July 29, 1998 WASHINGTON, D.C. - Over-the-horizon (OTH) radar systems were first developed during the Cold War as part of the early warning defense network. With the end of the Cold War, research into OTH radar continued, with a new focus: detection of illegal aircraft engaged in drug smuggling. Current research had its genesis in U.S. Air Force and Navy laboratories in the 1960s, as well as at Stanford University. At present, the United States, Russia, China, and Australia employ OTH radar for surveillance purposes. Much of the current effort is concentrated on the development of relocatable OTH radar (ROTHR), first demonstrated by the Navy in the early 1980s.
A special 224 page section of the July-August issue of the journal Radio Science, published by the American Geophysical Union, reviews OTH radar technology, with emphasis on recent progress. The 19 papers were edited by F. Tom Berkey of the Space Dynamics Laboratory Division of Utah State University in Logan. Berkey notes that the papers evolved from research and development of OTH radar in both the U.S. and Australia.
Following is a brief summary of the Radio Science papers from Berkey's introduction:
"The Navy's work in OTH is summarized in Paper 1 by J.H. Headrick and J.F. Thomason; an overview of OTH work undertaken in China over the past two decades is presented by L.-W. Li in Paper 2. Paper 3, by S.J. Anderson and Yu. I. Abramovich, describes a global algorithm for mitigating against the simultaneous occurrence of multiple distortion mechanisms, while the significance of FMCW waveform generator spectral purity in the context of limitations to OTH radar performance is discussed in the work of G.F. Earl (Paper 4). The utilization of known terrain features (cities, mountain peaks and islands) to improve the location accuracy of targets is discussed in the manuscript by J.R. Barnum and E.E. Simpson (Paper 5).
"To improve the capability of ship detection with OTH radar, B.T. Root describes a coherent sea-clutter cancellation method that subtracts the first-order Bragg peaks appearing in backscattered ocean spectra (paper 6). Using a Maximum likelihood adaptive neural system, L.I. Perlovsky, V.H. Webb, S.R. Bradley and C.A. Hansen discuss an advanced detection and tracking system that provides improved capabilities in clutter-dominated environments (Paper 7).
"Range-folded equatorial spread-F severely contaminates the ROTHR systems under certain conditions; a means of mitigating against what is termed spread Doppler clutter is the application of a non-recurrent transmitted waveform, as described in Paper 8 by M.P. Hartnett, J.T. Clancy and R.J. Denton. Paper 9, B.S. Dandekar, G. Sales, B. Weijers, D. Reynolds discuss a synoptic study of equatorial clutter using the OTH-B radar system, which suggests that equatorial spread-F does not exhibit a dependence on the frequency of operation or on the level of global magnetic activity. In Paper 10, S.V. Fridman describes a method of inverting backscatter ionograms that enables the three-dimensional reconstruction of the down-range ionosphere.
"A method of rapidly synthesizing backscatter ionograms from a known ionosphere has been developed by C.Y. Ong, J.A. Bennett and P.L. Dyson and is described in Paper 11. C.J. Coleman has developed a simple two-dimensional ray tracing formulation which is applied to propagation problems encountered with oblique and backscatter radars as described in Paper 12. R.H. Anderson and J.L Krolik have applied statistical modeling of ionospheric parameters to derive a maximum likelihood method of coordinate registration, showing that it is a significant improvement over conventional methods (Paper 13). R.I. Barnes, S.A. Braendler, C.J. Coleman, R.S. Gardiner-Garden, and T.V. Hoang, have compared parameterization techniques used for modeling the ionospheric vertical profile with the goal of increasing prediction accuracy (Paper 14).
"A multi-source volumetric technique that combines information from several sensors with TEC data obtained from ionospheric tomography to derive three-dimensional maps of electron density distribution is presented in Paper 15 by C. Biswas and H. Na. The application of OTH radar techniques to measuring the spatial and temporal variability of the Florida current is illustrated in Paper 16 by T.M. Georges, J.A. Harlan, T.N. Lee and R.R. Leben. Sea truth measurements in the Gulf of Mexico are compared with the radial component measurements of ocean surface current by the ROTHR Texas facility in work carried out by J.A. Harlan, T.M. Georges and D.C. Biggs (Paper 17).
"A physics-based model has been developed by C. Lauer, L.J. Nickisch and W. Wortman to simulate the physical phenomena that characterize the equatorial ionosphere; the current version of this model invokes two-dimensional raytracing and other simplifications to increase the computational speed of the model (Paper 18). In Paper 19, L.J. Nickisch and P.M. Franke use the Finite Difference-Time Domain method to validate the Born approximation, which is invoked in most modeling work."
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