Marines Aviation BAA M67854-00-R-2012



Miniature Spy Planes: The Next Generation of Flying Robots

Stephen J. Morris

MLB Company

May 2002

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Figure 1: MLB Bat mini-UAV

Introduction

Affordable airborne access to information has long been desired by militarists, businesses, and individuals, but nearly 100 years after the Wright brothers first flew at Kitty Hawk this dream has not yet come true. If one wants to see what is happening across their neighborhood at a particular moment a full-size airplane, a pilot’s license, and a nearby airfield are required. The barrier to obtaining information from airborne platforms is rooted in the technological difficulties of controlling and navigating aircraft and in remotely sensing information. In the last three decades advancements in digital computers, satellite position sensing, solid-state inertial sensing, and video imaging has made possible the first generation of small affordable robotic aircraft that can provide useful image data while requiring only moderate operator skill. These vehicles are typically less than 6 foot wingspan and 10 pound weight, so they pose minimal safety hazard to the public. By flying at low altitudes (less than 500 feet), they are below the airspace where full-size aircraft operate and are allowed to operate freely within the framework of local city ordinances. The Miniature Unmanned Air Vehicle (i.e. mini-UAV) typically has a quiet engine and is difficult to spot in the air. It can cruise the skies day and night in a wide range of weather conditions while safely gathering information for law enforcement, traffic monitoring, air pollution control, farming, fire spotting, power line inspection, search and rescue, and weather monitoring. In the near future individuals will log on to a web site of a mini-UAV service and request image data from a specific region. This request will prompt teams of mini-UAVs to fly to the appropriate location and automatically gather and send the data to the customer over the internet for a reasonable price. The fact that mini-UAVs are doing the work will be invisible to the customer.

This paper discusses some examples of successful mini-UAV designs, the technical challenges of miniaturizing UAVs, and future applications.

A brief background of UAVs

The military has used unmanned aircraft for intelligence gathering since World War 2. In the Vietnam War drones built by Teledyne Ryan regularly flew reconnaissance missions and gathered valuable photographic data while under autonomous flight control. The Israeli airforce pioneered the use of smaller low-cost drones in military conflict for both reconnaissance and decoy missions. In all of these examples the UAVs were flown in the same airspace as man-carrying aircraft and could only be operated this way because of military necessity.

The FAA has not decided how it will incorporate UAVs into civilian airspace and this has kept their commercial use to a minimum. Mini-UAVs offer a solution to this problem because they can operate safely at altitudes below full-size air traffic and their size, speed, and weight poses a much smaller threat to the public.

Until the introduction of the Global Positioning System in the 1980’s, UAVs needed expensive inertial guidance systems that were often adapted from man-carrying aircraft or long range missiles. This resulted in large and costly aircraft that only the military could operate. Most of the current military UAVs remain large and costly, primarily because they are designed for long-range, high-altitude missions and they carry heavy mil-spec sensor payloads.

Notable mini-UAVs

In the last few years mini-UAVs have become effective surveillance systems. Here are a few examples of notable mini-UAVs.

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Figure 2: AeroVironment Pointer UAV

Pointer

In the late 1980’s AeroVironment Inc. of Simi Valley California developed a small, low cost, remotely piloted drone for the Marines which could be carried in two back packs and be flown with moderate pilot skill. This nine-foot wingspan aircraft was the first true mini-UAV system to be commercially produced. The Pointer carries a forward looking color camera and is powered by an electric motor which provides up to 90 minutes of duration with present Lithium battery technology.

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Figure 3: AeroVironment Black Widow MAV

MAVs

In 1997 the Defense Advanced Research Project Agency funded the development of micro-reconnaissance aircraft defined by a largest dimension of 6 inches. Lockheed-Sanders and AeroVironment have studied these micro air vehicles (or MAVs) and each has developed successful flying examples. The typical MAV mission requires a modest 1/2 hour duration and a range of 1 mile while operating in winds up to 20 mph. AeroVironment’s Black Widow is a 6 inch wingspan aircraft that uses an electric motor for propulsion and weighs 80 grams (ref. 1). The Lockheed-Sanders MicroStar MAV has a 12 inch wingspan, electric propulsion, and is capable of autonomous flight GPS navigation. Both MAVs carry a color camera and use Lithium batteries for propulsion. The Black Widow and MicroStar MAVs have demonstrated the possibility for significant miniaturization in reconnaissance aircraft so long as mission duration and range remain small.

