It is no secret that technology across many disparate engineering and scientific disciplines continue to advance very rapidly. Perhaps nowhere is this more observable in the public eye than in the field of small, unmanned aircraft systems (sUAS), commonly referred to as “drones”. sUAS are at the forefront of efforts to synergistically combine numerous technologies into fully integrated platforms for use in an ever-expanding number of commercial, humanitarian, military, law enforcement, and research applications. But today’s commercial drones need improvements to their autonomous capabilities to safely divorce themselves from human control (Floreano, 2015). Recent Federal Aviation Administration (FAA) regulatory guidance in the form of 14 Code of Federal Regulations (CFR) Part 107, Small UAS Operation and Certification, has enabled the increased commercial, governmental, and private use of sUAS by providing sUAS users a framework in which to operate their aircraft safely within the National Airspace System (NAS) (FAA, 2016).

The Small Atmospheric Sensing Aircraft (SASA) is a sUAS concept that has the potential to change how critical atmospheric data is captured by offering an alternative platform to improve controllability, enable dynamic re-tasking and routing, and significantly reduce cost compared to the current system. Upper air observation stations capture vital atmospheric data across the Earth every day (“NWS Radiosondes”, 2015). In the United States, the National Weather Service (NWS) uses this captured data to develop weather forecasts that support all types of daily aviation operations, amongst a host of other national activities; countries across the globe conduct similar operations for the same reasons. Moreover, to improve our collective knowledge of atmospheric phenomena, weather and climate scientists can make great use of this spatial-temporal atmospheric data to tease out complex physical mechanisms that are not yet atmospheric data beyond pressure, temperature, winds, and relative humidity data, to aid in assessing the planet’s changing climate and weather patterns.

Climate Change is troubling to the scientific community because significant rises in global temperature can adversely affect many different areas of active research such as species habitat, agriculture and associated famines, global sea level rise, the spread of contagious diseases, and delicate marine ecosystems. Considering that atmospheric data is gathered every day across the globe, there seems to be a clear need to gain consistent, reliable, and cost-effective access to the Earth’s atmosphere to better understand how our atmosphere is changing.

The Conference of Paris 21 (COP 21), an international conference on Climate Change held in Paris, France from 30 November 30th – December 12th, 2015, concluded that significant steps must be made, globally, to ensure the average global temperature is not allowed to increase beyond 2 degrees centigrade (°C), and actions should be taken to hold the global average temperature at or below 1.5°C (Article 2, Paris Agreement). Global average temperature from January to June 2016 reached 1.3°C warmer than the preindustrial baseline temperature (NASA, 2016). The current atmospheric data sampling system continues to operate each day, but there is no plan to increase its operational frequency now or in the near future. The SASA sUAS has the potential to be revolutionary in sampling atmospheric data around the globe, combining the latest technology with a prime focus on aviation safety, sampling accuracy, precision, and aircraft recoverability and reuse.

Problem Statement

In the United States, the NWS has been launching Radiosondes, the meteorological hardware and software portion of the upper air observation system, via weather balloons for

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 4

decades. These instruments are considered expendable–most Radiosondes are lost with no chance for recovery (NWS Radiosonde Observations, 2016). This practice is also the staple method for countries across the globe for those able to afford it. While this method is reliable and extremely safe, it is expensive costing tens of millions of dollars, annually, in the U.S. alone (H. Escabi Jr., personal communication, November 5, 2015).

The NWS launches Radiosondes from over 100 locations, two times per day, every day of each year. This amounts to an annual total of about 75,000 units in the U.S.–over 80% are never recovered (NWS, 2016). At hundreds of dollars per launch, this portion of the NWS’s upper air observing program is tremendously expensive. This fact alone is testament to the value the NWS, and agencies they support, place on the daily atmospheric data that is gathered. The ability to employ a recoverable system that has the potential to significantly increase the amount of data gathered and simultaneously reduce cost could provide tangible benefits.

Research Design

An aviation and aerospace conceptual assessment was conducted on a novel, sUAS equipped with a meteorological instrument payload, such as SASA, that is both a feasible replacement to the legacy Radiosonde weather balloon system, and has the potential to be a more capable system without introducing a significant amount of added risk to the safety of manned aviation operations.

Literature Review

A review of current literature regarding weather balloon operations, similar use of sUAS for atmospheric data sampling, FAA sUAS recommendations and regulations, safety assessments of sUAS operating in the NAS, and significant technological advances in aviation, aerospace systems pertinent to SASA was accomplished.

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 5

Statistical Analysis

Because safety considerations are vital to any sUAS poised to operate in the NAS, a parametric t-test analysis of historical aviation aircraft bird collision data and historical aircraft-aircraft midair collision data was conducted. Because sUAS are only recently beginning to fly in numbers that have driven the FAA to develop regulatory guidance for their use, there is very little data on sUAS aviation incidents with other aircraft. Without a historical sUAS incident data pool to draw from, an analysis of aviation accident data that is available was used. The intent of this analysis was to review the available aviation midair collision data, and attempt to draw conclusions that may be applicable to sUAS that operate within the NAS. Any available literature pertaining specifically to sUAS collisions with commercial and general aviation aircraft were also examined.

Results

Criticality of Atmospheric Data Collection Via Radiosondes

Radiosondes give weather forecasters a vertical sampling of atmospheric wind direction and speed, pressure, temperature and relative humidity from the surface up to approximately 100,000 feet. This data is then used in myriad ways. Two-dimensional charts are produced to illustrate the vertical structure of the atmosphere where forecasters can visualize moist and dry areas, identify temperature inversions at different levels that can help forecast fog, severe thunderstorms, and wind patters, for example. This data is also the bedrock for numerical weather models that seek to ascertain the atmosphere’s initial characteristics at a given time and provide solutions for short and long-range weather forecasts.

