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AUTONOMY AS AN ENABLER OF ECONOMICALLY-VIABLE, BEYOND-LINE-OF-SIGHT, LOW-ALTITUDE UAS APPLICATIONS
WITH ACCEPTABLE RISK
Ella M. Atkins*
This paper describes a practical vision for low-altitude UAS operations based on a Class
G airspace subdivision that will support safe near-term UAS deployment without impact
to existing manned aircraft operations. An agriculture reference mission is defined as a
case study for which low-altitude UAS offer the landowner tangible benefit without in-
troducing unacceptable risk. A Class U airspace designation is proposed for surface to
500 feet above ground level below existing Class G airspace. Reasonable operational re-
quirements for Class U are shown to significantly depend on overflown property owner-
ship and type. A candidate sub classification of Class U airspace based on property own-
ership (private or public) and type (rural, suburban, and urban) is proposed along with
candidate requirements for the vehicle, its safety features, and its operator(s). Autono-
mous geofencing is proposed as a means to ensure low-altitude UAS do not exit their
designated Class U operating area. A certified geofencing capability can ensure safe
flight in rural areas without the need to wait for system-wide detect-and-avoid so long as
manned aircraft remain clear of the occupied Class U region. A deterministic and simple
geofencing (or electronic leashing) algorithm is presented. Geofencing represents auton-
omy in that it guarantees operating boundaries are respected even if operator inputs must
be overridden. Such algorithms are available today and can ultimately be safety-certified
to provide the backbone autonomy necessary to ensure UAS will not leave their designat-
ed operating area despite flight operations beyond line of sight. The paper concludes
with a discussion of additional technologies needed to achieve adequate safety and priva-
cy management for suburban and urban operations.
INTRODUCTION
Unmanned aircraft system (UAS) missions such as crop inspection will overfly one landown-
er’s property at low altitude. If a UAS can be guaranteed to remain in a box within immediate
reach of the landowner’s property, this UAS does not pose a hazard to aircraft or property outside
this box regardless of whether the UAS is within line of sight (LOS) or beyond line of sight
(BLOS). The expanding “hobbyist” community has embraced this viewpoint by offering autopi-
lots with an “electronic leashing” or “geofencing” option, controversially advertising such autopi-
lots to support beyond-line-of-sight (BLOS) flight capability. Although this technology is not yet
certified for civil use, the idea behind the geofence is compelling and itself suggests a regulatory
strategy by which low-altitude UAS might be supported: create a new classification for very low-
altitude airspace that supports UAS flight in areas currently either considered non-navigable, with
immediate reach of privately-held property, or both.
* Associate Professor, Aerospace Engineering Dept., University of Michigan, 1320 Beal Ave., Ann Arbor, MI 48109.
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The FAA’s recent roadmap for integration of civil UAS1 provides little provision for cost-
effective large-acreage (BLOS) UAS coverage even at low altitudes sufficiently far from airports
to pose no risk to existing manned aircraft traffic. The roadmap instead states that UAS either
must be certified for integrated operation in the NAS, implying high cost and significant delay
associated with certifying detect-and-avoid (previously sense-and-avoid2) and requiring it on all
aircraft, or that operations must occur within line-of-sight (LOS), not a practical constraint for
large-area BLOS applications. Behind this position is the ICAO-backed hypothesis that all UAS
are remotely-piloted aircraft (RPAs) without accommodation of fully-autonomous operations.
