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Marine radar
Published in Alexander Arnfinn Olsen, Core Principles of Maritime Navigation, 2023
The term “radar” is an acronym for Radio Detection and Ranging. The marine radar works on the basic principle of electromagnetic waves. The radar antenna sends the high-speed electromagnetic waves to establish the location of a target, which is the distance, the velocity, and the direction the wave travelled together with the altitude of the object, whether moving or stationary. Electromagnetic energy travels through the air at a constant high speed, equivalent to the speed of light, or 186,411 miles (300,000 km) per second. The object may vary from ships, boats, terrain, weather formations, coastal formations, buildings and so forth. The radar system sends out electromagnetic waves as a high-speed signal which travels several miles in the same direction the radar is facing. If there are no objects in the direction of the wave, the radar screen will be blank. If there is an object, this will reflect the wave and bounce it back to the radar receiver. Once the ship has received the returned signal, the onboard radar computer calculates the distance between the ship and the object along with its location. Subsequently, radar provides three critical pieces of information: the location of an object, the range of the object, and the direction the object is travelling.
Fundamentals of Indoor Radar
Published in Moeness G. Amin, Radar for Indoor Monitoring, 2017
Aboulnasr Hassanien, Braham Himed
The essence of conventional radar operation is to emit short pulses with very high peak power and listen to the echoes reflected by targets of interest. Figure 1.1 shows a simplified block diagram of a radar system that uses a single antenna. Transmitting pulses with maximum peak power and very short pulse duration usually results in low average power, which then limits the maximum achievable detection range. The development of pulse compression techniques permits reducing the required pulse peak power while maintaining high average power. The radar antenna focuses the transmit energy into narrow beams to localize the target in angle, then intercepts the target returns for additional processing. The receiver enhances the received signal through amplification, down-conversion of the radar signal to a low intermediate frequency, applying matched filter to the radar returns, detection of the signal envelope, and signal digitization for further processing. The postprocessing stage is intended to further separate target returns from disturbance (clutter + interference) signals, and estimate target parameters, including range, angle, and velocity. If the object is moving, either closer or farther away from the radar, then there is a slight change in the frequency of the radio waves, and this effect is referred to as the Doppler effect. The tracker then processes these detections to provide target history over time and predict future positions. Most radar systems are equipped with displays that show the operator the location of all detectable objects within the operating range of the radar system.
Sources of Radio Frequency Radiation
Published in Riadh W. Y. Habash, Electromagnetic Fields and Radiation, 2018
Typical radar measures the strength and round-trip time of the RFR signals (short pulses) that are emitted by a radar antenna and reflected off a distant surface or object. About 1500 high-power pulses per second are transmitted toward the target, with each pulse having a pulse width of typically 10-50 ps. The pulse normally covers a small band of frequencies, centered on the operating frequency of the radar. The radar antenna alternately transmits and receives pulses at particular wavelengths (in the range 1 cm to 1 m, which corresponds to the frequency range of about 300 MHz to 30 GHz) and polarization (waves polarized in a single vertical or horizontal plane).
Simulation and Synchronous Generation of Radar Signals at Geographically Distributed Sensors for Testing Emitter Location Systems
Published in IETE Journal of Research, 2023
Sudha Rani Suram, Niranjan Prasad, Sasibhushana Rao Gottapu
The signal power received at the sensor is the result of gains and losses it undergoes during its transmission. The Transmitter power is modulated by the radiation pattern of the Radar antenna. For broad beam emitters, the pattern can be considered as omni transmission. The polarization of antenna used for the receiver is chosen to match the desired signals to be intercepted. However, if the transmission polarization does not match with the polarization of the receiving antenna, the antenna gain is replaced with loss due to a mismatch in the signal path. In practical scenarios, the broad beam transmissions can be expected from aircraft transmissions like IFF, Mode-S, ADS-B, etc. Active antennas tuned to the frequency provide good gain and thus overall system sensitivity.
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
A single HF radar antenna can only measure the component of the velocity in line with the antenna (Figure 5). Coastal observation systems therefore often consist of several HF radars, such that independent current components of neighbouring stations can be used to project both horizontal current components. Diagnostic models can be applied to fill regions of the radar domain that are covered by only one radar, assuming flow constraints such as convergence-free flow and tidal modes (Barrick et al. 2012). However, the quality of the estimated current components weakens as the intersecting beams of two radars become less perpendicular, a problem that is more pronounced along the coastline. Another approach to fill data gaps is to assimilate the radial currents into an ocean circulation model (e.g. Gopalakrishnan and Blumberg 2012), as performed by the model plotted in Figure 6.
Coupled model simulation of the internal wave wakes induced by a submerged body in SAR imaging
Published in Waves in Random and Complex Media, 2022
Letian Wang, Min Zhang, Liuying Wang
Among the three modulation processes, tilt modulation is mainly manifested as the change of discrete facet slope due to the long gravity wave, the size of which is larger than the radar resolution. The facets elevated or tilted towards the radar antenna have a larger effective area for the radar. Then the hydrodynamic modulation is associated with the variation of the wave height spectral density of the Bragg waves, which could resonate with the incident electromagnetic wave. Its wavelength is much smaller than the radar resolution. This small-scale change is also called the modulation of the surface roughness. The Bragg-wave scattering has been verified as the main radar imagery mechanism of ship wakes with insignificant wave heights, such as narrow-V wake [39], centerline wake [40], and internal waves [41]. The demonstration of the scattering model with the tilt and hydrodynamic modulations is shown in Figure 4. In this study, a modulation facet scattering model was proposed to deal with the first two modulation effects. As shown in the Figure 4, the convergence current will make the sea surface rougher and the divergent current will make it smoother. Since the existence of a stable wave current would change the roughness of the ocean surface, the rougher region of the ocean surface is an enhanced scatter, and smooth regions appear darker. Therefore, internal waves usually appear as the alternating light and dark stripes in the SAR imagery. We implanted the modulation procedure in the original facet scattering model to describe this spectrum changes by the current effect.