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FengYun-3C (FY-3C) is the second-generation polar-orbiting meteorological satellite in China. As one of the most important microwave payloads deployed onboard FY-3C, microwave radiation imager (MWRI) has been continuously observing radiance originating from land and sea surface and supports numerical weather predictions at regional and global scales. With a long-term monitoring of the cold-end reference, MWRI onboard FY-3C was found suffering from discontinuous observations due to its temporary power-OFF/power-ON in the years of 2015 and 2018, resulting in anomalous brightness temperature (TB) jump up to 2–3 K in magnitude at some channels. Analysis of the operating status of FY-3C MWRI indicated that deviation of the receiver system from its optimal operating status affects the nonlinearity characteristics significantly. A correction algorithm, which relates nonlinearity to the autogain control (AGC) operating voltage of the sensor, is thus proposed on the basis of on-ground thermal/vacuum (T/V) test results in the prelaunch phase in order to mitigate the calibration anomalies of FY-3C MWRI. Nonlinearity correction of FY-3C MWRI is further corroborated by comparisons of simultaneously overlapping observations of global precipitation measurement (GPM) microwave imager (GMI). The results demonstrated that after nonlinearity correction the performance of FY-3C MWRI has been improved at all channels except for the 36-V channel where technical failures have been detected since October 2016.

This study presents a correction algorithm to remove the backlobe intrusion from the hot-load reflector of microwave radiation imager (MWRI) on-board China FengYun-3C (FY-3C) meteorological satellite. Discontinuities in the radiometric gain of MWRI, due to inaccurate backlobe spillover of the hot-load reflector, have been diagnosed. A physical correction method, relating the radiometric gain difference between two distinct backlobe scenes to the spillover of the hot-load reflector, is thus established to estimate the in-orbit spillover factor of the MWRI hot-load reflector. Meanwhile, it is found that brightness temperature (TB) at each 1° × 1° grid could not address the backlobe pattern appropriately, leading to abrupt changes in the radiometric gain occurring near the coastlines. To better represent the backlobe spillover, the effective backlobe TB is now averaged over a 4° × 4° area from advanced microwave scanning radiometer-observed TBs, which is now in a finer resolution of 0.25° × 0.25°. With the adjusted spillover for the hot-load reflector, anomalous behaviors of the radiometric gain are mitigated effectively and the improvement at the coastlines is pronounced, inducing TB variation up to 2 K in magnitude at the 10.65 GHz channel. Although the algorithm presented in this study is only restricted to channel frequencies up to 23.8 GHz due to its accuracy limitation, it could tackle similar issues for those microwave radiometers not capable of in-orbit backlobe maneuvers and receiving the radiances inevitably from the outer edge of the hot-load or cold-sky reflectors.

Consistency between the brightness temperatures observed with a ground-based microwave radiometer and the brightness temperatures computed by forward modeling is important in many different data applications. Using the National Centers for Environmental Prediction-Final Operational Global Analysis (NCEP-FNL) dataset as a reference, the brightness temperature was obtained through the radiation transfer model for forward calculation. The problem of segmented features in long time of observational data from ground-based microwave radiometers (the so-called “jumping problem”) was identified. By analyzing the deviation and correlation between the observational bright temperature data and the forward calculated data under clear sky conditions, a revised scheme is proposed for the bright temperature observational data. Data obtained with a ground-based microwave radiometer in Beijing from January 1, 2010 to December 31, 2011 around the date of liquid nitrogen calibration show that the correlation between the observed brightness temperatures and the forward computed brightness temperatures is better after correction, especially at 28 and 30 GHz. The “jumping” problem in the observational data for the brightness temperature is eliminated after correction and the time continuity of the observational data and its consistency with the forward calculated data based on the NCEP-FNL dataset are improved. The proposed correction scheme can be used both for real-time data quality control and to improve the accuracy of historical datasets obtained with poorly calibrated microwave radiometers or radiometers working in polluted environments.

This paper describes the technical approach to generate homogeneous warm brightness temperature scene binary mask over the earth for the intercalibration of microwave imagers. The objective is to identify homogeneous desert scenes as targets in radiometric intercalibration of multisource multisensor systems such as Global Precipitation Measurement (GPM) constellation. In this paper, a method is developed to generate mask over continental Australia using 10, 18, and 37 GHz vertical and horizontal polarized radiance observations from GPM Microwave Imager data. Major factors considered are diurnal variation, second Stokes parameter, spatial homogeneity, and surface emissivity. Major advantage of this tool is its minimal, almost negligible, dependence on any external ancillary modeled or observed data.

