Melissa L. Nischan

NLOS UV communication for distributed sensor systems

Atmospheric scattering of ultraviolet light is examined as a mechanism for short-range, non-line-of-sight (NLOS) communication between nodes in energy-constrained distributed sensor networks. The physics of scattering is discussed and modeled, and progress in the development of solid state sources and detectors is briefly summarized. The performance of a representative NLOS UV communication system is analyzed by means of a simulation model and compared to conventional RF systems in terms of covertness and transceiver power. A test bed for evaluating NLOS UV communication hardware and modulation schemes is described.

Compact active hyperspectral imaging system for the detection of concealed targets

We have recently conducted a series of laboratory and field test to demonstrate the utility of combining active illumination with hyperspectral imaging for the detection of concealed targets in natural terrain. The active illuminator, developed at MIT Lincoln Laboratory, is a novel microlaser-pumped fiber Raman source that provides high- brightness, subnanosecond-pulse-length output spanning the visible through near-IR spectral range. The hyperspectral- imaging system is comprised of a compact, grating-based spectrometer that uses a gateable, intensified CCD array as the detector element. The illuminator and hyperspectral imaging system are mounted on a small platform that is itself mounted on a tripod and scanned in azimuth to build an image scene of up to several hundred spectral bands. The system has been deployed under a variety of environmental conditions, including night-time illumination, and on a variety of target scenes, including exposed and concealed plastic and metallic mine-like targets. Targets have been detected and identified on the basis of spectral reflectance, fluorescence signatures, degree of polarization, and range-to-target information. The combination of laser-like broadband illumination and hyperspectral imaging offers great promise in concealed or obscured target detection. On-going developments include the incorporation of broadband illuminators in the 1 to 2 micrometers and 3 to 5 micrometers spectral bands, with corresponding increases in spectral coverage of the imaging and detection systems.

Field testing and evaluation of a solar-blind UV communication link for unattended ground sensors

Non-line-of-sight ultraviolet (UV) communication technology to support unattended ground sensor communication is described. The concept exploits atmospheric scattering of ultraviolet light to achieve modest data rates under non line-of-sight (ground-to-ground) and obstructed line-of-sight (foliage penetration) conditions. The transmitter consists of a digitally modulated UV source and the receiver employs a sharp cutoff solar-blind absorption filter coupled to a channel photomultiplier module. Prototype semiconductor UV sources with center wavelengths in the solar blind region (<280nm) already offer higher power efficiency than lasers, along with advantages in size, simplicity, and flexibility relative to both lasers and traditional mercury sources. Once commercialized, semiconductor UV sources will also offer significant cost savings over traditional gas-discharge and solid-state UV sources. In this paper, the temporal and spectral properties of a number of prototype semiconductor UV sources are presented and compared to a low-pressure mercury vapor source. Efficient modulation and data coding methods compatible with the output characteristics of both sources are discussed, and measurements from recent test bed experiments are presented.

Analysis of HYDICE noise characteristics and their impact on subpixel object detection

A number of organizations are using the data collected by the HYperspectral Digital Imagery Collection Experiment (HYDICE) airborne sensor to demonstrate the utility of hyperspectral imagery (HSI) for a variety of applications. The interpretation and extrapolation of these results can be influenced by the nature and magnitude of any artifacts introduced by the HYDICE sensor. A short study was undertaken which first reviewed the literature for discussions of the sensors noise characteristics and then extended those results with additional analyses of HYDICE data. These investigations used unprocessed image data from the onboard Flight Calibration Unit (FCU) lamp and ground scenes taken at three different sensor altitudes and sample integration times. Empirical estimates of the sensor signal-to-noise ratio (SNR) were compared to predictions from a radiometric performance model. The spectral band-to-band correlation structure of the sensor noise was studied. Using an end-to-end system performance model, the impact of various noise sources on subpixel detection was analyzed. The results show that, although a number of sensor artifacts exist, they have little impact on the interpretations of HSI utility derived from analyses of HYDICE data.

Demonstration system and applications for compact wireless ultraviolet communications

In battlefield situations, as well as other distributed sensing applications, networks of small, low-cost wireless sensors require short-range communication links that are low-power and difficult to detect at standoff distances (covert). Currently, short-range (< 100m) state-of-the-art ground-to-ground radio frequency (RF) links require line-of-sight for reliable connectivity, and may require 50 to 100 times more power for the transceiver electronics than what is radiated by the transmitter. Furthermore, the RF transmit power necessary to overcome R4 losses near the ground makes the links easily detectable at stand-off ranges unless sophisticated waveforms or highly directive antennas are employed, both of which are inconsistent with low-cost, low-power transceivers. In contrast, baseband optical communication links in the mid-ultraviolet (UV) band can exploit atmospheric scattering to achieve non line-of-sight (NLOS) operation with low-power transceivers at wavelengths that are difficult to detect at stand-off ranges. This paper reviews NLOS UV communication concepts, phenomenology, and the evolution of device technology. A portable communications test bed is described, and recent outdoor tests with 340nm semiconductor emitters are summarized. An indoor FM voice link is described, as an example of the compact form-factor that can be achieved with current technology. The paper concludes with a discussion of potential applications.

Short-range NLOS ultraviolet communication testbed and measurements

Atmospheric scattering of ultraviolet light is examined as a mechanism for short-range, non-line-of-sight (NLOS) communication between nodes in energy-constrained distributed sensor networks. A test bed for evaluating NLOS UV communication hardware and modulation schemes is described, and the bit error rate measured in the test bed is used to validate a numerical performance model. Design tradeoffs for a baseband UV transceiver are discussed and performance estimates obtained from the validated numerical model are presented.

Collaborative sensing test bed and experiments

A test bed to support research in collaborative sensing by means of energy-constrained, wireless networks is described. The test bed incorporates commercially available sensors and wireless networking technology, with the emphasis on providing a low cost, high-performance, easy to use development environment. The sensors comprising the baseline test bed, acoustic/seismic multimode sensors and panoramic color cameras, are described. A strategy for incorporating new sensors and deploying elements of the test bed to support field tests is described. Results of a field test conducted to demonstrate collaborative tracking, geolocation, and targeting of a tank are summarized. An example of motion detection and image extraction from a panoramic camera is presented.

Detection of manmade objects

Hyperspectral imagers have the unique capability of doing both material identification and anomaly detection. However, hyperspectral imagers with hundreds of co-registered contiguous bands are difficult to field particularly if real-time processing is required. With judicious choice of bands, the anomaly detection performance of a multispectral sensor can rival that of hyperspectral sensors. In order to achieve this performance, the choice of multispectral bands relies on the presence of exploitable target or background spectral features. The universality of these features will determine the overall utility of a multispectral system. We have discovered that water vapor features in the SWIR (Short Wave InfraRed) can be used to distinguish manmade objects from natural backgrounds. As an example, we will show that two broad bands chosen to exploit these features make most manmade objects detectable in the presence of natural clutter with few false alarms.