1. technology and computing

Practical Challenges Faced by Cellular Radio and Unmanned Aircraft System (UAS)
Research and Development – A Work in Progress
Capstone Final Paper
April 30th, 2015
Stefan Tschimben
Mason Ryan
Aaditya Goyal
Assyl Zhakupov
Interdisciplinary Telecom Program
University of Colorado Boulder
Abstract— When search and rescue operations set out to save
an avalanche victim, locating the victim within a reasonable
amount of time is critical for survival. Also, as 4G LTE
smartphones are becoming more ubiquitous within The United
States, those emissions can be gathered using unmanned and
automated drones to provide crucial locational information for
search and rescue (SAR) teams. This project set out to utilize
technological advances in compact drones and a mounted
enterprise small cell to improve the current state of SAR
procedures with the purpose of more efficiently locating a buried
victim in an unserviceable area during an avalanche. The main
goal of this project was to utilize 4G LTE emissions and drones to
provide emergency communications to SAR efforts, while
expanding on the previous research completed at CU, which
proved the feasibility of receiving a cell signal under several feet of
snow within the presence of a common carrier signal. In order to
conduct this research, our team made the decision to operate
legally and adhere to all regulatory requirements. However, this
decision exposed significant practical challenges which were not at
first anticipated. A Certificate of Authorization (COA), issued by
the Federal Aviation Administration (FAA), is required to legally
fly an unmanned aircraft system (UAS) both for research and
regular operation. This proved very challenging for our team as it
required extensive knowledge in avionic engineering and our team
was the first to file a COA for a rotary UAS, compared to the
traditional fixed wing or quad UAS. Additionally, a Special
Temporary Authority (STA) had to be requested from the Federal
Communications Commission (FCC) in order to operate the
enterprise small cell as an active transmitter for each geographic
area. Due to our innovative idea of a completely mobile low power
transmitting base station on a drone, or what we refer to as Cell
on Drones (COD), the attempt to legally follow regulatory
requirements created a substantial challenge for our research
team due to the unique nature of our solution. While these barriers
proved to be more substantial than first thought, the team was able
to establish a viable geolocation strategy and gain significant
experience when working with real world FCC and FAA
regulatory requirements. Our research furthermore garnered
substantial endorsement and cooperation from industry experts
such as Cisco and Northrop Grumman, as well as government
agencies including NTIA and the Department of Commerce
working on FirstNet. These high levels of interest and support
shown to our solution reinforced the critical importance of not
only our research, but also the challenges our team faced, while
creating various opportunities for our team to continue this
research beyond our school careers.
Search Terms—4G LTE; SAR; COA; FAA; UAS; FCC; COD
Dr. Harvey Gates Team
Assistant Professor
University of Colorado Boulder
Nicholas Little
CEO/Industry Advisor
Forge Aero LLC
I.
INTRODUCTION
A. Statement of the Problem
Time and communication are of essence in search and rescue
operations. In fact, the ability to communicate is a crucial
element of emergencies as it ensures that first responders are
able to react as quickly as possible. Failure, or the absence of
communications infrastructure can be fatal in such situations.
Unfortunately, during large-scale disasters or emergencies in
remote areas, such infrastructure is often one of the first things
to fail, either due to direct damage, power outage or network
overload [1], [2]. Its absence also considerably slows down first
responder efforts. During the floods in Boulder County of 2013
both landlines and cell services had been interrupted in remote
locations; some areas could even only be reached by radio [3].
This research will focus on another type of noteworthy
natural disaster, more specifically avalanches. Especially in
Colorado, where a considerable part of all avalanche related
deaths within the United States occur [4], this topic gains in
importance. Avalanches present a special challenge for SAR
teams. The situation demands a great amount of efficiency and
speed as a complete burial of the victim reduces survival chances
significantly after only 15 minutes due to the high risk of
asphyxiation [5], [6]. As skiers and snowboarders increasingly
seek out untouched snow in back country areas [7] the number
of potential deaths increase. These continuing developments
make it of utmost importance to develop a solution that makes it
possible to shorten the recovery time by determining the
victim’s location as fast as possible by relying on opportunities
offered by current technology such as a smartphone.
While special beacons make it possible to speed-up the SAR
operations to a certain degree, they still come with two
considerable disadvantages: they have to be activated and are
comparatively expensive. The ubiquity of smartphones on the
other hand continues to grow. By now, 169 million people in the
US own a smartphone, 70% of all mobile phone users [8]. More
importantly, over one third of mobile broadband usage in the
first quarter of 2014 has occurred using LTE (Long Term
Evolution) [9]. With the current advances in LTE, this offers an
incredible opportunity for SAR operations to improve available
communication technology on site, as it is rarely one of the first
aids to arrive, slowing down the rescue operations by delaying
coordination and the communication of crucial information [10].
Indeed, the United Nations Foundation not only emphasizes
the importance of information sharing during emergencies, but
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also highlights the potential of these new mobile technologies
[11]. Combined with the ability of Unmanned Aircraft Systems
(UAS) to carry a number of different payloads along with the
potential to be rapidly deployable over an extended amount of
time [10], this development opens up a great amount of
incredible opportunities [2]. Different UAS could therefore be
deployed in emergency situations for SAR operations in order to
fulfill a variety of purposes and solve traditional SAR issues.
While the propagation of the phone’s signal while buried under
considerable amounts of snow might present a challenge, recent
research by CU determined that the cellular signal in fact
propagates through up to 7 feet of snow with only very little loss
[12].
Regardless of the specific circumstances of the disaster and
the SAR operation, one important aspect will always remain,
determining the location of the victim to be rescued. This can be
significantly sped up using the signals emitted by the victim’s
smartphone. Especially in remote areas with no communication
and difficult access due to terrain, UAS could offer an optimal
platform to speed up SAR efforts by enticing a cell phone into
connecting to its payload and revealing its location through its
cell signal. The signal could be processed by the UAS using a
variety of passive direction finding methods and algorithms.
