"On the Bragg scattering observed in L-band synthetic aperture radar images of flooded rice fields," IEICE Trans. Commun., vol.E89-B, 2006 (in press) .
This article presents the analysis of the Bragg scattering phenomenon which has been observed in the images of machine-planted rice paddies acquired by the JERS-1 L-band synthetic aperture radar (SAR). The simultaneous measurements of rice plants were made at the SAR data acquisition times. Large differences of 20-25 dB in image intensity between the transplanting and ripening stages are found to be dependent on the planting direction and bunch separation. This selective image enhancement is a result of the Braggresonance backscatter due to the double-bounce of incident L-band microwave between the flooded water surface and periodically plantedbunches of rice plants. Support for the idea of double-bounce scattering is provided by the decomposition analysis of L-band and X-band polarimetric Pi-SAR data; and a simple numerical simulation basedon the physical optics model shows fairly good agreement with the JERS-1 SAR data. The results presented in this paper is mainly of academic interest, but a suggestion can be made on the selection of suitable microwave band for monitoring rice fields.
"合成開口レーダ画像からの情報抽出技術と研究動向," 信学論（B), , vol.J89-B, 2006 (in press).
近年における合成開口レーダ（SAR: Synthetic Aperture Radar）技術の発展にはめざましいものがあり，従来の単波長・単偏波の振幅画像の生成はもちろん，陸域での地表高度と地殻変動量の計測および海域での海流流速を計測する干渉SAR（InSAR: Interferometric SAR）や，偏波SAR(PolSAR: Polarimetric SAR ) による高精度な画像分類手法などが考案されている．SARリモートセンシングは，高度の情報処理を駆使した最先端技術として地球科学分野で必要不可欠な計測技術となっている．本稿では，SARの振幅画像とInSAR, PolSARのデータからの情報抽出技術と地球科学への応用，そして現在の研究動向について解説する．
"Interlook cross-correlation function of speckle in SAR images of sea surface processed with partially overlapped subapertures," IEEE Trans, Geosci. Remote Sens., vol.43, pp.695-701, 2005.
In the present paper, a general integral expression is derived and
discussed for the cross-correlation function (CCF) of speckle patterns in
synthetic aperture radar (SAR) images processed by using partially
overlapped subapertures of arbitrary Doppler center frequencies (or
equivalent azimuth times). It is shown that, under the white noise
approximation for the backscattered field, the CCF of the interlook speckle
intensity patterns is given by the squared modulus of the autocorrelation
function of the amplitude weighting function of subapertures where the time
lag is the center time difference. It is also shown that the CCF of the
interlook speckle patterns is independent of the surface coherence time of
sea surface. The integral expression for the intensity CCF is then
evaluated for a rectangular weighting function, and comparison is made with
the JERS-1 L-band and RADARSAT-1 C-band SAR images of sea surface to test
the theory. The CCFs computed from the JERS-1 SAR data show excellent
agreement with the theory, and good agreement is obtained with the
RADARSAT-1 data.
"Single look classification accuracy for polarimetric SAR," it Int. J. Remote Sens., vol.26, 2005 (in press).
Polarimetric SAR systems such as ALOS-PALSAR and Radarsat-2 can operate in
many different modes. The use of additional polarisations may require
additional time and operating power and it is important to justify this by
an improvement in final classification accuracy. A fully polarimetric, dual
frequency AirSAR scene from a rice growing area in Japan is classified by a
maximum likelihood method based on the Wishart distribution. It is shown
how the measured covariance matrices determine the separation accuracy
between two classes. Closed form expressions are then given for the
expected single look accuracy of the maximum likelihood classifier as a
function of the class covariance matrices. This can be used to quickly
compare the high resolution classification performance for different
polarimetric systems to decide on a particular operating mode.
"Ship Detection Based on Coherence Image Derived from Cross Correlation
of Multilook SAR Images," IEEE Geosci. Remote Sens. Lett., vol.1, no.3, pp.184-187, 2004.
A new simple technique is presented to extract ships from synthetic aperture
radar (SAR) images. The procedure is to compute the cross-correlation values
between two images extracted by moving windows of a small size from the
multilook SAR intensity (or amplitude) images. A coherence image, consisting
of the cross-correlation values of the intensity images, is then produced.
