VII. REVIEW AND SCORING OF CONCEPTS

In this section, we examine briefly each of the 28 concepts listed in Table 3 highlighting the operational concept, operational enhancement, key enabling technologies, and primary challenges. A detailed assessment is beyond the scope of this study and, in many concepts, references are given that more fully evaluate the technical and operational merits.

Based on the author’s evaluation as well as other sources, scores are listed at the end of the discussion of each concept. Also, at the end of this section, Table 5 summarizes all the scores, including the total scores. The order of the listing of the concepts is the same as in Table 3 for continuity.

The first two concepts offer special advantages to the operational commands, meriting a more comprehensive development in the following two sections, in part because they hold little commercial potential. Space-based laser communication systems also offer great near-term potential and are being developed by DOD, NASA, and industry. The space-based remote sensing is another highly scored concept that has been investigated by NASA.
 

Space-based Laser Target Designator

 

Operational Concept

As part of the force enhancement mission area, the space-based laser target designator (SB-LTD) directly extends the current LTD systems.53,54 A neodymium:YAG laser on-board a LEO satellite projects a beam onto a target on the earth’s surface in order to give an aim point for a laser-guided weapon. The choice of the Nd:YAG laser is predetermined because this is the source that is used for all US LGWs. The system would likely include a moderately high-resolution imaging system and a video data link to an operator because safety and positive control of deadly force require human oversight. The operator could be located anywhere, including CONUS, an AWACS aircraft, or a theater of operations. The satellite would have to have a clear view of the target area as it passes overhead, meaning that weather, smoke or other obscurants could defeat the LTD, which is a constraint shared with current LTDs.
 

Operational Enhancement

Moving the LTD platform out of the theater eliminates the risk to the LTD operator. The LGW could also be released at a long range, eliminating risk to that platform as well. The appropriate LGW would probably be a powered munition, such as a cruise missile rather than a gravity bomb. The recent misses of cruise missiles during the DESERT STRIKE attacks against Iraqi air defense sites might have been reduced if a SB-LTD had provided aim points for the appropriate PGM. It is conceivable that the target could even be mobile and still be designated for destruction. Even a few SB-LTDs give a significant enhancement to the current capability for limited strikes and shows of force.
 

Key Enabling Technologies

Clearly a sufficiently powerful laser is needed. However, the 1.064 micron wavelength of the Nd:YAG propagates with low loss through the atmosphere, and diode lasers have been successful in pumping the neodymium, offering the possibility of an all-solid-state, electrically powered laser source with adequate power. The output optics would have to be large enough to keep a small enough spot on the ground for accurate weapons delivery. Calculations discussed in a later section suggest that a one-meter diameter output optical system, possibly consisting of a cassegrainian telescope, could suffice. The same optical system could be used for imaging the target area by sharing the aperture and using a high-resolution CCD array. A microwave communication system would be needed to send that image to a controller. Acquiring the target requires highly accurate position information of the SB-LTD as well as foreknowledge of the target’s exact location. Given this information, computation could generate the required pointing vector to drive the imaging and laser systems. Pointing stability is critical, but could be satisfied by a recent advance in inertial reference units discussed in the next section.
 

Challenges

It would appear that most of the technology required for the SB-LTD is within reach. The primary challenges are operational. A large number of LEO satellites would be needed to give adequate coverage for sustained military operations. Opportunities of engagement need to be assessed with careful consideration of adverse weather obscuring the potential target areas. The cost of the individual satellites and the total system would be high.
 

Scoring

The concept was rated as part of the “virtual presence” paradigm by the NWV study as an interesting concept with high payoff, but they placed it in the long-term (30 year) development period, suggesting that they had concerns that the technology was not maturing soon.55 The Laser Mission Study also seemed to make a similar assessment when it placed the SB-LTD concept in the “keep in the database” category.56 There does not appear to be any fundamental principles that prohibit the concept from being developed, and the maturity of most of the technologies is high. The assessment of the two studies may not have considered the synergy of advances in pointing, imaging, diode-pumped lasers and high frequency microwave down-links. The acceptable cost of a small number of systems gives a unique capability for limited strikes.

Technical feasibility: 4. Technical maturity: 4. Operational enhancement: 5. Cost: 3. Total Score: 16
 

Battlefield Illuminator

Operational Concept

Offering potential for the force enhancement mission area, the concept is to project a laser beam over a large area on the earth’s surface to aid existing low light imaging systems in discerning targets.57,58 There are already fielded laser illuminators to aid night vision devices (NVD) using semiconductor laser arrays such as gallium-arsenide (GaAs) lasers operating around 830 nm. Forward-looking infrared (FLIR) systems could be augmented by using CO2 lasers operating at 10.6 microns. The illuminator could even be used to illuminate targets for infrared reconnaissance systems. The space-based battlefield illuminator (SB-BI) concept is a straightforward extension of the idea of using a flashlight to improve seeing in the dark, although, in this concept, the beam would be generated from a LEO satellite onto a target area. Because the illumination beam would likely pose no risk to ground personnel due to the large spot size, the illumination would not need a controller in the loop. The target location and the time of the illumination could be preprogrammed. Note that SB-LTD and the battlefield illuminator cannot share the same laser because the required wavelengths are different. The SB-LTD laser must operate at 1.064 microns because of LGW sensor requirements, while the SB-BI laser operates in the wavelength region of the imaging system that it is intended to enhance, which is typically the 800-900 nm region for NVDs and 8-12 microns for FLIR systems. A more detailed discussion of this concept is contained in Section IX.
 

Operational Enhancement

Applications include improved target acquisition from FLIR systems, augmented infiltration and exfiltration of special operations teams, enhanced landing under low light conditions, and increased effectiveness for night security of high value sites. Because the beam would likely be so large so that the energy density is at eye-safe levels, the illuminator could also be used for PSYOP missions. The friendly observer is less likely to be compromised because the illumination originates from a different location.
 

Key Enabling Technologies

A sufficiently powerful laser is required to provide enough illumination for the sensitivity of the fielded imaging systems. This levies a large requirement on the spacecraft power systems because of the inherent inefficiencies of lasers. (However, both semiconductor and carbon dioxide lasers are much more efficient than most lasers.) Highly accurate position information is required for both the satellite and the ground site to be illuminated to permit the pointing vector calculation for the laser beam. Highly precise pointing systems are required, but these are now available using advanced inertial reference units discussed in the next section. The output optical system needs to be large enough to permit the spot size on the earth to be as small as 100 meters in diameter. This should not be a limiting specification. It is possible the beam could be scanned over an area to expand the coverage, but this would significantly increase the system complexity.
 

Challenges

The principal challenge will be achieving the laser power requirements. Weather again will limit the opportunities for using the battlefield illuminator. By using more illumination satellites, limitations in coverage due to orbits could be reduced although at increased cost.
 

Scoring

There are no breakthroughs required, only engineering improvements in existing systems. A low number of systems could be deployed that would provide a significant capability for limited military operations. A moderate number of satellites would be required for larger scale theater operations or extended illumination of high value sites. The cost of the individual satellites would be moderately high due to the laser requirements.

Technical feasibility: 4. Technical maturity: 3. Operational enhancement: 5. Cost: 3. Total Score: 15.
 

Alignment and Docking (Guidance Systems)

Operational Concept

The low divergence of many lasers provides a very narrow, straight beam that can be used as a guidance beam for this space support concept. The construction industry already uses He-Ne lasers for surveying and for active control of blade height on road grading equipment. The Laser Mission Study lists the concept of using lasers as a “space reference grid” without further development.59 One realization of the concept could be to project three laser beams of different wavelengths from points around a docking port on a space station. By locating three optical detectors with wavelength filters at matching points on a space vehicle, the docking process could be automated. Using pulsed lasers, highly accurate range information could be obtained, further automating the docking process. Other approaches could use a laser homing beam and guidance system like that used on laser-guided munitions as they zero in on their target.
 

Operational Enhancement

By exploiting the pencil-like beam properties, the pulsed operation, and the monochromatic nature of the laser, autonomous guidance systems could be developed that greatly reduce the need for manpower-intensive, risky docking maneuvers. Refueling or re-supply of satellites would be more feasible with these types of guidance systems.
 

