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Glaciated regions are known to be particularly sensitive to climate change. Historical archives of glacier volume change are important, as they provide context for present-day changes. Although photogrammetric archives exist for many regions, their usefulness is often limited by a lack of contemporary ground control. High quality digital elevation models (DEMs) underpin a range of change analysis activities. This paper presents a cost-effective solution which utilizes Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) DEMs as control for the scaling and orientation of archival data sets. Instead of relying upon ground-control points, a robust surface matching algorithm is employed to automatically determine the transformation required to register two overlapping DEMs. Through application to the Slakbreen glacier system in Svalbard, Norway, the strategy is assessed by first matching an ASTER DEM to a fixed lidar reference surface. This demonstrates that ASTER DEMs are effectively correct in scale, supporting their use as a control surface. The second stage of the research implements this by matching an aerial photogrammetric DEM to an ASTER reference surface. Resultant volumetric and annual elevation change rates are compared to those derived from lidar data, which are considered in this paper as a truth data set. ASTER-based matching produced a mean annual elevation change rate of -4.12 [ma.sup.-1], compared to a value of -4.11 [ma.sup.-1] derived from the lidar data. In volumetric terms, this equates to a difference of 0.6%. A major advantage of this approach is the near-global coverage offered by ASTER data and the opportunity that this presents for remote glacial change analysis over regional extents. Index Terms--Geodesy, remote sensing, terrain mapping.

Methods described in the IEEE standards for frequency generator stability can be used to estimate the precision of brightness temperature measurements made by a microwave radiometer. The application of these methods and the inherent assumptions are reviewed. A simple statistic is proposed for use in estimating accuracy drift. Index Terms--Calibration, microwave radiometry, microwave receivers, radiometry, remote sensing.

We focus on inverting the surface temperature (or heat) equation to obtain the surface velocity field in the coastal ocean and compare the results with those from the maximum cross correlation (MCC) technique and with the in situ velocity fields measured by the Rutgers University Coastal Ocean Dynamics Radar (CODAR). When compared with CODAR fields, velocities from the heat equation and MCC have comparable accuracies, but the heat equation technique better resolves the finer scale flow features. We use the results to directly calculate the surface divergence and vorticity. This is possible because we convert the traditionally underdetermined heat inversion problem to an overdetermined one without constraining the velocity field with divergence, vorticity, or energy statements. Because no a priori assumptions are made about the vorticity, it can be calculated directly from the velocity results. The derived vorticity field has typical open-ocean magnitudes (~5 x [10.sup.-5]/s) and exhibits several structures (a warm core ring, Gulf Stream filament, and a diverging flow) consistent with the types of flows required to kinematically deform the sea surface temperature patterns into the observed configurations. Index Terms--Geophysical measurements, HF radar, infrared (IR) imaging, remote sensing, satellite applications, sea coast, sea surface.

We present a detailed analysis of the airborne passive microwave remote-sensing data that were collected at a broad range of microwave bands and at a high spatial resolution during the 2002 and 2003 National Aeronautics and Space Administration Cold Land Processes Experiment (CLPX). An accurate measurement of snowpack properties using passive microwave observations requires the detailed knowledge of the relationship between snowpack geophysical parameters and the upwelling polarimetric brightness signature. The principle microwave instrument used for the CLPX was the polarimetric scanning radiometer (PSR), which provided ~100-m resolution maps of the snow emissivity at all Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) bands during several intensive observation periods over the Colorado Rocky Mountains. The observed conditions included drought, normal snowpack, and spring snowmelt. The PSR and related ground-based observations of snowpack properties made during the 2002 and 2003 CLPX campaigns provide a comprehensive high-resolution passive microwave data set. Results show that the high-resolution PSR data exhibit emissivity modes that are similar to those observed in the historical data sets, and that the empirical relationships between the emissivity and the snow water equivalent (SWE), after the effects of macrovegetation are removed, closely match those found in the past theoretical studies. The use of the 89-GHz channel in the empirical relationships provides improved accuracy under dry snow conditions and a small SWE; however, the variability of the SWE--emissivity relationships increases with an increasing SWE. A summary of the observed relationships between the emissivity spectra of snow and snowpack properties is presented. Comparison of the total water content from the AMSR-E and PSR observations shows that the satellite measurements underestimated the total volume of water storage from airborne observations on the average by a factor of five. Index Terms--Airborne polarimetric scanning radiometer (PSR), beam polarization correction, georegistration, microwave emissivity, passive microwave remote sensing, retrieval algorithm development, snowpack, snow water equivalent (SWE), topography, validation.

