PREFACE
Radar land clutter constitutes the unwanted radar echoes returned from the earth’s surface that compete against and interfere with the desired echoes returned from targets of interest, such as aircraft or other moving or stationary objects. To be able to knowledgeably design and predict the performance of radars operating to provide surveillance of airspace, detection and tracking of targets, and other functions in land clutter backgrounds out to the radar horizon, radar engineers require accurate descriptions of the strengths of the land clutter returns and their statistical attributes as they vary from pulse to pulse and cell to cell. The problem of bringing statistical order and predictability to land clutter is particularly onerous at the low angles (at or near grazing incidence) at which surface-sited radars illuminate the clutter-producing terrain, where the fundamental difficulty arising from the essentially infinite variability of composite terrain is exacerbated by such effects as specularity against discrete clutter sources and intermittent shadowing. Thus, predicting the effects of low-angle land clutter in surface radar was for many years a major unsolved problem in radar technology.
Based on the results of a 20-year program of measuring and investigating low-angle land clutter carried out at Lincoln Laboratory, Massachusetts Institute of Technology, this book advances the state of understanding so as to “solve the low-angle clutter problem” in many important respects. The book thoroughly documents all important results of the Lincoln Laboratory clutter program. These results enable the user to predict land clutter effects in surface radar.
This book is comprehensive in addressing the specific topic of low-angle land clutter phenomenology. It contains many interrelated results, each important in its own right, and unifies and integrates them so as to add up to a work of significant technological innovation and consequence. Mean clutter strength is specified for most important terrain types (e.g., forest, farmland, mountains, desert, urban, etc.). Information is also provided specifying the statistical distributions of clutter strength, necessary for determining probabilities of detection and false alarm against targets in clutter backgrounds. The totality of clutter modeling information so presented is parameterized, not only by the type of terrain giving rise to the clutter returns, but also (and importantly) by the angle at which the radar illuminates the ground and by such important radar parameters as carrier frequency, spatial resolution, and polarization. This information is put forward in terms of empirical clutter models. These include a Weibull statistical model for prediction of clutter strength and an exponential model for the prediction of clutter Doppler spreading due to wind-induced intrinsic clutter motion. Also included are analyses and results indicating, given the strength and spreading of clutter, to what extent various techniques of clutter cancellation can reduce the effects of clutter on target detection performance.
The empirically-derived clutter modeling information thus provided in this book utilizes easy-to-understand formats and easy-to-implement models. Each of the six chapters is essentially self-contained, although reading them consecutively provides an iterative pedagogical approach that allows the ideas underlying the finalized modeling information of Chapters 5 and 6 to be fully explored. No difficult mathematics exist to prevent easy assimilation of the subject matter of each chapter by the reader. The technical writing style is formal and dedicated to maximizing clarity and conciseness of presentation. Meticulous attention is paid to accuracy, consistency, and correctness of results. No further prerequisite requirements are necessary beyond the normal knowledge base of the working radar engineer (or student) to access the information of this book. A fortuitous combination of national political, technological, and economic circumstances occurring in the late 1970s and early 1980s allowed the Lincoln Laboratory land clutter measurement project to be implemented and thereafter continued in studies and analysis over a 20-year period. It is highly unlikely that another program of the scope of the Lincoln Laboratory clutter program will take place in the foreseeable future. Future clutter measurement programs are expected to build on or extend the information of this book in defined specific directions, rather than supersede this information. Thus this book is expected to be of long-lasting significance and to be a definitive work and standard reference on the subject of land clutter phenomenology.
A number of individuals and organizations provided significant contributions to the Phase Zero/Phase One land clutter measurements and modeling program at Lincoln Laboratory and consequently towards bringing this book into existence and affecting its final form and contents. This program commenced at Lincoln Laboratory in 1978 under sponsorship from the Defense Advanced Research Projects Agency. The United States Air Force began joint sponsorship several years into the program and subsequently assumed full sponsorship over the longer period of its complete duration. The program was originally conceived by Mr. William P. Delaney of Lincoln Laboratory, and largely came into focus in a short 1977 DARPA/USAF-sponsored summer study requested by the Department of Defense and directed by Mr. Delaney. The Phase Zero/Phase One program was first managed at Lincoln Laboratory by Mr. Carl E. Nielsen Jr. and by Dr. David L. Briggs, and subsequently by Dr. Lewis A. Thurman and Dr. Curtis W. Davis III.
Early site selection studies for the Phase Zero/Phase One program indicated the desirability of focusing measurements in terrain of relatively low relief and at northern latitudes such as generally occurs in the prairie provinces of western Canada. As a result, a Memorandum of Understanding (MOU) was established between the United States and Canada implementing a joint clutter measurements program in which Canada, through Defence Research Establishment Ottawa, was to provide logistics support and share in the measured data and results. Dr. Hing C. Chan was the principal investigator of the clutter data at DREO. Dr. Chan became a close and valued member of the Phase One community; many useful discussions and interactions concerning the measured clutter data and its analysis occurred between Dr. Chan and the author down to the time of present writing. Substantial contracted data analysis support activity was provided to Dr. Chan by AIT Corporation, Ottawa. Information descriptive of the terrain at the clutter measurement sites was provided in a succession of contracted studies at Intera Information Technologies Ltd., Calgary.
The government of the United Kingdom through its Defence Evaluation Research Agency became interested in the Lincoln Laboratory clutter program shortly after its commencement. DERA subsequently became involved in the analysis of Phase One clutter data under the aegis of The Technical Cooperation Program (TTCP), an international defense science technical information exchange program. The U.S./Canada MOU was terminated at the completion of measurements, and the sharing of the measurement data and its analysis was thereafter continued between all three countries under TTCP.
