by Mark E. Rogers, Lieutenant Colonel, USAF
November 1997
Occasional Paper No. 2
Center for Strategy and Technology
Air War College
Maxwell Air Force Base, Alabama

from AirUniversityCenterForStrategyAndTechnology Website
 

Contents

• Disclaimer

• The Author

• Abstract

1. Why Lasers In Space?

2. Exploitable Characteristics Of Space

3. Exploitable Characteristics Of Lasers

4. A Taxonomy For Lasers In Space

5. Strategic Planning Studies

6. Criteria For Evaluating The Concepts

7. Review And Scoring Of Concepts

8. Space-based Laser Target Designators

9. Space-based Battlefield Illumination

10. Moving Concepts Into The Field

11. Conclusions And Recommendations

• Acronyms, Abbreviations And Symbols

• Useful Terms

• Notes

Disclaimer


The views expressed in this publication are those of the author and do not reflect the official policy or position of the United States Government, Department of Defense, or the Air War College Center for Strategy and Technology.

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The Author

Lieutenant Colonel Mark Rogers, USAF, a 1976 graduate of the USAF Academy where he majored in physics, has been involved in defense-oriented research and development for the past twenty years, focusing primarily on applications of lasers for military systems. His background includes test range support for future space programs and capabilities analysis for optical tracking systems while at Vandenberg AFB. After completing his MS and Ph.D. in laser/optics, he conducted and managed research on various high energy laser weapons concepts at the AF Weapons Laboratory, including coupled laser technology for space-based lasers as part of the Strategic Defense Initiative. While teaching undergraduate physics at the USAF Academy, he led a research team investigating stimulated Brillouin scattering and stimulated Raman scattering for possible use in nonlinearly coupling laser devices.

He headed a laser biophysics team at the Armstrong Laboratory, where they established new safety thresholds for sub-nanosecond pulses and near infrared wavelengths as well as studied various nonlethal technologies based on optical systems. Prior to entering the Air War College, he served as the deputy chief scientist for Armstrong Laboratory, spearheading the Lab’s information warfare program and overseeing multiple basic research and scientific personnel programs. His current interests include directed energy weapons, nonlinear dynamics, and laser-tissue interactions. A 1997 graduate of the Air War College, Lt. Colonel Rogers conducted this research under the auspices of the Center. His current assignment is in the Pentagon.

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Abstract

The emerging importance of space-based systems is matched by the maturing of laser technology, giving a potential synergy to enhance military capability. For example, global awareness is one of the AF goals to give the US military the competitive advantage in future conflicts. Obtaining global awareness requires a tremendous amount of information being acquired and transferred over vast distances. Space-based laser communication satellites offer the potential of greatly increased data rates, which is just one example of how lasers in space could significantly improve US military capabilities.

Recent strategic planning studies have identified various concepts for lasers in space, including both laser weapons and collateral applications such as communication and remote sensing. Four functional classes of systems (enabling, information-gathering, information-relaying, and energy delivery) serve to organize the various concepts and relate them to the new AF core competencies as well as the traditional AF roles.

This study analyzes these concepts, scoring them for technical feasibility, technical maturity, operational enhancement and operational cost. The most promising concepts include space-based laser target designation, space-based battlefield illumination, laser communication, and active remote sensing for battle damage assessment and weather characterization. Several strategies can accelerate the development of space-based laser systems, such as using the new AF battlelabs and advanced technology demonstrations.

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I. WHY LASERS IN SPACE?

Both laser technology and space operations have matured substantially in the recent decades, offering synergistic possibilities of using lasers from space-based platforms to improve US military capabilities. Coherent laser light offers a number of unique advantages as does the space environment, permitting speed-of-light applications such as optical communication, illumination, target designation, active remote sensing and high-energy weapons. Many of these concepts have been discussed in recent strategic studies, but it will take innovative leadership and close cooperation between researchers and operators to bring the concepts from the laboratory to the field.

One of the recent themes in US military thought has been achieving global awareness in order to establish dominant battlespace awareness. According to General Ronald Fogleman, AF Chief of Staff,

“The reality is that in the first quarter of the 21st century it will become possible to find, fix or track and target anything that moves on the surface of the earth.”¹

Whomever has such awareness, the theory goes, will have the upper hand in any military operation. Awareness equates to the possession of adequate information. Achieving global awareness will require obtaining, processing, and relaying massive amounts of information in near-real time across vast distances. Space-based laser systems bring many unique characteristics to the battlefield, and thus represent powerful tools in achieving global awareness.

Each of the new Air Force core competencies — Air and Space Superiority, Global Attack, Rapid Global Mobility, Precision Engagement, Information Superiority, and Agile Combat Support — highlights an area of expertise for the AF in accomplishing both warfighting and military-operations-other-than-war (MOOTW) missions. As discussed more fully in a later section, lasers in space can enhance each of these competencies. (A comprehensive list of acronyms is included at the end of this report.) Space-based laser systems offer unique opportunities to help the warfighters of the AF and the other services achieve these missions.

The Department of Defense and NASA are testing various space-based laser concepts, many of which have high military utility. While some projects are good candidates for collaboration, the overall development is ad hoc. There is a clear need for more overarching coordination across the agencies and between the researchers and the warfighters. This study proposes a common framework for lasers in space, and should serve as a catalyst to further cooperative development of the more attractive concepts.
 

Objectives

The thesis of this study is that many emerging military requirements can be met through the use of laser systems deployed on space platforms. Laser technology has matured sufficiently in the past decade to provide highly reliable, cost-effective, energy-efficient and wavelength-flexible systems that can be applied to a variety of missions, such as remote sensing and communication. Access to space is maturing with new launch vehicles on the horizon. The unique characteristics of the space environment greatly enhance the utility of deploying lasers in space. These include the lack of any medium to attenuate the beam and the ready access to the entire global surface. This synergy of lasers in space offers the warfighter a new and vastly more competitive tool in future conflicts. However, technology developers must move aggressively to field prototypes that demonstrate the capabilities and potential of space-based laser systems for a variety of missions. The various avenues to expedite bringing the most promising concepts into fielded systems is the final focus of this report.