Vicarious cold calibration (VCC) for spaceborne microwave radiometers is analyzed and modified for application to conical scanning microwave imagers at frequencies from 6 to 90 GHz. The details of the algorithm are modified to account for additional frequencies and polarizations that were not included in the development of the original algorithm. The modified algorithm is shown to produce a more stable cold reference brightness temperature (TB) than the original algorithm. An analysis is performed of this updated algorithm to show the global regions that contribute to the derivation of the cold reference TB and to show which geophysical parameters contribute to the coldest TBs. The analysis suggests that water vapor variability has the largest impact on the TBs in the VCC algorithm. The modified VCC algorithm is applied to microwave imager data and is used as an intercalibration method. It is shown to agree well with other intercalibration methods, demonstrating that it is a valid and accurate method for calibration of microwave imagers.

The self-designed HaiYang-2B (HY-2B) satellite was launched on 24 October 2018 in China at 22:57 UT in a 99.34&deg, inclination sun-synchronous orbit. The Scanning Microwave Radiometer (SMR) on the core observatory has the capability to provide near-real-time multi-channel brightness temperature (Tb) observations, which are designed mainly for improving the level of marine forecasting and monitoring, serving the development and utilization of marine resources. After internal calibration and ocean calibration, the first effort to retrieve land surface snow parameters was performed in this study, which obtained extremely low accuracy both in snow extent and snow mass. Accordingly, land inter-sensor calibration was carried out between SMR and the Advanced Microwave Scanning Radiometer 2 (AMSR2) in order to broaden the research and application of SMR data on the Earth&rsquo, s land surface. Finally, we evaluated the consistency of the snow extent and snow mass derived from the initial and land-calibrated SMR data. The results indicated that a systematic SMR cold deviation whose magnitude depends on the channel is present for all the compared channels. After intercalibration, the conformity of the snow extent and snow mass were substantially improved compared to before, the relative bias of the snow extent and snow mass decreased from &minus, 49.97% to 2.97% and from &minus, 51.71% to 3.01%, respectively.

Miniaturized microwave radiometers deployed on nanosatellites in Low Earth Orbit (LEO) are now demonstrating the ability to provide science-quality weather measurements, such as the 3U Micro-sized Microwave Atmospheric Satellite-2A (MicroMAS-2A). The goal of having cost-effective miniature instruments distributed in LEO constellations is to improve temporal and geospatial coverage. The Time-Resolved Observations of Precipitations structure and storm Intensity with a Constellation of Smallsats (TROPICS) is a constellation of six 3U CubeSats, based on MicroMAS-2A, scheduled to launch no earlier than 2020. Each CubeSat hosts a scanning 12-channel passive microwave radiometer. TROPICS will improve temporal resolution to less than 60 minutes compared to larger satellites in polar orbit, such as NOAA-20 which hosts the Advanced Technology Microwave Sounder (ATMS) and has a revisit rate of 7.6 hours. [1] The improved refresh rate will provide high value observations of inner-core conditions for tropical cyclones [2]. In order to effectively use miniaturized microwave radiometers on small satellites such as MicroMAS-2A and TROPICS operationally, new approaches to calibration are needed to achieve state-of-the-art performance. Calibration on nanosatellite platforms present new challenges, as standard blackbody targets are too bulky to fit on 3U CubeSats. Instead, internal noise diodes are used for calibration, with long-term drifts measured on the order of 0.2% to 3.0% (approximately 0.4 K to 6.0 K) [4]. Blackbody calibration targets such as used on ATMS have 0.14 K error or better for warm calibration [3]. In order to provide state of the art calibration for CubeSats, methods must be developed to trend and correct noise diode drift. We develop a new way to continuously calibrate CubeSat constellations, such as TROPICS, by incorporating frequent and periodic solar and lunar intrusions as an additional source of information to counter noise diode drift. These solar and lunar intrusions also occur for existing satellites hosting microwave radiometers in polar orbits, but are much more infrequent than for the scanning payload on the TROPICS constellation, and are typically treated as an observational and calibration limiting constraint. The higher occurrence rate of intrusions for TROPICS motivates the novel idea of using the intrusions to support calibration. An algorithm is developed to compare expected effective brightness temperature from solar and lunar measurements to actual measured brightness temperatures. Future work includes developing a detailed error budget for the algorithm and testing the algorithm using actual sun and moon measurements taken by MicroMAS-2A and ATMS. In this paper we discuss background information, provide our approach, and show initial results.