Key Enabling Technologies

Highly accurate position information would be required to get the two space vehicles relatively close. GPS signals could provide this capability if the spacecraft are able to receive the signals. This is not a major issue for LEO systems, but would be for MEO and GEO satellites. The laser power requirements could be met with current diode laser technology and the same semiconductor technology provides suitable detectors. The principal challenge would be the control algorithms to maneuver the vehicle along the guidance beams. Some of these schemes have already been worked out for terrestrial systems and could be readily adapted for spacecraft use.
 

Challenges

Initial acquisition of the guidance or homing beams may be an issue, although a low power radio-frequency beacon could be used for the coarse acquisition.
 

Scoring

Most of the components of this concept exist but have not been assembled in a space-qualified package. The guidance concept could be tested with airborne or underwater systems. The cost of the system should be low because of the availability of efficient semiconductor lasers. However, the operational enhancement is not overwhelming.

Technical feasibility: 5. Technical maturity: 4. Operational enhancement: 2. Cost: 5. Total Score: 16.
 

Deep Space Laser Altimeter

Operational Concept

This concept falls in the space support mission area of aiding on-orbit support. By sending laser pulses at an object with a reasonable reflectivity and then measuring the time between the transmission of a laser pulse and the time when the reflection is received back at the laser system, the distance from the object can be calculated with very high accuracy. The time-of-flight concept is the heart of the currently fielded military laser range-finders. NASA has developed a deep-space laser altimeter based on this concept for use on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft.60 The NASA system should provide 2-meter accuracy. The package only weighs about 5 kilograms and consumes 15 Watts. The instrument should have an effective range of over 300 km. A useful instrument in itself, it also demonstrates that laser-based instruments are effective enabling systems in general.
 

Operational Enhancement

Accurate distance information is important for obstacle avoidance, determining orbital altitude, docking maneuvers, and landing operations. While military spacecraft are not expected to make interplanetary missions in the foreseeable future, there may be many applications for operations near the earth. Having a small, affordable, space-qualified package such as NASA has developed is a capability worth having “on the shelf” for future military spacecraft.

 

Scoring

The system already exists on the NEAR mission to a small asteroid. The immediate operational utility is low, but the concept of laser-based spacecraft instrumentation is important.

Technical feasibility: 5. Technical maturity: 5. Operational enhancement: 2. Cost: 4. Total Score: 16.
 

Satellite-to-Satellite Doppler Velocimeter

Operational Concept

Another laser-based spacecraft instrument is the Doppler velocimeter, aiding on-orbit operations as part of space support. The concept was considered in the Laser Mission Study but not ranked highly.61 The concept relies on the narrow linewidth of the laser and the fact that EM radiation that reflects off a moving object undergoes a shift in frequency (or wavelength) that is proportional to the velocity. The small beam divergence permits precise pointing at the object of interest, which implies that highly accurate measurements of relative speeds are possible.
 

Operational Enhancement

Measuring closure rates is important to automated docking maneuvers. It is also an important measurement for determining the absolute speed of other satellites, assuming the speed of the laser platform is known. The absolute speed is a part of the data that allows prediction of where the satellite will be in the future, which is an important part of the AF’s space track system.
 

Key Enabling Technologies

Ultra-stabilized laser systems provide the extremely narrow linewidth for the Doppler measurement. Suitable homodyne or heterodyne optical receivers are used to measure the Doppler shift and compute the relative speed. Accurate pointing systems based on inertial reference units and possibly GPS measurements permit placing the laser beam on the other satellite.
 

Challenges

Qualifying the existing laser Doppler measurement systems for space is the principal challenge as well as modifying these systems for the expected operational environment.
 

Scoring

Most of the components exist and the concept is well understood. The cost should be relatively low. However, the near-term military utility is probably low, as indicated by the ranking in the Laser Mission Study.

Technical feasibility: 4. Technical maturity: 4. Operational enhancement: 2. Cost: 5. Total Score: 15.
 

SPACE-BASED LASER REMOTE SENSING CONCEPTS
Remote sensing is a fairly mature technology area used for many applications.⁶²

Space-based remote sensing, as part of the force enhancement mission area, has primarily used passive multi-spectral imaging to obtain information about terrestrial and near-surface locations. The false-color images taken from Landsat are a good example of using remote sensing for assessing crop and soil conditions on a global scale. The amount of data is substantial: 200 to 300 megabytes is required to store the digital data from one scene obtained with the 30 meter resolution thematic mapper on Landsat.⁶³

Thus, the value of remote sensing is just coming into its own as computer hardware and software are developed to manipulate the massive amount of data in a timely manner. Active remote sensing using synthetic aperture radar is being developed in order to get around weather limitations in imaging systems.⁶⁴ Radar penetrates light rain, haze, clouds, some tree canopies, and even the ground to shallow depths under the right circumstances. Lasers can also be used to gather information for remote sensing, with obvious military applications.⁶⁵ The new trend is to use lasers from space to gather information,⁶⁶ as the next three concepts illustrate.

Active remote sensing can use lasers to gather information about remote locations by projecting a laser beam onto the target site and then gathering the weakly back-scattered or reflected light. The amplitude, polarization, and frequency of the back-scattered light can all be used to measure properties at the remote location. The AF Phillips Laboratory Lasers and Imaging Directorate has expertise in the area of multi-spectral and hyper-spectral imaging for remote sensing, and is now pursuing some of the active sensing concepts described below, such as measuring wind speeds from orbit.

One approach, the differential absorption LIDAR (abbreviated as DIAL) system, sends two laser beams of different wavelengths through a region of air and looks for differences in absorption in the transmitted or back-scattered beams. Assuming the right wavelengths are used, DIAL systems can detect a wide variety of chemical compounds in the air. Some of the current DIAL systems are used to test for pollution. As shown in Figure 3, NASA has recently orbited a DIAL system, called the “Laser In-space Technology Experiment,” or LITE, in Space Shuttle Mission STS-64 to test the concept.67

The experiment used Nd:YAG lasers with nonlinear optical crystals to provide output energies of 500 mJ for the fundamental (1064 nm) and frequency-doubled (532 nm) beams and 160 mJ for the frequency-tripled output operating at 355 nm. The laser generated short, Q-switched pulses at a prf of 10 Hz. A one meter telescope collected the back-scattered light, using photomultiplier detectors for the 355 nm and 532 nm returns and a silicon avalanche photodiode to detect the 1064 nm light. The LITE package successfully probed the atmosphere over Los Angeles to determine effluent levels.68 It also measured the properties of clouds and aerosols in the stratosphere and troposphere.

The important point about the LITE experiment for this paper is that the technology currently exists and was successfully demonstrated in a space environment. The resolution and timeliness would not meet current military requirements, but the concept has moved to the engineering stage.

Thus, the AF should aggressively pursue space-based laser remote sensing to provide new, highly useful information to the operator.

Figure 3. LITE As An Example of Space-Based LIDAR69
Remote Sensing for Battle Damage Assessment (BDA)

Operational Concept

The application for BDA would be to probe the atmosphere near a target site after it has been attacked to determine the effluents coming from the site.70,71,72 In particular, chemical warfare agents, propellants, and other militarily-related compounds would be of greatest interest. Sensing biological agents is more difficult but conceivable. For the space-based concept, the DIAL system would be located on a LEO satellite that would have to be overhead just after the attack in order to be effective.
 

Operational Enhancement

Assessing the effectiveness of attacks is paramount for military operations in order to assign follow-on missions and to determine enemy capabilities. Also, in an era when chemical and biological warfare (CBW) agents are increasingly likely to be stored in bunkers that are attacked, being able to quickly determine if such agents have been released into the air is a highly useful technology.
 

Key Enabling Technologies

The proper laser wavelengths must be available, and the rapid emergence of tunable solid state lasers (as discussed in Section 3) that can be pumped by diode lasers should provide reliable sources of adequate power. Because the back-scattered signal is likely to be weak, a large optical receiver with highly-sensitive detectors would be required along with relatively powerful laser sources.73 Rapid beam steering is needed to interrogate a large region of air as the satellite moves overhead. Advances in non-mechanical beam steering offer potential solutions.
 

Challenges

The receiver system is the most challenging technical issue. Having an adequate number of satellites to cover numerous target sites during an extended period of attack drives up the cost of a useful system, although even limited capability would be militarily useful given the high risk involved in unintentional dispersal of CBW agents. Clouds and haze will reduce the effectiveness of this concept.
 

Scoring

The Laser Mission Study team ranked remote sensing as a concept of high interest to the operational community. The feasibility and maturity of remote sensing is high with proven demonstration of a space-based DIAL system. However, finding the right wavelengths for substances of military interest may be a challenge.

