Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 91

Fasano et al.
generation of false tracks. As it happens for accuracy and rates, the
notion of integrity is mainly related to the output of the detecting
task, which is described in Section IV.

B. COOPERATIVE SENSORS
Cooperative sensing technology depends on each aircraft in an
airspace to broadcast their position and intent information to the
other aircraft in the area. With this information about any manned
aircraft in its vicinity, a UAS can sense where intruding aircraft
are, and include this information in decisions about how to avoid
these aircraft.
Manned aircraft operating in controlled airspace have long used
the TCAS. TCAS is based on the secondary surveillance radar transponder which is required on manned aircraft that carry more than
19 passengers (TCAS I) or 30 passengers (TCAS II). It operates independently of ground-based secondary radar, to provide aircraft position information to the pilot on potential conflicting aircraft. Initial
evaluation of TCAS for use by UAS, many of which have very different operating characteristics, demonstrates that TCAS would have
to be significantly adapted for these new aircraft types, or would not
be appropriate for use by UAS depending on their operation and
flight performance [15]. The new generation of TCAS, the Airborne
Collision Avoidance System for Unmanned Aircraft (ACAS-Xu) is
currently under development and test. When deployed, it would be
able to manage the differences in UA characteristics, in a cooperative sensing environment and can include noncooperative sensors.
ADS-B is an alternative cooperative sensing solution based on
GPS location information, which depends on GPS accuracy and
reliability information. ADS-B provides more information than
the secondary radars of TCAS-each aircraft broadcasts location,
velocity, and intent information, and the other aircraft in the area
can receive that information and use it to calculate the possibility
of collision. If it is operating in an environment where other aircraft are using ADS-B, a UAS can use that information to perform
SAA functions from those aircraft. Even if not all aircraft are using ADS-B, the UAS can use this method to manage separation
from cooperative aircraft, and use other noncooperative methods
to manage separation from the remaining aircraft.
Technology improvements are making cooperative sensors
smaller and lighter, making them more accessible to a wider variety of UA sizes. Having the ability to broadcast its state information provides the added advantage of ensuring all surrounding
aircraft equipped with receivers can be aware of where and how
the UAS is operating.

C. NONCOOPERATIVE SENSORS
Noncooperative SAA is performed when an aircraft is equipped
with sensors that allow for autonomous detection of a collision
threat without external support. This is the most genuine form of
SAA, since it replicates the capability of human pilots to detect potential collision threats by exploiting only onboard resources and
their own sensorial capabilities. Regulatory agencies pointed out
that a form of noncooperative system must be installed onboard
UA that will be authorized to fly in the National Airspace [16].
NOVEMBER 2016, Part II of II

A primary function of sensors is the surveillance of traffic in
a specific region of airspace surrounding the own aircraft that is
called the field of regard. This process is usually performed by scanning the region in order to assess if a specific term measured by the
sensor, i.e., the level of energy detected in a specific bandwidth, can
be associated to an object that poses a collision threat. A scan or image can be defined as a three-dimensional or two-dimensional map
of the property measured by the sensor on its surveillance region.
Surveillance sensors measure a specific form of energy transported by a wave, such as an RF wave, an acoustic wave, and light.
As a consequence, as anticipated above, a typical classification of
surveillance sensors is related to the source of energy. If the source
of energy is provided by the same instrument that performs the
estimate, the sensor is classified as active. This is the case of radars
and LIDARs. At the same time, if the sensor exploits an external
source of energy, such as the object heat emission or the sunlight,
the sensor is classified as passive. This is the case of EO cameras
that exploit sunlight or aircraft engine thermal emission in the thermal infrared (IR) bandwidth. Also, acoustic sensors measure the
acoustic energy emitted by engines. Even if the source of energy
is less efficient in these conditions, the path loss determined by the
distance R between the object and the passive sensor is proportional to the reciprocal of R2 instead of the reciprocal of R4 as for
active sensor. This is determined by the fact that the signal travels
only a single-way path instead of a two-way path from the source
to the receiver and no intermediate reflection is needed.
In general, active sensors are more accurate and more reliable
than passive sensors, but are much more demanding in terms of
onboard resources such as electrical power, space, and weight. As a
consequence, the selection of passive sensors is preferred for small
aircraft whereas large ones need to use active sensors. This fact is
also determined by the consideration that large aircraft are usually allowed to fly in a segment of airspace where the maximum
allowed speed can be larger than 250 knots true airspeed (kTAS).
At this level of speed, the required initial detection range must be
larger than 3 nautical miles (NM), the admitted tracking inaccuracy must be very small, and the requested detection reliability must
be very high. Standing current level of available technologies, this
performance is attainable only by active sensors.
Table I summarizes the properties of noncooperative sensing
sources with reference to the SAA application. Some systems reported in the table have no significance in the typical SAA scenarios and they have not been discussed in the following subsections.
Active Sensors:
Airborne radars: Radars are the best sensing solution for airborne surveillance [17]. The typical configuration of an anticollision radar comprises a monostatic pulsed (or monopulse) radar,
i.e., a radar with a single antenna that emits electromagnetic waves
modulated by pulses at an assigned frequency [14, 18, 19]. A simple emitter can be realized by means of a dipole connected to a
coherent oscillator, i.e., an oscillator that has a very good phase
and frequency stability [17]. Each time the electrons in the dipole
change the sign of their linear momentum, an electromagnetic
wave is emitted in the form of a photon, i.e., a quantum of energy that has the same oscillation frequency as the electron. This

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