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Figure 4: Insitu Group Aerosonde UAV

Aerosonde

In 1998 a 30 pound 9 foot wingspan aircraft flew 2500 miles in 27 hours to cross the Atlantic Ocean under autonomous control, mimicking the flight made by Charles Lindbergh 51 years earlier. The Aerosonde by Insitu Group of Bingham, Washington, demonstrated that scaling to smaller sizes does not restrict a UAV to reduced flight performance. Using a modified model aircraft engine, composite model airplane wings, and GPS navigation, the Aerosonde crossed the Atlantic while consuming only1.5 gallons of gasoline

[pic] Figure 5: MLB Bat shown folded to 4 ft. size

MLB Bat

The MLB Bat (figures 1 & 5) is designed to be safely operated in populated areas, easily transported by a single person, low cost, and able to deliver high quality image data. The Bat has a 5 foot wingspan, weighs 9 pounds, and folds for transportation. The entire aircraft and ground station easily fits in the trunk of a car and can be ready for flight in 10 minutes. Using a miniature flight control system (figure 7), it flies autonomously between specified waypoints and can take-off and land unaided. Duration is one hour and the Bat can transmit video and flight data over a 2 mile radius. The sensor package is a 3-axis gimbal mount containing two video cameras, each with a different field of view lens that can be switched in flight for a zoomed-in view of desired areas. The gimballed camera is inertially stabilized using the flight control computer and the UAV operator can remotely aim the camera. In the event of a flight control system or structural failure, a separate system automatically deploys a parachute to slow the aircraft to a safe descent speed.

A ground station based on a PC laptop makes the Bat mini-UAV easy to operate. Figure 6 shows a screen snapshot of the display and includes the Bat’s courseline track on a moving map, altitude, speed, system status, and other flight data parameters. The geographic location of the image being viewed by the camera is also shown and recorded so that all images are geo-referenced. The operator specifies flight plans by clicking on the map and inserting new waypoints. Once the aircraft is launched it begins to fly the course defined by the waypoints and the speed and altitude for each course leg.

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Figure 6: MLB ground station screen image

[pic] Figure 7: MLB Miniature flight control hardware

The Bat has been used in wildlife habitat research projects to provide image mosaics of wetland areas and also to deploy a miniature sensor network that used the Bat to transport data back to a remote ground station. Figure 8 shows the Bat dropping these sensors along a roadway during a military demonstration at 29 Palms Marine base in California. The sensors were developed by Professor Kris Pister at the University of California at Berkeley (ref. 4) and are designed to automatically form data networks, detect moving vehicles, and relay this data to the UAV.

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Figure 8: MLB Bat deploying micro-sensors along roadway

Mini-UAV scaling issues and technical challenges

How small can a UAV be? Nature provides some readily observable answers to this question. Small birds are capable of sensing images and they can migrate thousands of miles. Even insects can operate in a several mile radius, so surveillance aircraft could certainly be this small if only engineers could design systems as adaptable and capable as are found in nature. The major technical challenges facing engineers in scaling unmanned aircraft to smaller sizes are discussed below.

Aerodynamics

As wings become smaller the nature of the airflow over them changes because their interaction with air particles scales with size, speed, and air viscosity. The Reynolds Number is a dimensionless parameter that characterizes the flow regime over bodies and it involves the ratio of inertial to viscous forces in a fluid flow. As Reynolds Number decreases, viscous effect dominates and it becomes difficult to generate lift while maintaining low drag. Many birds, insects, and model aircraft operate at low Reynolds Number (approximately 200,000 to 20,000) and still have adequate performance for their purposes. At very low Reynolds Number the best strategy for generating lift with low energy input shifts from smooth shaped airfoils to rougher wing surfaces and unsteady movement through the fluid. Small insects use rapidly changing motion of their wings to generate unsteady vortices in the viscous flow and use them to make lift with minimal energy input.

Initially, researchers believed that Reynolds Number effects would produce the greatest challenge in miniaturizing UAVs, but aircraft like the Black Widow and Aerosonde have demonstrated that a high level of performance can be obtained with proper attention to aerodynamic design. The impact of increased drag due to reduced Reynolds Number on MAV performance was quantified by Morris (ref. 2) and shown to be of small importance when compared to propulsion efficiency and lift generating capability. For mini-UAVs a lift to drag ratio of 20:1 can be achieved if necessary for long range and duration.

Propulsion

As UAVs decrease in size below 20 pounds the choice of efficient propulsion systems decreases dramatically. Modified model aircraft engines that use methanol or gasoline as fuel are popular on mini-UAVs, but they often lack efficiency and reliability. Because fossil fuels have such a high energy density, these engines are still useful for most mini-UAV missions. Efficient internal combustion engines under one horsepower have not been fully researched and there is potential for much progress in this area. Electric power has been used successfully on short range and duration aircraft such as MAVs, but suffers from the low energy density of present battery technologies. Batteries with the highest energy densities (Lithium) often have limited power density which further complicates the design electric powered UAVs.

Flight control and sensing

Birds and insects have evolved complex brains and a high degree of sensory input and musculature. Distributed actuator, sensor, and computation ability enable living creatures that fly to be very agile, efficient, and adaptable. Miniature UAVs are often limited in their flight agility because high quality sensors and actuators with extremely small size and weight have not yet been developed. Scaling to smaller sizes compounds the flight control problem because the dynamics of the aircraft increase in frequency as size decreases in the same way that a smaller pendulum will have a higher natural frequency than a larger one. Therefore, mini-UAVs require higher bandwidth actuators than their larger counterparts. Recent advances in Micro Electrical Mechanical Systems (MEMS) technology has produced microscopic sized sensors (gyroscopes, accelerometers, pressure transducers, etc.) highly suitable for mini-UAVs. If advances continue in the MEMS field an entire flight avionics system will soon be available on a single chip and this will greatly reduce the weight and volume of the flight avionics, which tend to be a higher percentage of the total as UAV size shrinks. MEMS devices can also provide the bandwidth and accuracy needed in flight control sensing and computing, so that mini-UAV flight agility can improve.

