Larry B. Stotts

Automatic target detection and recognition in multiband imagery: a unified ML detection and estimation approach

Multispectral or hyperspectral sensors can facilitate automatic target detection and recognition in clutter since natural clutter from vegetation is characterized by a grey body, and man-made objects, compared with blackbody radiators, emit radiation more strongly at some wavelengths. Various types of data fusion of the spectral-spatial features contained in multiband imagery developed for detecting and recognizing low-contrast targets in clutter appear to have a common framework. A generalized hypothesis test on the observed data is formulated by partitioning the received bands into two groups. In one group, targets exhibit substantial coloring in their signatures but behave either like grey bodies or emit negligible radiant energy in the other group. This general observation about the data generalizes the data models used previously. A unified framework for these problems, which utilizes a maximum likelihood ratio approach to detection, is presented. Within this framework, a performance evaluation and a comparison of the various types of multiband detectors are conducted by finding the gain of the SNR needed for detection as well as the gain required for separability between the target classes used for recognition. Certain multiband detectors become special cases in this framework. The incremental gains in SNR and separability obtained by using what are called target-feature bands plus clutter-reference bands are studied. Certain essential parameters are defined that effect the gains in SNR and target separability.

Hybrid Optical RF Airborne Communications

The use of hybrid free-space optical (FSO)/radio-frequency (RF) links to provide robust, high-throughput communications, fixed infrastructure links, and their associated networks have been thoroughly investigated for both commercial and military applications. The extension of this paradigm to mobile, long-range networks has long been a desire by the military communications community for multigigabit mobile backbone networks. The FSO communications subsystem has historically been the primary limitation. The challenge has been addressing the compensation of propagation effects and dynamic range of the received optical signal. This paper will address the various technologies required to compensate for the effects referenced above. We will outline the effects FSO and RF links experience and how we overcome these degradations. Results from field experiments conducted, including those from the Air Force Research Laboratory Integrated RF/Optical Networked Tactical Targeting Networking Technologies (IRON-T2) program, will be presented.

Optical communications in atmospheric turbulence

Recent experiments conducted under the Optical RF Communications Adjunct program demonstrate and validate the viability of hybrid free space optical communications links in heavy atmospheric turbulence. Long range air-to-mountain link closures were established under extreme atmospheric turbulence. The system implemented adaptive mechanisms such as adaptive optics, an optical automatic gain controller, forward error correction coding, and link-level retransmission to achieve low packet error rates for long distance links with heavy turbulence. The system, experiments, and results are presented and comparisons are made to statistical prediction models.

Free space optical communications: coming of age

Information superiority, where for the military or business, is the decisive advantage of the 21st Century. While business enjoys the information advantage of robust, high-bandwidth fiber optic connectivity that heavily leverages installed commercial infrastructure and service providers, mobile military forces need the wireless equivalent to leverage that advantage. In other words, an ability to deploy anywhere on the globe and maintain a robust, reliable communications and connectivity infrastructure, equivalent to that enjoyed by a CONUS commercial user, will provide US forces with information superiority. Assured high-data-rate connectivity to the tactical user is the biggest gap in developing and truly exploiting the potential of the information superiority weapon. Though information superiority is much discussed and its potential is well understood, a robust communications network available to the lowest military echelons is not yet an integral part of the force structure, although high data rate RF communications relays, e.g., Tactical Common Data Link, and low data SATCOM, e.g, Ku Spread Spectrum, are deployed and used by the military. This may change with recent advances in laser communications technologies created by the fiber optic communications revolution. This paper will provide a high level overview of the various laser communications programs conducted over the last 30 plus years, and proposed efforts to get these systems finally deployed.

Optical RF Communications Adjunct

The capacity to integrate RF and free space optical hybrid communications now feasible given advances in adaptive optics and optical automated gain control. The ORCA program is developing on operationally capable of highly reliable hybrid communications. This paper provides an overview of the ORCA systems and discusses some of the key developments in making the systems a reality.

