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The Concept of a Microwave Radar with an Asymmetric Knifelike Beam for the Remote Sensing of Ocean Waves

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The Concept of a Microwave Radar with an Asymmetric Knifelike Beam for the Remote Sensing of Ocean Waves NOVEMBER 2005 K A R A E V E T A L . 1809 The Concept of a Microwave Radar with an Asymmetric Knifelike Beam for the Remote Sensing of Ocean Waves V. YU.KARAEV,M.B.KANEVSKY, AND G. N. BALANDINA Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia P. CHALLENOR,C.GOMMENGINGER, AND M. SROKOSZ Southampton Oceanography Centre, Southampton, United Kingdom (Manuscript received 2 December 2004, in final form 10 April 2005) ABSTRACT The features of near-nadir probing of the ocean surface from a satellite are investigated, and a new configuration of a microwave radar for the surface slopes measurement is proposed. Numerical modeling is used to examine the feasibility of a spaceborne implementation, to devise a measuring technique, and to develop a data processing algorithm. It is shown that the radar with a knifelike beam (1° ϫ 25°–30°) can be used to measure the variance of surface wave slopes. Using range selection and a new processing procedure to synthesize data, it is possible to achieve the spatial resolution required to study wave processes on the ocean surface from satellites. A new approach to wind speed retrieval at and near nadir is also discussed. 1. Introduction shortcomings, and new radars are constantly being de- veloped (e.g., Lin et al. 2000; Moccia et al. 2000; Hauser Remote sensing is widely used for oceanographic re- et al. 2001). These new systems aim to measure ocean search. At present, spaceborne radars allow us to ob- parameters more precisely or to obtain data across a tain a global overview of the ocean surface state and to wider swath to improve sampling. Second, in contrast to obtain information on its characteristics, for example, buoys, radars cannot directly measure the characteris- significant wave height (altimeter), wind speed (altim- tics of the sea surface. Therefore, it is important to eter and scatterometer), wind direction (scatterom- develop good data processing and retrieval algorithms. eter), and sea wave spectrum [synthetic aperture radar An altimeter is one of most advanced tools among (SAR)]. This information is useful for a wide range of modern radar systems for remote sensing due to its applications in oceanography, meteorology, and navi- capabilities. One may measure the level of the ocean gation. (and then derive an estimate of surface geostrophic cur- For active microwave radars, all retrieval algorithms rents), the significant wave height, and the wind speed are based on the fact that the electromagnetic waves (from backscattered power). The precision of retrieved scattered by the ocean surface contain information wind speed may be further improved by using sea state about its characteristics. Therefore, the problem of the data, and new retrieval algorithms are an active area of retrieval of useful information from the ocean surface research (see, e.g., Gourrion et al. 2000; Gommengin- includes two stages. First, we must determine the opti- ger et al. 2002). mum radar parameters and which characteristics of the Our earlier investigations (Karaev et al. 1999, 2002b) reflected signal we need to recover. Existing publica- show that there is a basic limit for the precision of wind tions make it clear that modern radar systems have speed retrieval from altimeter data because of the com- plicated correlation between wind speed and radar cross section (RCS). The development of new two- Corresponding author address: V. Karaev, Institute of Applied Physics, Russian Academy of Sciences, 46, Uljanov str., Nizhny parameter algorithms using both radar cross section Novgorod, Russia. and significant wave height permits us to improve mar- E-mail: [email protected] ginally the precision of wind speed retrieval in compari- © 2005 American Meteorological Society Unauthenticated | Downloaded 09/30/21 02:17 PM UTC JTECH1807 1810 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 22 ␦ ␦ son to one-parameter algorithms. However, it is clear where x, y are the half-power beamwidths in two mu- 2 2 that there is still room for improvement. At nadir, the tually perpendicular planes, Sxx and Syy are the vari- radar cross section depends on the effective reflection ances of surface slopes along and perpendicular to coefficient (i.e., local wind speed) and the variance of the footprint’s major axis, and U10 is the wind speed sea surface wave slopes (i.e., local wind speed and at 10-m height. The antenna gain is assumed to be a swell) (Karaev et al. 2002a; Bass and Fuks 1972; Valen- Gaussian function. The effective reflection coefficient zuela 1978). Modern spaceborne radar systems do not Reff(U10) is introduced instead of the Fresnel