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Evolution or Revolution? Rise of UAVs
VOLUME 25, NUMBER 3, FALL 2006
Evolution or Revolution? Rise of UAVs
JEFFREY M. SULLIVAN
Kettering Bug (WWI). Courtesy: U.S. AIR FORCE MUSEUM.
Less than fifteen years after Orville Wright took flight in December of 1903, the Army Signal Corps flew its first pilotless aircraft: the Kettering Bug flying bomb. Since unmanned aerial vehicles (UAVs) are often included in the ongoing revolution in military affairs, they are seen as a new concept, created for the age of “Information Warfare” and “Net-Centric Operations.” Many identity changes over the course of almost a century contribute to this impression. Some supposedly revolutionary aspects of UAVs are actually reframed from previous efforts in pilotless aircraft, while others truly do have potential for great change. By examining the history of UAV development, we can identify which aspects of this technology fall under existing paradigms, and which are revolutionary enough that more attention is warranted.
Trends vs. Revolutions
In analyzing the last ninety years of unmanned flight we can discriminate between trends and revolutions in military pilotless aircraft.
In 1918 Charles Kettering developed a gyroscope-controlled flying machine that fell and exploded after the propeller turned a preset number of times. At first, the Army Signal Corps thought of using Kettering’s Bug as a form of long-range artillery [1, vol. 1]. Since then, pilotless aircraft have been used in a variety of contexts: the Army Air Forces ordered missiles during World War II, drones have been used for target practice since the 1930s, and remotely piloted vehicles (RPVs) were flown over Vietnam to gather intelligence. This wide variety of applications makes tracking the development of autonomous technologies difficult, since the idea is often reframed. For instance, the “drones” of the 1950s became the “remotely piloted vehicles” of the 1960s and 1970s and changed again into the “unmanned aerial vehicles” of today.
These frequent re-categorizations have contributed to confusion over what is new and what is old, what is revolutionary and what is reframed. UAVs have been referred to as both “unproven, infant technologies” and “mainstream military weapons” [2], [3]. Given the myriad definitions and concepts available for the term “unmanned aerial vehicle,” both descriptors are accurate.
Terminology
The acronym UAV (unmanned aerial vehicle) refers to any reusable air vehicle that does not have a pilot on board. “Missile” refers to a one-time use vehicle with no pilot on-board. In this sense, the Kettering Bug is a missile, not a UAV.
There are some programs that do not fit cleanly into either category, such as the craft used in Project Aphrodite in WWII (see discussion below under heading “Missiles and UAV’s Split”). Project Aphrodite aircraft were technically reusable, since they were converted airplanes, but they were never used or intended as such after being repurposed.
“Pilotless aircraft” refers to such aircraft as well as to the broader category of any aircraft with no pilot on board.
Approach
Rather than appearing abruptly in a technological revolution, much of UAV development has been evolutionary, as can be shown by identifying clear, strong trends in thought and technology over time. Several trends have been behind the development of UAVs and missiles. I will show here that, rather than being a new and revolutionary idea, pilotless aircraft are a tried and true branch of military research and development.
Rather than appearing abruptly in a technological revolution, much of UAV development has been evolutionary.
There are possible exceptions to the evolutionary development of pilotless aircraft. UAVs are a family of technologies that have revolutionary potential, but not necessarily due to their advertised characteristics of saving lives and increasing military success. This is not to say that UAVs do not save lives or increase success, but any improvements in these areas are the result of long-standing efforts, not revolutionary change.
All the arguments in this article will focus on the United States and exclude unmanned platforms such as satellites, ground vehicles, and unpowered aircraft. A more comprehensive view of unmanned technologies is beyond the scope of this paper. This discussion will focus on pilotless concepts, more than technology, so representative programs will suffice as examples of key concepts.
Evolutionary Development
Evolutionary trends in pilotless aircraft need to be identified and put in context. Instead of focusing on trends in the technical development of pilotless aircraft, in this article the pilotless aircraft concept will be tracked through its various incarnations over time. When looking for trends, ideas are more important than technology because real change occurs when people behave differently, not when they behave similarly with different equipment. For example, Rosen points out that the tank took fifteen months from concept to production, but the officers fighting World War I took another forty months after the tank entered service to change the way the war was fought [5].