A new type of remote-sensing radiometer based upon the Fabry--Perot (FP) interferometric technique has been developed at NASA's Goddard Space Flight Center and tested from both ground and aircraft platforms. The sensor uses direct or reflected sunlight and has channels for measuring the column concentration of carbon dioxide at 1570 nm, oxygen lines sensitive to pressure and temperature at 762 and 768 nm, and water vapor (940 nm). A solid FP etalon is used as a tunable narrow bandpass filter to restrict the measurement to the gas of interest's absorption bands. By adjusting the temperature of the etalon, which changes the index of refraction of its material, the transmission fringes can be brought into nearly exact correspondence with the absorption lines of the particular species. With this alignment between absorption lines and fringes, changes in the amount of a species in the atmosphere strongly affect the amount of light transmitted by the etalon and can be related to gas concentration. The technique is applicable to different chemical species. We have performed simulations and instrument design studies for C[H.sub.4], [sup.13]C[O.sub.2] isotope, and CO detection. Index Terms--Absorbing media, atmospheric measurements, Fabry--Perot (FP) interferometers, optical interferometry, remote sensing.

We present a new compact and automated ground-based microwave mobile radiometer dedicated to the study of middle atmospheric water vapor, which is convenient for measurement, intercomparison, and validation campaigns, particularly in remote places. The instrument detects the [6.sub.16]-[5.sub.23] [H.sub.2]O transition line at 22.235 GHz by means of balanced beam-switching observation of the atmosphere. Tipping curves are performed to estimate the tropospheric opacity. The sky is used as a cold load for calibration, minimizing the use of liquid nitrogen. The main technical issues are caused by the small horn antenna (a beamwidth of ~12[degrees]) inducing large bias when using pencil-beam approximation for the calculation of elevation-dependent parameters. Numerical methods have been developed to include this effect in the data calibration process. Preliminary vertical profiles are retrieved with the Microwave Odin Line Estimation and REtrieval radiative transfer and inversion tool from 25 to 55 km with a vertical resolution of 10-20 km increasing with height, for an integration time of ~12 h and a measurement error of ~15% (~0.8 ppmv). Preliminary comparisons with the spaceborne instrument Aura/Microwave Limb Sounder show a good agreement (~5%) in the 35-55-km altitude range, whereas a negative bias is detected below (up to -30% at 25 km). Index Terms--Atmospheric measurement, calibration, radiometry.

The spaceborne thermal monitoring of the great eruption of Pu'u 'O'o-Kupaianaha in Hawaii (1987 to present) was attempted, integrating high-resolution SWIR-TIR and low-resolution MIR-TIR observations. Data acquired by multispectral payloads ASTER and MODIS onboard spacecraft Terra, and MODIS onboard Aqua, were processed behind real time by the use of computing routines 'MyVOL' and 'MyMOD' based on assessed theory and designed for real-time operations. The process was tested against the 2000-2005 eruptive history, both in geometry and volume dynamics of the lava flow. Here, instant lava effusion rates obtained from approximately 1800 cloud-free (over a total > 3000) day-/nighttime MODIS scenes and 12 cloud-free (over a total of 30) nighttime ASTER scenes are compared with each other and with volcanological ground truth composed of gas (S[O.sub.2])- and electromagnetic potential (very low frequency)-derived eruption rates. Remote-sensed eruption rates are consistent with ground truth from approximately 1 [m.sup.3]/s up to the peak effusion rates of 1.2-1.3 million [m.sup.3]/day observed in 2005. Index Terms--Aqua, ASTER, eruption rate, Hawaii, lava, MODIS, Terra, volcano.

WindSat has systematically collected the first global fully polarimetric passive microwave data over both land and ocean. As the first spaceborne polarimetric microwave radiometer, it was designed to measure ocean surface wind speed and direction by including the third and fourth Stokes parameters, which are mostly related to the asymmetric structures of the ocean surface roughness. Although designed for wind vector retrieval, WindSat data are also collected over land and ice, and this new data has revealed, for the first time, significant land signals in the third and fourth Stokes parameter channels, particularly over Greenland and the Antarctic ice sheets. The third and fourth Stokes parameters show well-defined large azimuth modulations that appear to be correlated with geophysical variations, particularly snow structure, melting, and metamorphism, and have distinct seasonal variation. The polarimetric signatures are relatively weak in the summer and are strongest around spring. This corresponds well with the formation and erosion of the sastrugi in the dry snow zone and snowmelt in the soaked zone. In this paper, we present the full polarimetric signatures obtained from WindSat over Greenland, and use a simple empirical observation model to quantify the azimuthal variations of the signatures in space and time. Index Terms--Greenland ice sheet, ice, polarimetric microwave radiometry, snow, WindSat.

An analytical model and supporting measured data are presented for a preamplified W-band radiometer with a zero-bias detector appropriate for commercial millimeter-wave imaging cameras. Basic radiometer parameters, including RF bandwidth, are computed directly from simple low-frequency measurements and compare well with those obtained from RF measurements. A detailed analytical model shows how radiometer performance depends on internal component parameters, such as low-noise amplifier gain, noise factor, reflection coefficient, detector responsivity, etc. The measurements suggest that performance is sufficient for operation without a Dicke switch or mechanical chopping. A measured noise equivalent temperature difference of 0.45 K was obtained, assuming a single sensor is scanned across a focal plane, forming 32 pixels with 3.125-ms integration time per pixel. This sensitivity is considered sufficient by commercial manufacturers to obtain quality images in low-contrast (e.g., indoor) environments. Index Terms--Direct detection, millimeter-wave imaging, radiometers, W-band sensors.

