1

Introduction

Active electro-optical (EO) sensors can measure range, velocity, vibrations, intensity, phase, polarization, and angular position. They can provide literal imagery that can be easily interpreted at ranges beyond what passive EO sensors can provide. They can provide near-photographic quality literal imagery, three-dimensional (3-D) imagery, and other discriminants, even in some cases when the target is obscured by camouflage, foliage, or weather. The richness of the information collected gives rise to a very wide range of applications. Particularly if they are not anticipated, some of these applications could be very damaging to U.S. national interests if pursued by an adversary. Conversely, they can be an advantage if used by the United States.

Active and passive EO sensing can be compared to radar sensing by looking at the electromagnetic spectrum shown in Figure 1-1. Active EO sensing systems have a higher angular resolution than microwave radar systems due to the shorter wavelengths of infrared and visible light and, like radar, also benefit from controlling the illumination. In that respect, active EO systems enjoy resolution advantages over microwave radar and illumination advantages over passive EO sensors.

Some types of active EO sensing go by the name ladar (LAser Detection And Ranging) and sometimes by lidar (LIght Detection And Ranging). Use of these terms is not standardized from one subcommunity to the next (Box 1-1). The National Institutes of Standards and Technology (NIST) has adopted ladar as the standard name for active EO sensing, while the U.S. Department of Defense (DoD) has tended to use lidar. Historically, conventional usage has made a distinction based on the type of target—ladar for “hard” targets (e.g., solids) and lidar for “soft” targets (e.g., gases or aerosols). In this report, the committee followed a similar convention in that active EO sensing of targets with surface scattering is called ladar and active EO sensing of target volume scattering is called lidar. The committee felt that distinguishing between surface or volume scattering was less ambiguous than distinguishing between soft or hard targets.

If one compares a typical 200-THz, 1.5-µm wavelength, active EO system to a typical 10-GHz, 3-cm-wavelength radar system, the EO angular resolution will be nominally 20,000 times finer than that of radar for the same aperture diameter. For a synthetic aperture radar compared to a synthetic aperture ladar, this means flying 20,000 times shorter distance to form the synthetic aperture, but requiring motion compensation that is 20,000 times more accurate. Range resolution can also be higher than microwave radar because the high carrier frequency of active EO sensing allows very high bandwidth modulation to still be a small fractional bandwidth of the carrier. Range resolution depends on the bandwidth of the transmitted signal, as discussed later. Using these same wavelength ratios, the time required for an EO measurement of target velocity is also shorter than for microwave radar by a factor of 20,000. The benefit of active systems as opposed to passive systems is that they provide illumination of the target that does not vary over time unless it is varied intentionally. This allows active EO sensing to be day- and night-capable at short wavelengths and provides greater control over shadowing phenomenology.

In fairness, active EO sensing will not have advantages over passive systems and microwave radar in all cases. Passive EO systems are stealthier and avoid the risk that an illuminator could reveal the location of the system. They do not raise eye-safety issues (see Box 1-4) and may have advantages in terms of lower life cycle costs, lower power requirements, and simplicity. Similarly, radar systems can be



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1 Introduction Active electro-optical (EO) sensors can measure range, velocity, vibrations, intensity, phase, polarization, and angular position. They can provide literal imagery that can be easily interpreted at ranges beyond what passive EO sensors can provide. They can provide near-photographic quality literal imagery, three-dimensional (3-D) imagery, and other discriminants, even in some cases when the target is obscured by camouflage, foliage, or weather. The richness of the information collected gives rise to a very wide range of applications. Particularly if they are not anticipated, some of these applications could be very damaging to U.S. national interests if pursued by an adversary. Conversely, they can be an advantage if used by the United States. Active and passive EO sensing can be compared to radar sensing by looking at the electromagnetic spectrum shown in Figure 1-1. Active EO sensing systems have a higher angular resolution than microwave radar systems due to the shorter wavelengths of infrared and visible light and, like radar, also benefit from controlling the illumination. In that respect, active EO systems enjoy resolution advantages over microwave radar and illumination advantages over passive EO sensors. Some types of active EO sensing go by the name ladar (LAser Detection And Ranging) and sometimes by lidar (LIght Detection And Ranging). Use of these terms is not standardized from one subcommunity to the next (Box 1-1). The National Institutes of Standards and Technology (NIST) has adopted ladar as the standard name for active EO sensing, while the U.S. Department of Defense (DoD) has tended to use lidar. Historically, conventional usage has made a distinction based on the type of target—ladar for “hard” targets (e.g., solids) and lidar for “soft” targets (e.g., gases or aerosols). In this report, the committee followed a similar convention in that active EO sensing of targets with surface scattering is called ladar and active EO sensing of target volume scattering is called lidar. The committee felt that distinguishing between surface or volume scattering was less ambiguous than distinguishing between soft or hard targets. If one compares a typical 200-THz, 1.5-μm wavelength, active EO system to a typical 10-GHz, 3- cm-wavelength radar system, the EO angular resolution will be nominally 20,000 times finer than that of radar for the same aperture diameter. For a synthetic aperture radar compared to a synthetic aperture ladar, this means flying 20,000 times shorter distance to form the synthetic aperture, but requiring motion compensation that is 20,000 times more accurate. Range resolution can also be higher than microwave radar because the high carrier frequency of active EO sensing allows very high bandwidth modulation to still be a small fractional bandwidth of the carrier. Range resolution depends on the bandwidth of the transmitted signal, as discussed later. Using these same wavelength ratios, the time required for an EO measurement of target velocity is also shorter than for microwave radar by a factor of 20,000. The benefit of active systems as opposed to passive systems is that they provide illumination of the target that does not vary over time unless it is varied intentionally. This allows active EO sensing to be day- and night- capable at short wavelengths and provides greater control over shadowing phenomenology. In fairness, active EO sensing will not have advantages over passive systems and microwave radar in all cases. Passive EO systems are stealthier and avoid the risk that an illuminator could reveal the location of the system. They do not raise eye-safety issues (see Box 1-4) and may have advantages in terms of lower life cycle costs, lower power requirements, and simplicity. Similarly, radar systems can be 6