A long-standing limitation regarding numerical weather models is their reliance on very sparse upper air data. The U.S. contains about 3.8 million square kilometers of territory that

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 6

amounts to an average of one Radiosonde sampling two times a day, per 37,000 square kilometers. Assuming each Radiosonde is launched at an even spacing (they are not), this clearly leaves an immense amount of vertical atmosphere that is not sampled. Numerical weather models are forced to interpolate for all the points in between where there is no data available. The sparseness of upper air data induces initial errors that become more pronounced as the numerical model calculates weather conditions farther and farther away from the initial conditions frame of reference. SASA’s usefulness for the weather and climate community would be a major step forward, but it would also produce secondary benefits for every person and organization that uses weather and climate information in their decision-making cycles.

SASA data would likely provide improvements for short and long-term agricultural production outlooks, environmental disaster response agencies, research institutions, military operations, and for all forms of aviation that rely on accurate weather forecasts to fly safely each day. For the atmospheric data collected by SASA to be useful, each integrated system must be meticulously designed and tested for safety and functionality. But before any design considerations can be firmly contemplated, a comprehensive analysis must be conducted to assess current and past sUAS design and testing efforts. The current and anticipated aviation regulatory guidance must also be reviewed. And arguably most important, a preliminary safety analysis must be designed and conducted to determine how SASA might compare to the legacy system’s safety record.

Federal Aviation Administration Regulatory Structure

SASA is envisioned to augment or completely replace the existing Radiosonde system. The case must be made that SASA can significantly surpass the capabilities of the current system while affording a margin of safety consistent with, or greater than, what has already been

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 7

demonstrated. Comparing the overall picture of how these differing concepts of operation (CONOPs) work provides a quick assessment of their primary similarities and differences.

SASA’s main similarity to the legacy system is the method by which it ascends. SASA will rise up to altitude via a standard weather balloon via CFR 14, Part 101 “Moored Balloons, Kites, Amateur Rockets and Unmanned Free Balloons” just as legacy Radiosondes do today. As long as SASA adheres to CFR Part 101 guidance and limitations, there is virtually no difference compared to the current Radiosonde system. The stark difference rests in SASA’s descent method. SASA will not be falling via an unguided parachute as all Radiosondes do now. Once the weather balloon bursts at about 100,000 feet, SASA will become an autonomous sUAS that can either act as an unpowered sUAS glider, or as a powered sUAS when situations arise that require powered flight. At this transition point, SASA becomes a guided aircraft, whereas current Radiosondes are unguided payloads, not considered aircraft in any manner. As such, SASA is compelled to adhere to further CFR guidance. And this is the point at which SASA makes its pivotal departure from the legacy system.

Regulatory guidance for sUAS does exist in the form of CFR Part 107, “Operation and Certification of Small Unmanned Aircraft Systems”. However, this new guidance only allows operations up to 400 feet above ground level (AGL), unless, in the case of use by a structure greater than 400 feet in height, the UAS remains within 400 feet of the structure. Operating in any other way above 400 feet AGL is forbidden. SASA will descend through airspace as high as 100,000 feet, so CFR Part 107 is only applicable to SASA at or below 400 feet. This leaves over 99,000 feet of altitude unaccounted for in SASA’s operating altitude. The primary question is whether or not SASA, being a sUAS, should be treated as any other, large commercial or general aviation aircraft. If so, then SASA would be subject to additional CFR Parts.

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 8

FAA notice JO 7200.23, effective October 3, 2016, prescribes how unmanned aircraft will operate within the NAS, and delineates small and large UAS—less than 55 pounds for the former, 55 pounds and greater for the latter. Notwithstanding guidance in the newly released CFR Part 107, this notice affirms that the FAA will not allow small UAS operations outside of “active restricted, prohibited or warning areas in the NAS without specific authority…” If SASA were authorized to fly within the NAS to collect essential atmospheric data, safety considerations will be paramount. Specifically, the specter of mid-air collisions with commercial and general aviation aircraft immediately comes to mind. With no data available on sUAS flying at all levels within the NAS, one can look to the outstanding aviation bird strike dataset that is readily available and perhaps draw some informative conclusions from this extensive data set.

SASA Flight Safety

An extensive review of the National Transportation Safety Board (NTSB) database revealed no confirmed collision with weather Radiosondes–this aviation safety record is extraordinary given millions of launches over several decades. The obvious question arises regarding SASA’s likelihood of attaining this legacy level of safety. SASA will have the ability to maneuver with and without power, incorporate GPS geo-fencing technology, the potential to integrate advanced Automatic Dependent Surveillance-Broadcast (ADS-B) capability, amongst other cutting-edge aviation technologies. Given these aforementioned safety features, one might be very confident that SASA could achieve, or even exceed, the level of safety already demonstrated by the legacy Radiosonde system. While possible, it would be a significant mistake to make such an assumption without conducting a comprehensive safety analysis with historical aviation data. Moreover, a comprehensive review of any research involving sUAS similar to

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 9

SASA, governing aviation regulatory guidance, and potential hardware and software engineering solutions must be considered.

The initial SASA prototype is expected to be in the micro UAS category at less than 4.4 pounds. There is little doubt that most aviation experts would agree that even a sUAS accidently ingested into an aircraft jet engine would result in some degree of damage; fortunately, to date there are no confirmed reports of an actual sUAS mid-air collision with a commercial passenger aircraft. If a sUAS were sufficiently large, one could expect significant, to possibly catastrophic damage in a jet engine ingest accident.

On October 7th, 2015 Dr. Mykel Kochenderfer, Assistant Professor of Aeronautics and Astronautics at Stanford University, provided testimony to the U.S. House of Representatives on the subject of Ensuring Aviation Safety in the Era of Unmanned Aircraft Systems. The subject of safety regarding the introduction of sUAS into the NAS is arguably the most important aspect of the entire sUAS enterprise. Dr. Kochenderfer’s testimony centered around two primary points: how is one to properly measure and analyze the risk posed by sUAS, and what technologies and policies can be employed to minimize risk. Since the sUAS proposed here will descend from very high altitude and autonomously guide itself back to a recovery location, there is some added risk to manned aircraft. However, the magnitude of that additional risk is the unknown that must be assessed. The initial target weight of SASA is 4.4 pounds or less, similar to a small to medium-sized bird.