This paper investigates how a modest rulemaking effort focused on reclassifying low-altitude
uncontrolled airspace and capitalizing on a simple, deterministic3 autonomy algorithm for
geofencing can enable safe, cost-effective BLOS UAS operations. A new Class U low-altitude
airspace is proposed and assigned common-sense operational rules based on overflown property
ownership (private vs. public) and type (rural vs. suburban vs. urban). An agriculture case study
is proposed as an initial means of enabling commercial UAS in a manner that poses risk to neither
people nor property. Subsequent progression toward more populated and publically-owned land
is then described. The first message of this paper is that low-altitude airspace rules must take the
characteristics of overflown property into account – otherwise operational approval will either be
unsafe (e.g., allowing unlimited hobbyist flight operations in urban areas) or repressive (e.g., pro-
hibiting agricultural flights at low-altitude over privately-owned rural property). The second
message of this paper is that autonomy without possibility of pilot override can actually enable
safer flight, particularly for BLOS operations where human perception is achieved only through
graphical ground station interfaces. Geofencing is a specific example of an autonomy technology
specifically focused on keeping a UAS within its designated operating area. With a trusted
geofence, loss of communication link will not result in emergency flight termination (ditching)
and will not allow exit from the test range. Waypoint entry errors also will not cause geofence
violation unless the operator enters both waypoint and the geofence data incorrectly.
The paper is organized as follows. Below, UAS agricultural operations are introduced as a
motivating case study, including a discussion of platform types, operational paradigms, and legal
precedent that collectively must be considered in UAS regulatory policy. Next, a new class of
low-altitude airspace (Class U) is proposed and discussed in the context of property attributes and
operating requirements. Geofencing to ensure airspace constraints are respected is then dis-
cussed, focusing on a simple deterministic algorithm available today in an open-source autopilot
to provide context on how autonomy might be realized without introducing nondeterminism or
unmanageable system complexity. The paper concludes with a summary and discussion of future
work required to move forward in the suite of technological and policy challenges required to
comprehensively support UAS operations.
UAS FOR AGRICULTURE: A RECOGNIZED AND MOTIVATING CASE STUDY
There are numerous low-altitude UAS applications, some necessitating flight over densely
populated areas. Perhaps the “killer app” for the near term, however, is agriculture. First, low-
altitude UAS flights over privately-owned rural property will not pose a risk to people or property
on the ground other than those who own the land (and have chosen to fly). Unless the property is
in close proximity to an airport, airspace at low-altitude is currently uncontrolled (Class G) and
rarely to never occupied by manned aircraft. These circumstances imply very low-altitude UAS
operations over rural farms are safe whether the vehicle lands intact or not so long as the vehicle
does not depart its low-altitude operating range or “box” over the privately-owned farm. Low-
altitude UAS flights over privately-owned farmland also cannot raise [valid] privacy concerns so
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long as these flights are conducted by or approved by the landowner, who would then have exclu-
sive rights to any collected data including the right to decide how this data is disseminated.
Agricultural UAS flights would serve two primary purposes: surveillance and chemical appli-
cation, as shown in Figure 1. Crop, livestock, or infrastructure (e.g., fencing) inspection by air
can significantly improve situational awareness for the farmer at reasonable cost. Such awareness
will enable irrigation and pesticide application to be performed more effectively and efficiently,
find or survey livestock and their food/water sources, and identify specific problems with fencing.
Pesticide application by UAS can reduce risk to crop duster pilots due to pesticide exposure as
well as the relatively dangerous flight pattern flown to minimize operational cost. The Yamaha
RMAX is under consideration for agriculture operations in the USA
(http://www.ainonline.com/aviation-news/2013-09-23/after-first-commercial-flight-other-uas-
types-advance), having already established a track record for successful crop spraying overseas
(http://rmax.yamaha-motor.com.au/history).
(a) CropCam for imaging (b) Yamaha RMAX for pesticide application
(http://www.robotshop.com/en/cropcam-unmanned-aerial-vehicle-uav.html) (http://www.suasnews.com/2014/01/26971/unlocking-the-potential-of-unmanned-aircraft-systems/)
Figure 1. Commercial UAS for agriculture applications.
At first consideration, one might think establishment of a ruling enabling small UAS flight
within line-of-sight (LOS) will be sufficient to support agriculture; however, this is not the case.
First, note that pesticide spraying craft such as the RMAX are necessarily heavier than what we
typically consider “small” UAS so it is important that risk be appropriately assessed4 which in the
case of rural agriculture does not require establishing specific size or weight constraints. Second,
the agriculture surveillance UAS will need greater flexibility than a LOS SUAS rule would offer.