Ships are deterministic targets, so that their interlook subimages possess
higher degree of coherence than the uncorrelated random images of the surrounding
sea surface. The main advantage of this method over the conventional CFAR
(constant false alarm rate) is its ability to detect, under favorable conditions,
``invisible'' images of ships embedded in the speckled image of the sea
surface. The technique is tested using a RADARSAT-1 image in which a known
and several unknown ships are present. The use of complex images and the
exploitation of short decorrelation times of small-scale ocean waves to
obtain an extra look are also discussed.
"水稲生育状況のマイクロ波特性による把握−Pi-SAR による児島湾干拓地水田の多波長・多偏波解析−," 日本リモートセンシング学会誌, vol.23, pp.473-490, Dec. 2003.
South-east Asia has a rainy-season at the crop growing period, and it is difficult to observe agricultural land in this season using optical remote sensing. Synthetic Aperture Radar (SAR) can observe the earth's surface without being influenced by clouds. However, it is less useful for observing agricultural land, because satellite SAR has only one data band. Recently, SAR is able to provide multi band and multi polarimetric dara. Pi-SAR, an airborne SAR developed by NASDA and CRL, can provide L and X bands and fully polarimetric data. Rice is the main crop in Asia, and we studied the chracteristicmicrowave scatter on rice paddy fields using Pi-SAR data. Our studyarea was the rice paddy fields in Kojima reclaimed land in Japan. We had two fully polarimetric data sets from 13 July 1999 and 4 October 2000. First, we processed the color polarimetric composite image. Next we calibrated the phase of each polarimetric data using river area by the Kimura method. After that we performed decompositionanalysis and drew polarimetric signatures for understanding the status of rice paddy fields. At the rice planting period, rice paddy fields are filled with water and rice plants are very small. The SARmicrowave scatters on water surfaces like a mirror, called `mirror (or specular) reflection'. This phenomenon makes backscatter a small value at the water-covered area. The image from July is about one month after trans-planting and rice plants are 20-40 cm in height. X-band microwave scatters on the rice surface, but L-band microwave passes through rice bodies and shows mirror reflection on water surfaces. Some strong backscatter occurs on rice paddy fields especially VV polarization because of bragg scattering. The fields where bragg scattering returns strong VV scatter because the space between rice stems cause resonation in the L-band wavelength. We can easily understand bragg scatter by using polarimetric data. Using theimage from October at just before harvest, L-band polarimetric datacan detect various rice statuses such as standing, inclining, or lying. We conclude that multi band and fully polarimetric SAR data can quantity detect crop growth, as do optical sensors in all weatherconditions.
"Segmentation of SAR images using multitemporal information,'' IEE Proc.
Radar, Sonar and Navigation, vol.150, no.5, pp.367-374, Oct. 2003.
The maximum likelihood method of SAR segmentation has the potential to
retain single pixel accuracy without requiring heuristic decisions.
Normally a probabilistic measure is used to merge individual regions
without assuming any prior knowledge for the underlying cross-sections.
However, for a reasonable multitemporal scene there may beconsiderable
information available from the varying cross-sections over time. An example
is given where this information can be extracted by an initial
classification. It is then shown how the segmentation scheme can be
modified to incorporate this information via anestimate of the
multitemporal underlying class distributions. Usingsingle-look Radarsat
data at 8 m resolution, it is demonstrated howthe final segment population
can be significantly reduced. From a comparison with ground survey data and
a high-resolution AirSAR image, the structural quality of the segmentation
is shown to be improved.
"Non-uniform azimuth image shift observed in the Radarsat images of ships in
motion," IEEE Trans. Geosci. Remote Sens., vol.40, pp.2188-2193, 2002.
This paper describes, for the first time to the authors' knowledge, the phenomenon of non-uniform azimuth image shift of a rigid body observed in the Radarsat SAR (synthetic aperture radar) images of cruising ships. The effect is caused by the different slant-range velocities of coherent scatterers across the hull associated with the ship motions. The slant-range
velocity is estimated from the SAR image of an identified ship, and compared with that computed from a numerical model using the ship's specification and meteorological data. The result indicates that the dominant contribution to
the non-uniform image shift is the pitching motion of the ship. Further comparison is made with the wave orbital velocity, and the results are shown to be in good agreement. Comparisons are also made between the SAR-derived slant-range velocities of two unknown ships and wave orbital velocities, and reasonable agreement is obtained. One of the ships' images exhibits not only non-uniform shift but also image skew. The latter skewing effect may be caused by rolling of the ship.
"A theory on the distribution function of backscatter radar cross section from ocean waves of individual wavelength," IEEE Trans. Geosci. Remote Sens., vol.38, pp.811-822, 2000.
