Defense

Mini UAVs would be more vulnerable to attack and loss due to

their low-altitude missions and could suffer high attrition rates.

Thus, they would have to be inexpensive enough to be expendable

while also capable of flying useful payloads. To meet these requirements, we believe that these UAVs would have to be small and electrically powered and use mass-produced, commercial off-the-shelf

(COTS) technology. Electrically powered motors were chosen for the

mini UAVs discussed herein because of their low acoustic signature,

ease of start, reliability, and relative performance insensitivity to

altitude and temperature. Although the internal combustion engine

produces a higher thrust-to-weight ratio than does the electric

motor, the benefits of electric motors are significant. Some experimentation in this regard has begun through programs such as the

Marine Corps Dragon Eye program.2

Aerodynamic Characteristics

To address the aerodynamic regime of interest, this paper

examines five UAVs. The first three vehicles (Extender, Dragon Eye,

and MITE) were developed by the air vehicles group at the Naval

Research Laboratory.3 The fourth vehicle was developed by the micro

air vehicle design team at the University of Notre Dame.4 The fifth

vehicle, Black Widow, was developed by AeroVironment, Inc.5 The

physical characteristics of theses vehicles are summarized in table 1.

All are electrically powered except for the Notre Dame one, which is

included because detailed lift and drag data were available providing

Overview

The successes of the unmanned aerial vehicle (UAV) in

Afghanistan most likely will accelerate the introduction of UAVs

into the military force structure. What emerged in Afghanistan

was a tiered observation-detection-targeting system consisting of

spacecraft, the Global Hawk UAV, the Predator UAV, and often a

spotter on the ground. However, in complex terrain, as in

Afghanistan, and in urban situations, airborne assets may be

needed much closer to the ground. A low-altitude tier of UAVs—

mini UAVs—could serve such a purpose.

This paper provides a basic understanding of the

aerodynamic scaling of mini UAVs and a sense of how their

capabilities could be matched with specific missions. Mini UAVs

have substantial limitations, but the low radar cross section,

low infrared signature, low acoustic signature, and birdlike

appearance of these vehicles, combined with the remarkable

capabilities of miniaturized payloads, make them contenders

for certain missions and potential valuable tactical assets.

A New Class of Aircraft

A class of low-altitude unmanned aerial vehicles (UAVs) is

currently under development. Several desirable features in their

composition warrant discussion. This class of fixed wing vehicles is

larger than the micro UAVs that have received publicity in the past

several years,1 but substantially smaller than the Predator class

UAV. This paper considers vehicles, henceforth called mini UAVs,

that have wingspans ranging between 6 inches and 10 feet and that

fly in the 20- to 50-mile-per-hour (mph) range (the aerodynamic

regime typical of model airplanes and birds). While this regime is

not glamorous, significant developments have occurred in recent

years. The new class of aircraft is more capable and rugged than

typical model airplanes but less capable than birds, especially in

terms of control systems. These developments, plus the arrival of a

variety of low-cost but capable payload technologies, make this a

subject worthy of consideration.

The Emergence of Mini UAVs for

Military Applications

by Timothy Coffey and John A. Montgomery

A publication of the

Center for Technology and National Security Policy

National Defense University

DECEMBER 2002

Number 22

Horizons

December 2002 Defense Horizons 1

Table 1. General Characteristics of UAVs

Maximum 

Unmanned Wing Span Aspect Weight of Gross 

Aerial Vehicle (inches) Ratio Structure Weight

Extender 122 11.35 5.76 lbs. 31.5 lbs.

Dragon Eye 45 3.75 1.5 lbs. 4.5 lbs.

MITE 18.5 1.85 4 oz. 10 oz.

Notre Dame vehicle 10 1 0.53 oz. 3.7 oz.

Black Widow 6 1 0.34 oz. 2 oz.
Other

13% Structure

18%

Propulsion

and Power

13%

Payload

56%

Extender

Other

9%

Structure

30%

Propulsion

and Power

35%

Payload

26%

Dragon Eye

Other

 6%

Structure

38%

Propulsion

and Power

38%

Payload

18%

MITE

Other

9%

Structure

17%

Propulsion

and Power

62%

Payload

12%

Black Widow

Timothy Coffey is a senior research scientist at the University of Maryland.

He currently holds the Edison Chair for Technology in the Center for

Technology and National Security Policy at the National Defense University.

John A. Montgomery is Director of Research, U.S. Naval Research Laboratory.

scaling to aspect ratio one. These data were not available for the

aspect-ratio-one Black Widow vehicle.