Consider the powered glider and imaging UAS such as that shown in Figure 1a. This platform
will have a range and endurance that can and should support beyond-line-of-sight (BLOS) flight.
BLOS flight is essential for such craft to be effective, as many farms capable of appreciable profit
have large acreage that extends well beyond human visual range. Further, some crops can be ob-
served more effectively by alternative instrumentation (e.g., thermal, infrared scanners) at night
due to stomata closing to reduce evapotranspiration during the day. Perhaps the true “killer app”
for crop inspection by UAS is to fly the powered glider BLOS at night. Such an operation would
be most cost-effective and user-friendly if the farmer could deploy the craft just before retiring
for the evening. The UAS would then fly its pre-programmed mission overnight, downloading
status and alert data throughout the flight. The UAS would end its flight by autonomously return-
ing to base and (optionally) downloading collected data before the sun rises. The farmer could
then get a good night of sleep and examine collected data the next morning.
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There are numerous benefits to the “overnight crop inspection” mission. First, collected data
may be more informative due to night-time crop state as mentioned above. Second, night-time
flight in a rural area is even less likely than daytime flight to pose risk to people on the ground or
in the air. Third, given that profit margins are already small for most agricultural operations, au-
tonomous flight frees the farmer or any employees to complete the usual farm work without the
need to somehow find several “free hours” to monitor a UAS ground station display.
As will be discussed below, for low-altitude UAS applications to be realized reasonable guide-
lines must be established with respect to both airspace and autonomy. As a baseline for the rural
agriculture application, farmers simply need recognition of their freedom to fly in a manner that
can beneficially impact their livelihood. This can be accomplished by rulemaking efforts that
will take significant time at potential loss of US competitiveness, or else through recognition of
the private property owner’s right to occupy the airspace within immediate reach of their property
without regulatory oversight.
A precedent for “immediate reaches airspace ownership” by the property owner has in fact al-
ready been established by the Supreme Court (United States v. Causby – 328 U.S. 256 (1946),
http://supreme.justia.com/cases/federal/us/328/256/case.html). In this case, a landowner and
farmer argued that low-flying aircraft were causing harm to his chickens thus his livelihood. The
Supreme Court ruled that the landowner did have ownership to some volume of “immediate
reaches” airspace above his land, but the Court did not offer a specific above-ground-level alti-
tude defining the extent of immediate reaches airspace. Excerpts from the ruling include: “We
have said that the airspace is a public highway. Yet it is obvious that, if the landowner is to have
full enjoyment of the land, he must have exclusive control of the immediate reaches of the envel-
oping atmosphere. Otherwise buildings could not be erected, trees could not be planted, and even
fences could not be run.” (Page 328 U.S. 264), and “The superadjacent airspace at this low alti-
tude is so close to the land that continuous invasions of it [by publically-regulated aircraft] affect
the use of the surface of the land itself. We think that the landowner, as an incident to his owner-
ship, has a claim to it, and that invasions of it [by publically-regulated aircraft] are in the same
category as invasions of the surface.” (Page 328 U.S. 265). Quite simply, UAS use within imme-
diate reaches of privately-owned (rural) property can impact the owner of said property for agri-
culture and other uses. Such impact must therefore be carefully considered in the context of the
United States v. Causby ruling. If the landowner wants UAS flights he/she should be free to con-
duct them or grant permission for others (e.g., commercial operators) to conduct such operations.
Conversely, if the landowner does not want any UAS operations over his/her property, he/she
should also be free to say “no” to UAS presence in his/her immediate-reaches airspace. Recogni-
tion of immediate reaches airspace will support the emerging commercial UAS industry while
simultaneously addressing privacy concerns. This is also consistent with United States v. Causby.
