Spatial resolution enhancement of SSM/I data

One of the limitations in using Special Sensor Microwave/Imager (SSM/I) data for land and vegetation studies is the relatively low-spatial resolution. To ameliorate this limitation, resolution-enhancement algorithms can be applied to the data. In this paper, the Backus-Gilbert inversion (BGI) technique and the scatterometer image-reconstruction (SIR) algorithm are investigated as possible methods for creating enhanced resolution images from SSM/I data. The two algorithms are compared via both the simulation and the actual SSM/I data. The algorithms offer similar resolution enhancement, though SIR requires significantly less computation. Sample results over two land regions of South America are presented     

Interpretation of SSM/I measurements over Greenland

Multispectral brightness temperature (T_B) measurements over Greenland are obtained from the Special Sensor Microwave Imager (SSM/I), which are flown aboard the DMSP satellites. This paper examines the different spectral characteristics over Greenland throughout the year. Although snow covers the vast majority of Greenland, the southern regions rarely exhibit the spectral characteristics associated with snowcover (i.e., T_B decreases at higher frequencies). In fact, the SSM/I polarization and frequency measurements over southern Greenland are more indicative of water than a snow-covered surface (i.e., T_B increases at higher frequencies). A simplified physical model is developed to help explain the anomalous measurements over southern Greenland. Model results indicate that high frequency radiation is mainly scattered by snow grains residing above the subsurface ice layers, whereas low frequency radiation is scattered throughout a much greater depth. Since low frequencies are scattered throughout a greater volume, they are depressed relative to high frequencies, and the typical snowcover signature is absent     

Noise reduction in oversampled filter banks using predictivequantization

We introduce two methods for quantization noise reduction in oversampled filter banks. These methods are based on predictive quantization (noise shaping or linear prediction). It is demonstrated that oversampled noise shaping or linear predictive subband coders are well suited for subband coding applications where, for technological or other reasons, low-resolution quantizers have to be used. In this case, oversampling combined with noise shaping or linear prediction improves the effective resolution of the subband coder at the expense of increased rate. Simulation results are provided to assess the achievable quantization noise reduction and resolution enhancement, and to investigate the rate-distortion properties of the proposed methods     

Error-rate characteristics of oversampled analog-to-digitalconversion

Accuracy of simple analog-to-digital conversion depends on both resolution of discretization in amplitude and resolution of discretization in time. For implementation convenience, high conversion accuracy is attained by refining the discretization in time using oversampling. It is commonly believed that oversampling adversely impacts rate-distortion properties of the conversion, since the bit rate, B, increases linearly with oversampling, resulting in a slow error decay in the bit rate, on the order of O(1/B). We demonstrate that the information obtained in the process of oversampled analog-to-digital conversion can easily be encoded in a manner which requires only a logarithmic increase of the bit rate with redundancy, achieving an exponential error decay in the bit rate     

Oversampled cosine-modulated filter banks with arbitrary systemdelay

Design methods for perfect reconstruction (PR) oversampled cosine-modulated filter banks with integer oversampling factors and arbitrary delay are presented. The system delay, which is an important parameter in real-time applications, can be chosen independently of the prototype lengths. Oversampling gives us additional freedom in the filter design process, which can be exploited to find FIR PR prototypes for oversampled filter banks with much higher stopband attenuations than for critically subsampled filter banks. It is shown that for a given analysis prototype, the PR synthesis prototype is not unique. The complete set of solutions is discussed in terms of the nullspace of a matrix operator. For example, oversampling allows the design of PR filter banks having unidentical prototypes (of equal and unequal lengths) for the analysis and synthesis stage. Examples demonstrate the increased design freedom due to oversampling. Finally, it is shown that PR prototypes being designed for the oversampled case can also serve as almost-PR prototypes for critically subsampled cosine-modulated pseudo QMF banks     

Comparison of two microwave radiobrightness models and validationwith field measurements