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INTRODUCTION 7 FIGURE 1-1 The electromagnetic spectrum, showing the roughly 20,000-fold difference between the frequencies used in active EO sensing (visible and infrared) and traditional radar (microwave). SOURCE: By Inductiveload, NASA [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by- sa/3.0/)], via Wikimedia Commons. See http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg. BOX 1-1 Names Used within the Active EO Sensing Community • Lidar (Light Detection and Ranging) – Has historically been used with atmosphere, or chemical vapor, detection; – Is used by the National Geospatial Intelligence Agency for active EO sensing (3-D imaging and mapping); and – Is usually used for commercial applications. – This report uses “lidar” for active EO sensing of volume scattering targets. • Ladar (Laser Detection and Ranging) – Has historically been used with hard targets and has been – Adopted by NIST at the standard term for active EO sensing. – This report uses “ladar” for active EO sensing of surface scattering targets. • Laser radar and laser remote sensing are sometimes used. • For reference: radar is Radio Detection And Ranging. very nearly all-weather, and can penetrate clouds better than optical sensors. Radars also have more favorable search detection statistics, and so might be superior for early-warning applications. Figure 1-2 is a highly simplified diagram of a ladar. It shows a laser generating photons, which are transmitted to an exit aperture to illuminate a target. Light is scattered from the target and returned to the same aperture. 1 The returned light is routed to a detector array and then processed. Some ladars extract a sample of the outgoing signal and interfere it against the return signal in order to obtain phase information. 1 This is called a monostatic system. In a bistatic system, separate apertures are used to transmit and receive the optical radiation.

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8 LASER RADAR FIGURE 1-2 Highly simplified block diagram of a ladar. Active EO sensing modalities are very rich and, as previously mentioned, will support a very wide range of measurements. The field of active EO sensing can be subdivided with respect to wavelength of operation, detection technique employed, measurements to be performed, or application. One detailed taxonomy is shown in Figure 1-3, where these modalities are divided into three categories: (1) imaging, (2) spectral sensing, and (3) other, including sensing of vibrational and volume scattering. These modalities are discussed in greater detail in Chapters 2 through 4. Range resolution, accuracy, and precision (see Box 1-2) of active EO sensors are not dependent on the diffraction limit, so they do not directly benefit from the wavelength difference compared to microwave radar. Instead, range resolution depends on the bandwidth of the transmitted and received electromagnetic signal. This is the same as with microwave radar; however, in ladar the carrier frequency is orders of magnitude larger. Because of this, it is possible to use much higher bandwidth if desired and thus to obtain much higher target range resolution. For example, a recent paper 2 showed that with 3.6- THz transmit bandwidth (about 1.8 percent of the carrier frequency), a 50-μm range resolution could be achieved using ladar. Other aspects of ladar performance are discussed in Box 1-3. The laser illuminators in active EO sensing systems can raise eye safety issues for humans or animals in the beam path, as discussed in Box 1-4. Lasers operating in the wavelength region from about 400 nm to 1500 nm (blue to short wave infrared) are considered less eye-safe than lasers operating in other regions because the eye efficiently focuses light on the retina in this wavelength range, with the potential for damage to the retina. Lasers that operate outside of this wavelength range are sometimes referred to as eye-safe, even though eye-safe really only means a higher maximum permissible exposure (MPE). A number of countries have developed commercial ladar systems at shorter wavelengths (830- 850 nm) with a novel approach to eye safety. An example is the Elbit system. 3 Eye safety is handled using lower power interrogation pulses such that if no object is determined to be in the beam, the laser power is increased until an object is determined to be in the beam. These systems have a higher resolution than eye-safe systems due to the shorter wavelength. Such approaches may be particularly appropriate for shipboard and aviation platforms where the eye-safe “keepout” zone can be assured with altimeter devices and underway status. 2 R.R. Reibel et al., 2009, “Ultrabroadband optical chirp linearization for precision length metrology applications,” Optics Letters, 34: 3692-3694. 3 O. David, R. Schneider, and R. Israeli, 2009, “Advance in active night vision for filling the gap in remote sensing," Proc. SPIE 7482, Electro-Optical Remote Sensing, Photonic Technologies, and Applications III, 748203; doi:10.1117/12.830378.

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INTRODUCTION 9 FIGURE 1-3 Schematic showing various modalities of laser remote sensing, divided broadly into imaging, spectral sensing, and other, including sensing of vibrational and volume scattering. “Imaging” here refers to both 2-D and 3- D active imaging, where range is determined by one form or another of time of flight measurement. JAUDIT: Jungle Advanced Under Dense vegetation Imaging Technology, an NGA-developed airborne linear mode lidar. It later became TACOP (tactical operational lidar) as a program of record; Riegl: Commercial company selling scanning linear mode lidars; Ball: Ball Aerospace; FLASH: A term used in the community generically denoting modest arrays of linear-mode (conventional large signal) APDs for lidar; SAIL: The Aerospace Corporation's demonstration of Synthetic Aperture Imaging Ladar using FMCW (Frequency Modulated-Continuous Wave) waveforms. The same basic technology as used in the Raytheon implementation of the DARPA SALTI program; SALTI: Synthetic Aperture Lidar Tactical Imaging, a DARPA program to demonstrate synthetic aperture ladar from an airborne platform; RRDI: Range Resolved Doppler Imaging, a coherent ladar technique; PRN: Pseudo Random Number, a coded approach to correlation-based ranging; DIAL: Differential Absorption Lidar; PNR Photon Number Resolving (i.e., can measure 1, 2, 3, 4, ... photons, as opposed to Geiger mode that can count one at a time); GLAS: NASA's Geoscience Laser Altimeter, a ranging ladar flown on the first IceSat mission.