B. Research Question
In emergency situations, communication is critical for search
and rescue teams to be effective and swift in their operations
[11]. Communication becomes even more important when a
natural disaster ceases all cellular communications. By utilizing
leading mobile technology, SAR teams can become more
effective in times of an emergency in an unserviceable area. By
utilizing the unique features of new mobile and drone
technology, the opportunity for saving lives could be
significantly improved. Emergency communication, other than
immediate life safety, is the most crucial aspect within a disaster.
However, significant regulatory hurdles exist and must be faced
before implementing this new technology. This proposal will
seek to overcome these hurdles and improve the current state of
the art in SAR by answering the following question:
What challenges do researchers and search & rescue teams
face when developing innovative solutions with the intent of
better locating a victim’s position in an emergency situation in
absence of a cellular network?
C. Sub Problems
Improving the current state of the art in SAR is a challenging
task. Search and rescue teams choose to not regularly implement
unproven methods due to the very life or death nature that comes
from their duty. For the sake of this proposal and to limit the
scope for the reader, this research will focus on search and
rescue teams in response to natural disasters, more specifically
avalanches. Avalanches are the most prevalent natural disaster
in Colorado. In fact, one third of all avalanche related deaths in
the United States occur in Colorado [4]. When exploring the
current SAR methods in response to avalanches, we seek to
improve current standards and push the envelope in developing
groundbreaking rescue methods enhanced by new technologies.
Within our innovative solution lie three major problems. This
section will present the three sub problems in the form of a
question, the assumptions used, the background of the problem
and what action steps are suggested in order to solve each of
them.
Discovery: What is the best method for awakening a mobile
phone device in sleep mode within an unserviceable area?
Assumptions: A major assumption involved with this
problem is that a person needs rescue assistance due to a natural
disaster. Along with this assumption, the person must have a
mobile smart phone turned on, but he/she does not have cellular
service or any other means of communication during the
emergency. If the cellular device is within an unserviceable area,
we assume the phone will enter a “sleep mode”, where the phone
stops actively seeking out the nearest base station [13]. These
assumptions form the basic premise of why SAR teams do not
currently rely on smart phones and will have a significant impact
on our proposed solutions. We assume there is a possible way to
awaken a phone out of sleep mode, by convincing the smart
phone of the presence of a nearby base station available for
connection. Associated with that assumption, we conclude the
UAS being deployed is capable of holding and powering a small
cell during flight. This could prove to be a challenge as both the
UAS and the small cell will rely on limited battery life,
ultimately limiting flight duration. As the industry is developing
solutions, our final assumption is that we will be able to obtain
the enterprise small cell, weighing less than fifteen pounds,
which was offered to us as a loan by Global Wireless
Technology vendor. This programmable and flexible small cell
will be essential for our purposes to awaken a phone from a
designated sleep mode.
Problem Background: When a mobile smart phone device
enters an unserviceable cellular area, it still continues to search
for the nearest base station. Even though mobile phones are
constantly searching for an active signal, this process is greatly
reduced when within an unserviceable area. When no base
station can be found, a smart phone will enter sleep mode, where
the phone stops searching for a connection to a base station to
conserve battery life [13]. In order to wake up a phone from
sleep mode, it requires a burst from a base station signaling a
connection is present and ready to be made [14], [15]. If a person
is downed with no cellular service, it presents a tough challenge
for SAR to locate that person via the person’s cell phone, and is
the main reason why SAR teams do not currently rely on smart
phones. Within a mountainous environment, there are multiple
back country areas where cellular service is not provided. Even
if a cellular network is nearby, when a person is buried under
several feet of snow, the signal’s propagation might be impacted
and the smart phone could have a harder time searching for an
active signal.
Proposed Solution: Our team conducted extensive
interviews of industry professionals and research in order to
discover the best means of waking up a smart phone out of sleep
mode. Our proposed solution was centered on an enterprise
small cell, or an emulated base station, to provide service to an
otherwise unserviceable cellular area. The small cell would be
attached to an UAS which is then actively seeking out the
downed smart phone and theoretically would emit a burst strong
enough to notify the smart phone of a nearby connection. The
benefits of using a small cell include the capability of operating
on different mobile bands and the ability to be programmed to
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suit our SAR needs. The payload from the small cell would
notify the smart phone of a nearby base station, therefore putting
the phone back into an active search mode and become visible
for SAR discovery. This proposed solution would require no
interaction by the downed person and could be used in a variety
of emergency situations to either find a missing person or to even
provide basic coverage for a cellular phone network.
Locational Tracking: How can SAR teams locate a downed
person’s cell phone based on its 4G LTE emissions using a UAS
and geolocation techniques?
Assumptions: Following the previous problem’s
assumptions, this problem will contain some similarities. A
person would still require rescue assistance due to a natural
disaster and the SAR team must be ready to initiate their rescue
process. One major difference in assumptions related to this
problem is the method of passive direction finding. Passive
direction finding indicates the searching techniques and analysis
performed are not in real time. In order to achieve real time
results, which will be crucial for future SAR operations, this
theory first must be tested and then further adopted amongst the
industry. Another assumption is the SAR team has a capable and
rapidly deployable UAS on hand, with the intention of
performing passive direction finding. We also have to assume
we can perform field tests by obtaining a Leptron Avenger
helicopter, a UAS with military grade features, and a UAS is
the preferred vehicle for rescue operations [16]. The final
assumption is the UAS must be programmable and capable of
flight time long enough to support SAR operations.
