We directly quantify the effect of infrequent calibration on the stability of microwave radiometer temperature measurements (where a power measurement for the unknown source is acquired at a fixed time, but calibration data are acquired at variable earlier times) with robust and nonrobust implementations of a new metric. Based on our new metric, we also determine a component of uncertainty in a single measurement due to infrequent calibration effects. We apply our metric to experimental data acquired from experimental ground-based calibration data acquired from a NASA millimeter-wave imaging radiometer and a NIST radiometer (Noise Figure Radiometer-NFRad). Based on a stochastic model for the NFRad, we determine the random uncertainty of an empirical prediction model of our stability metric by a Monte Carlo method. For comparison purposes, we also present a secondary metric that quantifies stability for the case where calibration data are acquired at a fixed time, but power measurements for the unknown source are acquired at variable later times.

Background (what): SI (International System of Units)-traceable Microwave Radiometer calibration; Motivation (why): NWP (Numerical Weather Prediction), FCDR (Fundamental Climate Data Record); Technology (how): NIST (National Inst. of Standards and Technology) blackbody target for ; Standards: Status & Future Plans. Development of a National Standard for Microwave Brightness Temperature (TB) at NIST.

We discuss the analytical derivation of the absolute brightness temperature uncertainty of a hollow conical blackbody source for radiometer calibration. We introduce a Monte Carlo uncertainty propagation analysis method to quantify uncertainty contributions from nonideal emissivity, physical temperature, and antenna pattern, which are the three major factors contributing to the uncertainty of the brightness temperature radiation from the source. The low reflectance of the hollow conical geometry depends on multiple bounces on the absorber surface. To quantify total brightness temperature over a nonuniform temperature surface, each individual bounce must be considered. We derive a recursive analytical relationship to quantify this multiple-bounce effect as a function of a view angle. The resulting effective blackbody brightness temperature uncertainty is a function of frequency, temperature, antenna pattern, measurement distance, and the measurement environment. We also propagate the uncertainty to the antenna flange of a radiometer viewing the conical blackbody. This includes additional effects, such as spillover, illumination efficiency, and antenna efficiency. We demonstrate the propagation method with an example case. We use fictional input values to investigate the response of the uncertainty to input variables and distance. We find that as the distance between antenna and blackbody increases, the uncertainty due to spillover dominates, but at close distances, the dominant uncertainty contributor is linked to the physical temperature of the absorber.

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.

This paper describes the development of a waveguide radiometer to measure noise from millimeter wave electronic components from 75 GHz to 110 GHz. The radiometer will estimate the noise temperature of a device under test (DUT) based on comparison with room temperature and 77K noise standards. This is a standard physical approach in other NIST microwave radiometers. The radiometer is particularly amenable to performing noise temperature as well as noise parameter measurements for amplifier and transistor characterization. As wireless communications progresses towards millimeter wave systems, noise characterization of related components and subsystems becomes essential. We report our progress in radiometer design, construction and verification for millimeter wave noise metrology at NIST.

A modified measurement technique and nonlinear least-squares solution method is introduced for determining complex permittivity and permeability in transmission lines. In addition to a two-port S-parameter measurement, a one-port measurement of the shorted sample is employed. For low-loss samples, material property determination using the traditional method can be plagued by resonances when the sample thickness is an integer multiple of the guided half-wavelength in the sample medium. The introduction of the shorted reflectivity measurement reduces the large uncertainties inherent in the resonance effect, and increases the likelihood of determining the correct zero in the fitting of the material properties. The short-circuited sample S 11 measurement has an increased magnitude compared with the standard two-port measurement, which greatly reduces the uncertainty of this term. The resulting complex permeability and permittivity values obtained with this method are smoother improved solutions, and have lower uncertainty. Results are presented comparing the traditional Nicolson-Ross-Weir solution and National Institute of Standards and Technology iterative method with the proposed technique for a cast epoxy and a ferrite-loaded microwave absorber sample in WR42 waveguide. We also investigate finite element simulated S-parameters of a fictional material and compare the derived material parameters obtained with the proposed method to the "true" permittivity and permeability values.

We outline the theoretical formulation of radiometry of the free-space radiation emitted by a blackbody target. Simulation shows a much smaller drop of radiation intensity of a Lambertian source than that of an incoherent source in the near-field region, indicative of a powerful influence produced by the coherence property of the blackbody source. Further, the coupling of the radiation to a radiometer is formulated by the plane-wave scattering theory of the radiation field.

This paper discusses the design of a microwave blackbody to be used as a primary laboratory standard for passive remote sensing applications. This temperature adjustable design is required to operate and be fully characterized from 10 to 220 GHz. We discuss the challenges involved in designing this type of calibration source and address how improvements can be made to increase performance over blackbodies typically flown on airborne and space-borne instruments. A simplified electromagnetic model for absorber layer optimization is introduced as the precursor to a finite-element, full-wave solution for the calculation of emissivity. A temperature simulation predicts the physical temperature of the blackbody and surrounding chamber. The simulated data are used as inputs to a rigorous calculation of the microwave brightness temperature radiated by the blackbody source. This calculation provides an estimate of the offset between measured physical temperature and radiometrically measured brightness temperature.