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10 LASER RADAR BOX 1-2 Explanation of Terms: Range Resolution, Precision, and Accuracy Range Resolution • Quantifies the ability to detect two objects separated in range along a single line of sight. • Limited to c/(2 × B) where c is the speed of light and B is the system bandwidth. Range Precision • Quantifies the relative uncertainty of a range measurement to an object. • Limited by the range resolution and the signal-to-noise ratio of the measurement. It can be significantly better than the range resolution. Range Accuracy • Quantifies the degree to which a range measurement yields the "true value" of the absolute range. • Depends on range precision as well as systematic errors (e.g., clock rate, drift, and timing offsets) BOX 1-3 The Laser Radar Range Equation The performance of an active EO system can be predicted by use of the laser radar range equation. The ladar equation calculates the laser power collected by the receiver.1 There are multiple forms of the ladar equation, depending on definitions and assumptions. One form is shown in equation 1. σ A PR ≈ PT * * rec *η atm *η sys . 2 (1) Aillum πR 2 where PR = power received, PT = power transmitted, σ = cross section in square meters, Aillum = area illuminated, Arec = area of the receiver, R = range, ηatm = transmission efficiency through the atmosphere, ηsys = receiver system optical systems efficiency. It is assumed the object to be viewed is within the illumination area. “The power received is the power transmitted × two ratios of areas × appropriate efficiency terms.”1 “The first ratio of areas is the cross section divided by the illuminated area at the object plane.”1 The cross section considered is the reflection within a given range bin. “The second ratio of areas is the receiver aperture area divided by the effective average area illuminated by Lambertian reflection.”1 Radar practitioners, as well as some of the people working active EO sensing, use a different definition of cross section based on reflection from a gold ball. In that case, instead of π in the denominator, representing Lambertian scattering on return over an effective angle of π steradians, the committee uses 4π, representing specular scattering from a round ball over 4π steradians. The EO handbook uses this definition.2 It is very important to be aware of the definitions used regardless of which form of the ladar equation is used. __________ 1 P.F. McManamon, 2012, “Review of ladar: a historic, yet emerging, sensor technology with rich phenomenology,” Optical Engineering 51(6), 060901. 2 C.S. Fox, ed., The Infrared and Electro-Optical Systems Handbook, Volume 6: Active Electro-Optical Systems.

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INTRODUCTION 11 BOX 1-4 Eye Safety in Active EO Sensing Systems Active EO sensing systems have the potential to cause eye damage to humans or animals exposed to the illuminating laser beam. The potential for damage depends on the light irradiance at the eye (for continuous wave systems) or pulse fluence, pulse duration, and pulse rate (for pulsed systems) as well as the wavelength. Such damage may include surface heating of the cornea if the laser light has a wavelength that is strongly absorbed, or a burn on the retina if sufficient light passes through the lens and is focused there. Maximum permissible exposure (MPE) levels for laser light in the workplace are set by the U.S. Occupational Safety and Health Administration.1 The American National Standards Institute also publishes laser safety standards (ANSI Z136.1). In evaluating the eye safety characteristics of procured active EO systems, at least one U.S. government agency uses a software code that takes system inputs and evaluates eye exposure against the OSHA standards.2 Of course, observers in the beam path using binoculars or telescopes will experience a higher fluence than would the unaided eye, so eye safety for a person looking with binoculars would require a lower MPE. Figure 1-4-1 shows the MPE levels for laser pulses of various durations as a function of wavelength. Note the MPE is much lower in the wavelength region from 400 nm to 1500 nm. This is because the eye efficiently focuses light on the retina in that wavelength range. Lasers that operate outside of that wavelength region are sometimes referred to as eye-safe, even though eye-safe really only means a higher MPE. As can be seen in the Figure 1-4-1, lasers operating at wavelengths longer than about 1500 nm or shorter than about 400 nm are safer because molecular absorption occurring in the volume of the eye attenuates the beam and reduces the intensity of the light that would be focused on the retina.3 There is a slight decrease in the MPE at wavelengths longer than 1500 nm because light is absorbed more strongly at longer wavelengths, and so is absorbed in a smaller portion of the eye volume. At 10,000 nm (approximately the wavelength of a CO2 laser), the eye has strong absorption, so the light is absorbed near the surface in the cornea, and burns may result. A second factor to consider in relation to eye safety is laser pulse width. Short pulses have higher peak power for the same energy per pulse, and so may cause more damage to the eye. As shown in Figure 1-4-1, at all wavelengths the MPE decreases with decreasing pulse length. Of course at a high enough power, a laser operating at any wavelength and pulse duration can cause damage, but the permissible exposure levels are lower in certain wavelength bands and for shorter pulses. The wavelength chosen for any laser remote sensing system depends on: (1) the phenomenology being sensed (reflectivity/absorption, atmospheric transmission, spatial resolution, etc.), and (2) technology constraints, including compatibility with sensors, maturity, and eye safety. Whether or not the laser is eye-safe, the system should be designed to be eye-safe at the range of operation, possibly with hardware or software constraints to preclude operations in non-eyesafe conditions (say before the platform reaches operational altitude.) Some applications require operational wavelengths not There are two main types of optical active EO sensing: (1) direct detection, which measures only the intensity of the return signal, and (2) heterodyne, or coherent, active EO sensing, which measures the field, including both phase and intensity. 4 Owing to its relative simplicity, the direct detection technology base is mature and proliferating rapidly. Direct detection ladars are widely used today to provide excellent 3-D images, maps, and infrastructure models for commercial, civil, and military applications. This has fostered the emergence of a sophisticated global enterprise to obtain and exploit a high- 4 As illustrated in Figure 1-3, there are also a variety of lidar techniques that utilize spectroscopic interactions of the laser illuminator with the target volume, such as Raman or Rayleigh scattering, fluorescence, etc.

