An internal calibration subsystem operating in the W-band that is integrated in the frontend of a microwave radiometer has been developed: the calibration signals are generated by noise diodes and matched waveguide loads and coupled to the receiver by RF switches and directional couplers. The brightness temperatures of the injected calibration signals, calibration path losses, the isolation of the switches, as well as the transient behavior of the noise diodes and the switches have been determined using an accurate, temperature stabilized radiometric testbed. The testbed is described, and the measurement results are presented.

We present the design and characterization of the onboard calibration target (OBCT) for the arctic weather Satellite radiometer. The arctic weather satellite (AWS) is a single-instrument mission consisting of a cross-track scanning microwave radiometer. The radiometer optics consists of a feed cluster with four horns (54, 89, 183, and 325 GHz) directly illuminating a primary scanning mirror. The OBCT is a wedge-shaped cavity with an absorber consisting of an epoxy-based mixture, developed and produced by the University of Bern. The coupling into the target via a secondary mirror is simulated using Ticra Tools and the effects of the divergent beams are investigated. We present the development of the target and laboratory measurements of the flight model showing a return loss of 55 dB or better for all bands in TM mode. A worst case thermal simulation is presented, highlighting possible temperature gradients in the target.

This paper presents the first detailed description of the innovative measurement set up of an indoor tropospheric microwave radiometer (TROWARA) that avoids water films on radome. We discuss the performance of a commercial outdoor microwave radiometer (HATPRO) for measuring tropospheric water parameters in Bern, Switzerland. The HATPRO is less than 20 meters from the TROWARA and has different instrument characteristics. Brightness temperatures measured by HATPRO are analyzed by comparing them with coincident measurements from TROWARA and radiative transfer simulations based on the ECMWF operational analysis data (denoted as RTSE). To find the source of brightness temperature bias, a gradient boosting decision tree is used to analyze the sensitivity of eight feature factors to bias. Data processing routines of the two radiometers use different algorithms to retrieve integrated water vapour (IWV) and integrated cloud liquid water (ILW), whereas the same physical algorithms based on the radiative transfer equation are applied to obtain the opacity and rain rate. Using 62 days of data with varied weather conditions, it was found that TROWARA brightness temperatures are in good agreement with RTSE. HATPRO brightness temperatures are significantly overestimated by about 5 K at 22 GHz, compared to TROWARA and RTSE. HATPRO brightness temperatures at 31 GHz agree well with TROWARA and RTSE (within about +/-1 K). The overestimated brightness temperatures in the K-band and the HATPRO retrieval algorithm lead to an overestimation of IWV and ILW by HATPRO. The opacities at 31 GHz match very well for TROWARA and HATPRO during no rain with a verified R2 of 0.96. However, liquid water floating or remaining water films on the radome of the outdoor HATPRO radiometer induce an overestimation of the rain rate. The physical reason for the overestimated 22 GHz brightness temperatures of the HATPRO is mainly the result of the combined effect of instrument calibration, the surrounding environment of the instrument, and the sun elevation angle. This can be a problem with the Generation 2 HATPRO radiometer and this problem was resolved in the Generation 5 HATPRO radiometer.

This paper addresses the further development of the ground based TEMPerature RAdiaometer TEMPERA, which was used successfully for temperature retrievals up to an altitude of 50 km in the period from 2013–2017 [1], [2]. The latest innovation is a new front-end, which allows a fully polarimetric analysis of the measured microwave radiation, in combination with improved inversion methods. In the new forward model the Zeeman effect (see Section 2) and the circular polarization of the microwave radiation is taken into account. Based on this new technology, it should be possible to retrieve atmospheric temperature and Earth's magnetic field up to the lower mesosphere.