The agriculture applications described in this section may perhaps be accomplished without
specific regulatory oversight except formal definition and recognition of low-altitude airspace
over privately-owned property as “immediate reaches” (thus not taken for public use), “non-
navigable” (in that it is sufficiently low or close to obstacles to pose a hazard at least to manned
fixed-wing aircraft),4 or both. Care does need to be taken that such operations will remain within
designated lateral and altitude airspace boundaries, much like farmers are responsible for keeping
livestock within fenced property boundaries (but perhaps with a higher level of confidence for the
UAS). BLOS UAS operations, day or night, will necessarily rely on instrumentation and soft-
ware to remain within designated boundaries. Geofencing systems, already available on some
UAS, can provide such a capability through autonomy that can and will override a pilot who di-
rects the aircraft outside property bounds. Specifics on potential airspace designations and the
necessary autonomy required for robust and certifiable geofencing are discussed below.
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CLASS U: PRACTICAL LOW-ALTITUDE AIRSPACE ALLOCATION FOR UAS
The National Airspace System (NAS) is currently divided into a set of classes (A, B, C, D, E,
G) that distinguish operational requirements. The NAS has a “static” structure that is specified on
aviation charts as shown in Figure 2 with the operational requirements summarized in Figure 3.
As shown, very low-altitude airspace is designated as uncontrolled (Class G) except near pub-
lished airports. Commercial transport aircraft and their passengers primarily occupy airspace
Class A enroute and Class B for approach and departure. Commercial and charter flights also
operate out of smaller airports surrounded by Class C or Class D airspace. General aviation
flights typically operate out of smaller airports (Classes C, D, E) and then typically fly above the
Class E airspace floor to minimize risk due to terrain proximity. Exceptions to this rule include
missions involving overflight of a particular property (e.g., aerial inspection or pesticide applica-
tion). For fixed-wing aircraft, altitudes less than 500’ above ground level (AGL) are not consid-
ered safely navigable per the Federal Aviation Regulations (FARs)* thus should never be occu-
pied except on final approach and initial departure where obstacles must be mapped and marked.
Figure 2. The National Airspace System (NAS): Current classification. (https://www.aopa.org/-/media/Files/AOPA/Home/Pilot%20Resources/ASI/various%20safety%20pdfs/airspace2011.pdf)
Rotorcraft (helicopters) may operate within or transit Class G airspace. However, rotorcraft
have no real need for extremely low flight (e.g., less than 500’ AGL) over rural private property
unless performing a law enforcement operation with appropriate search warrant or emergency
service mission5 (e.g., search-and-rescue, medical evacuation, firefighting) for which landowners
would readily ground their own UAS and grant temporary and immediate permission to overfly
and also to land as needed. In urban and suburban areas rotorcraft typically fly across a series of
public and private properties necessitating rulemaking to safely support existing manned and new
unmanned aircraft operations. Once approved it is likely that commercial low-altitude UAS oper-
ations will soon outnumber low-altitude manned operations of any type;† the question is how to
support both safely and effectively. Clearly the answer cannot be to continue labeling all low-
altitude airspace as Class G (uncontrolled) as this creates the current chaotic environment in
* Per 14 CFR 1.1 General Definitions: “Navigable airspace means airspace at and above the minimum flight altitudes
prescribed by or under this chapter [14 CFR], including airspace needed for safe takeoff and landing.” Per 14 CFR
91.119 Minimum safe altitudes: provided no emergency landing is required, aircraft must operate above 500’ AGL
(rural) or 1000’ AGL (congested areas including urban, suburban, and other assemblies of persons). Exceptions are
given to helicopters, powered parachutes and weight-shift-control aircraft. The latter two types tend to operate locally
thus would not overfly many different properties unless [unwisely] operating over small-tract urban/suburban areas. † In fact, the number of LOS hobbyist operations per day already likely outnumbers the typical number of manned ro-
torcraft flight operations per day.
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which manned rotorcraft attempt to see-and-avoid, hobbyists fly anywhere they please, and com-
mercial operators are told not to fly.
