent

* Infrared

Monostatic RADAR

Bistatic/Multistatic RADAR (using signalsof-opportunity-cell, FM radio)

Bistatic/Multistatic RADAR (known signal
source)
Sound

Passive
(no electronic interrogation)

SONAR

Acoustic (microphone)
* Array
* Vector

is called the photoelectric effect [17]. Signal amplification must be
applied to the oscillator output signal in order to have a significant
power associated to the electromagnetic wave. Just after emitting
a pulse, the radar switches the antenna from the transmitter to the
receiver by means of a controlled switch that is called a diplexer.
The pulses hit the object to be detected and a certain amount of energy is backscattered to the radar antenna depending on the reflective properties of the object surface. If this energy is enough to be
detected by the radar, the presence of the object can be recognized
and the relative range can be measured by estimating the travel
time. The radar head can be rotated mechanically or electronically
in order to cover an assigned field of regard, thus performing the
requested surveillance function. This configuration has several advantages in terms of the SAA application, such as the following.

In the following, an overview of basic sensor properties related
to SAA application is presented in order to assess the main design
issues related to the development of an airborne anticollision radar.

1. It can estimate all terms of the track state, such as range, range
rate, angles, and angular rates.

trx is the time when the pulse is received,

2. The reliability of detection and the initial detection distance can
be easily regulated by selecting the level of transmitter power.
3. If a proper carrier frequency is selected, it is an all-weather and
all-time system. Indeed, its performance can be degraded by bad
weather conditions but the degradation is well modeled and it can
be well compensated by further increase of the emitted power.
Radars can be developed also with continuous wave configuration [20]. In this case, the radar configuration is called bistatic because it comprises two antennas, i.e., a transmitting antenna and a
receiving antenna. This system measures the range rate by Doppler
processing. The range can be determined by performing a proper
triangular frequency modulation of the carrier. Even if this system
is less complicated to be realized in terms of electronics, it is also
much larger than a standard pulsed radar, since it requires two antennas. For this reason, it is not commonly considered for SAA
applications that require compact configurations to be installed
onboard UA. It is a much more possible application for groundbased radars that do not have to be constrained by size, weight, and
power limitations.
92

Terms Measured by the Sensor and Sensor Measurement Performance: A pulsed radar is capable to determine the range by estimating the round-trip distance travelled by a pulse that moves at
the speed of light c, since it is carried by an electromagnetic wave
generated by the transmitter. The resulting measured range R is
given by (15):

(15)

where

trx is the time when the pulse is transmitted.
An important question that must be assessed in the design of an airborne radar is the selection of the pulse repetition frequency (PRF),
i.e., the frequency of repetition of pulses. This term is correlated
to the maximum nonambiguous distance that can be measured by
the radar. Indeed, if a pulse is transmitted before the previous one
is received after being backscattered by an object, the radar system
can be affected by an ambiguity in the determination of the range
since the value of trx is not referred to the reception of the last pulse
transmitted but to the previous one. As a consequence, false close
objects can be detected if the PRF is not selected properly. They can
determine a threat in the case of collision avoidance, since unnecessary own aircraft maneuvers can be executed in consequence of the
detection of these false objects. The best way to select the PRF is to
ensure that it is smaller than the reciprocal of the time needed for a
pulse to travel a distance that is the double of the maximum range
Rmax that is assigned to the region that forms the radar detection coverage area. As a result, the PRF can be computed by means of (16):

IEEE A&E SYSTEMS MAGAZINE

(16).

NOVEMBER 2016, Part II of II

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