Spaceborne microwave radiometers with high frequency channels (150 – 183 GHz) are important for retrieving many geophysical parameters such as snowfall, ice water path, and atmospheric water vapor profiles. In order to obtain accurate retrievals from the brightness temperature measurements, the radiometers must be properly calibrated. Several methods have been developed to analyze the on-orbit calibration, including comparisons with radiative transfer models, comparisons with radiosonde profiles, and cross-calibration with similar radiometers. This paper introduces a new method to calibrate high frequency channels that utilizes TB histograms. This new method gives an independent approach that can be used by itself to analyze radiometer calibration or in conjunction with other methods to corroborate results.

Conventional satellite intercalibration for microwave imaging radiometers involves a linear two-point calibration approach, over cold ocean scenes and warm ends of the scene brightness temperatures, traditionally over tropical rain forest. Unfortunately, this approach ignores the possibility of non-linear radiometer transfer function, which is often important. This paper discusses a new tool developed to identify large-scale homogeneous desert scenes as possible third calibration targets with brightness temperatures that fall between the conventional cold and warm earth scenes. Previously a prototype “ShowMask” procedure was developed over continental Australia using 10, 18 and 37 GHz Vertical and Horizontal polarized brightness temperatures from GPM Microwave Imager (GMI) data. This paper focus on extending this tool to global scale and including 89 GHz channel in the analysis. Further, by applying the mask, we examine intercalibration results over selected desert targets using GMI-TMI and GMI-AMSR-2 match-ups. Initial results indicate a scene temperature dependence of calibration factor particularly for horizontally polarized channels.

High emissivity temperature-controlled microwave blackbodies, or calibration targets, are often used as brightness temperature reference sources for radiometer calibration. Calibration targets are, in practice, often viewed from a range of angles due to the scanning nature of operational radiometers (e.g. a conical scanning instrument). Ideally, the calibration target has view-angle-independent emissivity of unity, but any deviation from ideal can bias the brightness temperature radiating from the target. Microwave emissivity is not a directly-measurable quantity, and instead must be inferred through measurements of reflectivity. We measure reflections from calibration targets by quantifying the magnitude of the standing wave formed by the target as it is linearly stepped through space at sub-wavelength increments. We present monostatic reflectivity results over a range of incidence angles for two types of calibration targets; a periodic pyramidal absorber array and a conical cavity blackbody. Measurements are presented at 165.5 GHz and 183.3 GHz, two channels of interest for environmental remote sensing. We find that the pyramidal array has higher reflectivity than the conical cavity at both frequencies and across the range of incidence angles. Additionally, we find that the pyramidal array has a larger range of reflectivity across the range of incidence angles. The reflectivity magnitude decreases as a function of incidence angle for both geometries, with local maxima at normal incidence except for the pyramidal array at 165.5 GHz. The considerable angular variation in reflectivity observed for the pyramidal array could potentially cause significant calibration bias as large as 0.12 K.

In microwave radiometric remote sensing, undetected radio frequency interference (RFI) can adversely affect the accuracy of the science products. A method is presented to adaptively tune the parameters of an RFI detection algorithm which controls the equivalent brightness temperature of undetected RFI. The method is adaptive in the sense that it adjusts to variations in the RFI environment, e.g., from high RFI conditions near some population centers to low RFI conditions in the tropical Pacific Ocean. The RFI environment is characterized by inferring the distribution of low-level undetected RFI from that of high-level detected RFI using appropriate scaling arguments. The resulting tuned algorithm adjusts its detection threshold to equalize the brightness temperature calibration bias due to RFI at the expense of the now variable measurement precision (noise equivalent delta temperature). This tradeoff between calibration bias and measurement precision can be represented as a modified version of the classic receiver-operating-characteristic curve. The radiometer on the Aquarius/SAC-D mission is used as an example.

Here, we describe the characteristics of brightness temperature (Tb) measured by the Advanced Microwave Scanning Radiometer-2 (AMSR2) onboard the Global Change Observation Mission 1st-Water (GCOM-W1). This mission aims to achieve long-term global monitoring of Earth using two polar-orbiting satellite observation systems with three consecutive generations. GCOM-W1, the first satellite of the GCOM-W (Water) series, was launched successfully on May 18, 2012. AMSR2 is a single-mission instrument onboard the GCOM-W1 satellite. The basic characteristics of AMSR2 are similar to those of its predecessor, AMSR-E; this allows the continuation of AMSR-E observations but with several improvements, including a larger main reflector (2.0-m diameter), additional channels at C-band frequency, an improved calibration system, and increased reliability imparted by the addition of a redundant momentum wheel. Since July 3, 2012, the instrument has functioned properly and has accumulated a data set of Tb measurements. During the initial calibration and validation period, Tb values are being evaluated and characterized according to various methodologies, including intercalibration between similar microwave radiometers [e.g., the Tropical Rainfall Measuring Mission Microwave Imager (TMI)] based on radiative transfer computations. The intercalibration results for the ocean area as the cold end and the rainforest area the as warm end demonstrate that Tb measured by AMSR2 exhibits no apparent seasonal variation, with a maximum calibration difference of approximately 5 K compared to TMI and AMSR-E. This calibration difference appears to depend on the Tb of the observed object.

