Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 96

Sense and Avoid for Unmanned Aircraft Systems
1. It is a coherent source, i.e., all photons emitted at the same
time have the same phase. This property can be obtained by
properly sizing the cavity.
2. It is a monochromatic source, i.e., all photons emitted have
almost the same wavelength or frequency. The wavelength depends on the type of gas adopted.
3. It is a collimated source, i.e., all photons emitted move on parallel rays rather than on large beams as happens for the radar.
Indeed, the last property can be justified by applying (22) to the
typical wavelength of light that is on the order of 500 NM for visible
light and 1000 NM for near IR. After being emitted from the source,
the light impacts on a mechanism that supports the scanning of the
field of regard. This mechanism can be a movable mirror or prism
that permits control of the orientation of the beam boresight axis.
For typical application, Nd-Yag lasers are adopted that have a peak
at 1064 NM wavelength. From (22) for d= 5 10-3 m the resulting half
beamwidth is Δθ = 1.5 10-2 deg. This means that at a distance of 10
km from the emitter, the footprint of the beamwidth covers a circular
area with a diameter of less than 5 m. As a consequence, the number
of cells to be defined would require a very slow scanning process. In
order to speed up this process, customized scanning strategies must
be adopted such as transmitting pulses only for a reduced percentage of cells at each scan and skipping the other cells. This strategy
determines a risk of detecting an object, in particular a small size
one, with a considerable delay with respect to its theoretical initial
distance of detection. Moreover, since the wavelength is very small,
the atmospheric attenuation effects are very strong and they can reduce the maximum range Rdet < 1.5 NM, when high energy output is
feasible. As a consequence, LIDARs can be a feasible solution for
SAA only if we consider two specific sensor architectures, such as:
1. a system that must be used as a primary sensor for short range
operations, such as the one executed by multirotors or small
fixed wing UA, that are carried out in the segment of the airspace where the aircraft velocity is very limited (< 100 knots);
2. a system that can be used as secondary sensor in conjunction
with EO cameras in order to perform the determination of
range and range rate for close objects.
An important issue related to the use of LIDAR is eye safety.
The risk that a beam enters through the cockpit window and hits
the eye of a pilot is not remote. The level of emitted power must
be kept within a safety envelope and a proper frequency must be
selected. For this reason, 1000 NM lasers are preferred, since they
are at a frequency that is not harmful for the human eye.
In summary, airborne LIDARs can be a compact alternative to
radars for small aircraft applications if they fly in conditions where
the level of speed is reduced with respect to one of the standard
airspace classes. Indeed, they could be a reasonable option for UA
flying in Class G airspace.
Passive Sensors:
Airborne acoustic sensors: A microphone is a passive sensor
that measures the level of acoustic pressure emitted by an object
[24]. In the case of SAA, the acoustic pressure could be emitted by
96

the engine of an intruder aircraft. Several forms of microphones
are available. In order to provide information about the bearing of
an intruder aircraft, directional microphones are needed.
Two basic types of microphones are available, pressure-operated microphones and pressure-gradient microphones. All microphones operate by sensing the pressure difference over the two side
faces of a thin diaphragm. However, in pressure-operated microphones, one side face is exposed to open atmosphere whereas the
other side face is contained in a closed volume where a reference
constant pressure is preserved. As a consequence, this system is
sensitive to pressures incoming from any direction, i.e., it is an omni-directional microphone. Conversely, in pressure-gradient, both
side faces of the thin diaphragm are exposed to open atmosphere.
In this case, a pressure that acts in the plane of the diaphragm does
not produce any output since it determines the same pressure on
both sides and the net difference is null. At the same time, a sound
pressure that arrives from a direction that is normal to the plane
of the diaphragm determines a maximum displacement of the diaphragm, since it determines values of the pressure that have the
same magnitude and different direction. Fig. 14(a) and Fig. 14(b)
show the polar plot of pressure-operated and pressure-gradient
microphones, respectively. The polar plot depicts the trend of the
gain that the microphone has for a sound incoming from a direction with an assigned azimuth. It results that an omni-directional
microphone has the same gain for all directions while a bidirectional microphone has two large main lobes, i.e., it is very sensitive for sounds incoming from azimuth 0° and 180° and it is deaf
to sounds incoming from azimuth 90° and 270°. It is worth noting
that the signal sensed by the left lobe of the bidirectional microphone has the same amplitude as the one of the right lobe but it has
a phase shift of 90°. This property has been exploited in order to
realize directional microphone. Indeed, the integration of an omnidirectional microphone and a bidirectional microphone results in
a combined microphone that has the polar pattern of a cardioid
as reported in Fig. 14(c). This is a basic form of directional microphone since the gain for 0° is maximum and it is null for 180°.
However, the angular dispersion of the main lobe is very wide,
i.e., a half sound pickup angle of 50°, and the directivity is poor. In
order to increase the directivity, the proportions of the omni and bidirectional terms can be varied and configuration, including more
than one bidirectional microphone, can be adopted. The resulting
polar plots include the hypercardioid microphone [Fig. 14(d)] and
the shotgun microphone [Fig. 14(e)] [24]. The last one has a half
sound pickup angle in the order of 15° that permits a rough estimation of the direction of the boresight. This is also the best angular
resolution that can be obtained by acoustic sensors.
A proper installation strategy must be considered for acoustic
sensors in order to reduce the risk of disturbance generated by the
own aircraft or other unwanted sources. Moreover, the installation
should prevent the microphones from rapid degradation generated
by oil exhaust of the engine or dirt collected during landing and
takeoff.
Regarding the detection distance, the analysis is more complicated than that of radars and LIDARs. Indeed, the noise sources that can disturb the transmission of sound are much more uneven than the ones of previous sensors. Moreover, the speed of

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NOVEMBER 2016, Part II of II

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