Hideo Maeno

Nature of radio echo layering in the Antarctic ice sheet detected by a two-frequency experiment

A two-frequency radio echo sounding experiment was carried out at Dome Fuji, the second highest dome in East Antarctica, and along a 1150-km-long traverse line from the dome to the coast. The goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions. From the two-frequency (60 MHz and 179 MHz) radio echo responses at various sites, we distinguished four zones. Each of the zones is characterized by a dominant cause of radio echo internal reflection as follows. In the “PD” zone, changes in dielectric permittivity are mainly due to density fluctuations; in the “PCOF” zone, changes in dielectric permittivity are mainly due to changes in crystal-orientation fabrics; and in the “CA” zone, changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions. In each of these three zones, the changes occur commonly along isochrones. In addition, a basal echo-free zone, the fourth zone, was found to appear always below the PCOF zone. These four zones and their distribution suggested variations of the physical conditions within the ice sheet.

Texture statistics for classification of land use with multitemporal JERS-1 SAR single-look imagery

Texture statistics are studied using multitemporal JERS-1 synthetic aperture radar (SAR) single-look imagery. A correct autocorrelation function of texture is derived and utilized to calculate textural features. It is demonstrated that a significant improvement in the classification accuracy results if the textural features are used as additional inputs to the classifier.

Polarimetric SAR Image Analysis Using Model Fit for Urban Structures

This paper describes a polarimetric feature extraction method from urban areas using the POLSAR (Polarimetric Synthetic Aperture Radar) data. The scattering characteristic of urban areas is different from that of natural distributed areas. The main point of difference is polarimetric correlation coefficient, because urban areas do not satisfy property of azimuth symmetry, = 0. The decomposition technique based on azimuth symmetry can not be applied to urban areas. We propose a new model fit suitable for urban areas. The proposed model fit consists of odd-bounce, even-bounce and cross scattering models. These scattering models can represent the polarimetric backscatter from urban areas, and satisfy ¬= 0 and ¬= 0. In addition, the combination with the proposed model fit and the three component scattering model suited for natural distributed areas is examined. It is possible to apply the combined technique to POLSAR data which includes both urban areas and natural distributed areas. The combined technique is used for feature extraction of actual X-band POLSAR data acquired by Pi-SAR. It is shown that the proposed model fit is useful to extract polarimetric features from urban areas.

Radio-wave depolarization and scattering within ice sheets : a matrix-based model to link radar and ice-core measurements and its application

Crystal-orientation fabric (COF) has a large influence on ice-sheet flow. Earlier radar studies have shown that COF-based birefringence occurs within ice sheets. Radio-wave scattering in polar ice results from changing physical properties of permittivity and conductivity that arise from differing values of density, acidity and COF. We present an improved mathematical model that can handle all these phenomena together. We use this matrix-based model to study the two-way propagation of depolarized radio waves that scatter at both isotropic and anisotropic boundaries. Based on numerical simulations, we demonstrate how COF affects the radar signals in terms of radar polarization and frequency. We then compare the simulated features with VHF radar data obtained at two contrasting inland sites in East Antarctica, where COF is known from ice-core studies. These two sites are Dome Fuji, located near a dome summit, and Mizuho, located in a converging ice-flow region. Data at Dome Fuji are dominated by typical features resulting from birefringence. In contrast, both birefringence and anisotropic scattering affect the radar data at Mizuho. We argue that radar methods can be used to determine principal axes and strength of birefringence in the ice sheets.

A study on extraction of urban areas from polarimetric Synthetic Aperture Radar image

This paper discusses the polarimetric correlation coefficient to extract the urban areas from polarimetric Synthetic Aperture Radar (POLSAR) image. For classification of POLSAR image, several methods have been proposed to extract polarimetric feature, such as Polarimetric Entropy-Alpha, three-component scattering model, Huynen parameters and so on. However, there is a possibility that the polarimetric correlation coefficient has a potential for the objective of this paper, too. In order to verify the capability of polarimetric correlation coefficient, we examine the behavior of this coefficient between the urban areas and the natural distributed areas with respect to the several polarimetric scattering models and the difference of polarization basis. Moreover, we apply the polarimetric correlation coefficient to the actual polarimetric SAR data acquired by Pi-SAR/X-SAR.