In an increasingly resource-constrained environment, the Air Force must successfully blend strategy and technology. One straightforward definition of strategy is “a broad concept, embracing an objective, resources, and a plan for using those resources to achieve the objective.” ² There are two relevant types of strategies.

• First, acquisition strategy applies R&D resources (funds, manpower, facilities, etc.) to develop new technologies that match operational deficiencies identified by the warfighting commands.

• Second, operational strategy examines the threats to the security and national interests of the US and its allies, matches current capabilities against the threats to achieve military objectives, and highlights areas where improvements in capabilities could enhance military success.

It is at this point that the two strategies interact. The warfighter must face any conflict with the tools at hand, striving for victory with diligence and ingenuity. The military researcher must work with equal diligence and ingenuity to find new or more effective tools for achieving military objectives, which in some cases will require new technology and in others might mean repackaging existing technology into new systems. It is increasingly important that the system developers and the operational users work closely together. One new AF attempt to generate this synergy is the concept of battle labs that are discussed at the end of this report.
 

Scope

The field of laser technology has greatly expanded since the laser was first demonstrated in 1960. Innovative minds have found many applications of these technologies, including active remote sensing, active imaging, optical communication, power beaming, and high-energy weapons. Since the early 1960s, the complexity of the military missions has dramatically increased, with more diverse theaters of operation, expanded spectrums of conflict, and tremendously increased requirements for information delivered in almost immediately to the warfighter. It would be impossible in a short report to comprehensively address all the unique aspects of lasers in the space environment as well as the potential military applications.

The scope of this paper is limited to surveying a subset of “lasers in space” concepts to establish a basis on which they can be compared and development decisions can be made. Each concept could be examined in more depth, and some of the concepts have been discussed in other, more focused reports, but that is beyond the scope of this report. Also, the operational applications could be discussed in more detail, which would lead to concepts of operations (CONOPS) that consider operational employment, doctrinal implications, constraints, proper force size, interfaces with other systems, and so forth. Again, this type of discussion is beyond the scope of this study. While it is impossible to give sufficient detail about each concept to fully explain the range of benefits and costs, this discussion will give the reader a firm understanding of the relevant technological issues.

In the near term, most applications for lasers in the space environment involve non-weapons systems. Although this study discusses laser weapons in some detail, it focuses on non-weapons applications that could be developed in the near future to enhance the warfighters’ capability. As the analysis will show, a plethora of maturing concepts exist that can increase military effectiveness.

The intended audience consists of the individuals in both the research laboratories and the operational commands, including the innovators in the battlelabs, who are building their program plans and looking to the future technological and operational requirements of the Air Force. Hopefully, these groups will find value in the study of the integration of laser technology in the space environment with the needs of the warfighter. However, the scientist at the bench will find the technical details lacking and the warfighter will find the operational details inadequate. This study serves simply as a compilation of various concepts for space-based laser systems and a brief analysis of the most likely near-term concepts.
 

Military Uses of Space

Access to outer space began in the 1950s with the Soviet launch of Sputnik I on 4 October 1957 and the US launch of Explorer I on 31 January 19583. Immediately, a vast range of potential applications was possible that had been the dream of scientists and science-fiction writers for years. Many ideas were pursued that were of immediate use to the military. Those that have found the most success can be broadly grouped into systems that gather information and those that relay it.

The first class of information-gathering systems includes weather satellites that image the earth in various spectral bands to determine cloud cover, moisture content, and related information. These systems include both geosynchronous satellites that remain relatively stationary over a region of the earth and low orbiting systems like the Defense Meteorological Satellite Program (DMSP) satellites that gather more highly resolved data for the military operator. Also, earth resources satellites like Landsat have imaged the earth in various spectral bands to determine such information as crop usage, environmental changes, and shifts in the rural/urban mix of the population.

Both weather and resources satellites exploit the visible and infrared regions of the electromagnetic (EM) spectrum, passively collecting the data and then sending it to ground stations via radio and microwave links. Multispectral imaging (MSI) systems have been developed to collect data at different wavelengths simultaneously providing much more information. The MSI systems can determine such information as the health of forests and crops. They can also differentiate between wet and dry ground and the composition of structures, providing measurement and signature intelligence (MASINT) to the military users. Image Intelligence (IMINT) gathering from orbital platforms began early with the Corona program, a recently declassified imaging satellite system.⁴

Most of these space assets are still cloaked in secrecy, but clearly the information is collected across a wide range of the EM spectrum, including radio and microwave transmissions as well as visible and infrared images. Some of the systems are passive, collecting the signals with extremely sensitive antennas and optical receivers, while others are active, using radar to penetrate cloud cover and observe targets on the ground. Other passive imaging systems have been placed in orbit to detect missile launches that might signal a hostile act against the US or its allies. For example, the Defense Support Program (DSP) surveillance satellites provided launch detection of Scud missiles during the Gulf War that permitted impact point prediction and early warning.⁵ With the exception of space-based imaging radar satellites, all of these satellites are passive collectors of information. The laser, as discussed later, offers unique advantages in gathering information by actively illuminating targets.

The second class of information-relaying systems transfers voice, data and image information, and encompasses numerous commercial systems as well as dedicated military systems. For example, communications satellites typically use microwave frequencies to carry the information from the earth to the satellite, between satellites, and back to the earth. The Global Positioning System (GPS) provides a second example, relying on a constellation of satellites to generate and transmit highly accurate time signals for precisely determining location. These guidance or navigation systems offer incredible capabilities that are beginning to revolutionize military and civilian travel on or near the surface of the earth. In this second class of information-relaying systems, the laser offers unique capabilities for extremely high data rates and highly accurate guidance systems.