Problem Background: In order to implement passive
direction finding as a viable option for search and rescue teams,
our team had to first validate the advantages it would have over
traditional SAR methods. In the avalanche scenario, SAR teams
typically rely on an avalanche transceiver. The transceiver is
used mostly in back country skiing areas and can be very costly,
averaging around $300 [17]. There are many limitations to
transceivers, but they have proven to be reliable in avalanche
disasters in the past and are still heavily used today. Search and
rescue teams rely on what works, but also need to explore future
technologies so they are not behind the curve when a critical
disaster strikes. Henceforth, assuming every skier or
snowboarder has a smart phone, this technology will be far more
advantageous than a traditional avalanche beacon due to the
ubiquity of smartphones today.
Another major problem with current SAR methods is the
timeliness and limitations of an avalanche mountain rescue
operation. SAR teams are limited by the mountainous terrain and
dependent on a downed person having activated an avalanche
transceiver. A challenge that was not tackled in this report are
the resources needed to deploy an active SAR solution with real
time results. This means allowing for real time locational
tracking, while establishing a communication infrastructure for
SAR teams on the backbends of most unserviceable
mountainous areas. This active SAR solution will require app
development, extensive funding, and additional testing even
before initial implementation. Implementing an active SAR
solution is more suited towards a business and was out of scope
for our project purposes.
Proposed Solution: Our project sought to find a viable and
inexpensive solution via passive direction finding when
compared to a traditional avalanche transceiver. Passive
direction finding will be used to measure emitting signals from
the person’s smart phone and measure the spikes at a specific
location. Our proposed solution focused on the UAS performing
the passive direction finding in a programmed search pattern on
the mountain. This pattern will be programmed to swing inwards
and outwards, similar to an “S” shape, in order to cover a wide
area of land, but still keep the search area well defined. The data
will be gathered in 30 minute flight intervals and the
transmission spikes will be analyzed to determine the location.
Assuming the smart phone has no signal, it is not possible to
locate the device via cooperative communication or even a
traditional GPS sense. With future testing and funding UAS
would also be capable of using a camera to display a live feed of
the suggested area of entrapment.
Although the majority of the passive direction finding will
be conducted by a UAS, it will also be important to test this
solution using ground elements. Motor vehicles such as cars and
snowmobiles can cover a good area of land, and could even be
used simultaneously during the active UAS search. By utilizing
a passive direction finding technique, it would allow SAR teams
to traverse treacherous terrains and deploy rescue efforts at a
more rapid rate. Helicopters are used in conjunction with
avalanche transceivers but are limited by the mountainous
terrain and slow deployment period. When time is of the
essence, UAS could provide a significant advantage by flying
over hard to traverse land, leading to an improvement of the state
of the art in search and rescue.
Regulatory Barriers: What regulatory restrictions are currently
in place and present considerable practical challenges to future
research and implementation?
Assumptions: For the scope of this project we assumed that
these regulatory barriers were not big enough barriers to hinder
our research efforts. For sake of the research we also assumed
operating in the public safety Band 14 frequency at 700 MHz
using 4G LTE, was the best means to simulate the intended
conditions. Testing was focused on areas where cellular service
is very limited or ideally not available, also known as radio
silent. Another assumption is that the Leptron Avenger UAS
model would be provided and the Certificate of Authorization
(COA) to properly fly it would be completed in a timely manner.
These assumptions evolved to be breaking assumptions and had
a significant impact on our project scope.
Problem Background: These barriers proved to be a
noteworthy practical challenge for our research team. When
attempting to use an enterprise small cell the base tower would
be operating on a specific frequency that is regulated by the
Federal Communications Commission (FCC). For plausible and
practical implementation, the SAR teams would have to comply
with FCC spectrum policies and definitely obtain a license in
order to legally operate in the needed frequency range. With
further research, emergency communications may rely on
nonpublic frequency bands and would require a series of action
items in order to operate on those frequencies. In regards to
emergency communications, the FCC has unique policies during
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a time of crisis and these policies would have to be made well
known and clear. [18]
For our purpose, along with the FCC is the FAA, or the
Federal Aviation Administration, the controlling authority on
the ability to occupy air space. Similar to the limited and
regulated spectrum, airspace is a regulated zone and requires a
license and certificate to operate in. For our purposes of field
testing, we had to acquire not one, but two Certificate of
Authorizations in order to fly the UAS legally [19]. For flying
the UAS on a local ski mountain it would require the owners of
the land to approve of the UAS flying around and acquiring a
COA for the specific area as well. In order to receive a COA,
one must file the specific tail number and model of the registered
UAS to be used with the FAA, and the purpose of UAS use.
Additionally, extensive aeronautic information had to be
submitted which we and Leptron had to provide. Operating in a
realistic field testing scenario such as a ski resort posed potential
problems. Due to the nature of ski resorts, many visitors carry
smartphones which could potentially interfere with our data
collection due to the countless emissions being broadcasted.
Also, the mountainous terrain at a ski resort could prove to be
too difficult to conduct tests on as the terrain is hard to traverse
for a team of non-skiers. The back country areas where the tests
would have been conducted require accessibility via
snowmobile and snow shoes, making it an ordeal to perform a
simple test scenario.
Proposed Solution: In order to comply with the challenging
regulatory requirements, it was necessary to take extra
precaution and work diligently to resolve these issues as early as
possible. Towards the end of 2014, we established contact with
James Mack of the University of Colorado at Boulder and have
provided him with the information needed and also the timely
importance of filing a COA. On our behalf, Professor Mack was
supposed to file a COA for the use of the Leptron Avenger UAS
with the registration and tail number for the duration of the
spring 2015 semester. By starting this process as early as fall
2014, it was supposed to ensure the near two month long
authorization process would be approved and not hinder us from
conducting research. However, our support fell short due to a
hectic schedule and we were forced to file the intricate COA
ourselves. We also received approval to use the local range from
the Research and Engineering Center for Unmanned Vehicles
(RECUV). This field would have provided us with a great range
to test different equipment, flight of the UAS, and simulate an
unserviceable area. However, another COA would have had to
be filed for this location as well, totaling our COA submissions
to the FAA to two.
