Radiometric and spectral performance and calibration of the GHz bands of EOS MLS

This paper describes radiometric performance and prelaunch radiometric and spectral calibrations of the GHz component of the Microwave Limb Sounder (MLS) experiment on NASA's Aura spacecraft. Estimated systematic scaling uncertainties (3/spl sigma/) on limb port radiances are /spl sim/0.5% from radiometric calibration and /spl sim/0.5% to /spl sim/1% from spectral calibrations. Operational noise performance is consistent with prelaunch expectations, and in-orbit measurements to date indicate no changes in noise characteristics, and no observable calibration drifts. Spectral baseline has remained stable to /spl sim/20 mK since launch. Refinements to calibrations based on in-flight data are discussed, and radiometric calibration algorithms are described.     

Analysis of surface roughness and morphology of first-year sea ice melt ponds: implications for microwave scattering

Variations in wind forcing over summer first-year sea ice (FYI) melt ponds occur at hourly to weekly scales and are a significant contributor to microwave backscatter (/spl sigma//spl deg/) variability observed from spaceborne synthetic aperture radar (SAR) platforms (e.g., ENVISAT-ASAR and RADARSAT-1). This variability impairs our ability to use SAR to derive information on summer sea ice thermodynamic state and energy balance parameters such as albedo and melt pond fraction. The surface roughness contribution of FYI melt ponds in the Canadian Arctic Archipelago to like-polarized, C-band /spl sigma//spl deg/ estimates is analyzed through a spectral and statistical analysis of surface wave height profiles for varying wind speeds, upwind fetch lengths, and melt pond depths. A unique derivation of melt pond surface wave height spectra is presented based on digital video of melt pond surface wave trains. Significant scale surface roughness was observed even at wind speeds of 3 m/spl middot/s/sup -1/, resulting in small perturbation model estimates of /spl sigma//spl deg/ (HH) ranging from -5 dB at 20/spl deg/ incidence to -22 dB at 50/spl deg/ incidence. Results from a multivariate linear regression analysis show that 53.5% of observed variance in /spl sigma//spl deg/ (HH or VV) can be explained by wind speed, upwind fetch from melt pond edges, and melt pond depth, with no appreciable difference in the relative contribution of explanatory variables. Modeled omnidirectional /spl sigma//spl deg/ as a function of wind speed and incidence angle for 100-m transects collected throughout the melt pond season act to elaborate the role of fetch and depth, as well as the modulating effect of hummocks, on /spl sigma//spl deg/.     

A CMOS smart temperature sensor with a 3/spl sigma/ inaccuracy of /spl plusmn/0.5/spl deg/C from -50/spl deg/C to 120/spl deg/C

A low-cost temperature sensor with on-chip sigma-delta ADC and digital bus interface was realized in a 0.5 /spl mu/m CMOS process. Substrate PNP transistors are used for temperature sensing and for generating the ADC's reference voltage. To obtain a high initial accuracy in the readout circuitry, chopper amplifiers and dynamic element matching are used. High linearity is obtained by using second-order curvature correction. With these measures, the sensor's temperature error is dominated by spread on the base-emitter voltage of the PNP transistors. This is trimmed after packaging by comparing the sensor's output with the die temperature measured using an extra on-chip calibration transistor. Compared to traditional calibration techniques, this procedure is much faster and therefore reduces production costs. The sensor is accurate to within /spl plusmn/0.5/spl deg/C (3/spl sigma/) from -50/spl deg/C to 120/spl deg/C.     

Incidence angle dependence of the statistical properties of C-band HH-polarization backscattering signatures of the Baltic Sea ice