Significant analyses of selected subsets of the Phase One measurement data occurred with DERA sponsorship in the U.K. at Smith Associates Limited and at GEC Marconi Research Centre. The principal coordinator of these interactions at DERA was Mr. Robert A. Blinston. Mr. John N. Entzminger Jr., former Director of the Tactical Technology Office at DARPA, provided much encouragement to these joint U.S./Canada/U.K. clutter study interactions in his role as head of the U.S. delegation to Subgroup K (radar) in TTCP.
In its early years, the Lincoln Laboratory clutter program was followed by Mr. David K. Barton, then of Raytheon Company, now of ANRO Engineering, who stimulated our thinking with his insights on the interrelationships of clutter and propagation and discussions on approaches to clutter modeling. Also in the early years of the clutter program, several interactions with Mr. William L. Simkins of the Air Force Research Laboratory, Rome, N.Y., influenced methodology to develop correctly at Lincoln Laboratory in such matters as clutter data reduction and intrinsic-motion clutter spectral modeling. In the latter years of the Phase One program, Professor Alfonso Farina of Alenia Marconi Systems, Italy, became interested in the clutter data. An informal collaboration was organized by Professor Farina in which some particular Phase One clutter data sets were provided to and studied by him and his colleagues at the University of Pisa and University of Rome. These studies were from the point of view of signal processing and target detectability in ground clutter backgrounds. A number of jointly-authored technical journal papers in the scientific literature resulted.
The five-frequency Phase One clutter measurement equipment was fabricated by the General Electric Co., Syracuse, N.Y. (now part of Lockheed Martin Corporation). Key members of the Phase One measurements crew were Harry Dence and Joe Miller of GE, Captain Ken Lockhart of the Canadian Forces, and Jerry Anderson of Intera. At Lincoln Laboratory, the principal people involved in the management and technical interface with GE were David Kettner and John Hartt. The project engineer of the precursor X-band Phase Zero clutter program was Ovide Fortier. People who had significant involvement in data reduction and computer programming activities include Gerry McCaffrey, Paul Crochetiere, Ken Gregson, Peter Briggs, Bill Dustin, Bob Graham-Munn, Carol Bernhard, Kim Jones, Charlotte Schell, Louise Moss, and Sharon Kelsey. Dr. Seichoong Chang served in an important consultant role in overseeing the accurate calibration of the clutter data. Many informative discussions with Dr. Serpil Ayasli helped provide understanding of the significant effects of electromagnetic propagation in the clutter data. Application of the resultant clutter models in radar system studies took place under the jurisdiction of Dr. John Eidson.
The original idea that the results of the Lincoln Laboratory clutter program could be the basis of a clutter reference book valuable to the radar community at large came from Mr. Delaney. Dr. Merrill I. Skolnik, former Superintendent of the Radar Division at the Naval Research Laboratory in Washington, D.C., lent his support to this book idea and provided encouragement to the author in his efforts to follow through with it. When a first rough draft of Chapter 1 of the proposed book became available, Dr. Skolnik kindly read it and provided a number of constructive suggestions. Throughout the duration of the clutter book project, Dr. Thurman was a never-failing source of positive managerial support and insightful counsel to the author on how best to carry the book project forward. Mr. C.E. Muehe provided a thorough critical review of the original report material upon which much of Chapter 6 is based. Dr. William E. Keicher followed the book project in its later stages and provided a technical review of the entire book manuscript. Skillful typing of the original manuscript of this book was patiently and cheerfully performed through its many iterations by Gail Kirkwood. Pat DeCuir typed many of the original technical reports upon which the book is largely based. Members of the Lincoln Laboratory Publications group maintained an always positive and most helpful approach in transforming the original rough manuscript into highly finished form. These people in particular include Deborah Goodwin, Jennifer Weis, Dorothy Ryan, and Katherine Shackelford. Dudley R. Kay, president of SciTech Publishing and vice-president at William Andrew Publishing, and the book’s compositors, Lynanne Fowle and Robert Kern at TIPS Technical Publishing, ably and proficiently met the many challenges in successfully seeing the book to press.
It is a particular pleasure for the author to acknowledge the dedicated and invaluable assistance provided by Mr. John F. Larrabee (Lockheed Martin Corporation) in the day-to-day management, reduction, and analyses of the clutter data at Lincoln Laboratory over the full duration of the project. In the latter days of the clutter project involving the production of this book, Mr. Larrabee managed the interface to the Lincoln Laboratory Publications group and provided meticulous attention to detail in the many necessary iterations required in preparing all the figures and tables of the book. Mr. Larrabee recently retired after a long professional career in contracted employment at Lincoln Laboratory, at about the time the book manuscript was being delivered to the publisher.
Many others contributed to the land clutter project at Lincoln Laboratory. Lack of explicit mention here does not mean that the author is not fully aware of the value of each contribution or lessen the debt of gratitude owed to everyone involved in acquiring, reducing, and analyzing the clutter data. Although this book was written at Lincoln Laboratory, Massachusetts Institute of Technology, under the sponsorship of DARPA and the USAF, the opinions, recommendations, and conclusions of the book are those of the author and are not necessarily endorsed by the sponsoring agencies. Permissions received from the Institute of Electrical and Electronics Engineers, Inc., the Institution of Electrical Engineers (U.K.), and The McGraw-Hill Companies to make use of copyrighted materials are gratefully acknowledged. Any errors or shortcomings that remain in the material of the book are entirely the responsibility of the author. The author sincerely hopes that every reader of this book is able to find helpful information within its pages.