This paper deals with the potential application of National Oceanic and Atmospheric Administration (NOAA)/ Advanced Very High Resolution Radiometer (AVHRR) data to detect subsurface fire (subsurface hotspots) by proposing an efficient contextual algorithm. Most of the solutions proposed to date are mainly focused on the problem of surface fires, and very few research works have been performed to develop techniques for the subsurface fire problem. Although few algorithms based on the fixed-thresholding approach have been proposed for subsurface hotspot detection, however, for each application, thresholds have to be specifically tuned to cope with unique environmental conditions. The main objective of this paper is to develop an instrument-independent adaptive method by which direct threshold or multithreshold can be avoided. The proposed contextual algorithm is very helpful to monitor subsurface hotspots with operational satellite data, such as the Jharia region of India, without making any region-specific guess in thresholding. Novelty of the proposed work lies in the fact that once the algorithmic model is developed for the particular region of interest after optimizing the model parameters, there is no need to optimize those parameters again for further satellite images. Hence, the developed model can be used for optimized automated detection and monitoring of subsurface hotspots for future images of the particular region of interest. The algorithm is adaptive in nature and uses vegetation index and different NOAA/AVHRR channel's statistics to detect hotspots in the region of interest. The performance of the algorithm is assessed in terms of sensitivity and specificity and compared with other well-known thresholding techniques such as Otsu's thresholding, entropy-based thresholding, and existing contextual algorithm proposed by Flasse and Ceccato. The proposed algorithm is found to give better hotspot detection accuracy with lesser false alarm rate. Index Terms--Contextual algorithm, entropy-based thresholding, image analysis, National Oceanic and Atmospheric Administration (NOAA)/Advanced Very High Resolution Radiometer (AVHRR), Otsu's thresholding, remote sensing, satellite imaging, subsurface hotspot, threshold decoding.

A new consistent long-term normalized difference vegetation index (NDVI) time series at a l-[km.sup.2] resolution for Southern Africa that is based on the data from Satellite Pour l'Observation de la Terre VEGETATION (VGT) (1998-2006) and the National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) (1985-1998) has been produced for vegetation-dynamics monitoring purposes. This paper presents the evaluation of the newly processed AVHRR data set, as well as the integration of this data set with the VGT archive. First, the AVHRR processing chain and the resulting AVHRR data set have been investigated with respect to calibration accuracy, cloud masking, and atmospheric and geometric correction. Second, different calibration approaches, spectral response (SR) functions, spatial resolutions, overpass times, and geometries of observation for the VGT and AVHRR data sets have been compared for a common observation period. The application of published correction functions accounting for the SR differences for both sensors considerably improved the consistency between both data sets. An [r.sup.2] of 0.85 is obtained between paired samples of the NDVI from the VGT and the newly processed AVHRR archive. After the application of the correction functions, the slope of the regression line between the two NDVI data sets was much closer to the 1:1 line. The performance of the correction functions differed among vegetation types. The largest reduction in the root-mean-square error between the NDVI of both sensors is obtained from areas with higher biomass. Large parts of the remaining variability are suggested to be attributed to the bidirectional reflectance distribution function effects, as demonstrated by the intersensor NDVI time-series variability versus the intrasensor NDVI time-series variability. Index Terms--lmage processing, spectral response function (SRF), time series.

The weekly 0.144[degrees] resolution global vegetation index from the National Oceanic and Atmospheric Administration (NOAA) National Environmental Satellite, Data, and Information Service (NESDIS) has a long history, starting late 1981, and has included data derived from Advanced Very High Resolution Radiometer (AVHRR) sensors onboard NOAA-7, -9, -11, -14, -16, -17, and -18 satellites. Even after postlaunch calibration and mathematical smoothing and filtering of the normalized difference vegetation index (NDVI) derived from AVHRR visible and near-infrared channels, the time series of global smoothed NDVI (SMN) still has apparent discontinuities and biases due to sensor degradation, orbital drift [equator crossing time (ECT)], and differences from instrument to instrument in band response functions. To meet the needs of the operational weather and climate modeling and monitoring community for a stable long-term global NDVI data set, we investigated adjustments to substantially reduce the bias of the weekly global SMN series by simple and efficient algorithms that require a minimum number of assumptions about the statistical properties of the interannual global vegetation changes. Of the algorithms tested, we found the adjusted cumulative distribution function (ACDF) method to be a well-balanced approach that effectively eliminated most of the long-term global-scale interannual trend of AVHRR NDV1. Improvements to the global and regional NDVI data stability have been demonstrated by the results of ACDF-adjusted data set evaluated at a global scale, on major land classes, with relevance to satellite ECT, at major continental regions, and at regional drought detection applications. Index Terms--Advanced Very High Resolution Radiometer (AVHRR), land surface, normalized difference vegetation index (NDVI), remote sensing, satellite-based vegetation, vegetation index.