Incidence angle dependence of three statistical parameters-the mean of the backscattering coefficient (/spl sigma//spl deg/), standard deviation, and autocorrelation coefficient of texture (/spl sigma//sub T/ and /spl rho//sub T/)-of the C-band horizontal-horizontal (HH) polarization backscattering signatures of the Baltic Sea ice are investigated using RADARSAT ScanSAR Narrow images and helicopter-borne Helsinki University of Technology Scatterometer (HUTSCAT) data. The analysis of the large amount of data shows that the relationship between the mean /spl sigma//spl deg/ in decibel scale and the incidence angle in the range from 19/spl deg/ to 46/spl deg/ is usually well described by a linear model. In general, the RADARSAT and HUTSCAT results agree with each other, and they are also supported by theoretical backscattering model calculations; the more deformed the ice, the smaller the slope between /spl sigma//spl deg/ and the incidence angle, and the higher the moisture content of snow or ice, the larger the slope. The derived /spl sigma//spl deg/ incidence angle dependencies can be used to roughly compensate the /spl sigma//spl deg/ incidence angle variation in the SAR images to help their visual and automated classification. The variability of /spl sigma//sub T/ and /spl rho//sub T/ with the increasing incidence angle is insignificant compared to the variability within each ice type. Their average changes with the incidence angle are so small that, in practice, their trends do not need to be compensated. The results of this study can be utilized when developing classification algorithms for the RADARSAT ScanSAR and ENVISAT HH-polarization Wide Swath images of the Baltic Sea ice.     

Microwave backscatter modeling of erg surfaces in the Sahara desert

The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter (/spl sigma//spl deg/) modulation with azimuth angle (f). We use /spl sigma//spl deg/ measurements observed at various incidence angles and f from the NASA Scatterometer (NSCAT), the SeaWinds scatterometer, the ERS scatterometer (ESCAT), and the Tropical Rainfall Measuring Mission's Precipitation Radar to model the /spl sigma//spl deg/ response from sand dunes. Observations reveal a characteristic relationship between the backscatter modulation and the dune type, i.e., the number and orientation of the dune slopes. Sand dunes are modeled as a composite of tilted rough facets, which are characterized by a probability distribution of tilt with a mean value, and small ripples on the facet surface. The small ripples are modeled as cosinusoidal surface waves that contribute to the return signal at Bragg angles only. Longitudinal and transverse dunes are modeled with rough facets having Gaussian tilt distributions. The model results in a /spl sigma//spl deg/ response similar to NSCAT and ESCAT observations over areas of known dune types in the Sahara. The response is high at look angles equal to the mean tilts of the rough facets and is lower elsewhere. This analysis provides a unique insight into scattering by large-scale sand bedforms.     

Bias point thermal shift growth in Z-cut LiNbO/sub 3/ Modulators

Growth of a bias point thermal shift induced by dc bias in z-cut LiNbO/sub 3/ optical intensity modulators is quantitatively discussed from the standpoint of its impact on field service. During 20 years of device operation under a worst-case dc bias condition, the thermal shift slope grows almost symmetrically with respect to a bias polarity. An initial small thermal shift of around /spl plusmn/7 mV//spl deg/C (3/spl sigma/ distribution bounds) increases continuously over time with biased operation at 55/spl deg/C and approaches /spl plusmn/70 mV//spl deg/C after 20 years. This increased temperature sensitivity would generate approximately a 2-V bias point shift toward the bias rail when modulator is exposed to temperature variation from 55/spl deg/C to 25/spl deg/C.     

Analysis of the equal-delay topology for transformers and hybrid networks

A complete scattering matrix representation for the ideal equal-delay topology for transformers and hybrid networks is presented. It is shown that while the operation of the hybrid as a 180/spl deg/ power combiner, current balun, or voltage balun is essentially frequency independent, the operation as a 0/spl deg/ power combiner or splitter is not. Instead, the isolation between the 0/spl deg/ and 180/spl deg/ ports is finite and frequency dependent. Moreover, the reflection coefficient at the sum port is nonzero and frequency dependent. These characteristics lead to the conclusion that while the equal-delay 180/spl deg/ power splitter/combiner is fundamentally frequency independent, its 0/spl deg/ counterpart is limited to operation well below the fundamental quarter-wave frequency of the constituent transmission lines. Full three-port scattering parameter representations, which are compatible with the calibration and analysis approach given in the CISPR 16-1 specification, are given for the three fundamental transformer and balun types derivable from the equal-delay hybrid: 1 the Guanella voltage balun, 2 the Guanella current balun, and 3 the 180/spl deg/ power divider or terminated hybrid balun, as specified in the CISPR 16-1 specification.     