For the purposes of this paper, we assume that the three design

teams that developed the vehicles in table 1 have produced an optimized design for each aircraft. Our intention is to investigate the scaling of these aircraft as we vary the aircraft size and aspect ratio and

determine how this relates to the payloads that this class of UAVs can

carry for a specific mission and duration. As can be seen from table 1,

the vehicles cover an order of magnitude range in wingspan and in

aspect ratio and cover two orders of magnitude in vehicle weight. An

immediate practical conclusion can be drawn from table 1 relating to

the ability of these vehicles to fly autonomously. A competent, commercially available autopilot for low-flying vehicles will weigh from 4

to 6 ounces.6 MITE and smaller size vehicles with their low weight

restrictions will be hard pressed to undertake autonomous flight with

present technology. Dragon Eye-class and larger mini UAVs have

ample capacity to carry an autopilot, and we assume that they do in

the remainder of this article.

Payload carrying capacity is key to assessing the mission capabilities of these vehicles. Figure 1 provides an estimate of the weight

budgets for each electrically powered vehicle when sufficient battery

power is provided for a 20-minute flight. Twenty minutes is selected

because it begins to tax the smaller vehicles’ ability to carry any sort

of meaningful payload. Even for such short flight times, it is clear

from figure 1 that power and propulsion begin to dominate the

weight budget for the smaller vehicles.

Figure 1 shows that as one progresses from Extender to Black

Widow, the available payload weight reduces almost 5 times more

rapidly than the vehicle weight. For the smaller UAVs, the gross

available payload weight of less than 7 grams severely limits the

availability of useful sensors. It is also clear from figure 1 that one

can trade battery weight for payload weight and vice versa. The 

maximum weight available for payload (Wm) is achieved when battery weight is reduced to zero and the maximum powered flight time

is achieved when payload weight (Wp) is zero, with the payload being

replaced totally by batteries. Assuming a linear variation between

these two limits and using the data provided by the Naval Research

Laboratory (NRL), we constructed figure 2, which estimates powered mission duration versus payload weight for each NRL vehicle.

Data was not available for Black Widow, but it is expected to follow

a curve similar to MITE. The Notre Dame vehicle was not electrically

powered and therefore is not included in figure 2. Figure 2 is used in

the following sections to examine various payloads that might fly on

these or similar vehicles.

The term powered flight is used deliberately above. It will

become clear in the following sections, as it does from figure 1, that

powered flight duration will be very limited for the small, aspectratio-one vehicles. Increasing the reach of these mini UAVs will be

necessary, and there are very desirable techniques to accomplish

this. For example, these vehicles can undertake unpowered flight by

gliding, providing an opportunity to increase range. From a 5-mile

release altitude, the Extender has the capability for a 77-mile glide;

the Dragon Eye, 37 miles; MITE, 34 miles; and Black Widow, 25

miles. Of course, technical issues connected with the release of the

mini UAVs would need to be addressed.

These glide distances assume zero windspeed. Depending on

windspeed and direction, the range could be reduced or extended.

For some missions, the payload might be operated during the glide

phase. Climb and glide strategies could be used for some missions

due to ease of motor restart. Advanced strategies could exploit soaring for much longer duration in mountainous terrain, analogous to

the seagull soaring stationary relative to a bridge updraft. One could

speculate that under the right conditions, the aircraft might soar

and windmill its props to generate direct current (DC) power from

the DC motors. The mini UAVs could also be glided to rendezvous

with users on the ground who would electronically capture and program them in flight for the local mission need.

The glide distance, therefore, is a definite factor to consider

in the creative use of the mini UAVs. A “mother ship” might release

the UAVs into unpowered flight at a safe altitude and distance from

2 Defense Horizons December 2002

Figure 1. Weight Budgets for 20-Minute Flight
1.0

0.8

0.6

0.4

0.2

0

0 1 2 3 4 5 

Mission Duration (hours)

Relative Payload (Wp/W

m)

Extender (Wm = 18.7 lbs.)

Dragon Eye (Wm = 1.8 lbs.)

MITE (Wm = 0.15 lb.)

an area of interest that is reachable by gliding. Upon gliding to the

mission area, the UAVs could then power up and undertake the

desired mission. This scenario would exploit the ease of starting an

electrically powered UAV as well as the small cross sections and low

acoustic signatures. Depending on the mission, the UAVs might

remain in contact with the mother ship. A variation on the mother

ship approach would be to incorporate the mini UAV into a round

that is fired from a gun or other launcher. When the round nears

the end of its range, the UAV aerodynamically deploys and conducts

its mission.

These scenarios assume that retrieval of the mini UAV will not

be necessary. This requires an acquisition strategy that treats mini

UAVs as expendable, much like the sonobuoys deployed by Navy P–3

aircraft. The use of COTS components would keep the price down

and reduce the prospect of releasing sensitive technologies.

Since cost will be an issue regarding the employment of mini

UAVs, we provide the following estimates for the cost to purchase

the NRL vehicles in lots of 1,000 in a ready-to-fly state, but without

the payloads: the Extender, $50,000; Dragon Eye, $10,000; and

MITE, $700. The Extender and Dragon Eye numbers incorporate

military frequency transceivers into the UAVs. The MITE class UAVs

cannot carry the heavier, more capable military frequency transceivers and are priced assuming the incorporation of much lower

cost COTS transceivers.