Figure 3. Operational requirements by airspace class. (https://www.aopa.org/-/media/Files/AOPA/Home/Pilot%20Resources/ASI/various%20safety%20pdfs/airspace2011.pdf)
This paper proposes introduction of a new airspace class, say Class U, that establishes the sort
of common-sense requirements to enable low-altitude UAS that mirror the existing airspace clas-
ses that historically have made a great deal of sense for manned operations. As a start, the new
Class U designation could be exclusively defined as the airspace from ground level up to, say,
500’ AGL, directly below all existing Class G airspace. With this definition the new Class U
airspace would have absolutely no impact on commercial transport operations* and almost no im-
pact on other manned operations with the exception of the rotorcraft missions discussed above.
Figure 4 illustrates the proposed Class U airspace designation, shown in yellow under all Class G
airspace. While this appears as one airspace type, appropriate policy will require further sub clas-
sification by attributes such as property ownership and type. This additional complexity in classi-
fication is required because operations in Class U will necessarily occur close to the ground
where operational approval criteria, level of risk, and potential for privacy violation are strongly
influenced by the attributes of the property being overflown.
ClassB
Class E
Class U(*,*)
Figure 4. Class U airspace (<500’ AGL).
The (*,*) notation indicates property ownership and type as shown in Table 1.
* Provided Class U occupants did not illegally enter other airspace.
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Table 1 proposes a specific sub classification for Class U airspace. Property can either be pri-
vately or publically owned, while property type can be rural, suburban, or urban. Collectively
these attributes generate six distinct sub classifications. Although a quantity of six sub classes
may seem a bit cumbersome from a regulatory perspective, the diversity in operation types makes
it absolutely essential for rulemaking to reflect the appropriate needs and concerns for each class
of property. Without such distinctions, common-sense rulemaking will be impossible. To sup-
port this claim, consider two of the example missions listed in Table 1: flight over a national for-
est versus flight through an urban canyon. For these examples, both properties are publically-
owned thus subject to rulemaking rather than immediate-reaches/imminent domain per the above
agriculture case study. Low-altitude overflight of a national forest is challenging because of po-
tentially complex terrain and obstacles (trees), but loss-of privacy and safety risks to people and
[improved] property on the ground are minimal so long as the UAS doesn’t create a forest fire,
stalk tourists, or spill chemicals following an emergency flight termination. Conversely, flight
down an urban canyon supports the well-publicized “delivery drone” mission with high potential
to generate capital, but this mission is fraught with situations that place both people and property
at risk. Both payload and flight sensors also have potential to violate privacy in the urban can-
yon. Rulemaking must directly deal with these differences if common-sense policy is to emerge.
Table 1. Class U Airspace Overflown Property Designations.
Airspace
Class
Property
Class
Property
Ownership
Property Type
(Population)
Examples
U (P,R) Private (P) Rural (R) Farm, utility (e.g., pipeline), unimproved (e.g.,
forest, desert, mountain, water)
U (P,S) Private (P) Suburban (S) Residential, business
U (P,U) Private (P) Urban (U) Multi-tenant residence, business
U (G,R) Govt / Public (G) Rural (R) National park, national forest, waterway
U (G,S) Govt / Public (G) Suburban (S) Transportation infrastructure, public utility
easements
U (G,U) Govt / Public (G) Urban (U) Public parks, public buildings, city streets,
bridges
Table 2 proposes an initial draft of operational requirements for the six Class U property-
based sub classifications. This draft would of course require substantial analysis and revision to
ensure an adopted policy is appropriate. The fields in Table 2 intentionally mirror those in Figure
3 to facilitate comparison. New link requirements and safety requirements achieved primarily
through onboard automation technology are also proposed. As shown, the privately-owned, rural
operation has no link or pilot training requirement, consistent with the concept that the landowner
or lessee with explicit landowner permission is responsible for authorization and safety of UAS
operating in his/her “immediate reaches” airspace. For Class (P,R) the only safety requirement is
a geofence capability to ensure the UAS does not exit the private property owner’s airspace.
While both of the remaining privately-owned property classes require owner permission or a
search warrant for entry, additional requirements are prudent because these operations will occur
on smaller-acreage properties, e.g., residential neighborhoods. Confined operations over private-
ly-owned properties, e.g., for real estate videos, can likely be accommodated through local link.