This paper evaluates the performance of the global precipitation rate retrieval algorithm for the Advanced Microwave Sounding Unit (AMSU) that was described in Part I of this paper. AMSU is in polar orbit on several National Ocean and Atmospheric Administration (NOAA) operational weather satellites. Predicted rms retrieval errors based on a 15-km resolution 0.5-1.0-mm/h MM5 truth were 0.88, 0.83, 1.13, and 3.04 for stratiform, warm rain, ice-free rain, and convective rain, respectively, which were averaged over all view angles ['or land and sea up to 73[degrees] latitude. For MM5 rates of 4-8 mm/h, these rms errors increased to 2.8, 3.4, 3.9, and 4.9 nun/h, respectively. The corresponding rms retrieval accuracies for MM5 hydrometeor water paths between 0.125 and 0.25 mm for rainwater, snow, and graupel were 0.19, 0.10, and 0.22 mm, respectively. The rms retrieval accuracy for the 0.125-0.25-m/s peak vertical wind was 0.08 m/s. Biases are small for cumulative precipitation estimates, although an upward correction factor of 1.37 is derived for convective precipitation rate probability distributions. Differences between these retrievals and those from the conically scanned Advanced Microwave Scanning Radiometer for the Earth Observing System instrument and an alternate NOAA AMSU algorithm are also characterized. Index Terms--Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), Advanced Microwave Sounding Unit (AMSU), microwave precipitation estimation, nficrowave radiative transfer, precipitation.

To better understand how terrestrial vegetative ecosystems respond to climate and/or anthropogenic effects, the scientific community is increasingly interested in developing methods of employing satellite data to track changes in land surface phenology (e.g., timing and rate of green-up, amplitude and duration of growing season, and timing and rate of senescence of plant classes). By increasing the inherent resolution of signal extraction procedures while minimizing the effects of cloud cover and prolonged data gaps, such tools can significantly improve land cover classification and land cover change monitoring on multiple scales. This report describes an intuitive approach for tracking the intra-annual details and interannual variability of multiyear time series, employing a sequence of annual high-order polynomial splines (up to the 14th order), stabilized by minimizing model roughness and weighted to fit the upper data envelope to minimize cloud cover bias. The algorithm is tested using multiyear time series for three very different classes of vegetation-stable agriculture, high elevation montane shrubland, and semiarid grassland with high interanuual variability. The results accurately track both short--and long-term land surface phenology and illustrate a robust potential for extracting temporal and spatial detail from a variety of satellite-based multiyear vegetation signals. Index Terms--Advanced Very High Resolution Radiometer (AVHRR), Fourier analysis, harmonic series, multiyear, normalized difference vegetation index (NDVI ), phenology, polynomial lit, signal processing, spline, time series.

The University of Michigan Microwave Geophysics Group participated in the Cold Lands Processes Experiment from February to April 2003 by deploying its Truck Mounted Radiometer System-3 (TMRS-3) to perform temporal monitoring of the snow pack at the local scale observation site (LSOS). The LSOS was located at the Fraser Experimental Forest headquarters in the mountains near Fraser, CO. The small clearing in which the TMRS-3 was deployed was adjacent to tall evergreen trees. To quantify the amount of the downwelling brightness from these trees onto the snow pack, the TMRS-3 periodically observed these trees. Microwave brightness data were collected from the trees every 15 [degrees] from horizontal incidence to 45 [degress] from zenith. Both polarizations were observed for 6.7, 19, and 37 GHz. A rapid decrease in brightness is evident as the radiometers were pointed progressively upward. The next May, an upward-looking hemispherical ('fish-eye') photograph was taken from the center of the clearing, and it reveals a significant sky background through the incomplete canopy. By superimposing Gaussian approximations to the microwave antenna gain pattern of the individual TMRS-3 radiometers onto the photograph, we estimated the amount that the main beams were filled with canopy and with sky. Comparison of the measured data to that expected for a partially filled main beam indicates that the needle-leaf canopy is roughly an isotropic emitter having emissivities at frequencies between 6.7 and 37 GHz of between 0.93 and 0.97 with air temperature as a proxy for tree temperature. Index Terms--Emission, microwave radiometry, snow, vegetation.

An improved Backus--Gilbert resampling scheme is developed and applied on Advanced Microwave Scanning Radiometer-EOS (AMSR-E) brightness temperature swath data. The new resampling scheme has two improvements over the Special Sensor Microwave Imager and AMSR-E resampling schemes currently used to produce standard brightness products. First, the use of a circular Ganssian footprint as the reference footprint achieves near-circular synthesized footprints for all channels. The near-circular synthesized footprints diminish the effect of different orientations of the synthesized elliptical footprints produced by the standard algorithm. Second, a better synthesized footprint spatial resolution for the 6.925- and 10.65-GHz channels in the across scan direction is achieved with a significant reduction in noise level. Oversampling by AMSR-E at these frequencies enables this improvement. Index Terms--Image processing, microwave radiometry.