Dr. Kochenderfer referred to the dual engine failure of U.S. Airways flight 1549 that occurred on January 15th, 2009. The A320 aircraft ingested a flock of Canadian geese into both engines just after takeoff and suffered dual, catastrophic engine failures that led to an emergency landing on the Hudson River. A swarm of similarly sized sUAS comparable to the weight of a

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 10

Canadian goose, about 6-14 pounds, could surely do equivalent damage. But large groups of sUAS flying near airports and at low altitude are not at all consistent with SASA’s envisioned flight profile. Analyses that estimate potential risk of collision with a single, very small UAS seem reasonable versus a large, concentrated group.

The seminal difference in the NWS legacy Radiosonde system and SASA involves autonomous guidance as it makes its way to a recovery point. This capability should not be viewed as a risk liability, necessarily, and the FAA has previously affirmed that an autonomous sUAS glider does afford greater safety than an unguided payload falling via parachute.

In 2013, the National Oceanic and Atmospheric Administration’s (NOAA) UAS Program office obtained FAA approval to operate their SkyWisp autonomous sUAS. The SkyWisp sUAS glider used standard weather balloons to ascend to an altitude of about 100,000 feet, and then glided autonomously to a recovery point after balloon burst. NOAA argued to the FAA that a “glider controlled recovery” afforded a reduction in risk compared to parachute-aided recovery and that extensive, documented SkyWisp test flights showed an exemplary safety record. After reviewing the SkyWisp program testing history and associated safety record, the FAA responded, “We have evaluated the SkyWisp program. The UAS Integration Office does not currently regulate unpowered, unmanned gliders. Consequently, the SkyWisp program can be operated in the same manner as your other unmanned free balloon operations…” The FAA’s decision makes perfect sense when one considers that a “glider controlled recovery” has the potential to autonomously navigate to a recovery point, which implies that such a device has inherent flexibility that an unguided, parachute-aided payload recovery system lacks.

SkyWisp’s high level of safety in restricted airspace is an important risk indicator, but it doesn’t address the potential risk a sUAS might pose in the NAS at large. Conducting a

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 11

comparison of aircraft-on-aircraft midair collisions and bird-on-aircraft mid-air collisions could provide valuable information. FAA and NTSB databases provide historical data regarding aircraft-on-aircraft mid air collisions, aircraft-on-aircraft near mid-air collisions (NMACs), and bird-on-aircraft mid-air collisions, also known as “bird strikes”, and annual accident statistics that involve what the NTSB calls major, serious, and injury accidents resulting from diverse causes. Bird strikes on all forms of aircraft are by far more frequent than aircraft colliding in mid-air with one another. The first obvious difference is the sheer number of birds that exist in the U.S.—10 to 20 billion birds, depending on the time of year (Manville, 2005). Bird strikes occur in all sectors of aviation.

Considering there are 109 NWS locations currently sending up weather balloons twice a day, the total number of SASA launches, if SASA were being used today, would be about 218 sUAS in a 24-hour period. SASA is comparable to a small or medium size bird, but SASA’s envisioned operational numbers are nearly insignificant compared to the amount of birds flying in the NAS at any given moment. Nor will SASA be launched from the airport runways where birds do have the potential to impact aircraft on takeoff and landing.

In addition to SASA being carefully integrated into airspace, each aircraft will require robust communications and telemetry capabilities to ensure positive control of all SASA aircraft flying in the NAS. Each capability will have to work synergistically to avoid collision mishaps with manned aircraft. SASA must be on par with the legacy Radiosonde system’s perfect record to date. Anything less than an exemplary safety record may only serve to question why the legacy system was abandoned in favor of the SASA sUAS.

Aircraft Mid-air Collisions and Bird Strikes Analysis

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 12

 

According to the NTSB, from 1997 – 2014 there were 32 major, 28 serious, and 284 injury aviation accidents involving Part 121 commercial air carrier operations in the U.S. Over this period 18 million flight hours were flown on average, annually, with 0.098 major accidents per million hours flown. Over the 18-year period, this amounts to less than two Part 121 major accidents every year. The NTSB considers a major accident as one that involves the destruction of the aircraft, multiple fatalities, or at least one fatality and substantial damage to a Part 121 aircraft. These descriptive statistics alone show that major Part 121 accidents are extremely rare; however, there were 202 General Aviation (GA) mid-air collisions in the same 18-year period, including 120 instances where fatalities occurred that resulted in a total of 295 deaths.

Though no mid-air collisions involving Part 121 aircraft occurred from 1997 – 2014, there were 341 pilot reports of critical NMACs; a critical NMAC is one in which the pilot lacked sufficient time to execute an evasive maneuver, and the estimated separation of the aircraft was less than 100 feet (FAA, 2016). Put another way, on average almost 19 critical NMACs involving Part 121 aircraft occur each year, and an average of 946,578 Part 121 aircraft hours are flow between each NMAC occurrence. Again, while certainly more prevalent than actual mid-air collisions, NMACs are a fairly rare Part 121 occurrence.

Bird strikes on all forms of aircraft are by far more frequent than aircraft colliding in mid-air with one another because of the sheer number of birds living in the U.S. It turns out that from 1997 to 2014 there were 170 mid-air collisions between aircraft in the U.S. and 11,043 damaging bird strikes on aircraft, within an annual average of over 58 million aircraft operations (NTSB, 2016). Intuitively, one can deduce that with only 218 hypothetical SASA launches per day, and considering SASA’s very small mass, the incidence of mid-air collisions with other commercial or general aviation aircraft would be at or below the incidence of aircraft on aircraft mid-air

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 13

 

collisions. Recalling the fact that the legacy Radiosonde system has never collided with an aircraft, one can argue that there is the potential for a guided sUAS such as SASA to have a real potential to continue this outstanding safety trend. SASA would have the capability to actively increase the safety margin by not drifting into unwanted sections of airspace. SASA will have the ability to avoid airspace where the current system cannot in any way. If SASA incorporates many of today’s modern technology, surpasses the legacy Radiosonde system in operational tempo, and can match its safety record, then this could be the right time to design the next-generation atmospheric sampling platform in the form of the SASA sUAS concept.