Safety features such as the geofence plus flight termination are prudent given the small test range,
and line-of-sight (LOS) restrictions consistent with the local link are reasonable. This type of
operation is already being conducted by “hobbyists” with only basic transmitter “stick skills”. It
is reasonable to expect that such LOS operations can continue to be conducted safely with train-
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ing consistent with that recommended by AMA (the Academy of Model Aeronautics); it is less
reasonable to require an LOS operator of Class U (P,S) aircraft who will likely operate the UAS
while standing in their back yard or sitting in their lawn chair to pass an aviation medical exam
and/or earn a manned aircraft pilot’s license. For urban operations over privately-owned proper-
ty, the expectation is that this property size will be smaller (e.g., above a high-rise roof), so an
additional VTOL (vertical takeoff and landing) restriction may be prudent.
Flight above publically-owned property will necessarily be governed through rulemaking,
perhaps a combination of federal, state, and locally-specified guidelines. Examples distinguish-
ing “common-sense” differences between types of public property overflown were given in Table
1, which in turn suggest the requirements listed in Table 2 below. Low-altitude flight over rural
areas, whether private or publically-owned, does not pose risk to people or property. However,
any operator should expect to assume responsibility to recover his/her UAS if it does not return to
the designated recovery site. To support this need, Table 2 recommends return-to-base and flight
termination capabilities as well as “facility training” for the UAS operator. Note that facility
training would be aimed at ensuring the operator understood how to operate safely above and on
the public property in additional to having AMA-level skills. Suburban and urban flight over
publically-owned properties, including roads, bridges, waterways, and parks, will require the
most regulatory oversight of the proposed six Class U sub classes. Both (G,S) and (G,U) opera-
tions have potential to pose loss-of privacy and safety risks to people and property. Additional
requirements for (reliable) long-range communication, UAS pilot certification, and detect-and-
avoid (with ground and air) are proposed. Note that the designation “UAS pilot” implies training
that leads to certification. A competent UAS pilot will require understanding of their craft, in-
cluding link and automation behaviors. The UAS pilot will also need to know how to interact
with ATC and manned aircraft (e.g., rotorcraft) that might occupy shared airspace. However,
specifics on the UAS pilot rating require further research; such a rating must not be presumed to
require either an aviation medical exam or any manned aircraft pilot license as the UAS pilot will
never be exposed to low-density atmosphere or high-g maneuvers and will not have the immer-
sive sensory feedback available to an onboard pilot.
Table 2. Candidate Operational Requirements for Each Class U Property Sub Class.
Property
Class
Entry Requirements Link
Requirements
Safety
Requirements
Minimum Pilot
Qualifications
(P,R) Owner permission or
search warrant
None Geofence None
(P,S) Owner permission or
search warrant
Local Class U(P,R), flight termina-
tion, LOS
AMA
(P,U) Owner permission or
search warrant
Local Class U(P,S), VTOL AMA
(G,R) Public agency permis-
sion
None Class U(P,R), flight termina-
tion, return-to-base
AMA, facility train-
ing
(G,S) Community permis-
sion, certification
Long-range Class U(G,R), detect-and-
avoid (ground)
UAS pilot
(G,U) Municipality permis-
sion, certification
Long-range Class U(G,S), VTOL, detect-
and-avoid (air)
UAS pilot
While most low-altitude missions can be conducted in the proposed permanent Class U air-
space, some missions may require flight above the Class U boundary. Certainly, comprehensive
integration including detect-and-avoid will enable such flights but this is a very long-term solu-
Proc. AUVSI North America, May 2014
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tion as most general aviation and low-cost UAS will not be equipped with certified detect-and-
avoid for some time. An alternative to full detect-and-avoid is dynamic airspace allocation. In
the modern age of connectivity, Class U airspace “boxes” can be reserved and carved from Class
E and G airspace. Such usage can be communicated system-wide through the web / cell network
or means such as the NOTAM system plus regular announcements on local ATC voice communi-
cation frequencies.