Delay Difference Calibration of TWSTFT Earth Station Using Multichannel Modem

Delays in signal transmission and reception paths of an Earth station must be calibrated accurately, and their variation should be reduced in the two-way satellite time and frequency transfer (TWSTFT). We have developed a portable TWSTFT station using a modem built by the National Institute of Information and Communications Technology (NICT) and performed calibration between NICT and Telecommunication Laboratories. An uncertainty of about 1 ns in the differential calibration was achieved. In addition, we have monitored the delay variation in the Earth station using a measurement system installed at NICT to improve accuracy of the TWSTFT. The contributed instability was 50 ps/day

Scattering of VHF radio waves from within an ice sheet containing the vertical-girdle-type ice fabric and anisotropic reflection boundaries

Abstract We studied the scattering of radio waves off strata within the ice sheet at Mizuho station, Antarctica, to determine the most plausible scattering mechanisms at this location. We measured the effects of birefringence and anisotropic scattering boundaries on the return signal using the following set of experimental conditions: (1) co-polarization and cross-polarization antenna arrangements, (2) all orientations of the antenna system, (3) 60 and 179 MHz frequencies, and (4) pulse lengths of 150–1000 ns. Analyses of the propagated radio waves suggested that the signal is dominated by anisotropic scatter-ingboundaries at 179 MHz, but effects from birefringence also occurred. At depths of 250– 750 m, the scattering was stronger when the polarization plane was along the flowline. In contrast, at depths of about 900–1500 m, scattering was stronger when the polarization plane was perpendicular to the flowline. We suggest that the scattering below about 250 m is related to a layered ice stratum of crystal-orientation fabrics with different types or different cluster strengths. Although our study was at a single site, similar remote measurements over wider regions should provide valuable information about the physical structure of this vast ice sheet.

A ground-based, multi-frequency ice-penetrating radar system

Abstract To better understand how ice sheets respond to climate, we designed a new multi-frequency ice-penetrating radar system to investigate subsurface structures of ice sheets. The system is mounted on a single platform and handled by a single operator. Three radio frequencies, 30,60 and 179 MHz, were used. An underlying principle of these multi-frequency observations is that the lower frequencies are more sensitive to electrical conductivity changes, whereas the higher frequencies are more sensitive to dielectric permittivity fluctuations in the ice. The system is composed of three single-frequency pulse radars, a trigger-controller unit and a data-acquisition unit. The trigger controller is the key component of this system. It switches transmitters on at different timings to prevent mixing of signals among the three radars. The timing difference was set as 50 μs, which is equivalent to the two-way travel time for radio waves reflecting from 4250m below the surface. A field test was done along a 2000 km long traverse line in east Dronning Maud Land, Antarctica. The multi-frequency system successfully acquired data that are equivalent in quality to our earlier single-frequency measurements along the same traverse line. The details of the system and preliminary data are described.

CRL/NASDA airborne SAR (Pi-SAR) observations of sea ice in the Sea of Okhotsk

Abstract Multi-frequency, multi-polarization airborne synthetic aperture radar (SAR) observations of sea ice in the southern Sea of Okhotsk were carried out in February 1999 in conjunction with RADARSAT SAR observations. The final goal of this study is to clarify the backscattering characteristics and to understand the scattering mechanisms of sea ice in the Sea of Okhotsk by using microwave multiparametric SAR. The airborne SAR (Pi-SAR) has two frequencies (X- and L-band) and multi-polarization (HH, VV, HV, VH) with 1.5 m (X-band) and 3.0 m (L-band) resolution. It was developed by the Communications Research Laboratory (X-band) and the National Space Development Agency of Japan (L-band). We show the frequency dependence and polarization properties of radar backscattering from sea ice. We find that it is possible to distinguish ice types by comparing backscattering from sea ice in the X- and L-bands. Investigation of the polarization characteristics at X-band was very useful for detecting the thin-ice area (e.g. nilas and gray ice).

Deriving sea-ice thickness and ice types in the Sea of Okhotsk using dual-frequency airborne SAR (Pi-SAR) data

Abstract Dual-frequency, multi-polarization airborne synthetic aperture radar (Pi-SAR; developed by the Communications Research Laboratory and National Space Development Agency of Japan) observations of the seasonal sea-ice region off the Okhotsk coast of Hokkaido, Japan, were carried out in February 1999 using X- and L-band radar frequencies with a resolution of 1.5 and 3.0 m. In conjunction with the SAR observations, the sea-ice thickness (draft) and velocity were measured by a moored Ice Profiling Sonar (IPS) and an Acoustic Doppler Current Profiler (ADCP). Tracks of the sea ice passing over the IPS were estimated from the time series of the ADCP ice-velocity and -direction data. Along these tracks, the SAR backscattering coefficient profiles were compared with the IPS ice-draft profiles. The results showed that the L-band SAR backs cattering profiles correlated well with the IPS ice-draft data, particularly in the thicker part (a few meters thick) of the rim of first-year ice, which had a large backscattering coefficient. Although the X-band SAR backscattering profiles did not correlate well with the IPS data, thin ice (<10 cm thick) showed a large backscattering coefficient. The L-band SAR and IPS data did not distinguish thin ice from open water.