Technical feasibility: 4. Technical maturity: 4. Operational enhancement: 5. Cost: 3. Total Score: 16.
 

Environmental Remote Sensing

Operational Concept

This concept is a generalization of the remote sensing concept just discussed. Here, the space-based laser is used to interrogate a region of space or a target in order to determine some of its physical properties. A space-based laser system can help the Measurement and Signature Intelligence (MASINT) mission by measuring target properties. For example, the smoothness of the surface affects the polarization of the reflected beam and might provide a way of aiding target recognition.⁷⁴ The degree of reflectivity from a soil surface can estimate the moisture content, giving advanced warning of soft soil for the movement of heavy equipment such as tanks as well as troop movement.⁷⁵ For some of these concepts, a single frequency laser that can be polarized could suffice for some of the applications, avoiding the complexity of DIAL systems. However, tunable lasers would substantially extend the capabilities of this concept. The beam would be scanned over the target region, while a sensitive optical receiver processed the scattered laser light.
 

Operational Enhancement

The ability to gather greater information about a landing zone before inserting troops or equipment substantially increases global awareness. Searching a large area for distinctive signatures provides a new dimension to intelligence. Both of these applications, as well as other similar ideas, can be realized with a space-based remote sensing system.
 

Key Enabling Technologies

See the preceding concept for a discussion of critical technologies.
 

Challenges

See the preceding concept for some of the challenges of remote sensing. Here, specific wavelengths might be required for certain targets and suitable lasers of high enough power need to be developed.
 

Scoring

The technical challenge of target discrimination is greater than the effluent sensing of the preceding concept, lowering the technical scores. Also, the increased complexity of the system increases cost.

Technical feasibility: 4. Technical maturity: 3. Operational enhancement: 4. Cost: 3. Total Score: 14.
 

Weather Monitoring and Characterization

Operational Concept

This concept is another specialized application of the remote sensing discussed earlier for BDA. In this case, the space-based laser is used to measure meteorological parameters such as wind speed, cloud density, the height of cloud tops, water vapor content, temperature, and pressure.^76,77,78,79 Measuring time-of-flight for short laser pulses can measure cloud top heights and infer potential storms. Using a time-gated heterodyne optical receiver, the space-based weather monitoring system can measure Doppler shifts from a specific region of the atmosphere and thus compute the wind speed. The two examples highlight the potential of this concept. Currently, these measurements are measured at fixed points on the earth’s surface, measured by weather balloons, or calculated by indirect, passive measurements from satellites.

Operational Enhancement.

Direct measurement of winds over a target region can substantially improve the accuracy of unguided bombs, the insertion of airborne troops, and the operation of aircraft on unimproved airfields. More accurate measurement of atmospheric parameters extends the time interval for accurate weather prediction, which clearly improves military operations.

Key Enabling Technologies.

As already discussed, sufficient laser power and a highly sensitive optical receiver are required as well as accurate pointing and scanning systems. Selection of the proper wavelengths to optimally sense the various atmospheric parameters is a key technological investigation which could be met with tunable, infrared, solid-state lasers. On-board processing can fuse the large amount of collected data into information to reduce the communications bandwidth required. The weather monitoring and characterization systems could be sequentially developed, with direct measurement of winds being the most valuable initial objective due to the many military requirements for this data.

Challenges.

Tunable lasers with moderate energy, such as the titanium:sapphire laser, would need to be developed and qualified for space operations. Also, fixed wavelength lasers such as holmium:YAG or thulium:YAG lasers are nearly ideal for sensing water vapor but need to be scaled to higher powers. Range-gated heterodyne optical receivers with high gain collecting telescopes are required. On-board, high-speed computers for data fusion would greatly enhance the system’s utility. Other challenges for remote sensing systems were discussed in the previous two concepts.

Scoring.

This concept was ranked as having “definite interest” by the Laser Mission Study. Substantial development has been done for near-earth wind sensing as well as ground-based LIDAR measurements for meteorology. However, the hardware for a complete space-based system is complex and requires some extensive engineering development. Even a relatively small number of satellites could give significant capability.

Technical feasibility: 4. Technical maturity: 3. Operational enhancement: 5. Cost: 3. Total Score: 15.
 

Space Debris Cataloging

Operational Concept.

Space debris is an increasing problem due to the ever-growing number of defunct satellites, fragmented spacecraft, and spent rocket boosters. According to the New World Vistas study, there are about 300,000 pieces of debris, many in the LEO region.⁸⁰ Natural debris such as small meteorites and dust also orbit the earth. The potentially high relative velocity of the debris makes the impact of even small debris on orbiting systems very serious. Using its globally distributed Space Surveillance Network (SSN), the Air Force maintains an extensive catalog of space objects that includes debris. However, ground-based radar and optical systems can only measure objects larger than about 10 centimeters.

 

A concept that received high ranking by the Laser Mission Study team and one that aids space control role via the space surveillance mission is to catalogue space debris with space-based laser surveillance systems that locate, track, and potentially identify a much greater amount of debris. This includes smaller objects in the 1 to 10 cm range that pose a high risk.⁸¹ The concept could use one laser with a large beam divergence to obtain an optical reflection from the debris and a second, pulsed laser with small beam divergence (operating as a Doppler LIDAR) to measure the position and velocity of the debris. By varying the wavelength of the LIDAR, it might be possible to determine the composition of the debris or at least determine if it is natural or man-made debris. This information can be useful in removing the debris, a concept to be considered later.

Operational Enhancement.

A more extensive knowledge of the debris field can be used to reduce the risk to operational spacecraft which have some maneuvering capability. The database can also be used to select orbits for new satellites that minimize the probability of collision.

Key Enabling Technologies.

A relatively low number of satellites would be required if they could be designed for autonomous searching of different regions of the near-earth environment. A substantial on-board computer capability would be needed to process all the possible hits. The laser systems would likely consist of low power devices given the loss-free environment of space, but the optical receivers need to be very sensitive because the amount of reflected light from a small piece of debris would be low. Scanning systems would be needed to accurately point the laser beams. Because the search satellites might illuminate reconnaissance and early-warning satellites during its scans, the wavelengths for the lasers would need to be chosen to prevent damage to the sensitive optical receivers.

Challenges.

The principal challenge will be in developing a sufficiently sensitive optical receiver in a reasonably small and affordable package. Recent advances in inertial pointing systems and non-mechanical beam steering should be exploited to develop a space-based “space debris cataloging” system.

Scoring.

The operational need is high, and the technology should be attainable with a concerted engineering push, but the cost of a network of search satellites is likely substantial.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 4. Cost: 2. Total Score: 12.
 

Integrated Tactical Warning and Attack Assessment

Operational Concept.

The Integrated Tactical Warning and Attack Assessment (ITW/AA) concept links the data from multiple sensor systems to provide a rapid, near-real-time (NRT) determination of a launch of a theater missile launch and an accurate estimate of its trajectory and impact point. A potentially valuable part of the force enhancement mission area, the concept is listed in the Laser Mission Study, but it was not a high priority item and no details were given to suggest how the team conceived of using space-based lasers to augment the ITW/AA systems.

 

However, as mentioned in the environmental remote sensing concept, a space-based laser radar could be used for automatic target recognition to identify transporter erector launchers (TEL) before the launch occurs and a SB-LTD could guide stand-off munitions onto the target. Space-based battlefield illuminators could also be used to increase the likelihood of detecting and tracking the missiles after they are launched as well as the warheads after they are deployed. Thus, the ITW/AA concept would combine a number of other space-based laser concepts.

Operational Enhancement.

Space-based laser systems could provide improved detection of the threat and enhanced likelihood of negating the targets before launch or in the boost phase.

Key Enabling Technologies.

See earlier discussion of the SB-LTD, SB-BI, and environmental remote sensing systems.

Challenges.

See earlier discussion of the SB-LTD, SB-BI, and environmental remote sensing systems.

Scoring.

The need to field multiple systems of the different types in order to provide adequate coverage increases the risk and complexity of this concept. The LMS ranking placed this concept of higher technical risk and lower operational relevance.

Technical feasibility: 2. Technical maturity: 2. Operational enhancement: 3. Cost: 2. Total Score: 9.
 

Active Illuminator/Imager for Space Surveillance

Operational Concept.