If UAVs are going to spark any revolutions, some break in continuous development must occur. Possible breaks are analyzed in the next section, but those discontinuities cannot be analyzed out of context, since their potential for abrupt change is directly related to the connection between the present and the past.
Unified Development: Interwar Pilotless Aircraft
The early years of pilotless aircraft saw no distinction between UAVs and missiles. Rather than debating single-use versus reusable vehicles, military planners were investigating whether pilotless aircraft had any military purpose at all. The period from World War I to World War II was a period of exploration to determine what was possible, not a period of focused development.
If UAVs are going to spark any revolutions, some break in continuous development must occur.
The U.S. military began research into missiles during the First World War, under the idea of the “flying bomb” or “aerial torpedo.” The gyroscope-controlled bombs designed by Charles Kettering for the Army Signal Corps and by Elmer and Lawrence Sperry for the Navy were stand-off weapons meant to attack targets, such as German U-boat facilities, from a distance of up to one hundred miles [1, vol. 1], [6]. For various reasons, not the least of which was the fact that they rarely hit their targets, missiles did not catch on until much later [7]. Incorporating the idea into existing combat categories, the Army thought of the missiles as long-range artillery; with this, the confusion over what to call such weapons began.
Abandoning autonomy for a few years, the military began conducting radio control experiments in the 1920s to achieve better accuracy for the Army’s Messenger Aerial Torpedo and the Navy’s Curtiss N-9. Both services lost interest in the programs after a few years, just as they had lost interest in the autonomous programs of the decade before. Radio-controlled aircraft re-emerged in the 1930s as target drones; thus, UAVs gained their first permanent foothold in the U.S. military. In 1938 the U.S. Navy began using UAVs, including the N2C-2 drone, for anti-aircraft gunnery practice [6].
This new use of pilotless aircraft marks a shift in thinking about the platforms. Using their own pilots for target practice was clearly out of the question, so UAVs were developed to fill a role that could not be filled by manned aircraft. Conversely, missiles were a substitute for existing artillery, and so missile development was shaped by competition with established weapons. From this point onward, UAVs have filled roles that manned aircraft would not take, while missiles compete more directly with other weapons.
Missiles and UAVs Split
World War II prompted a number of innovations and experiments in pilotless aircraft that highlight the differences between missiles and UAVs.
Project Aphrodite (1944) was the only major Army use of pilotless aircraft during World War II and it was a general failure. By using war-weary B-17s stripped of armor and loaded with nine tons of explosive, these bombs were more vulnerable to German air defenses than manned B-17s. None of the Aphrodite aircraft ever hit their targets [7].
The Navy’s Operation Option (1944) was an evaluation program in the Pacific to examine the use of TV-guided assault drones. These TDR-1 and TDN-1 drones were flown over radio control by escort planes, just as the Aphrodite planes were, but a TV camera in the nose of each enabled the operators to hit artillery emplacements, bridges, caves, etc. The Navy’s Operation Anvil loaded war-weary Navy bombers with explosives and control equipment. These bombers met the same fate as those from Aphrodite despite the more developed radio control [6].
The goal of Aphrodite and Anvil was to destroy German missile facilities by flying the bombers into the hangar doors, as the hangars were impervious to overheard bombardment. Demonstrating the advantage of an unusual attack direction, the projects started the services on the drive towards cruise missiles for hard-to-hit targets. This desire for precision bombing has driven missile developments for the last sixty years. The latest incarnations of this desire (smart bombs, Tomahawk missiles, etc.) continue this trend.
The Army JB-2, a copy of the German V-1 cruise missile, lost the bureaucratic battle against manned aircraft before its performance was even tested in the field. It was an original production missile, not converted from war-weary aircraft as other Army and Navy missiles. Production was ordered at the rate of one thousand per month. By the end of January 1944, production had been canceled because the Army refused to shift any personnel or resources away from bombs and artillery [7].
Hap Arnold, Commanding General of the Army Air Forces in 1945, predicted that the emphasis in air power would shift from pilots to scientists and machinery [5]. Arnold’s endorsement of replacing soldiers with machines was a distinct break from the past, when American missile and assault drone production was not allowed to interfere with any other elements of the war effort.