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12 LASER RADAR FIGURE 1-4-1 Maximum permissible exposure (MPE) levels for laser pulses of various durations as a function of wavelength. SOURCE: Hankwang at en.wikipedia [CC-BY-SA- 3.0 (http://creativecommons.org/licenses/by-sa/3.0/), GFDL (http://www.gnu.org/copyleft/ fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-2.5 (http://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons. See http://en.wikipedia.org/wiki/Laser_safety. considered eye-safe (very near infrared spectroscopy, bathymetry, etc.). Nonetheless, it is often possible to design such systems to be eye-safe by choice of illumination, dwell time, receiver parameters, and concept of operations. The committee notes that the active EO systems developed by potential U.S. adversaries may not operate under the same eye safety restrictions as U.S. systems. The implications of this possibility are discussed in the text. _______________________ 1 See https://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_6.html#2. 2 Dr. Richard Heinrichs, Program Manager, DARPA/STO, personal communication to the committee on October 17, 2013. 3 See http://en.wikipedia.org/wiki/Laser_safety, downloaded October 15, 2013. resolution database of the Earth. 5 The National Enhanced Elevation Assessment (NEEA), sponsored by the United States Geological Survey (USGS), has documented the enormous economic value to the United States of obtaining and maintaining this information. With over 600 applications documented in the NEEA, 6 it is clear that the exploitation of 3-D data is just beginning. 5 The 2013 International Lidar Mapping Forum (ILMF) hosted attendees from over 30 countries. 6 Dewberry, 2011, Final Report of the National Enhanced Elevation Assessment (revised 2012): Fairfax, Va., Dewberry, 84 pp. plus appendixes. Available at http://www.dewberry.com/Consultants/GeospatialMapping/ FinalReport-NationalEnhanced ElevationAssessment.

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INTRODUCTION 13 Coherent laser radar was originally developed to mitigate the fact that detectors at CO2 laser wavelengths were rather poor, and heterodyne detection was employed to overcome detector noise and reach the shot noise limit by using a powerful local oscillator. Later, the ability to measure small frequency shifts in coherent ladar was used to measure the velocity of moving targets, using both CO2 and solid-state ladars. Since those early days, coherent ladar has blossomed to include ultraprecise range measurement, laser vibrometry, synthetic aperture ladar, spatial heterodyne imaging, ladar using multiple transmit and receive apertures, and to encompass a wide array of laser and detector technologies (see Chapter 3). Coherent ladar is now part of the engineering trade space for many active EO needs, and enables the use of inexpensive detectors and commercial off-the-shelf (COTS) laser components. The reasons for selecting coherent ladar as a solution to achieve desired sensing goals have evolved as new sensing concepts emerge, and as a result the technology has evolved in new directions. One of the main reasons to use coherent ladar is to be able to capture the field (phase and intensity), rather than just the intensity, which is captured by a direct detection ladar. Given the dramatic expansion of coherent active EO in the past decade or so, the committee expects both applications and technological solutions to expand and proliferate. Fiber lasers are emerging as powerful and flexible sources for active EO systems. As discussed in Chapter 4, fiber lasers are best suited to low peak power waveforms (e.g., high pulse-repetition-frequency pulse trains, modulated continuous-wave waveforms) The advances in high-sensitivity, low-noise detectors (Chapter 4) are thus synergistic with the advances in fiber laser technology as applied to active EO systems. The frequency agility and spectral quality of seed sources for fiber lasers enable a wide range of high bandwidth waveforms for both direct detection and coherent systems. Fiber lasers can also be combined, using a variety of coherent and incoherent beam combining techniques to produce high- power beams with the waveform quality of the modest-power individual fiber lasers. Finally, all of these properties are available with COTS or near-COTS components with heritage in the optical communications industrial base. These properties combined, in concert with either high-sensitivity, low- noise detector arrays or coherent detection, lead the committee to expect fiber laser solutions to become ever more prevalent in advanced active EO systems. This will also lower the barrier for entry into advanced active EO systems to groups with access to the COTS market. Active EO sensing systems are sophisticated electro-opto-mechanical systems and are the product of a complex set of interdependent system engineering trades. The ultimate engineering solution for a given active remote-sensing problem will depend strongly on both the sensing problem at hand (parameters to be measured, operational range, propagation path, and so on) and any externally imposed constraints (system size, weight and power, operational environment, concept of operations, etc.). Given that the range of sensing possibilities and phenomenologies for active EO sensing is so much broader and richer than either passive EO sensing or radar, the committee expects the range of technological solutions available to the system designer to further broaden in the foreseeable future. One corollary to this situation is that observations of specific technology development choices and paths can provide significant insight into the interests and intentions of the technology developers. An early example of a range-resolved image using a CO2 laser is shown in Figure 1-4, which illustrates many of the important features of laser radar systems. As a result of the availability of long wave infrared detectors and optics, CO2 lasers were the coherent illumination sources of choice for active EO systems from the early 1970s until the 1990s. Most CO2 laser radars were at a wavelength of 10.6 µm, although some used different isotopes of CO2 to avoid atmospheric absorption at that wavelength. 7 The system that produced the image shown in Figure 1-4 achieved the angular resolution of an optical system and the range resolution and precision of a very advanced radar. Specifically, Figure 1-4 demonstrates milliradian angular resolution and better than 30 cm range precision through only a 2.5 cm clear aperture. 7 P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, “A history of laser radar in the United States,” Proceedings of SPIE, 7684: 76840T.