This paper compares microwave brightness temperature (T_B) estimated by two radiobrightness models: a multilayer coherent radiative transfer (CRT) model and a single-layer Fresnel reflectance model. Two dielectric mixing schemes were used along with the models to calculate permittivity (real part of the dielectric constant). Model T_B and permittivity estimates were compared and validated against Huntsville, AL 1998 field experiment measurements. Model differences can be attributed to the mixing scheme, the radiobrightness model, or the vertical profile representation. Two sets of simulations were performed to quantify the sources of variation, one using observed son temperature and moisture profiles as input, and another using uniform profiles. Using uniform profiles, systematic differences in permittivity estimated by the mixing schemes resulted in T_B differences as large as 15 K. However, for uniform profiles, differences in T_B estimated by the radiobrightness models for a given permittivity value were less than 2 K. For cases using observed profiles, near-surface drying of the profiles resulted in T_B values from the CRT model 6-10 K higher than estimates from the Fresnel model, which determines T_B based on 0-5 cm mean moisture and temperature. Therefore, the major sources of T_B variations were the dielectric mixing scheme and the shape of the near-surface moisture profile. No radiobrightness/mixing scheme combination exhibited superiority across all plots and times     

Data-recursive algorithms for blind channel identification inoversampled communication systems

Data-recursive algorithms are presented for performing blind channel identification in oversampled communication systems. Novel on-line solutions with complexities that are only linear in the oversampling rate are considered, and mean convergence conditions are provided. Numerical results are presented for a binary phase-shift keyed (BPSK) system     

16-channel oversampled analog-to-digital converter

Oversampled analog-to-digital conversion has been demonstrated to be an effective technique for high resolution analog-to-digital (A/D) conversion that is tolerant to process imperfections. The area and power budget of conventionally designed oversampled analog-to-digital converters has precluded their application from areas where a large number of low frequency signals need to be converted simultaneously. A new oversampled A/D design methodology is proposed to cut the area and power budget per channel of an oversampled analog-to-digital converter. The design and implementation of a 16-channel oversampled analog-to-digital converter is presented which can be used as the core of the multichannel data acquisition system. The prototype achieved 80 dB of signal-to-noise-plus-distortion over 1 kHz, -80 dB of crosstalk and used less than 20 mW of power excluding clock generation     

A technique for enhancing and matching the resolution of microwavemeasurements from the SSM/I instrument

The authors describe and apply a correction technique for matching the resolution of all the frequencies of the Special Sensor Microwave/Imager (SSM/I) to the 25-km spatial resolution of the 37-GHz channel. To accomplish this, it is necessary to increase the spatial resolution of the 19- and 22-GHz channels while degrading that of the 85-GHz channel. It is found that the approach produces adequate enhancement of the spatial resolution to make such a correction worthwhile. Results suggest that this technique decreases brightness temperature differences stemming solely from spatial resolution differences by over 50% for the low resolution channels with only a modest increase in random noise. The correction can also help to better resolve small features which would otherwise be lost due to the lack of resolution     

Pipelined multibit oversampled digital-to-analog converters with capacitor averaging

A combination of pipelined architecture and dynamic element matching technique is applied to multibit oversampled D/A (digital to analog) converters. The approach translates the harmonic distortion components of the nonideal internal DAC (digital-to-analog converter) of the oversampled DAC to high-frequency components, which can then be filtered out by the analog low-pass filter for anti-imaging. Computer simulations have confirmed that with this approach a third-order oversampled DAC employing a 3-bit quantizer, a 3-bit pipelined internal DAC with a random mismatch of 0.1%, can achieve a 94-dB dynamic range with an oversampling ratio of 64 while eliminating the harmonic distortion.This work was supported by NSERC (Canada).     

Partial-response decoding of rate 1/2 modulation codes for digitalstorage

In digital storage systems, receivers for rate 1/2 modulation codes are usually oversampled by a factor of two with respect to the data stream that they attempt to reconstruct. It is shown that oversampling may be avoided by using partial-response techniques to detect, instead of the encoded binary signal, a decimated ternary one, from which the original data can be recovered by means of a simple decoder. A method is described to find all such decoders for a given rate 1/2 code. Examples treated are FM, MFM, Miller-squared, (2,7), 3PM, and quad-phase. The mean-square performances of these reception schemes are analyzed and compared to those of their predecessors for a Lorentzian channel. The relative merits are found to depend quite heavily on the information density, favoring some of the studied schemes at high densities     

An Ocean Surface Wind Vector Model Function for a Spaceborne Microwave Radiometer