ClassB
Class EClass U(t0,tf)
PermanentClass U
under Class G
Dynamic allocation of (temporary) Class U from Class E or G
Figure 5. Dynamic Class U airspace allocation.
ELECTRONIC GEOFENCING (OR LEASHING) FOR SAFETY MANAGEMENT
The above discussion proposes a Class U airspace and suggests its progressive introduction,
starting with the “easiest to implement” rural, privately-owned sub class. As shown in Table 2,
flight in privately-controlled (immediate reaches) airspace will be safe to entities outside this pri-
vately-controlled airspace so long as each aircraft (UAS) remains within designated altitude and
lateral airspace boundaries. Ensuring a UAS remains within designated boundaries, especially
when flying BLOS and/or night-time operations, requires a geofencing or electronic leashing au-
tomation aid. This automation aid in reality represents autonomy in that the pilot-in-command
(UAS operator) will not have full override authority should said pilot be directing the UAS to exit
the designated Class U airspace when the geofencing software intervenes to ensure the aircraft
remains within the designated area.*
Figure 6 shows a candidate geofencing algorithm that is straightforward in the software and
for the operator. This algorithm is a high-level summary of the geofence software implemented
in the open-source Ardupilot / Arduplane system recently repackaged as the APM Autopilot suite
with upgraded processing capabilities. Software-based geofencing is optional in this system – the
operator can choose no geofence, altitude-only, or lateral plus altitude. Because the Ardupilot is
intentionally simple, the geofence never prompts replanning of the flight – it instead triggers de-
scent and/or a turn back to a central “base” or “return-to” waypoint.
* Common arguments against autonomy involve “what if…” (anomalous) scenarios. UAS operating over private prop-
erty can flight terminate at any juncture with risk only to the private property and its occupants; a property owner’s
decision to fly (or do anything else for that matter) on or immediately above their own property is always at their risk.
Proc. AUVSI North America, May 2014
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Function geofence (altitude_check_only_flag, geofence_violation_flag)
1. Check if geofence is enabled and load any new geofence definition. a. If geofence is not enabled, check if pilot/operator has activated
the geofence control reset switch, reset switch per direction from
operator, and RETURN.
b. If geofence is enabled, check for new boundary definition and load as needed. CONTINUE if new geofence is enabled; otherwise RETURN.
2. If geofence minimum or maximum altitude is violated (new violations only; do not trigger on existing violations indicated by a set violation_flag):
a. Set new reference/waypoint altitude to return value (either user-specified if acceptable or exactly between minimum and maximum
values otherwise).
b. Activate autopilot altitude flight control law as needed.
3. If altitude_check_only flag is false and geofence lateral boundary lati-tude, longitude is violated (new violations only; do not trigger on ex-
isting violations indicated by a set violation_flag):
a. Set lateral latitude, longitude waypoint to a default “return-to-base” value.
b. Activate autopilot lateral flight control law as needed.
4. If operator has triggered a reset, reset violation_flag to zero and re-sume regular manual or waypoint flight.
Figure 6: Ardupilot Geofencing Summary (http://ardupilot.com/).
Processor
DatalinkAutopilotGeofence
Processors
DatalinkAutopilot
Pilot Controls
Laptop
Ground station
UAV
Geofence
UAV
Pilot Controls
Laptop
Ground station
Servos SensorsServos Sensors
(a) Single processor architecture (b) Multi-processor architecture
Figure 7: Candidate geofencing processor architectures.
Figure 7 shows the two basic processor configuration strategies for implementing a software
geofencing capability. On the left, a single onboard processor handles all nominal functionality
plus geofencing. This is desirable and was the default Ardupilot (with Arduino) configuration
because a single processor minimizes cost, weight, power, and wiring harness complexity. On
the other hand, if other software on this shared processor experiences a problem, the geofence
Proc. AUVSI North America, May 2014
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also will fail, suggesting an architecture (Figure 7b) where the geofence software is pulled onto a
separate (micro)processor with the sole purpose of ensuring the vehicle remains within its desig-
nated operating area. The Figure 7b design is likely best for research groups who will be modify-
ing nominal autopilot functionality thus potentially introducing software problems; the Figure 7b
design can also provide computational redundancy so long as both processors nominally guide
the aircraft inside its operating area, although for full redundancy the processors would need to
utilize independent servo connections and inertial navigation sensors as well as providing logic
such as a watchdog-triggered bypass for servo output source selection. A key feature of the Fig-
ure 7b design is the possibility of certifying only the geofencing module independent of the rest
of the autopilot system, which should be sufficient for safety certification in applications such as
LOS test flights and rural agriculture.