Sea surface salinity can be remotely measured by means of L-band microwave radiometry. However, the brightness temperature also depends on the sea surface temperature and on the sea state, which is probably today one of the driving factors in the salinity retrieval error budgets of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission and the NASA-Comision Nacional de Actividades Espaciales Aquarius/SAC-D mission. This paper describes the Passive Advanced Unit (PAU) for ocean monitoring. PAU combines in a single instrument three different sensors: an L-band radiometer with digital beamforming (DBF) (PAU-RAD) to measure the brightness temperature of the sea at different incidence angles simultaneously, a global positioning system (GPS) reflectometer [PAU-reflectometer of Global Navigation Satellite Signals (GNSS-R)] also with DBF to measure the sea state from the delay-Doppler maps, and two infrared radiometers to provide sea surface temperature estimates. The key characteristic of this instrument is that both PAU-RAD and the PAU-GNSS/R share completely the RF/IF front-end, and analog-to-digital converters. Since in order to track the GPS-reflected signal, it is not possible to chop the antenna signal as in a Dicke radiometer, a new radiometer topology has been devised which makes uses of two receiving chains and a correlator, which has the additional advantage that both PAU-RAD and PAU-GNSS/R can be operated continuously and simultaneously to perform the sea-state corrections of the brightness temperature. This paper presents the main characteristics of the different PAU subsystems, and analyzes in detail the PAU-radiometer concept. Index Terms--Correlator, Global Navigation Satellite Signals (GNSS), radiometry, reflectometry, sea salinity, sensitivity.

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 ([T.sub.B]) 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 TB'S (specifically A * [T.sub.BV] - [T.sub.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. Index Terms--Active and passive microwave, microwave radiometry, ocean surface wind vector, scatterometry.

In this paper, a research version of the Meteorological Service of Canada (MSC) Global Environmental Multiscale (GEM) mesoglobal weather forecasts model is evaluated by comparing the simulated Special Sensor Microwave/Imager brightness temperatures (Tbs) with the observed ones. Several comparisons based on two 15-day periods, one in winter and one in summer, have been done. Results are compared to those obtained for a study conducted on the European Centre for Medium-Range Weather Forecast (ECMWF) model. The overall performance of the GEM model is similar to that of the 2001 version of the ECMWF model. The model appears to simulate with realism the large-scale rainy systems but with frequent mislocations. Moreover, the model has a tendency to produce intense small-scale precipitating areas that are not observed. The occurrence frequency of cloud and rain is overestimated by the model. Finally, these results are encouraging enough to continue investigating the assimilation of Tbs in cloudy and rainy skies at MSC. Index Terms--Forecast-model evaluation, model-to-satellite method, passive-microwave radiometer, Special Sensor Microwave/Imager (SSM/I).

For future remote sensing applications the potential of the millimeter wavelength range for precipitation observations from geostationary orbits is investigated. Therefore, a database consisting of hydrometeor profiles from various mid-latitude precipitation cases over Europe and corresponding simulated brightness temperatures at 18 microwave frequencies was built using the cloud resolving model Meso-NH and the radiative transfer model MicroWave MODel. The information content of the database was investigated by applying simple statistical methods, as well as developing first-order retrieval approaches. The results show that, particularly for snow and graupel, the total column content can be retrieved accurately with relative errors smaller than 25% in dominantly stratiform precipitation cases over land and ocean surfaces. The performance for rain-water path is similar to the one for graupel and snow in light precipitation cases. For the cases with higher precipitation amounts, the relative errors for rain-water path are larger particularly over land. The same behavior can be seen in the surface rain rate retrieval with the difference that the relative errors are doubled in comparison to the rain-water path. Algorithms with reduced number of frequencies show that window channels at higher frequencies are important for the surface rain rate retrieval because these are sensitive to the scattering in the ice phase related to the rain below. For the frozen hydrometeor retrieval, good results can be achieved by retrieval algorithms based only on frequencies at 150 GHz and above which are suitable for geostationary applications due to their reduced demands concerning the antenna size. Index Terms--Hydrometeor, millimeter wave radiometry, remote sensing, retrieval, satellite applications.

This paper presents the results of a simple information content study between the Advanced Microwave Sounding Unit/Microwave Humidity Sounder (AMSU/MHS) and the Advanced Technology Microwave Sounder (ATMS). When a single field of view is considered for both instruments, the AMSU/MHS generally outperforms the ATMS for temperature and moisture information due to its better noise performance. However, when footprint matching is employed to use oversampled ATMS observations, the ATMS consistently shows improvement in temperature and moisture information over the AMSU/MHS. Index Terms--Electromagnetic propagation in absorbing media, geophysical inverse problems, information retrieval, Jacobian matrices, maximum likelihood estimation, microwave measurements, microwave radiometry, remote sensing, satellite applications.

