Lincoln Laboratory Journal - Volume 15, Number 1
Standoff Acoustic Laser Technique to Locate Buried Land Mines
By Robert Haupt and Kenneth Rolt
The worldwide proliferation of land mines leads to thousands of civilian casualties each year and threatens military forces who patrol hostile territories. To reduce these casualties, military and humanitarian organizations seek methods to detect the large variety of mines being deployed. Many mine detection systems currently under development can detect only metal or a specific mine feature, have limited standoff range, or are impractical for field operations. A promising approach uses acoustic waves to induce mechanical vibrations in both plastic and metal mines. The vibration field above these mines can then be measured remotely with a laser Doppler vibrometer. This article describes a method to advance acoustic land-mine detection by increasing standoff range from the minefield and by developing a more practical lightweight system. We take a novel approach to excite mines by using a parametric acoustic array (PAA) source to transmit a highly directive sound beam from a safe distance. We discuss the standoff system concept, the process of PAA wave generation, and the coupling of acoustic waves to the ground to excite mines. A proof-of-concept system, built at Lincoln Laboratory, deploys a commercial PAA and a commercial laser vibrometer. We tested the system at a land-mine facility and measured distinct vibration signatures from buried anti-personnel mines. The overall concept shows promise. The PAA tested in these experiments, however, was developed for home entertainment and has marginal power for land-mine detection, even at close range. A system suitable for standoff detection requires more acoustic power and substantial modification. We estimate that power gains up to 50 dB may be achievable, and we discuss alternatives to the commercial PAA design.
Jigsaw: A Foliage-Penetrating 3D Imaging Laser Radar System
By Richard Marino and William Davis Jr
Situation awareness and accurate target identification are critical requirements for successful battlefield management. Ground vehicles can be detected, tracked, and imaged by using airborne or space-borne microwave radar. Obscurants, however, such as camouflage net and tree-canopy foliage can degrade the performance of these radars. Foliage can be penetrated with long-wavelength microwave radar, but generally at the expense of imaging resolution. The DARPA Jigsaw program includes the development of high-resolution three-dimensional (3D) imaging laser radar (ladar) sensor technology and systems that can be used in airborne platforms to image and identify military ground vehicles hiding under camouflage or foliage. Lincoln Laboratory has developed a rugged and compact 3D imaging ladar system that successfully demonstrates this application. The sensor system, including a microchip laser and novel focal-plane arrays, has been integrated into a UH-1 helicopter. The sensor operates day or night and produces high-resolution 3D spatial images by using short laser pulses and a focal-plane array of 32 × 32 Geiger-mode avalanche photodiode (APD) detectors with independent digital time-of-flight counting circuits at each pixel. With appropriate optics, the 32 × 32 array of digital time values represents a 3D spatial image frame of the scene. Successive image frames from the multi-kilohertz pulse-repetition-rate laser pulses are accumulated into range histograms to provide 3D volume and intensity information. In this article, we describe the Jigsaw program goals, our demonstration sensor system, and the data-collection campaigns, and we show examples of 3D imaging with foliage and camouflage penetration. Other applications for this 3D imaging direct-detection ladar technology include robotic vision, navigation of autonomous vehicles, manufacturing quality control, industrial security, and topography.
Simulation of 3D Laser Radar Systems
By Michael O’Brien and Daniel Fouche
We describe a computer model that was developed to simulate the performance of three-dimensional (3D) laser radars (ladars) that use arrays of Geiger-mode avalanche photodiode (APD) detectors. The model allows considerable flexibility in the specifications of sensor characteristics, 3D scene, background light, and dynamics of the sensor platform. It is used to help design and predict the performance of 3D ladars used for surveillance, city topography, combat identification, and other applications. In particular, the model was used to help design the Laboratory’s foliage-penetrating airborne 3D ladar for the DARPA-sponsored Jigsaw program. Results of the model’s simulations of Jigsaw Phase 2 experiments agree quantitatively with actual measurements of a tank in the open. In addition, the model’s simulations agree well qualitatively with actual measurements of a tank under trees. Both the simulation and the Jigsaw data demonstrate an ability to obtain detailed 3D images of objects under thick foliage.
Pose-Independent Automatic Target Detection and Recognition Using 3D Laser Radar Imagery
By Alexandru Vasile and Richard Marino
Although a number of object-recognition techniques have been developed to process terrain scenes scanned by laser radar (ladar), these techniques have had limited success in target discrimination, in part due to low-resolution data and limits in available computation power. We present a pose-independent automatic target detection and recognition system that uses data from an airborne three-dimensional imaging ladar sensor. The automatic target recognition system uses geometric shape and size signatures from target models to detect and recognize targets under heavy canopy and camouflage cover in extended terrain scenes. The system performance was demonstrated on five measured scenes with targets both out in the open and under heavy canopy cover, where the target occupied between 1% to 10% of the scene by volume. The automatic target recognition section of the system was successfully demonstrated for twelve measured data scenes with targets both out in the open and under heavy canopy and camouflage cover. Correct target identification was also demonstrated for targets with multiple movable parts in arbitrary orientations. The system achieved a high recognition rate along with a low false-alarm rate. Immediate benefits of the presented work are in the area of automatic target recognition of military ground vehicles, in which the vehicles of interest may include articulated components with variable position relative to the body, and may come in many possible configurations. Other application areas include human detection and recognition for homeland security, and registration of large or extended terrain scenes.
Iterative Demodulation and Decoding for Protected Satellite Communications
By Wayne Phoel
Military satellite communications (Milsatcom) systems use frequency hopping over a wide bandwidth to provide protection from hostile jamming, interception, and detection. These systems are being challenged to transport more information at higher rates, among smaller and more mobile terminals, with little or no increase in allocated bandwidth. Meeting these challenges requires advanced bandwidth-efficient and power-efficient signaling techniques. This article describes an approach to error-control coding and modulation that can help achieve the requirements of future Milsatcom systems. The receiver in this approach iterates between demodulation and decoding, which enables near-coherent performance with minimal reference symbol overhead. Also, the decoding process is augmented so that, in the presence of jamming, the receiver estimates the jammer state and combines information appropriately from different hops.
MIMO Wireless Communication
By Daniel Bliss, Keith Forsythe, and Amanda Chan
Wireless communication using multiple-input multiple-output (MIMO) systems enables increased spectral efficiency for a given total transmit power. Increased capacity is achieved by introducing additional spatial channels that are exploited by using space-time coding. In this article, we survey the environmental factors that affect MIMO capacity. These factors include channel complexity, external interference, and channel estimation error. We discuss examples of space-time codes, including space-time low-density parity-check codes and space-time turbo codes, and we investigate receiver approaches, including multichannel multiuser detection (MCMUD). The ‘multichannel’ term indicates that the receiver incorporates multiple antennas by using space-time-frequency adaptive processing. The article reports the experimental performance of these codes and receivers.