Surface wind vector measurements over the oceans are vital for scientists and forecasters to understand the Earth's global weather and climate. In the last two decades, operational measurements of global ocean wind speeds were obtained from passive microwave radiometers (Special Sensor Microwave/ Imagers); and over this period, full ocean surface wind vector data were obtained from several National Aeronautics and Space Administration and European Space Agency scatterometry missions. However, since SeaSat-A in 1978, there have not been other combined active and passive wind measurements on the same satellite until the launch of Japan Aerospace Exploration Agency's Advanced Earth Observing Satellite-II in 2002. This mission provided a unique data set of coincident measurements between the SeaWinds scatterometer and the Advanced Microwave Scanning Radiometer (AMSR). The AMSR instrument measured linearly polarized brightness temperatures (TB) over the ocean. Although these measurements contained wind direction information, the overlying atmospheric influence obscured this signal and made wind direction retrievals not feasible. However, for radiometer channels between 10 and 37 GHz, a certain linear combination of vertical and horizontal brightness temperatures causes the atmospheric dependence to cancel and surface parameters such as wind speed and direction and sea surface temperature to dominate the resulting signal. In this paper, an empirical relationship between AMSR T_B's (specifically A ^. T_BV - T_BH) and surface wind vectors (inferred from SeaWinds' retrievals) is established for three microwave frequencies: 10, 18, and 37 GHz. This newly developed wind vector model function for microwave radiometers can serve as a basis for wind vector retrievals either separately or in combination with active scatterometer measurements.     

The use of optically coherent detection techniques for true-timedelay phased array and systems

We analyze a true-time-delay, optically controlled phased array antenna system whereby beam forming is accomplished using a large number of antenna elements that can receive any of several different microwave true-time-delays via the use of coherent optical carriers transmitted through a single fiber. The transmitter and receiver architectures are described in detail. We present calculations for the signal-to-noise ratio (SNR), the signal-to-interchannel interference ratio (SIR) and dynamic range (DR) of one channel for various modulation-demodulation schemes (i.e., AM-heterodyne, FM, PM). We show for an example system that can accommodate 128 antenna elements and provide 128 different delays (7 bits), assuming AM modulation with a bandwidth of 1 MHz, a transmitter power of -5 dBm and a laser linewidth of 10 MHz, we can obtain DR≃50 dB for one channel using a channel separation twelve times of that of the microwave frequency (f_m). For the FM scheme, DR≃60 dB can be obtained for one channel with a laser linewidth of 1 MHz, a FM modulation index of one, and a channel spacing of about 13f_m. With a local oscillator laser tuning range of 100 Å, the system can provide seven-bit resolution for a 128 element antenna array. It is found that coherent optically controlled phased array antennas provide improved controllability over direct detection method, and can have the SNR, SIR and DR that meet the stringent requirements of modern high resolution microwave antenna systems     

Broadband,High Gain,Narrow Width Rectangular Dielectric Resonator Antenna with Air Gap

The broadband, narrow width, rectangular dielectric resonator antenna (RDRA) of aluminum nitride (ε_r=8.6) was designed and the effect of inclusion of air gap at the bottom of the dielectric resonator antenna (DRA), above the ground plane, was investigated. Gain around 7 dBi was obtained for DRA with air gap (DRAAG) over a broad bandwidth in upper X, K_u, and K bands. Further enhancement in gain could be obtained by placing a metal wall parallel to the length of DRA. However, due to the presence of metal wall, bandwidth was reduced. These structures with the metal wall are capable of operating over a wide band extending from K_u band to lower K band with the gain of around 10 dBi. CST Microwave Studio Software was used to simulate all these structures. Performance parameters of DRA with air gap were compared with several broadband DRA structures reported in recent literature. The proposed DRAAG with the metal wall in this paper is capable of operating over a wide bandwidth along with a significant gain.     

Optimum interpolation of imaging microwave radiometer data

A technique of interpolating imaging microwave radiometer data is presented as an application of the Backus-Gilbert theory. The interpolation process is optimal in the sense that it attempts to preserve the spatial resolution of the antenna gain function associated with the sampled radiometer data. The technique is applied to the Special Sensor Microwave/Imager (SSM/I) satellite data and is found to enhance the high-resolution features of the imagery     

A Direct Design of Oversampled Perfect Reconstruction FIR Filter Banks

We address a problem to find optimal synthesis filters of oversampled uniform finite-impulse-response (FIR) filter banks (FBs) yielding perfect reconstruction (PR), when we are given an analysis FB, in the case where all the filters have the same length that is twice a factor of downsampling. We show that in this class of FBs, a synthesis FB that achieves PR can be found in closed form with elementary matrix operations, unlike conventional design methods with numerical optimization. This framework allows filter coefficients to be complex as well as real. Due to the extra degrees of freedom in a synthesis FB provided by oversampling, we can determine optimal coefficients of synthesis filters that meet certain criteria. We introduce in this paper two criteria: variance of additive noise and stopband attenuation. We show theoretical results of optimal synthesis filters that minimize these criteria and design examples of oversampled linear-phase FIR FBs and DFT-modulated FBs. Moreover, we discuss applications to signal reconstruction from incomplete channel data in transmission and inverse transform of windowed discrete Fourier transform with 50% overlapping.     