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14 LASER RADAR Original Image First Range Gate Second Range Gate Last Range Gate FIGURE 1-4 Early CO2 ladar images. The original gray-scale encoded image is shown at the top left. The subsequent range images were produced by selecting a range window and displaying all returns within that window in white and all other ranges in black. The second and last range windows are more distant than the first. By gating out scattered light returning at different times, different features in the foreground and background of the scene can be isolated. SOURCE: P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, “A history of laser radar in the United States,” Proceedings of SPIE, 7684: 76840T. Another very interesting feature of active EO sensing is the ability to gate out the light backscattered from obscurants. This results from the fact that it utilizes a high-bandwidth sensor with fine range resolution. A ladar can look at returns from a given range and ignore those that come back sooner or later. This is illustrated in Figure 1-4, where the power lines (bottom left) are clearly visible

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INTRODUCTION 15 FIGURE 1-5 The first laser designator is shown on the left. The Pave Way laser-guided bomb, shown on the right, has revolutionized tactical air-to-ground warfare since its introduction in 1968. SOURCE: P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, “A history of laser radar in the United States,” Proceedings of SPIE, 7684: 76840T. FIGURE 1-6 The Thanh Hoa (Dragon’s Jaw) bridge in Vietnam off its abutment after attack. SOURCE: See http://en.wikipedia.org/wiki/File:ThanhHoaBridge1.jpg. through and behind the tree foliage (top right). So long as there is sufficient return signal penetrating the obscurant to be detected, backscatter can be eliminated. This can be a significant advantage in military engagements in which smoke or fog may be encountered. Active EO sensors have long played an important role in tactical military systems. While the technology has the potential for a wide range of applications, implementation has been limited by cost. Precision-guided munitions based upon semiactive EO technologies (Figure 1-5) have revolutionized modern warfare since laser designators first demonstrated their effectiveness by destroying the Thanh Hoa Bridge in Vietnam (Figure 1-6) over 40 years ago. 8 The introduction of precision-guided munitions, enabled by semi-active laser designators and weapon seekers, was a revolution in military affairs. Laser designators and seekers are a bistatic form of active EO sensing—the illuminator and sensor are located in different places. “Wind sensing, navigation, long-range target tracking, terrain following, two-dimensional (2-D) and three dimensional (3-D) imaging, and velocity detection are other laser radar uses that have been pursued.” 9 CO2 laser-based ladar navigation systems have been deployed but have gone out of common use with the advent of global positioning system (GPS) navigation. 10 Modern ladar systems do, however, offer an alternative for navigation in GPS-denied environments. 8 J.T Correll, 2010, “The emergence of smart bombs,” Air Force Magazine, March: 60. 9 P.F. McManamon, M. Huffaker, and G. Kamerman, 2010, “A history of laser radar in the United States,” Proceedings of SPIE, 7684: 76840T. 10 Ibid.

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16 LASER RADAR APPLICATIONS Numerous other applications of active EO technologies are discussed in this report. The specific applications mentioned below give a flavor of some of the major opportunities. More discussion of these applications can be found in Chapters 2 and 3. 3-D Mapping Ladar systems are rapidly emerging as the global standard for commercial 3-D mapping and are increasingly used to support mapping for U.S. military operations. Commercial 3-D mapping ladars are proliferating around the world and operate effectively in secure airspace.11 According to the NEEA, lidar data had been collected over 28 percent of the conterminous United States and Hawaii as of 2011. 12 The availability of ladar-generated 3-D maps has enabled immediate responses to humanitarian and emergency situations. After the earthquake in Haiti, the affected area was 3-D mapped by the Airborne Ladar Imaging Research Testbed (ALIRT) system to assure that victims were not migrating into flood-prone areas, to monitor the movement of displaced persons, and to evaluate the ability of candidate routes for disaster relief supplies to carry traffic. 13 After the attacks on the World Trade Center on September 11, 2001, 3-D ladars were used to map the rubble at ground zero to determine if it was shifting or sinking. Movement of the rubble could have posed a hazard to emergency response personnel. In a recent Defense Advanced Research Projects Agency (DARPA) presentation 14 the director highlighted the 3-D mapping High Altitude Ladar Operational Experiment (HALOE) system. HALOE can map large areas in 3-D in a relatively short time. During Operation Enduring Freedom, over half of Afghanistan has been mapped with a fleet of airborne ladars. 15 High-resolution mapping of the bottom of water bodies has become an important tool for littoral and riverine navigation. This optical bathymetry typically operates in the blue-green wavelength region for improved water penetration. The committee expects that ladar will eventually replace most standard surveying techniques to provide native 3-D maps with significant economic benefits. Wind Sensing Global wind sensing can significantly improve weather prediction. Lidars are ideal for mapping global winds if one or more lidars can be placed in orbit. These can be either coherent Doppler lidars or direct-detection lidars that measure velocity via Doppler-induced spectral shifts. Wind sensing has been used around large wind turbine farms to assist in planning how to best harvest electricity from the winds; it can be used by the military for improved cargo or bomb drop accuracy, or for improved aim point accuracy on unguided weapons. Lidars can also be used on a sailboat to find the best method of using the winds or on an aircraft to detect clear air turbulence. At airports, a lidar can help detect microbursts and wind shear and can assist in planning aircraft separation. Global wind sensing from orbit could significantly improve weather prediction accuracy. Wind sensing is expected to continue to be an important application of future lidar systems. 11 Michael S. Renslow editor, 2012, Manual of Topographic Lidar, American Society for Photogrammetry and Remote Sensing, 34. 12 Available at http://nationalmap.gov/3DEP/neea.html. 13 Available at http://www.ll.mit.edu/news/pathfindercover.html. 14 Regina Dugan, former director of DARPA, Plenary Presentation, 2011, Defense Security, and Sensing Symposium, International Society for Optical Engineering (SPIE), Orlando, Fla., April 26, 2011, available at http://spie.org/x48217.xml. 15 Personal communication from Walter Buell, committee member, August 26, 2013.

