The integration of the unmanned aerial systems (UAS) into the
National Aerospace System (NAS) brings out the subject of maintaining operational
safety. Several important topics pertaining to the safe UAS operations have
been discussed in recent years. They include establishing regulatory standards
for UAS, communication link security, operator training and currency, vehicle
maintenance requirements, and privacy concerns. This paper will focus on the
UAS traffic separation requirements in the NAS. This issue is directly
connected to UAS operational safety, since potential air traffic collision may
result in a loss of life of manned aircraft occupants and persons on the
ground.
Air traffic separation considerations
The main difference between the UAS and manned aircraft is the
absence of a human pilot onboard of UAS. Traditionally, the pilot at the
controls of a manned aircraft is in charge to see-and-avoid the nearby air
traffic when operating in the visual meteorological conditions (Federal Aviation Administration
[FAA], 2013). When flying in instrument conditions, air traffic control
(ATC) provides traffic separation to all manned aircraft under their control.
Pilots also rely on traffic collision avoidance technology that is available in
the cockpit (Ramasami &
Sabatini, 2015). For line-of-sight UAS operations, the operator on the
ground has the authority to see and avoid other air traffic. However, visual
traffic detection may be complicated at night and in adverse weather
conditions. When operating a UAS beyond-line-of-sight, the vehicle controller
is responsible for avoiding other traffic by relying on visual information and
other available technology. However, the UAS operator’s situational awareness
may be limited due to the restricted field of view, control signal latency, and
lack of visual cues (Lam,
Boschloo, Mulder, & Paassen, 2009). These limiting factors may
negatively affect the UAS pilot’s ability to see and avoid other traffic.
Therefore, UAS separation from other aircraft should be based on
sense-and-avoid (SAA) technology installed on board the vehicle. Additional
traffic separation can be provided by ATC, which can issue traffic advisories
and alerts. Instead of communicating with the UAS itself, ATC would provide
this information to the operator located at the ground control station (GCS).
The disadvantage of this method is the possible delay in the communications and
delayed collision avoidance execution due to signal latency.
Sense-and-avoid considerations for different UAS groups
The UAS operational altitudes, speeds, and sizes vary considerably
depending on the UAS group. Therefore, the SAA technology selection should be
based on the specific characteristics of the UAS. There is no one-size-fits-all
SAA system currently available to fulfill the SAA requirements for all groups
of the UAS (Gageik, Benz, &
Montenegro, 2015). For instance, the Traffic Alert and Collision Avoidance System
(TCAS), which is presently widely employed in manned aircraft could potentially
be used in UAS. However, the large size and excessive weight of current TCAS
equipment may prohibit its usage on some lighter UAS. Some of the smaller UAS
may also be difficult to detect visually by a manned aircraft pilot due to
their size. For example, a pilot of a manned aircraft travelling at 300 knots
may not have enough time to see, react and avoid a small UAS. Pilots who
routinely scan and locate full sized aircraft may have a difficult time
locating small UAS and this may result in mid-air collision.
Consideration should also be given to different airframe designs
of the UAS. For example, lighter than air UAS may lack maneuverability when
compared to fixed-wing aircraft and may be unable to execute the evasive
maneuver in a timely manner. However, their large size makes them more visible
and their slower speeds decreases their closure rate with conflicting traffic.
Current
sense-and-avoid technology
The current technology employed by manned aircraft includes both the
ground-based traffic detection and separation and the airborne traffic detection
and avoidance technologies. ATC employs radars
to monitor and manage aircraft traffic. Current ATC radars feature clutter reduction
technology to include only primary traffic detection. Due to their small size
some UAS may not be displayed on the radar screen. Therefore, modifications to radar
processing may be required to allow UAS detection by the ATC (Lacher, Zetlin, Maroney, Markin,
& Ludwig, 2010). Another option is to design the UAS to be easier to
detect both visually and on radar. This is just the opposite of military UAS
that need to be stealthy and hard to detect. Commercial UAS operation in the
NAS should be designed to be easily seen and detected on radar. Day glow
reflective paints schemes for visual detection and the use of sharp angles and
highly reflective materials for radar detection will make the job of detection
much easier for both pilot and controllers.