Emerging from World War II were two different concepts of pilotless aircraft. The autonomous missiles used by the Germans were a weapon, much like artillery, that could be fired at the enemy from a great distance and at high velocity. The U.S. military continued missile development along those lines, using cruise and ballistic missiles as platforms for delivering nuclear weapons. Second, assault drones, which were slower and could not operate outside line-of-sight, lent themselves to roles in surveillance where television capability was used to gather intelligence.
Missile Developments
In recent years, cruise missiles have acquired sophisticated autonomous capabilities that will be relevant in discussions of the future of pilotless aircraft, so a few salient points will be reviewed here.
Air-launched missiles took many forms following World War II. Significant advances included the infrared-homing AIM-9 (operational in 1953), the onboard programmable AGM-28 (1959), the anti-radar AGM-45 (1966), the television-guided AGM-65 (1973), and the radar-guided AGM-84A (mid- 1980s) [10].
In addition to using cruise missiles for direct attacks, missiles such as the ADM-20 Quail were used as bomber-protecting decoys starting in 1961. They simulated the heat and radar signatures of B-52 strategic bombers in order to confuse and distract opposing air defenses [10].
The Navy’s Tactical Tomahawk, a next generation cruise missile, uses multiple guidance systems to find its way to the target and can even be re-targeted during flight. This version of the Tomahawk also has a loiter capability; it can deploy to an area and wait for further instructions in the case of time-sensitive or opportunistic targets [11].
Methods for guiding and controlling cruise missiles have become increasingly sophisticated, especially with the recent additions of loiter patterns and two-way communication. The concept of a loitering missile is similar to the ideas behind current UAV weapons platforms, and this convergence will be examined in the context of future UAV systems.
From Drones to RPVs to UAVs
Gathering information on the enemy is a dangerous task. Current UAV proponents argue that the ability to gather intelligence over hostile areas without risking pilots’ lives will markedly change how the military operates. Far from being revolutionary, unmanned reconnaissance has been a function of UAVs since the 1950s.
By building on guidance and control improvements from cruise missiles, as described in Newcome [6, ch. 10], UAVs became more reliable and autonomous in their operation. These technical improvements were accompanied by changes in name and concept.
During the 1950s and 1960s, the aircraft were called drones, signifying limited abilities and mindless flights into enemy fire. The word “drone” also conjures the idea of something that can be easily discarded or replaced, unlike a pilot.
One example of an expendable reconnaissance UAV is the Firebee drone, originally designated as the Q-2 from Ryan Aeronautical and flown since 1951. The Firebees were jet-powered, air-launched, remotely piloted UAVs used to gather information over hostile areas.
Firebees of many types flew over 3400 sorties during the Vietnam War, with only 211 lost. These aircraft were primitive compared to the modern Predator, but had enough utility to warrant their widespread use. In fact, their simplicity most likely encouraged widespread production and use [12].
Since the UAV missions over Vietnam resembled those that a manned aircraft would perform, the pilots assigned to fly the Firebees began referring to them as remotely piloted vehicles (RPVs) instead of drones. A piloted vehicle, remote or otherwise, is expected to show intelligence, innovation, and other human qualities. This term made the pilots more comfortable by making it clear that they were still pilots, not drone operators [6].
In 1976, the Air Force acquired the AQM-34V to perform electronic warfare ahead of manned attack aircraft. Up to eight of these Firebee UAVs could be controlled from one DC-130. By 1987, 13 existing UAV models were for surveillance while only one had an attack role [12].
One of the Air Force goals for the future is “Maintain Global Awareness” of the activities and capabilities of potential enemies [13]. Most UAV programs give prominence to information gathering.
The High-Altitude Endurance UAV Advanced Concept Technology Demonstration program, which resulted in the Global Hawk UAV, represents the latest stage in the drive toward more altitude and longer endurance times for reconnaissance UAVs. The concentrated effort to develop high-altitude long-endurance (HALE) UAVs began in the late 1960s in order to combine the safety of a spy satellite with the flexibility and higher resolution of a manned spy plane [6].
Global Hawk (1990s to today). (U.S. DOD.)
Global Hawk and similar surveillance systems are attractive because they are “24-hour-a-day capable and are adverse-weather capable” unlike manned aircraft that must refuel more often and land to change crew [14]. Another advantage to pilotless aircraft is that they can continue to operate after being damaged without posing a risk to any operators [15]. Rather than being a new development, however, UAVs have been considered as U-2 replacements since Gary Powers was shot down over the Soviet Union in 1960. Conceptually, these vehicles are not different from their manned counterparts; they perform similar missions in similar ways.