Ground-based active imaging of space objects is a rapidly maturing technology at the AF’s Starfire Optical Range (SOR) in New Mexico.^82,83 These experiments have developed laser imaging in an environment that requires adaptive optics to reduce atmospheric distortions. If the same imaging technology was placed on a spacecraft, the atmospheric effects are eliminated. If the range to the target can be reduced by manipulating orbital parameters, active imaging could be exploited to give three-dimensional images of the target satellite, its location and velocity data, and (possibly) MASINT data on the composition of the vehicle by assessing the reflectivity of the various surfaces. This concept supports space surveillance and the space control mission area.

Operational Enhancement.

The space-based active imaging system would enhance the current space surveillance database, particularly in the area of space object identification. This increased awareness is an essential ingredient of effective space control.

Key Enabling Technologies.

Assuming the Starfire imaging technology can be utilized, the new requirements include highly accurate pointing systems that rely on recent developments in inertial reference units. The concept would also rely on active target acquisition systems using either microwave radar or large-divergence optical beams in order to provide coarse target location. Fine tracking would be accomplished through the high resolution imaging receiver, which is itself a stressing technology.

Challenges.

The major challenges involve pushing the engineering limits of APT and imaging systems. The laser power could be kept reasonably low because of the loss-free environment of space.

Scoring.

The packaging of the complex system makes this concept immature, but the underlying technology has been demonstrated by the SOR system.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 3. Cost: 3. Total Score: 12.
 

Sensor Pointing Accuracy Beacon Network

Operational Concept.

This is another concept identified in the LMS as a low priority concept but with no additional detail. However, it would likely aid force support for on-orbit operations or space control by improving space surveillance. Thus, it is not possible to determine exactly what the study meant, but the title suggests using very low divergence laser beams broadcast from satellites of known position as a calibration system. By fixing the beams on the satellite to aim at the nadir (the point on the earth’s surface that is directly below the satellite on a line that connects the satellite to the center of the earth), a ground optical sensor that intercepts the beam can use it as a reference to determine precisely where the sensor is pointing.

Operational Enhancement.

Improvement in the pointing accuracy of ground sensors that are part of the Space Surveillance Network would generate more accurate position and velocity data on the numerous space objects in the database. However, other methods can be used, such as celestial objects and GPS, which makes the concept redundant.

Key Enabling Technologies.

Relatively low-power lasers coupled with highly accurate pointing systems and highly stabilized alignment platforms would be required. Also, very accurate knowledge of where the satellite is located would be required; this data could be obtained from GPS for LEO satellites.

Challenges. The technical challenges appear to be small. However, the cost of deploying and operating a dedicated network of calibration satellites seems excessive given the added benefit. If the packages could be built as low weight, autonomous add-ons for other satellites, the concept would be more viable.

Technical feasibility: 4. Technical maturity: 3. Operational enhancement: 2. Cost: 2. Total Score: 11.
 

Satellite Traffic Management/IFF

Operational Concept.

This is another concept identified in the Laser Mission Study as a low priority concept with no additional detail. It could aid either space control or space support mission areas. The concept of “identification-friend-or-foe” is a familiar one, and usually operates as a transponder that transmits identifying information. Applying this concept to spacecraft, a low power, large-divergence diode laser could be pulse-code-modulated with the identity of the spacecraft and other relevant information.

 

Proper optical systems could make this transponder cover 360 degrees around the satellite. Power requirements would be modest due to the high efficiency of semiconductor lasers. An alternative concept would be to use a passive system that consists of a retro-reflector such as a corner cube or a “cat’s eye” that is modulated (vibrated) so that it sends an encoded reflection back to any laser that illuminates it.

Operational Enhancement.

As the near-earth space environment becomes more crowded and as spacecraft become more maneuverable, some sort of traffic management system similar to the current air traffic control system will likely be required. However, radio-frequency systems that already exist serve the same purpose, which suggests that the advantage of the laser system over these RF systems is not evident.

Key Enabling Technologies.

High efficiency semiconductor lasers with PCM circuitry to send out the encoded signal as well as the broad-coverage output optical system would be needed.

Challenges.

There are only a few pressing technical challenges, but the key challenge is establishing operational usefulness.

Technical feasibility: 4. Technical maturity: 1. Operational enhancement: 1. Cost: 5. Total Score: 11.
 

Laser Communications and Data Relay

Operational Concept.

Transmitting information over a laser beam is a well-established concept, both using optical fiber and free-space pathways. Commercial fiber optic communication networks already handle tremendous volume for terrestrial telecommunications including voice, video, and data transmission. Free-space links are generally designed for short paths in areas where other communication networks are impractical.⁸⁴ There is a variety of methods for modulating the laser beam, including pulse code modulation (PCM), amplitude modulation (AM), frequency modulation (FM), and polarization modulation.

 

Semiconductor diode lasers are ideally suited to this application because they operate by direct electrical-to-optical conversion, are highly efficient, and can be modulated at extremely high rates. Digital methods, like PCM, offer the best data rates with lowest error rates, but not all lasers can be modulated in this manner. Free-space links can also take advantage of the “wavelength division multiplexing” (WDM) concept used in fiber optical systems where different wavelengths are used as discrete channels for carrying information.

As a space support and force enhancement concept, space-based laser communication has been proposed by the LMS, NWV, and AF2025 studies.⁸⁵ This concept also is actively being pursued by the AF Phillips and Rome laboratories as well as by NASA and the commercial sector. ⁸⁶ The laser communication links can exist between satellites or between earth stations and the satellites, as shown in Figure 4. Different transmitter-receiver schemes and laser wavelengths may be better suited to the different type of links.
 

Figure 4. Schematic of Space-Based Optical Communication

McDonnell Douglas has developed technologies for space-to-space laser links for the Defense Support Program early warning satellites and a space-to-ocean link to communicate with submerged submarines.⁸⁷ The space-to-earth link could readily be aimed at an unmanned aerial vehicle (UAV) or C2 aircraft, such as the E-3A AWACS.

The Ballistic Missile Defense Organization (BMDO) is also active in developing space-based laser communications. It is funding efforts that would provide high-data-rate links from space capable of transferring 1.24 Gbps at distances up to 1800 km. This satellite laser communications package is scheduled to fly on the Space Technology Research Vehicle in 1998.88 Thus, space-based laser communication systems are maturing rapidly.

Operational Enhancement.

The ability to relay massive amounts of information is becoming a critical determinant of military success. The concept of global awareness, which relies on having dominant battlespace knowledge, is possible only if a steady stream of information reaches the operators, from the platoon leader in the field to the Joint Force Commander in theater and the military support staffs in CONUS.

Laser communications offer a number of advantages over radio-frequency and microwave systems.

• First, the potential data rate is much higher because of the carrier frequency is much higher. The NWV study states that the data rate could exceed 10 billion bits per second (Gbps).⁸⁹

• Second, the link can be more secure given the narrow laser beam. The implication is that someone would need to get inside the beam to intercept the message, interrupt the beam to the intended receiver, and thus signal that an intrusion had occurred. This fact also makes it very hard to jam laser communication links.

• Third, if the carrier wavelength operates outside the visible region, the communication would invisible, and thus covert, unless electro-optical systems are used to search for the beam. These last two advantages are captured in the notion of a Low Probability of Intercept/Low Probability of Detection (LPI/LPD) communication system.

• Fourth, because of the linearity of the atmosphere for low peak-power laser beams, there is no “cross-talk” between laser beams, reducing this source of signal degradation.

• Fifth, the laser communication system will be much smaller and lighter than comparable radio-based systems because the output “antenna” is much smaller, the laser is much smaller than microwave generators, and the power consumption is lower. These factors produce substantial savings in payload launch costs.

 

Key Enabling Technologies.

Acquisition of the receiving satellite in order to establish the communication link is a challenge in view of the small divergence of the laser beam. A higher divergence diode laser and a corner cube on the receiving satellite could solve this problem. Non-mechanical beam steering is desired to minimize spacecraft jitter. This technology is maturing rapidly for diode laser arrays, and sensitive detectors are required for receivers. Lasers with useful wavelengths, adequate output power, and the ability to modulate the output at the desired data rates are a key technology area that is developing rapidly without DOD involvement given interest in the commercial sector.

Challenges.