A Ground Based Sense and Avoid (GBSAA) capability especially
developed by the U.S. Army for UAS. The GBSAA is designed for traffic
separation in the busy airspace segments, such as terminal areas. However, ground
based sense-and avoid has limitations and needs to be supplemented with
airborne sense-and-avoid technology (Department of Defense [DoD], 2016).
For example, ground based SAA is susceptible to signal latency and may not be
usable during lost link scenarios.
The airborne sense-and-avoid technologies includes both cooperative
and non-cooperative sensors. Cooperative sensors obtain radio signals from
another aircraft to determine its position and altitude. Cooperative sensors
include: transponders, Automatic Dependent Surveillance – Broadcast (ADS-B),
Automatic Dependent Surveillance – Contract (ADS-C) (Ramasami & Sabatini, 2015).
TCAS is also a cooperative technology employed in manned aircraft, which is based
on the aircraft's transponder equipment. It provides the pilot with traffic
information, automated traffic alerts, and smart conflict resolution
advisories. The disadvantage of cooperative sensors for SAA is that cooperative
technology will only function if all participating traffic is equipped with
transponders (Lacher et al.,
2010). ADS-B and ADS-C is based on the satellite surveillance. This
technology does not require ground based interrogation by a radar. Currently,
cooperative sensors are not required in all classes of airspace. (Delves &
Angelov, 2012). Figure 1 depicts UAS SAA system.
Figure 1. SAA system diagram for UAS.
Adapted from “MIDCAS sense-and-avoid” by MIDCAS, 2011. Copyright 2011 by
MIDCAS.
Non-cooperative sensors passively acquire target information or
actively emit energy to perceive the surrounding traffic. The non-cooperative
SAA sensors will function even if other aircraft are not carrying similar equipment.
Non-cooperative sensors include millimeter wave radar, electro-optical/
infrared, thermal, and acoustic sensors (Lacher, Maroney, & Zeitlin, 2007).
Different sensors has their own advantages and limitations (Ramasami & Sabatini, 2015).
A full description of these sensors go beyond the scope of this paper. Some
researchers offer a sensor fusion approach for UAS SAA in order to satisfy
collision avoidance requirements. Figure 2 depicts sensor data fusion
schematic.
Figure
2. Sensor data fusion for
SAA technologies and system process. Adapted from “A unified approach to
cooperative and non-cooperative sense-and-avoid,” by S. Ramasamy, S., & R.
Sabatini, 2015. Copyright 2015 by S. Ramasamy, S., & R. Sabatini.
This is just a brief description of available SAA technology. As
we can see, there are a variety of SAA methods employed in manned aircraft. It
is possible to use some of these traffic avoidance methods for UAS. The author
would recommend employing ground based surveillance together with airborne
sensors for UAS to ensure proper traffic separation in the NAS. However, due to
size, weight, and power limitations of different UAS groups, there is no one
universal airborne sensor available for SAA. Therefore, it is important to
select appropriate sensor based on the unique characteristics and operational parameters
of the UAS. For added safety an automated
collision avoidance and evasive maneuver execution will be necessary to make
the SAA protocols effective. Automated traffic avoidance execution will be
especially useful in case of loss of link. Similar to car that will apply the
brakes themselves when no input from the driver is detected. A last line of
defense avoidance maneuver should be incorporated.
With fast technological progress, it is likely, that SAA
capabilities will increase rapidly. Smaller, and more capable sensors together
with ground and satellite based traffic separation methods will enable safe UAS
operations in the NAS.
References
Department
of Defense. (2016). Airborne based Sense and Avoid (ABSAA) sensor for tracking
non-cooperative aircraft for RQ-7 Shadow and larger UAS. Retrieved from
https://www.sbir.gov/sbirsearch/detail/870673
Lam, T., Boschloo, H. W., Mulder, M., & Paassen, M.
V. (2009, November). Artificial force field for haptic feedback in UAV
teleoperation. IEEE transactions on systems, man, and cybernetics, 39,
1316-1331. Retrieved from
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5263033&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D5263033
MIDCAS. (2011). http://www.midcas.org/News/News/Midcas-Flight-tests-press-release,i196.html
