Microwave Scatterometer Research Articles

Sensitivity of satellite microwave and infrared observations to soil moisture at a global scale: Relationship of satellite observations to in situ soil moisture measurements

This study presents a systematic and integrated analysis of the sensitivity of the available satellite observations to in situ soil moisture measurements. Although none of these satellites is optimized for land surface characterization, before the launches of the SMOS‐ and HYDROS‐dedicated missions they are the only potential sources of global soil moisture measurements. The satellite observations include passive microwave emissivities, active microwave scatterometer data, and infrared estimates of the diurnal amplitude of the surface skin temperature. The Global Soil Moisture Data Bank provides in situ soil moisture measurements in five separate regions. This simultaneous analysis of various satellite observations and the large amount of in situ measurements has two major advantages. First, this analysis helps identify and separate the physical mechanisms that affect the satellite observations. For example, we show that the passive microwave polarization differences at 19 GHz and above are essentially sensitive to the vegetation and not to the soil moisture (i.e., the correlation between microwave observations and soil moisture is only indirect and comes from the statistical correlation between vegetation and soil moisture). Second, this analysis enables an objective comparison of the relative potential of the various satellite observations for soil moisture retrieval when other conditions are held constant. The second part of this study benefits from this synthesis to derive a relationship between satellite observations and soil moisture at a global scale.

Measurement of Concrete Highway Rough Surface Parameters by an X-Band Scatterometer

A nondestructive evaluation (NDE) method is developed to detect the dielectric constant and surface roughness of concrete highway pavement. The system is based on Leader's slope-expansion scattering theory. Different from most other scattering theories, the slope-expansion method does not pose stringent requirements about the nature of the rough surface. Yet, it can calculate the in-plane scattered field (or power), including the nonzero slope terms up to the second order. The explicit expression of the scattered power is given, provided that the incident wave is plane wave, and the surface is Gaussian distributed. A microwave scatterometer operating in X-band is implemented. The measurement system consists of a pair of horn antennas. The measured scattered power is used as the input to the software application. The program then computes the dielectric constant and surface roughness inversely by the least square inversion technique. Several samples have been measured in the laboratory. To verify the test results, a laser profiler and a radar system were used to provide a direct measurement result.

Systematic error of microwave scatterometer wind related to the basin‐scale plankton bloom

Chlorophyll‐a concentration derived from satellite ocean color sensor are compared with coincident wind speed difference between microwave scatterometer QuikSCAT product and reanalysis product from the National Center for Environmental Prediction over global oceans. The objective is to explore if a natural surface film, which originates in biological productivity, causes the underestimation of surface wind in scatterometer measurement by damping the small capillary wave through the surface tension. The wind speed difference (Reanalysis wind – QuikSCAT wind) is found to correlate positively with chlorophyll‐a over the biologically highly productive areas, especially from intra‐seasonal to seasonal time scales, indicating the surface film effect. The wind speed difference increases from ∼−1 m s−1 to ∼0.5 m s−1 as chlorophyll‐a increases from ∼0.1 mg m−3 to ∼3 mg m−3, which is comparable with the results of previous laboratory experiments with artificial oil.

Sea Surface Measurement by Using Microwave Scatterometer

Sea Surface Measurement by Using Microwave Scatterometer

Measurement of concrete highway rough surface parameters by an X-band scatterometer

A nondestructive evaluation (NDE) method is developed to detect the dielectric constant and surface roughness of concrete highway pavement. The system is based on Leader's slope-expansion scattering theory. Different from most other scattering theories, the slope-expansion method does not pose stringent requirements about the nature of the rough surface. Yet, it can calculate the in-plane scattered field (or power), including the nonzero slope terms up to the second order. The explicit expression of the scattered power is given, provided that the incident wave is plane wave, and the surface is Gaussian distributed. A microwave scatterometer operating in X-band is implemented. The measurement system consists of a pair of horn antennas. The measured scattered power is used as the input to the software application. The program then computes the dielectric constant and surface roughness inversely by the least square inversion technique. Several samples have been measured in the laboratory. To verify the test results, a laser profiler and a radar system were used to provide a direct measurement result.