The Tropical Rainfall Measurements Mission (TRMM) Microwave Imager (TMI), has served as the radiometric transfer standard for the TRMM constellation radiometers for over 17 years. The final legacy processing for the TMI is in progress for the TMI 1B11 Tb Version-8 (V8), which will be released in late 2017. This paper presents a radiometric evaluation of the Tb differences between the current TMI Version-7 (V7) and the newly derived V8 dataset. For this evaluation, we perform an inter-satellite radiometric calibration (XCAL) between the Global Precipitation Mission Microwave Imager (GMI) Tb product and TMI Tb products for both V7 and V8. The results of XCAL using different datasets are compared to assess the TMI V8 calibration biases relative to V7.

Soil Moisture Active Passive (SMAP) mission of NASA was launched in January 2015. Currently, SMAP has an L-band radiometer and a defunct L-band radar with a rotating 6-m mesh reflector antenna. On July 7th, 2015, the SMAP radar malfunctioned and became inoperable. Consequently, the production of high-resolution active-passive soil moisture product got hampered, and only ~2.5 months (April 15th, 2015 to July 7th, 2015) of data remain available. Therefore, during the SMAP post-radar phase, many ways were examined to restart the high-resolution soil moisture product generation of the SMAP mission. One of the feasible approaches was to substitute the SMAP radar with other available SAR data. Sentinel-1A/Sentinel-1B SAR data was found most suitable for combining with the SMAP radiometer data because of its nearly similar orbit configuration that allows overlapping of their swaths with a minimal time difference, a key feature/requirement for the SMAP active-passive algorithm. The Sentinel interferometric wide swath (IW) mode acquisition also provides the co-polarized and cross-polarized observations required for the SMAP active-passive algorithm. However, some differences do exist between the SMAP and Sentinel SAR data. They are mainly: 1) Sentinel has a C-band SAR whereas SMAP operates at L-band; 2) Sentinel has multiple incidence angles within its swath, and SMAP has one single incidence angle; and 3) Sentinel 1A/B Interferometric Wide (IW) swath width is ~250 km as compared to SMAP with 1000 km swath width. On any given day, the narrow swath width of the Sentinel observations significantly reduces the overlap spatial coverage between SMAP and Sentinel as compared to the original SMAP radar and radiometer swath coverage. Hence, the temporal resolution (revisit interval) suffers due to narrow overlapped swath width and degrades from 3 days to 12 days. One advantage of using very high-resolution resolution Sentinel-1A/Sentinel-1B data in the SMAP active-passive algorithm is the potential of obtaining the disaggregated brightness temperature and thus soil moisture at a much finer spatial resolution of 3 km and 1 km at global extent. The assessment of high-resolution product at 3 km and 1 km using the soil moisture calibration and validations sites shows reasonable accuracy of ~0.05 m3/m3. The SMAP-Sentinel1 active-passive high-resolution product is now available to the public (new version released in October 2018) through NSIDC (NASA DAAC). The duration of this product is from April 2015 to current date.

The microwave radiometer (MRM) onboard the Chang' E-1 (CE-1) lu- nar orbiter is a 4-frequency microwave radiometer, and it is mainly used to obtain the brightness temperature (TB) of the lunar surface, from which the thickness, tem- perature, dielectric constant and other related propertie s of the lunar regolith can be derived. The working mode of the CE-1 MRM, the ground calibration (including the official calibration coefficients), as well as the acquisiti on and processing of the raw data are introduced. Our data analysis shows that TB increases with increasing fre- quency, decreases towards the lunar poles and is significant ly affected by solar illumi- nation. Our analysis also reveals that the main uncertainty in TB comes from ground calibration.

estimating the regolith depth from brightness temperature (TB) measured by a multi-channel radiometer at 3GHz, 7.8GHz, 19.35GHz and 37GHz. To establish the accurate microwave radiation or scattering model of lunar surface, firstly we need to determine an accurate model of dielectric constant of lunar regolith. However, until now there is no model of dielectric constant suitable for microwave band between 3GHz–37GHz.In this paper, we extract the normalized temperature deviation τ from CE-1 TB data, which filter out the diurnal variation and the impact of latitude. Then we established a new dielectric constant model for microwave band, through analyzing the different relationship that between τ and the content of iron and titanium. The parameters in the model are inverted by comparing the TB measured and the TB calculated. At last, we compared the proposed dielectric constant model with the measured results of lunar regolith samples.

The Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) had been operated in special slow rotation mode to obtain continuous brightness temperatures for the inter-comparison with those of AMSR2 on the Global Change Observation Mission-Water (GCOM-W) for three years. Although its observation coverage is sparse and some calibration uncertainties exist, the data are still very useful for straightforward inter-comparison.

A robust XCAL double difference (DD) approach for radiometric calibration has been successfully applied between the TRMM Microwave Imager, TMI, (previous calibration transfer standard for NASA's Precipitation Measuring Mission) and a number of precipitation measuring radiometers in polar sun-synchronous orbits. Now that the TRMM Mission has ended (April 2015), the radiometric transfer standard was changed from TMI to the current GPM Microwave Imager (GMI). The use of an ocean radiative transfer model (RTM) is an integral part of our XCAL DD approach. Because the RTM physics is imperfect and because the environmental parameters are likewise only estimates of their true values, it is important to assess the sensitivity of the derived brightness temperature biases to these limitations. Therefore, in this paper, we conduct the inter-calibration between GMI and other constellation satellite microwave radiometers using the XCAL DD approach with two different ocean surface emissivity models. Results are presented that demonstrate the robustness of the XCAL DD approach for multiple instruments over a wide range of channel frequencies.

The method of two-point calibration of multi-band microwave radiometer is investigated in this paper. The radiometer calibration results are verified with synchronized measurements, compared between the thermometer and the calibrated radiometer for the 6.6GHz, 13.9GHz, 19.35GHz, 22.235GHz and 37GHz bands. Results show that the root-mean-square error is below 0.5K in each channel for both brightness temperature measured by radiometer and the temperature from thermometer. Therefore, assuming good radiometer receiver linearity, the calibration coefficient of radiometer obtained by two-point calibration is valid in the dynamic range of measurement. The error induced by parameter variation and the system error have been analyzed. Absolute calibration accuracy of brightness temperature is better than 1K within the radiometer measuring range.

Time series analysis on clear-sky brightness temperature (TB) data observed in the morning with ground-based microwave radiometer for atmospheric remote sensing is adapted to judge the working state of the radiometer system according to meteorological data variation features in terms of radiative transfer. The TB data taken as the first example in this study was for the ground-based microwave radiometer at Nanjing during the period from Nov. 27, 2010 to May 29, 2011. The radiometer has 12 channels including five channels at 22.235, 23.035, 23.835, 26.235, and 30 GHz for sensing air humidity and liquid water content and seven channels at 51.25, 52.28, 53.85, 54.94, 56.66, 57.29, and 58.80 GHz for sensing air temperature profiles. The correlation coefficients between the TB readouts from the radiometer and the simulated TB with radiosonde temperature and humidity profiles as input to radiative transfer calculation are greater than 0.9 for the first five channels while the correlation coefficients for the last seven channels are quite poor, especially for the channels at lower frequency such as 51.25, 52.28, and 53.38 GHz, at which the TB readout values in time series do not show the right atmospheric temperature variation features as time goes from November (winter) through spring to early summer (late May). The results show that the first five channels worked well in the period while the last seven channels didn’t, implying that the radiometer need to be maintained or repaired by manufacture. The methodology suggested by this paper has been applied to the similar radiometers at Wuhan and Beijing for quality control and calibration on observed TB data.

The double difference method of inter-calibration between spaceborne microwave radiometers is combined with the vicarious cold calibration method for calibrating an individual radiometer. Vicarious cold calibration minimizes the effects of geophysical variability on radiative transfer models (RTMs) of the brightness temperature (TB) data and it accounts for frequency and incidence angle dissimilarity between radiometers. Double differencing reduces the sensitivity of the inter-calibration to RTM error and improper accounting for geophysical variables in the RTM. When combined together, the two methods significantly improve the confidence with which calibration differences can be identified and characterized. This paper analyzes the performance of the vicarious cold calibration double difference method for conical scanning microwave radiometers and quantifies the improvement this method provides compared to performing a simpler inter-calibration by direct comparison of radiometer measurements.

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.