For our realistic mountain field testing, we obtained official
permission to conduct testing at Copper Mountain Ski Resort by
the Ski Patrol manager Hagan Lyle. Copper would have allowed
our team to conduct tests as long as the school could cover a
three million dollar liability coverage for the resort. As a student
research group, it was difficult to cover this large liability and
we could only get the school to cover half, leaving us to seek out
vendors to pay the other half. When faced with spectrum
regulations we received assistance from the University of
Colorado at Boulder Law School. This allowed us to perform
due diligence towards the regulations we had to adhere to. In the
end, the goal of this solution was to follow all regulations and
practical challenges faced by a SAR operator with intentions to
operate completely legally, considering airspace as well as
spectrum. Following a legal path however proved to create more
problems than solutions unfortunately.
II. LITERATURE REVIEW
The possibility of using UAS as an airborne multipurpose
communications relay has been examined on a theoretical basis
in a number of reports and journals [2], [10], [20], [21]. The
general consensus is that, compared to alternatives like cells on
wheels (COW) that could be used for this purpose, UAS have
the advantage of being rapidly deployable, able to cover a
greater area, less affected by the terrain and they use the
spectrum more efficiently [10]. The system is in fact comparable
to the reliability of GEO satellites, with the advantage of
increased flexibility and reduced costs [20].
This advantage can furthermore be exploited to determine
the location of user equipment more rapidly and reliably using
passive direction finding (DF) techniques. In fact, the literature
further suggests the idea of using airborne vehicles like drones
in combination with special antenna systems is intelligent [22]–
[24]. Thanks to new compact directional finding antenna designs
which could be easily mounted onto UAS of different sizes [24],
and the fact that passive systems weigh less due to lighter
hardware requirements, UAS offers a very suitable platform for
the deployment of a passive direction finding system [23]. While
it is true that passive systems provide lower accuracy than real
time active systems, it should be noted that such systems are not
only less complicated and require less power, but furthermore
can be enabled by a single aircraft in order to locate signal
sources [23], [25].
Direction finding, in the literature is often referred to as angle
of arrival (AOA) determination, which can operate under the
important advantage that no knowledge about specific features
of the signal is required [26]. A single UAS can therefore be used
in SAR situations in order to determine the AOA of signals and
consequently the location of user equipment (UE). Besides
simple mechanical rotation of antennas, three main methods for
passive direction finding are commonly used: time difference of
arrival (TDOA), amplitude comparison, and phase
interferometry [24]–[28].
Phase interferometry determines the AOA by comparing the
phase difference of a wave front arriving at spatially separated
antennas [28]. While this method allows accuracy with
measurements in short distance from each other, it faces
however size and bandwidth limitations. In order for phase
interferometry to produce minimal error, antennas should not be
further apart from each other than the wavelength of the
frequency to be measured. On the other hand, the further apart
the antennas are, the greater the time delay and the better the
measurements [25].
Alternatively, amplitude comparison methods are not limited
by bandwidth or frequency. They are usually implemented by
using two squinted antennas that receive the signal at different
power levels and consequently allow to derive the angle of
arrival [27]. However, amplitude comparison is much more
sensitive to noise fluctuation, which can differ significantly in
different locations. Still, in an environment with no or only little
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radio communication this might not present a noticeable
challenge [25].
Finally, time difference of arrival (TDOA) methods measure
the time difference between the arrivals of a signal at multiple
antennas. However, in order to determine the AOA with
accuracy these antennas would have to be placed kilometers
apart from each other. This necessity however makes such
systems impractical for airborne SAR systems that would rely
on quick measurements over a short distance [25]. The literature
suggests this will not be a practical solution for our research.
As the literature highlights, each of these methods has its
advantages and disadvantages, making some more useful in
specific situations. The effectiveness of each is further
dependent upon the algorithm that is used for the direction
finding, such as Wattson-Watt, Butler Matrix, and MUSIC [28],
[29] What the literature in general lacks is a practical application
of the theories in the context of search and rescue emergency
operations. Therefore we will have to conduct experiments using
multiple passive direction finding methods in order to determine
the most practical solution for SAR.
By being able to equip a drone with a programmable micro
cell, such as a GWT Small Metro Cell 053-2 [30], it would be
possible to determine how it might be best accomplished to
make a phone reveal its location. We could theoretically test any
other similar device that would make it possible to conduct field
tests, by measuring the signals such a system would receive and
the signals being exchanged. The technology to intercept calls
and spoof GSM cell towers has in fact already been in use by
law enforcement and successfully recreated in the form of homebrewed devices that entice cell phones to connect to them [31].
The result of these tests would be of great assistance in the search
for cheap, robust, and efficient location finding equipment to
improve the current state of SAR operations.
III. RESEARCH METHODOLOGY
Research Sub-Problem: What is the best method for awakening
a mobile phone device in sleep mode within an unserviceable
area?
Methodology: Industry interviews and library research
In most emergencies, there is a high likelihood of
communications being ceased within an area caused by a
disaster. The use of commercially available programmable small
cells could negate this issue. The micro cell simulates a mini cell
tower for a mobile phone to latch onto. They are also used by
operators to extend and improve the coverage within a small area
of an existing network. In the scope of this research, small cells
were intended to be used to mimic the behavior of cell towers
and introduce an efficient enough signal within the disaster area.
This would theoretically enable the phone to ping its
identification information to the tower in an effort to connect
onto the network. In order to solve this first sub problem, our
team focused on the distinct advantages of small cells and
alternative solutions. Interviews with industry experts where
then used to discover the practicality of this solution.
Small cells, have a huge competitive advantage over most
current technologies, but they are difficult to acquire and
implement for emergency purposes. Relying on small cells 053
and 054 from Global Wireless Technologies would have
provided us with these needed capabilities and seemed to be a
viable option for our team to acquire. These small cells have a
built-in real time interference mitigation mechanism, which is
helpful for identifying specific bands [32]. This mechanism
helps to avoid any unwanted radio signals or any other signals
that can provide interference and lead to potential path loss.