In January 2019 a Project Authorization Request was submitted to the IEEE standards association with the title "Standard for Calibration of Microwave Radiometers in the 300 MHz to 1 THz Frequency Range for Geoscience Applications". An open committee is being assembled to draft this standard with the purpose of unifying and documenting calibration procedures for a wide range of microwave radiometers. The committee includes members, collaborators, and contributors from academia, international government and private industry. We include ground-based, air-borne, and space-borne systems. The standard will also define standardized terminology, and address procedures required to obtain traceability to fundamental units or constants. The scope of the standard encompasses various radiometer geometries, Dicke switching, total power, and differential, as well as different polarization configurations including fully polarized (full Stoke's) radiometers. The standard will also separately address free-space and single-mode (e.g. transmission-line) radiometer calibration techniques.

A highly sensitive receiver for the microwave ozone profiling instrument MOPI 5 has been developed. The receiver allows the simultaneous measurement of the pressure-broadened line shape of the thermally-excited rotational emission lines of middle atmospheric ozone and carbon monoxide at 110.836 GHz and 115.271 GHz, respectively. It exhibits a single side band noise temperature of 550-625 K. This paper describes the receiver design and compares the results of the first spectral line measurements obtained using two different digital fast Fourier transform spectrometer back-ends.

A conical cavity has been designed and fabricated for use as a broadband passive microwave calibration source, or blackbody, at the National Institute of Standards and Technology. The blackbody will be used as a national primary standard for brightness temperature and will allow for the prelaunch calibration of spaceborne radiometers and calibration of ground-based systems to provide traceability among radiometric data. The conical geometry provides performance independent of polarization, minimizing reflections, and standing waves, thus having a high microwave emissivity. The conical blackbody has advantages over typical pyramidal array geometries, including reduced temperature gradients and excellent broadband electromagnetic performance over more than a frequency decade. The blackbody is designed for use between 18 and 230 GHz, at temperatures between 80 and 350 K, and is vacuum compatible. To approximate theoretical blackbody behavior, the design maximizes emissivity and thus minimizes reflectivity. A newly developed microwave absorber is demonstrated that uses cryogenically compatible, thermally conductive two-part epoxy with magnetic carbonyl iron (CBI) powder loading. We measured the complex permittivity and permeability properties for different CBI-loading percentages; the conical absorber is then designed and optimized with geometric optics and finite-element modeling, and finally, the reflectivity of the resulting fabricated structure is measured. We demonstrated normal incidence reflectivity considerably below -40 dB at all relevant remote sensing frequencies.

A rigorous numerical technique to compute the brightness temperature of arbitrarily shaped microwave calibration targets is presented. The proposed method allows the brightness temperature of calibration targets to be investigated depending on frequency, absorber material, geometry, antenna pattern, field incidence, and temperature environment. We have validated the accuracy and studied the numerical complexity of the approach by means of analytical reference solutions. Fundamental brightness temperature investigations of pyramid absorbers are shown for various thermal environments in different frequency bands between 20 and 450 GHz. Based on these analyses, a novel pyramid geometry was designed, which features a superior electromagnetic and thermal performance compared with conventional pyramid designs. Using the theoretical findings, we have developed reduced-order models of pyramid targets for rapid brightness temperature studies.

Blackbody targets are needed for the calibration of radiometers for remote sensing applications. The use of hot and cold references is essential for an accurate calibration. Commonly, the cold loads are made with liquid nitrogen, but for a campaign instrument, this is not a sustainable option. In this paper, we present the design, construction, and characterization of a new cold calibration target for a microwave radiometer operated at 110 GHz for the detection of pressure-broadened spectra from atmospheric ozone. The target is cooled with Peltier elements. Heat transfer simulations were made to optimize the design and minimize temperature gradients. Several tests were performed with a vector network analyzer, a radiometer, and a network of thermometers in order to characterize the blackbody built. Results show a homogeneous temperature along the absorber surface, notably low reflections, and a very high emissivity.

Microwave radiometers are widely used for remote sensing and radio astronomical observations. Their absolute accuracy depends on the performance of the blackbody targets which are used for the radiometric calibration. In this paper we summarize the performance aspects of such calibration targets and present different designs for upcoming ESA missions.

Images can be made in different spectral ranges, particularly in the microwave, infrared and visible part of the spectrum. Each of these images contains different information about the scene. The goal is to combine interesting information of different images in one image, so that a fast recognition of all relevant information is possible. The aim of this paper is to illustrate the fundamental image processing steps applied to millimetre wave (MMW) images of our 91 GHz Scanning Polarimetric Imaging Radiometer (SPIRA). After a short description of the instrument we present three sections concerning image registration, image deblurring and image fusion.

