Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 108


Sense and Avoid for Unmanned Aircraft Systems
cessing power of these controllers means that there are more limited time frames and alternatives. In each case, the primary purpose
of these displays is to provide information to the pilot about the
track of the target aircraft (or location of the obstacle of concern)
and the alternatives of where to direct the UA, to avoid the target.
For the latter, algorithms that continuously calculate the encounter geometry and display the results are the most useful to
the pilot. A number of methods of display have been developed,
including an arc on an aircraft situation display (with indication of
which direction to fly and not fly), and geometrically constrained
airspace areas from which to steer away. In both cases, if the pilot
changes course to avoid these areas, the situation will unfold in
real-time as the conflict is averted and the display improves.
Basic Concepts about Command and Control Link:  In general, remotely operated collision avoidance strategies establish a
logical dependency of the SAA function on the C2 link, creating a
connection between the two main roadblocks to UAS integration.
Though a detailed explanation of C2 architectures and the relevant
technologies is well beyond the scope of this tutorial, some basic
information is provided in this subsection.
Fundamentally, the C2 link provides the remote pilot with the
capability to direct the flight and understand the UA state. Since
this information usually requires only tens or low hundreds of kilobits per second, C2 is generally low bandwidth. This is compatible
with relatively low frequencies (very high frequency (VHF) and
ultrahigh frequency (UHF) spectra) which offer reliable propagation and are rather insensitive to atmospheric absorption.
On the other hand, reliability of the C2 link is of paramount
importance for flight safety, and the link is "flight critical" when
the UA is incapable of autonomous operation and landing. Thus,
protection from intentional jamming/unintentional interference
and from unauthorized use via encryption capabilities, are usually
recommended. Link redundancy (using at least one backup C2
link) is also commonly adopted, and proper "lost link" behaviors
have been defined in ad hoc developed standards.
In general, loss of the C2 link can be due to several reasons,
such as:
1.	 Failures of system components
2.	 Geographic features (including Earth curvature and topography) and obstacles blocking the signal
3.	 Weakening of the received power due to excessive range between transmitting and receiving antennas, with the SNR at
the receiver falling below the minimum threshold required for
closing the link. The threshold usually depends on the bit error
rate (BER) that is required for the C2 link. Transmit power can
be limited by regulations.
4.	 Intentional/unintentional jamming. In fact, avoiding frequency
conflicts is a major challenge for unmanned systems, especially considering the evolution towards dense low altitude
environments with many UAS operating in the same scenario.
At the architectural level, the C2 link can work in LOS or
BLOS. LOS communications utilize a direct connection between
the GCS and the UA. This is usually achievable at relatively short
distances, on the order of a very few hundreds of kilometers at
108	

most. At all but very close-range operations, directional tracking
antennas are used, at least on the ground, exploiting human-in-theloop or automatic mechanical/electronic antenna steering systems.
BLOS communications occur when the UA and the GCS use relay
nodes to retransmit the signal. These are other airborne platforms
or, more commonly, satellites.
Interestingly, because of the low data rate requirements of C2
link, medium size and even small UAS can also leverage satellitebased communications (SATCOM), exploiting constellations of
satellites flying in low Earth orbits (LEO). In fact, unlike high data
rate communications exploiting high frequencies and geostationary (GEO) satellites (adopted on large UAS), C2 SATCOM do not
require highly directional antennas onboard the aircraft, which are
large and need accurate pointing.
In general, BLOS communications are typically characterized
by a larger latency compared with LOS (on the order of one or
more seconds), which impacts SAA performance since it affects
the previously discussed time delay between sensor detection and
avoidance execution.
Besides the "classical" vision of LOS/BLOS communications
between the GCS and the UA, the latest developments regarding
small UAS have introduced the concept of communications using
existing network infrastructures. This concept can also be exploited for C2 needs. Reliability and latency issues need to be taken into
account, but a significant progress is being made, and the introduction of "cloud-based" C2 concept may have a significant impact on
future SAA architectures in low altitude environments.
The interested reader is referred to [49] and [50] for a comprehensive discussion of UAS communications.

B.  AUTONOMOUS SENSE AND AVOID
For autonomous SAA, the same "avoid" functions must take place,
though they must all be performed algorithmically onboard the
UA. Instead of displaying the situation to the pilot with guidance
on where to fly, the automation must have access to this information. Factors for deciding on an avoidance option must be built
into the avoid algorithm, which must be able to evaluate the alternatives on several dimensions of environment and safety, before
directing the UA on a preferred course.
Fundamental conflict probing algorithms and avoidance strategies are used by TCAS for manned aircraft, and more recently, are
being developed for UAS as ACAS-Xu [51]. Many research activities are underway in this area, to explore possible decision-making
strategies that include parameters of when to start an avoidance
maneuver (while the UA is well clear, or wait until the UA and
target approach the self-separation or collision avoidance time
frames) and how much of a deviation should be taken (minimum
to optimize flight, or more to provide a safety margin).
Fully autonomous avoidance maneuvering remains a research
area, since there are a wide variety of sensing possibilities, as well
as detection methods. Based on the time frame to collision, the
avoidance strategy should be tailored to make best use of the information available, and provide an avoidance solution that is appropriate for the time frame. Bear in mind that a deliberate maneuver
to avoid a target that is sensed well in advance may be very dif-

IEEE A&E SYSTEMS MAGAZINE	

NOVEMBER 2016, Part II of II



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