These cells are highly programmable and can search for any type
of signal in a variety of bands. They already have an inbuilt
network listening capability for rapid location identification and
are highly reliable and robust in providing connectivity for
wireless devices to the core network. These models can be tuned
to multi channels and offers support for 2G, 3G and 4G LTE
signals. Additionally, they can also be configured with an
operations, administration and management module (OAM)
which can provide alarms and performance management data to
the core network using SNMP (simple network management
protocol). However, this is done in real time and is beyond the
scope of this research.
These small cell features are advantageous and would
provide search & rescue teams with a feasible option of
awakening a mobile phone in an unserviceable area. The plan
therefore was to use these features to perform multiple tests
within a variety of locations and cellular variations. Initial
testing was planned to occur in simulated environments with the
main objective to simulate a base signal and have a phone search
and find the emissions form the small cell. These tests would
have been completed with a variation of cellular signals and
bands, to include 4G LTE, and within areas with and without
established cellular service. This would have included enabling
a small cell capable of transmitting a base signal and a mobile
device becoming aware of this unverified and newly introduced
signal. The mobile phone and small cell would have to be in very
close proximity for this purpose, as the small cell’s
transmissions would be limited to very low power to negate any
interference to local carriers such as AT&T.
The second round of field testing would have included
different locations and variations in distance and signal strength.
We would have explored the limitations of the small cell in
regards to height, distance, signal strength and reliability. This
can be achieved through the use of motor vehicles to test
distance and UAS to test for height variation. In order to analyze
signal strength and reliability of multiple records, emissions
would have been collected in order to determine the duration for
which the mobile phone awakens in order to maximize data
collection. When all of the requirements would have been
fulfilled, this testing could have been performed at the RECUV
range, or in a more realistic environments such as the back
country of Copper Mountain Ski Resort, which already
expressed their support for our testing.
Ultimately, due to the small cell vendor’s failure to deliver a
promised item and similar FCC regulatory hindrances faced, we
were only able to conduct several interviews with industry
experts from Cisco and Northrop Grumman in order to obtain
knowledge about similar techniques and industry opinions on
implementing a small cell in this environment. We furthermore
interviewed Hagen Lyle, ski patrol manager at Copper, and
discussed the current communication infrastructure at Copper
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Mountain, reliability of the network in a disaster, and opinions
on realistic implementation.
implemented in the preliminary lab and ground tests and in order
to determine how it operates in a more realistic environment.
Research Sub-Problem: How can SAR teams locate a
downed person’s cell phone based on its 4G LTE emissions
using a UAS and geolocation techniques?
Research Sub-Problem: What regulatory restrictions are
currently in place and present considerable practical challenges
to future research and implementation?
Methodology: Library research and industry interviews
Methodology: Extensive research and industry interviews
Direction finding generally requires two or more antennas
operating simultaneously in order to determine a difference
either in amplitude, time of arrival, or phase of the signals
arriving at the antennas. While TDOA methods would require
the UAS to fly over large distances in order to acquire significant
time differences in the signal, both power and phase
methodologies can acquire the AOA of the signal within short
distances [27]. The antennas used for the passive direction
finding are also an important consideration. While directional
antennas such as Horn and Yagi antennas are very suitable for
determining the source of the signal by collecting data on
amplitude differences, omnidirectional antennas like aerial loop
antennas with equal gain in all directions are also an option [25].
In fact, phase interferometry for example can be conducted using
arrays of Log-periodic antennas or passive vector sensor arrays
consisting of multiple dipoles [22], [24].
This final sub problem was the most crucial to solve and
overcome as it had dire implications in accomplishing our
capstone project. Regulatory barriers and privacy concerns are
not something a traditional methodology such as field testing
can solve for. The methodologies that were used to solve for this
problem are qualitative instead of quantitative in nature. Library
research, presentations, meetings, and discussions with industry
and policy experts was a key methodology to ensure our team
stayed in compliance with current policies by operating legally
and represented our university accordingly. In order to
overcome this sub problem, proper due diligence was performed
and this sub problem was always in the forefront of our mind in
regard to regulation and policy.
By collecting additional information regarding antenna
specifications, advantages and disadvantages, we were able to
make preliminary decisions regarding the proposed solutions.
Library research provided further assistance in establishing the
specific needs and constraints our proposed system would
potentially put on a UAS, which in turn allowed us to determine
the optimal choice regarding the UAS specifications.
While later tests with multiple antennas are still necessary,
initial tests to determine the theoretical feasibility would have to
be conducted first. Using a highly interactive and flexible
software would make it possible to determine the best,
theoretical solution for the presented problem and plan the field
testing accordingly. A software such as MATLAB makes it
possible to answer questions relating to the UAS available
power, payload capacity, number and type of antennas, and
flown distance. Conducting theoretical testing in lab-like
conditions would have important beneficial effects on the time
needed for the eventual field testing, but this is another research
project entirely on its own.
We propose field testing would mainly consist in verifying
lab results. Ground tests would again be conducted under
simplified conditions. This makes it possible to verify the type
and number of antennas determined to be the best solution in
MATLAB, or other software tests. Lab results on the three
different methodologies, Cell ID, Angle of Arrival, and RSSI,
would further determine which algorithm should be utilized in
realistic environments, ultimately making it easier to develop a
viable solution.
In the end, the goal was to do the final testing at Copper
Mountain under less controlled conditions. In order to simulate
such conditions at the ski resort, a cell phone would be buried
under a considerable amount of snow while a UAS with a small
cell payload would fly the optimized direction finding pattern.