CONCLUSION AND FUTURE WORK
This paper has described a practical vision for low-altitude UAS operations that will enable
their immediate (or near-term) deployment through judicious use of autonomy to assure safety
using techniques such as software-based geofencing. Agriculture is defined as a compelling ref-
erence mission for low-altitude UAS that must operate above the “small” category and BLOS to
be effective. A Class U airspace designation is proposed for surface to 500’ AGL below airspace
currently designated as uncontrolled (Class G). Operational requirements for Class U necessarily
and for the first time depend on property ownership and type to ensure safety and privacy while
also supporting rather than unnecessarily restricting commercial UAS operations. Autonomous
geofencing is proposed as a means to ensure low-altitude UAS do not exit their designated Class
U operating area. A certified geofencing capability can in fact ensure safe flight in rural areas
without the need to wait for system-wide detect-and-avoid capability so long as manned aircraft
remain clear of Class U or else obtain specific authorization thus clearance through specific pri-
vate or publically-owned Class U airspace regions.
For urban areas, the geofence must be supplemented by additional safety and even privacy
management capabilities. Perhaps most important is detect-and-avoid not just for other aircraft
but also for people and property on the ground. Given the need to collect a large volume of sen-
sor data both to ensure safe operations and to support surveillance missions, policy to ensure ap-
propriate privacy of collected data is indeed needed for urban and potentially suburban opera-
tions. Privacy must be specifically recognized as not being a concern for rural UAS operations
conducted by private property owners; otherwise privacy issues will continue needlessly delaying
approval of these rural operations.
While Class U airspace could be created in the near-term through a concerted rulemaking ef-
fort, it will be prudent to phase in deployment of Class U from the “easiest” case of private prop-
erty, rural (P,R) to the more challenging case (G,U) for which the public expects privacy and de-
mands safety. Additionally, public entities (e.g., municipalities, department of transportation au-
thorities) must commission or at least approve UAS overflight of their properties. Such entities
must gain a better and more realistic understanding of the needs and desires of their constituents
with respect to benefits of the operations as well as safety and privacy concerns. Only then can
these needs and desires be translated to policy that makes sense. Urban UAS operators will then
Proc. AUVSI North America, May 2014
12
only be allowed to “darken the skies” with delivery drones should their activities truly be safe and
supported overall by the community being overflown.*
This paper motivates and proposes a regulatory scheme to enable low-altitude UAS operations
with neither the need for manned aircraft detect-and-avoid nor real-time air traffic control over-
sight. This strategy will enable commercial UAS operations and will finally enable both com-
mercial and academic entities to routinely flight test emerging technological capabilities. How-
ever, this operational paradigm is incomplete. Substantial additional research, procedure, and
policy developments are required to accomplish the end-goal: comprehensive integration of UAS
into the NAS. It is this author’s conviction, however, that the proposed Class U airspace will
continue to fill a long-term rather than a short-term need, as even when technology and training
are ready to accommodate full integration, property ownership and type must still factor promi-
nently into low-altitude airspace ownership, operational approval, certification, and training re-
quirements.
ACKNOWLEDGMENTS
The author would like to thank Jerry Lin for his work to decode and flow chart the Ardupilot
geofencing algorithm. This work was supported in part by NASA contract NNX11AO78A.
REFERENCES
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* Such endorsement will likely involve financial compensation in the form of operator taxation and/or public airspace
usage fees, not just the public’s overwhelming desire to have packages or pizzas delivered by air.