The potential of a ground-based microwave temperature profiler to combine full tropospheric profiling with high-resolution profiling of the boundary layer is investigated. For that purpose, statistical retrieval algorithms that incorporate observations from different elevation angles and frequencies are derived from long-term radiosonde data. A simulation study shows the potential to significantly improve the retrieval performance in the lowest kilometer by combining angular information from relatively opaque channels with zenith-only information from more transparent channels. Observations by a state-of-the-art radiometer employed during the International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and Its Impact on High-Resolution Modeling (LAUNCH) in Lindenberg, Germany, are used for an experimental evaluation with observations from a 99-m mast and radiosondes. The comparison not only reveals the high accuracy achieved by combining angular and spectral observations (overall, less than 1 K below 1.5 km), but also emphasizes the need for a realistic description of radiometer noise within the algorithm. The capability of the profiler to observe the height and strength of low-level temperature inversions is highlighted. Index Terms--Atmospheric boundary layer (ABL) profiles, ground-based microwave radiometry, remote sensing, vertical resolution.

The Atmospheric Radiation Measurement (ARM) Program maintains a suite of instruments in various locations to provide continuous monitoring of atmospheric parameters. Temperature and humidity retrievals are two of the key parameters used by the climate-modeling community. Accuracy in the spectroscopy adopted by the various radiative transfer models is crucial for obtaining accurate retrievals. While the accuracy of the spectroscopic parameters used for water-vapor retrievals is satisfactory, temperature retrievals continue to be affected by uncertainties in oxygen line parameters leading to discrepancies between the modeled and observed brightness temperatures. In this paper, we compare the model calculations in the oxygen-band channels with the measurements collected by the ARM-operated 12-channel Microwave Radiometer Profiler (MWRP). The dataset used spans a wide range of atmospheric temperature conditions, with ground temperatures varying between -40 [degrees]C and +20 [degrees]C. Model calculations are performed by using line parameters from the high-resolution transmission molecular-absorption (HITRAN) database and from a set of newly published parameters. Our comparison shows that the newly published parameters agree more closely with the MWRP measurements and confirms the need to update the HITRAN database for the oxygen lines. We show the effect of line parameters on the retrievals of temperature, water vapor, and liquid water, and show that improved oxygen absorption is essential to reduce the clear-sky bias in the liquid-water path retrievals. Index Terms--Microwave radiometry, oxygen absorption, temperature retrieval, water-vapor retrieval.

ProSensing Inc. has developed a G-band (183 GHz) water Vapor Radiometer (GVR) for long-term unattended measurements of low concentrations of atmospheric water vapor and liquid water. Precipitable water vapor (PWV) and liquid water path (LWP) are estimated from zenith brightness temperatures measured using four double-sideband receiver channels, which are centered at 183.31 [+ or -] 1, 183.31 [+ or -] 3, 183.31 [+ or -] 7, and 183.31 [+ or -] 14 GHz. A prototype ground-based version of the instrument was deployed at the Department of Energy Atmospheric Radiation Measurement program's North Slope of Alaska site near Barrow, AK, in April 2005, where it collected data continuously for one year. This paper presents design details, laboratory test results, and examples of retrieved PWV and LWP from measured brightness temperature data. Index Terms--Millimeter-wave radiometry, precipitable water vapor (PWV) and liquid water path (LWP) retrieval, remote sensing.

A variational method to retrieve profiles of temperature, humidity, and cloud is described, which combines observations from a 12-channel microwave radiometer, an infrared radiometer, and surface sensors with background from short-range numerical weather prediction (NWP) forecasts in an optimal way, accounting for their error characteristics. An analysis is presented of the error budget of the background and observations, including radiometric, modeling, and representativeness errors. Observation errors of some moisture channels are found to be dominated by representativeness, due to their sensitivity to atmospheric variability on smaller scales than the NWP model grid, whereas channels providing information on temperature in the lowest 1 km are dominated by instrument noise. Profiles of temperature and a novel total water control variable are retrieved from synthetic data using Newtonian iteration. An error analysis shows that these are expected to improve mesoscale NWP, retrieving temperature and humidity profiles up to 4 km with uncertainties of < 1 K and < 40% and 2.8 and 1.8 degrees of freedom for signal, respectively, albeit with poor vertical resolution. A cloud classification scheme is introduced to address convergence problems and better constrain the retrievals. This Bayesian retrieval method can be extended to incorporate observations from other instruments to form a basis for future integrated profiling systems. Index Terms--Atmospheric measurements, microwave radiometry, remote sensing, variational methods.

A new G-band (183.31-GHz) water-vapor radiometer, developed and built by ProSensing Inc., was deployed in Barrow, AK, in April 2005. The radiometer is part of a suite of instruments maintained by the Atmospheric Radiation Measurement program. The instrument measures brightness temperatures from four double-sideband channels centered at [+ or -]l, [+ or -]3, [+ or-]7, and [+ or -]14 GHz from the 183.31-GHz water-vapor line. Atmospheric emission in this spectral region is primarily due to water vapor, with some influence from liquid water. In this paper, data collected in November 2005, December 2005, and January 2006 are analyzed. Measurements are compared with simulations obtained by using a radiative transfer model. We show that the measurements agree well with model simulations. Precipitable water vapor (PWV) and liquid water path (LWP) are retrieved with a nonlinear physical algorithm, and results are compared with those from the colocated dual-channel microwave radiometer and radiosondes. Retrieval errors are estimated to be below 5% for PWV and of the order of 0.006 mm for LWP. Index Terms--Microwave radiometry, remote sensing, water-vapor retrieval, 183 GHz.