Proximity Effect and Radar Jamming

The general theme of this paper is that sensor technology, materials technology, power technology, and autopilot technology, including miniature high-accuracy global positioning system (GPS) navigation, have progressed to the point where rather small platforms can

carry capable but inexpensive payloads and possibly accomplish

meaningful missions. This offers the opportunity to place payloads

close to targets and to benefit from this proximity. As a simple illustration, we consider the case of main-beam barrage jamming. In particular, we examine a recently developed jammer,7 which produces a

50-milliwatt (mW) jamming signal with a 250-megahertz (MHz)

bandwidth for 4 hours operating in S band and with a total weight of

20 grams. This jammer could be flown in any of the UAVs discussed

above in powered flight mode, except perhaps for the Black Widow. If

the mini UAV could place this jammer within 500 feet of a nominal 1

megawatt S-band radar, with an antenna gain of 1,000 and an instantaneous bandwidth of 1 MHz, then a 1-square-meter radar cross section aircraft could be screened to within a range of 10 kilometers

(km) from the radar (this assumes a jammer antenna gain of one). If

this were accomplished with a jammer standing off at 200 miles, then

it would require an effective radiated power of 207 kilowatts, as compared with the 50 mW for the UAV-placed jammer.

The standoff jammer, of course, has some survivability advantage due to the standoff distance (however, new long-range missiles

designed to attack such jammers may jeopardize this advantage).

The price to be paid for this advantage is the need for large jammer

power and a large aircraft. Additionally, high processing power is

needed to handle the millions of pulses per second seen; great

dynamic range is needed to handle weak distant sources and strong

close sources; and high angular resolution is required to separate

proximate emitters at great distance. Inadequate isolation between

transmit and receive antennae means one must shut off the jammer

briefly to detect signals. Modern radars can detect aircraft during this

interval. Much of this is avoided for proximate jammers. If it were

possible to place the jammer close to the target radar, then the power

requirements would drop substantially. The advent of capable mini

UAVs and technology opens possibilities in this regard.

The 50-mW case, of course, would be viable only if the jammer

could be covertly placed 500 feet from the radar and turned on at the

beginning of the attack. If one of the mini UAVs were able to accomplish this, then there could be a great cost advantage, since the small

jammer package would be inexpensive enough ($50–$100) to be

disposable. Indeed, if the covert placement problem were solved,

placing a ring of small jammers around the target radar to mask a

number of approach directions might be considered.

One way to accomplish this might be to have a Dragon Eye class

vehicle carry approximately 10 of the miniature jammers discussed

above. A 5-mile-high release would allow a 37-mile glide. Figure 2

shows that the UAV would have about 30 minutes of powered flight,

allowing about 20 miles of flight. If the release were made in the

dark, the UAV might be able to approach the radar at low altitude

using its GPS navigation, circle the radar in a 500- to 1,000-foot

radius, and precisely drop the miniature jammers, which could be

either timed to activate or command-activated. After the drop, the

UAV would fly some predetermined path until the power expired. If

ground forces were within a few miles of the target radar, the Dragon

Eye class UAV could be ground launched, fly the mission, and return

to the launch point—obviating the need for a mother ship.

The case discussed above is extremely elementary, but it does

illustrate that proximity, combined with inexpensive payloads, could

be effective if the element of surprise can be maintained. The intrinsically low radar, infrared, and acoustic signatures of the mini UAVs

may have some merit in this regard, and even modest attention to

signature management might reduce these signatures below

detectable levels at any range. Of course, the actual jamming scenarios would be far more sophisticated than that described above.

December 2002 Defense Horizons 3

Figure 2. Payload vs Mission Duration
On the other hand, the general disposable character and proximity

effect would be maintained. Also, if the radar were on a school, hospital, or mosque, hard-kill options might well be unacceptable, making the mini jammer approach even more attractive.

Optical and Infrared Payloads

In the previous section, substantial reductions in jamming

power were achieved by placing the jammer in proximity to the radar

being jammed. A similar situation develops in electro-optics. A

nadir-looking camera flying 0.5-centimeter (cm) optics at an altitude of 100 meters (m) has the same spatial resolution as a camera

with 30-cm optics flying at 6,000 m altitude. The obvious question is

whether there are cameras that could fit into the UAVs discussed

above. A search of the literature demonstrates that a wide variety of miniature cameras are available. There are miniature color

video cameras on the market today in which

the camera weighs about 2 grams, consumes

about 150 milliwatts (mW) of power, and

costs about $200, including the lens.