Large-scale inverse Ku-band backscatter modeling of sea ice

Polar sea ice characteristics provide important inputs to models of several geophysical processes. Microwave scatterometers are ideal for monitoring these regions due to their sensitivity to ice properties and insensitivity to atmospheric distortions. Many forward electromagnetic scattering models have been proposed to predict the normalized radar cross section (/spl sigma//spl deg/) from sea ice characteristics. These models are based on very small scale ice features and generally assume that the region of interest is spatially homogeneous. Unfortunately, spaceborne scatterometer footprints are very large (5-50 km) and usually contain very heterogeneous mixtures of sea ice surface parameters. In this paper, we use scatterometer data in a large-scale inverse modeling experiment. Given the limited data resolution, we adopt a simple geometric optics forward-scattering model to analyze surface and volume scattering contributions to observed Ku-band signatures. A model inversion technique based on recursive optimization of an objective function is developed. The result is a least squares estimate of three surface parameters: the power reflection coefficient at nadir, the rms surface slope, and the volume scattering albedo. Simulations demonstrate the performance of the method in the presence of noise. The inverse model is implemented using Ku-band image reconstructed data collected by the National Aeronautics and Space Administration scatterometer. The results are used to analyze and interpret /spl sigma//spl deg/ phenomena occurring in the Antarctic and the Arctic.

Space based techniques for remote sensing of oceanic winds : A Review

Observations of surface winds over the global oceans at regular intervals from space platforms have been demonstrated. This paper focuses on different techniques developed over the past few decades. The most successful wind sensor has been the microwave scatterometer, which can provide vector winds with reasonable accuracy. With the launch of METOP-1, scheduled in 2005, scatterometer will be part of routine satellite system for weather forecast. Among the research efforts has been retrieval of winds from Synthetic Aperture Radar data, available in high spatial resolution and in both open oceans and coastal waters. Radiometric, altimetric and optical wind measuring techniques are also reviewed in the paper.

Sea Surface Measurement by Using Microwave Scatterometer

Microwave remote sensing is effective technique for sea surface measurement. For example, it enables us to know the wind vector on global scale. In order to develop more advanced measurement, it is necessary to understand the mechanism of microwave scattering at sea surface. This phenomenon consists of two physical processes. One is the formation of sea surface by the composite action of wind, waves and currents, and the other is microwave scattering at that surface. Because these processes are so complicated, appropriate models are not established. In this paper, the relation between sea surface condition and microwave scattering is investigated. Microwave scattering from wind wave surfaces is measured in the experimental tank and the results are analyzed by scattering coefficients and Doppler spectra. In order to connect these values with water surface parameters, surface condition is investigated minutely. The energy of high frequency waves is expressed by the friction velocity, and so it is presumed that there is the universal law between friction velocity and scattering coefficient. Since wind-generated current exist, the usual dispersion relation is not available. Furthermore, very high frequency components move at the phase velocity of components near the spectral peak. The results of Doppler spectra agree with that phenomenon.

QuikSCAT reveals the surface circulation of the Catalina Eddy

The Catalina Eddy, a small, recurring cyclonic vortex in the ocean off Los Angeles, is of keen interest to local weather forecasters because of the moderating oceanic effect it brings to the city. Its small size and shallow vertical extent have made it difficult to monitor and predict using conventional data. The microwave scatterometer on the QuikSCAT spacecraft has generated high‐resolution surface wind vectors that provide the first visualization of the complete cyclonic flow of the eddy. Moreover, the superior performance of the QuikSCAT scatterometer demonstrates the relative inaccuracy and inconsistency of predictions of the eddy based upon numerical weather prediction models.

マイクロ波リモートセンシングによる土壌水分検出 : 表面粗度の影響(学生 / 若手技術者発表会および一般)

マイクロ波リモートセンシングによる土壌水分検出 : 表面粗度の影響(学生 / 若手技術者発表会および一般)

Sea Surface Measurement by Using Microwave Scatterometer

The microwave scatterometer is one of the useful remote sensors for the sea surface measurement. The basic principle of the measurement is a scattering at sea surface. Therefore, it is necessary to understand the characteristic of the microwave scattering at sea surface in order to measure the sea surface condition. The purpose of this study is to investigate characteristics of microwave scattering under the various conditions in the experimental tank and also the consideration of measurement technique and data analysis method. Bragg scattering model is applied to the experimental condition, and the tendency in the measuring results of scattering strength is explained in this model. For the quantitative evaluation, however, it is necessary to more accurately grasp water surface profiles. The Doppler shift of scattering microwave has related to the speed of the scatterer.