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INTRODUCTION 17 FIGURE 1-7 The AGM-129A is a subsonic, turbofan-powered, air-launched cruise missile. To reduce electronic emissions from the missile, the radar was replaced with a combination of inertial navigation and terrain contour matching (TERCOM) enhanced with highly accurate speed updates provided by a laser Doppler velocimeter. 16 SOURCE: See http://commons.wikimedia.org/wiki/File:Agm-129_acm.jpg. Robotic Vehicle Control The concept of robotic, or at least remotely controlled, combat vehicles is not new, but could be significantly enabled by active EO sensing. The German V-1 “buzz bomb” of the Second World War, the modern cruise missile, and the naval torpedo are operational examples of robotic vehicles. The AGM- 129A, shown in Figure 1-7, even relies upon active EO for its guidance. Previous robotic guidance systems required manual selection of the target, or a course to the target. The emergence of high-resolution 3-D imaging, near-real-time processing, and automatic target recognition algorithms could result in the deployment of autonomous, robotic combat vehicles. The high precision of ladar ranging makes it an attractive technology solution for fine-grained self-navigation of autonomous vehicles. Ladar has additional advantages for vehicular applications, such as coping with real-world obscurants such as fog, dust, and rain. As an example, all of the vehicles in the DARPA Grand Challenges (2004 and 2005) 17,18,19 and 2007 DARPA Urban Challenge 20 competitions were equipped with ladar sensors. The essential role of ladar technology for this application was local navigation. Stanley, Stanford University’s winning entry in the 2005 race (now in the Smithsonian), is shown in Figure 1-8. 16 P. McManamon, M. Huffaker, and G.W. Kamerman, 2010, “A brief history of laser radar,” Military Sensing Symposia Specialty Group on Active-EO Systems, Orlando, Fla., September 28-30. 17 Umit Ozguner, Keith A. Redmill and Alberto Broggi,
2004, “Team Terramax and the DARPA (1) Grand Challenge: A General Overview,” IEEE Intelligent Vehicles Symposium: 232. 18 R. Behringer, S. Sundareswaran, B. Gregory, R. Eisley, B. Addison, W. Guthmiller, R. Daily and D. Bevly, 2004, “The DARPA grand challenge: development of an autonomous vehicle,” IEEE Intelligent Vehicles Symposium: 226. 19 Martin Buehler, Karl Iagnemma, and Sanjiv Singh, eds., 2007, The 2005 DARPA Grand Challenge: The Great Robot Race, Springer. 20 Martin Buehler, Karl Iagnemma, and Sanjiv Singh, eds., 2009, The DARPA Urban Challenge: Autonomous Vehicles in City Traffic, Springer, 2009.

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18 LASER RADAR FIGURE 1-8 Stanford University’s 2005 DARPA Grand Challenge winner, Stanley. SOURCE: Linda A. Cicero, Stanford University News Service, copyright 2005 The Board of Trustees of the Leland Stanford Junior University. Autonomous vehicles are typically equipped with a suite of sensors including radars, cameras, and GPS. GPS is ideal for course-following—for a set of waypoints for a multiple-mile course through desert terrain—but it cannot cope with the obstacles and hazards encountered on a more local scale, such as boulders and ditches or, in the case of the Urban Challenge, other movers. These autonomous vehicle competitions can be viewed as technology trials for self-driving automobiles, which are now being evaluated experimentally with the intent to commercialize. 21 This is potentially a substantial market for ladar technology. Experience with solutions for self-driving automobiles is likely to provide insights into related domains requiring motion coupled to considerable autonomy. Examples of domains include extraterrestrial rovers as well as possible future military applications such as battlefield robotics that may exploit other ladar technology features such as object identification for targeting or identify-friend-or-foe (IFF). The market consideration is important, since it will be a spur for reducing cost and advancing performance as well as maintaining an industrial base for ladar technology. Battlefield robotics is a particularly compelling national security consideration, as it is a priority for U.S. armed services 22 and is clearly a battle space game changer. While primarily focused on the logistics task, Boston Dynamics’ ladar-equipped “BigDog” 23 (Figure 1-9) is one precursor of battlefield robots that can traverse rough terrain, with considerable implications for national security, since BigDog enables fighting with fewer soldiers carrying less equipment with them. The possibilities for a weaponized version of BigDog can also alter battlefield tactics for both offense and defense. 21 Available at http://spectrum.ieee.org/automaton/robotics/artificial-intelligence/how-google-self-driving-car- works. 22 National Research Council. 2002, Technology Development for Army Unmanned Ground Vehicles, Washington, D.C.: The National Academies Press. 23 Marc Raibert, Kevin Blankespoor, Gabriel Nelson, Rob Playter, and the BigDog Team, 2008, “BigDog, the rough-terrain quadraped robot,” Proc. International Federation of Automatic Control.

