China
Africa
Explore chapters and articles related to this topic
Radar Cross Section and Target Scattering
Published in James D. Taylor, Introduction to Ultra-Wideband Radar Systems, 2020
In the optical or high frequency (not to be confused with the HF radar band) scattering region, the body is much larger than the wavelength of the incident wave. Unlike the Rayleigh and the resonance regions, the scattering here is not a result of the field interacting with the total body. In the optical region it is useful to think of the scattering as coming from a collection of independent scattering centers distributed over the target. Most of the scattering contribution comes from specular points, edges, and shadow boundaries. Hence, small target dimensions and details are important in this region. In fact, edges, metal seams, and discontinuities as small as rivets can be big contributors to the RCS in the optical region.
Over-the-Horizon Radar
Published in Habibur Rahman, Fundamental Principles of Radar, 2019
The most common type of OTHR system, sometimes called skywave radar, uses skywave propagation in which HF radar waves are reflected from the ionosphere. Skywave propagation beyond the horizon is made possible by the existence in the atmosphere of approximately spherically stratified ionized layers concentric with the earth's surface. The radar installation in the skywave system operates like an HF communication system, radiating energy in a narrow azimuthal beam, but with a broad vertical beam extending typically from 5° to 25° elevations. The radar signal is transmitted toward the ionosphere that absorbs part of the energy; but the major part of the energy is reflected from the ionosphere, the amount being dependent on the actual electron density in the ionosphere, frequency and the angle of incidence of the HF signal. The specific frequency to be used by a skywave radar is a function of the desired range and the character of the ionosphere. Since the ionosphere varies with many factors such as time of the day, season, and solar activity, the optimum frequency must be selected from time to time to ensure that refraction in the ionospheric layers results in the maximum illumination of the target.5 Thus, such radars can operate over a wide portion of the HF band. The maximum usable frequency (MUF) and the least usable frequency (LUF) are the two terms used to describe the frequency-dependent characteristics of the ionosphere that determine its ability to support HF skywave propagation. Both MUF and LUF are dependent on ionospheric conditions, and provide a portion of the HF band available to support skywave propagation at any given time and space.
Epilogue
Published in Abhijit Pandit, Mathematical Modeling using Fuzzy Logic, 2021
In high-frequency (HF) radar systems, the classical platform for signal processing uses DSP and field programmable gate array (FPGA). However, the seizure of these two sites is very high, and the scalability is bad for both sites. It is therefore important to find new ways to process HF radar signals. Inspired by the software-limited radar concept and the excellent hardware speed performed in parallel processing to the GPU, we explored how to implement the HF radar algorithm on a GPU on a regular PC platform. Numerical tests were carried out to demonstrate the effectiveness of the proposed method.
Suitability of the Southern Australia Integrated Marine Observing System’s (SA-IMOS) HF-Radar for operational forecasting
Published in Journal of Operational Oceanography, 2019
Charles James, Matt Collopy, Lucy R. Wyatt, Andrew Middleditch
The main role of HF-Radar is to measure the radial surface currents, which is done by measuring the Doppler-shifted power spectrum of the received signals. This is the information used to provide near real-time updates of currents to the IMOS data portal (IMOS 2017, Cosoli et al. 2018). However, phased array systems like the SAG WERA system can also provide information on a variety of other physical properties including wave information and wind direction (Heron and Prytz 2002; Wyatt et al. 2006). The ACORN facility has recently begun processing the SAG data and producing high-quality wave and wind time series. The sources of error mentioned above are more significant when measuring waves since these use more of the radar spectrum and signal-to-noise is also lower.
Surface currents in operational oceanography: Key applications, mechanisms, and methods
Published in Journal of Operational Oceanography, 2023
Johannes Röhrs, Graig Sutherland, Gus Jeans, Michael Bedington, Ann Kristin Sperrevik, Knut-Frode Dagestad, Yvonne Gusdal, Cecilie Mauritzen, Andrew Dale, Joseph H. LaCasce
The choice of frequency for a HF radar systems requires to balance maximum range versus resolution. Frequencies of 30 MHz yield a range of 80 km at and a range resolution of about 1 km (Gurgel et al. 1999). HF radar applications are therefore restricted to coastal seas, and particular focus on near-shore and harbour currents require to chose VHF radars. Frequencies of 50 MHz achieve a range resolution up to 250 m but are restricted to 8km total range (e.g. Shay et al. 2002).