The biggest challenge for space-to-space links appears to be the acquisition, pointing, and tracking problem. For earth-to-space links, the biggest problem is attenuation due to clouds, haze, or other atmospheric effects. For this reason, the AF2025 study concluded: “Laser communication systems are a poor choice for communications between satellites and individual earth stations.”⁹⁰ However, they noted that the use of spatially distributed ground stations that are linked by very high-speed communication lines could ameliorate this problem. Numerous minor engineering challenges remain in all sub-systems but steady progress is being made.

Scoring.

This is one of the most promising space-based laser concepts and should be aggressively pursued by AF research laboratories and operational commands. The cost is likely to be high because it is an entirely new communication system.

Technical feasibility: 5. Technical maturity: 5. Operational enhancement: 5. Cost: 2. Total Score: 17.
 

Space Track Accuracy Improvement

Operational Concept.

The concept of supporting space control through improved space surveillance was ranked by the LMS team as one of its top seven concepts, which was the subject of a mini-study to more fully develop the concept.⁹¹ As described in LMS, the laser system would be a ground-based laser configured as a LIDAR to make highly precise measurements of satellite position and velocity. These state vectors would update the ephemeris information in the AFSPC Space Catalog database. An alternative configuration would be to use space-based lasers as described above for tracking space debris. The same data would be collected, with improved capability to measure the orbital characteristics of MEO and GEO satellites.

Operational Enhancement.

A relatively small number of satellites could provide enough information over an extended period to augment the Space Surveillance Network. The improved ephemeris data reduces the risk of accidental collision, and permits better traffic management in determining orbits for new spacecraft.

Key Enabling Technologies.

Moderately powerful, pulsed lasers are needed for detection and ranging. A number of infrared lasers are potential candidates. The wavelengths should be chosen to not interfere with any optical sensors on the illuminated satellites. Highly accurate pointing capability is required to acquire the satellites, assuming that the approximate position is known from other components of the tracking network. An autonomous target acquisition system could be developed using a large-divergence laser ‘floodlight’ and a highly sensitive detector to look for very weak reflections. The optical receivers must be sufficiently sensitive to measure weak returns of the satellites.

Challenges.

The principal challenge will be in developing a sufficiently sensitive optical receiver in a reasonably small and affordable package. Another major challenge will be the pointing system. Recent advances in inertial pointing systems and non-mechanical beam steering should be exploited to meet the APT requirements of the space debris cataloging system.

Scoring.

The operational requirement for improved accuracy in the Space Catalog is high and both the ground-based and space-based LIDARs offer feasible approaches to collecting the data.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 4. Cost: 2. Total Score: 12.
 

Space-Based Reference Grid

Operational Concept.

The straightness of a laser beam can be used as a reference grid or beacon, providing improved on-orbit operations for space support. Although the LMS only listed this concept by title because it did not rank high enough for further development, one possible realization of this concept would be an extension of the VOR and TACAN systems employed on earth for aircraft navigation. These systems broadcast signals that permit aircraft to determine the range and bearing to the navigation aid.

 

A GEO satellite that sends laser beams with slightly different wavelengths or pulse encoding along different angles could serve a similar function. Another, smaller scale realization would be to use a grid of laser beams to maintain alignment of a distributed space vehicle like a large space antenna.⁹² This version is similar to the guidance system discussed earlier. By placing mirrors on the outer edges of the antenna and illuminating them with laser beams from the central hub, the reflections can be detected back at the central hub by detectors that generate a control signal to maintain the position of the antenna.

 

Active alignment with a laser reference grid could also be used to hold multiple spacecraft into fixed relative positions, forming a large ‘virtual’ spacecraft. The NWV report discusses using distributed spacecraft constellations to give much wider coverage of the earth or much more accurate interrogation of the earth’s surface with reduced cost, lowered time of deployment, and reduced vulnerability.93 A laser reference grid could be used to hold this constellation together.

Operational Enhancement.

The first concept of a navigation grid could improve the performance of maneuverable space vehicles that are likely in the next revolution in orbital operations. The second concept offers a method to increase the efficiency of single or multiple spacecraft in performing their mission.

Key Enabling Technologies.

Pulsed lasers with low beam divergence are required to provide narrow beams. Sensitive detectors that can discern angular shifts in the incident signal, such as quad cell detectors, are needed to generate the control data. Computation of the control signals and the thrust generating systems are essential components of using the information relayed by the laser reference grid.

Challenges.

Numerous engineering challenges exist to make the various possible applications of laser reference grids come to fruition. The specific challenges depend on the application.

Scoring.

The need for reference grids is predicated on significant advances in other spacecraft, which suggests that it is unlikely to be needed in the near future.

Technical feasibility: 3. Technical maturity: 2. Operational enhancement: 3. Cost: 2. Total Score: 10.
 

Holographic Projector

Operational Concept.

This concept, which would fall into the force enhancement mission area, was considered in the Spacecast 2020 study, and as a truly novel idea provides evidence that the strategic studies did consider “out of the box” ideas. However, the concept ignores the fundamental physics of generating holograms. The concept is a

“system that could project holograms from space onto the ground, in the sky, or on the ocean anywhere in the theater of conflict for special operations deception missions. This system would be composed of either orbiting holographic projectors or relay satellites that would pass data and instructions to a remotely piloted vehicle or aircraft that would then generate and project the holographic image.”⁹⁴

The apparent intention is to generate three-dimensional images of sufficient quality to make the observer believe an actual object is being seen. For example, projecting the face of Allah over Baghdad has been mentioned as one application of this concept for PSYOP missions; projecting the image of a tank would be a deception mission that could force enemy troops to move out of their position, exposing themselves to attack.

 

There have even been suggestions by anonymous sources that these holographic images could be made to produce speech as well, which is theoretically possible using the photo-acoustic effect in air. This effect has been proposed by Oak Ridge National Laboratory for a laser-based emergency broadcast system.⁹⁵

Operational Enhancement.

If this concept were achievable, there would be great operational enhancement. There would be a tremendous “dual-use” benefit to the entertainment industry if the technology were releasable.

Key Enabling Technologies.

A hologram is basically an interference pattern that has been recorded in some medium, such as film or a nonlinear crystal. The pattern is generated by overlapping a laser reference beam that has a very smooth phase front with a laser beam that has been scattered off the object to be imaged. The two beams interfere, creating bright and dark regions that, when reilluminated with a suitable optical source, recreate the image to an observer who looks at the hologram from the right point of view. Ideally, a laser is used to view the hologram, although “white light holograms” produce fairly good images using regular incandescent bulbs.

 

The key technologies here are finding some way to generate the interference pattern in air and then illuminate that pattern with a visible laser beam, or possibly sunlight if the angle is correct, to create the image for the intended observers. Modulating the interference pattern would be required to create the illusion of motion.

Challenges.

The ability to create holographic images is well established, but creating them in an uncontrolled environment like the open air is almost inconceivable. Making images that are realistic enough to confuse an enemy is highly unlikely in the next 30 years. The ancillary concept of auditory project, however, is feasible and demonstrated, but probably would not be done from a space-based platform given the difficulty of controlling the region of air that is modulated.

Scoring.

Even the Spacecast 2020 study ranked this idea as “so far in the future” that it is not worth further consideration.

Technical feasibility: 1. Technical maturity: 1. Operational enhancement: 5. Cost: 1. Total Score: 8.

 

See "Project Blue Beam"
 

Laser Rocket Propulsion

Operational Concept.

Using laser beams to generate propulsion is another concept identified in the Laser Mission Study, but was ranked low and was not described further.⁹⁶ If successful, it would aid the space support mission area by giving alternatives for spacelift. The concept has been investigated by NASA, the Department of Energy and the Air Force. As described by Muolo,

[T]he basic scheme involves transmitting laser energy from some platform (ground, airborne, or orbiting) through an optical window to heat a working fluid. This fluid could be hydrogen, perhaps seeded with cesium or carbon to improve energy absorption. Very high temperatures and high Isp [specific impulse] (over 1,000 seconds) would be possible.⁹⁷

The New World Vistas study also mentions generating thrust by sending energy to the spacecraft via a high power laser beam.⁹⁸ They indicate that both electric propulsion and hot hydrogen gas propulsion engines could be driven by off-board laser energy, significantly decreasing the weight of the vehicle. Such engines have low thrust, making them most appropriate for orbital transfer.