There are also many S-band video

transmitters that radiate about 100 mW of

power, consume about 600 mW of power,

weigh about 3 grams, and cost about $400. These transmitters have

ranges for video detection of about 1 to 3 miles, depending on the

receiver being used. Hence, at the low end, a color video camera with

a 1- to 3-mile transmitter range will weigh about 5 grams without the

batteries to power the camera. In order to estimate the payload

weight, we need an estimate of the battery weight required to power

the camera. For purposes of illustration, we assume the payload is

powered by a standard 9-volt (V) lithium battery that weighs about

35 grams and can deliver 1,200 milliampere hours of current. We further assume that the weight of the battery required to accomplish a

desired mission scales linearly with the required milliampere hours.

For the case of the MITE vehicle, the maximum mission duration is,

according to figure 2, 0.67 hours. Assuming a 9 V power supply, this

payload would draw 83 milliamperes, leading to a requirement of

55.8 milliampere hours. From the assumption of linear scaling of battery weight, we estimate that this payload requires a battery weight

of 1.6 grams or 0.06 ounces, leading to a total payload weight of 0.23

ounces. Since the maximum payload weight for MITE is 2.4 ounces,

the ratio of payload to maximum payload becomes 0.09. In figure 2,

we find that MITE could fly this payload for 37 minutes. Even if our

payload weight estimate were underestimated by a factor of 2, the

mission duration would still last 33 minutes. The Black Widow aircraft flew a similar payload for just over 30 minutes. Extender and

the Dragon Eye UAVs could fly this particular payload for their maximum flight times.

A more capable video camera, which typically weighs 70 grams

and requires 2.7 watts, clearly is not applicable to a MITE or Black

Widow class UAV; however, it could fly on a Dragon Eye class vehicle.

The Dragon Eye has flown with two such cameras mounted in it, one

looking down, the other looking to the side. Repeating the calculation done above for these two 70-gram cameras flying on Dragon Eye,

the powered flight time is 58 minutes. The images obtained from this

class of camera on a Dragon Eye class UAV are of reconnaissancelevel quality.

Considerable advances in the area of uncooled infrared cameras

warrant notice. Indigo Systems recently announced the development

of the UL3 infrared camera.8 This camera, employing a 160-by-120-

micro bolometer detector array, achieved a sensitivity of better than

80 MK using an F1.6 optic. The sensor weighs less than 200 grams,

including the optics, requires a volume of 3 cubic inches, and also

requires approximately 1 watt of power. It outputs analog video as well

as 14-bit digital. The weight of this camera clearly precludes its use on

MITE or Black Widow class UAVs, but Dragon Eye class aircraft should

be able to fly the camera. Using the linear battery scaling, we estimate

a required battery weight of 7 grams, leading

to a total payload weight of 207 grams or 0.457

pounds. This produces a payload weight to

maximum payload ratio of 0.253 and leads to

a 54-minute powered flight time for the

Dragon Eye vehicle.

The previously discussed small video

camera could be added to this payload without substantially affecting the flight duration. Additionally, the two 70-gram cameras

discussed above, if added to the UL3 payload, could fly on a Dragon

Eye class vehicle for 39 minutes of powered flight. This combined

electro-optic/infrared (EO/IR) payload could also be placed on

Extender, which would have a nearly 5-hour powered flight duration.

Large production runs likely will bring the cost of the UL3 camera

down to several hundred dollars, making it expendable (assuming

that there are no technology loss issues).

The examples above illustrate that a very capable EO/IR payload could be placed on a Dragon Eye class or larger mini UAV. At the

Dragon Eye class, the cameras must be hard mounted to the airframe since no weight budget is available for gimbals and their control system. This means that the motion of the UAV will limit the

quality of the image and the ability to do image processing. It should

be noted, however, that the vehicles are inherently stable and have

very low vibration levels due to electric propulsion. Although the

angular rates at which they move about their axes are greater than

those of larger aircraft, their distances from the targets are so much

shorter that their images are surprisingly clear.

The imagery from this UAV class should be useful for reconnaissance. Its value for targeting depends on a number of factors.

For example, if targeting quality data requires location to within

30 m, and the vehicle overflies the target vertically from 100 m and

knows its location to about 7 m (available with undithered GPS) as

well as its altitude, its proximity might well give targeting quality

data. An Extender class vehicle should certainly be able to provide

targeting quality data.

Chemical and Biological Surveillance

A topic of considerable concern today is a means for monitoring

the release of chemical and/or biological agents. It is logical to ask if

any of the mini UAVs might have a role here. As in the previous examples, small, commercially available, chemical agent detectors are now

4 Defense Horizons December 2002

a very capable electrooptic/infrared payload

could be placed on a

Dragon Eye class or

larger mini UAV
entering service. Some of these may be compatible with mini UAV use.