A new and simple method is presented of interpreting the `distribution' of the backscatter radar cross section (RCS) from ocean waves of individual wavelength. Using the Kirchhoff scattering (Physical Optics) theory, the `cumulative' RCS from the ambient waveheight spectrum is first computed as a function of the wavenumber. Differentiating this cumulative RCS yields the distribution function of the RCS from ocean waves of different wavenumbers. The present study shows, as a general feature, that the maximum RCS contribution tends to come from the ocean waves of wavenumbers closer to the Bragg wavenumber as the radar incidence angle increases. Under low wind speeds (〜 2m/s) and at the incidence angles between 20° and 45°, the dominant RCS contribution at both C- and L-bands comes from ocean waves of wavenumbers close to the Bragg wavenumber. Under intermediate to high wind speeds (〜10-15 m/s) and at the small incidence angle of 20°, the radar backscatter is dominated by long ocean waves with little contribution from the Bragg waves at both bands. At the intermediate incidence angles (〜35°to 45°), the RCS distribution is centered at wavenumbers near the Bragg wavenumber at all wind speeds from 2 m/s to 15 m/s. These features are more pronounced at C-band than L-band. The present theory is based on the Kirchhoff scattering model, and as such, its validity may also be limited to a range of small to intermediate incidence angles where the polarization effect is not very significant. The RCS distribution is evaluated for the ambient sea surface and the surface perturbed by varying currents induced by the interaction between the current and bottom topography, and the consequence of the result is discussed.
"Determination of ocean wave propagation direction by split-look processing using JERS-1 SAR data," IEEE Trans. Geosci. Remote Sens., vol.37, pp.849-855, 1999.
The purpose of the present article is to show that the JERS-1 synthetic aperture radar (SAR), primarily designed for global land observation, can be applied to deriving ocean wave spectra and determining the wave propagation direction. Split-look (multilook) processing is used to produce a sequence of individual sub-images of ocean waves. This processing technique has an inherent property that the successive sub-images are formed at different discretely delayed times. Wave propagation direction can, therefore, be determined from a pair of wave images in different looks. The L-band JERS-1 SAR, despite its low signal-to-noise ratio, has an advantage over C-band spaceborne SARs for having a longer integration time, suitable for the wave analysis using split-look processing. Several different methods are applied to the wave data collected near the coasts of Japan, including the methods of look sum (taking the standard Fourier spectrum of look-summed image), spectral sum, spectral phase shift, and cross-correlation function of sub-images.
"Multi-Frequency SAR images of ship-generated internal waves," Int. J. Remote Sens., vol.18, pp.3709-3718, 1997.
During the joint U.K./U.S.A. experiment conducted in the Loch Linnhe, Scotland, U.K., in 1989, the Jet Propulsion Laboratory (JPL) multi-frequency airborne synthetic aperture radar (SAR) was deployed to investigate the relation between the SAR images and ship-generated internal waves. One of the intriguing results, consistent throughout the experiment, was the striking difference in phase (position) between the images of internal wave wakes in the P-band (〜68 cm) wavelength and those in L-band (〜24 cm) and C-band (〜6 cm) wavelengths. An explanation for this difference is found in the sensitivity of different radar wavelengths to oceanic surface waves in different ranges of wavelengths, that are perturbed by varying surface currents by different amounts and at different positions, depending on the wavelength of surface waves.
"合成開口レーダと海洋のリモートセンシング I ：合成開口レーダの基本," 可視化情報学会誌, vol.15, pp.41- 48, 1995.
"合成開口レーダと海洋のリモートセンシング II：海洋リモートセンシングへの応用," 可視化情報学会誌, vol.15, pp.118- 125, 1995.