The change from RPV to UAV denotes a move away from direct piloting of the reconnaissance vehicles to semi-autonomous control with the goal of one operator controlling multiple vehicles simultaneously. The Predator UAV, for instance, currently has three operators for each vehicle. It is popular in the CIA as well as in the Air Force not only because it gives a bird’s eye view but also because it can be safely operated from halfway across the world. “Although the Predator was uninhabited, it was very much ‘manned’ in the sense that a soldier controlled its every move, seeing what it saw and, ultimately, pulling the trigger” [16].
Predator (1990s to today). (U.S. DOD)
After the Firebee drone and its descendants showed the capability to penetrate dense air defenses and perform limited missions over Vietnam, it was suggested that UAVs would be gradually absorbed into the USAF force structure. However, over 25 years after the prediction of gradual absorption was made, UAVs have still made little headway into standard operations in the Air Force [17].
UCAVs and Smart Munitions: Unified Development Once More
If UAVs are incorporated into the U.S. military, their digital tethers to human operators will not be cut. As cruise missiles gain two-way communication and UAVs are again thought of as weapons platforms, not just eyes in the sky, their development tracks have begun to merge.
Search-and-destroy missiles are the current missile focus for suppression of enemy air defense missions. This goal typically entails the most dangerous part of an air campaign [18]. Undertaking this operation along with the two-way communication technology incorporated into the Tactical Tomahawk leads to weapons systems that can search territory, send gathered information to the base station, receive changes in orders, and destroy targets. These are often referred to as “smart munitions.”
The latest “smart” UAV is the Boeing X-45 unmanned combat air vehicle (UCAV). The goal of the project is to “demonstrate the technical feasibility, military utility and operational value of a UCAV system to effectively and affordably prosecute 21st century lethal and non-lethal suppression of enemy air defenses and strike missions within the emerging global command and control architecture” [19]. According to Boeing’s X-45 program manager, the UCAV is not meant to compete with any manned platforms [20]. If the X-45 is not competing with bombers, then it must be competing with missiles.
These visions of smart munitions and UCAVs are similar in nearly every aspect, with the main difference being that the former systems are designed for one-way trips. The smart munition has even been described as “a small UAV ... with kill power” [21]. In some future projects, the merging of UCAV and missile concepts, not just technology, is explicit, such as the AC-17 “Mother-ship.” This is a large aircraft that carries and launches a dozen UCAVs at distant targets. This is the current concept of operations for cruise missiles, except that air-launched cruise missiles do not return to pick up another bomb [22]. While the technological challenges in building an airborne launch-recovery station for UCAVs are substantial, the conceptual leap would be a small one.
Reconnaissance UAV operational concepts are more difficult to categorize because they come in two different forms. The Air Force operates its Predators and Global Hawks at medium to high altitudes (15 000 feet and 60 000 feet) and uses them to gather photographic and video information. The pilots are far removed from the battlefield (one operations center is in Las Vegas, Nevada). While soldiers on the ground may receive the information in real time, they have no control over the flight path or camera orientation, just as with manned aircraft [23].
The Army and Marine Corps have their own reconnaissance UAV programs which are conceptually different from traditional and Air Force UAV systems. These will be discussed in the next section.
Possible Discontinuities
While progress from aerial torpedoes to autonomous UCAVs has been smooth, there are a few places where current or near-future developments could break sharply away. In addition, some possibly revolutionary UAV characteristics can be recast as trends and long-standing goals of the U.S. military. These non-revolutionary ideas are touted as selling points for UAV research and development, but are really reframed goals and concepts from previous UAV efforts. This section will point out one example from each category as illustrations of how to look at revolutionary claims with a critical eye.
Tactical Intelligence
Army and Marine Corps UAV operations have a different structure from those in the Air Force, and this structure gives them potential to spark dramatic changes. When intelligence is derived from satellite imagery and high-flying spy planes, there is a long loop between when information is requested and when it is utilized. With Army and Marine Corps soldiers gaining direct control over the UAVs that deliver their information, this delay has grown small enough to change how soldiers collect, process, and act on information [24].