Micro Sensor Systems, for example, has recently released a hand-held

detector, HAZMATCAD, that detects and classifies chemical warfare

agents, as well as industrial chemicals.9 It uses both surface acoustic

wave sensor arrays and electrochemical cells, and it also employs

signal-processing techniques to provide low susceptibilities to false

alarms. It detects the nerve and blister agents VX, GA, GD, GF, HD,

HN3; the blood agents hydrogen cyanide and cyanogen chloride; and

the choking agent Phosgene with a “fast response” mode of 20 seconds

and a “sensitive” mode of 120 seconds. It has a data logging capability

of 8 hours, recording alarm, level, time, and

date. The basic instrument without batteries

and case weighs about 7 ounces and costs

about $8,000. While it is not suitable for

MITE or Black Widow class air vehicles, it

should fit in Dragon Eye.

For a Dragon Eye mission, the extra

weight of the batteries to power this sensor

can be ignored. Therefore, the payload would

weigh 7 ounces, which is the same as the UL3

camera payload discussed earlier. Hence,

Dragon Eye could fly the HAZMATCAD

chemical agent sensor for 54 minutes or

about 35 miles. This means it could sample a

suspicious cloud 10 miles away and return with results or radio those

results back to a receiving station. If a 30-minute mission could be

settled upon, then, from figure 2, it is clear that about another halfpound of payload could be added. This could be a biological collection payload that collects samples at distances of about 5 miles and

returns them to a laboratory for biological agent determination testing. The addition of the small video camera discussed above to this

payload would have little impact on the mission duration. The resulting payload provides real-time in situ testing for chemical agents,

real-time visible imagery, and remotely collected biological samples

for laboratory testing. An Extender class UAV could fly this payload

for several hours and cover more than 100 miles.

As far as we know, there is no proven technology that can do an

in situ determination of biological agents with a half-pound payload

and report results via a communication link. There are, of course,

antibody-based coupons that can detect biological agents and that

could be read upon the return of the UAV. Devices capable of in situ

detection and reporting have been built, but they are too large to fit

on a Dragon Eye class vehicle. One such device is the RAPTOR, built

by Research International in cooperation with the Naval Research

Laboratory.10 This device, when stripped of its casing and provided

with batteries suitable for an Extender mission, weighs about 8

pounds. This leads to a payload ratio of 0.42, which, again from figure 2, shows that Extender could accomplish approximately a 3-hour

mission. The agents that this device can detect include SEB, cholera

toxin, ricin, plague, Bacillus anthracis, botulinum toxin, Brucella

abortus, and F. tularensis.

We can imagine employing the mother ship approach in which

Dragon Eye or Extender class vehicles with suitable chemical and

biological detection sensors are transported large distances by

another aircraft and then released at an altitude safe for the mother

ship. The mini UAVs would then glide/fly to the area of interest, perform their measurement mission, and either radio the findings back

to the mother ship or collect samples and fly them to a predetermined location. The Defense Threat Reduction Agency (DTRA) is

operationally demonstrating the chemical portion of this concept as

part of the Second Counterproliferation Counterforce Advanced

Concept Technology Demonstration (CP2 ACTD).

The ACTD provides kits for the Air Force MQ–1 Predator UAV

to employ two NRL Flight Inserted Detection Expendables for

Reconnaissance (FINDER) mini UAVs (103

inches in wing span) along with a passive

standoff sensor as part of the chemical combat assessment system. The two FINDERs

are carried on the outboard wing hardpoints

of a Predator UAV. The FINDER sensor payload weighs about 14 pounds and consists of

dual ion mobility spectrometers and a sample collection subsystem. The mission is to

detect, identify, track, and characterize

chemical plumes that might result from

strikes on adversary chemical weapon production and storage sites. The FINDER mini

UAVs will be released from the Predator, fly

into the plumes, and report their findings in near-real-time through

the Predator air vehicle to a ground control station for distribution

to the warfighter. The collected samples can be retrieved for laboratory testing and further exploitation and reporting post-strike. As a

follow on, DTRA has formulated an ACTD to provide the biological

solution called the Biological Combat Assessment System.

Signals Collection

The collection and interpretation of electronic signals are important aspects of modern military operations. Substantial resources, as

well as very capable and elegant technologies, are routinely deployed

in this quest. It is logical to ask whether the mini UAVs might have a

contribution to make in this area. For obvious and valid reasons, much

of what is done in this area is highly classified and would lie outside

the scope of this article. Our objective is to stimulate a general discussion on the potential use of the mini UAVs rather than to design a

solution to a particular problem. Therefore, to address the potential

use of mini UAVs in the signal collection area, we illustrate with an elementary discussion of cellular telephone signal collection. Perhaps

the simplest way to intercept cell phone signals is with a cell phone. A

cell phone, after all, is a scanner, receiver, and transmitter. Each

phone is identified by an electronic serial number (ESN) and a mobile

identification number (MIN).