The methodology highlighted as optimal would then be
Our team researched current FCC policies in regard to small
cells and spectrum issues. It was not enough to just research
current policies for spectrum use, we also had to consider how
these polices can fluctuate when given an emergency situation.
Emergency disasters allow public safety officials authorization
to use specific bands, otherwise unallocated or restricted. The
FCC has well defined policies during emergency situations and
due to spectrum being a scarce resource, licenses play a major
factor in using given frequencies [18]. We also had to take into
account whether it would be feasible for SAR teams to rely on
the FirstNet bands for introducing a base tower turned small cell.
The final crucial aspect regarded mobile credentials and network
policies. Our team had to research possible repercussions due to
the result of a mobile phone located on an existing major
network attempting to connect to an unverified small cell. The
plan was to avoid any distress with network providers when
attempting to introduce a base tower at Copper Mountain. The
goal of this research methodology was to gather pertinent
information to regulations, while protecting the integrity of our
research findings in a legal manner.
Another major regulatory figure is the FAA. Since we will
be using an Unmanned Aircraft System, it was essential to our
team to be in compliance with all FAA standards. The first
action step we had to take was to obtain permission to use the
regulated airspace via obtaining a COA. The COA is very
specific authorization request and can only be used in certain
areas, with a specific registered aerial vehicle, and for a limited
amount of time [19]. Unfortunately, it turned out that the
Leptron Avenger B lacked an appropriate FAA airworthiness
certificate, which resulted in additional regulatory paperwork.
The Leptron Avenger is a rotary UAS, not a standard quad, and
we were one of the first people to file a COA for this type of
aircraft.
The COA we requested would have only been sufficient
enough for the spring semester and within a limited geographic
area within the state of Colorado. If anything happened to the
Leptron Avenger, such as it crashing or malfunctioning, we
would be unable to obtain another aerial vehicle to complete our
6
project. This being said, it never was our responsibility to pilot
or program the UAS, for legal liability reasons. Obtaining the
COA was only the first part of complying with FAA regulations.
This is where additional research had to be completed to ensure
no other airspace barriers posed any issue. Additionally, we had
to research how the FCC and the FAA policies intertwine when
in regard to emergency situations. This factor was crucial due to
the nature of two authoritative figures coming together and
which policies are restrictive or enabling.
Interviews, meetings and presentations with government and
industry representatives was another method used to overcome
these policy concerns. The industry advisors came into contact
with were a substantial factor in closing in on these barriers.
Advisors such as Nicholas Little and Ben Posthuma were able
to bring their experience in communication and regulations to
our project, which was without any doubt, a great help. With our
limited industry knowledge, we relied on interviews to ensure
we stayed within the scope of the project and solved for the
regulatory restrictions.
Along with the multitude of regulatory barriers we also had
to consider certain privacy concerns. Searching for a person’s
phone without his/her explicit permission is a potential invasion
of privacy that might still be neglected thanks to its main
purpose, as for our research and scenario testing, the passive
direction finding was only supposed to be used during
emergencies and in SAR operations. However, nothing would
prevent this technology from being used for other tracking or
signal acquisition purposes, which might pose significant
privacy concerns.
IV. RESEARCH RESULTS
While we were able to research the most effective direction
finding methods and also the best methods of waking up a phone
to transmit temporarily, our main research results consisted in
the great amount of obstacles and challenges a search and rescue
team would have to surmount in order to implement our solution
and save lives in a lawful manner adhering to all regulations. The
barriers we encountered could not only potentially result in the
loss of lives, but furthermore discourage first responders from
following these strict regulations. This will lead to a limitation
in innovation as any advances would not be available to the
public.
The following section represents a breakdown of the results
for each of our proposed and analyzed by each sub-problem:
Results Sub-Problem: What is the best method for awakening
a mobile phone device in sleep mode within an unserviceable
area?
Cell phones have to communicate regularly with a base
station, however as determined in our preceding research,
phones enter into a “sleep mode” when no cell tower is detected
in its vicinity in order to reduce power consumption and have
more left for the time when it is needed most [13]. This
technological characteristic leads to the phone transmitting at a
much lower frequency in such an environment.
While those signals can be used to determine the phone’s
location, they would be transmitted too infrequently to
accurately determine the device’s location. The goal therefore
was to introduce an element into the cell phone’s limited
environment which would result in the phone transmitting for a
certain amount of time at a much higher frequency.
A so-called “Cell on Wheels” (COW) could be brought in by
the rescue team to provide a cell tower for the phone to connect
with, resulting in the phone transmitting at a normal frequency.
However, COWs are usually the size of a truck and very difficult
to deploy in some environments, and would be entirely
unfeasible in backcountry areas that are only accessible by
helicopter.
The alternative solution our research therefore focused on
was the development of a custom platform using an enterprise
class small cell carried and powered by the drone it is mounted
onto. The small cell is such a disruptive technology, only beta
versions have been created and our promised unit never came in
on time. Not only are enterprise small cells like the GWT 052
highly customizable and programmable and can therefore
collect and use additional emissions such as WiFi and Bluetooth
for direction finding, but additionally offer the prospect of
providing communication to search and rescue teams. In fact,
given a cellular operator’s cooperation, one small cell could be
deployed on a ridge as a repeater while another Cell on Drone
locates the victim and provides network infrastructure to the first
responders working within its reach. Therefore, instead of
simply pinging a victim’s location, an enterprise class small cell
could provide meaningful communication in areas without
connectivity in conjunction with a backhaul cell on a ridge.
Result Sub-Problem: How can SAR teams locate a downed
person’s cell phone based on its 4G LTE emissions using a UAS
and geolocation techniques?
Once the smartphone has been provided with a reason to
transmit, that collected data has to be interpreted correctly in
order to determine the devices location. Research has shown that
a variety of methods are currently being used, further indicating
that a single direction finding method alone would not be
optimal. Instead, it is often suggested to combine different
methods in order to maximize results and the location
probability [24]–[28].