Lawrence Livermore National Laboratory (LLNL) has investigated an alternate approach to laser propulsion that would work in both the atmosphere and in space. LLNL conducted tests “in which a laser beam was directed at a pusher plate with machined paraboloid dimples. Light was focused by each dimple on a spot behind the plate. The focused beam heated air pockets and the expanding pockets imparted a thrust to the plate. This concept provides respectable thrust in the atmosphere. In space (vacuum) the dimpled plate is jettisoned to expose a block of solid propellant which is ablated by the laser beam to produce thrust.”⁹⁹

The Phillips Laboratory recently conducted experiments that support the concept of power beaming for propulsion, projecting that “chemical rocket engines can be replaced by lightweight, efficient ion or atomic propulsion engines powered by thermal or electrical energy produced from a ground-based laser.”¹⁰⁰ In partnership with NASA, the Department of Energy’s Sandia National Laboratory and the COMSAT Corporation, Phillips Laboratory personnel used the 1.5 meter telescope at the Starfire Optical Range to transmit a laser beam over 3 million kilometers onto the Galileo spacecraft, demonstrating both the concept and a pointing accuracy sufficient to place the 40 microradian beam on the target.

Operational Enhancement.

Reducing payload weight decreases the launch costs, or, conversely, the weight saved by the laser propulsion engine can be used for increases in productive payload. A Phillips Laboratory report suggested a 50 percent reduction in launch weight of geosynchronous satellites is possible using laser propulsion.¹⁰¹

Key Enabling Technologies.

Efficient high-energy lasers are critical to this concept. Also, designing the engine requires windows with high transmission and gases that have high absorption for the laser wavelength, under the current concept.

Challenges.

The laser source is most likely to be ground-based for any near-term applications due to difficulties in packaging a high-energy laser into a spacecraft. However, ground-based lasers face substantial propagation challenges in getting the laser energy up to the spacecraft. Some concepts such as solar-pumped chemical lasers may alter this bias in ten years or more, permitting space-based HELs to drive laser propulsion engines..

Scoring.

Technical feasibility: 2. Technical maturity: 2. Operational enhancement: 3. Cost: 2. Total Score: 9.
 

Power Beaming (Earth-to-Space)

Operational Concept.

This concept falls in the space support mission area of on-orbit support. Providing energy on-board a spacecraft has long been a major challenge because it consumes a significant amount of space and weight. Various methods of producing electricity are used, including photovoltaic (e.g., solar panels), electrochemical (e.g., batteries and fuel cells) and thermoelectric (e.g., solar-thermal conversion, nuclear reactors) systems.¹⁰² The concept of using a ground-based laser to beam power up to a satellite has been considered by a number of sources, including LMS and NWV.¹⁰³

 

The ground-based laser system can be a large facility that has access to the abundant energy of terrestrial sources. The energy in the laser beam is collected on the spacecraft and converted into energy either directly by using a photoelectric process, or indirectly by heating up a substance. That substance then generates electricity or by driving a chemical reaction that stores the energy in the reaction products that can be later used in a fuel cell or battery. Other conversions of light to energy should be considered to identify the most efficient method. The key advantage is that the small beam divergence of the laser beam means that most of the energy can be collected by a relatively small telescope on the satellite.

Operational Enhancement.

Increased lifetime of spacecraft, decreased total weight for the same mission payloads, and increased mission payload for the same total weight are some of the potential operational enhancements. One application would be to recharge a spacecraft while it is in the shadow of the earth and cannot receive solar energy. For some orbits, this could substantially reduce payload weight by decreasing the need for on-board energy sources.

Key Enabling Technologies.

For ground-based laser power beaming, compensation for atmospheric distortions is critical for efficient delivery to the spacecraft. The research at the AF Starfire Optical Range has matured this technology. Also, high efficiency collectors are required on the satellite, with photoelectric or photothermoelectric being the most promising concepts. These collectors must be able to handle large amounts of power without being damaged. Stabilized platforms with sub-microradian accuracy are needed on both the ground and space systems to insure the laser beam hits the right part of the satellite to avoid damaging the satellite and to improve the efficiency of the power transfer.

Challenges.

The inefficiencies and high cost of ground-based lasers is a major challenge. Also, clouds and haze can attenuate or totally block the beam, requiring that the laser be located where clear weather is prevalent most of the time or that multiple sites be constructed.

Scoring.

The concept is feasible but both laser and collector technologies are immature. Operational enhancement is likely but limited because of the mature state of current power generation systems. Future systems would have to be specifically designed to benefit from laser power beaming.

Technical feasibility: 2. Technical maturity: 2. Operational enhancement: 3. Cost: 2. Total Score: 9.
 

Power Beaming (Space-to-Space, Space-to-Earth)

Operational Concept.

Similar to the concept above, the space-to-space power beaming would fall in the space support area while the space-to-earth power beaming would lie in the force enhancement mission area. This concept beams energy from a space-based laser to either another satellite or down to the earth, as mentioned in NWV.¹⁰⁴ The advantage for space-to-space power beaming is avoiding atmospheric distortion and losses as well as weather-related mission cancellation.

 

The range between the satellites may also be less than from earth to the receiving satellite, and thus reduce the power requirements on the laser source. Beaming power from space to earth offers the possibility of providing significant amounts of power at remote sites where other sources of energy are not readily available, such as unattended ground sensors. Also, the atmospheric effects appear to be less for lasers pointing down into the atmosphere, in part because there is less remaining path length when the beam enters the more dense regions.

 

Although this prospect needs further modelling, it is clear that power-beaming from earth-to-space is not the same as from space-to-earth due to the asymmetry of the atmosphere. Also, electrically-powered, high-altitude UAVs could be recharged by space-based lasers. These UAVs could provide communication links, tactical surveillance, theater missile defense, and temporary navigation aids.

Operational Enhancement.

As before, by reducing the requirements to carry energy-generating and energy-storing equipment, the cost, size and weight of the satellites and terrestrial equipment powered by the laser power beaming could be reduced.

Key Enabling Technologies.

Compact, high-energy lasers of suitable wavelengths and long operational lifetimes are required. Carbon dioxide lasers are a possible candidate due to the high efficiency (about 30 percent), as are solar-powered atomic bromine lasers. The AF2025 study recommended considering solar-powered lasers for the various HEL missions.¹⁰⁵ A critical part of this concept is providing the power to the laser for the beaming. Solar energy and a nuclear reactor are viable candidates. As mentioned in the previous concept, high efficiency, durable power collectors and accurate pointing systems are needed as well.

Challenges.

The principal challenge for this variant of power-beaming is the laser source and its associated systems.

Scoring.

Although the concept appears feasible, most of the components remain to be developed. The enhancement resulting from space-to-ground power beaming could be significant.

Technical feasibility: 2. Technical maturity: 2. Operational enhancement: 4. Cost: 2. Total Score: 10.
 

Space Debris Clearing

Operational Concept.

As discussed earlier, space debris is a growing threat to space operations. High-energy lasers offer the possibility of removing the debris. Although the NWV study specifically suggests using a ground-based laser for clearing space debris,¹⁰⁶ basing the laser in space may offer better angles from which to irradiate the debris. The concept, supporting the space control and space support areas, would be to project a laser beam from a space-based, pulsed HEL and vaporize a portion of the surface of the debris, creating a small burst of thrust from the blow-off of material.

 

By repeatedly hitting the debris, sufficient impulse could be transferred to cause the debris to de-orbit, and burn up in the atmosphere. Tens to hundreds of pulses may be necessary. Obviously, a great deal of care must be taken to insure the thrust slows down the debris, which would cause it to move to a lower orbit, and that the lower orbit does not result in a collision with any other space objects. Even a collision with other debris should be avoided as it would likely generate more pieces of debris that are smaller and more difficult to track. This concept could be merged with the space debris identification concept, using the laser operating at low power for detection and at high power for debris removal.

Operational Enhancement.

There is currently no method to clear the debris from space. A space-based laser offers one of the few viable options to remove the threat. With greatly increased emphasis on exploiting space for future AF operations, this concept may be well worth the substantial investment required.

Key Enabling Technologies.

A high-energy, pulsed laser with a long operational life is required. Open-cycle chemical lasers would not be good choices. Current technology offers a number of candidate lasers that can produce tens of kilojoule pulse energies that would be sufficient for the mission, if the output optics are large enough to focus the beam in a small spot on the debris. For example, a self-contained, two kilowatt, master oscillator-power amplifier (MOPA), Nd:YAG system (called “MODS” for Mobile Ordnance Disposal System) has been built for anti-landmine missions that fits in a tracked vehicle. Also, electrically pumped, closed cycle, carbon dioxide lasers are a mature technology used for industrial welding. Large, lightweight optics would clearly be required. Accurate pointing of the laser beam is an additional requirement that can be met with current technology.