Recommendations

SASA should build on the work of NOAA’s SkyWisp autonomous, sUAS glider. This effort has already provided a successful proof of concept regarding many of the aspects discussed in this paper. The addition of a battery-powered propulsion system, passive and active sense and avoid technologies, and operations outside of carefully controlled airspace are the primary differences between what has been accomplished to date and the next evolution of this concept.

The sUAS proposed here, designed with the latest technology in mind, could be the basic template for other sUAS that seek to operate in the NAS beyond CFR Part 107 requirements. Modeling and simulating SASA operations in the NAS will be a key aspect of the overall developmental and testing plan.

Embry Riddle Aeronautical University’s Next-Generation ERAU Advanced Research Lab’s (NEAR) Real-Time Distributed Simulator (RTDS) can simulate historical U.S. air traffic to investigate SASA’s virtual performance within the NAS alongside manned aircraft. This future effort would be very useful in providing both qualitative and quantitative data prior to

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 14

actual SASA flight-testing. Upon completing modeling and simulation, SASA could begin controlled range flight-testing at low altitudes and works its way to ever-higher altitudes alongside the legacy system to evaluate SASA’s performance.

Conclusion

The legacy NWS Radiosonde has performed exceedingly well for decades, but it suffers from excessive cost, limited capability, and most significantly, it cannot provide enough data to address 21st Century challenges.

Our Earth’s atmosphere and oceans are experiencing considerable warming. The COP 21 meeting in Paris, France set a goal of no more than a 1.5°C increase in average global temperature. This goal seems to be at risk of being broken in the very near future. Whether climate change is due to anthropogenic forcing or some other “natural” phenomena, SASA has the potential to provide researchers with a wealth of new atmospheric data. Moreover, SASA simultaneously provides a more robust data set for weather forecasts that are already critical for our daily lives. Any civilian or military activity that currently relies on Radiosonde data can only benefit from SASA’s operational introduction.

Designed with the latest technologies in mind, SASA could be the basic template for other sUAS that seek to operate safely in the NAS outside of CFR Part 107 boundaries. The idea that a sUAS concept such as SASA could actually help improve weather forecasts, and through these forecasts, serve to make flying safer for all other forms of aviation, is both satisfying and exciting.

 

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References

Aeronautics and Space, 14 Code of Federal Regulations - C.F.R. § 1-1399 et seq. (2016). Dolbeer, R. A. (2006, February 26). Height distribution of birds recorded by collisions with civil aircraft. Journal of Wildlife Management, 70,5, 1345-1350. Retrieved from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1496&context=icwdm_usdan wrc

Drone Aircraft Privacy and Transparency Act of 2015, S. S.635, 114th Cong. (2015)

Ensuring Aviation Safety in the Era of Unmanned Aircraft Systems, 114th Cong. 1 (2015) (testimony of Dr. Mykel Kochenderfer).

Federal Aviation Administration. (2012). Pilot’s handbook of aeronautical knowledge. Retrieved from http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/

Federal Aviation Administration. (2014). Wildlife strikes to civil aircraft in the united states 1990-2014 (Serial Report Number 21). Retrieved from http://www.faa.gov/airports/airport_safety/wildlife/media/Wildlife-Strike-Report-1990-2014.pdf

Federal Aviation Administration Accident and Incident Data System. (2016). Retrieved from http://www.asias.faa.gov/pls/apex/f?p=100:12:0::NO:::

Feinstein, D., & Schumer, C. (2015). Consumer drone use threatens public safety. Retrieved from http://www.feinstein.senate.gov/public/index.cfm/2015/10/feinstein-schumer-increased-use-of-consumer-drones-threatens-public-safety

Floreano, D., & Wood, R. J. (2015). Science, technology and the future of small autonomous drones. Nature, 521(7553), 460-466. Retrieved from

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 16

http://search.proquest.com.ezproxy.libproxy.db.erau.edu/docview/1685003576?accountid

=27203

Joslin, R. (2015). Synthesis of unmanned aircraft systems safety reports. Retrieved from Journal of Aviation Technology and Engineering: http://docs.lib.purdue.edu/jate/

Lynch, P. (2016). 2016 Climate trends continue to break records. Retrieved from https://climate.nasa.gov/news/2465/2016-climate-trends-continue-to-break-records/

Manville, A. M. II. (2005). Bird strikes and electrocutions at power lines, communication towers, and wind turbines: state of the art and state of the science - next steps toward mitigation. Retrieved from https://www.fs.fed.us/psw/publications/documents/psw_gtr191/psw_gtr191_1051-1064_manville.pdf

Marsh, S., & Schrab, K. (2010). National Weather Service Manual 10-1401: Operations and Services Upper Air Program Rawinsonde Observations. Retrieved from http://www.nws.noaa.gov/directives/sym/pd01014001curr.pdf

National Weather Service Radiosonde Observations. (2016). Retrieved from https://www.weather.gov/gjt/education_corner_balloon

National Transportation Safety Board Aviation Accident Database & Synopses. (2016). Retrieved from https://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx

Nuckolls, L. (2015). FAA Operation and Certification of Small Unmanned Aircraft Systems

(RIN-2120-AJ60). Washington, DC: Government Printing Office.

United Nations Framework Convention on Climate Change, Dec. 12, 2015, Article 2.