In this paper, a new and improved L-band version of the correlated noise calibration standard (CNCS) has been developed to aid in the characterization of polarimetric microwave radiometers. The CNCS generates a series of known polarimetric test signals using a two-channel commercial arbitrary waveform generator (AWG) and a pair of frequency-upconversion modules. When it is inserted in place of the antenna used by a radiometer-under-test (RUT), it can fully characterize the polarimetric response of the receiver portion of the RUT. Both hardware and software improvements have been made over a previous version of the CNCS. The frequency upconverters now include integral warm and cold calibration-reference targets to automatically compensate for small drifts in AWG output-signal strength. The procedure used to calibrate the RUT has been generalized to correct for nonideal characteristics of the CNCS itself. Moreover, the effects on the derived polarimetric gain matrix of impedance mismatches between the RUT, and either the CNCS or the antenna have been determined. Details of the CNCS improvements are presented. The results of an experimental demonstration of its use and of a series of validation tests of its performance are also presented.

We present the results of a study that shows that a simple design modification is sufficient to avoid a major shortcoming in the layout of external warm loads commonly used in the calibration of spaceborne microwave radiometers. The modification consists of placing a layer of Plastazote, a polyethylene foam, over the opening of the warm load enclosure. The foam is transparent at micrometer and millimeter wavelengths, is opaque in the infrared and visible, and isolates the warm load from the environment, keeping the temperature of the radiometric warm load constant. The proposed solution can be easily implemented and is suitable even for retrofitting on instruments that have already been built but not yet launched, and the material presents no obvious shortcomings that could prevent its intended application in space.

Results are presented from the long-term monitoring and calibration of the National Aeronautics and Space Administration Jason Microwave Radiometer (JMR) on the Jason-1 ocean altimetry satellite and the ground-based Advanced Water Vapor Radiometers (AWVRs) developed for the Cassini Gravity Wave Experiment. Both radiometers retrieve the wet tropospheric path delay (PD) of the atmosphere and use internal noise diodes (NDs) for gain calibration. The JMR is the first radiometer to be flown in space that uses NDs for calibration. External calibration techniques are used to derive a time series of ND brightness for both instruments that is greater than four years. For the JMR, an optimal estimator is used to find the set of calibration coefficients that minimize the root-mean-square difference between the JMR brightness temperatures and the on-Earth hot and cold references. For the AWVR, continuous tip curves are used to derive the ND brightness. For the JMR and AWVR, both of which contain three redundant NDs per channel, it was observed that some NDs were very stable, whereas others experienced jumps and drifts in their effective brightness. Over the four-year time period, the ND stability ranged from 0.2% to 3% among the diodes for both instruments. The presented recalibration methodology demonstrates that long-term calibration stability can be achieved with frequent recalibration of the diodes using external calibration techniques. The JMR PD drift compared to ground truth over the four years since the launch was reduced from 3.9 to -0.01 mm/year with the recalibrated ND time series. The JMR brightness temperature calibration stability is estimated to be 0.25 K over ten days.

A constellation of disparate radiometers is inherent to the Global Precipitation Measurement (GPM) mission concept. The task of the Intersatellite Calibration Working group is to generate adjustments to make the measurements of all these radiometers physically consistent. A key role of the GPM Microwave Imager (GMI) on the GPM Core satellite is to serve as a transfer standard among the constellation radiometers. The TRMM Microwave Imager (TMI) has served this role during the development phase and for interim corrections early in the GPM mission. The stability of GMI appears to be very good and a physically based calibration has been generated that appears to be accurate at the 1K level or better.

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.

Sea-level change studies from altimetric satellites are reliant on range stability of the sea surface heights computed from orbital positioning and geophysically corrected data. One such correction, namely the wet tropospheric delay induced by the highly variable atmospheric water vapor content, is provided by radiometers onboard ERS-2 and TOPEX/Poseidon (T/P). In this study the long-term stability of the ERS-2 microwave radiometer (E2MR) and the T/P microwave radiometer (TMR) are investigated with the observed drift in the brightness temperatures approximated by reference to the coldest temperatures over the oceans. The E2MR stability is characterized by a gain anomaly fall in 1996 and a drift in the 23.8 GHz channel. For the TMR, investigations show that the dominant drift is about 0.2 K/year in the 18 GHz channel over the first 7–8 years but stabilizing and even decreasing slightly thereafter. In contrast, the 21 GHz and 37 GHz channels are comparatively stable. Utilizing correction formulae a modified...

