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High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters. Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or "skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting stations (3.95–25.82 MHz), aviation communication, government time stations, weather stations, amateur radio and citizens band services, among other uses.
Millimeter-wave cloud radars, also denominated cloud radars, are radar systems designed to monitor clouds with operating frequencies between 24 and 110 GHz. Accordingly, their wavelengths range from 1 mm to 1.11 cm, about ten times shorter than those used in conventional S band radars such as NEXRAD.
Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, asphalt, metals, pipes, cables or masonry. This nondestructive method uses electromagnetic radiation in the microwave band of the radio spectrum, and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, fresh water, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks.
Over-the-horizon radar (OTH), sometimes called beyond the horizon radar (BTH), is a type of radar system with the ability to detect targets at very long ranges, typically hundreds to thousands of kilometres, beyond the radar horizon, which is the distance limit for ordinary radar. Several OTH radar systems were deployed starting in the 1950s and 1960s as part of early warning radar systems, but airborne early warning systems have generally replaced these. OTH radars have recently been making a comeback, as the need for accurate long-range tracking becomes less important with the ending of the Cold War, and less-expensive ground-based radars are once again being considered for roles such as maritime reconnaissance and drug enforcement.
Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the location of something through the use of radio waves. It generally refers to passive uses, particularly radar—as well as detecting buried cables, water mains, and other public utilities. It is similar to radionavigation, but radiolocation usually refers to passively finding a distant object rather than actively one's own position. Both are types of radiodetermination. Radiolocation is also used in real-time locating systems (RTLS) for tracking valuable assets.
A scatterometer or diffusionmeter is a scientific instrument to measure the return of a beam of light or radar waves scattered by diffusion in a medium such as air. Diffusionmeters using visible light are found in airports or along roads to measure horizontal visibility. Radar scatterometers use radio or microwaves to determine the normalized radar cross section of a surface. They are often mounted on weather satellites to find wind speed and direction, and are used in industries to analyze the roughness of surfaces.
The NASA QuikSCAT was an Earth observation satellite carrying the SeaWinds scatterometer. Its primary mission was to measure the surface wind speed and direction over the ice-free global oceans via its effect on water waves. Observations from QuikSCAT had a wide array of applications, and contributed to climatological studies, weather forecasting, meteorology, oceanographic research, marine safety, commercial fishing, tracking large icebergs, and studies of land and sea ice, among others. This SeaWinds scatterometer is referred to as the QuikSCAT scatterometer to distinguish it from the nearly identical SeaWinds scatterometer flown on the ADEOS-2 satellite.
Ocean acoustic tomography is a technique used to measure temperatures and currents over large regions of the ocean. On ocean basin scales, this technique is also known as acoustic thermometry. The technique relies on precisely measuring the time it takes sound signals to travel between two instruments, one an acoustic source and one a receiver, separated by ranges of 100–5,000 kilometres (54–2,700 nmi). If the locations of the instruments are known precisely, the measurement of time-of-flight can be used to infer the speed of sound, averaged over the acoustic path. Changes in the speed of sound are primarily caused by changes in the temperature of the ocean, hence the measurement of the travel times is equivalent to a measurement of temperature. A 1 °C (1.8 °F) change in temperature corresponds to about 4 metres per second (13 ft/s) change in sound speed. An oceanographic experiment employing tomography typically uses several source-receiver pairs in a moored array that measures an area of ocean.
A multibeam echosounder (MBES) is a type of sonar that is used to map the seabed. It emits acoustic waves in a fan shape beneath its transceiver. The time it takes for the sound waves to reflect off the seabed and return to the receiver is used to calculate the water depth. Unlike other sonars and echo sounders, MBES uses beamforming to extract directional information from the returning soundwaves, producing a swathe of depth soundings from a single ping.
Coastal ocean dynamics applications radar (CODAR) describes a type of portable, land-based, high frequency (HF) radar developed between 1973 and 1983 at NOAA's Wave Propagation Laboratory in Boulder, Colorado. CODAR is a noninvasive system that permits to measure and map near-surface ocean currents in coastal waters. It is transportable and offers output ocean current maps on site in near real time. Moreover, using CODAR it is possible to measure waves heights and it provides an indirect estimate of local wind direction.
Wave radar is a type of radar for measuring wind waves. Several instruments based on a variety of different concepts and techniques are available, and these are all often called. This article, gives a brief description of the most common ground-based radar remote sensing techniques.
The Q band is a range of frequencies contained in the microwave region of the electromagnetic spectrum. Common usage places this range between 33 and 50 GHz, but may vary depending on the source using the term. The foregoing range corresponds to the recommended frequency band of operation of WR22 waveguides. These frequencies are equivalent to wavelengths between 6 mm and 9.1 mm in air/vacuum. The Q band is in the EHF range of the radio spectrum.