We apply coherence-propagation theory to model the radiation generated by a planar passive thermal source of any state of coherence. Of our particular interest is the blackbody calibration source with partially coherent characteristic that produces the radiant intensity with its angular distribution following Lambert's cosine law in the far field. A closed-form expression of the Poynting vector of the electromagnetic field is obtained from the theoretical framework. The formulation links the radiation field to the correlation function of the sources in a straightforward manner, though numerical computation of the Poynting vector involves evaluation of a quadruple integral and is difficult to implement directly, especially when the observation of radiation occurs at a close distance from the source. The study of the close-range radiation would, in particular, benefit the microwave remote sensing radiometric calibration that is encountered in terrestrial laboratories and space-borne satellites. To tackle the challenges in numerical calculation, we have made a few mathematical adjustments to develop a feasible scheme for improved computational efficiency, including reformulation in the angle-impact notation and various simplifications of the integration. We apply the theory and numerical techniques to simulate thermal radiation in some illustrative examples such as an isothermal blackbody source, a blackbody misaligned from the on-axis position, and a nonuniformly heated blackbody target. The coherence property of the blackbody source is shown to possess influential impacts on the radiation arising from such a source, especially in the near-field range where most measurements of the radiation take place in a practical system. The theory and technical approaches provide a systematic and reliable way to quantify the Poynting vector radiated by the blackbody source in a microwave remote-sensing radiometer.

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.

Microwave brightness temperature (Tb) observations by satellite sounders represent some of the most important input data for numerical weather prediction (NWP) models as well as providing vital records for climate trend detection. Each flight instrument, even if built to nominally identical specifications, has a unique calibration, typically based on the implicit assumption that the on-board black body target is well understood and characterized, but even more importantly that the transfer function between the Tb received at the antenna and the thermometry embedded in the target is well determined.

We present a geometric error model associated with calibration-target misalignment in passive microwave remote-sensing systems. The developed analytic formulation is universally applicable to both lateral and rotational misalignment conditions. Numerical simulations are performed on two practical blackbody targets of different sizes used as radiation references for passive microwave remote sensing. The significance of this work is to furnish a framework of uncertainty analysis due to target misalignment and to provide a reference for alignment requirements based on passive radiometer measurement sensitivity.

We introduce a method to determine the gradient between measured physical temperature and true radiating surface temperature of a passive microwave calibration target (load or blackbody). An empirical cooling-curve fit is employed to determine heat-transfer coefficients that then allow commercial finite-element software to solve for the physical temperature at the surface of the target. Only gradients in the direction parallel to the target's pyramidal structures are determined. Two target insulation thicknesses are investigated and a mean surface radiating temperature is determined. This surface temperature differs from the internally measured physical temperature by a maximum of 0.3 K in an ambient environment. Use of a thicker insulation assembly decreases this temperature bias by 0.1 K.

We report measurements and uncertainty analysis on a cryogenic low-noise amplifier (LNA) with a very low noise temperature (NT), among the lowest noise performances reported at microwave frequencies. The LNA consists of three stages of InP high electron mobility transistors with a gate length of 130 nm. It exhibits about 44 dB gain and less than 2 K average NT in the operational band of 4 GHz to 8 GHz. A detailed uncertainty analysis is outlined to evaluate a variety of error sources in the measurement. The calculated uncertainty shows as low as 0.1 dB on the measured gain of about 44 dB and 0.18 K on the measured NT of 1.65 K, indicating excellent measurement accuracy. A breakdown of the uncertainty components helps identify the major causes of the overall uncertainty and enlightens us about how to further improve accuracy. It is important to know the actual physical temperature of the passive termination that is used as a cryogenic noise source in experiments. Due to its large temperature gradients, the commercial matched load is replaced by a custom-made attenuator that is isothermal and consequently provides reliable NT measurements of the LNA. The precision measurement technique developed at the National Institute of Standards and Technology is independent from the manufacturers' characterization method. This study marks the first time that such a low NT from a cryogenic LNA is verified independently with such a low uncertainty.

We compare three methods of quantifying illumination efficiency (IE). The ratio IE describes the contribution of energy emitted from a blackbody target to the total energy measured at an antenna aperture in a free-space microwave calibration target radiometric measurement. Measurements are compared at three frequencies: 18 GHz, 22.5 GHz, and 26 GHz. An antenna pattern integration method is compared with a recently developed target-temperature fitting method. These two experimental approaches are also compared to a computational antenna pattern simulation. Results show that the simulation agrees with the experimental fitting method more closely at far-field distances, whereas the antenna pattern integration and experimental fitting method agree at closer distances.

We describe and demonstrate a standard radiometer for making microwave brightness-temperature measurements that are traceable to fundamental noise standards. The standard radiometer is based on a National Institute of Standards and Technology (NIST) waveguide radiometer for 18-26.5 GHz, fitted with an antenna to measure radiated power. The fraction of the antenna pattern subtended by the radiating target is determined by anechoic-chamber measurements in which we vary the temperature of the target and measure the received power. Sample measurement results with uncertainties are presented. The typical standard uncertainty for a brightness temperature of around 340 K is about 1 K. The approach should be extendable to other waveguide bands where NIST has radiometers and standards.