To date, no weapon systems have been stationed in space, primarily due to technological and treaty limitations. Both the Soviet Union and the United States have conducted tests of anti-satellite systems using various types of kinetic and chemical energy warheads to intercept and destroy orbiting platforms. The use of directed energy weapons (DEW) such as lasers, high power microwaves (HPM) or charged particle beams (CPB) has been considered in great detail by such programs as the US Strategic Defense Initiative (SDI). Some space-based DEW components, such as the ALPHA laser, have been constructed and tested on the ground, but no systems have been tested in orbit. Without question, space-based lasers could be fielded in 10 to 20 years that can destroy targets in space as well as on or near the earth’s surface. The challenges involve engineering and cost, rather than the fundamental laws of physics.

Treaties, such as the Outer Space Treaty of 1967 and the Anti-Ballistic Missile (ABM) Treaty, restrict the United States from placing certain types of weapons in space and need to be carefully considered as lasers move into the space environment. Other pending international agreements such as the “Blinding Laser” ban could affect space-based lasers even though space-based lasers would not be specifically designed for blinding personnel.⁶ Technologists and operational commanders need to be aware of the political issues that can radically alter the new systems under development.
 

Military Uses of Lasers

For many years, the laser was touted as a “solution in search of a problem,” as most of the early applications remained in the research laboratory.⁷ In the past twenty years, lasers have solved myriad problems. The advances in laser technology have truly revolutionized a variety of areas, including medicine, telecommunications, industrial welding and cutting, and data processing. The ubiquitous laser bar code scanners and compact disc players provide almost daily contact with lasers for most people in the developed world. The military was one of the first services to see the potential for lasers in many applications. The early hopes of fielding a high-energy laser (HEL) weapon have yet to be fully realized by the United States, although the technology is well in hand. The AF’s Airborne Laser (ABL) program aims to field an operational HEL to negate theater ballistic missiles in their boost phase by early in the 21st century.⁸

The laser was quickly employed as an aid to other weapon systems. Innovative scientists and engineers used the beam to point at a target and generate an aim-point that could be used to guide a bomb precisely to the target.⁹ As a type of precision guided munitions (PGM), the PAVE (“Precision Avionics Vectoring Equipment”) series of laser target designators (LTD) and the associated PAVEWAY laser-guided bombs (LGB) have been tremendously useful in conflicts from the Vietnam War to the Gulf War. Fielded military laser systems also include highly accurate range-finders and secure communication systems. Lasers have also been very useful for training aids such as the MILES system, the military equivalent of “laser tag” available now to the general public. Recently, laser spotlights have provided both visible and infrared illumination for improved use of night vision devices (NVD). As widespread as the laser has become in the US military, it has yet to be effectively employed in space. It is this shortcoming that motivates this study.


Military Use of Lasers in Space

This study begins with a brief discussion of the exploitable characteristics of both the space environment and the laser to create the backdrop for the subsequent analysis of various concepts for lasers in space. After defining a taxonomy for space-based laser systems and relating it to the operational concerns, the study examines several recent strategic studies with respect to how different groups of analysts envisioned using lasers in space for military purposes. These concepts are scored with a simple set of technological and operational criteria in order to determine the most attractive near-term concepts for technology demonstrations.

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II. EXPLOITABLE CHARACTERISTICS OF SPACE

The region outside the atmosphere offers a unique environment for military operations. The ability to exploit the characteristics of space will give the warfighter a competitive edge in virtually all conflicts. Highlighting the differences between “air” and “space” in terms of doctrinal development, one study identifies the three characteristics of space systems — emplacement, pervasiveness, and timeliness — that benefit from the features of space.¹⁰ A brief review of these features provides background to understanding how space-based laser systems can be most effectively deployed.

A slightly different phrasing of these characteristics is given in an Air Force Space Command publication:

The space high ground offers tremendous advantages not found on Earth. Space allows countries like the United States to watch the entire globe on an almost “real time” basis [timeliness]—getting information nearly the instant it’s needed. Space assets are always available [emplacement] when America needs them. Space also gives military planners the added advantage of seeing the entire battlefield—the highest possible vantage point [pervasiveness] allows friendly forces to watch over an enemy, serving as both a watchdog and a deterrent. Space systems also offer the distinct advantage of longevity. Aircraft missions last a few hours, while satellites are constantly operational for years.¹¹ [Emphasis added.]

While these unique characteristics of space are only beginning to be exploited effectively, an important caveat is that space is an international environment over which no nation has sovereign control. The fact that space systems do not need basing rights or over-flight approval increases the freedom to conduct operations, but this liberty not only creates vulnerabilities but increases the possibility of conflict with nations that attempt to interfere with the space systems of other nations.
 

Emplacement

Emplacement means that space systems can be pre-positioned in orbits which offer optimal support when needed. This characteristic might also be called persistence or presence. With proper ground support systems and sufficient satellites, space systems can be maintained in a state of wartime readiness, and thus are “inherently ready to support military operations at all times, which avoids the potential complications of basing rights and over-flight permission.”¹² Given the remoteness of space as well as the difficulty and expense of deploying systems there, the characteristic of emplacement demands foresight by planners so that these systems can be pre-positioned to ensure availability in a crisis.
 

Pervasiveness

Outer space begins between 50 to 100 miles above the earth’s surface, depending on the criteria used.¹³ This region surrounds the earth’s surface and thus permits a presence over all land, sea, and air targets. Surveillance systems positioned in space reduce the risk of being surprised and complicate a potential adversary’s ability to hide. Systems in low earth orbit (LEO) move at high velocities, traveling over 7 kilometers every second and completing an orbit every one to two hours, depending on altitude. LEO satellites give the best resolution due to their proximity to earth, but more satellites are required to permit continuous coverage of various ground and sea locations.

Systems in middle earth orbit (MEO) have longer orbital periods, like the 12-hour period for the semi-synchronous orbits of the GPS satellites. This means that fewer satellites are required—only 24 satellites are needed for GPS to provide adequate coverage for global navigation. Satellites in geosynchronous earth orbit (GEO) have a 24-hour period and remain roughly over the same point on the earth’s surface. With just three satellites, the entire surface of the earth can be covered, with the exception of the higher latitudes. However, due to the greatly increased altitude (22,300 nautical miles (NM)), achieving high resolution with GEO systems (e.g., the ability to discern or point at small targets) is challenging.
 