coeffi- measure surface wave slopes, and this uncertainty leads cient. This coefficient takes into account the diffuse to errors in wind speed retrieval by existing algorithms. scattering of the reflected field due to the small-scale The purpose of the present research is to increase the waves. Hence, the radar cross section depends on both number of characteristics of the ocean surface that can large-scale waves and little ripples. be measured by microwave radars at and near nadir, In the general case of mixed sea (wind waves plus and to show the way to improve the precision of wind swell) the characteristics of large-scale waves do not speed retrieval. Section 1 describes the fundamental correlate strongly with local wind speed. Only the wind assumptions that are used. Section 2 presents the meth- waves depend on the local wind speed. The presence of ods for measuring large-scale wave slopes and wind swell makes the connection between the wind speed speed by a radar with an asymmetric knifelike beam. In and large-scale waves less strong [this is particularly section 3 the spatial resolution problem for such a sat- true for significant wave height (SWH), although a little ellite-borne radar is discussed. Section 4 contains a less critical for slopes]. At low wind speed (Ͻ6msϪ1), brief description of a new model for the Doppler spec- swell may increase the variance of surface slopes two- trum of such a system. Section 5 gives details of the or threefold (Hwang and Shemdin 1988). special synthesis procedure used for data processing. In The correlation between short waves and wind speed section 6, numerical simulations are used to illustrate is strong, and it is demonstrated by the short time re- the features of the new radar, and section 7 gives an laxation for ripples when wind stress suddenly disap- example of data processing. pears. On the other hand, NRCS depends on the vari- ance of large-scale waves. As a result, the connection 2. Initial assumptions between the reflected power and wind speed is ambigu- ous, and this is one of the reasons for errors in one- and As is well known, at small incidence angles, micro- two-parameter retrieval algorithms. wave backscatter from the ocean is quasi-specular and The direct measurement of the variance of surface takes place from the facets of the ocean surface ori- wave slopes by radar would permit us to take these into ented perpendicularly to the incident electromagnetic account and make the connection between NRCS and wave (Bass and Fuks 1972; Valenzuela 1978). There- wind speed less ambiguous. Therefore, the measure- fore, it is well recognized that the radar cross section ment of surface wave slopes would allow us to improve depends on the variance of the slopes of large-scale the precision of the wind speed retrieval in comparison waves and on the spectral density of the small ripples to the present situation where two variables [see Eq. responsible for the diffuse scattering. (1)] are unknown (long-wave slopes and wind speed). Here we will analyze the properties of microwave Also, we should note that the radar will measure slopes signal scattered by sea surface. Namely, centimeter that differ significantly from those measured by buoys. waves (1–10 cm) contain the maximum information A buoy cannot measure sea waves shorter than ϳ10 m, about the state of a near-surface layer. while the new radar will measure the slopes of waves In the framework of the Kirchoff approximation, shorter than 1 m (Bass and Fuks 1972; Karaev et al. which does not account for radar frequency depen- 2002b). dence, a theoretical analysis of the backscattering of Of course, there are already methods for measuring electromagnetic waves from the ocean surface at nor- wave slopes from aircraft (see, e.g., Walsh et al. 1985; mal incidence has been carried out for a wide radar Walsh and Vandemark 1998; Hauser et al. 1992). These beam, and a new theoretical model of the normalized are provided by good spatial resolution achievable with radar cross section (NRCS) has been obtained (Karaev radars mounted on aircraft. Such radars can see a cell and Kanevsky 1999; Karaev et al. 2002b): less than 0.3 m ϫ 0.3 m. Special algorithms allow the Խ ͑ ͒Խ2 ␦2 Ϫ1ր2 ␦2 Ϫ1ր2 retrieval of detailed information about the ocean sur- Reff U10 x y ␴ ϭ ͩS2 ϩ ͪ ͩS2 ϩ ͪ , face, but this approach is too complicated for use from 0 2 xx 5.52 yy 5.52 a satellite. Here we suggest another approach to this ͑1͒ problem. Unauthenticated | Downloaded 09/30/21 02:17 PM UTC NOVEMBER 2005 K A R A E V E T A L . 1811 At small incidence angles the radar cross section de- From this, it is possible to determine the direction of pends on the variance of long-wave slopes. An increase wave propagation by using a rotating antenna system or in incidence angle leads to a decrease in normalized by organizing a circular flight of the aircraft.
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