It is no secret that technology across many disparate engineering and scientific disciplines continue to advance very rapidly. Perhaps nowhere is this more observable in the public eye than in the field of small, unmanned aircraft systems (sUAS), commonly referred to as “drones”. sUAS are at the forefront of efforts to synergistically combine numerous technologies into fully integrated platforms for use in an ever-expanding number of commercial, humanitarian, military, law enforcement, and research applications. But today’s commercial drones need improvements to their autonomous capabilities to safely divorce themselves from human control (Floreano, 2015). Recent Federal Aviation Administration (FAA) regulatory guidance in the form of 14 Code of Federal Regulations (CFR) Part 107, Small UAS Operation and Certification, has enabled the increased commercial, governmental, and private use of sUAS by providing sUAS users a framework in which to operate their aircraft safely within the National Airspace System (NAS) (FAA, 2016).

The Small Atmospheric Sensing Aircraft (SASA) is a sUAS concept that has the potential to change how critical atmospheric data is captured by offering an alternative platform to improve controllability, enable dynamic re-tasking and routing, and significantly reduce cost compared to the current system. Upper air observation stations capture vital atmospheric data across the Earth every day (“NWS Radiosondes”, 2015). In the United States, the National Weather Service (NWS) uses this captured data to develop weather forecasts that support all types of daily aviation operations, amongst a host of other national activities; countries across the globe conduct similar operations for the same reasons. Moreover, to improve our collective knowledge of atmospheric phenomena, weather and climate scientists can make great use of this spatial-temporal atmospheric data to tease out complex physical mechanisms that are not yet

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 3

fully understood. SASA is further envisioned to serve as an advanced and reconfigurable platform that can gather different types of atmospheric data beyond pressure, temperature, winds, and relative humidity data, to aid in assessing the planet’s changing climate and weather patterns.

Climate Change is troubling to the scientific community because significant rises in global temperature can adversely affect many different areas of active research such as species habitat, agriculture and associated famines, global sea level rise, the spread of contagious diseases, and delicate marine ecosystems. Considering that atmospheric data is gathered every day across the globe, there seems to be a clear need to gain consistent, reliable, and cost-effective access to the Earth’s atmosphere to better understand how our atmosphere is changing.

The Conference of Paris 21 (COP 21), an international conference on Climate Change held in Paris, France from 30 November 30th – December 12th, 2015, concluded that significant steps must be made, globally, to ensure the average global temperature is not allowed to increase beyond 2 degrees centigrade (°C), and actions should be taken to hold the global average temperature at or below 1.5°C (Article 2, Paris Agreement). Global average temperature from January to June 2016 reached 1.3°C warmer than the preindustrial baseline temperature (NASA, 2016). The current atmospheric data sampling system continues to operate each day, but there is no plan to increase its operational frequency now or in the near future. The SASA sUAS has the potential to be revolutionary in sampling atmospheric data around the globe, combining the latest technology with a prime focus on aviation safety, sampling accuracy, precision, and aircraft recoverability and reuse.

Problem Statement

In the United States, the NWS has been launching Radiosondes, the meteorological hardware and software portion of the upper air observation system, via weather balloons for decades. These instruments are considered expendable–most Radiosondes are lost with no chance for recovery (NWS Radiosonde Observations, 2016). This practice is also the staple method for countries across the globe for those able to afford it. While this method is reliable and extremely safe, it is expensive costing tens of millions of dollars, annually, in the U.S. alone (H. Escabi Jr., personal communication, November 5, 2015).

The NWS launches Radiosondes from over 100 locations, two times per day, every day of each year. This amounts to an annual total of about 75,000 units in the U.S.–over 80% are never recovered (NWS, 2016). At hundreds of dollars per launch, this portion of the NWS’s upper air observing program is tremendously expensive. This fact alone is testament to the value the NWS, and agencies they support, place on the daily atmospheric data that is gathered. The ability to employ a recoverable system that has the potential to significantly increase the amount of data gathered and simultaneously reduce cost could provide tangible benefits.

Research Design

An aviation and aerospace conceptual assessment was conducted on a novel, sUAS equipped with a meteorological instrument payload, such as SASA, that is both a feasible replacement to the legacy Radiosonde weather balloon system, and has the potential to be a more capable system without introducing a significant amount of added risk to the safety of manned aviation operations.

Literature Review

A review of current literature regarding weather balloon operations, similar use of sUAS for atmospheric data sampling, FAA sUAS recommendations and regulations, safety assessments of sUAS operating in the NAS, and significant technological advances in aviation, aerospace systems pertinent to SASA was accomplished.

Statistical Analysis

Because safety considerations are vital to any sUAS poised to operate in the NAS, a parametric t-test analysis of historical aviation aircraft bird collision data and historical aircraft-aircraft midair collision data was conducted. Because sUAS are only recently beginning to fly in numbers that have driven the FAA to develop regulatory guidance for their use, there is very little data on sUAS aviation incidents with other aircraft. Without a historical sUAS incident data pool to draw from, an analysis of aviation accident data that is available was used. The intent of this analysis was to review the available aviation midair collision data, and attempt to draw conclusions that may be applicable to sUAS that operate within the NAS. Any available literature pertaining specifically to sUAS collisions with commercial and general aviation aircraft were also examined.

Results

Criticality of Atmospheric Data Collection Via Radiosondes

Radiosondes give weather forecasters a vertical sampling of atmospheric wind direction and speed, pressure, temperature and relative humidity from the surface up to approximately 100,000 feet. This data is then used in myriad ways. Two-dimensional charts are produced to illustrate the vertical structure of the atmosphere where forecasters can visualize moist and dry areas, identify temperature inversions at different levels that can help forecast fog, severe thunderstorms, and wind patters, for example. This data is also the bedrock for numerical weather models that seek to ascertain the atmosphere’s initial characteristics at a given time and provide solutions for short and long-range weather forecasts.

