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In this paper, we present the design and radiometric characterization of a water-based conical blackbody calibration target to be applied as a precise reference source in laboratories and for the accurate calibration of ground-based microwave instruments. The concept of aqueous blackbody targets circumvents the conflict between the electromagnetic and thermal properties of traditional calibration target designs. The temperature controllable target consists of a conical low loss plastic shell, which has been manufactured using a stereolithography three-dimensional printer to define the water–air interface. An advanced exponential profile of the shell is used to compensate the comparatively high refractive index of water and to obtain a high emissivity. Simulations and active measurements of the coherent backscattering S11 have been performed verifying its excellent electromagnetic properties. Furthermore, radiometric measurements at 110 GHz demonstrate the outstanding thermal performance at various target temperatures between 10 and 60 °C. The differences between the brightness temperature and the water temperature are measured to be less than 40 mK and therefore significantly smaller in comparison to traditional calibration targets. In addition, the measured baseline ripples in the spectra caused by the water-based calibration target are small compared to established blackbody concepts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

Advanced shapes for conical blackbody targets, as applied in millimeter wave remote sensing instruments, are designed and analyzed for frequencies between 20 and 600 GHz. We propose exponential shaped targets and show that exponential cones are superior to commonly used linear cones. Design and scattering analyses are carried out using the body of revolution method of moments and the finite-element method. Furthermore, a highly efficient ray-tracing approach is introduced and applied. Numerical studies regarding absorbing materials, beam waists, potential manufacturing artifacts, as well as several types of beam misalignment are performed to study practical challenges. Finally, starting from an exponential profile, the blackbody shape is further improved using numerical optimization techniques. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

The reliability of millimeter and sub-millimeter wave radiometer measurements is dependent on the accuracy of the loads they employ as calibration targets. In the recent past on-board calibration loads have been developed for a variety of satellite remote sensing instruments. Unfortunately some of these have suffered from calibration inaccuracies which had poor thermal performance of the calibration target as the root cause. Stringent performance parameters of the calibration target such as low reflectivity, high temperature uniformity, low mass and low power consumption combined with low volumetric requirements remain a challenge for the space instrument developer. In this paper we present a novel multi-layer absorber concept for a calibration load which offers an excellent compromise between very good radiometric performance and temperature uniformity and the mass and volumetric constraints required by space-borne calibration targets. [ABSTRACT FROM AUTHOR]

We present a conceptual prototype model of a focal plane array unit for the STEAMR instrument, highlighting the challenges presented by the required high relative beam proximity of the instrument and focus on how edge-diffraction effects contribute to the array's performance. The analysis was carried out as a comparative process using both PO & PTD and MoM techniques. We first highlight general differences between these computational techniques, with the discussion focusing on diffractive edge effects for near-field imaging reflectors with high truncation. We then present the results of in-depth modeling analyses of the STEAMR focal plane array followed by near-field antenna measurements of a breadboard model of the array. The results of these near-field measurements agree well with both simulation techniques although MoM shows slightly higher complex beam coupling to the measurements than PO & PTD. [ABSTRACT FROM AUTHOR]

The Scanning Polarimetric Imaging RAdiometer at 91 GHz with an angular resolution of 0.5 ^\circ 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. [ABSTRACT FROM AUTHOR]

This paper presents the design, manufacturing, and characterization of a waveguide based on electromagnetic-bandgap (EBG) technology working in W-band. A modified silicon EBG woodpile structure was used in order to improve the matching performance of the EBG waveguide to a standard rectangular waveguide. The transition between the silicon EBG woodpile waveguide and the conventional WR10 waveguide was optimized and a 13.5% bandwidth around 90 GHz was achieved. The measured insertion losses remained better than 3 dB in the overall working bandwidth. [ABSTRACT FROM AUTHOR]

Results are presented for the telescope optics verification of the TELlS instrument which is developed and constructed at the Deutsches Zentrum für Luft- und Raumfahrt (DLR), Germany. TELlS is a balloon-borne limb-sounding radiometer which measures pressure-broadened emission lines of various species of the earth's atmosphere. Due to the strong brightness gradient in the limb-viewing geometry the antenna beam of the instrument needs to be well-defined in order to ensure proper operation. Near-field measurements made at 637 GHz of the TELlS telescope were used to derive the far-field pattern. Error compensation included correction of the phase error of the near-field data due to cable flexing. The phase error due to cable flexing was measured using a technique based on heterodyne reception of the signal reflected at the end of the cable. The results are compared to an optics model based on physical optics. Discrepancies between measurements and the model are resolved by misaligning the mirrors in the optics model while minimizing the deviations from theory and experiment. [ABSTRACT FROM AUTHOR]

The Institute of Applied Physics (lAP) 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. [ABSTRACT FROM AUTHOR]

The design, fabrication, and characterization of a 500-GHz electromagnetic bandgap (EBG) based heterodyne receiver array is presented. The array contained seven planar dipole antennas that were photolithographically defined on a common 2Oµm-thick quartz substrate. Each antenna incorporated a Schottky diode and was connected to coplanar transmission lines that conveyed the down-converted 500-GHz signals to detectors. The quartz substrate was backed by a silicon EBG woodpile structure, which reduced antenna crosstalk and increased directivity. An off-axis parabolic mirror completed the beam-forming network. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We propose a new quasi-optical setup for a nulling interferometer at short millimeter and submillimeter wavelengths employing a single Martin-Puplett interferometer (MPI) for side- band separation. The new design allows the simultaneous use of both sidebands, thus avoiding the loss of sensitivity from conventional sideband filters. Although intended for solar flare observations, the quasi-optical setup can be applied to cancel any extended circular source, while at the same time preserving the full sensitivity toward a point-like source on or near the extended one. Numerical simulations show that the sinusoidal transmission characteristic of the MPI limits the usable relative bandwidth to one-third of the IF. Compared to an instrument employing lossless sideband filters, an increase of the response to a point-like continuum source by a factor of ∼1.6-2 is expected, depending on the source position and relative bandwidth. [ABSTRACT FROM AUTHOR]

In the first half of this paper, an efficient method to calculate transmission and reflection characteristics of wire-grids consisting of periodic arrays of cylindrical conductors for arbitrary directions of incidence is presented by extending exact theory of wire-grids proposed by Yasumoto et al. In the latter half of this paper, an approximate calculation method for wire-grids with slight random irregularity in their spacings between wires is proposed by treating the irregularity as a small perturbation to the theory for regular grids. A comparison with preliminary measurements for actual wire-grids in millimeter- and submillimeter-wave regions demonstrated that this method is effectively applicable to wire-grids whose irregularity is not too much. [ABSTRACT FROM AUTHOR]

This paper presents a new configuration of quasi-optical polarization-rotating dual-beam interferometer, which uses a pair of frequency-selective polarizers (FSPs) consisting of a wire-grid placed in front of a flat mirror, and has a function similar to the conventional Martin-Puplett interferometer (MPI). Advantages of this new configuration over the conventional MPI are lower residual reflection at the input and output ports and suitability to fixed-tuned applications. An experiment has shown it to have an MPI-like frequency characteristic as calculated. Careful machining was successful in achieving accuracy needed for a specified filter characteristic. This FSP-based quasi-optical device is to be used as a sideband separator in a space-borne submillimeter receiver for atmospheric research. [ABSTRACT FROM AUTHOR]