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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.

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.

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.

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.

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.

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 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.

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.

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.

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.

The launch of the Advanced Microwave Sounding Unit (AMSU) on the polar platforms, NOAA-K, L, M and N will provide vertical temperature and humidity soundings for inclusion in numerical weather prediction. It is important therefore, that calibration and validation of the data generated takes place. The microwave radiometers, MARSS and Deimos, onboard the Met. Offices C-130 are to be used to this end, so it is vital that their calibration be well understood. This presentation investigates the factors that can effect the calibration of airborne radiometers.

The dynamical properties of microwave attenuation have been stuied in tenns of fade duration. The attenuation levels due to only atmospheric water vapour, liquid water and oxygen have been estimated by a ground-based radiometer with channels at 20.6, 31.65 and 52.85 GHz at Denver; USA. The measurements period covers about one year. The statistic features of the fade duration have been indivduated and estimation of model parameters has been performed. The Log-Normal model has provided a fairly accurate description of number of occurrences and durations of fades for all frequencies and thresholds considered.