To use a cell phone, the ESN and MIN must be passed between

the phone and a cell tower. When the information being passed is not

encrypted, which is often the case, collecting the ESN and MIN is a

straightforward process. Once these numbers are known, the cell

phone collector can be instructed to report when a particular phone

becomes active and then record the conversation associated with

that phone. If the information is encrypted and the encryption

algorithm has been obtained, then the same game can be played. If

December 2002 Defense Horizons 5

the FINDER mini UAVs

will be released from

the Predator, fly into

the plumes, and

report their findings

in near-real-time

through the Predator
all else fails, one could simply record all cell traffic in a particular

area and bring it back for processing. Today, flash memory cards are

available that hold about 8 hours of cell phone conversation.

Since cell phone radio frequency (RF) sections without batteries and housing weigh only 15 to 20 grams, one could imagine a cell

phone suitably reprogrammed and modified for cell phone signal collection flying on a small UAV. If the UAV flies at an altitude of 1,000

feet, then the area to which the vehicle has a line of sight is approximately 1,500 square miles. A collection made at 20,000 feet would

cover a region about 30,000 square miles, while a collection made at

60,000 feet would cover 90,000 square miles. Although the higher

altitude has an advantage, it also requires greater on-board processing power and dynamic range because of

the increased traffic collected against.

Additionally, more sophisticated collectors are required because of the reduced

signal strength.

For certain applications in which

there is interest in collecting cell phone

signals in a small area, the mini UAVs

might be useful. An appropriately modified cell phone could serve as the collector, presumably on a mission in which long duration is important. One

would likely use an Extender class vehicle where several hours of collection could be made. Of course, if the UAV landed and then collected, it might do so for quite a while over a more limited region.

Also, the UAV might call home literally via the targeted cellular network to deliver data or receive commands.

Other Considerations

We have seen that video transmission distances from small

UAVs are typically a few miles. This requires a power investment of

about one-half watt, and the transmission is omnidirectional. At the

smaller end of the mini UAVs, the omnidirectional transmission is

required because the vehicle is unable to house and point a directional antenna. Therefore, for these vehicles, the power required to

achieve larger ranges will increase with the square of the distance.

Achieving an order of magnitude increase in range will require two

orders of magnitude increase in power (that is, 50 watts). This may

be possible for the Extender class vehicles, but it is not available for

the Dragon Eye class or smaller vehicles. At the larger end of the

mini UAVs, the payload carrying capacity is sufficient to allow directional antennae, and enough power may be available to reach long

distances and communication satellites. Under some circumstances,

the smaller mini UAVs may be able to communicate to a mother ship,

which would act as a communication relay. This is how the FINDER

UAV communicates via Predator, which launches it. In other circumstances, it may be possible to communicate with other mini UAVs

that could act as communication relays, but this option will not be

viable in most applications envisioned here. This communication

limitation is a serious restriction on the missions that the smaller

mini UAVs can undertake.

The omnidirectional transmission is also a problem with the

covert applications of these vehicles. Some attempts have been

made to remove the transmission problem from the small UAV by

placing the transmitters on the ground. One such approach has been

discussed by Gilbreath et al., using the concept of the modulated

retro-reflector.11 The basic idea is to place a semiconductor optical

switch based on multiple quantum well (MQW) technology in front

of a retro-reflector that reflects an incoming laser beam back to its

origin. The MQW has the property of blocking the transmission of the

incident light when it is in its quiescent state. When a moderate voltage is applied to the MQW, the absorbance shifts, and light is transmitted to the retro-reflector, where it is sent back to its original

direction. This allows a signal to be produced that can be encoded in

an on-and-off keying format. In other words, an incoming laser beam

can be encoded with information gathered by the UAV and sent back to its

source where it is collected and decoded.

Each retro-reflector weighs a few

grams (plus housing), has a field of view

of about 20 degrees, and requires 15 to 50

milliwatts depending on the size of the

multiple quantum well modulator. Five of

these MQW devices properly arrayed give

a field of view of about 60 degrees, weigh a

few tens of grams (plus housing), and have a power requirement of

85 to 250 milliwatts. No gimbaled telescope or laser need be flown on

the UAV. This burden is transferred to the ground or another platform

where power and weight may not be such a problem. A 5-watt, 1,550-

nanometer eye-safe laser will provide ranges of about 10 km in clear

air and about 5 km in light rain. This system has been shown to

achieve 10 megabit-per-second transfer rates.

In addition to potentially relieving the communications power

problem for the smaller UAVs, a laser could provide for encryption

and low observability of the communication signal. This approach,

while promising, has a fundamental limitation in that the atmosphere, under the worst conditions in which one would likely fly a

mini UAV, will limit the range to about 10 miles regardless of the

laser power. Also, at the time this article was written, the data compression and encoding electronics weighed about 1 pound and

required about 10 watts of power. This would limit this technique to

an Extender class vehicle, which already has sufficient payload

capacity to carry more conventional communications technology. If

progress can be made to reduce the size and power requirements of

the compression and encoding electronics, then the modulated

retro-reflector could be useful for providing a stealthy communications link for the small mini UAVs to ranges of 10 miles or more. One

might adapt the laser designator on, for example, Predator to perform this function as an alternative mode for some applications. The

UAV could report its location via a low data rate channel to allow the

required pointing by the Predator laser.