Evaluation of a compound probability model with tower-mounted scatterometer data

Six months of data from the YSCAT94 experiment conducted at the CCIW WAVES research platform on Lake Ontario, Canada, are analyzed to evaluate a compound probability model. YSCAT was an ultrawideband small footprint (/spl ap/1 m) microwave scatterometer that operated at frequencies of 2-18 GHz, incidence angles from 0/spl deg/ to 60/spl deg/, both h-pol and v-pol, and which tracked the wind using simultaneous weather measurements. The probability distribution function of the measured instantaneous backscattered amplitude (p(a)) is compared to theoretical distributions developed from-the composite model and a simple wave spectrum. Model parameters of the resulting Rayleigh/generalized lognormal distribution probability density function (pdf) (C, a/sub 1/, and a/sub 2/) are derived directly from the data and are found to demonstrate relationships with wind speed, incidence angle, and radar frequency.

On airborne measurement of the sea surface wind vector by a scatterometer (altimeter) with a nadir-looking wide-beam antenna

A pilot needs operational information about wind over sea as well as wave height to provide safety for a hydroplane landing on water. Near-surface wind speed and direction can be obtained with an airborne microwave scatterometer, a radar designed for measuring the scatter characteristics of a surface. Mostly narrow-beam antennas are applied for such wind measurement. Unfortunately, a microwave narrow-beam antenna has considerable size that hampers its placement on flying apparatus. In this connection, a possibility to apply a conventional airborne radar altimeter as a scatterometer with a nadir-looking wide-beam antenna in conjunction with Doppler filtering for recovering the wind vector over sea is discussed, and measuring algorithms of sea surface wind speed and direction are proposed. The obtained result can be interesting for many studies in oceanography, meteorology, air-sea interaction, and climate change and for creation of an airborne radar system for amphibious airplane safe landing on the sea surface, in particular for search and rescue operations in coastal areas.

Azimuth variation in microwave scatterometer and radiometer data over Antarctica

While designed for ocean observation, scatterometer and radiometer data have proven very useful in a variety of cryosphere studies. Over large regions of Antarctica, ice sheet and bedrock topography and the snow deposition, drift, and erosional environment combine to produce roughness on various scales. Roughness ranges from broad, basin-scale ice-sheet topography at /spl sim/100 km wavelengths to large, spatially coherent dune fields at /spl sim/10 km wavelength to erosional features on the meter scale known as sastrugi. These roughness scales influence the microwave backscattering and emission properties of the surface, combining to introduce azimuth-angle dependencies in the satellite observation data. In this paper, the authors explore the use of NASA scatterometer (NSCAT) data, European remote sensing (ERS) advanced microwave instrument (AMI) scatterometer mode data, and special sensor microwave/imager (SSM/I) data to study surface roughness effects in Antarctica. All three sensors provide strong evidence of azimuth modulation, which is correlated with the surface slope environment and results in a katabatic wind flow regime. Due to its broad azimuth coverage, NSCAT data appears to be the best suited for azimuth-angle observations. A simple empirical model for the azimuth variation in the radar backscatter is developed, and an algorithm for computing the parameters of the model from NSCAT data at a fine scale is presented. Results indicate relationships exist between the azimuthal variation of the data and the orientation of the surface slope and small-scale roughness relative to the sensor-look direction.

ANALYSES OF GLOBAL LAND COVER INFORMATION USING BACKSCATTERING COEFFICIENTS BY TRMM-PR

The backscattering coefficients observed by satellite-based microwave scatterometer are influenced by surface geometric roughness and incident angles. Land surface information can be obtained using these dependences of backscattering coefficients on a different land cover. The Precipitation Rader (PR) on-board the Tropical Rainfall Measuring Misson (TRMM) offers a global and seasonal data set of backscatter coefficients as a by-product. A set of TRMM-PR2A21 data in whole 1998 is utilized in this present paper to investigate land 3urfacc information. The backscattering coefficients at selected regions which belong to different land cover types are analyzed in order to clarify the dependence of backscattering coefficients on incident angle. It is found that the backscattering coefficients significantly depend on land type and incident angle. The seasonal change of backscattering coefficients at each land type is also assessed which should depend on the seasonal change of soil moisture and vegetation. Then, the PR-data are divided into 3 groups based on incident angle, and a global land cover map is produced by classifying land cover into 16 categolies using the 3 groups. The global land cover map is presented and the characteristics of it is described in detail. It is expected that the seasonal change of global land surface information can be obtained utilizing this method in the future.