We discuss work at NIST aimed at developing a passive microwave brightness temperature standard. By reducing uncertainty, we can provide better calibrations for future weather and climate-monitoring radiometers. We discuss the calibration procedure used, measured data, and various theoretical and simulated results that have led to an improved understanding of the various uncertainty contributions in the measurement. We overview a Monte Carlo simulation to determine the uncertainty in target brightness temperature as a function of measurement distance and target size. We also discuss other future improvements including an improved blackbody design. The achievable calibration source brightness temperature uncertainty is expected to be reduced from the current 0.7 to 1.0 K from 18 to 65 GHz to less than 0.3 K.

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.

Thermal radiation of complex man-made objects as well as of our natural environment contains fully polarimetric in-formation in many cases. A quasioptical, imaging radiometer system was designed and built up at DLR for the measu-rement of the four Stokes vector components. A polarimetric calibration procedure was developed and verified. Selec-ted measurements have been carried out which indicate new possible applications. 1. Introduction The information content of the measured natural thermal radiation of man-made targets and of our environment dependson the frequency, observation angle, surface roughness, chemical composition and the polarization. Dependencies ofbrightness temperature signatures on polarization are based on prefered orientations of the electric field vector. For-mally, with respect to an orthogonal coordinate system x m , y m , z m and an observer looking into the direction z m , thebrightness temperature T B (p) for any polarization ppx py xm y m can be expressed by the following equation

This paper reports characteristics of the AMSR2 L1B brightness temperature (Tb) measurement through intercalibration approach among other microwave radiometers including the TRMM/TMI with help of radiative transfer models. The intercalibration results indicate that AMSR2 has calibration difference, 5K at most, compared with TMI and the relative differences of some channels depend on observed scene temperature. There is no apparent seasonal variation and no undesirable discrepancy between daytime and nighttime orbits on the AMSR2 Tbs. The cause of the calibration difference is under investigation.

A styrofoam layer is introduced to the cryogenic calibration load for microwave radiometers in order to keep the antenna at ambient temperature while calibrating. Obviously the insulation layer has nonuniform temperature profile. A novel approach based on the transmission-line theory is presented to calculate the emission from the load. According to the sample calculation through this new approach, the contribution of the insulation layer to the load radiation can not be neglected.

A method for estimating the pitch and roll errors of a satellite with an onboard conical scanning microwave radiometer is described. The method makes use of the vicarious cold calibration algorithm which derives a stable cold brightness temperature (TB) over ocean. This cold TB is sensitive to the Earth Incidence Angle (EIA) of the radiometer. Given no pitch or roll errors, the EIA can be modeled as a function of the Earth radius and altitude of the satellite. Deviation from this EIA can then be used to estimate the pitch and roll errors. The pitch/roll algorithm is applied to the current spaceborne microwave radiometer WindSat to show its performance, and the results are compared to the derived pitch and roll of WindSat that are found using a different attitude analysis method.

Characterization of the drift in the 18-GHz brightness temperature (TB) measured by the TOPEX microwave radiometer (TMR) is presented. The appropriate correction is not a constant drift but varies with TB. Using this correction results in a more accurate path delay retrieval that varies appropriately with cloud cover and surface wind speed.

Brightness temperatures from NOAA-18 and NOAA-19 MHS measurements are investigated over Antarctica to establish it as a natural vicarious calibration target for determination of the Inter-satellite/inter-sensor Calibration Biases (ICB) of the two MHS radiometers. Establishment of a natural test site for calibration references is important for calibration and validation of space-borne microwave instruments

Radiometric digital count data are required to test the operational software of the Advanced Technology Microwave Sounder (ATMS) prior to its launch in 2011 to verify effective calibration procedures and optimize performance. These data, however, do not exist prior to launch, thus necessitating a method to build a model of the ATMS that synthesizes realistic data as though the sensor were space-borne. A method to build such a model is described in this paper. The procedure and application are specific to the ATMS but could be applied to other passive microwave radiometers as well.

Accurate characterization of the brightness temperature (T B ) of black-body targets used for calibrating microwave remote-sensing radiometers includes many inputs: antenna pattern and loss, target temperature, target emissivity, mechanical alignment, and radiometric T B measurements, all of which must be calibrated against physical standards. Here, we describe measurements made using several black-body targets and two different antennas within the WR-42 (18 to 26.5 GHz) waveguide band. Uncertainty estimates are also shown for the retrieved target T B measurements.