Comparisons of precipitation measurements by the Advanced MicrowavePrecipitation Radiometer and multiparameter radar

The NASA airborne Advanced Microwave Precipitation Radiometer (AMPR) and the National Center for Atmospheric Research (NCAR) CP-2 multiparameter radar were jointly operated during the 1991 Convection and Precipitation/Electrification experiment (CaPE) in central Florida. The AMPR is a four channel, high resolution, across-track scanning total power radiometer system using the identical multifrequency feedhorn as the widely utilized Special Sensor Microwave/Imager (SSM/I) satellite system. Surface and precipitation feature are separable based on the T_B behavior as a function of the AMPR channels. The radar observations are presented in a remapped format suitable for comparison with the multifrequency AMPR imagery. Striking resemblances are noted between the AMPR imagery and the radar reflectivity at successive heights, while vertical profiles of the CP-2 products along the nadir trace suggest a storm structure consistent with the viewed AMPR T_B     

低频/甚低频电磁脉冲测量系统研究

为了准确地测量电磁脉冲（electromagnetic pulse,EMP）的波形,开展低频/甚低频（low-frequency/very low-frequency,LF/VLF）EMP的形成和传播机理研究,识别不同EMP的特征,并以此为依据来识别、确认雷电及核爆炸等现象,提出了一种高分辨率的EMP原始波形测量系统方案,同时针对LF/VLF这一较低的频段提出了一套有效的系统标校方法,实现了对EMP的连续、准确测量和快速有效判别,并能通过网络实时上传数据.系统采用正交环磁场天线和平板电容电场天线实现信号的接收,设计了低噪声高保真的信道调理电路和高速数据采集电路来实现信号的采集,利用高精度的授时模块为EMP信号标记时间戳,最后结合多点监测波形实现EMP定位估计.实测结果表明:该系统能够给出高分辨率、高精度的LF/VLF EMP波形;利用该系统组网可以实现远距离EMP源的定位,定位精度与目前近距离的定位手段相当.通过长期的运行,验证了该系统具有高可靠、低失真、判别准确、实时性强等特点.     

Throughput Evaluation of Oversampled Receiver: An Example of WiMAX System

In this paper, we propose an oversampled WiMAX receiver as an attempt to improve throughput of WiMAX system. In communication, oversampling is a technique that a signal is sampled with a higher sampling frequency than its original bandwidth at the receiver side. Thus more signal sequences, which is received through different paths, may be obtained compare to system with conventional sampling. These additional sequences generate diversity. This study theoretically analyzes an oversampled wireless communication system in case of capacity with outage over Rayleigh distributed flat fading channel and presents throughput of the oversampled WiMAX receiver that obtained by simulation over a realistic channel model and by experimental tests. Since the performance of an oversampled system very much depends on the channel conditions, realistic channel model and experimental study are essential to exhibit realistic performance benchmarks. Theoretical analyzes, simulations and experimental results clearly show that the oversampled receiver significantly outperforms the spectrum efficiency of the communication system.     

Excellent effects of hydrogen postoxidation annealing on inversionchannel mobility of 4H-SiC MOSFET fabricated on (112¯0) face

Effects of hydrogen postoxidation annealing (H₂ POA) on 4H-silicon carbide (SiC) MOSFETs with wet gate oxide on the (112¯0) face have been investigated. As a result, an inversion channel mobility of 110 cm²/Vs was successfully achieved using H₂ POA at 800°C for 30 min. H₂ POA reduces the interface trap density by about one order of magnitude compared with that without H₂ POA, resulting in considerable improvement of the inversion channel mobility to 3.5 times higher than that without H₂ POA. In addition, 4H-SiC MOSFET with H₂ POA has a lower threshold voltage of 3.1 V and a wide gate voltage operation range in which the inversion channel mobility is more than 100 cm²/Vs