合成開口レーダ（SAR: Synthetic Aperture Radar）は波長が数ミリから数十センチのマイクロ波を使い、地海面の２次元画像を高分解能で生成する結像レーダで、現在および将来のリモートセンシングにとって欠かせない最も重要なセンサーの一つである。1960年代から1970年代の開発段階のSARは、マイクロ波ホログラムとも呼ばれ、その基本原理はホログラムを可視光の代わりにマイクロ波で作成し、それを光学的処理をすることで画像を生成することにある。この段階での研究は航空機を使った小規模なものであった。1978年にNASAによって打ち上げられた海洋観測衛星SEASAT搭載のSARは、陸海面の鮮明な画像とそれに内含している多大な潜在的情報と応用の可能性を提供した。以後の研究は非常に活発となり、シャトルや航空機を使った大規模な研究と実験が続けられてきた。現在ではヨーロッパ宇宙局（ESA）のERS-1、日本のJERS-1そしてロシアのALMAZ-1衛星搭載のSARが活動しており、21世紀に向けたSAR搭載のプロジェクトが各国で作成されつつある。さらに、コンピューターの急激な発達により現在ではほとんどの画像はコンピューター処理により生成されている。この処理法の利点は画像が複素形で得られることで、その結果光処理法では得られない画像の位相が直接得られることである。位相に内含されている情報は振幅のそれよりも多大であると言われている。今、脚光を浴びているSAR干渉計はこの位相を利用したものである。なぜSARがこのように重要なセンサーであるかということには主に三つの理由があげられる。まず第１に、合成開口技術を使うことにより従来の実開口レーダーの欠点であったアジマス（レーダー進行）方向の分解能を大幅に改良し、２次元での高分解能画像が得ることができる。第２にSARはマイクロ波をつかっているので昼夜の別なく雲や雨にも影響されない全天候型のセンサーであるということにある。これは１年の半分が夜である極地や、雲などに隠れがちな熱帯森林地方での情報解析に特に有効である。第３にマイクロ波で得られる情報は可視光や赤外線で得られる情報と違うことである。このようにSARはセンサーとしてそれ自体の有用性を持つが、他のセンサーからの情報と組み合わせた多重センサー、多重パラメタータの情報を総合的に解析することにより潜在情報の抽出と結果の信頼性が増大することは明らかである。このSARを利用した海洋のリモートセンシングは、現在と将来の地球環境にとって重要な研究テーマの一つである。海洋は地球の３分の２を占め、天候を左右し、その資源が人類に及ぼす影響ははかりしれない。しかしながら、その広さゆえ船舶などによる情報収集はほんの部分的でその応用性もおのずから限られてくる。一方、衛星による情報収集は地球規模で、特に上記したマイクロ波の特性を考慮に入れると海洋リモートセンシングにとって欠かせない方法であろう。本講義の第１部ではSARの基本が説明されている。レンジ（レーダー照射）方向の高分解能を得るパルス圧縮技術とアジマス方向の合成開口技術は良く似ており、理論的にも比較的簡単であるので数式を使って解説してある。数式記述が必要でない読者はSARの原理の「まとめ」に直接進まれたい。ここでは一応専門的数式を使わず理解できるようにまとめてある。海洋は常に動いており、散乱体の動きが画像似与える影響はSAR独特なものがあるのでその点も考慮に入れてある。第２部（次号掲載）では海洋のリモートセンシングへの応用として、SARで画像化が可能な海洋現象を画像を使って説明してある。これらの現象としては波浪などの表面波、内部波、海底構造、海流、潮目、渦、海上風、冷水域や暖水域の水隗、そして海氷や氷山などがある。限られたスペースでSAR画像生成の詳細や多くの画像を呈示することはできないので、それらについては参考文献を参照されたい。
"Resolution of a controversy surrounding the focusing mechanisms of synthetic aperture radar images of ocean waves," IEEE Trans. Geosci. Remote Sens., vol. GE-32, pp.1004- 1016, 1994.
This paper addresses key problems regarding the focusing of synthetic aperture radar (SAR) images of ocean surface waves, explaining why applying a processor defocus will generally yield an enhanced image, why the same defocus applies to both image modulations brought about by the radar cross section and by the velocity bunching process, and why the effects apply to both single-look and multilook systems independently of look relocation. Two interpretations are given for the case when surface scatterers are stationary, but modulated in reflectivity (radar cross section) by a propagating wavefield. The first interpretation is what will be called a "degrade-and-shift" model. In it, a processor focusing adjustment degrades a point image. However, the overall image can be enhanced because an appropriate defocus results in a shifting of points in such a way that the image can most closely resemble the image of the time-invariant (or "frozen") reflectivity. The second interpretation is a "defocus-and-refocus" model in which the image of a time-varying reflectivity is defocused and may be refocused to enhance the image. In justifying this "defocus-and-refocus" model, it is shown that the radar return from stationary scatterers of time-varying reflectivities is identical to that from physically moving scatterers of constant reflectivity. Thus, the two interpretations are not contradictory; they are, fundamentally, equivalent. The models support the use of a processor defocus corresponding to one half the wave phase velocity. Both qualitative and quantitative illustrations of the effects are given. Finally, it is shown that the same defocusing effect applies to image modulations brought about by the velocity bunching process.
