Wind direction over the ocean determined by an airborne, imaging,polarimetric radiometer system

The speed and direction of winds over the ocean can be determined by polarimetric radiometers. This has been established by theoretical work and demonstrated experimentally using airborne radiometers carrying out circle flights and thus measuring the full 360° azimuthal response from the sea surface. An airborne experiment, with the aim of measuring wind direction over the ocean using an imaging polarimetric radiometer, is described. A polarimetric radiometer system of the correlation type, measuring all four Stokes brightness parameters, is used. Imaging is achieved using a 1-m aperture conically scanning antenna. The polarimetric azimuthal signature of the ocean is known from modeling and circle flight experiments. Combining the signature with the measured brightness data from just a single flight track enables the wind direction to be determined on a pixel-by-pixel basis in the radiometer imagery     

Analysis of correlation and total power radiometer front-ends using noise waves

A complete and systematic noise analysis of radiometer front-ends, including both total power and correlation measurements, is presented. The procedure uses the concepts of noise waves and S-parameters, widely used in microwave systems design and takes into account full noise characterization of receivers including mismatch effects. The general formulation is compatible with known total power radiometer analysis and is specially appropriate in correlation radiometers for which the effect of nonideal components, such as input isolators, is analyzed. Along with numerical simulations, simple formulas are given to compute the measured visibility in nonideal conditions. The analysis is validated using experimental results consisting of correlation measurements of four receivers placed inside an anechoic chamber. Good agreement between theoretical predictions and experimental data is observed.     

Correlation Denormalization in Interferometric or Polarimetric Radiometers: A Unified Approach

This paper presents a general analysis of correlation measurements in an interferometer or a radiometer based on noise injection and/or switching and measurement of normalized correlations (e.g., imaging synthetic aperture or polarimetric radiometers). A compact unifying notation for denormalizing the measured normalized correlations in the presence of noise injection in one or both of the receiving channels is presented. Technological limitations are also assessed by evaluating the effect of associated approximations. Finally, the approach is validated by experimental results of the measurement and calibration of related front-end nonidealities, namely, the finite isolation of the front-end switch. The methods presented in this paper are illustrated by a thorough analysis of the so-called mixed baselines of microwave imaging radiometer using aperture synthesis, which refer to those baselines which are formed between the regular receivers (light-weight cost-efficient front-end) and the reference radiometers. These baselines require special attention, since the reference radiometers are noise-injection radiometers, which inject noise to the measured signal, whereas the regular receivers are total power receivers.      

数字增益补偿及实时定标微波辐射计

微波辐射计的长期稳定性,是影响最小可检测信号的主要因素。研究出数字增益补偿与实时定标两种新型微波辐射计。能完全消除系统增益波动的影响。有极好的长期稳定性。实时定标型还能消除系统噪声的波动,完成实时定标。在０．６ｓ时间常数下最小可检测信号分别达到０．１３和０．１９Ｋ,达到国内外先进指标。在实际中已得到应用。     

TWO NEW TYPES OF MICROWAVE RADIOMETER: A COMPUTER GAIN COMPENSATIVE DESIGN AND A REAL-TIME CALIBRATED DESIGN

Long term stability is the main factor that influences the minimum detectable signal of microwave radiometers. Two new types of microwave radiometer were studied: a computer gain compensative microwave radiometer and a real-time calibrated microwave radiometer. The long term stability of both designs was optimal because they were insensitive to system gain fluctuations. The continuous calibrated microwave radiometer was also insensitive to system noise fluctuations. The minimum detectable signals were 0.13 and 0.19K respectively under an integration time of 0.6s.     