Spacebased observations of the seasonal changes of south Asian monsoons and oceanic responses

The seasonal changes of monsoons in the South China Sea and the Arabian Sea are compared, using observations by spaceborne microwave scatterometers and radiometers. The oceanic responses to the forcing of wind stress and latent heat flux resulting from the monsoon changes are identified through surface temperature tendency and sea level changes.

Radar signatures of marine mineral oil spills measured by an airborne multi-frequency radar

Radar signatures of mineral oil spills consisting of heavy and light fuel were measured by an airborne five-frequency ( L - S - C - X - and K -band) multi u polarization microwave scatterometer flown on a helicopter during a controlled oil spill experiment in the North Sea. The damping ratio, defined as the ratio of the backscattered radar power from an oil-free and an oil-covered sea surface, was measured at different radar frequencies and incidence angles such that the Bragg wavenumbers, k, between 20 radm 1 and 500 radm 1 were covered. The B following results were obtained: for the five oil spills deployed in the experiment the damping ratio, in general, increases monotonically from k 20 radm 1 to B k 500 radm 1 . At S - C - X - and K -band, the damping ratio is larger for heavy B u fuel than for light fuel spills, while at L- band it is almost the same. For heavy fuel, the damping ratio increases with increasing thickness of the oil layer. Furthermore, for wind speeds between 6 m s 1 and 10 m s 1 the damping of the backscattered radar power is independent of the polarization of the radar and its look direction relative to the wind. The results obtained in the present investigation agree well with those obtained from similar measurements with heavy fuel under high wind conditions.

The observation of the surface roughness characteristics of the Rhine plume frontal boundaries by simultaneous airborne thematic mapper and multifrequency helicopter-borne radar scatterometer

In this paper, we describe how high spatial resolution (10 m) multisensor remote sensing techniques can be used to study the surface roughness characteristics of large scale frontal boundaries (in this case associated with the Rhine Plume). The instrumentation employed in the research consisted of a Daedalus AADS 1268 Airborne Thematic Mapper (ATM) operated by the UK National Environment Council, the HELISCAT helicopter-borne multifrequency microwave scatterometer of the University of Hamburg, and research vessels (R.V.s) from the University of Wales and the Dutch Rijkswaterstaat. The data we present were gathered on 24 April 1991 when calm wind conditions developed within the test area. A sequence of thermal infrared images gathered by the ATM provides a record of the motion of a frontal boundary through this experimental region which is then used to identify the frontal signature in the HELISCAT data. ATM sunglint images show that the front is characterized by a zone of reduced surface roughness, some 75m in width, which is detected on the 'upstream' side of the front (as defined relative to the tidal flow direction), where surface current convergence can be expected. Radar backscatter levels at X and C bands are reduced by 10 dB in this region but with increase in radar wavelength, the signature weakens and is rarely detected at L band. On crossing the front in the downstream direction, radar backscatter levels are rapidly restored. The available evidence indicates that the reduced backscatter signature is caused by a surface slick which is formed at the frontal interface rather than by short gravity wave damping from shear in local surface currents.

Inversive Technique of Surface Roughness Estimation Using Multi-incident Angular Data by Microwave Scatterometer

This paper explores the possibility of inversion technique of surface roughness estimation using multi-incident angular data by microwave scatterometer. We made field experiments to measure soil moisture, surface roughness and soil backscatter by C-band microwave scatterometer mounted on a car. Surface roughness observed 1 dimensionally is scattered widely even in a same land cover and we have to make more than eight observations to get the representative roughness. Algorithm of the inversion technique is developed based on the surface scattering model and surface roughnesses are estimated inversely. However the observed and estimated roughnesses are quite different each other and the latter one proves to be suitable for decision of relational curve for soil moisture-backscattering coefficients.

An Experimental Study on Microwave Backscattering from Laboratory Wave Surfaces

This paper describes the experimental results backscattering from wave surfaces by using microwave scatterometer. Active microwave remote sensing is considered to be very useful to observe the vast ocean at one time, while the ocean phenomena change quickly. The characterization of the microwave backscattering from wave surfaces is investigated in a wind wave tunnel and a square basin by using a C-band (5.2 GHz) microwave scatterometer. This paper clears the relationships between microwave backscattered power and wind driven waves. The significant wave height of wind wave and the average period, the wave direction contribute to Backscattering. The HH and VH-polarization are effective for monitoring the wind wave than the VV and HV-polarization.