"Modulation of waveheight spectrum and radar cross section by varying surface currents," IEEE Trans. Geosci. Remote Sens., vol. GE-32, pp.995- 1003, 1994.
This paper investigates the validity of the relaxation model which is generally used to describe the modulation of wind-generated ocean waves by slowly varying surface currents, and the applicability of the model to illustrating features observed in the radar images of internal wave wakes. The amplitude and phase modulations in the waveheight spectrum are examined for sinusoidal current variations: these changes are then translated into the radar cross section (RCS) modulation using the Kirchhoff scattering theory. The present study concludes that: 1) the relaxation model is valid under intermediate to strong wind conditions and/or weak current signatures, and the conditions for the validity are given; 2) the relaxation model combined with the Kirchhoff model could, under certain conditions, explain the differences in amplitude and phase between the P -band and L (C - and X -) band SAR images of ship-generated internal wave wakes observed during the 1989 Loch Linnhe experiment; and 3) the directional dependence of the relaxation rate enhances these differences.
"Microwave remote sensing of the ocean: a review," 日本リモートセンシング学会誌, vol.12, pp.92-101, 1992.
海洋のマイクロ波リモートセンシング、特に、合成開口レーダーによる海表面の観測を中心とする能動型マイクロ波リモートセンシングの原理と応用について述べている。
"Dependence of SAR azimuth image displacement of range moving scatterers on processor focal setting," IEEE Trans. Geosci. Remote Sens., vol. GE-30, pp.180- 184, 1992.
This paper is a reappraisal of the principal theory on the azimuth displacement of the synthetic aperture radar (SAR) image of a point scatterer moving in the range direction with a constant velocity. New results are presented which describe the dependence of the image shift on the processor focal setting. It is shown that, provided the bandwidth of the processor is greater than or comparable with the bandwidth of the signal, there is a strong dependence of the azimuth image shift on the processor focusing. Simple geometric illustrations based on the optical processor are provided to interpret the effects.
"Statistical analysis of azimuth streaks observed in digitally processed CASSIE imagery of sea surface," IEEE Trans. Geosci. Remote Sens., vol. GE-29, pp.727-735, 1991.
During the Canberra SAR Sea Imaging Experiment in 1987 (CASSIE'87), numerous azimuth streaks were observed by the X-band synthetic aperture radar (SAR). The data are digitally processed, enabling us to make accurate statistical analysis. The probability density function (PDF) and higher order intensity moments are first calculated and compared with theory for both the image areas containing a few and a large number of streaks. The results show that the PDF corresponding to the area of a few streaks fits well with a log-normal distribution, while the image intensity containing many streaks is found to obey K-distribution. The two-dimensional autocorrelation function (ACF) is shown to consist of a central peak followed by a slow decay. The presence of a slow decay in range direction indicates that the backscattered radar cross section (RCS) is spatially correlated over the extent of breaking waves that are considered to be the source of azimuth streaks. The slow decay in azimuth direction is more pronounced than that in range direction due to the scatterers' motions in addition to the spatial correlation of RCS. The comparison of the ACF's of the streaked images processed at different processor focal settings suggests that the dominant motion effect is caused by the scatterers' random motion, although a little dependence, but not dominant, has been observed to indicate the presence of uniform scatterers' motions and/or unknown aircraft motion errors. Based on the experimental observations, a theoretical model is postulated to account for the correlated non-Rayleigh scattering amplitude statistics and the random motion of scatterers. Attempts are then made to recover the range ACF of RCS from the image ACF and to predict the azimuth component of the image ACF.
"Speckle cross-correlation function in multilook SAR images of moving discrete scatterers," Int. J. Remote Sens., vol.12, pp.1933-1946, 1991.
The properties are described of the cross-correlation function (CCF) of the non-Gaussian speckle in the images of a moving random rough surface produced by a synthetic aperture radar (SAR) operating in the multilook mode. The scattering surface is considered to contain discrete scatterers, giving rise to the backscattered complex field which consists of spiky non-Gaussian random amplitude fluctuations. This spatial complex field is subjected to temporal changes due to the scatterers. motions which include random motion, uniform azimuth velocity and uniform range acceleration, since they are the major components to cause the motion effects which appear in the CCF. This type of scattering model can be found in the radio wave backscatter from breaking or near-breaking sea waves. The multilook impulse responses from a moving point scatterer are briefly described; they are then convolved with the backscattered spatial field to yield the subimages of different looks. A general integral form for the correlation function of image speckle intensity is expressed in terms of the degraded multilook impulse responses and the number of discrete scattering amplitude fluctuations. Explicit expressions of the speckle CCF between the subimages are analysed for different scatterers' motions, highlighting the motion dependent effects inherent to multilook SARs.