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INTRODUCTION 19 FIGURE 1-9 Boston Dynamics’ BigDog quadraped robot. SOURCE: BigDog Robot image courtesy of Boston Dynamics. Object/Material Identification Object identification is a primary use for active EO sensing because of the power of the various ladar modes for object discrimination, discussed below. For example, a range-gated 2-D active EO image is far superior to a passive EO image at night. With active EO, quality illumination is assured, and one can use 1-μm or 1.5-μm illumination rather than the mid-wavelength infrared (MWIR, 2.7-6.2 μm) or long-wavelength infrared (LWIR, 6.2-15 μm) cameras required at night for passive sensors. This means that with active EO, the diffraction-limited spatial resolution can be at least 2-3 times better than with a passive camera, 24 and there is no crossover issue 25 where available signal becomes limited. 3-D imagery provides even more information, including the possibility of very high-resolution range information in each pixel. Laser vibrometry (see below) can provide high-quality object identification for most objects with moving parts, such as engines. Synthetic aperture ladar can provide 2-D or possibly 3-D images at long ranges. Use of multiple illuminating laser polarizations and wavelengths simultaneously can provide additional discriminants. Speckle characteristics caused by interference in the light scattered by the target can also be used to help identify an object, as can surface material identification, using surface fluorescence or other methods. 24 Passive short-wave infrared (SWIR) imaging is possible at night using airglow (Meinel bands in the 1.3-1.9 µm range). This would offer spatial resolution comparable to that of the active EO system discussed here. 25 At crossover the contrast of the object compared to the background goes to zero, making it difficult to see. Crossover occurs twice a day for passive IR sensing, once as an object is heating up, and once as it is cooling off.

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20 LASER RADAR FIGURE 1-10 Retroreflectors on the Moon (left) and a satellite (right) aid in precise laser ranging. SOURCE: See http://library01.gsfc.nasa.gov/gdprojs/images/LAGEOS.jpg (left) and see http://www.hq.nasa.gov/office/pao/ History/alsj/a11/AS11-40-5952.jpg (right). Vibration Measurement/Object Characterization The ability to measure very fine vibrations using active EO sensors enables status determination in many situations and is a method of characterizing objects. 26 Vibration measurements can potentially determine whether power is flowing through a transformer or whether liquid is flowing through a pipe, based on the acoustic or vibrational signature produced by such activities. It may be possible to determine not only how many cylinders an engine has but even if it needs a tune-up. Laser vibrometers have been used to determine based on harmonic variations, whether a bridge is degrading. Companies concerned about noise from machinery use laser vibrometers to find noise sources in the machinery, and they can be used to make quieter cars and airplanes. In the future, oil companies may be able to explore for oil using laser vibrometers. Anytime there is a need to characterize object motion, a laser vibrometer will be a useful sensor. Satellite Laser Ranging Precise long-distance ranging can be facilitated by attaching retroreflectors on the target. Retroreflector arrays have been deployed on the moon for precise ranging. In Figure 1-10 the array left by Apollo 11 is shown on the left. On the right of Figure 1-10 is shown the LAGEOS-1 satellite, which was launched in 1976 and is covered with 426 corner-cube retroreflectors to enhance laser ranging. 26 An example of one commercial system can be found at: http://www.polytec.com/us/.

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INTRODUCTION 21 FIGURE 1-11 The Airborne Laser Mine Detection System (ALMDS). SOURCE: Photo credit: Northrop Grumman Corporation (left) and http://www.navy.mil/management/photodb/photos/100608-N-0001S-002.jpg (right). Underwater Mine Detection Water is very opaque to microwave radiation because of its conductivity. While blue-green lasers may not penetrate to large depths, they do penetrate to depths of tens of meters, depending on water turbidity. Just as blue-green lasers can be used for short-range underwater communication, they can be used to find objects in the water. 3-D mapping of coastal underwater areas can locate mines, a very valuable service for the military. Figure 1-11 shows a mine detection system. Weapon Seekers A 3-D ladar can provide range as well as angle/angle information. This will make closing on a target easier. Also, coherent ladar can provide target velocity, and direct-detection ladars can use multiple range measurements to obtain velocity. Systems such as the Air Force Research Laboratory’s (AFRL’s) Low Cost Autonomous Attack System (LOCAAS) have used ladar to make a 3-D image of objects and automatically recognize them. This could provide an autonomous seeker capability. Under the usual offensive rules of engagement, the United States does not permit a kill decision to be made without the involvement of a responsible human decision maker, as would be possible with robotic attackers. It is not obvious, however, that a potential adversary will employ the same operational constraints that the United States employs. The same observation could be made about U.S. adherence to eye-safety standards (see Box 1-4). The uses of active EO sensing could be a technological surprise if a state or nonstate actor were to impose fewer ethical or policy constraints on an EO-sensor-equipped weapon system. Police Speed Measurement Police forces around the world now use ladar to identify vehicles that are speeding. These systems are relatively inexpensive, on the order of thousands of dollars. For a police force, the ladar systems pay for themselves very rapidly. Most, if not all, current police speed measurement ladars are direct-detection ladars, using multiple range measurements to estimate speed.

