ASCI 530 Module 4 Research: UAS in the NAS

Introduction

With the recent release of Chapter 14 of the Code of Federal Regulations Part 107 regulating commercial small unmanned aircraft systems (sUAS) weighing less than 55 pounds and operating within visual line of sight, the Federal Aviation Administration (FAA) has opened the door to a massive aviation industry. As part of the new regulations, the FAA takes great care to preserve the safety of all aircraft in the National Airspace System (NAS), which includes maintaining the requirement to both see and avoid other aircraft. To satisfy this requirement, the FAA currently requires all sUAS to be flown within visual line of sight (VLOS). The next regulatory hurdle will be to regulate UAS flights flown beyond line of sight (BLOS). Currently, the distinction in regulations will not depend on aircraft size, but on the type of operation as determined by altitude and speed (Elias, 2016). For the purposes of this paper and a best guess based on current regulations, the following segments of UAS will be analyzed: 

·      Category 1: sUAS flown within VLOS below 400’ above ground level (AGL)

·      Category 2: sUAS flown BLOS below 400’ AGL (below controlled airspace of the NAS)

·      Category 3: UAS flown above 400’ AGL with max airspeed less than 100 kts (within controlled airspace of the NAS)

·      Category 4: UAS flown above 400’ AGL with max airspeed greater than 100 kts

(These categories may not reflect the actual regulations, but are based on available predictions of FAA intentions)

General Considerations

Of these categories, some UAS airframe types will require additional consideration to satisfy separation of manned and unmanned aircraft. All current UAS are smaller than their manned aircraft counterparts, due to the lack of a cockpit. Regardless of shape, their smaller visual profiles will not provide pilots of manned aircraft with accurate references to estimate distance, and their small profiles and mostly composite construction does not provide passive radar systems with a sufficient profile to enable accurate resolution (Strain, DeGarmo, Moody, 2007).

Fixed-wing airframes present a challenge to aircraft separation because of their necessary forward speed to maintain flight. Fixed-wing aircraft on a converging path are not able to simply “put on the brakes” and yield to one another, and the visual judgement of distance and speed is largely based on the size of the aircraft.

Technology use in Manned Aircraft

A traffic collision avoidance system (TCAS) is available for use in manned aircraft, and required for turbine-powered aircraft carrying more than 10 passengers (FAA, 2011). Of the available TCAS systems within the U.S., TCAS I provides pilots with traffic advisories (TA) to assist with visual identification of other aircraft, and TCAS II adds resolution advisories (RA) that provide recommendations for escape maneuvers to increase vertical separation (climb or descend). TCAS depends on the same transponder signals that aircraft use to provide monitoring stations with altitude and aircraft information. As a matter of fact, the FAA states that “…TCAS provides no protection against aircraft that do not have an operating transponder.” TCAS systems use standardized display symbols to show the position of other aircraft in the vicinity, usually as an overlay on a secondary moving map display (FAA, 2011). 

Figure 1. TCAS Volume. Eurocontrol: http://www.eurocontrol.int/msa/public/standard_page/ACAS_Overview_Principles.html

The Automated Dependent Surveillance-Broadcast (ADS-B) system is an additional technology that will be required for all manned aircraft starting in 2020. Equipment for both commercial and general aviation aircraft has already been made available for installation, often as a compliment to a pre-existing TCAS. ADS-B uses different frequencies than the transponder/TCAS system, and provides similar but better information.

Applications for Unmanned Aircraft

Category 1: sUAS in this category are commercially regulated. For sUAS flights in Category 1, an ADS-B receiver is recommended for situation awareness of any manned aircraft in the immediate area. For about $35, a simple 1090 MHz receiver USB dongle can be connected to an antenna, and any ADS-B or transponder signals coming from transmitting aircraft can be depicted on a computer display (RTL-SDR.com, 2013). Another possible application of this low-cost system is the potential for the sUAS GPS position as reported to a computer-based ground station to be sent to the ADS-B system via internet connection, and re-transmitted to airborne aircraft using ADS-B receivers. This is not a “true” ADS-B solution, but does provide additional awareness to manned aircraft.

 Figure 2. ADS-B Virtual Air Radar with RTL-SDR. YouTube user Superphish.

Category 2: for BLOS flights that remain below 400’ AGL, the UAS will be below the minimum altitude for most fixed-wing aircraft. However, helicopters are permitted below 500’ AGL. To increase aircraft separation for BLOS flights and flights at night (Category 2 and Category 1 operating with a waiver) a low-cost ADS-B transceiver is recommended. A commercial-off-the-shelf solution from uAvionix brings full ADS-B capabilities to sUAS. Their ADS-B transceiver weighs only 20 grams but is capable of transmitting a 20 watt ADS-B UAT signal (uAvionix, 2016), alerting all ADS-B-equipped aircraft within 20 nautical miles. The receiver is also sensitive to manned aircraft signals within 100 nautical miles. Using open-source autopilot platforms, various alerts can be configured to provide sUAS pilots with near-TCAS capability. Even more exciting is the potential for programmed automatic responses to manned aircraft in the vicinity i.e. “hover” or “loiter” in place if a manned aircraft signal is received within 1 NM of the UAS. 