It is clear that mini UAVs with wingspans smaller than a few

feet have very limited capability and flight duration. This makes it

difficult to find meaningful missions. One application that may be

worth consideration is that of leave-behind capability. Often, a

military aircraft will spot something on the ground that it does not

have time to investigate. The aircraft may also be painted by an electromagnetic field, such as a radar pulse, whose origin is not obvious

but may be of concern. For example, through the use of camouflage

6 Defense Horizons December 2002

communication

limitation is a serious

restriction on the

missions that the smaller

mini UAVs can undertake
and thermal signal suppression techniques, a prepared adversary

can make it difficult for a high-flying imager to detect his presence.

A small leave-behind mini UAV may be able to detect activity, which

begins after the primary surveillance asset leaves the scene. Under

these circumstances, it might be useful to be able to eject a small

mini UAV to glide and fly to investigate the area of interest. The mini

UAV payload may consist of a small GPS sensor, a small camera, or an

electronic warfare support measures sensor, or a signal intelligence

sensor and some limited communications capability, through which

it intermittently transmits its location and a short description of

what it has found. Low-power tracking technology such as that

developed by Skybitz12 for reporting the position of transportation

containers may be useful here.

The most significant limitation on the mini UAVs is power supply. As one approaches the small end of the mini UAVs, power and

propulsion become the dominant component of the weight budget.

Improvements in the energy storage capacity of batteries would

translate directly into increased payload or increased mission. Suggestions have been made to move from batteries to solid oxide fuel

cells in which the energy density is at least twice that of today’s batteries. One problem with this approach is that the solid oxide fuel

cells operate at temperatures between 600 and 1,000 degrees centigrade. Isolating these fuel cells from the structure will undoubtedly

introduce weight in an already weight-constrained environment. The

fuel cells will also introduce a thermal signature not present with

batteries. Currently, it is not clear that science or technology has a

solution for the power problem. However, investment in military specific technologies that might not be commercially viable might have

very high payoff in terms of mission capability. The need for a great

advance here is even more urgent for the micro UAVs.

Additional Potential Uses

As examples of the potential uses of mini UAVs, this article discussed radar jamming, EO/IR imaging, chemical and biological

detection, and cellular telephone signals collection. There are many

more applications to which these vehicles can be put. Tables 2 and 3

December 2002 Defense Horizons 7

Table 2. Potential Sensor Payload for Mini UAVs (read columns vertically)

Electro-optic/Infrared Acoustic Passive Relays/ Chemical/Biological/

Sensors Sensors RF/mm Wave Radar Responders Radiological Others

Thermal ESM Search/Track Communications Chemical

Long-wave Imagers FMCW detection Microwave Repeater Collectors Gravitational

Short-wave Range finders mm Wave receivers mm Wave Transponder Analyzers Meteorological

Sky-shine Scanners Superhetrodyne Impulse Translator Biological Magnetic

receivers

Firing locations Motion Channelized Homing Satellite Collectors Electric

receivers

Electro-optic Shock wave Unintentional Navigation Navigation Analyzers

radiation

Intensifiers Firing locations PCS receivers FMCW GPS surrogate Radiological

Low light TV Hand-held radio Imaging Beacons Collectors

detectors

Color TV Radiometric SAR Transponders Analyzers

Black and white TV mm Wave imager High resolution Tracking beacons/

Tags

Laser range finders Passive geolocation Through wall Markers

Laser imagers Bi-static GPS tags

Laser velocimeters Cell phone towers Spectral

Flow rate sensors TV/radio

Radio illuminators

Special illuminators

Table 3. Countermeasures Payloads for Mini UAVs

Radar Communication EO/IR Decoys and Navigational

Jammers Jammers Jammers Deception Denial

Smart PCS jammers Detection EO/IR decoys GPS

jammers deception denial

Coherent Satellite Imager Radar decoys GLONASS

jammers receiver disruption

jammers

Preemptive Hand-held Trackbreak False target LORAN

jammers radio generators

jammers

Networked Denial of Coherent Networked Beacons

jammers service

False target Relay Incoherent Clutter 

generators jammers generators

TV/radio LED based
summarize where we believe some of these applications will be

found in the area of sensors and countermeasures.

Each of the applications outlined in tables 2 and 3 requires a

detailed tradeoff analysis among desired capabilities, class of mini

UAV, and available technologies in order to lead to an optimal mission. Because of the relatively low cost of pursuing the mini UAV

approach to the missions envisioned in tables 2 and 3, a “build a little, test a little” strategy would be quite viable.

