Amanda Cox

Errors resulting from the reflectivity of calibration targets

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurements are described that enable one to estimate the resulting uncertainty in the measured brightness temperature. The measurement results are presented for several combinations of antenna and calibration target. The resulting uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered.

Proposed Development of a National Standard for Microwave Brightness Temperature

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

Standard radiometers and targets for microwave remote sensing

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.

Initial Results from the Infrared Calibration and Infrared Imaging of a Microwave Calibration Target

As part of the ongoing effort at NIST to develop a microwave brightness temperature standard, we are exploring the electromagnetic and thermal characteristics of microwave calibration targets. We investigate the thermal properties of a microwave calibration target using NIST infrared calibration facilities and techniques. An infrared radiometer is used to measure the radiance at the surface of a microwave calibration target and to compare it with the values reported from contact temperature sensors. We find temperature gradients within the target greater than 1 K. Infrared imagery shows temperature gradients within the geometric structure of the target. Keywords-brightness temperature; infrared calibration; microwave calibration; microwave radiometry; radiometer calibration; remote sensing

Comparison of microwave black-body target radiometric measurements

Accurate characterization of the brightness temperature (TB) 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 TB 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 TB measurements.

Reflectivity studies of passive microwave calibration targets and absorptive materials

We report on the characterization of blackbody reflections as a part of the recent progress on the development of brightness standards for microwave remote sensing at National Institute of Standards and Technology (NIST). Three blackbody targets at variable temperatures used for airborne and/or satellite systems along with an aluminum plate were measured in terms of their reflection coefficients by horn antennas in connection with a vector network analyzer (VNA) in the WR-42 waveguide band. Precision measurements of reflection are needed for blackbody emissivity computation to check against the brightness temperature measurement of blackbody targets. All experiments were conducted in two distance ranges by free-space methods in an anechoic chamber. Preliminary results show negligible reflections from the calibration targets, indicating near ideal blackbody characteristics in the measured frequency range.

Errors due to the reflectivity of calibration targets

For a microwave total-power radiometer, we consider the error introduced by neglecting the difference in the antenna reflection coefficient between when it views a distant scene and when it views a nearby calibration target. An approximate expression is presented for the error, and measurement results are presented that enable one to estimate the resulting uncertainty in the measured brightness temperature. This uncertainty ranges from about 0.1 K to several kelvins for the representative cases considered