"Autocorrelation function of non-Gaussian speckle in synthetic aperture radar images of dynamic sea surfaces," Int. J. Remote Sens., vol.12, pp.315-327, 1991.
This paper presents a detailed theory on the statistical properties of non-Gaussian speckle in the synthetic aperture radar (SAR) images of dynamic sea surfaces, where breaking or near-breaking waves are present. The backscattered complex field is modelled as the sum of two independent fields: one arises from breaking waves which occur discretely and therefore the amplitude fluctuations are also discrete, and the other represents the weak background field of an uniform amplitude due to non-breaking sea waves and/or system noise. The elementary scatterers are considered to be either in random motions, and the magnitude of the motion is considered to be much greater for breaking waves than the background non-breaking waves. This compound backscattered field is then imaged by a SAR, resulting in a non-Gaussian speckle pattern which appears as the pronounced azimuth streaks. A general and simple expression is derived for the autocorrelation function (ACF) of the speckle intensity in terms of the statistical properties of the spatial backscattered field and the impulse response degraded by the scatterers' motions. The ACF is evaluated and discussed for a Gaussian impulse response and for different scatterers' motions.
"Synthetic aperture radar imagery of range travelling ocean waves," IEEE Trans. Geosci. Remote Sens., vol. GE-26, pp. 30-37, 1988.
In synthetic aperture radar (SAR) imagery of ocean waves, an image modulation by radar foreshortening exists in addition to backscatter radar cross sections. The foreshortening effect is due to surface height differences making sections of a rough surface fall into different range bins than they would if the surface were flat. Since the degree of this modulation changes according to the local waveheight, foreshortening alone could produce the image pattern corresponding to the wave field. The effect termed as "range bunching" is maximum for range traveling ocean waves and vanishes for azimuth traveling waves. It also decreases with decreasing radar look angle. In this paper the imaging processes of range traveling waves are investigated by taking account of both the cross section and range bunching. The SAR transfer function and image modulation functions are defined to describe the relative importance and the coupling effects of the two contributions. It is shown that in low to moderate sea states where the major variation in backscatter arises from local surface tilt, the image modulation by the cross section is enhanced by range bunching and the effect increases with increasing wave slope and also with decreasing look angle. For ocean waves in high sea states with a small variation in the cross section and steep wave slopes, range bunching may become an important mechanism for the interpretation of the images formed by SAR with small look angles.
"Dependence of speckle statistics on backscatter cross-section fluctuations in synthetic aperture radar images of rough surfaces," IEEE Trans. Geosci. Remote Sens., vol. GE-25, pp.623-628, 1987.
A theory is described to relate the statistical properties of the fields backscattered from rough surfaces to those of speckle in synthetic aperture radar (SAR) images. The expressions are derived for the autocorrelation and cross-correlation functions of speckle intensity in both single-look and multilook images of stationary random rough surfaces in terms of the SAR system parameters and the autocorrelation function of backscatter radar cross-section fluctuations. It is shown that if the correlation scale of cross-section fluctuations is comparable with or greater than the SAR resolution, the correlation functions of speckle intensity depend on those of the cross-section fluctuations. This property, therefore, may be applied to image classification. Comparison of the theory with computer simulation shows good agreement.
"Multilook images of ocean waves by synthetic aperture radars," IEEE Trans. Antennas Propagat., vol. AP-35, pp.313-317, 1987.
A property of multilook processing of synthetic aperture radar (SAR) data is that a time lapse exists between subapertures, so that they contain information about a scattering surface at different times. Reported here is a theoretical study on the images of dynamic ocean waves processed by this technique. It is shown that due to the time lapse the subimages of a moving ocean wave differ in position depending on the look number and the wave phase velocity. Such images cannot be enhanced by the incoherent addition so much as those of stationary surfaces. The difference in image position can be corrected by defocusing the azimuth reference signal by the same amount as for the correction of defocusing induced by the wave motion. Discussions are presented on the correction of image positions and on the effect of defocusing. The property of the time lapse could be applied to estimating not only the phase velocity of ocean waves but also temporal changes in general scattering surfaces.
"On the images of ocean waves by synthetic aperture radars," in Wave Propagation and Scattering, Oxford: Oxford University Press, (ed. B.J.Uscinski), pp.297-332, 1986.