The Predator does provide real-time intelligence and offers unlimited control range through use of satellite relays. By early 2001, the Predator had acquired a laser designator Hellfire-C missile launcher, which it used to destroy a stationary Army tank [25]. As pointed out above, however, the Predator is controlled from remote base stations and the operators must communicate through proper channels to the forces they are supporting. While this standardization of communications makes incorporating new platforms easier, the soldier on the ground sees no difference between a Predator and a manned asset.
Army and Marine units have much more direct control over their UAVs which generally fly below 8000 feet [23]. This improves reaction time and also decreases reliance on other services to fill their information needs. It also allows ground forces to increase the amount of self-observation and criticism, which could improve their effectiveness [24].
The U.S. Air Force asked in Spring 2005 to become the executive agent for all UAV programs in the Department of Defense. The Army and Marine Corps fear that “the Air Force would intentionally or inadvertently undermine the needs of its sister services in the name of commonality and savings” [26].
The disagreement is rooted in two competing methodologies for UAV command and control. Currently, most Army and Marine Corps platforms are organic, meaning they are tightly coupled to the units that use them. The Air Force vision is of centralized control under the joint commander to simplify air traffic control and more quickly shift resources [27]. Under current Army and Marine practice, the possibilities for rapid reaction and adjustment inherent in local control of UAV assets may spark a revolution, especially in the tight quarters of urban warfare. The Air Force vision would incorporate UAVs into the existing air power methods and practices, negating any revolutionary potential.
No Human at the Trigger
While removing human decision making from warfare is not something that should be undertaken without careful analysis, it has already happened to a large extent. Focusing on the UAV “revolution” as the turning point in automated decision-making obscures a much larger trend.
During World War II, bombers used Norden bombsights to locate and attack targets. During a bombing run, the bombardier could actually relieve the pilot of control in order to make best use of his judgment [10]. While airmen did not choose their targets, they were generally aware of their actions; when they dropped bombs at night or through cloud cover, they knew that reduced accuracy would cause bombs to fall off-target.
Cruise and ballistic missiles introduced unintelligent guidance. The gyroscopes at the heart of most navigation systems make no judgments, cannot adapt to mitigating circumstances, and are susceptible to many sources of error. London was spared the full force of German V-I attacks during World War II because a double agent convinced the operators to move the aimpoint, causing most of the missiles to fall in the countryside [12]. Unlike the bombardier, technicians could not observe the results of their actions, preventing self-criticism and removing the actors from some of the consequences.
The increasing use of fire-and-forget missiles has also removed the human factor from both target selection and decisions to attack. By their very nature, fire-and-forget weapons do not communicate back and so cannot ask for clarification if the guidance systems become confused [10]. The Patriot missile system, for instance, is notorious for erroneously firing upon friendly and non-existent aircraft despite its human operators [28].
Current work in smart munitions is focusing on missiles that can change course or target mid-flight. Even when a human decision is required to make any of these changes, the human interface can be designed to create a “moral buffer that diminishes a controller’s sense of responsibility and autonomy” [29]. This automation bias cedes effective control over the munitions to the computer, even when human intervention is technically necessary.
Of most concern is the use of automation in bombing where civilians may be present. Precision-guided munitions currently take two main forms. The first is a laser-guided bomb, which moves towards a target designated by a person who can see the target and thus is aware of and responsible for the results. The second type of precision munition is guided by the Global Positioning System towards a pre-programmed set of coordinates. There is no way for a pilot to determine what is at those coordinates, so he can make no intelligent decisions about whether to abort the mission. This disconnect became apparent in May of 1999, when NATO accidentally bombed the Chinese embassy in Belgrade. The intended target was a Yugoslav arms agency, “[b]ut the detailed, two-year-old map used for targeting did not show the numbers of the buildings on that street, so the officer used the numbering of buildings on parallel streets to mark what he thought was the Yugoslav arms agency” [30].
The evolutionary development of stand-off missiles never underwent such an examination because the changes were gradual. This led to a level of autonomy in weapons that may not be acceptable in a conscious design. Since the Predator, the current armed UAV of choice, was not designed to carry weapons, a discussion of the morality and accountability of pilotless weapons has still not taken place. There is a possibility that the X-45 could provoke such a discussion, but with explicit limits placed on the vehicle’s autonomy, this is unlikely.