The analysis and discussion in this paper is intended to provide

the reader some rough sense of the scaling of the mini UAVs, the

technologies that might be compatible with them, and some of the

problems confronted by mini UAVs. Because of the serious constraints confronting the mini UAV, a very detailed and painstaking

design is required that consistently makes the necessary tradeoff to

construct a viable mini UAV. It should be clear that within the constraints of current technology, there are many missions that are simply not credible for mini UAVs. This is especially true for the smaller

of the mini UAVs. However, the simple arguments and scaling presented here indicate that a carefully designed mini UAV built around

carefully selected technology can conduct some important missions.

Conclusions

This paper has attempted to provide insights into what role

mini UAVs might have in military operations. Major findings include

the following:

■ The power of proximity to the area of interest tremendously

enhances the capability of mini payloads (for example, higher ratio of

received power level of a jamming signal and the desired signal, lower

receive sensitivity, fewer signals to process, lower required resolution for the

same feature size).

■ The ability to arrive at lower cost by drawing on commercial production quantities allows for expendability.

■ The potential exists for extreme stealthiness and covert advance

preparation of the battlespace.

■ GPS allows precise autonomous navigation and position reporting

for the mini UAVs, which are critical to the military application of these

vehicles.

■ Currently available technology offers useful surveillance, reconnaissance, and other missions for mini UAVs with wingspans of about 4 feet.

Capability increases rapidly for wingspans greater than 4 feet.

■ For mini UAVs with wingspans less than 2 feet, currently available

technology appears to offer very few missions. However, there may be a limited leave-behind mission.

■ Power technology appears to be the biggest problem inhibiting the

mission capabilities of the mini UAVs. It is not clear that anything on the

horizon will resolve this problem. A high-impact opportunity certainly exists

for the investment of science and technology funds.

■ Communications from the mini UAVs is a serious mission inhibitor,

especially so for the smaller mini UAVs.

■ The employment of mini UAVs from a mother ship shows promise in

enhancing the capabilities of the mother ship and improving the communication prospects for the mini UAVs.

The aerodynamic regime of the mini UAVs is that occupied by

birds. Birds have prospered for over 50 million years working out

strategies for both short- and long-distance flights. While we have

not discussed control systems here, it should be clear that by millions of years ago, birds had evolved control systems that are far

more sophisticated and capable than those we have available for the

mini UAVs. Perhaps the rapid advances in computer power, information storage technology, and artificial intelligence will provide an

opportunity to narrow this gap. At the present time, birds have far

more sophisticated control of their aerodynamic structures than do

mini UAVs. They change the wingspan and aspect ratio at will and

vary the location of center of gravity relative to the center of lift by

shifting wing positions. Perhaps advances in smart materials will

hold promise in this regard for future mini UAVs. Advances in

biotechnology may also have a role to play. More than 100 years ago,

modern aerodynamics emerged from studies of birds. It may now be

time to return to the study of birds if we are to make the most of the

potential offered by the mini UAVs.

Notes1 See J.M. McMichael and M.S. Frances, “Micro Air Vehicles—Toward a New

Dimension in Flight,” accessed at <www.darpa.mil/tto/mav/mav_auvsi.html>; and M.

Hewish, “A Bird in the Hand,” Jane’s International Defense Review (October 1999),

22–29.2 “Sight for Corps Eyes,” Popular Science 46 (December 2001); and “War in the

Sky,” Popular Science 46 (December 2001), 71–72. 3 Private correspondence from the Tactical Electronic Warfare Division, Naval

Research Laboratory, January 2002. 4 G. Torres and T.J. Mueller, “Micro Aerial Vehicle Development: Design, Components, Fabrication, and Flight Testing,” accessed at <www.nd.edu/auvsi>. 5 J.M. Grassmeyer and M.T. Keennon, “Development of the Black Widow Micro

Air Vehicle,” AIAA–2001–0127 (1–9). 6 See <http://www.micropilot.com>.

7 Private communication from the Electronic Science and Technology Division,

Naval Research Laboratory, February 2002. 8 See <http://www.indigosystems.com>.

9 See <http://www.raeco.com>.

10 See <http://www.resrchintl.com>.

11 G.C. Gilbreath et al., “Large-Aperture Multiple Quantum Well Modulating

Retro-Reflector for Free-Space Optical Data Transfer on Unmanned Aerial Vehicles,”

Optical Engineering 40, no. 7 (July 2001), 1348–1356. 12 See <http://www.skybitz.com>.

The authors gratefully acknowledge helpful discussions with and valuable

input from Wayne Evrett, Loyd Feldman, Richard Foch, Charmaine Gilbreath,

Fritz Kub, Joel Schnur, Warren Schultz, and William Rabinovich.

8 Defense Horizons December 2002

Defense Horizons is published by the Center for Technology and National Security Policy through the

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Defense Horizons and other National Defense University publications are available online at

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The opinions, conclusions, and recommendations expressed or implied within are those of the

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department or agency of the Federal Government.

Center for Technology and National Security Policy

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Director