In-flight spectral calibration of the Atmospheric Infrared Sounder

Preflight testing of AIRS determined that the shapes of the detector spectral response functions (SRFs) do not vary under different instrument conditions. This reduces in-flight spectral calibration to the determination of detector spectral centroids. A spectrometer grating model has been developed to calculate detector centroids. Only two parameters of this model need to be determined in orbit. An algorithm is presented for determining these two parameters in orbit by correlating observed upwelling radiance spectra with modeled spectra. The method of selecting spectral regions against which to correlate is detailed. The in-orbit spectral calibration algorithm was tested on one day of simulated global AIRS radiances, showing that the uncertainty in the frequencies of the SRF centroids is between 0.006 /spl Delta//spl nu/ and 0.01 /spl Delta//spl nu/, compared to the spectral calibration requirement of 0.01 /spl Delta//spl nu/, where /spl Delta//spl nu/ is the SRF full width at half maximum. The simulation also indicates that the stability of the spectral calibration can be monitored at the 0.001-/spl Delta//spl nu/ level on a daily basis.     

Semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces

A semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces is presented. Based on existing scattering models and data sets measured by polarimetric scatterometers and the JPL AirSAR, the parameters of the co-polarized phase-difference probability density function, namely the degree of correlation /spl alpha/ and the co-polarized phase-difference /spl sigmav/, in addition to the backscattering coefficients /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/ and /spl sigma//sub /spl nu/h//sup 0/, are modeled empirically in terms of the volumetric soil moisture content m/sub /spl nu// and the surface roughness parameters ks and kl, where k=2/spl pi/f/c, s is the rms height and l is the correlation length. Consequently, the ensemble-averaged differential Mueller matrix (or the differential Stokes scattering operator) is specified completely by /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/,/spl sigma//sub /spl nu/h//sup 0/,/spl alpha/, and /spl zeta/.     

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.     

On the detection of Faraday rotation in linearly polarized L-band SAR backscatter signatures

The potentially measurable effects of Faraday rotation on linearly polarized backscatter measurements from space are addressed. Single-polarized, dual-polarized, and quad-polarized backscatter measurements subject to Faraday rotation are first modeled. Then, the impacts are assessed using L-band polarimetric synthetic aperture radar (SAR) data. Due to Faraday rotation, the received signal will include other polarization characteristics of the surface, which may be detectable under certain conditions. Model results are used to suggest data characteristics that will reveal the presence of Faraday rotation in a given single-polarized, dual-polarized, or quad-polarized L-band SAR dataset, provided the user can identify scatterers within the scene whose general behavior is known or can compare the data to another, similar dataset with zero Faraday rotation. The data characteristics found to be most sensitive to a small amount of Faraday rotation (i.e., a one-way rotation <20/spl deg/) are the cross-pol backscatter [/spl sigma//spl deg/(HV)] and the like-to-cross-pol correlation [e.g., /spl rho/(HHHV/sup */)]. For a diverse, but representative, set of natural terrain, the level of distortion across a range of backscatter measures is shown to be acceptable (i.e., minimal) for one-way Faraday rotations of less than 5/spl deg/, and 3/spl deg/ if the radiometric uncertainty in the HV backscatter is specified to be less than 0.5 dB.     

Lognormal selection with applications to lifetime data

We consider selection problems on k lognormal (/spl mu//sub i/,/spl sigma//sub i//sup 2/) populations when the /spl sigma//sub i//sup 2/ are unequal, under both known & unknown cases. Selection based on linear functions of /spl mu/ & /spl sigma//sup 2/ are considered. For the unequal & unknown /spl sigma//sub i//sup 2/ case, a two-stage Rinott-type asymptotic procedure is proposed. In consideration to reliability & survival analysis applications, an asymptotic procedure is also proposed for the parametric selection based on the /spl alpha/-quantiles. Simulation studies to evaluate the procedures & to compare the latter procedure to the nonparametric procedure based on the /spl alpha/-quantiles are presented. The procedures are demonstrated using examples from reliability & quality control.     