When concerns arise about who is responsible for the results of a combat action, it is too simplistic to focus on the pilot. In many cases, modern pilots have no way of verifying that their actions are correct and justifiable, so we must look further afield. A thorough examination of who would be responsible for the use of an autonomous attack UCAV is necessary, and the results would be applicable to manned platforms as well. The determination of the combat-responsible agent is a revolution-in-waiting, but nothing under near-term development is likely to spark any such examination.
Most UAV Development Evolutionary
Most UAV development has been evolutionary. Current operational concepts are not radically different from their predecessors. Even technology has been recycled and repackaged; for instance the TV guidance systems that were used on WWII drones resurfaced in the AGM-65 in 1973. Most pilotless reconnaissance aircraft are flown just as manned aircraft would be. Weaponized pilotless aircraft fall under standard missile concepts of fire-and-forget. UCAV systems may be able to change target during flight, but so can the Tactical Tomahawk cruise missile.
There are two potentially revolutionary aspects of UAVs; one is likely to succeed, while the other is likely to go unnoticed. The introduction of small, locally controlled UAVs deviates from most pilotless aircraft development, since these vehicles are prompting real behavioral change in ground operations based on real-time control of surveillance assets.
In addition to finding where revolutions are possible, the evolutionary framework can also be used to counter specific arguments about revolutionary potential. The idea that UAVs present a new and unexamined danger in their autonomous deployment of weapons has been investigated and found to be misleading. While there truly is danger in removing people from decisions involving lethal force, this trend has been underway for decades, with UAVs only the latest step. An examination of moral accountability for all weapons systems is in order, not just for UAVs.
Extensions
There are other characteristics of pilotless aircraft that can be examined using this framework. One example is the idea that deploying UAVs will save the lives of pilots who would otherwise be put at risk. Another is that pilotless aircraft offer potential cost savings when compared to manned aircraft, and that UCAVs in particular are more affordable than cruise missiles.
While the idea of saving lives seems straightforward at first, a brief example drawn from Predator operations can show that pilots are generally safe to begin with and UAVs increase only success rates.
The Predator UAV was designed as “a rapidly deployable reconnaissance and surveillance” aircraft [31]. It flew over 50 sorties during Operation Allied Force over Kosovo as a targeting aid using two cameras and a synthetic-aperture radar [32]. Predators were used to direct smart bombs using a laser designator from altitudes of about 8000 feet while manned aircraft flew above an “artificial deck” of 15 000 feet. At these altitudes, the manned aircraft were safe from ground fire, but often wound up flying above the clouds with Predators flying below. In these cases, Predators enabled more accurate guidance for the bombs, but did not save any pilots from danger; the pilots flew above the threat zone regardless of UAV presence [33].
Future Projections
Extending the trends and considering the possible revolutions presented above must be coupled with a look at new technologies that may change the face of pilotless aircraft.
The constant march towards smaller components has now enabled mini- and micro-UAVs, with maximum altitudes of 250 to 350 meters.
Micro-UAV (in development). (SEIKO EPSON CORPORATION.)
These platforms could fill niches that did not previously exist, including close-proximity electronic jamming and gathering information inside of structures. Micro-UAV proponents exist not only inside the military, but within organizations like Unmanned Vehicle Systems International [34]. Recalling that UAVs first established a permanent foothold in a previously unfilled niche, the target drone, this is one area where UAV usage may grow substantially. Should the necessary reductions in payload weight and increases in endurance be realized, these vehicles will increase the speed of the current tactical-UAV revolution.
Development of the Predator, Global Hawk, X-45, and other large UAVs will proceed in small steps. The X-45 will evolve along lines similar to the more advanced cruise missile concepts, creating competition for cost reductions and performance increases in both programs. The Predator did not spark any revolutions in 1995 when it entered service and similar platforms in the future will likely be used the same way. As long as the Air Force continues to incorporate its pilotless systems into the existing command and control hierarchy for manned systems, the programs will not make large waves. We may see increased use of the Global Hawk and Predator instead of manned spy planes, but the UAVs will be used and tasked in the same way.
Author Information
The author is on leave from the Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853. Email: jms276@cornell.edu.
Acknowledgment
I would like to thank Professors Judith Reppy and Kathleen Vogel for providing valuable comments, insights, and information.
An earlier version of this article was presented at ISTAS 2005, Loyola Marymount University, Los Angeles, CA.
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