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Summary of the Sensing and Positioning Technology Workshop of the Committee on Nanotechnology for the Intelligence Community: Interim Report TOPIC 5: RADIO/RADAR/OPTICAL TAGS Four papers were presented on these topics, by Mark Shellans, Pathfinder Technology, Inc.; Bill Hurley, Inkode; Dennis Prather, University of Delaware; and Stephen Griggs, DARPA. VESTA/PARD TECHNOLOGY Mark Shellans discussed two related technologies for the acquisition or analysis of data on subjects at considerable distances: vibro-electronic signature target analysis (VESTA) and passive acoustic reflection devices (PARDs). These technologies could be used to identify vehicles on the battlefield as well as communicate covertly with forces on the ground. The VESTA technology is based on the fact that physical motions of target objects will cause slight modulations of the reflected signals—for example, the sound vibrations of a moving vehicle modulate the return signal when the vehicle is illuminated by radar. Based on the modulated return, empirical, non-pattern-matching algorithms can be used to identify the class of vehicle and even to recognize a particular vehicle’s unique vibrational pattern, provided the algorithm was previously “trained” on that vehicle’s signal. Shellans’s approach is based on the principle that the number of features that describe the physical state of a system can be represented by a multidimensional polynomial, and any arbitrary multidimensional polynomial can be matched by expansion of a McLauren series. A typical recognition problem might involve analyzing a polynomial surface in 12 dimensions; the sheer number of possible patterns in a library of templates for pattern matching would make a pattern-matching approach unmanageable. Instead, one must limit the dynamic range of the variables or cluster around variable values for targets one expects to see. Shellan’s algorithm uses a “Twenty-Questions” approach to narrow down the phase space. The VESTA approach enables fine-grain discrimination of signals requiring orders of magnitude less storage and processing than would otherwise be needed to analyze the signal, and Shellans believes this property will make the technology more useful for nanoscale devices with limited processing capabilities. Shellans has used radar analysis techniques and recorded the acoustic characteristics of five different cars from a distance to train the system using a single 2- to 3-second scan of a Doppler radar. Later, the cars were driven past the detector many times in random order and could be identified with 100 percent accuracy. A similar technique was also used successfully with multiband, polarimetric synthetic aperture radar images of forested terrain to locate objects—for example, downed aircraft, tanks, or even cars—under the foliage. The technique can be applied to other kinds of sensor data, including biometric sensors. Shellans also described a passive acoustic reflection device that superimposes information onto reflected radar signals. A radar receiver would be able to extract the superimposed information using the same analytical methods used to identify unique vehicle sounds. Because the PARD is not a transmitter,
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Summary of the Sensing and Positioning Technology Workshop of the Committee on Nanotechnology for the Intelligence Community: Interim Report it could be used by ground-based friendly forces to communicate covertly with radar systems on aircraft. A simple message (e.g., 30 bits) could be coded into the PARD and read by a VESTA radar system passing overhead without the message being detectable to hostile forces. COVERT RADAR TAGS AND APPLICATIONS Bill Hurley discussed his company’s technology, which involves incorporating a large number of radar reflectors in a substrate to form passive RFID tags. The radar reflectors used by Inkode are usually simple aluminum fibers that form half-wave resonators within the object to be tracked (e.g., a piece of paper). The radar-reflecting fibers are approximately the same diameter as paper fibers (typically 6.5 mm long and 1.5 μm in diameter). Randomly oriented radar-reflecting fibers provide a unique backscatter pattern that can be read and stored in a database for future identification. Ordered patterns can also be designed so that individual resonators are coupled or decoupled, whatever is likely to give the optimum backscatter pattern. Tagging an object with this technology costs about 1 cent. When illuminated with radar, the backscattered fields interact to create a unique interference pattern that enables one tagged object to be identified and differentiated from other tagged objects. In the near field (that is, where there is an interaction between the tag and antenna), the backscatter depends on the position of the tag in the substrate, and information is represented in a scalar waveform. Beyond the near field, the backscatter is like a traditional radar, with no coupling between tag and antenna. For nonmilitary applications, the reader is less than 1 meter from the tag. For military applications, the reader and tag could theoretically be separated by a kilometer or more. The tags are the same in these two cases, but the reader is different. The normal operating point is 10 mW at 24 GHz. The three most commonly used tags are free, individual resonators, continuous filaments, and photolithographically printed patterns of filaments. They consist of aluminum/glass fibers, either coaxial or side by side. These may be adapted for inclusion in a variety of objects, including paper, airline baggage tags, book bindings, clothing and other fabrics, and plastic sheet. The reflectance of the half-wave resonators is very efficient. In a typical 8½ by 11 inch piece of paper, there are over 8,000 radar reflecting fibers. This can theoretically be seen from a distance comparable to the distance from which a 1 m2 target can be seen. NANOSCALE DESIGN AND FABRICATION CONSIDERATIONS FOR PHOTONIC TAGS AND RADAR DEVICES Dennis Prather discussed applications of nanoscale photonics technology and some of the tools being developed to fabricate these devices. Applications include (1) diffractive (as opposed to refractive) lenses that can reduce the size and weight of millimeter wave imaging systems; (2) polarization-dependent tags or reflectors that can be read with a CO2 laser; (3) integrated sensors using components made from photonic crystal devices—that is, devices that guide light based on the scattering properties created by tailoring the material profile—such as beam splitters, optical switches, and couplers; and (4) motion microsensors, both rotational and linear. The beam routing in photonic crystal devices is strongly wavelength-dependent; for instance, such devices exhibit a sharp spike in reflectivity over a narrow frequency bandgap. This property can be exploited in photonic tags. Prather discussed the capabilities of various fabrication methods for creating nanoscale patterned structures, such as e-beam and interferometric lithography, methods for optical drilling of nanoscale cylindrical holes through silicon, and techniques for making tapered connectors to link microscale