A long-standing limitation regarding numerical weather models is their reliance on very sparse upper air data. The U.S. contains about 3.8 million square kilometers of territory that

 

SMALL ATMOSPHERIC SENSING AIRCRAFT (SASA) 6

amounts to an average of one Radiosonde sampling two times a day, per 37,000 square kilometers. Assuming each Radiosonde is launched at an even spacing (they are not), this clearly leaves an immense amount of vertical atmosphere that is not sampled. Numerical weather models are forced to interpolate for all the points in between where there is no data available. The sparseness of upper air data induces initial errors that become more pronounced as the numerical model calculates weather conditions farther and farther away from the initial conditions frame of reference. SASA’s usefulness for the weather and climate community would be a major step forward, but it would also produce secondary benefits for every person and organization that uses weather and climate information in their decision-making cycles.

SASA data would likely provide improvements for short and long-term agricultural production outlooks, environmental disaster response agencies, research institutions, military operations, and for all forms of aviation that rely on accurate weather forecasts to fly safely each day. For the atmospheric data collected by SASA to be useful, each integrated system must be meticulously designed and tested for safety and functionality. But before any design considerations can be firmly contemplated, a comprehensive analysis must be conducted to assess current and past sUAS design and testing efforts. The current and anticipated aviation regulatory guidance must also be reviewed. And arguably most important, a preliminary safety analysis must be designed and conducted to determine how SASA might compare to the legacy system’s safety record.

 

Federal Aviation Administration Regulatory Structure

SASA is envisioned to augment or completely replace the existing Radiosonde system. The case must be made that SASA can significantly surpass the capabilities of the current system while affording a margin of safety consistent with, or greater than, what has already been

 

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demonstrated. Comparing the overall picture of how these differing concepts of operation (CONOPs) work provides a quick assessment of their primary similarities and differences.

SASA’s main similarity to the legacy system is the method by which it ascends. SASA will rise up to altitude via a standard weather balloon via CFR 14, Part 101 “Moored Balloons, Kites, Amateur Rockets and Unmanned Free Balloons” just as legacy Radiosondes do today. As long as SASA adheres to CFR Part 101 guidance and limitations, there is virtually no difference compared to the current Radiosonde system. The stark difference rests in SASA’s descent method. SASA will not be falling via an unguided parachute as all Radiosondes do now. Once the weather balloon bursts at about 100,000 feet, SASA will become an autonomous sUAS that can either act as an unpowered sUAS glider, or as a powered sUAS when situations arise that require powered flight. At this transition point, SASA becomes a guided aircraft, whereas current Radiosondes are unguided payloads, not considered aircraft in any manner. As such, SASA is compelled to adhere to further CFR guidance. And this is the point at which SASA makes its pivotal departure from the legacy system.

Regulatory guidance for sUAS does exist in the form of CFR Part 107, “Operation and Certification of Small Unmanned Aircraft Systems”. However, this new guidance only allows operations up to 400 feet above ground level (AGL), unless, in the case of use by a structure greater than 400 feet in height, the UAS remains within 400 feet of the structure. Operating in any other way above 400 feet AGL is forbidden. SASA will descend through airspace as high as 100,000 feet, so CFR Part 107 is only applicable to SASA at or below 400 feet. This leaves over 99,000 feet of altitude unaccounted for in SASA’s operating altitude. The primary question is whether or not SASA, being a sUAS, should be treated as any other, large commercial or general aviation aircraft. If so, then SASA would be subject to additional CFR Parts.

 

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FAA notice JO 7200.23, effective October 3, 2016, prescribes how unmanned aircraft will operate within the NAS, and delineates small and large UAS—less than 55 pounds for the former, 55 pounds and greater for the latter. Notwithstanding guidance in the newly released CFR Part 107, this notice affirms that the FAA will not allow small UAS operations outside of “active restricted, prohibited or warning areas in the NAS without specific authority…” If SASA were authorized to fly within the NAS to collect essential atmospheric data, safety considerations will be paramount. Specifically, the specter of mid-air collisions with commercial and general aviation aircraft immediately comes to mind. With no data available on sUAS flying at all levels within the NAS, one can look to the outstanding aviation bird strike dataset that is readily available and perhaps draw some informative conclusions from this extensive data set.

SASA Flight Safety

An extensive review of the National Transportation Safety Board (NTSB) database revealed no confirmed collision with weather Radiosondes–this aviation safety record is extraordinary given millions of launches over several decades. The obvious question arises regarding SASA’s likelihood of attaining this legacy level of safety. SASA will have the ability to maneuver with and without power, incorporate GPS geo-fencing technology, the potential to integrate advanced Automatic Dependent Surveillance-Broadcast (ADS-B) capability, amongst other cutting-edge aviation technologies. Given these aforementioned safety features, one might be very confident that SASA could achieve, or even exceed, the level of safety already demonstrated by the legacy Radiosonde system. While possible, it would be a significant mistake to make such an assumption without conducting a comprehensive safety analysis with historical aviation data. Moreover, a comprehensive review of any research involving sUAS similar to

 

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SASA, governing aviation regulatory guidance, and potential hardware and software engineering solutions must be considered.

The initial SASA prototype is expected to be in the micro UAS category at less than 4.4 pounds. There is little doubt that most aviation experts would agree that even a sUAS accidently ingested into an aircraft jet engine would result in some degree of damage; fortunately, to date there are no confirmed reports of an actual sUAS mid-air collision with a commercial passenger aircraft. If a sUAS were sufficiently large, one could expect significant, to possibly catastrophic damage in a jet engine ingest accident.

On October 7th, 2015 Dr. Mykel Kochenderfer, Assistant Professor of Aeronautics and Astronautics at Stanford University, provided testimony to the U.S. House of Representatives on the subject of Ensuring Aviation Safety in the Era of Unmanned Aircraft Systems. The subject of safety regarding the introduction of sUAS into the NAS is arguably the most important aspect of the entire sUAS enterprise. Dr. Kochenderfer’s testimony centered around two primary points: how is one to properly measure and analyze the risk posed by sUAS, and what technologies and policies can be employed to minimize risk. Since the sUAS proposed here will descend from very high altitude and autonomously guide itself back to a recovery location, there is some added risk to manned aircraft. However, the magnitude of that additional risk is the unknown that must be assessed. The initial target weight of SASA is 4.4 pounds or less, similar to a small to medium-sized bird.

