The sUAS Guide Issue 02, July 2016 | Page 75


I. BACKGROUND

The proliferation of Unmanned Aerial Systems (UAS), also known as Unmanned Aerial Vehicles (UAV) or Remotely Piloted Systems (RPS), promises to revolutionize many areas of modern life. Examples of UAS usage include law enforcement, disaster relief, mapping, entertainment, and product transport and delivery. Figure 1 gives some examples of uses and expected trends in coming years.



















Figure 1. UAS Uses and Proliferation

However, this proliferation presents a unique challenge for traditional sensor and aircraft traffic management systems.

The Federal Aviation Regulation (FAR) Section 91.113 dictates that an aircraft must be able to “see and avoid” other aircraft. With no pilot on board, an alternate means of compliance must be established. Current rules require either operation within LOS of the controller or a chase plane to provide the visual avoidance.

Given the huge economic and societal benefits of UAS integration, a more efficient and cost effective manner of ensuring safety must be developed. The most common approach is to use some combination of sensors to replicate the pilot’s vision.

While cooperative (transponder based) solutions are welcome, there will always be non-cooperative targets that also require mitigation. These non-cooperative targets can include manned aircraft without transponders, other UAS, craft wishing to be undetected, and birds.

A. Definition of UAS Sizes
UAS are generally defined by their maximum range (how far they can fly without landing to refuel) and their maximum payload. Systems can then be categorized by group number, with Group 1 being very small (such as a DGI Phantom) to Group 5 (such as a Global Hawk). Some put the smallest craft such as the Phantom in the “Group 0” or “Micro” classification. Figure 2 gives an example of several current UAS with approximate groupings.

















Figure 2. Group Assignment for Selected UAS

The technology required to successfully integrate UAS is partially dependent on the UAS class. Group 3 and above are analogous to current manned aircraft in terms of capabilities. These aircraft generally have the payload and power handling to integrate sensors directly on the platform. Small UAS (Group 2 and below for the purpose of this paper) often do not have the payload or power capacity for sensors capable of sufficient detection ranges to avoid manned aircraft. In all cases, the mission and environment of the specific craft dictates the optimal method of providing safe airspace deconfliction.

B. Sensor Trade Space
Radar systems have well documented advantages over other sensor types such as optics, acoustics, and lasers. Optical systems have limited range (especially on small craft) and are severely degraded in fog, rain, and darkness. Cameras are limited in tracking multiple targets simultaneously over a wide angular extent.

Acoustic sensors can be used to detect the sound of UAS, especially those with propellers. However, acoustics are much less accurate and performance suffers in noisy environments (urban, vehicular, etc).

Finally, lasers can provide high accuracy in a small package but lack the range and volumetric coverage for DAA operations. Lasers are best left for terminal deconfliction of the craft when landing or mitigating stationary obstructions.

There are advantages to non-radar sensors as well and these sensors can augment radar to aid in target recognition in a System of Systems (SoS) construct. However, any robust solution is likely to require a radar system for primary detection. Table 1 shows the trade space of radar-only options that include Ground Based Radar, Airborne Radar, and Transponder solutions.

C. Ground Based Radar Details
The largest benefits of Ground Based Radars include:

• Large Size, Weight, and Power (SWaP) availability to survey a large volume
• Single sensor can cover multiple UAS, which saves cost and reduces RF spectrum usage in dense target environments
• No need for extra equipment on the craft, saving payload for the primary mission
• Emplacement (and freqeuncy) can be optimized ahead of time, including coordination with the FCC
• Redundant source of position/velocity of craft if communications are lost
• Ability to detect non-cooperative targets

The largest drawbacks of Ground Based Radars include:

• Line of Sight (LOS) limitations for low flying targets
• Required infrastructure for long range or remote operations
• General radar/transponder inability to detect stationary targets such as water towers or telephone poles for collision avoidance

The pros and cons make the optimal sensor mission dependent. For example, if instrumenting a city or large area for UAS usage, a ground based sensor can be sited to avoid LOS limitations, operate at a single frequency carefully coordinated with the FCC, and cover many UAS with a single sensor to provide a robust and cost effective solution. Multiple airborne radars requiring mobile coordination of spectrum would be very problematic in this case.