The theory of synthetic aperture radar (SAR) imaging of dynamic ocean surface waves is presented. The principles are described of single- and muli-look processing of SAR data from a stationary and moving point target with special emphasis on the motion effects inherent to multi-look processing. The theory is extended to the imaging of diffusely scattering ocean waves, where image modulation by backscatter cross sections, velocity bunching and tilt or range bunching is discussed. Image degradation by defocusing and the finite lifetime of scatterers are also considered.
"Multilook processing of synthetic aperture radar data from dynamic ocean surfaces," Pattern Recognition Lett., vol.4, pp.305-314, 1986.
The basic theory is described of multi-look processing of synthetic aperture radar (SAR) data from dynamic ocean surfaces. The principal effects inherent to multi-look SAR on the impulse response function are first illustrated using a moving point target model. The theory is then extended to the multi-look imagery of diffusely scattering ocean surface waves. Finally, the use of a sliding synthetic aperture is suggested to investigate the short life-times of small scale surface waves that are considered to be the predominant scatterers on the ocean surface.
"On the multilook images of moving targets by synthetic aperture radars," IEEE Trans. Antennas Propagat., vol. AP-33, pp.823-827, 1985.
In the multilook processing of synthetic aperture radar (SAR) data, subapertures are synthesized at different center times so that a time-lapse exists between looks. This does not affect the imaging of stationary targets but if targets are in motion, the information content about the targets differs from look to look. The purpose of this paper is to investigate the effects of motions on the SAR multilook images of moving targets. Expressions are derived for the impulse response function from a moving point target in terms of the look number and the nature of the motion. Discussions are given only on the effects inherent to multilook processing.
"Two-dimensional imaging mechanisms of ocean waves by synthetic aperture radars," J. Phys., D : Appl. Phys., vol.17, pp.25-42, 1984.
Expressions are derived and discussed for the local mean intensity of the two-dimensional images of ocean waves produced by a synthetic aperture radar (SAR). Accounts are taken of the spatial and temporal changes of both back-scattered radar cross-section and ocean wave height. It is shown that the images of ocean waves are distorted as a result of radar layover. This imaging process is similar to velocity bunching but the effect is largest for range waves and it vanishes for azimuth waves. The mechanism of defocusing is also investigated. If reference signals are designed for stationary objects, the images of dynamic waves are always defocused, irrespective of their propagation direction and of the types of imaging processes, including radar layover, velocity bunching and back-scattered cross-section. Defocusing originates from the systematic degradation and upgradation of point spread (impulse response) functions associated with the periodic structure of the waves. The images can be enhanced by applying a defocused azimuth reference signal and the amount of defocusing for optimum images depends only on the wave phase velocity and propagation direction.
"Effect of defocusing on the images of ocean waves produced by synthetic aperture radars," in Satellite Microwave Remote Sensing, ( Chichester: Ellis Horwood, ed. T.D.Allan ), pp.209-234, 1983.
The effects of defocusing on the images of dynamic sea surfaces produced by SAR are of considerable current interest (Shuchman & Zelenka 1978, Jain 1978, Valenzuela 1980, Alpers & Rufenach 1979, Raney 1981). Two main theories have been postulated to explain the experimental observations in which the images of ocean waves are enhanced by adjusting the azimuth focus of a SAR processor. One of them is that the azimuth component of the wave phase velocity changes the relative velocity of the radar platform (Shuchman & Zelenka 1978, Jain 1978, Valenzuela 1980), and the other is that defocusing is caused by the slant-range component of the acceleration associated with the wave orbital motion (Alpers & Rufenach 1979). In this chapter, we investigate the defocusing effect by applying a different approach to the formulation of SAR imagery of dynamic objects from the conventional phase perturbation method. The main advantage of the present approach is that it is rigorous and could lead to simple analytical results (Rufenach & Alpers 1981). We consider that the main contribution to the final image is backscattered amplitude (power) modulation arising from surface roughness and/or surface tilt (Elach & Brown 1977), and the velocity bunching effect (Valenzuela 1980, Alpers & Rufenach 1979, Rufenach & Alpers 1981, Ouchi 1982a) is not included. The cases where the two effects contribute to the images of azimuth waves have been previously discussed by Ouchi (1982b). We also consider the spatial and temporal random fluctuations of backscattered amplitude which in effect degrade image quality (Ouchi 1982c, Ouchi 1981, Raney 1980, Raney & Shuchman 1978, Raney 1981). We then derive and discuss expressions for the local mean intensity and constrast of the images of ocean waves propagating in an arbitrary direction. It is shown that if the reference signal of the SAR processor is matched to a stationary point target, the images of waves are always defocused, irrespective of their propagation direction. The images can be enhanced by applying a defocused reference signal, and the amount of defocus depends on the phase velocity and propagation direction of ocean waves.