Ground-based observations at millimeter (mm) and submillimeter (submm) wavelengths were collected at the Atmospheric Radiation Measurement Program site at Barrow, AK, during the Arctic winter by a new 25-channel radiometer. A weighting function analysis is presented to demonstrate the enhanced sensitivity of mm- and submm-wave (50--400 GHz) radiometers to low vapor and liquid water contents with respect to conventional instruments such as the ones operating at centimeter (cm) wavelengths (20-30 GHz). In addition, based on measurements, we carried out a quantitative analysis of mm-and submm-wavelength sensitivity, yielding improvement factors from 1.5 to 69 for precipitable water vapor (PWV) and 3 to 4 for liquid water path (LWP) when compared to 20-30 GHz radiometers. Furthermore, using a simulated data set, we evaluate the effect of hydrometeor scattering: Given the conditions occurring during the experiment, the scattering contribution is within the instrumental noise for most, but not all, of the considered channels. With the same data set, we demonstrate that in the dry conditions of the Arctic, a simple linear regression yields satisfactory results when applied on selected mm- and submm-wave channels. For a dual-channel combination, the expected accuracy is ~0.23 (0.007) mm for PWV (LWP), when using mm- and submm-wavelengths, whereas it is 0.37 (0.012) mm using cm-wave channels. When the retrieval is applied to real observations, the accuracy is found in agreement with theoretical expectations. Index Terms--Atmospheric measurements, microwave radiometry, remote sensing.

The Helsinki University of Technology's airborne fully polarimetric profiling radiometer at 36.5 GHz has been used for wind-vector measurements over the Gulf of Finland. The results, collected in a series of measurements over a period of two years, are presented in this paper. The Fourier coefficients of the harmonics of the first three modified Stokes parameters (in brightness temperature) have been solved, and their behavior as a function of the measurement incidence angle and the wind speed has been examined, resulting in a linear model in the measurement range. In this paper, we show a clear relationship between the incidence angle and the third modified Stokes parameter (in brightness temperature), which has been used to compensate for aircraft motion during measurements. Furthermore, the sensitivity of the wind-speed measurement to the incidence angle has been studied, and a model for wind-speed retrieval as a function of the harmonic coefficients and incidence angle was developed. Index Terms--Polarimetric radiometer, Stokes parameters, wind speed.

In order to correct the altimeter range for tropospheric humidity, a microwave radiometer is added to altimetry missions [Envisat/microwave radiometer, Jason/Jason Microwave Radiometer, and TOPEX-Poseidon/TOPEX Microwave Radiometer (TMR)]. Over open ocean, the combination altimeter/radiometer is satisfactory. This is not the case in coastal zones, where the signal coming from the surrounding land surfaces contaminates the radiometer measurement and makes the humidity-retrieval method unsuitable. In this paper, a radiometer simulator is built, using data from a field experiment (in situ measurements and collocated TMR measurements) and analyses from a mesoscale-forecast model. This simulator is used to perform sensitivity tests and to evaluate the current methods to retrieve the wet tropospheric correction in transition areas. The purpose of this paper is to analyze and compare the performances of these methods. After examining simple correction methods (extension of the open-sea wet tropospheric correction and use of the meteorological model value), we evaluated the feasibility and performances of two methods, which propose to take into account the land-surface effect in the brightness-temperature estimation. The latter was found to give significantly better results. Index Terms--Coastal, microwave radiometry, wet tropospheric correction.

A large database of radiosoundings, brightness temperatures, and satellite beacon data was used to verify the ability of radiometers in estimating the attenuation in nonprecipitating conditions at millimeter wavelengths. The mass absorption models proposed by Liebe and Rosenkranz have been used coupled with the Helsinki University of Technology (TKK) cloud-detection model to derive coefficients for the calculation of the attenuation due to integrated water vapor and liquid water contents. Comparison of predicted and measured attenuation cumulative distribution functions in the 20-50-GHz band confirms radiometers as a unique tool for retrieving attenuation in nonrainy condition and shows the very good performance of the TKK model for propagation applications. Index Terms--Attenuation, radiometry.

A consistent radiometric model of the ocean is an important part of the interpretation of passive microwave radiometric data. Empirical relations between measured brightness temperature and environmental parameters such as wind, surface temperature, salinity, etc., are often used for processing satellite and airborne data. There are some difficulties when using these approaches when a nonstationary environment is being studied, such as wind and/or surface currents that vary in time and space. In this case, a consistent model based on physical parameters of the sea surface and atmosphere is needed. We consider a radiometric model of the ocean based on a wave-action balance equation. Following Kudryavtsev and Makin, we modified the source term to account for parasitic ripples. Along with surface roughness, the model provides wave-breaking statistics and foam coverage, which is especially important for the microwave radiometry due to high foam emissivity. Comparison between the model prediction and the experimental observations shows a good agreement. Index Terms--Microwave radiometry, sea-surface electromagnetic scattering.