The Scanning Polarimetric Imaging RAdiometer at 91 GHz with an angular resolution of 0.5° was used to investigate the dynamics of the atmosphere. We introduced a new imaging method by continuously scanning the sky over a range of elevation angles in a fixed azimuth direction. The measurements were realized during three different situations: clear sky, sky with water clouds, and sky with cirrus clouds. In most situations, the scan direction was nearly parallel to the mean atmospheric flow. Particularly interesting structures were found in the images with water clouds. In contrast, cirrus clouds are highly transparent. Simulations of the applied imaging method helped to interpret the cloud images, particularly concerning the cloud movement. Characteristic shapes were identified as signatures of motions along the scan line, which were used to estimate the horizontal velocity of water clouds. It was also possible to estimate the integrated water vapor from the clear sky images. They allow a visualization of water vapor parcels. Some of these images contain similar signatures as the clouds, indicating advection of water vapor along the scan line. In the future, we plan to extend these measurements and to combine them with multifrequency observations.

The Stratospheric WAter vapor RAdiometer (SWARA) is a microwave radiometer designed for ground-based measurements of water vapor$(\hbox{H}_{2}\hbox{O})$in the middle atmosphere (20 to 80 km), including the stratosphere and mesosphere. The instrument is operating in a noncryogenic balancing calibration mode. Since its deployment, features have been observed in the spectrum which can be attributed to resonant variations of the antenna pattern of the corrugated horn. This paper presents copolar and crosspolar antenna pattern measurements of two sister antennas of the SWARA horn, as well as water vapor measurements from both antennas on the ground-based microwave radiometerMIddle AtmosphericWAter vaporRAdiometer. We show that small irregularities in the frequency spectrum at the$-$20-dB level are visible in the copolar pattern, which, due to the balancing operation scheme used for the radiometer, lead to features in the spectrum that have the same or even higher brightness temperature as the line of interest.

The Institute of Applied Physics (IAP) of the University of Bern is active in the field of remote sensing of middle atmospheric trace gases such as ozone and water vapor by microwave radiometry. From the measured pressure broadened spectral lines, it is possible to retrieve the vertical distribution of the observed species. The vertical range is dependent on the bandwidth and resolution of the spectrometers. The radiometers from the IAP perform ground-based and airborne measurements. For the spectral analysis, they used acoustooptical spectrometers (AOS) or low-resolution filterbanks until recently. Unfortunately, AOS proved to be deficient under conditions encountered in an aircraft. For this reason, the approach of using novel real-time digital fast Fourier transform (FFT) spectrometers of bandwidth 1 GHz was chosen. In this paper, we present measurements of atmospheric trace constituents using digital FFT spectrometers and compare the results with measurements from the AOS and filterbank. The FFT spectrometer is superior in resolution and system stability as well as in the linearity and stability of the frequency axis. An important point is also the lower costs per bandwidth and resolution. The measured intensities of emitted radiation from the atmosphere from all spectrometer types were in good agreement.

The Scanning Polarimetric Imaging Radiometer (SPIRA) is a versatile fully polarimetric imager operating at 91 GHz. It is designed for measurements of polarimetric signatures of the Earth's surface and man-made objects. SPIRA combines a method for the measurement of the complete polarization state with a relatively fast high-resolution imager, which is suitable for a range of applications. The instrument measures all four Stokes parameters simultaneously and delivers images by mechanically scanning the scene with an elevation over azimuth scanner and an offset parabolic antenna. A two-channel heterodyne receiver is used for the reception of polarized radiation. The Stokes parameters are obtained by correlating two linear orthogonal-polarization components in a broadband analog adding correlator. In this paper, we present the design of the instrument and analyze its radiometric and polarimetric characteristics. The polarimetric calibration and a method for the characterization of the polarimetric calibration device are described. First polarimetric measurements are presented and discussed.