In this work, a novel 670-GHz integrated direct-detection receiver using 25-nm InP HEMT technology is presented. This is the first demonstration of an integrated direct-detection radiometer architecture at these frequencies. The receiver exhibits a noise figure of 11.4 dB with a total dc power consumption of 0.25 W. The integrated receiver measures only 0.8 cm × 1.3 cm × 4.8 cm (0.3” × 0.5” × 1.9”). These results show that transistor-based direct-detection receivers are a viable technology for submillimeter-wave applications, with low SWaP with no compromise in performance.

Satellite-based retrieval of forest soil moisture (SM) and vegetation optical depth (VOD) are two long-standing unresolved issues hindering advances in hydrology, ecology, and Earth system science. A key obstacle is the lack of adequate reference data in forested regions. NASA's Soil Moisture Active Passive (SMAP) mission, with its partners, conducted the SMAP Validation Experiment 2019-2022 (SMAPVEX19-22) to improve the SMAP SM and VOD retrievals in temperate forests of the northeastern USA. The scope and scale of the campaign exceeded anything done thus far to develop forest satellite-based SM and VOD retrieval algorithms. The field campaign measured SM, surface conditions, and vegetation properties, with results demonstrating the value of tree sensors with SM measurements and destructive sampling of the vegetation water content of branches and leaves to capture the water distribution in soil and trees. Using low-cost zenith-pointing cameras proved effective in tracking vegetation phenology, aiding the interpretation of brightness temperature (TB). Airborne and mobile terrestrial laser scanning measurements captured the three-dimensional forest structure necessary for microwave measurement interpretation. Challenges included characterizing SM in organic forest soils and determining volumetric SM due to spatially variable soil bulk density. Comparisons of the field measurements with SMAP data revealed its ability to retrieve the soil permittivity (correlation of 0.68 and 0.75 for the two experiment sites) alongside VOD, including the frozen conditions. The findings indicated that L-band scattering albedo is temporally variable, and L-band TB is sensitive to deciduous forest leaves, influencing the development of SM and VOD retrieval algorithms.

Calibration plays an important role in improving the accuracy of the microwave and millimeter-wave radiometric measurements. Several calibration techniques have been used in radiometers including external calibration targets, vicarious sources, and internal calibrators such as noise diodes or matched reference load. A new calibration technique based on deep learning has recently been developed to calibrate microwave and millimeter-wave radiometers. The deep-learning calibrator has been previously demonstrated on a computer noise-wave modeled Dicke-switching radiometer. This article applies the new deep-learning calibration technique for the calibration of the high-frequency airborne microwave and millimeter-wave radiometer (HAMMR) instrument. A deep-learning neural network model is built to calibrate the 2014 West Coast Flight Campaign antenna temperature measurements of the HAMMR. The deep-learning calibrator antenna temperature estimates are obtained from the radiometric measurements. The deep-learning calibration results are compared with the existing conventional calibration techniques used in HAMMR 2014 field campaign. The results have shown that the deep-learning calibrator is in agreement with the conventional calibration techniques. In this article, it is demonstrated that the deep-learning calibrator can be employed for calibrating the radiometers with high accuracy.

Global measurements of ice in clouds, both the amount and particle size distribution, are critically needed to reduce uncertainties in global climate models. The retrieval of this information is best achieved with a range of receiver channels across the submillimeter wave range. Advancement of low noise Indium Phosphide (InP) MMIC amplifier technology enabled us to develop miniature submillimeter-wave receivers for a CubeSat scale instrument that achieves 6 km spatial resolution. The heritage of InP MMICs includes continuous operation for four years in the PLANCK Low Frequency Instrument and providing world-record sensitivity with high reliability in several airborne instruments. We have developed a set of InP MMICs for receivers at 240, 310, 380 and 670 GHz with significantly lower noise than previously reported. The noise temperature is NT=450 K at 240 GHz, NT=550K at 310 GHz and NT=650 K at 380 GHz. The 670 GHz LNAs provide a NT of 2400 K. These channels are complemented by the previously-developed and airborne-proven 183 and 118 GHz MMIC receivers that were tested to TRL 6.

Global measurements of cloud ice particle size, total ice water content and water vapor content in the upper troposphere are critically needed to improve knowledge of the role of ice clouds in Earth's climate, precipitation and cloud processes. Such observations will enable improvement in cloud and moisture simulations in global climate models as well as precipitation forecasts on a global basis. Measurements at a range of frequencies in the millimeter- and submillimeter-wave frequency range provide sensitivity to ice particle size distribution in the range of tens to hundreds of micrometers. To perform this measurement on a global basis, a new millimeter- and submillimeter-wave instrument is currently under development with suitable mass, power and volume to be deployed on 6U-Class nanosatellites. To achieve miniaturized receivers in this frequency range, InP MMIC technology has been scaled to 25-nm gate length transistors, enabling demonstration of low-noise amplifiers with sufficient gain at frequencies up to 1 THz. This technology is being applied to develop direct-detection submillimeter-wave receivers at 240 GHz, 310 GHz and 670 GHz. InP MMIC technology has also enabled multipliers for local oscillators and subharmonic mixers for temperature and humidity sounding channels near atmospheric absorption lines at 118.75, 183.31 and 380.20 GHz.