Challenges.

The large-scale optics may be the most pressing technology for this concept. Also, the orbital mechanics calculations pose a significant challenge in determining when to apply the laser pulses.

Scoring.

Technical feasibility: 3. Technical maturity: 2. Operational enhancement: 4. Cost: 3. Total Score: 12.
 

SPACE-BASED LASER WEAPONS CONCEPTS

The next five concepts envision using the laser in direct, offensive roles against adversarial targets, while the sixth concept targets extraterrestrial objects.

All these concepts fall in the space control and force application mission areas. The seventh concept, falling more into the force enhancement area, aims the high power beams from space back at the earth to alter weather patterns. Space-based laser weapons (abbreviated here as SBL with the assumption that it refers to an HEL weapon) have been studied extensively over the past twenty years and numerous references describe the technology from favorable,^107,108 neutral,¹⁰⁹ and unfavorable¹¹⁰ perspectives. Scientists and engineers have been diligently pursuing the dream of high-energy laser weapons for over thirty years¹¹¹ and their efforts are paying off in maturing technology. The high power Alpha laser developed by TRW is a good example of their success, as reflected in renewed congressional funding.¹¹²

Obviously, the next few pages cannot give a thorough summary of this complex topic, but the generalizations below should give a fairly accurate appraisal of these seven concepts. Most of the concepts are variants of the SBL weapon concept, with the exception of the GBL ASAT concept where the laser is based on the ground. This concept is included (1) for comparison to SBLs and (2) because the GBL beam may be bounced off relay mirror satellites to accomplish its mission. Some of the relay mirror challenges overlap with some of the technological challenges for SBLs.

A few general comments should be made about laser weapons. There is no doubt that a high-energy laser can cause substantial damage to a target, as is routinely done with laser welders for industrial applications and medical lasers for a wide variety of surgeries. Damage of militarily significant targets has been demonstrated with the destruction of air-to-air missiles with the Airborne Laser Laboratory and a pressurized booster tank with the MIRACL laser. The key advantages of a laser weapon is the speed-of-light, straight-line delivery of the energy with little concern about windage and ballistic effects, as discussed in an earlier section. However, laser weapons are inherently inefficient ways of destroying targets.

The production of the laser energy is usually difficult, with single-digit efficiencies common for high-energy systems such as the Alpha laser and MIRACL systems. The coupling the energy to the target usually occurs at the surface where the light is absorbed, unlike the deep penetration common to kinetic energy systems like bullets. Thus, it may be difficult to kill some targets like reentry vehicles and it may be easy to develop countermeasures such as thicker skins or spinning the target to dissipate the laser energy. CW lasers need to dwell on the target for a sufficient time to damage it by thermal effects, while pulsed lasers damage the target by blowing off part of the surface, causing a plasma.

This plasma becomes an absorbing surface itself, so the laser energy may end up heating the blown-off material rather than further damaging the target. All of these challenges are well known to the laser weapon community and a number of innovative solutions have been found. There is great value in pursuing laser weapons because they will offer a capability that is not available in any other weapon system. However, this brief discussion highlights the fact that this pursuit is a very challenging one.
 

Space-Based Counterforce Weapon

Operational Concept.

This concept is an overarching term that applies to any application of SBLs against military targets such as military satellites, nuclear warhead reentry vehicles (RV), missiles in the boost phase, high-flying military aircraft, and anti-satellite kinetic energy missiles. The term was used in the LMS¹¹³ but was also discussed in Spacecast 2020¹¹⁴ and AF2025¹¹⁵.

 

The NWV report discusses a similar concept that they called the Global Precision Optical Weapon (GPOW) that would have a “clearly dominant role in warfare.”¹¹⁶ Carryovers from the Cold War, the term “counterforce” is juxtaposed against “countervalue” used to describe targets that are usually predominantly civilian such as cities. Treaties such as the Anti-Ballistic Missile Treaty restrict placing operational ABM systems in space and may affect other SBL applications.

Operational Enhancement.

The capability to negate adversarial targets at great distances at the speed-of-light would be a revolutionary capability and has served as the motivation behind decades and billions of dollars of research. HELs are one of the few technologies with the potential of destroying ballistic missiles in the boost phase. Even a limited number of SBLs offer significant capability for a limited number of ballistic missile launches that might characterize a terrorist or rogue-state attack against the United States or the limited number of theater missile attacks during a conflict like the Gulf War. The Spacecast 2020 study proposed the idea, echoed in AF2025, of using the SBL for a range of missions such as passive imaging using the large telescope by itself and active imaging using the laser operating at low power.¹¹⁷

Key Enabling Technologies.

High-energy lasers are the obvious central technology for this concept. Most of the contemplated designs use CW chemical lasers that would destroy their targets by thermal effects. The hydrogen fluoride laser operating at 2.7 microns is the most mature candidate. Large diameter (>10 meters), lightweight mirrors are required in order to focus the laser beam on the target. Near-real-time target acquisition is required, perhaps by linking the SBL to early warning satellites. The pointing and tracking systems must also be robust. The interval between launch of a ballistic missile and the end of the boost phase can be as short as 60 seconds, levying a requirement for automated battle management software. The ability to rapidly slew the beam to a new target is a key requirement for engaging multiple targets. NWV discusses their view of the technical requirements for the GPOW in some detail.¹¹⁸

Challenges.

Each of the SBL systems (e.g., laser device, large output optics, APT, battle management software) has significant engineering challenges to overcome. The SBLs are very expensive systems and could be attacked by anti-satellite weapons such as the Soviet co-orbital interceptor developed in the late 1970’s. Thus, a defensive system would also be required for space-based battle station. This system could use either kinetic or directed energy weapons to counter the ASAT weapon.

Scoring.

While scoring a general concept is difficult and less meaningful than considering the specific concepts discussed below, the overall concept is feasible but faces monumental engineering challenges. The operational enhancement would have strategic implications for national missile defense as well as provide a force multiplier for theater operations. However, deploying even a limited number of SBLs would be very expensive.

Technical feasibility: 3. Technical maturity: 2. Operational enhancement: 5. Cost: 1. Total Score: 11.
 

Space-Based Ballistic Missile Defense (BMD) Weapon

Operational Concept.

Already in the research phase but given a significant impetus by President Reagan’s speech on March 23, 1983, the idea of building an effective defensive shield against a massive ICBM attack included the use of space-based lasers to destroy the boosters before the reentry vehicles were released. The laser beam would be directed at the side of the booster, weakening it by heating and letting the internal pressures rupture the booster.

 

The RVs are much harder targets because they are covered with an ablative cover capable of sustaining the heat of reentry. A large number of SBLs would be needed to provide an effective defense due to the orbital movement of the systems (likely to be deployed at about 1300 km with an orbital period of a little less than two hours¹¹⁹) and the large number of boosters that an adversary with ICBMs might launch.

Operational Enhancement.

Destroying the boosters in the boost phase is the best solution because (1) the booster is the softest link in the chain of events that send the nuclear warheads to the target, and (2) the warheads and other debris falls back on the launching nation. An effective SBL BMD system would provide a unique, highly valuable capability to the warfighter and the nation.

Key Enabling Technologies.

The discussion in the previous concept applies here. A significant amount of R&D has resulted in proof-of-concept demonstrations of the Alpha hydrogen fluoride laser, the Large Advanced Mirror Program (LAMP) and the Large Optics Demonstration Experiment (LODE) beam control system. The Ballistic Missile Defense Office (BMDO) is continuing the SBL development with the Alpha/LAMP Integration (ALI) program.

Challenges.

Again, the discussion in the SBL Counterforce concept applies to this concept. The systems management challenges are significant. There must be an autonomous system that would detect the launch, activate the SBL, acquire and track the target, point the laser at the target, engage the target with the laser for a sufficient time to destroy it, and then rapidly move to the next target. The complexity and brevity of the engagement has been studied in great detail by the battle management portion of the SDIO program. Also, the laser propagation and target interaction issues are as important as the challenges of making very high-energy laser devices.

Scoring.

This is perhaps the most challenging and important mission for an SBL.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 5. Cost: 1. Total Score: 12.
 

Ground-Based Laser Anti-Satellite (ASAT) Weapon

Operational Concept.