Superheterodyne Radiometers for Use at 70 Gc and 140 Gc

In this paper four different millimeter wave equipments, which have been made for plasma diagnostic work, are described. They are: 1) A straightforward 70-Gc superheterodyne radiometer with an over-all noise factor of 13 db; 2) An early 140-Gc radiometer, with second harmonic mixing, which has an over-all noise factor of about 25 db; 3) A later and more sensitive 140-Gc radiometer which contains a fundamental local oscillator, VX 3352 mixer crystals and a 408-Mc IF amplifier commencing with an Adler tube; 4) A very simple 140-Gc transmission measuring equipment containing a 1-watt source and a crystal video receiver which has a tangential sensitivity of ---42 dbm. The last part of this paper discusses the minimum temperature changes which can be detected, at short millimeter wavelengths, with various types of superheterodyne radiometers, the Golay cell, the barretter, the crystal video radiometer, the 1.5/spl deg/K carbon bolometer and the 1.5/spl deg/K InSb photoconductive detector. The performances expected from straight traveling-wave tube radiometers and traveling-wave masers at short millimeter wavelengths are also considered. The Appendices are devoted to mixer crystal performance in the millimeter and submillimeter regions, a theory of second harmonic mixing and the voltage sensitivity of a forward biased detector crystal.     

Signal interception: performance advantages of cyclic-featuredetectors

The problem of detecting the presence of spread-spectrum phase-shift-keyed signals in variable noise and interference backgrounds is considered, and the performances of four detectors are evaluated and compared. The detectors include the optimum radiometer, the optimum modified radiometer that jointly estimates the noise level and detects the signal, and the maximum-SNR spectral-line regenerator for spectral-line frequencies equal to the chip rate and the doubled carrier frequency. It is concluded that the spectral-line regenerators can outperform both types of radiometers by a wide margin. The performance advantages are quantified in terms of receiver operating characteristics for several noise and interference environments and receiver collection times     

International comparison of the SIAR measurement and the WRR standard

We experimentally compare the solar irradiance absolute radiometer (SIAR) measurement with the world radiometric reference (WRR) standard to improve the accuracy of instrument. The SIAR joined in the international pyrheliometer comparison (IPC) in 2000. The comparison results show that the calibration factors for SIAR to WRR are 0.999 220, 1.001 694, 0.998 334 and 0.997 439 in the 9th IPC, the 10th IPC, the 11th IPC and the 12th IPC, respectively. These results are added to the measurement uncertainty budget of SIAR. The repeatability of the SIAR-type absolute radiometers is also investigated. The relative error introduced by two SIAR-type absolute radiometers is within 0.25%.     

Two-load radiometer precision and accuracy

The advent of fast computers and digital signal processing permits many aspects of radiometer operation to be decided in real time so that precision and accuracy are maximized. Closed-form expressions for precision, optimal antenna/load dwell times, and maximum chopping period are derived for a class of radiometers that uses two loads to calibrate its measurements of antenna brightness temperature. The importance of load brightness temperature selection emphasizes the need to develop "cold" calibration loads that can operate at ambient temperature. A method of incorporating critical thermistor data into the radiometer brightness temperature calibration can be used to help stabilize measurements against drift and may permit radiometers to operate without thermal control in some low-accuracy applications.     

Calibration and applications of polarization-correlatingradiometers

Hardware and associated software for nonmechanical rotation of the polarization basis of a dual linearly polarized cross-correlating radiometer are demonstrated. The technique requires precise measurement of two orthogonal-mode antenna temperatures along with cross-correlation of the two orthogonal-mode field amplitudes. Collectively, these are the first three Stokes parameters. A polarized blackbody load was developed for accurate calibration of the orthogonal-mode and cross-correlating channels. Using the hardware, rotation of the radiometer antenna's polarization basis by a matrix transformation was demonstrated by 91.65 GHz near-Brewster-angle scans of a polarizing water surface. The experimental results demonstrate the viability of the rotation technique and suggest practical calibration schemes for airborne and spaceborne polarimetric radiometers. Application of the rotation technique in mechanically scanned polarization-sensitive imaging radiometers will eliminate polarization coupling inherent in conventional fixed-feed dual-polarization scanner designs     

A Very Sensitive Airborne Microwave Radiometer Using Two Reference Temperatures

The accuracies of conventional microwave radiometers are influenced by instabilities mainly in their RF components A radiometer is described that uses two reference temperatnres. The signal processing eliminates the influences of all RF parameter changes on the radiometer indication. Thus the overall stability depends only on the reference temperatures and the low-frequency and dc components. These can be made extremely stable by careful design. The paper presents a detailed analysis of the radiometer performance. The minimum detectable signal is greater than that of a comparable Dicke radiometer, but approaches this, if the reference temperatures are appropriately chosen according to the particular measurement problem. The great advantage of the radiometer is that no measurement time is lost for frequent recalibration. Practical results of an X-band radiometer for airborne use are given at the end of the discussion. They are in good agreement with the theory and demonstrate the feasibility and superiority of the concept.     