Timeliness

Depending on the type of orbit, a sufficient number of space systems can provide near-instantaneous coverage of every point on the globe, at the cost of increased complexity in controlling the network. Because the network is linked by EM radiation that travels at the speed of light (186,000 miles per second), a properly fielded space system permits “near-real-time transfer of information” to war-fighters and facilitates “rapid application of force” against almost any type of target.¹⁴ Also, the high orbital speeds mean that space systems are frequently overhead, which translates into more engagement opportunities for the military commander.
 

Unattenuated Propagation of Electromagnetic Radiation

A vacuum is the ideal environment for propagating electromagnetic radiation. Any intervening medium, such as air, bends, scatters or absorbs the radiation, depending on the frequency or wavelength of the radiation and the type of medium. In the near vacuum of space, all frequencies propagate with essentially the same low attenuation.¹⁵

For space-based systems that transmit EM radiation toward the earth, how far this radiation penetrates into the atmosphere depends strongly on the wavelength due to both absorption and scattering. For example, clouds attenuate visible light much more than microwave frequencies used in radar systems. Thus, radar gives better all-weather coverage than lasers when propagating through the air. From space, radar systems are able to penetrate some clouds to image objects on the ground when optical imaging systems cannot discern the target through cloud cover. Many optical wavelengths do penetrate through clear air with low absorption, allowing space-based lasers to focus optical energy on targets at or near the earth’s surface for applications such as illumination, communication, remote sensing, target designation, or target destruction.

By virtue of the negligible attenuation of EM radiation in space and the short wavelengths of lasers, laser beams can propagate for extremely long distances with relatively little growth in the cross-sectional area of the beam. This helps maintain the energy density in the beam, meaning that more energy can be delivered to a target, both cooperative ones like communications satellites and uncooperative ones like reentry vehicles.
 

Challenges

The characteristics of space pose many challenges to both systems and people who transit through space. A brief discussion of a few of these challenges will support the subsequent discussion of laser systems that must operate in space for long periods.

Environmental Hazards.

Ambient ionizing radiation poses unique risks to systems deployed in space. The atmosphere filters out a wide range of threats to people and systems near the surface. Ionizing radiation means any radiation that has sufficient energy to knock electrons out of their orbits around the atomic nucleus. It includes very short wavelength EM radiation, such as extreme ultraviolet and x-rays and energetic particles consisting primarily of protons, alpha particles, and electrons. Much of the ionizing radiation comes from the sun. Very high-speed particles, cosmic rays, come from other sources. This naturally occurring radiation can destroy electronics, cause software errors by changing memory values, and degrade hardware.

While the EM radiation from the sun is fairly constant, the intensity of particles varies greatly with time and peaks with solar flares. The particles interfere with satellite operations and can disable satellites. Forecasting the space environment is an emerging area of meteorology. Thus, space systems that must remain in orbit for many years can face a large cumulative exposure as well as brief, high doses. Shielding can reduce the effects of ionizing radiation but at the expense of increased weight. Careful selection of materials can also reduce the deleterious effects. For this reason, extensive radiation hardening programs for critical components are underway within NASA and DOD.

A further complication is the existence of the Van Allen radiation belts, which are regions of high-energy particles trapped in the earth’s magnetic field. Primarily consisting of protons and electrons, these particles spiral around the magnetic fields, reflecting back and forth between the earth’s magnetic poles, where the magnetic flux becomes more concentrated. The interaction of this radiation with the air gives rise to the beautiful aurora borealis and aurora australis. There are two Van Allen belts. The inner belt begins at an altitude between 250 and 750 miles (depending on latitude), extends to about 6,200 miles and covers from about 45 degrees north to 45 degrees south in latitude; the outer belt begins at about 6,200 miles and extends out to as much as 52,000 miles.¹⁶ Due to orbital constraints most space systems will operate within one of these two belts. Spacecraft in LEO appear to receive an insignificant amount of radiation from the Van Allen zones, while spacecraft in MEO or GEO may pick up substantial doses.¹⁷

Another challenge posed by space is extreme temperatures. With no atmosphere to help conduct heat, systems can become either extremely hot or extremely cold, depending on exposure to the sun. As a spacecraft orbits, it moves into the earth’s shadow where the sun’s heating is eliminated and radiative cooling occurs. When it again enters the sunlight, the heat builds up on the surfaces exposed to the sun. Such thermal cycling causes expansion and contraction of materials and needs to be considered in the design of spacecraft. The cycling may eventually cause cumulative damage to the spacecraft and could cause the misalignment of sensitive optical systems.

Cost.

Currently, people launch objects into space by using chemical rockets to give the object sufficient altitude and velocity to attain orbit. The costs of building and launching the space boosters include both the cost of the booster and the cost of the facilities and manpower to accomplish the task. Current costs of orbiting a LEO satellite are very high, ranging from $3,000 to $10,000 per pound.¹⁸ The Space Shuttle payloads cost roughly $8,000 to $9,000 per pound to launch.¹⁹ Several programs are underway within NASA and DOD to explore ways of bringing the cost down below $1,000 per pound. These programs include reusable chemical rocket systems, such as the single-stage-to-orbit (SSTO) concept tested in the Delta Clipper rocket; a reusable space plane that would take off and land like a conventional aircraft; and novel ideas, such as electromagnetic rail guns and air guns to put satellites in orbit without chemical rockets. The relevant point for this discussion is that because cost determines the feasibility of putting systems in space, anything that can be done to reduce weight is highly valued. As will be discussed later, laser systems may offer significant reductions in weight while maintaining or expanding system capabilities (such as communication bandwidth).

Manpower-Intensive Operations.