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22 LASER RADAR Entertainment The use of active EO sensing is only limited by the imaginations of people using it; for example, the Microsoft Kinect game system shows an innovative use of active EO sensing. The committee was also briefed on research aimed at developing a handheld video camera that would generate “real” 3-D movies. 27 Such future commercial entertainment applications could provide profitable opportunities for developing the technology. REPORT SCOPE AND COMMITTEE APPROACH By comparison with passive EO sensing systems reviewed in an earlier NRC report, 28 the added complexity of laser illumination in active EO systems means that the scope of this report is considerably broader, as reflected in the committee’s statement of task (see the Preface). In order to focus the report as much as possible on the sponsors’ priorities, the committee chose to bound the discussion in several ways. First, it distinguished between what can be called the “engineering limits” of key technologies (those that may be exceeded by the invention of new materials or methods) and the fundamental limits (those imposed by the laws of physics) and tended to focus on the latter. Second, the committee focused on active EO technologies that might be developed by potential adversaries in the next 10-15 years to generate technological surprise. The adversaries considered included peer countries with highly developed technological bases as well as nonstate actors whose capabilities might derive only from access to commercial off-the-shelf systems. Third, the committee attempted to identify areas of foreign strength in active EO technologies (as well as areas in which the United States is falling behind) based on the open literature and its own expertise, as required in the statement of task (see the Preface). It did not attempt to describe and characterize active EO R&D programs in terms of either content or level of funding. Such an undertaking would not only have been time- and resource-intensive, it would likely have been futile because many such government programs are classified or not discussed in the open literature. Thus, the committee chose to focus on technologies and key application areas rather than programs. Fourth, a comprehensive examination of active EO capabilities would include those for tasking EO assets, collecting data, processing data into useful information, exploiting the information, and disseminating the products in a timely way to the warfighter or other ultimate user in a user-friendly form. The acronym TCPED is sometimes used to describe this comprehensive chain of events. Communication methods, including considerations of speed, data fidelity, and data security, are critical at each stage of TCPED. The committee recognizes that each of these aspects of active EO sensing is important, but the statement of task places emphasis on the front-end technologies of TCPED such as lasers, optics, and detectors, and this is where the committee put its emphasis, although it did consider some aspects of processing sensor data into useful information. Finally, in discussion with the study sponsors, it was decided that this report would focus on active EO sensing systems and not on technologies for damaging them or other kinds of countermeasures that might be employed to reduce their effectiveness. However, active EO technologies that could be used for battle damage assessment or to assess the remaining capabilities of EO assets that had been attacked did fall into the sensing category and therefore were included in the committee’s scope. 27 Paul Banks, Tetravue, presentation to the committee on March 6, 2013. 28 National Research Council, 2010, Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays, Washington D.C.: National Academies Press.

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INTRODUCTION 23 STRUCTURE OF THIS REPORT This chapter has introduced active EO technologies and their applications. The remaining chapters roughly follow the tasks identified in the statement of task (see the Preface). Chapter 2 provides an overview of existing active EO approaches, including applications, methods, and what the data products look like. In Chapter 3, emerging active EO systems are explored, particularly those that might be realized in the next 10-15 years. The individual components of active EO systems, including lasers, detectors, associated optics, and signal processing, are discussed in Chapter 4. Finally, Chapter 5 explores the fundamental limits of these technologies and their implications for the future. Appendix A provides the biographies of members of the committee. Appendix B lists the committee’s public meeting sessions and the presentations made in them. Appendix C discusses various types of laser sources and their fundamental and engineering limits in a series of tables. Subsequent appendixes are classified or otherwise restricted and are available in the full classified version of this report. CONCLUDING THOUGHTS Active EO sensing provides a very rich phenomenology to explore. The laser, detector, and aperture technologies to support active EO sensing are maturing and becoming cheaper. The telecommunications revolution over the last few decades has dramatically improved the quality and lowered the cost of many components used in active EO sensing. Whether it is referred to as ladar, lidar, or laser radar, the application space is large and growing, and is being pursued diligently by the United States, its allies, and its potential adversaries around the world. Understanding the historical and current development trajectories of passive EO and radar systems will serve as a useful guide for projecting the future course of active EO sensing technologies. The committee performed a Web search of the annual number of publications in different regions of the world over the past 10 years that use the terms “lidar” or “ladar” in their titles or as their “topic;” the results are shown in Figure 1-12. The data show that there has been a steady increase in the number of such publications in each region. However, the annual number of publications in China has been increasing rapidly over the past several years. Active EO sensing is a game-changing sensing technology. Conclusion 1-1: Active EO sensing enables measurements not possible with passive EO sensors by exploiting the control of the illumination. Exploiting control of the illumination includes its coherence properties. Conclusion 1-2: Active EO sensing offers many of the controlled illumination advantages of radar, and by operating at much shorter wavelengths enables greater range precision and angular resolution as well as more literal interpretability. Recommendation 1-1: To avoid the possibility of technological surprise, the U.S. intelligence community should consider all potential uses of active electro-optical sensing, including those not allowed by the ethics and policies of the United States.

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24 LASER RADAR 700 600 500 400 China USA 300 W. Europe 200 100 0 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 FIGURE 1-12 The number of publications on lidar or ladar per year is growing in the labeled countries/regions. It appears that the growth in the United States is comparable to that in Western Europe, while the recent rapid increase in the research in China is remarkable. The number of publications in Western Europe was determined by summing those from Germany, France, Great Britain, and Italy, and then increasing this number by 19 percent, which is the fractional increase in the number of publications due to the other Western European countries during this 10-year period.