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Summary of the Sensing and Positioning Technology Workshop of the Committee on Nanotechnology for the Intelligence Community: Interim Report components to nanoscale components. He showed how silicon could be patterned on the nanoscale by E-beam lithography to create dramatically different images when viewed with horizontally (as opposed to vertically) polarized infrared laser light. Prather showed how a micro-ring laser patterned in silicon (~10 μm in diameter, with coupled output-sampling waveguides) could measure rotation rate and an optical waveguide can be inserted into a microcantilever MEMS device to measure linear motion. The small dimensions of nano-optical waveguides mean that relatively small electric fields (millivolts) can be used to modulate light transmission through the electro-optic effect, because these fields equate to kilovolts per meter. By coupling an RF antenna to a nano-optical waveguide fabricated from silicon and a few micrometers in width, for instance, one can impose millimeter-wave sidebands on the optical carrier using the electro-optic effect, separate the sidebands from the carrier with a dispersive element, and measure the sideband strength with a low-frequency detector, thus avoiding the need for any RF circuits. Future challenges include further development of nanoscale integration techniques and refining the compatibility of manufacturing processes for both micro- and nanostructures. DYNAMIC OPTICAL TAGS Stephen Griggs described a DARPA-funded program starting to develop small, retro-reflecting optical tags that can be attached to targets, assets, and precision special reference points. These tags would provide non-RF location and tracking, with covert, two-way data exchange in friendly and denied areas. The specifications are as follows: size, 25×25×5 mm (a small thickness is critical for covertness); operating temperature −40° to 70°C; data rate, >100 kbps; optically readable from a distance of 10 km (line of sight) by an airborne or handheld interrogator; operating time >2 months; acceptance angle >+/−60°; cost, <$100 per tag; and non-visually alerting. As the tagged object moves through a region, the Department of Transportation could record location information (via GPS) as well as other data (imagery, audio, etc.) that can provide vital information and decrease target ambiguity. The main technical challenges are to develop (1) thin, retro-reflecting optics that can be modulated and (2) tag-specific transceiver systems that are eye-safe at the tag (range of λ=1.3–2 μm) and that can search and interrogate quickly and automatically (both handheld and airborne versions). The power requirements for the tag are estimated to be about 75 mA-hours total over 2 months. Commercial lithium coin batteries can be used. They are available in the appropriate thickness and can provide up to 120 mA-hours at approximately 3 volts. The range requirement also appears achievable based on the sensitivity of present photomultiplier tubes relative to the expected return signal intensity. The program will be structured in three phases. The first, beginning in FY04, will develop various tag technologies and utilize a bench interrogator. The second, beginning in FY05 (assuming a positive go-no go decision), will focus on dynamic optical tag system design, including a handheld interrogator. If progress through FY05 is acceptable, prototype dynamic optical tag systems will be demonstrated using an airborne interrogator in FY 06 and 07. PANEL 5 DISCUSSION The discussion consisted largely of questions directed at individual presenters. Shellans was asked what materials are used in the PARD, given that one must modulate at rates greater than 100 kbps. He described the use of indium phosphide, which can be modulated at rates greater than 200 kbps (by
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Summary of the Sensing and Positioning Technology Workshop of the Committee on Nanotechnology for the Intelligence Community: Interim Report moving just the electron cloud). In the future, it should theoretically be possible to transmit voice using PARD. Hurley was asked how one can read a specific passive RF tag at a random angle if the tag is made up of randomly oriented fibers. The answer is that the computer processing the return signal can calculate what the return signal from each tag in its library should look like as it is rotated with respect to the interrogating radar and match the pattern to the observed pattern. Instead of inserting fibers into the material to be tagged, it is also possible to print metal lines on the surface. Currently, the main applications of these passive tags are in security documents (e.g., passports) and currency. Prather was asked why, given the potential value of photonic band gap materials, the technology is not in use. For example, one can control the reflection spectra off various materials (though it is hard to make the reflection isotropic) and one can use the band gap to confine light effectively in waveguides and achieve a very high photonic “wiring density” and overlay capability with no crosstalk between waveguides. His answer was that satisfactory processes for integrating hybrid devices on silicon still need to be developed, and losses need to be reduced. Griggs was asked how small the optical retroreflectors can be made. With UV interrogating light, it is possible to make them with micrometer-scale dimensions. All of the corner cube reflectors being evaluated can modulate light at frequencies of at least 200 kHz. To make retroreflectors more covert, one can make them reflective at only a single frequency using photonic band gap materials. The reflection wavelength can be tuned to suppress the visible wavelength signal. Gratings can also be used as retroreflectors if they are oriented at the right angle, but of course the angle is wavelength-dependent. Griggs stressed that if one wants to make the tags very small and have reasonable data transmission rates, they must be optical. No RF device can match the size and data rate of optical devices. For example, using an optical device one can detect voice signals on the ground from a U2 aircraft using a 2-ft. by 2-ft. box. RF may be appropriate if lower data rate, power, and range are acceptable. The disadvantage of optical devices is that they must operate within the line of sight. Covert dynamic optical tags are expected to be manually placed on noncompliant targets.
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