Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 87


Fasano et al.

Fig. 7.

Cylindrical collision volume and collision avoidance/safe separation
thresholds [8].

Fig. 6.

SAA encounter timeline [8].

and obstacles. Included is a review of sensor types (cooperative
and noncooperative, ground-based, and airborne) and how they
contribute to filling the sensor requirements.

III.  SENSING FOR UNMANNED SYSTEMS
A.  SENSING REQUIREMENTS BASICS
From a qualitative point of view, a surveillance system that is designed to provide obstacle detection and tracking within an SAA
architecture needs to fulfill a set of requirements. Conversely, for
a given sensing system, these represent the basic specifications in
view of SAA applications. While these requirements are described
separately in what follows, it is worth underlining that strong logical links exist among them, making an integrated approach to sensing system design/integration necessary.
The basic specifications/requirements are:
1.	 minimum range of initial detection for an obstacle big enough
to be considered a nonnegligible threat
2.	 FOV to be monitored
3.	 sensing accuracy
4.	 measurement rates and latencies
5.	 integrity.
The main focus is set on minimum range, while other aspects are
covered mainly at a qualitative level. As is also clarified in Subsection III-D, though this discussion mostly assumes that sensors are
airborne, the mathematical foundations can also be used within a
ground-based sensing perspective.
Minimum Detection Range:  Detection range represents the
basic requirement of an obstacle sensing system aimed at separation assurance or collision avoidance. Indeed, detection range
has an intrinsic stochastic character, meaning that any sensing
NOVEMBER 2016, Part II of II	

system is better defined in terms of a probability of detection.
In this regard, a requirement level detection range can be considered as the range where the probability of detection is close
enough to 1.
Given the sequence of operations described in Section II, sensor detection by itself does not suffice for evaluating collision risk
and planning and executing eventual maneuvers to reduce collision
threat. Indeed, collision detection and avoidance maneuvers can
be executed only when a confirmed track is present. The range at
which this happens is usually defined as declaration range, and the
related probability is named probability of declaration.
As firm tracking requires a minimum number of associated
measurements, a close connection exists between detection range,
probability of detection, measurement rate, and declaration range.
Given the typical differences between detection ranges for cooperative and noncooperative systems, the distinction between detection and declaration is typically important for noncooperative
systems.
The basic requirement behind minimum sensing range is that
an intruder has to be detected/declared at a range where there is
still enough time to perform an avoidance maneuver which keeps a
minimum separation between aircraft.
The notion of minimum distance implies the concept of a
spherical bubble that does not have to be violated. This is consistent with the concept of near midair collision that is verified if the
distance between two aircraft falls below 500 ft [10].
However, a more recent approach foresees a cylindrical collision volume, with a horizontal radius of 500 ft and a vertical range
of 200 ft [8]. A larger volume, whose horizontal shape actually
depends on time-to-collision requirements, is usually considered
to correspond to the concept of "well clear," related to separation
assurance. (See Fig. 7.) Indeed, minimum detection range is usually associated with the collision avoidance function, as separation
assurance maneuvers should be executed at much larger ranges.
For a given intruder range, it is intuitive that a head-on collision scenario gives the minimum time to collision. Thus, this is
usually considered as worst case to evaluate detection range. Of
course, this performance parameter becomes conservative when
nonfrontal geometries are considered.

IEEE A&E SYSTEMS MAGAZINE	

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