A Simplified Calculation of the Kurtosis for RFI Detection

Microwave radiometers detecting geophysical parameters are susceptible to radio-frequency interference (RFI) from anthropogenic sources. The kurtosis statistic can be a powerful means of identifying some types of low-power RFI, as thermal noise has a distinct kurtosis value of three, while thermal noise contaminated even with low-power nonthermal RFI often has other values of kurtosis. This paper presents a method for calculating the kurtosis and brightness that significantly reduces the radiometer data download requirements.      

L波段微波辐射计周期脉冲式干扰时域检测方法研究

L波段微波辐射计是探测土壤湿度和海水盐度的有效遥感器。但是,全球定位系统(GPS)信号、雷达信号以及一些商用电子产品的电磁辐射造成的频谱污染都可以对微波辐射计的探测造成干扰,使得被动微波遥感对地观测结果具有一定的偏差,降低了地表参数的反演精度。该文通过实验模拟脉冲式噪声干扰,观测其在L波段(全功率接收型式)微波辐射计系统中的传输特性,分析输出信号特性与辐射计参数(积分时间、灵敏度)的相关性,获取其数字特征参数,结合脉冲检测法(APB),提出一种新的自相关检测(ACD)算法,能够有效用于周期性的脉冲式辐射干扰的检测,在微波辐射计系统积分时间1 ms的情况下,能够检测1.5 K的噪声干扰,满足卫星遥感探测反演地表参数精度的需求。     

The measurement of atmospheric water vapor: radiometer comparisonand spatial variations

Two water vapor radiometer (WVR) experiments were conducted to evaluate whether such instruments are both suitable and necessary to correct for propagation effects that are induced by precipitable water vapor (PWV) on signals from the Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI). WVRs are suitable for these corrections if they provide wet path delays to better than 0.5 cm. They are needed if spatial variations of PWV result in complicated, direction-dependent propagation effects that are too complex to be parameterized in the GPS or VLBI geodetic solution. In the first experiment, the suitability of radiometers were addressed by comparing six WVRs at Stapleton International Airport in Denver, Colorado, for two weeks. The second experiment addressed the question whether radiometers are needed for the detection of inhomogeneities in the wet delay. Three JPL D-series radiometers were operated at three sites in Colorado approximately 50 km apart. The WVRs simultaneously sampled PWV at different azimuths and elevations in search of spatial variations of PWV     

Digital measurement of narrowband noise power

A digital measurement method for narrowband noise power is described and analyzed in the case of flat spectrum Gaussian noise. Noise power is measured by counting the number of times the noise voltage crosses upwards a fixed voltage level. The method should be useful in noise-figure measurements, and by applying it a digital radiometer can be constructed. This digital radiometer has the same theoretical (about 9 percent worse than the best possible radiometer) as conventional radiometers now in use but offers several advantages in comparison with them. A switched digital radiometer is somewhat more sensitive to large gain variations than a normal switched radiometer.     

Ground-Based Microwave Radiometric Observations of the Temporal Vanation of Atmospheric Geopotential Height and Thickness

Since 1981, the Wave Propagation Laboratory of NOAA has operated a ground-based zenith-viewing microwave radiometer. This radiometer, designed to measure precipitable water vapor, cloud liquid, and temperature profiles, has two moisture-sensing channels and four temperature-sounding channels. Data from this system, taken at Denver, Colorado, are used to derive geopotential heights and thicknesses from the surface (about 830 mbar) to 300 mbar. Time series and spectra of several directly measured and inferred quantities are analyzed for different meteorological situations: a period of unusual calm in surface pressure, a frontal passage, and a gravity wave event. The three cases presented illustrate how rapid variations in meteorological variables can be studied using ground-based radiometers. These radiometers provide temporal continuity not hitherto available. The performance of the radiometer, both in observing a blackbody target and during an unusually calm pressure event, shows high sensitivity to changes in geopotential height and thickness and to integrated water vapor. Consequently, the combination of high temporal resolution and high sensitivity allows unique monitoring of rapidly changing conditions, such as frontal passages and gravity wave events. Comparisons of these data with various sources of ground truth, including radiosondes, satellite cloud observations, and arrays of microbarographs, show excellent agreement.     