The GBL ASAT is included (1) for comparison to SBLs and (2) because the GBL beam may be bounced off relay mirror satellites to accomplish its mission, with the relay mirror satellites having some of the same technological challenges for SBLs. Having some potential for BMD through the use of relay mirrors, the GBL ASAT system could disable satellites at lower powers than that required for the BMD mission because satellites are softer targets and the ranges are much shorter.

 

According to NWV, satellites are “particularly vulnerable to laser attack...Target irradiances of several to ten watts/cm² are adequately lethal.”¹²⁰ The GBL ASAT weapon would likely be located at a low latitude site that has exceptionally clear weather most of the year so that it could attack satellites when they pass overhead. The concept is proposed in both NWV and AF2025 studies.¹²¹

Operational Enhancement.

Having a non-nuclear capability to deny adversarial satellites would provide a critical force enhancement tool for space control.

Key Enabling Technologies.

Projecting a high power laser beam from the ground requires a high power laser, large telescopes, atmospheric compensation, high accuracy pointing and tracking systems, and imaging systems for kill assessment. Because the laser does not have to be compact and can access the local power grid or other substantial energy sources, the GBL is more readily developed than the SBL ASAT weapon. Candidate lasers would include the COIL operating at 1.315 microns that has relatively good transmission through the atmosphere. High average power Nd:YAG systems operating at 1.064 microns are also feasible.

Challenges.

Most of the technologies for a GBL ASAT are maturing. Only moderate engineering challenges remain to provide a limited capability. However, operational issues over where to base the systems and how to employ them remain. Significant political issues are likely to arise with the ASAT mission because attacking a nation’s satellites would likely be taken as a military attack on the nation.¹²²

Scoring.

The GBL ASAT is a relatively mature concept aimed at relatively soft targets.

Technical feasibility: 4. Technical maturity: 4. Operational enhancement: 5. Cost: 2. Total Score: 15.
 

Space-Based Anti-Satellite (ASAT) Weapon

Operational Concept.

The same operational concept described for the GBL ASAT would be the basis for the SBL ASAT, except that that laser would be placed on an MEO orbiting platform. The relative softness of the satellites, the lack of any atmospheric effects and the decreased range at the time of engagement reduce the power requirements for the SBL ASAT system as compared to the BMD mission.

Operational Enhancement.

The operational enhancement is the same as that provided by the GBL ASAT weapon.

Key Enabling Technologies.

Developing a high power, lightweight, reliable laser device with a ‘deep magazine’ is the critical technology but the large, lightweight optical elements are also crucial. The APT system requirements are similar to those discussed in many of the preceding concepts.

Challenges.

The placement of a SBL ASAT in orbit would likely raise serious international issues and may be restricted by the Outer Space Treaty of 1967. Significant technical issues also remain as discussed for the SBL Counterforce concept.

Scoring.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 5. Cost: 2. Total Score: 13.
 

Space-Based Counter-Air Weapon

Operational Concept.

The SBL could be used to attack high-flying aircraft (above 30,000 feet or so) because high power lasers can penetrate to that altitude with relatively little loss of energy, even if the wavelength is one that is strongly absorbed at lower altitudes. In fact, having the wavelength stopped by the lower air layer provides the requisite laser safety for unintentional illumination of targets. (If the wavelength is not strongly absorbed, it would be conceivable to attack soft targets on the surface.¹²³)

 

The hostile aircraft could be tracked either from the SBL via infrared or radar techniques or by an off-board sensor and the location relayed to the SBL. With sufficiently large optics, the beam could be focused to a small enough spot to cause structural failure at critical parts of the aircraft. The size of the optics and the power of the laser should be significantly lower than that required to destroy an ICBM because the aircraft’s slower speed permits longer dwell times.

Operational Enhancement.

The capability of attacking aircraft without putting friendly forces at risk and in regions where anti-aircraft weapons are not available would be a significant force enhancement and contribute to air and space superiority. General Fogleman’s goal to “find, fix...and target anything that moves on [or near] the surface of the earth” referenced in the opening paragraphs of this study would be greatly aided by SBL counter-air weapons.

Key Enabling Technologies.

Because this concept is akin to the SBL BMD, the key technologies include the laser device, large, lightweight optics, APT systems, and self-defense systems.

Challenges.

Although significant challenges remain in each technology area, the power requirements for the laser and the size of the optics would likely be somewhat lower than those of the SBL BMD weapon.

Scoring.

Technical feasibility: 3. Technical maturity: 3. Operational enhancement: 5. Cost: 1. Total Score: 12.
 

Planetary Defense Weapon

Operational Concept.

The threat of an asteroid or large meteor colliding with the earth is remote but not impossible. The results of such a collision would be cataclysmic, delivering the energy of hundreds to thousands of megatons of energy and possibly destroying life on the earth. At present, we may have the rudiments of a space surveillance system that could warn us of the impending collision but we lack any technology to counter the threat. In the Spacecast 2020 study, the team considered an asteroid negation system that could include directed energy weapons such as lasers.¹²⁴

 

The idea discussed in clearing space debris, of creating multiple, cumulative thrusts via laser ablation, could be applied to the asteroid to change its direction slightly, causing it to miss the earth. The momentum of even a small asteroid is very large, making any attempts to alter its trajectory challenging. Engaging the target at extremely long ranges would give the best opportunity to affect its path, but that imposes demanding requirements on the laser system.

Operational Enhancement.

No operational requirement exists for this mission, although it would clearly be in mankind’s interest to prevent such a disaster.

Key Enabling Technologies.

Extremely powerful lasers would be required for this concept, possibly requiring multiple HELs or lunar-based lasers.

Challenges.

The challenges are so imposing that the Spacecast 2020 operational analysis team did not consider the concept further.

Scoring.

Technical feasibility: 2. Technical maturity: 1. Operational enhancement: 1. Cost: 1. TOTAL COST: 5.
 

Weather Modification System

Operational Concept.

A concept proposed by Spacecast 2020 and echoed by AF2025 is the notion of modifying the weather using a directed energy source such as HPM and HEL systems.¹²⁵ The concept is to deliver enough energy to a region of the atmosphere to change the local weather. For example, heating the air could raise its temperature above the dew point, which could disperse fog or clouds. This would allow surveillance systems to see previously obscured ground targets. Other effects could include creating local storms or winds, and grounding enemy aircraft.

Operational Enhancement.

Weather has always been a dominant factor in military operations and any ability to control it would give significant military capability.

Key Enabling Technologies.

Extremely high power DE systems would be required to be able to heat a large volume of air quickly enough to affect the weather. The laser wavelength would have to be chosen to be strongly absorbed in the correct type of air. For example, hydrogen fluoride lasers operate at 2.7 microns, a wavelength that is strongly absorbed by water vapor. Laser beams from carbon dioxide lasers operating in the region of 10.6 microns are attenuated by atmospheric carbon dioxide. Numerous other technological breakthroughs would have to take place in energy sources, large optics, and modeling of nonlinear dynamics before this concept could be given serious consideration.

Challenges.

The required power levels are so high that this idea must await multiple breakthroughs in laser technology. Further, weather is inherently nonlinear and any attempt to manipulate it may cause adverse unintended consequences. Careful, validated modeling should be undertaken before any field tests are considered. Finally, international agreements may restrict weather modification technology.

Scoring.

Technical feasibility: 1. Technical maturity: 1. Operational enhancement: 5. Cost: 1. Total Score: 8.
 

Summary of Concept Scores

Table 5 summarizes the scores of the 28 concepts that were examined in this section. Not surprisingly, the highest ranked concepts tend to be those that augment other systems or build on existing technology.

The top concepts are listed here in order of total score (shown in parentheses):

• Laser communication and data relay (17)

• Target designation (16)

• Alignment and docking guidance systems (16)

• Deep space laser altimeter (16)

• Remote sensing for battle damage assessment (16)

• Battlefield illumination (15)

• Satellite-to-satellite Doppler velocimeter (15)

• Weather monitoring and characterization (15)

• GBL ASAT (15)

• Environmental remote sensing (14)

• SBL ASAT weapon (13)

The top concept, space-based laser communication, is maturing in the commercial sector as well as within NASA and DOD. It needs no further elaboration but should continue to be pursued in a coordinated manner. In the subsequent sections, the SB-LTD and SB-BI concepts are discussed in more detail as they are militarily unique applications.

Table 5. Composite Scores for Concepts
 

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