At present, each US space launch requires large teams of highly trained personnel to design, build, launch, and operate space systems. The launch facilities are expensive, as are the global network of ground stations that are currently necessary to maintain operational control over the spacecraft. Research is underway to make spacecraft more modular and standardized, which would reduce design and manufacturing costs. Advances in software and increasing space infrastructure (such as GPS as a navigational grid) will make spacecraft more autonomous and reduce the need for large numbers of highly skilled engineers to operate the systems. Manpower reductions should be included in the design of all space systems.

Self-Protection.

Systems placed in space face both accidental and intentional threats. As more satellites have been launched, the amount of man-made debris (“space junk”) has increased. The Air Force currently tracks 8,168 objects in orbit, only 7 percent of which are active satellites.²⁰ When combined with naturally occurring meteorites, the risk of colliding with a hypervelocity projectile is not insignificant, particularly if spacecraft remain in orbit for years. Proper orbit selection, additional shielding, or maneuvering are options to increase the spacecraft’s survivability.

Since from 1950s, military researchers have considered ways to destroy satellites.²¹ Anti-satellite (ASAT) weapons using nuclear warheads, conventional explosives and hypervelocity kinetic kill warheads as various ways to destroy a satellite were tested by the United States and the Soviet Union. There were suspicions that Soviets blinded optical sensors on US early warning satellites in the mid-1970s but, even if true, the laser systems would likely have been R&D prototypes rather than fielded hardware.²² Of course, a successful R&D system leaves a residual operational legacy, as the Russians may possess at their laser research facility in Sary Shagan, which is located in Kazakstan.²³ The US Air Force has an on-going ground-based laser anti-satellite (GBL ASAT) research program. Certainly directed energy systems such as lasers or high power microwave (HPM) weapons could be used to disable or negate satellites, but they have not been fielded at this time due to substantial technical challenges and treaty agreements. At present, only a residual capability exists within the United States or Russia for ASAT missions. However, the predictability of orbits and the remoteness of space complicate the ability to protect spacecraft from attack.

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III. EXPLOITABLE CHARACTERISTICS OF LASERS

The laser is a unique source of optical radiation which has a number of characteristics that can be exploited for military space systems. As a number of these applications carry over to the civilian market, NASA and the private sector are interested in exploiting the laser for space applications. There are a number of sources that describe the basics of lasers.²⁴

Briefly, the laser uses the phenomenon of stimulated emission to generate a very narrow beam of light that is usually highly monochromatic. Such a beam has a high degree of coherence as compared to other optical sources like the sun or incandescent light bulbs. The coherence permits the beam to propagate long distances with little spreading and to be focused onto a small area. Within the ever-expanding variety of types of lasers, with different wavelengths, power levels, temporal characteristics and operating efficiencies, the unifying characteristic is the generation of coherent radiation.

In analyzing any laser application, it is convenient to break the process into three parts:

• the laser system

• the propagation medium through which the beam travels

• the target where the beam is absorbed

The laser system can be further divided into the laser, where the coherent radiation is generated, and the optical system that takes the beam from the laser to the output aperture where the beam enters the propagation medium. At the target, the light from the laser must be absorbed to cause an effect, whether that effect is an electrical response in a detector or a destructive effect for a weapons application. Each part (the laser system, the propagation medium, and the target) plays an important role in evaluating the feasibility of a laser application. Keeping this taxonomy in mind will help avoid fixating on any single part to the exclusion of the others.
 

Directionality

One of the key properties of lasers is that the output beam is highly directional. Typical laser beams have a divergence of less than a milliradian,* and some systems can be designed to have sub-microradian divergences. Because of their small size, semiconductor diode lasers usually have divergences measured in degrees, expanding rapidly. However, this beam divergence can be substantially reduced by using external optics. A laser system with an output beam diameter of one meter could readily have a 0.05 milliradian beam divergence, expanding to only about 25 meters after traveling 500 kilometers.

This pencil-like beam of light permits highly accurate placement of energy on a target for a variety of applications such as target designation and efficient communication links. Additionally, the beam can be used for covert applications because it is very difficult to detect the beam without intercepting it. The disadvantage, of course, is that pointing the beam requires a high degree of precision.
 

Wavelength, Bandwidth, and Tunability

A laser operates in the infrared, visible and ultraviolet regions of the electromagnetic spectrum, from one millimeter to 100 nanometers in wavelength. Typically, lasers are described by their wavelength (l) as contrasted with radar systems that are characterized by frequency, because the laser’s frequency is from 10,000 to 1,000,000 times higher than typical microwave radars. Both microns (mm or 10-6 meters) and nanometers (nm or 10-9 meters) will be used in this study to characterize lasers. Radar systems usually have wavelengths on the order of millimeters to centimeters. Many lasers generate light in a very narrow band around a single, central wavelength.

Because this characteristic manifests itself in visible lasers as a very pure, single color, the narrow linewidth is termed monochromaticity. For example, the neodymium laser used in most laser designators (the ubiquitous “Nd:YAG”) generates an output beam at 1.064 microns, with a typical bandwidth of 0.00045 microns, an amazingly narrow linewidth of 0.04 percent of the central wavelength. This spectrally pure output is critical for a multitude of applications, including remote sensing for specific chemical constituents and high signal-to-noise ratio (SNR) communications. Some types of lasers operate on several different wavelengths simultaneously, such as the argon ion laser that emits most of its light at 488 nm and 514.5 nm. Multiline emission can be both a benefit and a detriment, depending on the application.

While most lasers will only operate on discrete wavelengths, some types can be tuned over a range of wavelengths, giving an additional agility that has multiple uses. Examples of tunable lasers include,

• the titanium sapphire (Ti:S) laser

• the chromium:LiSAF laser (where the host material is a crystal of LiSrAlF6)

• the chromium:LiCAF lasers (where the host material is a crystal of LiCaAlF6)

These three lasers are solid state systems that have great potential for space applications, such as remote sensing of the atmosphere from orbit. They also have the added potential of being pumped by diode lasers or other solid state lasers that are diode-pumped. Thus, all-solid-state systems can be constructed with much improved reliability and durability. Table 1 shows the tuning range of these three lasers.
 