However, a mission that included a Long Endurance (LE) UAS traveling hundreds of kilometers in remote locations (say for wildlife monitoring) would be better suited for an airborne sensor. LOS limitations and the large area requiring instrumentation would make ground based technology prohibitive; not counting localized operations that could use aground mobile solution.

As previously discussed, transponders are very useful but are not sufficient for all situations due to non-cooperative targets.
II. DEVELOPMENT
A. Research Tasks
The following tasks are proposed as steps to determining the requirements for BVLOS operations. These are research oriented and are meant to be completed through analysis and simulation.

1) Targets of interest
a) Survey of present sUAS characteristics: altitude of operation, ground speeds, ascent/descent rates, turn rates, physical and RF size, mission specific behaviors, failure modes, etc. These should be extrapolated based on design trends.
b) Survey of manned aircraft expected to occupy the same airspace as the sUAS. Use similar characteristics list as above.
c) Analysis of other non-cooperatives expected, for example looking at bird density fluctuations over time for different regions.

2) Concept of operation
a) Select a few representative, varied missions requiring BVLOS operation of sUAS : Precision Agriculture, Emergency Responders, Power Line inspection, and Commercial Deliveries.
b) Specify hypothetical System of Systems (SoS) to enable each one of the missions for BVLOS oepration. This includes the full sensor suite required both on the ground and on the vehicles.
c) Define an error budget for the System of Systems including the craft characteristics, mission specifics, and residual needed by ground and air sensors for DAA along with tracking and fusion algorithms needed to make real-time decisions. This task includes basic simulation to test the bounds of the problem space.

3) Analysis
a) Establish a conservative estimate for well clear definition based on the error budget above, desired probability of near mid air collision for each mission safety case, and what can be attained from sUAS and sensor technologies.
b) Determine minimum requirements for sUAS and sensors involved in DAA given the above. For example range and volume coverage, bandwidth, update rates, LOS limitations and blockage mitigation, reaction time, necessary failure modes, etc. Target density will also be a driving factor for these requirements and several could be parameteric on density.
c) Generate a report that will ultimately guide the testing phase. Outputs from simulation should point to edge case scenarios to find potential weaknesses in the theory. These can then be tested with physical craft to reinforce the theory with real data.

B. Facilities
The design and maturation of DAA requires controlled infrastructure to develop requirements and standards for all applicable equipment and to test different scenarios. These facilities can provide a venue for rapid technology progress by providing the infrastructure that is often costly and time consuming to stand up. Examples include:

• Cleared airspace for UAS operations
• Approved frequency plan for various sensor types
• Truth data instrumentation to measure performance
• Network connectivity and standard interfaces
• Data collection ability on aircraft and sensors
• Testing of multiple mission scenarios
• Longer term test improvement

C. Test
With access to facilities and the above analysis tasks done, the testing phase can begin. The goal is to construct a test plan per requirement with the purpose of matching up predicted performance with actual performance.

Tests should be broken up by mission space and to minimize risk, virtual targets can be flown in tandem with real targets to exercise the DAA algorithms based on prior simulation. This allow for quick scalability with low overhead. It is also much safer than flying manned craft in close proximity of sUAS until performance can be verified.

III. CONCLUSION

This paper summarizes a few of the high level trades regarding technology that enables sUAS integration into the airspace for BVLOS operation. Radar systems are necessary to provide all weather, day/night surveillance of large volumes for DAA operations. The exact mix of sensors is scenario dependent, but many will be optimized through the use of ground based radar.

Instrumented test facilities require development in order to develop system requirements, understand con-ops, verify simulation models, and advance the technology required to successfully integrate UAS into the airspace.

A clear research, analysis, and test plan was laid out as a list of smaller tasks.