Figure 3. PINGNAV-2020. uAvionix: http://www.uavionix.com/wp-content/uploads/2016/04/pingnav_ping2020_pixhawk_autopilot.jpg

Category 3 and 4: as all UAS that operate above 500’ AGL will potentially share airspace with manned aircraft, there is a high probability that ADS-B will be required equipment. Sagetech is another company that produces transponders and ADS-B transceivers for UAS, with the advantage that most of their products are designed to FAA technical service order (TSO) standards (Sagetech, nd).

Conclusion

To date, a large number of operating UAS originated from hobby-grade equipment and hobby-grade technology. To successfully integrate UAS into the NAS and ensure separation from manned aircraft, it will benefit UAS operators and manufacturers to select equipment that is compatible with manned aircraft systems and compliant with manned aircraft regulations.

References:

Elias, B. (2016, January). Unmanned aircraft operations in domestic airspace: U.S. policy perspectives and the regulatory landscape. Congressional Research Service: Washington, D.C. Retrieved from https://www.fas.org/sgp/crs/misc/R44352.pdf

Federal Aviation Administration. (2011, February). Introduction to TCAS II version 7.1. U.S. Department of Transportation: Washington, DC. Retrieved from http://www.faa.gov/documentLibrary/media/Advisory_Circular/TCAS%20II%20V7.1%20Intro%20booklet.pdf

RTL-SDR. (2013, April). RTL-SDR tutorial: cheap ADS-B aircraft radar. Retrieved from http://www.rtl-sdr.com/adsb-aircraft-radar-with-rtl-sdr/

Sagetech. (n.d.) XP family of transponders. Retrieved from https://sagetechcorp.com/xp-transponders.html

Strain, R.C., DeGarmo, M.T., and Moody, J.C. (2007). A lightweight, low-cost ADS-B system for UAS application. MITRE Corporation: McLean, VA. Retrieved from https://www.mitre.org/sites/default/files/pdf/07_0634.pdf

uAvionix. (2016). Ping-2020 ADS-B transceiver data sheet [Brochure]. Retrieved from http://uavionix.com/downloads/ping2020/docs/uAvionix-ping2020-data-sheet-ap0.pdf

ASCI 530 Module 2 Research: Overweight Crop-dusting UAS

In the role of a systems engineer, my first course of action when approaching a design conflict is to focus on the requirements and constraints of the initial project. In the given example, the navigation, payload, and safety groups are at odds with their given constraints. However, systems engineers are tasked with determining implementation of system functions. In the example of the overweight agricultural spray application UAS, the payload (spray equipment and capacity) are constrained by promises to customers and the overall requirement of the aircraft. The project requirements should “bring the most value to the customer and help the business improve innovation” (IBM, 2013). Timeline and budget should also be considered, but in this example, the highest priority would be improvement of the navigation and control system.

In order to carry the determined payload of spray application equipment and tank capacity (without lowering fuel load), either the navigation and control system needs to lose weight, or the overall lift and endurance of the aircraft must be increased. There are a multitude of potential technical solutions, including selection of an improved power plant, improved aerodynamics, or a lighter navigation system altogether. For each of these options, there should be a risk assessment conducted to determine the cost and benefit of each implementation. Since each technical option has the potential of bringing a new vendor/subcontractor to the design process, external partners can be utilized to fill the execution “gaps” present in the aircraft design (Terwilliger, Burgess, & Hernandez, 2013). Feasibility, schedule, and budget should be weighed to assess risk to the overall success of the design project.

In this particular design problem, a recommended technical solution would be to remove some of the onboard navigational processing equipment and replace it with a wireless sensor network (WSN). UAS application of agricultural chemicals can be achieved through implementation of a ground-based network of sensors that provide feedback to a computer that determines and commands the performance of the aircraft and sprayer payload in varying wind conditions. A 2012 paper by F.G Costa (et. al) predicted a 10-22% reduction of wasted pesticide through corrections provided every 15 seconds to a UAS via ground-based WSN and control loop algorithm.

A WSN solution brings added cost and complexity to the product, but it adds two important benefits. First, it performs the dual function of reducing the weight of the aircraft by removing some of the necessary sensors and processors to perform corrections on-board. Second, and more importantly, using a WSN increases efficiency by a large margin, which aids the success of meeting a capacity and endurance requirement. The solution also aids the business in future aircraft development by providing a path to swarm technology through mobile ad-hoc networking (MANET). 

References

Costa, F. G., Ueyama, J., Braun, T., Pessin, G., Osório, F. S., & Vargas, P. A. (2012, July). The use of unmanned aerial vehicles and wireless sensor network in agricultural applications. In Geoscience and Remote Sensing Symposium (IGARSS), 2012 IEEE International (pp. 5045-5048). IEEE.