Most traditional wireless localization techniques are based
on trilateration, a technique that uses the distance between the
mobile station and three base stations in order to locate the
device. Triangulation on the other hand is the method of
computing the distance by estimating the signal’s direction of
arrival. In combination with either deterministic or probabilistic
algorithms these are used to locate a device for communication
purposes [33].
Position location techniques can be divided into two main
categories: (1) network-based and (2) device-based. It should
be noted that the second category consists mainly of solutions
like the Global Navigation Satellite System (GNSS), which
however requires a direct line of sight between receiver and
transmitter, and is therefore not applicable for our purpose.
Consequently, a network-based localization technology is
necessary and can be implemented either at the base station
conducting the search or nearby devices.
7
The following are the main network-based techniques used
for location finding of mobile devices:
1) Cell ID (CID)
Cell ID and Enhanced Cell ID are the techniques used in
cellular networks to identify the location of the device based on
the cell site location [12], [33], [34]. Depending on the type of
small cell in use (micro, pico, or femto) this method might
present different results. It is considered the least precise
technique of position locating as its position accuracy depends
on the cell size.
2) Angle of Arrival (AoA)
AoA uses the angle of arrival of the received signal from a
mobile device with respect to a reference direction, usually the
geographic North, in order to determine the devices location [3].
As described in LTE 3GPP Rel.9 [5] the Sounding Reference
Signals or the Demodulation Reference Signals (DM-RS)
received from the mobile device are used for the angle
calculation purposes. The angle calculation could be performed
using the phase difference estimation across the array
(interferometry) or using the power density estimation across the
array (beamforming). The use of multiple antennas on the base
station and simple triangulation makes it possible to determine
the incident angle of an arriving signal and as a result, a direction
of the received signal.
3) Received Signal Strength (RSS)
RSS estimates the distance between an energy source and a
receiver based on the received energy levels. This technique
requires at least three reference points to determine the twodimensional location of a given mobile device, four to
determine its three-dimensional location. However, due to its
dependence on path loss and channel characteristics this
method is considered to be somewhat unreliable [33].
Equipment such as a small cells can be used to calculate the
received signal strength indication (RSSI) at all the coordinates
(x, y) collected by the drone. The stronger the RSSI value, the
closer the transmitter is. This makes it possible to calculate the
probability of the transmitter’s location along the drone’s
flightpath. The accuracy increases as more data on received
signal strength is collected from additional coordinates,
ultimately allowing the SAR team to narrow down a possible
location.
Based on our research, our solution consists of a multielement antenna connected to a base station – our enterprise
class small cell. In addition, the RSSI, the drone’s movement
and angle would be used for our direction finding solution.
While the multi-element antenna would make both an angle and
possible Doppler Effect measurable, the small cell could support
both a carrier, small cell, and public safety capability.
However, due to complications with multiple small cell
suppliers, it was unfortunately not possible to obtain a multielement antenna as our small cell solution was not delivered in
time. Since our solution proposes the option of using any cell
phone emission for the direction finding, WiFi and a phone
turned into a hotspot were considered instead. This however
proved not to be a viable alternative as it increasingly depends
on individual phone settings and is more prone to interference
from other WiFi transmitters.
Results Sub-Problem: What regulatory restrictions are
currently in place and present considerable practical challenges
to future research and implementation?
Major results in our attempt to develop a search and rescue
solution for avalanche victims were ultimately achieved in our
regulatory sub-problem. In order operate legally and in
accordance with regulations, the project team had to acquire
both a Certificate of Authorization from the FAA, and a highly
technical airworthiness certificate which details the specific
qualifications of the UAS and the emergency plan for a
multitude of scenarios. By complying with the FAA
requirements and establishing a standard of use for CU, this
would have allowed us to lawfully fly the UAS provided by
Leptron, the Avenger drone. However, several issues where
encountered on the path to the certificate.
First, the application for a COA is a highly technical
document that requires considerable aeronautical knowledge in
order to be able to complete it without the help of a seasoned
expert. While support was initially offered from the University,
it was ultimately not possible to garner enough support and
assistance in order to complete the meticulous application.
Additionally, experienced operators like CU’s Research &
Engineering Center for Unmanned Vehicles (RECUV) solely
applied for fixed wing COA’s in the past, while our project
relied on a 22lb helicopter with a 6’ rotor span. Different from
our anticipation, this new technology was different enough for
RECUV to not be able to assist us.
These hurdles were further exacerbated by the fact that it
would have been necessary to acquire a COA for each location
at which we wanted to test. While the original plan was to do
final testing at Copper Mountain, due to liability issues a Plan B
was outlined, which included operating on RECUV’s Table
Mountain range. This solution however would have required the
drafting of an additional application and air worthiness
document. While it would have been possible to fly one of
RECUV’s fixed wing drones on the range, it would not only
have missed our goals of a stationary drone but would have
further made it necessary to find an experienced pilot, who could
fly the drone. Leptron’s Avenger UAS in fact is equipped with
fully featured avionics and autopilot, which would have made it
possible for any person to fly the drone in a variety of
environments, including snow, rain, and winds gusting at 40
mph.
In addition to the hurdles presented by the required amount
of substantial aeronautical knowledge and a background in
aerospace due to the highly specialized information, our
research was furthermore confronted with the necessity of a
Special Temporary Authorization (STA) from the FCC.
Acquiring this license would have made it possible for us to
transmit a signal on Band 14 to the cell phone that would
simulate a cell tower appearing in the phone’s vicinity and
consequently lead to more frequent transmissions from the
phone. Our team even garnered permission from Adam’s county
to operate on Band 14, but unfortunately this regulatory barrier
added further hurdles to search and rescue efforts.