On the utility of SeaWinds/QuikSCAT data for the estimation of the thermodynamic state of first-year sea ice

The thermodynamic state of sea ice is important to accurately and remotely monitor in order to provide improved geophysical variable parameterizations in sea ice thermodynamic models. Operationally, monitoring the thermodynamic state of sea ice can facilitate eased ship navigation routing. SeaWinds/QuikSCAT (QuikSCAT) dual-polarization [i.e., horizontal (HH) and vertical (VV)] active microwave data are available at a sufficiently large spatial scale and high temporal resolution to provide estimates of sea ice thermodynamics. This analysis evaluated the temporal evolution of the backscatter coefficient (/spl sigma//spl deg/) and VV/HH copolarization ratio from QuikSCAT for estimating sea ice thermodynamics. QuikSCAT estimates were compared against RADARSAT-1 synthetic aperture radar (SAR) imagery and the Canadian Ice Service (CIS) prototype operational ice strength algorithm. In situ data from the Collaborative Interdisciplinary Cryospheric Experiment (C-ICE) for 2000, 2001, and 2002 were used as validation. Results indicate that the temporal evolution of /spl sigma//spl deg/ from QuikSCAT is analogous to RADARSAT-1. The QuikSCAT /spl sigma//spl deg/ temporal evolution has the ability to identify winter, snow melt, and ponding thermodynamic states. Moreover, the copolarization VV/HH ratio of QuikSCAT could provide a second estimate of the ponding state in addition to identifying the drainage state that is difficult to detect by single-polarization SAR. QuikSCAT detected thermodynamic states that were found to be in reasonable agreement to that of in situ observations at the C-ICE camp for all years. Operational implications of this analysis suggest QuikSCAT is a more effective and efficient medium for monitoring ice decay compared to RADARSAT-1 and can be utilized to provide more robust modeled ice strength thresholds.     

An ultrafast wide-band millimeter-wave (MMW) polarimetric radar for remote sensing applications

With the advent of high-frequency radio frequency (RF) circuits and components technology, millimeter-wave (MMW) radars are being proposed for a large number of military and civilian applications. Accurate and high-resolution characterization of the polarimetric radar backscatter responses of both clutter and man-made targets at MMW frequencies is essential for the development of radar systems and optimal detection and tracking algorithms. Toward this end, a new design is developed for ultrafast, wide-band, polarimetric, instrumentation radars that operate at 35 and 95 GHz. With this new design, the complete scattering matrix of a target (magnitude and phase) can be measured over a bandwidth of 500 MHz in less than 2 /spl mu/s. In this paper, the design concepts and procedures for the construction and calibration of these radars are described. In addition, the signal processing algorithm and data-acquisition procedure used with the new radars are presented. To demonstrate the accuracy and applicability of the new radars, backscatter measurements of certain points and distributed targets are compared with their analytical radar cross section (RCS) and previously measured /spl sigma//spl deg/ values, respectively, and good agreements are shown. These systems, which can be mounted on a precision gimbal assembly that facilitates their application as high-resolution imaging radar systems, are used to determine the MMW two-way propagation loss of a corn field for different plant moisture conditions.     

K-band HBT and HEMT monolithic active phase shifters using vector sum method

Two monolithic 3-bit active phase shifters using the vector sum method to K-band frequencies are reported in this paper. They are separately implemented using commercial 6-in GaAs HBT and high electron-mobility transistor (HEMT) monolithic-microwave integrated-circuit (MMIC) foundry processes. The MMIC HBT active phase shifter demonstrates an average gain of 8.87 dB and a maximum phase error of 11/spl deg/ at 18 GHz, while the HEMT phase shifter has 3.85-dB average measured gain with 11/spl deg/ maximum phase error at 20 GHz. The 20-GHz operation frequency of this HEMT MMIC is the highest among all the reported active phase shifters. The analysis for gain deviation and phase error of the active phase shifter using the vector sum method due to the individual variable gain amplifiers is also presented. The theoretical analysis can predict the measured minimum root-mean-square phase error 4.7/spl deg/ within 1/spl deg/ accuracy.     

A micropower logarithmic A/D with offset and temperature compensation

Logarithmic circuits are useful in many applications that require nonlinear signal compression, such as in speech recognition front-ends (SRFEs) and cochlear implants or bionic ears (BEs). A logarithmic current-input analog-to-digital converter (A/D) with temperature compensation and automatic offset calibration is presented in this paper. It employs a diode to compute the logarithm, a wide linear range transconductor to perform voltage-to-current conversion, and a dual-slope auto- zeroing topology with 60 dB of dynamic range for sampling the envelope of speech signals. The temperature dependence of the logarithm inherent in a diode implementation is automatically cancelled in our circuit topology. Experimental results from a 1.5-/spl mu/m 3-V BiCMOS process show that the converter achieves a temperature stability lower than 150 ppm//spl deg/C from 12/spl deg/C to 42/spl deg/C, and consumes only 3 /spl mu/W of power when sampling at 300 Hz. At this level of power consumption, we show that the design is thermal-noise limited to 8 bits of precision. This level of precision is more than adequate for deaf patients and for speech recognition front-ends. The power consumption is almost two orders of magnitude lower than state-of-the-art DSP implementations, and the use of a local feedback topology achieves a 2.5-bit improvement over conventional dual-slope designs.     