Telemetry

The power required to transmit data varies with the square of the distance between transmitter and receiver and is independent of the size of aircraft attempting to send the data. This poses one of the greatest scaling challenges for mini-UAVs: how to send data over great distances without requiring excessive power (and weight). Possible solutions to this problem include the use high gain antennas, reduced data rates, or burst transmission communication schemes. In general, telemetry range is reduced as UAV size decreases unless the vehicle is linked to a communication network. In a recent demonstration the MLB Bat was used to ‘truck’ data from a deployed sensor network back to a remote location. This eliminated the need for either system to have a long-range telemetry link and mimics the solution often seen in nature: store the data and retransmit it when in close range to others.

Data Quality

Most mini-UAVs carry a fixed camera as a primary image sensor and flight test results have shown that with this installation image quality is degraded by aircraft motion caused by maneuvering or gust disturbances. A desired feature for improved imaging is an inertially stabilized servo-controlled gimbal camera mount that adjusts for aircraft motion and allows the operator to position the field of view. Small gimbal camera systems are now being developed for the latest mini-UAVs.

Data quality is strongly linked to the post-processing of the image data collected by UAVs. Raw video images must be adjusted for camera alignment, combined into larger image maps (mosaic), and possibly have features identified and extracted. This data may then be combined with other databases (satellite, digital maps, etc.) to maximize data value. Currently, little commercial software exists to process the vast amount of video data that will be generated by fleets of mini-UAVs. The full potential of UAV fleets will only be realized when the data fusion bottleneck is eliminated.

Complete UAV system

UAV size is further influenced by the ground-based systems needed to operate the aircraft. As an example, consider a UAV so small that its ability to transmit data over great distances is poor. To have an acceptable telemetry range a large directional antenna will be required and the complete system may be bigger than if a larger UAV carrying a more powerful data transmitter was chosen. System size is often more important than UAV size because of transportation and cost factors. Achieving the smallest system size requires that the UAV and its ground support equipment be considered simultaneously in the design process.

Future applications

An example of a near-term commercial application for mini-UAVs is to aid precision agricultural. The amount of water, insecticide, and fertilizer used in agriculture is highly regulated and must be dispensed efficiently. High-value crops such as strawberries or vineyards require almost daily monitoring during specific points in the growing season to insure a high quality and yield per acre. Mini-UAVs can deliver the imagery data necessary to accomplish this and can outperform satellites and light aircraft in efficiency, cost, and data quality. Image data taken in the near-infrared and color spectra can be processed to make vegetation growth index maps that will show where fertilizer, water, or insecticide is needed. In the future, automated farming operations will use the data gathered by mini-UAVs to direct unmanned robotic tractors and harvesters to optimally manage crops.

Current satellite imagery is limited to 10 meter pixel resolution and image availability is affected by cloud cover and satellite trajectories. Flying at low altitudes, mini-UAVs will supplement satellite image databases with high-resolution imagery that can be collected during periods of cloud cover and when desired by the customer.

Mini-UAVs will evolve into ever more capable surveillance platforms that will be economical to operate and will fly safely over populated areas while being almost unnoticeable. Teams of these aircraft will be able to communicate with each other and self-organize to best accomplish data gathering. Mini-UAVs will become an essential part of information gathering systems and will supply near-real time data to customers through the internet. Because they are transportable and low cost, they will also be used in remote locations that are currently too costly to monitor.

References

1) Grasmeyer, J.M., Keennon, M.T., “Development of the Black Widow Micro Air Vehicle”, Proceedings of the 39th AIAA Aerospace Sciences Meeting, Reno, NV, January 2001.

2) Morris, S., “Design and Flight Test Results for Micro-Sized Fixed-Wing and VTOL Aircraft”, Proceedings of the First International Conference for Emergent Technologies of Micro Air Vehicles, Georgia Institute of Technology, Atlanta, Georgia, February 1997.

3) “Shephard’s Unmanned Vehicle Systems Handbook 2001”, The Shephard Press, Bucks, England, 2000.

4) J. M. Kahn, R. H. Katz and K. S. J. Pister, "Mobile Networking for Smart Dust", ACM/IEEE Intl. Conf. on Mobile Computing and Networking (MobiCom 99), Seattle, WA, August 17-19, 1999.

Photo Credits

Figures 1,5,6,7,8 Stephen Morris, MLB Company, Palo Alto, CA

Figures 2,3 Aerovironment Inc., Simi Valley, CA

Figure 4 “Shephard’s Unmanned Vehicle Systems Handbook 2001”, The Shephard Press, Bucks, England, 2000.

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