Forward model studies of water vapor using scanning microwave radiometers, global positioning system, and radiosondes during the cloudiness intercomparison experiment

Brightness temperatures computed from five absorption models and radiosonde observations were analyzed by comparing them with measurements from three microwave radiometers at 23.8 and 31.4 GHz. Data were obtained during the Cloudiness Inter-Comparison Experiment at the U.S. Department of Energy's Atmospheric Radiation Measurement Program's (ARM) site in North-Central Oklahoma in 2003. The radiometers were calibrated using two procedures, the so-called instantaneous "tipcal" method and an automatic self-calibration algorithm. Measurements from the radiometers were in agreement, with less than a 0.4-K rms difference during clear skies, when the instantaneous method was applied. Brightness temperatures from the radiometer and the radiosonde showed a bias difference of less than 0.69 K when the most recent absorption models were considered. Precipitable water vapor (PWV) computed from the radiometers were also compared to the PWV derived from a Global Positioning System station that operates at the ARM site. The instruments agree to within 0.1 cm in PWV retrieval.     

基于微波遥感的台风对海洋动力影响分析

台风是一种路径和强度变化不确定性很大的、短暂的、能量很强的灾害性天气系统。卫星遥感特别是星载微波遥感不仅具有大范围、快速重复观测的特点,而且具备全天候和全天时的优点,受恶劣天气影响较小,可以对台风活动路径进行较好的监测和预警,是分析台风的一种有效手段。本文结合3种微波卫星载荷：高级微波扫描辐射计(AMSR)、TOPEX&Jason-1微波高度计和WindSat全极化微波辐射计的数据,初步分析了台风对海洋动力环境的影响,结合与台风相关的海洋参数的卫星遥感测量结果,给出了目前常用的微波散射计和微波辐射计海洋风场遥感的比较。对于多云情况下的海表面温度测量,微波辐射计比红外辐射计更有优势。全极化微波辐射计还具有在恶劣天气条件下海洋风场测量能力。     

A Double Detector for RFI Mitigation in Microwave Radiometers

A double detector (DD) for radio-frequency interference (RFI) in microwave radiometers is demonstrated in theory and practice. The detector is based on the principle of using kurtosis to detect the presence of non-Gaussian signals and is shown to approximate the kurtosis of input. Theoretical response to continuous wave and pulsed RFI is derived and tested in two experiments. The DD hardware comprises two microwave detectors, two integrator-amplifiers, and a wideband video amplifier. The technique is compatible with existing direct-detection radiometer designs and desirable for applications requiring low technological risk.     

Sea Surface Emission Characteristics as Viewed by the Spin-Scanned Satellite Microwave Radiometer (SAMIR) on Board Bhaskara

Emission characteristics from ocean surfaces around the Indian subcontinent have been studied using data from the satellite microwave radiometer (SAMIR) on board Bhaskara, the first Indian satellite for Earth observations. SAMIR is a three-channel microwave radiometer, with two channels operating at around 19 GHz and one at 22.235 GHz. Each radiometer has a temperature sensitivity of better than 1 K, and the ground resolutions are 150 and 230 km, respectively. The radiometers are primarily intended for studies related to meteorology and oceanography, and for general land-related investigations. One of the special features of the payload is that it can be operated in two different modes of spin of the satellite. In the first (normal mode), the antennas scan along the satellite track, and in the other (alternate mode), the antennas scan across the track. The data of the alternate mode have been compared with theoretical emission models and also with the normal-mode analysis. The results correlate well with the established theoretical studies for both the horizontally and vertically polarized components of microwave emission from the sea surface. The effects of mixed polarization also correlate well with the data.