Dr. Kochenderfer referred to the dual engine failure of U.S. Airways flight 1549 that occurred on January 15th, 2009. The A320 aircraft ingested a flock of Canadian geese into both engines just after takeoff and suffered dual, catastrophic engine failures that led to an emergency landing on the Hudson River. A swarm of similarly sized sUAS comparable to the weight of a

 

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Canadian goose, about 6-14 pounds, could surely do equivalent damage. But large groups of sUAS flying near airports and at low altitude are not at all consistent with SASA’s envisioned flight profile. Analyses that estimate potential risk of collision with a single, very small UAS seem reasonable versus a large, concentrated group.

The seminal difference in the NWS legacy Radiosonde system and SASA involves autonomous guidance as it makes its way to a recovery point. This capability should not be viewed as a risk liability, necessarily, and the FAA has previously affirmed that an autonomous sUAS glider does afford greater safety than an unguided payload falling via parachute.

In 2013, the National Oceanic and Atmospheric Administration’s (NOAA) UAS Program office obtained FAA approval to operate their SkyWisp autonomous sUAS. The SkyWisp sUAS glider used standard weather balloons to ascend to an altitude of about 100,000 feet, and then glided autonomously to a recovery point after balloon burst. NOAA argued to the FAA that a “glider controlled recovery” afforded a reduction in risk compared to parachute-aided recovery and that extensive, documented SkyWisp test flights showed an exemplary safety record. After reviewing the SkyWisp program testing history and associated safety record, the FAA responded, “We have evaluated the SkyWisp program. The UAS Integration Office does not currently regulate unpowered, unmanned gliders. Consequently, the SkyWisp program can be operated in the same manner as your other unmanned free balloon operations…” The FAA’s decision makes perfect sense when one considers that a “glider controlled recovery” has the potential to autonomously navigate to a recovery point, which implies that such a device has inherent flexibility that an unguided, parachute-aided payload recovery system lacks.

 

SkyWisp’s high level of safety in restricted airspace is an important risk indicator, but it doesn’t address the potential risk a sUAS might pose in the NAS at large. Conducting a

 

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comparison of aircraft-on-aircraft midair collisions and bird-on-aircraft mid-air collisions could provide valuable information. FAA and NTSB databases provide historical data regarding aircraft-on-aircraft mid air collisions, aircraft-on-aircraft near mid-air collisions (NMACs), and bird-on-aircraft mid-air collisions, also known as “bird strikes”, and annual accident statistics that involve what the NTSB calls major, serious, and injury accidents resulting from diverse causes. Bird strikes on all forms of aircraft are by far more frequent than aircraft colliding in mid-air with one another. The first obvious difference is the sheer number of birds that exist in the U.S.—10 to 20 billion birds, depending on the time of year (Manville, 2005). Bird strikes occur in all sectors of aviation.

Considering there are 109 NWS locations currently sending up weather balloons twice a day, the total number of SASA launches, if SASA were being used today, would be about 218 sUAS in a 24-hour period. SASA is comparable to a small or medium size bird, but SASA’s envisioned operational numbers are nearly insignificant compared to the amount of birds flying in the NAS at any given moment. Nor will SASA be launched from the airport runways where birds do have the potential to impact aircraft on takeoff and landing.

In addition to SASA being carefully integrated into airspace, each aircraft will require robust communications and telemetry capabilities to ensure positive control of all SASA aircraft flying in the NAS. Each capability will have to work synergistically to avoid collision mishaps with manned aircraft. SASA must be on par with the legacy Radiosonde system’s perfect record to date. Anything less than an exemplary safety record may only serve to question why the legacy system was abandoned in favor of the SASA sUAS.

Aircraft Mid-air Collisions and Bird Strikes Analysis

 

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According to the NTSB, from 1997 – 2014 there were 32 major, 28 serious, and 284 injury aviation accidents involving Part 121 commercial air carrier operations in the U.S. Over this period 18 million flight hours were flown on average, annually, with 0.098 major accidents per million hours flown. Over the 18-year period, this amounts to less than two Part 121 major accidents every year. The NTSB considers a major accident as one that involves the destruction of the aircraft, multiple fatalities, or at least one fatality and substantial damage to a Part 121 aircraft. These descriptive statistics alone show that major Part 121 accidents are extremely rare; however, there were 202 General Aviation (GA) mid-air collisions in the same 18-year period, including 120 instances where fatalities occurred that resulted in a total of 295 deaths.

Though no mid-air collisions involving Part 121 aircraft occurred from 1997 – 2014, there were 341 pilot reports of critical NMACs; a critical NMAC is one in which the pilot lacked sufficient time to execute an evasive maneuver, and the estimated separation of the aircraft was less than 100 feet (FAA, 2016). Put another way, on average almost 19 critical NMACs involving Part 121 aircraft occur each year, and an average of 946,578 Part 121 aircraft hours are flow between each NMAC occurrence. Again, while certainly more prevalent than actual mid-air collisions, NMACs are a fairly rare Part 121 occurrence.