Methods are developed and evaluated to retrieve surface soil temperature information for the Advanced Microwave Scanning Radiometer on Earth Observing System for seven boreal forest and Arctic tundra biophysical monitoring sites across Alaska and Northern Canada. A multiple-band iterative radiative transfer process-based method producing dynamic vegetation and snow cover correction quantities and an empirical multiple regression method using several frequencies are employed. The seasonal pattern of microwave emission and relative accuracy of the soil temperature retrievals are influenced strongly by landscape properties, including the presence of open water, vegetation type and seasonal phenology, snow cover, and freeze-thaw transitions. The retrieval of soil temperature is similar for the two methods with an overall root-mean-square error of 3.1-3.9 K during summer thawed conditions, with a larger error occurring in winter during periods of dynamic snow cover and freeze--thaw state. These results indicate that at high latitudes, the influence of the atmosphere may be less important than that of surface conditions in determining the relative accuracy of the estimated soil temperature. Impacts of surface conditions on surface emissivity, observed brightness temperature, and estimated soil temperature are discussed. Index Terms--Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E), Arctic tundra, boreal forest, microwave radiometry, satellite remote sensing, soil temperature.

WindSat provides an opportunity to explore the passive microwave polarimetric signatures of land surfaces. In order to accommodate the large sensor footprint, large homogeneous regions with unique features were used. These included forest, rangeland, desert, and agricultural conditions. WindSat observations at horizontal and vertical polarizations over land surfaces were found to be well calibrated and consistent with other passive microwave sensors. Isotropic regions (e.g., Amazon rainforest) had no polarimetric response at all azimuth angles. Results showed that land surfaces with aligned features (topography or row structured vegetation) produced systematic variations in the third and fourth Stokes parameters. These responses were found to be in good agreement with previous sea surface studies. Analysis of the temporal trends of the variation in polarimetric measurements for a specific azimuth angle could be attributed to the crop growth cycle in the agricultural region. Further analyses will seek to isolate specific features that could be used in applications such as soil moisture retrieval. Index Terms--Microwave radiometry, polarimetric, soils, vegetation, WindSat.

The Antarctic plateau that extends for several hundred kilometers with an average altitude of close to 3000 m a.s.l. is the highest part of the east Antarctic ice cap. This area provides unique opportunities for various scientific disciplines, including glaciology and atmospheric and earth sciences. In addition, there is growing interest in using the Antarctic plateau, for calibrating and validating data of satellite-borne microwave radiometers, thanks to the size, structure, and spatial homogeneity of this area, and the thermal stability of deeper snow layers. In this paper, we analyze the temporal and spatial variabilities of multifrequency microwave emission from the area surrounding the Dome-C scientific station using Advanced Microwave Scanning Radiometer data collected throughout 2005. Moreover, a multilayer coherent electromagnetic model is used for estimating the contribution of snow layers to emission at various frequencies. The results are consistent with the physical structure of the ice sheet and with its seasonal and spatial variations. Index Terms--Advanced Microwave Scanning Radiometer-EOS (AMSR-E), Antarctica, electromagnetic models, microwave radiometry.

The Geostationary Synthetic Thinned Array Radiometer represents a promising new approach to microwave atmospheric sounding from geostationary orbit based on passive interferometry. Distortion due to mechanical or thermal constraints produces a displacement of the ideal antenna positions in the array that causes sampling errors. In this paper, the impact of array distortion on radiometric error is analyzed in detail so as to identify the dominant sources of error. A preliminary analysis showing that array distortion can be well corrected by means of an external phase reference is also presented. Index Terms--Array, distortion, error analysis, interferometry, radiometry, remote sensing.

A new type of microwave radiometer detector that is capable of identifying low-level pulsed radio frequency interference (RFI) has been developed. The Agile Digital Detector can discriminate between RFI and natural thermal emission signals by directly measuring other moments of the signal than the variance that is traditionally measured. The kurtosis is the ratio of the fourth central moment of the predetected voltage to the square of the second central moment. It can be an excellent indicator of the presence of RFI. A number of issues that are related to the proper calculation of the kurtosis are addressed. The mean and standard deviation of the kurtosis, in both the absence and the presence of pulsed sinusoidal RFI, are derived. The kurtosis is much more sensitive to short-pulsed RFI--such as from radars--than to continuous-wave RFI. The minimum detectable power for pulsed sinusoidal RFI is found to be proportional to [([M.sup.3]N).sup.-1/4], where N is the number of independent samples and Mis the number of frequency subbands in the receiver. Index Terms--Detectors, digital radio, interference suppression, microwave radiometry.