While the technical details required to obtain a license are,
with some help from the manufacturer and compared to the
details required for a COA, easier to obtain, other elements of
8
the application proved far more challenging. Having a moving
transmitter mounted onto a drone presented a considerable
challenge as it made it necessary to widen the area for which a
license needed to be acquired. While the small cell is only
transmitting at very low power, transmission licenses are limited
by location, making it necessary to specify a location within
which the transmission would occur. To our extensive
knowledge, we were one of the very first groups to file for a STA
for a moving transmitter on a UAS, which, lacking the benefit
of additional sources or experiences, further increased the
practical challenges presented by FAA and FCC regulations.
Coupling UAS with a low powered small cell 4G LTE
technology seemed to boggle the minds of most regulators and
enforcers of both the FCC and FAA when confronted with it.
This did gain us significant interest in our research, but not
enough substantial support was offered to overcome these
sophisticated documents.
While an STA application can take up to two months to be
granted, a co-existence analysis should show that no interference
is caused with neighboring cell site operators. These operators
additionally have within a limited amount of time the option to
provide opposition to an application. Even though we gained the
support of the Department of Commerce and Colorado’s Office
of Information Technology, we were not able to gain any
neighboring operator’s support. Additionally, one STA
application would not be sufficient as each geographic
transmission location would require an additional license.
These two major hurdles all derived from our decision to
operate legally and follow regulatory authorization as any search
and rescue team would hopefully do. These real life regulatory
problems proved to be too substantial for our small research
team, but the lessons learned and the interest shown from
industry professionals in our experiences validated our results to
our entire team. We will be presenting these challenges at the
2015 Public Safety Broadband Stakeholder Conference in San
Diego and we also gave a personal presentation to NIST, the
National Institute of Standards and Technology about our
results. Additionally, steps are now being taken by The
University of Colorado Law School to integrate FAA policy into
the curriculum and the popular Silicon Flatirons Conferences.
V. DISCUSSION OF RESULTS
Although research, as well as current developments in
Canada and the US have shown that Cell on Drones is a feasible
solution[35], the capacity of complying with all regulatory
requirements and of operating legally presented a great number
of challenges for any researcher and search and rescue group.
The goal was to be among the first to legally operate a drone
carrying a legally certified radio system operating on band 14,
the frequencies dedicated to the public safety first responder
network.
Instead of following admittedly burdensome regulatory
requirements, many conduct their research, development, and
operations under the banner of radio controlled model airplane
rules, established between the Academy of Model Aeronautics
(AMA) and the FAA, operating standards established back in
1981 [36]. In just recent months the emergence of the term of
COD has started to appear as a general idea, and more strides are
being taken outside of the United States due to the lack of
regulatory barriers. We were one of the first groups or business
to attempt COD within The United States of America.
Our research was further hampered by harsh liability
requirements. While AMA members and their hobby aircraft
flights are covered by AMA liability insurance, Non-AMA
flights operate illegally and hence don’t require liability
coverage. On the other hand, testing and conducting research at
Copper Mountain using a certified Drone and transmission
equipment would have made it necessary to provide a three
million dollar liability coverage, an amount that ultimately was
not possible to be achieved by a small university research team.
Additionally, the FAA does not have the staff to enforce
these rules and usually only catches those with flight
malfunctions that result in various levels of damage, and those
advertising their illegal flight operations. This project however
dedicated itself to respect our university’s reputation, to follow
all proper flight rules and to face all the practical challenges an
honest researcher and search & rescue team would have to
follow when trying to acquire the necessary certifications from
both FAA and FCC.
In the end the technical focus, modeling efforts, and
experimentation goals of this project were diverted by the time
dedicated to license applications. A great amount of time was
spent researching documents and obtaining technical details
necessary to file all required paperwork and forms. While the
FAA’s COA required deep aeronautical knowledge, usually
only available to long-time experts, the FCC’s STA provided the
vague potential outlook that license holders operating within our
research area could provide reasons to deny our application and
ultimately halt any effort to advance the state of the art of SAR
technology. This fact further diverted attention from our
technical focus by making it necessary to warn operators and
gather support for the pending application. However, apart from
these technical missteps we gained significant experience in real
life policies and the strenuous nature of government agencies.
A further challenge represented the fact that a mobile,
airborne transmitter close to ground stations can still be
considered a novel, thought-provoking initiative that combines
UAS, commercial small cell radio systems, and public safety
broadband radio services associated with Band 14 4G LTE. This
uncharted domain of research and development is still an area
even government stakeholders, agencies, departments, and
organizations are unfamiliar with. Trying to advance the state of
the art will always be met with uncertainty and a lack of general
advice.
VI. CONCLUSION AND FUTURE RESEARCH
After a yearlong of research, preparations, and establishing
a concept of operations, the project’s main goals were ultimately
impeded by bureaucratic challenges. These practical challenges
that many researches and SAR teams have to face not only
affected our original research plan, but even our back-up
alternatives. Still, our research provided many valuable lessons
learnt and highlighted regulatory barriers, which need to be
addressed in the future for any research group. Implementing our
suggested direction finding method and using a small cell
attached to a UAS will enable future researchers to collect a
smart phone’s emissions for radio geolocation. Moreover, our
9
research garnered remarkable attention from both industry
professionals and government, ultimately validating its value
and the importance of advancing the current state of the art for
both SAR operations and the deployment of small cells in the
form of Cell on Drones.
Future research needs to make sure that all regulatory
barriers are tackled early enough. Overcoming the painstaking
barriers of COA and STA would allow future researchers to
legally conduct flight trials and transmit signals. Additionally,
more
attention
of
both
CU’s
Interdisciplinary
Telecommunications Program and Law School should be
focused on the current state of regulatory barriers as both the
Department of Transportation and the Federal Aviation
Administration are proposing new rules for Small Unmanned
Aircraft Systems [37]. This opens up the opportunity to propose
rule changes that would remove barriers to similar research and
ultimately be an important contribution to saving lives in search
& rescue operations.
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