Bird strikes on all forms of aircraft are by far more frequent than aircraft colliding in mid-air with one another because of the sheer number of birds living in the U.S. It turns out that from 1997 to 2014 there were 170 mid-air collisions between aircraft in the U.S. and 11,043 damaging bird strikes on aircraft, within an annual average of over 58 million aircraft operations (NTSB, 2016). Intuitively, one can deduce that with only 218 hypothetical SASA launches per day, and considering SASA’s very small mass, the incidence of mid-air collisions with other commercial or general aviation aircraft would be at or below the incidence of aircraft on aircraft mid-air

 

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collisions. Recalling the fact that the legacy Radiosonde system has never collided with an aircraft, one can argue that there is the potential for a guided sUAS such as SASA to have a real potential to continue this outstanding safety trend. SASA would have the capability to actively increase the safety margin by not drifting into unwanted sections of airspace. SASA will have the ability to avoid airspace where the current system cannot in any way. If SASA incorporates many of today’s modern technology, surpasses the legacy Radiosonde system in operational tempo, and can match its safety record, then this could be the right time to design the next-generation atmospheric sampling platform in the form of the SASA sUAS concept.

Recommendations

SASA should build on the work of NOAA’s SkyWisp autonomous, sUAS glider. This effort has already provided a successful proof of concept regarding many of the aspects discussed in this paper. The addition of a battery-powered propulsion system, passive and active sense and avoid technologies, and operations outside of carefully controlled airspace are the primary differences between what has been accomplished to date and the next evolution of this concept.

The sUAS proposed here, designed with the latest technology in mind, could be the basic template for other sUAS that seek to operate in the NAS beyond CFR Part 107 requirements. Modeling and simulating SASA operations in the NAS will be a key aspect of the overall developmental and testing plan.

Embry Riddle Aeronautical University’s Next-Generation ERAU Advanced Research Lab’s (NEAR) Real-Time Distributed Simulator (RTDS) can simulate historical U.S. air traffic to investigate SASA’s virtual performance within the NAS alongside manned aircraft. This future effort would be very useful in providing both qualitative and quantitative data prior to

 

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actual SASA flight-testing. Upon completing modeling and simulation, SASA could begin controlled range flight-testing at low altitudes and works its way to ever-higher altitudes alongside the legacy system to evaluate SASA’s performance.

Conclusion

The legacy NWS Radiosonde has performed exceedingly well for decades, but it suffers from excessive cost, limited capability, and most significantly, it cannot provide enough data to address 21st Century challenges.

Our Earth’s atmosphere and oceans are experiencing considerable warming. The COP 21 meeting in Paris, France set a goal of no more than a 1.5°C increase in average global temperature. This goal seems to be at risk of being broken in the very near future. Whether climate change is due to anthropogenic forcing or some other “natural” phenomena, SASA has the potential to provide researchers with a wealth of new atmospheric data. Moreover, SASA simultaneously provides a more robust data set for weather forecasts that are already critical for our daily lives. Any civilian or military activity that currently relies on Radiosonde data can only benefit from SASA’s operational introduction.

Designed with the latest technologies in mind, SASA could be the basic template for other sUAS that seek to operate safely in the NAS outside of CFR Part 107 boundaries. The idea that a sUAS concept such as SASA could actually help improve weather forecasts, and through these forecasts, serve to make flying safer for all other forms of aviation, is both satisfying and exciting.

 

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References

Aeronautics and Space, 14 Code of Federal Regulations - C.F.R. § 1-1399 et seq. (2016). Dolbeer, R. A. (2006, February 26). Height distribution of birds recorded by collisions with civil aircraft. Journal of Wildlife Management, 70,5, 1345-1350. Retrieved from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1496&context=icwdm_usdan wrc

Drone Aircraft Privacy and Transparency Act of 2015, S. S.635, 114th Cong. (2015)

Ensuring Aviation Safety in the Era of Unmanned Aircraft Systems, 114th Cong. 1 (2015) (testimony of Dr. Mykel Kochenderfer).

Federal Aviation Administration. (2012). Pilot’s handbook of aeronautical knowledge. Retrieved from http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/

Federal Aviation Administration. (2014). Wildlife strikes to civil aircraft in the united states 1990-2014 (Serial Report Number 21). Retrieved from http://www.faa.gov/airports/airport_safety/wildlife/media/Wildlife-Strike-Report-1990-2014.pdf

Federal Aviation Administration Accident and Incident Data System. (2016). Retrieved from http://www.asias.faa.gov/pls/apex/f?p=100:12:0::NO:::

Feinstein, D., & Schumer, C. (2015). Consumer drone use threatens public safety. Retrieved from http://www.feinstein.senate.gov/public/index.cfm/2015/10/feinstein-schumer-increased-use-of-consumer-drones-threatens-public-safety

Floreano, D., & Wood, R. J. (2015). Science, technology and the future of small autonomous drones. Nature, 521(7553), 460-466. Retrieved from

 

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http://search.proquest.com.ezproxy.libproxy.db.erau.edu/docview/1685003576?accountid

=27203

Joslin, R. (2015). Synthesis of unmanned aircraft systems safety reports. Retrieved from Journal of Aviation Technology and Engineering: http://docs.lib.purdue.edu/jate/

Lynch, P. (2016). 2016 Climate trends continue to break records. Retrieved from https://climate.nasa.gov/news/2465/2016-climate-trends-continue-to-break-records/

Manville, A. M. II. (2005). Bird strikes and electrocutions at power lines, communication towers, and wind turbines: state of the art and state of the science - next steps toward mitigation. Retrieved from https://www.fs.fed.us/psw/publications/documents/psw_gtr191/psw_gtr191_1051-1064_manville.pdf

Marsh, S., & Schrab, K. (2010). National Weather Service Manual 10-1401: Operations and Services Upper Air Program Rawinsonde Observations. Retrieved from http://www.nws.noaa.gov/directives/sym/pd01014001curr.pdf

National Weather Service Radiosonde Observations. (2016). Retrieved from https://www.weather.gov/gjt/education_corner_balloon

National Transportation Safety Board Aviation Accident Database & Synopses. (2016). Retrieved from https://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx

Nuckolls, L. (2015). FAA Operation and Certification of Small Unmanned Aircraft Systems

(RIN-2120-AJ60). Washington, DC: Government Printing Office.

United Nations Framework Convention on Climate Change, Dec. 12, 2015, Article 2.

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