Deep-space calibration of the WindSat radiometer

The WindSat microwave polarimetric radiometer consists of 22 channels of polarized brightness temperatures operating at five frequencies: 6.8, 10.7, 18.7, 23.8, and 37.0 GHz. The 10.7-, 18.7-, and 37.0-GHz channels are fully polarimetric (vertical/horizontal, /spl plusmn/45/spl deg/ and left-hand and right-hand circularly polarized) to measure the four Stokes radiometric parameters. The principal objective of this Naval Research Laboratory experiment, which flys on the USAF Coriolis satellite, is to provide the proof of concept of the first passive measurement of ocean surface wind vector from space. This paper presents details of the on-orbit absolute radiometric calibration procedure, which was performed during of a series of satellite pitch maneuvers. During these special tests, the satellite pitch was slowly ramped to +45/spl deg/ (and -45/spl deg/), which caused the WindSat conical spinning antenna to view deep space during the forward (or aft portion) of the azimuth scan. When viewing the homogeneous and isotropic brightness of space (2.73 K) through both the main reflector and the cold-load calibration reflector, it is possible to determine the absolute calibration of the individual channels and the relative calibration bias between polarimetric channels. Results demonstrate consistent and stable channel calibrations (with very small brightness biases) that exceed the mission radiometric calibration requirements.     

Low-noise silicon carbide X-ray sensor with wide operating temperature range

A silicon carbide (SiC) sensor is presented with high energy resolution in X-ray spectroscopy over a wide temperature range (27-100/spl deg/C). The sensor, consisting of a Schottky barrier diode on high resistivity epitaxial SiC, is characterised by an extremely low noise due to its ultra-low reverse current density even at high operating temperature (15 pA/cm/sup 2/ at 27/spl deg/C and 0.5 nA/cm/sup 2/ at 100/spl deg/C). Equivalent noise charges as low as 17 electrons rms at 27/spl deg/C and 47 electrons rms at 100/spl deg/C have been measured, allowing X-ray spectroscopy with an energy resolution as low as 315 eV and 797 eV FWHM, respectively.     

Multi-angle Imaging SpectroRadiometer (MISR) on-board calibrator (OBC) in-flight performance studies

The Multi-angle Imaging SpectroRadiometer (MISR) consists of nine cameras pointing from nadir to an extreme of 70.5/spl deg/ in the view angle. It is a pushbroom imager with four spectral bands per camera. Instrument specifications call for each camera to be calibrated to an absolute uncertainty of 3% and to within 1% relative to the other cameras. To accomplish this, the MISR instrument utilizes an on-board calibrator (OBC) to provide updates to the instrument gain coefficients on a bimonthly basis (i.e. once every two months). Spectralon diffuse panels are used in the OBC to provide a uniform target for the nine MISR cameras to view. The radiometric scale of the OBC is established through the use of photodiodes. The stability of the MISR OBC system and its in-flight calibration are discussed.     

A chopped Hall sensor with small jitter and programmable "True Power-on" function

A chopped Hall sensor for camshaft applications is presented which provides a programmable "True Power-on" switching level effective after the power-up phase. The proposed chopping technique and self-compensation methods for temperature drift and technology spread provide a correct output state immediately after power-on even at zero speed of the target wheel. The circuits improve the magnetic offsets from 5 to 10 mT to below 200 /spl mu/T in a bandwidth of 30 kHz and stabilize the spread of the magnetic switching points from 20% to <2% in a temperature range from -40/spl deg/C to 175/spl deg/C. The novel combination of chopping and enhanced digital self-calibration algorithm adjusts the magnetic switching point and improves phase accuracy to <0.5/spl deg/, independent of air gap variations between sensor and wheel. An end-of-line calibration for the customer is implemented using surface micromachined cavity fuses which offer a reliable function higher than 195/spl deg/C.