"Defocus dependence on ocean wave shape in synthetic aperture radar imagery," Opt. Quant. Electron., vol.15, pp.355-357, 1983.
In a previous paper [1] we examined the effect of defocusing on the images of sinusoidal ocean waves produced by synthetic aperture radar (SAR) and suggested that defocusing may depend on the shape of waves. In this note, we investigate this dependence in a little more detail. For simplicity, the image structure in the azimuth (along-track) direction only will be considered.
"Imagery of ocean waves by synthetic aperture radars," Apl. Phys., vol. B-29, pp.1-11, 1982.
We consider the image structure of ocean waves produced by a synthetic aperture radar (SAR). The theory takes account of the spatial and temporal dependence of small scale capillary waves and large scale waves. Detailed discussions are given to the imaging process in which both amplitude modulation and velocity bunching contribute to the final images. Discussions are also presented on the effect of defocusing on the images of azimuth waves.
"Effect of random motion on synthetic aperture radar imagery," Opt. Quant. Electron., vol.14, pp.263-275, 1982.
We consider the effect of the random motion of capillary waves on the mean intensity and the contrast of the images of dynamic ocean surfaces, with such features as surface waves and current boundaries, produced by a synthetic aperture radar (SAR).
"Statistics of speckle in synthetic aperture radar imagery from targets in random motion," Opt. Quant. Electron., vol.13, pp.165-173, 1981.
We derive and discuss expressions for the autocorrelation functions of speckle patterns in synthetic aperture radar imagery both when the scattering surface is stationary and in random motion. The effects of the motion of a point target on the poin spread function are also discussed.
"First-order statistics of speckle from hard-edged apertures," Ph.D. Thesis, University of London, 1980.
The present thesis is concerned with the first-order statistics and contrast of speckle patterns in and near the far-field and image plane, formed by optical systems having hard-edged pupils. We start, in Chapter one, by reviewing some previous work on the subject. In Chapter two, we give a detailed analysis for the first-order statistics of speckle patterns in and near the far-field. The theory involves assumptions about the fisrt-order probability density function and autocorrelation function of the surface height of scattering media and these are assumed to have Gaussian forms. By applying small-angle scaler diffraction theory, we derive expressions for the constrast in terms of the statistical parameters of rough surfaces and the geometry of the optical system. It is shown that the contrast of speckle patterns produced by hard-edged circular pupils has maxima and minima in the focal plane and along the axis of the optical system: the results are compared with those when the system has a soft pupil. The experimental results show fairly good agreement with the theory. Chapter three deals with the first-order statistics of speckle formed in and near the image plane of a telecentric optical system. We show that when a hard-edged aperture is applied the contrast has maxima and minima along the optical axis; while for a soft aperture there is only one well defined contrast minimum at the Gaussian image plane. Experimental results are presented to verify the theory. In the experiments carried out here, ground glass surfaces were used as the scattering objects and we present, in Chapter four, the production and determination of the statistical properties of ground glass surfaces. The work reported in Chapter two, three and four provides new results in the fields of optical speckle.
"Statistics of image plane speckle," Opt. Quant. Electron., vol.12, pp. 237-243, 1980.
We examine the statistical properties of speckle patterns produced at an arbitrary axial point of a telecentric imaging system having a soft and hard-edged aperture. Expressions for the contrast of the pattern are derived and discussed.
"Statistics of speckle focused through a hard-edged pupil," Opt. Quant. Electron., vol.11, pp.345-352, 1979.
We examine the statistical properties of speckle patterns in and near the far-field, produced by an ordinary optical system having a hard-edged pupil. By applying Fresnel diffraction theory, expressions for the contrast in terms of the statistical parameters of rough surfaces and the geometry of the optical systems are derived and discussed. It is shown that the contrast of speckle has maxima and minima in the focal plane and also along the axis of the optical system.
"Measurement of contrast of speckle in and near the image plane," Opt. Commun ., vol.24, pp.273-275, 1978.
The contrast of a speckle pattern formed by a conventional imaging system has been experimentally investigated. A dip in contrast of the speckle pattern produced near the focal plane and image plane has been observed. It is also found that the variation of contrast along the optical axis of the imaging system with a hard-edged aperture differs considerably from that with a soft aperture.