Table 1. Typical Tunable Lasers²⁵
 

As discussed in more detail in Appendix B, some special types of materials respond nonlinearly to light passing through them and can generate new wavelengths of light. Such nonlinear optical (NLO) materials are the subject of contemporary research. The most common is the frequency doubling crystals that cut the wavelength in half, so that the infrared emission of a Nd:YAG laser (at 1064 nm) can be converted into a visible beam (at 532 nm). Frequency doubling can be fairly efficient, with reported values of 50 to 80 percent conversion from the fundamental wavelength to the doubled wavelength. Other nonlinear systems, like optical parametric oscillators (OPO), can generate a tunable output.

While the technical details of such systems are beyond the scope of this study, they highlight the possibility of “wavelength agility” or the ability to tune the output wavelength of the laser. However, at this time, only a limited number of NLO materials are available. The efficiency at which they operate tends to be low. Also, obtaining efficient nonlinear effects requires high peak powers from the laser beam, which can damage the NLO material. The threshold at which NLO materials are damaged is usually low, making scaling to systems with high average power challenging. Research in material science is likely to push back these limits.

Another NLO effect that can be used for wavelength shifting is stimulated Raman scattering (SRS). Raman scattering occurs when a beam of light passes through a material and excites a very weak transition within the material, leaving some of its energy. The emitted light is shifted to a longer wavelength. If the process is stimulated in a method analogous to the operation of a laser, a significant amount of the light can be shifted to the new wavelength. SRS is a complicated process beyond the scope of this study but offers great potential for laser systems.

The narrow bandwidth of the typical laser can be reduced even further by specialized design of the laser. If this narrow beam is reflected off a moving object, the frequency of the reflection will be shifted slightly by the Doppler effect, permitting direct measurement of the velocity of the object. The “object” can even be a region of air, allowing direct, remote measurement of wind speed.
 

Temporal Modulation

The output from the laser can be either continuous (called “continuous wave” or CW) or pulsed. Usually a laser is called CW if the output lasts more than 0.25 second.²⁶ A pulsed laser is characterized by the pulse duration, which is measured in seconds. If the laser is repetitively pulsed, the pulse repetition frequency (known as the prf and measured in Hertz) is the period from the beginning of one pulse to the beginning of the next pulse. The duty cycle of the laser is the product of the pulse duration and the prf and gives a measure of what percent of the time a laser is emitting. A duty cycle of 50 percent means the laser is emitting energy half of the time.

Most lasers of interest to the military are either CW or have very short pulses on the order of nanoseconds. For example, the laser being developed for theater missile defense in the Airborne Laser is a CW laser. In contrast, the typical laser designator emits pulses of about 10 nanoseconds in duration, with a prf of 10 Hz, giving a duty cycle of 0.0000001. Some lasers use much higher repetition rates, on the order of kilohertz or higher, such as semiconductor or diode lasers that have great potential for communication systems. Imaging laser radars under development may use pulses with durations of less than a nanosecond in order to achieve good spatial resolution. (Light moves about 1 foot in a nanosecond, so temporally resolving the reflections of pulses off a surface can give detailed three-dimensional information if the pulses are short enough.)
 

Output Power and Energy

The laser beam contains energy in the form of electromagnetic radiation that travels at the speed of light and has no mass. CW output is usually characterized by the power in the beam measured in Watts, while pulsed output is characterized by the energy in each pulse, in Joules. Repetitively pulsed systems are also characterized by their average power. The range of output power from useful CW lasers ranges from milliwatts to hundreds of kilowatts. Megawatt lasers are feasible but pose unique challenges when scaling the output power to that level. The MIRACL laser system is a megawatt-class laser at the High Energy Laser System Test Facility on White Sands Missile Range that is routinely used for damage testing and other studies. Commercial users can even buy time on this device for their own testing.²⁷

All of the laser power is concentrated in a small solid angle due to the narrow beam. This means that even small lasers, like the helium neon (He-Ne) lasers frequently used as pointers, have output beams that are brighter than the sun. Here the term brightness is rigorously used to mean the amount of power being emitted per unit area of the source per solid angle.

A laser is classified as a severe hazard (denoted Class IV by federal regulations) if the output power exceeds 0.5 watts—quite a contrast with a 100-watt lightbulb that emits its energy in all directions. The reason is the high brightness of these lasers. Class IV lasers, the most dangerous category, are sufficiently intense that the direct beam is a hazard to both eyes and skin, causing nearly instantaneous injury. The beam could also ignite some objects that it strikes. (Several fatalities have been reported due to laser-generated fires.) The Class IV laser is so dangerous that even the diffuse reflection of the beam from a wall might cause eye injury. Stringent control measures are required for the use of these lasers, even though the output power may appear to be low.

To give a feeling for these numbers, the damage threshold for “soft” targets like paper or skin is roughly one Joule per square centimeter, assuming a one-second exposure. Wood surfaces are damaged at approximately 10 J/cm2 while metal surfaces are damaged in the range of 100 J/cm2.²⁸ Note that these numbers are just order of magnitude values to illustrate the concentrated power of the laser because actual damage thresholds depend on a number of factors. For example, the New World Vistas study by the AF Scientific Advisory Board cited a damage threshold for Scud missiles as roughly 1,000 J/cm2 as a first-order design parameter.²⁹

Pulsed lasers emit light in short bursts. The energy per pulse for useful lasers begins in the millijoule range and reaches into kilojoules. A typical laser designator, like the LANTIRN system used by the AF, emits very short pulses (of about 10 nanoseconds in duration) that contain over 100 millijoules of energy. Assuming a square pulse, these pulses have a peak power of 10 megawatts (dividing 100 millijoules by 10 nanoseconds). It is the high peak powers in pulsed lasers that can be exploited for a variety of applications. If the output of even the modest LANTIRN is focused in the air, it can be sufficiently intense to ionize the air in a phenomenon known as air breakdown.
 