CE-2 Lunar Microwave Sounder (CELMS) is the second passive microwave radiometer in the world to sound the surface of the Moon on the orbit of 100 km. The scientific objective of CELMS is to further obtain global brightness temperature (TB) of the Moon, to retrieve information on lunar regolith, and to evaluate the distribution of helium-3 on the Moon. Before launch of CELMS, calibration experiments were carried out in laboratories to test the performances of the systems, and to derive transfer parameters on the transfer paths of CELMS. This paper the procedure of calibration is introduced and the method of computing the coefficients of each path is given. The calibration results on nonlinearity and accuracy of the system are summed and analyzed.

[1] The tipping curve calibration method has been an important calibration technique for ground-based microwave radiometers that measure atmospheric water vapor and cloud liquid water. The method calibrates a radiometer system using the brightness temperatures at various viewing angles in the atmosphere. In this paper, the tipping calibration was carried out by digital gain compensative microwave radiometer (DGCMR) in Inner Mongolia, China, in August 2006. The frequencies of the radiometers are 23.8 and 31.65 GHz with rectangle horn and paraboloid antenna. Using the relationship between correction value and scanning angle, the brightness temperature of the antenna sidelobe contribution is corrected. The comparison of the correction results for the two antenna types shows that the antenna sidelobe error for the rectangular horn antenna can be corrected more effectively.

The Special Sensor Microwave/Imager (SSM/I) instrument and scan geometry are briefly described. The results of investigations of the stability of the gain, calibration targets and spin rate, the radiometer noise and sensitivity, the coregistration, the beam width and main-beam efficiency of the antenna beams, and the absolute calibration and geolocation of the instrument are presented. The results of this effort demonstrate that the SSM/I is a stable, sensitive, and well-calibrated microwave radiometric system capable of providing accurate brightness temperatures for microwave images of the Earth and for use by environmental product retrieval algorithms. It is predicted that this SSM/I and the 11 future ones currently built or to be built will provide high-performance microwave measurements for determination of global weather and critical atmospheric, oceanographic, and land parameters to operational forecasters and users and the research community for the next two decades. >

The NOAA-18 satellite carries two microwave radiometers, the Advanced Microwave Sounding Unit-A and the Microwave Humidity Sounder. After its launch in May 2005, a systematic post-launch calibration and validation of the instrumental performances were conducted with on-orbit data. A brief report of the initial assessment of the two instruments' performances is presented

We review the advantages of a national standard for microwave brightness temperature and outline our proposed approach toward developing such a standard. The proposal is a combined standard that would comprise both a standard radiometer, traceable to primary noise standards, and a fully characterized standard target. Keywords-brightness temperature; microwave radiometry; radiometer calibration; remote sensing; standards

Beginning in the 1970s, continuous observations of the Earth have been made by spaceborne microwave radiometers. Since these instruments have different observational characteristics, care must be taken in combining their data to form consistent long term records of brightness temperatures and derived geophysical quantities. To be useful for climate studies, data from different instruments must be calibrated relative to each other and to reference targets on the ground whose characteristics are stable and can be monitored continuously. Identifying such targets over land is not straightforward due to the heterogeneity and complexity of the land surface and cover. In this work, we provide an analysis of multisensor brightness temperature statistics over ocean, tropical forest, and ice sheet locations, spanning the period from 1978 to the present, and indicate the potential of these sites as continuous calibration monitoring targets

The main results of the field testing of different microwave L-band radiometers designed for soil moisture measurements are presented. The measurements were made at two different experimental sites located near the Sofia airport. It is demonstrated that the brightness temperature results of the RM1 (18 cm) and RM2 (21 cm) compare very well after the RFI effects were removed. It is stated that the simultaneous use of both radiometers can provide reliable measurements even in noisy EM environment like this one described in the paper.

We describe the NIST effort to develop brightness-temperature standards for microwave and millimeter-wave frequencies. Results of preliminary measurements at 26 GHz are presented.

The Jason microwave radiometer is calibrated using hot and cold on-Earth theoretical brightness temperature references. The retrieved path delay values are validated using collocated Topex microwave radiometers and radiosonde values. The calibrated path delay values are demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay values is demonstrated to have no significant bias or scale errors. The absolute accuracy of the individual path delay measurement exceeds the mission goal of 1.2 cm RMS.

Brightness temperatures (TBs) from satellite microwave radiometers need to be calibrated on-orbit at the hottest and coldest ends of the dynamic TB spectrum. A method is developed to determine hot reference calibration TBs from SSM/I TB data for other radiometers operating between 18-40 GHz and 0-55 degrees incidence. Regions of optically thick vegetation in the Amazon rainforest are chosen as calibration targets. Hot reference temperatures for the on-orbit calibration of microwave radiometers are determined for these regions.