A spar buoy is a tall, thin buoy that floats upright in the water and is characterized by a small water plane area and a large mass. Because they tend to be stable ocean platforms, spar buoys are popular for making oceanographic measurements. Adjustment of the water plane area and the mass allows spar buoys to be tuned so they tend to not respond to wave forcing. This characteristic differentiates them from large water plane area buoys such as discus buoys that tend to be wave followers. Spar buoys are often used as stable platforms for wave measurement devices and air–sea interaction measurements. Spar buoys range in length from a few feet to the 354-foot RP FLIP. To avoid the difficulties inherent with shipboard launch and recovery, helicopter deployment of large spar buoys has been studied.
Bistatic sonar is a sonar configuration in which transmitter and receiver are separated by a distance large enough to be comparable to the distance to the target. Most sonar systems are monostatic, in that the transmitter and receiver are located in the same place. A configuration with multiple receivers is called multistatic.
Microwave engineering pertains to the study and design of microwave circuits, components, and systems. Fundamental principles are applied to analysis, design and measurement techniques in this field. The short wavelengths involved distinguish this discipline from electronic engineering. This is because there are different interactions with circuits, transmissions and propagation characteristics at microwave frequencies.
Frank Edwin Snodgrass was a physical oceanographer and electrical engineer. He spent nearly all of his career working with Prof. Walter Munk at Scripps Institution of Oceanography (SIO). The Cecil H. and Ida M. Green branch of the University of California Systemwide Institute of Geophysics and Planetary Physics (IGPP), in La Jolla, California has been strongly linked to Scripps since the 1960s through joint faculty appointments, research interests, and shared facilities. Other IGPP branches can be found at the Los Angeles, Irvine, Santa Cruz and Riverside campuses. Snodgrass spent many years researching and measuring the ocean tides and waves. During his career he had opportunities to work with fellow scientists around the world, including collaborations through the IGPP campuses, with Woods Hole Oceanographic Institution in Massachusetts and with the National Institute of Oceanography in England.
Bio-radiolocation is a technology for remote detection and diagnostics of biological objects by means of radar, even behind optically opaque obstacles. Devices based on this method are called bio-radars.
Elizabeth Laverick was a British engineer who became technical director of Elliott Automation Radar Systems. Laverick was the first female deputy secretary of the Institution of Electrical Engineers and president of the Women's Engineering Society. She was the first woman to receive a PhD in a scientific curriculum at Durham University, and was appointed an OBE in 1993.
Mahta Moghaddam is an Iranian-American electrical and computer engineer and William M. Hogue Professor of Electrical Engineering in the Ming Hsieh Department of Electrical and Computer Engineering at the University of Southern California Viterbi School of Engineering. Moghaddam is also the president of the IEEE Antennas and Propagation Society and is known for developing sensor systems and algorithms for high-resolution characterization of the environment to quantify the effects of climate change. She also has developed innovative tools using microwave technology to visualize biological structures and target them in real-time with high-power focused microwave ablation.
Akira Ishimaru is a Japanese-American electrical engineer and professor emeritus at Department of Electrical and Computer Engineering at University of Washington. He is best known for his contributions to the theory of wave scattering in random media.
References
- ↑ "Preface for the IEEE JOURNAL OF OCEANIC ENGINEERING Special Issue on HF/VHF Ocean Surface Radar". IEEE Journal of Oceanic Engineering. 31 (4): 739.
- ↑ Wyatt, Lucy (2000). "Limits to the Inversion of HF Radar Backscatter for Ocean Wave Measurement". Journal of Atmospheric and Oceanic Technology. 17 (12): 1651–1666. Bibcode:2000JAtOT..17.1651W. doi: 10.1175/1520-0426(2000)017<1651:LTTIOH>2.0.CO;2 .
- ↑ Wyatt, L.R.; Green, J.J.; Middleditch, A (2011). "HF radar data quality requirements for wave measurement". Coastal Engineering. 58 (4): 327–336. doi:10.1016/j.coastaleng.2010.11.005.
- ↑ Wang, W; Wyatt, L.R. (2011). "Radio frequency interference cancellation for sea-state remote sensing by high-frequency radar". IET Radar, Sonar and Navigation. 5 (4): 405–415. doi:10.1049/iet-rsn.2010.0041.
- ↑ Robinson, A.M.; Wyatt, L.R.; Howarth, M.J. (2013). "HF Radar data availability and measurement accuracy in Liverpool Bay before and after the construction of Rhyl-Flats wind farm". Journal of Operational Oceanography. 6 (2): 1–12. doi: 10.1080/1755876X.2013.11020144 .