“Deep Magazine”

The amount of power or energy depends strongly on the application. For communication systems, the use of sensitive detectors can permit the use of low energy pulses and lasers with average powers of a few watts. The operational lifetime of such systems could easily be decades. For weapons applications, megawatts or tens of kilojoules can be required to achieve structural damage on distant targets. By carrying an adequate fuel supply or by using energy from the sun either directly or indirectly to power the laser, many firings of the high-energy laser can be possible, thus giving a “deep magazine” when compared to conventional systems. Estimating the energy requirements is one of the key assessments of the laser concepts to be discussed later in order to appraise the feasibility of the concept.
 

Adjustable “Power on Target”

The ability to adjust the energy output of a laser system is also an exploitable characteristic.³⁰ A single, properly designed system could be used in a passive mode (no output beam) for surveillance, in a low power mode for target illumination for active imaging and for target designation for PGM delivery, and in a high power mode for negating a target.³¹
 

No Recoil from Light Beams

One unique aspect of directed energy beams like lasers is that there is no recoil in the weapon due to the output beam, as there is with kinetic energy weapons. This means that there is negligible effect on the orbital parameters in even a HEL due to the beam. However, there can be substantial thrust generated by the exhaust gases from a high-energy, chemical laser as it creates the output beam. The SBL systems, such as the Alpha laser, are designed to reduce the impact on the orbit of the space-based laser.


Speed of Light Delivery

Laser beams travel at about 300,000 km per second, or about Mach 1,000,000.³² This permits near-instantaneous engagement of targets, even for targets at very long distances. This greatly reduces the need to lead the target as compared to kinetic energy weapons like missiles. (There may still a need for a significant point-ahead angle if the relative velocities between the laser system and the target is large, as in the case of pointing a laser at a satellite from a ground-based laser.) If the target can be tracked visually, the laser beam can be placed on the target and, if sufficient energy is delivered, the desired effect can be achieved. This exploitable characteristic is particularly useful for operations in which time is critical or the engagement range is extremely long.
 

Freedom from Newtonian Constraints

Conventional weapons rely on kinetic energy in the form of a high-speed impact or chemical energy in the form of explosives to attack targets. These weapons are subject to the Newtonian laws of physics, such as gravitational attraction and aerodynamic forces. Gravitational attraction and aerodynamic forces, such as crosswinds, complicate the targeting by requiring trajectory considerations during the weapon delivery.

Aerodynamic forces such as drag and lift affect how much range can be achieved from the weapon. The laser beam is not significantly affected by gravity unless the propagation path is extremely long where bending of light in gravitational fields is significant. Aerodynamic forces do not slow the beam, although the atmosphere can scatter and absorb the optical radiation as discussed below. Thus, the laser is free from the usual constraints of Newtonian physics.
 

“Ilities”

One topic of constant concern for system developers is the “ilities” such as reliability, maintainability and affordability. This is particularly important for space systems because access for repairs is almost non-existent. The lengthy series of tests to “space qualify” hardware is one of the factors that drive up the cost of space systems. Some types of lasers are very reliable, relatively free of maintenance requirements, and have a long service life. For example, semiconductor diode lasers are now able to operate for tens of thousands of hours.

The fact that every compact disk player contains three diode lasers is quiet testimony to the reliability and affordability of these types of lasers. Semiconductor lasers are a leading candidate for some of the space-based applications such as communication. Also, all-solid-state laser systems are now being marketed commercially, such as diode-pumped Nd:YAG lasers, that are very rugged and reliable, require little maintenance, and have a long service life while generating substantial output power. Replacing the flashlamps with laser diodes was a key change to this advancement. Such lasers would be appropriate for applications such as space-based target designation and remote sensing.

Other systems, such as some closed-cycle gas lasers that can be scaled to fairly high powers, can be operated continuously in the laboratory for thousands of hours. These lasers have potential for laser illuminators, active imagers, and possibly even weapons. A final point related to affordability derives from the much shorter wavelength of lasers as compared to microwave systems. This means that the size and weight of the spacecraft can probably be reduced as compared to microwave systems that perform analogous functions, such as imaging or communication.
 

Challenges

The laser has a number of unique characteristics that can exploited for military applications, but there are challenges and inherent limitations. These limitations can be overcome or ameliorated by proper design. The challenge to understanding laser devices is to consider that the entire system, which consists of the laser device and its supporting systems on a space-based platform, its beam through space or air to impact on a target of some sort, and its ability to influence that target in some way. It is essential to consider the entire laser system, rather than just the laser itself, in order to identify the limiting technical challenges. In some concepts, it may be generating sufficient power from the laser device, as in the case of space-based laser weapons for national missile defense. In other cases, the prevailing challenge may be the acquisition, pointing, and tracking (APT) system or propagating the beam through the atmosphere, as in laser communication systems.

In still others, the challenge may be in gathering weakly scattered light from the target in order to gather the desired information, as in the case of direct wind measurement or remote sensing of effluents. Finally, the nature of the target may be the limiting constraint, as each target has its characteristic reflection and absorption properties. In assessing any concept, the entire end-to-end system needs to be considered.

Efficiency.

The process of generating the highly coherent laser beam is usually very inefficient.* The Nd:YAG laser is only about one percent efficient, while the popular helium-neon laser is only about 0.001 percent efficient. The unique features of the output beam make these inefficiencies bearable. Fortuitously, semiconductor lasers, which generate light by direct conversion of electrical current to photons, are very efficient, achieving 20 to 50 percent efficiencies. At present, these systems do not produce the power levels necessary for high-energy laser weapons.