Hybrid optical radio frequency airborne communications

Optical RF Communications Adjunct Program flight test results provide validation of the theoretical models and hybrid optical radio frequency (RF) airborne system concepts developed by the Defense Advanced Research Projects Agency and the U.S. Air Force Research Laboratory. Theoretical models of the free-space optical communications (FSOC), RF, and network components accurately predict the flight test results under a wide range of day and night operating conditions. The FSOC system, including the adaptive optics and optical modem, can operate under high turbulence conditions. The RF and network mechanisms of Layer 2 retransmission and failover provide increased reliability, reducing end-to-end packet error rates. Overall the test results show that stable, long-range FSOC is possible and practical for near-term operations.

Progress towards reliable free-space optical networks

Free-space optical communications (FSOC) links provide an appealing and complementary enhancement to current radio frequency (RF) systems because of their inherent benefits of high-bandwidth and directional communication. Although FSOC systems can be inoperable through clouds or thick fog, employing them in a hybrid RF/optical link configuration can yield a system that can operate under most weather conditions and provide high-bandwidth, secure, jam-resistant communications under most conditions. Beyond attenuation effects and line-of-sight limitations, FSOC link performance is primarily driven by optical turbulence along the beam path, which leads to severe fluctuation of the communications channel and distortion of the signal wavefront. Many methods have been either modeled or field-tested to reduce this fading with varying degrees of success. The approach taken in the DARPA Free Space Optical Experimental Network Experiment (FOENEX) program is a continuance of systems development work funded and developed by DARPA, the Air Force Research Laboratory (AFRL), and the Naval Research Laboratories (NRL). The use of QoS-based link-level techniques was successfully demonstrated under the AFRL Iron T2 and DARPA ORCA programs. The FOENEX program extends these methods via technology developments at the physical layer as well as implementing the network methods to ensure end-to-end high bandwidth connectivity. This paper focuses on progress to date in networking technologies that will support free space optical networks (FSON) out to 200 km ranges even when individual links are disrupted up to 5% of the time.

Clutter rejection using multispectral processing

ABSTRACT The DARPA MUSIC program is presently collecting data to support multi-spectral infrared target detection in clutter.The plan of the MUSIC program is discussed first, followed by the theoretical basis of multi-spectral and recursive moving target indicator (RMTI) processing. An example using data from the MUSIC sensor is presented. In this example spectral-spatial processing of two bands is compared to registration and temporal processing of a singleband.1. INTRODUCTIONThe MUSIC program was initiated in October 1988 to study the application of passive multi-spectral infrared to the detection of air and ground targets. The targets of interest are buried in background clutter and have very weak signal to noise ratios. Interest in using multi-spectral techniques was stimulated by an analysis of multi-spectral datacollected by the NASA Thermal Infrared Multi-Spectral Scanner (TIMS) sensor in Australia in 1985 (Reference 1).This analysis, as well as others, suggested that multi-spectral processing techniques could greatly improve detectionstatistics against targets that are stationary or have little apparent motion relative to a highly cluttered background.In order for multi-spectral processing to be beneficial, the radiance of the target must be proportionately different in

Free-space optical communications link budget estimation

This paper describes a new methodology of estimating free-space optical communications link budgets to be expected in conditions of severe turbulence. The approach is derived from observing that the ability of an adaptive optics (AO) system to compensate turbulence along a path is limited by the transmitter and receiver Rayleigh range, proportional to the diameter of the optics squared and inverse of the wavelength of light utilized. The method uses the Fried parameter computed over the range outside of the transmitter and receiver Rayleigh ranges, to calculate the Strehl ratios that yield a reasonable prediction of the light impinging on the receiving telescope aperture and the power coupling into the fiber. Comparisons will be given between theory and field measurements. These comparisons show that AO is most effective within the Rayleigh ranges, or when an atmospheric gradient is present, and lesser so when the total range is much greater than the sum of the Rayleigh ranges.

High-sensitivity DPSK receiver for high-bandwidth free-space optical communication links

A high-sensitivity modem and high-dynamic range optical automatic gain controller (OAGC) have been developed to provide maximum link margin and to overcome the dynamic nature of free-space optical links. A sensitivity of -48.9 dBm (10 photons per bit) at 10 Gbps was achieved employing a return-to-zero differential phase shift keying based modem and a commercial Reed-Solomon forward error correction system. Low-noise optical gain was provided by an OAGC with a noise figure of 4.1 dB (including system required input loses) and a dynamic range of greater than 60 dB.