IBM Corporation, Software Group. (2013). Ten steps to better requirements management. Somers, NY: Author. Retrieved from http://public.dhe.ibm.com/common/ssi/ecm/en/ raw14059usen/RAW14059USEN.PDF

Terwilliger, B., Burgess, S., and Hernandez, D. (2013). ASCI 530 Module #2 global system design concepts, requirements, and specifications overview [PowerPoint slides]. Retrieved from https://erau.instructure.com/courses/39651/files/6702704/download?wrap=1

Coaxial Rotorcraft Design in UAS Development

Figure 1. Consumer-grade coaxial rotorcraft UAS.

Figure 2. Gyrodyne QH-40 DASH.

The Gyrodyne QH-50 DASH (Drone Anti-Submarine Helicopter) was the first vertical takeoff and landing (VTOL) UAS, which is presently one of the most ubiquitous designs among consumer and professional users alike. The original QH-50 design was developed to carry a single reconnaissance Marine via a coaxial rotorcraft vehicle. In 1960 the design was changed to an unmanned vehicle and the QH-50 began service. President John F. Kennedy personally approved the aircraft program after observing a demonstration in 1962. The aircraft consisted of two counter-rotating lift surfaces rotating on a single axis. A reciprocating Porsche engine was originally used to power the craft, but a turboshaft engine was eventually used in it’s place. The aircraft was designed to carry two anti-submarine torpedoes and could also be fitted with transmitting cameras for remote reconnaissance. The QH-50 was used extensively during the Vietnam War for reconnaissance and spotting of naval gunfire (observing the accuracy of impact to assist accurate corrections) (Evans, 2011).

Figure 3. Aerialtronics Altura Zenith ATX8

Modern rotorcraft UAVs come in many shapes and sizes. One design that has gained recognition in professional and heavy-lift applications is a coaxial multi-rotor. In this application, an even number of electric motors are placed in counter-rotating pairs at equal axis around a central airframe. An example is the Aerialtronics Altura Zenith, an octo-rotor in the “X8” configuration (four main arms with a coaxial pair at each end). A flat-axis quad-rotor aircraft is considered by many to be the most efficient design for simplicity, low power consumption, and low weight (Bouabdallah, Becker, and Siegwart, 2007). However, for professional videography and other applications, safety is an important consideration in the engineering design process, specifically in redundancy of controls.

The main similarity between the two systems is the use of coaxial rotor technology. Coaxial rotors counteract the main problem of torque in traditional single-rotor helicopters. They also prevent a condition called “retreating blade stall” that occurs at high forward speeds. A 1997 NASA Technical Paper by Colin Coleman found that a coaxial rotorcraft system “requires 5% less power than an equivalent single rotor.”

The main difference between the QH-50 and Altura Zenith (besides the obvious size difference) is the flight control, specifically in the yaw direction. Yaw of a rotorcraft is accomplished by increasing or decreasing relative airframe torque in opposition to the direction of the rotor, which turns the aircraft. However, the QH-50 was a turbine-driven aircraft and could not easily speed up and slow down the rotors to create the torque effect needed to turn. The solution was to install flaps on the blade tips that could be deployed to quickly slow the rotation of a rotor and decrease the torque in that direction. Modern electric multi-rotor aircraft such as the Altura Zenith are much more responsive. Torque effect is induced by quickly speeding up and slowing down rotor pairs via computerized control of motor speed.

Electric VTOL aircraft are still relatively inefficient and do not have the endurance of fixed-wing aircraft. One technology that could be applied to future unmanned rotorcraft is called “slowed rotor/compound,” where a main lift rotor is slowed during forward flight while the aircraft is driven forward by a propeller. It is different from an auto gyro aircraft in that the rotor is powered. This would allow true VTOL access to small landing areas while maintaining the speed and efficiency of fixed-wing flight (Carter Aviation Technologies, 2016).

References:

Bouabdallah, S., Becker, M., & Siegwart, R. (2007, September). Autonomous miniature flying robots: coming soon! - Research, development, and results. Robotics & Automation Magazine, IEEE, 14(3), 88-98. doi: 10.1109/M-RA.2007.901323

Carter Aviation Technologies. (2016, April). Carter Aviation seeking partners for UAS business development. [Press release]. Retrieved from http://www.cartercopters.com/pdfs/ CarterCopterBrochure-June2015.pdf

Coleman, C.P. (1997). A survey of theoretical and experimental coaxial rotor aerodynamic research (NASA Technical Paper 3675). Moffett Field, CA: Ames Research Center.

Evans, S.S. (2011). The incredible story of the QH-50 DASH – The first unmanned helicopter turns 50. Vertiflight, 57(1), pp. 36-39. Retrieved from http://www.aero.psu.edu/Facilities/ images/36_DASH_QH-50.pdf