But a laser, such as the carbon dioxide laser (used in the Airborne Laser Laboratory (ALL) to shoot down several Sidewinder missiles), has an efficiency on the order of 20 to 30 percent, which can achieve output powers of hundreds of kilowatts. The chemical efficiency of hydrogen fluoride (HF) lasers, being considered for space-based laser weapons, can be up to 20 percent or more, while the electrical efficiencies can exceed 150 percent, because the energy in the beam comes from a highly exothermic chemical reaction.³³

It is critical to determine where the remaining energy goes, which inevitably ends up as waste heat and must be removed from the laser system. In some lasers, like the HF laser, the exhaust gases carry away the heat. In other lasers, such as the Nd:YAG or semiconductor laser, some method must be used to extract the heat from the laser, such as flowing cooled water within the laser. If it is allowed to remain in the laser, the performance of the laser is likely to be degraded or, in the extreme, the laser may be damaged. Dissipating heat in a spacecraft can pose serious problems.

Refueling.

Some types of lasers consume fuel while they are operating. For example, hydrogen fluoride lasers need hydrogen, fluorine, and carrier gases such as helium. The designs usually include enough consumables for the expected number of engagements. However, as scenarios shift and requirements change, the capability to refuel these types of laser systems through a space transportation system would be very useful, although quite expensive.

The difficulty in refueling certain types of lasers makes them less attractive. It also increases the desirability of other lasers that obtain their energy from electricity or directly from the sun. Semiconductor lasers, Nd:YAG lasers, and tunable lasers, such as the titanium sapphire laser, can be efficiently operated using electricity. An atomic bromine laser, operating at 2.714 microns, has the potential of being directly pumped by sunlight without expending any fuel. This laser is currently under investigation at the AF Phillips Laboratory.

Target Acquisition.

The disadvantage of the small divergence of the laser beam is the challenge of accurately pointing it at the desired target, whether that target is a communications satellite or a nuclear warhead. The basic issue is acquiring the target in order to know where to point the beam. (This is part of the acquisition, pointing, and tracking (APT) process that is inherent in many potential laser applications.) For cooperative targets, such as a satellite in a communications network, a corner cube reflector could be used that reflects any incident light directly back at the source.

By using a laser beam that has a larger divergence, essentially a laser floodlight, a wider region of space can be scanned until the retroreflection is sensed and the narrow beam pointed at the target. For uncooperative targets, other methods of acquiring the target need to be considered, such as passive optical sensors and active microwave radar systems. Using the GPS system and recently-developed inertial reference units, space-based laser systems will be able to point at targets on or near the surface of the earth with very high accuracy. Non-mechanical beam steering, which involves tracking with no moving parts, is also being developed to permit high speed tracking of targets such as missiles or satellites.

Propagation into the Atmosphere. Some of the possible applications of space-based lasers involve aiming the laser back into the atmosphere. This includes such concepts as remote sensing of chemical effluents, measuring wind speeds, and negating enemy targets on the ground, on the sea, or in the air. However, the atmosphere attenuates many wavelengths, which greatly reduces the amount of energy that can be put on the target. For example, the leading candidate for a space-based laser weapon is the TRW’s hydrogen fluoride Alpha laser, which lases at 2.7 microns, a wavelength that is strongly absorbed by water vapor in the air. Thus, this laser could only attack targets at or above 30,000 feet.³⁴

Visible wavelengths penetrate far deeper into the air, but another phenomenon that attenuates light is Rayleigh scattering that increases as the fourth power of frequency. This means that shorter wavelength beams will scatter more significantly than infrared beams, reducing the energy that reaches the target and increasing the size of the laser beam. The optimum wavelengths appear, at first order analysis, to be the near infrared regions of 0.7 to 1.4 microns. Fortunately, a large number of lasers exist in this region, although few that are scalable to high powers. For example, the laser for the Airborne Laser is a chemical oxygen-iodine laser (COIL) that operates at 1.315 microns in this preferred region.

Target Coupling.

Another challenge to using lasers in space is to determine the most efficient way to couple the laser energy onto the target. In order to cause an effect, the laser energy must be absorbed by the target. For cooperative targets, this means finding a suitable detector that has a good response to the particular wavelength. Detector materials are fairly mature and improving, so this is unlikely to limit laser applications. However, for uncooperative targets like warheads, the laser radiation is usually absorbed at or near the surface of the target. Some of the light will be reflected as well. Thus, the laser beam will have to work its way into the target in order to cause structural damage. For fairly “soft” targets like satellites, this may not be difficult—parts like solar panels and optical systems can be readily damaged. But burning through a reentry vehicle’s outer skin would be very difficult.

Laser Safety.

Laser beams can injure people, particularly if a person looks directly into the beam of a visible or near infrared laser with wavelengths from 400 to 1400 nm.³⁵ Even a millijoule entering the cornea can cause a catastrophic hemorrhage of the retina, because the optics of the eye increase the intensity of the laser beam by reducing the size of the beam by about a factor of 100,000 as the light travels from the cornea to the retina. Retinal injuries are permanent, leading to some loss of sight. Even Class IIIa lasers with output powers in the 1 to 5 milliwatt range can burn the retina. Thus, space-based lasers in this wavelength range could pose a safety hazard if pointed toward the earth.

Unfortunately, the term “eye-safe” has recently become popular when referring to lasers that emit wavelengths longer than 1400 nm, even if the output power or pulse energy puts the laser in Class IV as a severe hazard. The notion is that the retina would be safe and that the cornea might be injured but would heal. This ignores the reality that deep cornea burns are permanent, although corneal transplants are possible. Some people are even referring to deuterium fluoride HEL weapon prototypes (operating at 3.8 microns) and 50 watt carbon dioxide laser radars (operating at 10.6 microns) as “eye-safe” given the operating wavelength, even though this ignores the high power output. Such systems are certainly not eye-safe in any aspect, and the term sends the wrong message to those not well versed in laser safety.

Almost all the risks with lasers can be avoided by carefully controlling where the laser is pointing before it is activated. A thorough laser safety analysis should be included early in any laser program, particularly if the laser has new or unusual output characteristics, such as extremely short pulses with high peak powers. The biological database that is used to set safety standards may not be appropriate for some new types of lasers, and gathering new data to set new safety thresholds is time-consuming, challenging research. Thus, the program manager must assess the safety risk early in coordination with DOD laser safety experts.
 

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