The paper presents a commercially available, low-power digital spectrometer Application Specific Integrated Circuit (ASIC). We describe the ASIC architecture, its implementation aspects as well as board level spectrometer solutions. ASIC application examples as well as testing data are also presented. Data show adequate performance parameters for spectroradiometer applications at exceptionally low Size, Weight and Power (SWaP), compared to solutions based on off-the-shelf components. The ASIC development was funded through several NASA Small Business Innovative Research (SBIR) program awards.

Future smart sensors will be able to utilize the maximum information content from data products, while minimizing the resources required to acquire, downlink, and process data. In-orbit calibration is required for space-borne radiometers in order to correct for gain fluctuations. Many sensors like radiometers are generally only able to produce calibrated scene measurements after reaching steady state. Waiting to reach thermal equilibrium to obtain useful data results in wasted power, excess useless data, and delays in obtaining useful data. Instrument power cycling provides a way to lower power use, but at the cost of pauses in data collection when the instrument is cycled off. Rapid power cycling can be used to reduce the average power draw of a radiometer, at the cost of increased measurement uncertainty. These power cycling techniques have been used on real systems, including the IceCube radiometer [1]. Using a convolutional neural network trained on synthetic data, a simulated radiometer can produce calibrated measurements with lower uncertainties and errors than conventional least-squares-regression (LSR) - based estimators. This approach presents an opportunity to reduce the average power draw of a radiometer by minimizing uncertainties of calibrated data products collected during rapid power cycling.

We present a comprehensive Earth Planetary Boundary Layer temperature and water vapor retrieval improvement demonstration by the use of hyperspectral microwave measurements. Our results indicate that the use of a hyperspectral sampling in the oxygen and water vapor sounding lines alone provides significant improvements in the lower and free tropospheric thermodynamic fields (up to 40%), when compared against the program of record (i.e., the Advanced Technology Microwave Sounder, ATMS). Our experiments also demonstrate the essential role played by extending the coverage in the so called spectral window regions, leading to an overall PBL temperature and water vapor improvement of up to 50%.

In the future, smart sensors will be designed to extract maximum value information, while minimizing the resources required to acquire, downlink, and process data. Many sensors like radiometers are only able to produce calibrated measurements after reaching steady state. However, waiting until reaching thermal equilibrium to obtain useful data leads to wasted power, excess useless data, and delays in obtaining useful data. Power cycling a radiometer is one way to circumvent this requirement, but leads to other challenges, as turning power off to instrument not only stops data acquisition until it is powered on, but also past power-on until it reaches thermal equilibrium again. This paper introduces a framework which will use machine learning algorithms to enable the calibration of a radiometer during its transient state after power-on and in the presence of power cycling, aiming to further reduce resource utilization.

This paper presents an overview of the Hyperspectral Microwave Photonic Instrument (HyMPI), a 2021 NASA Instrument Incubation Proposal funded project aimed at developing the very first hyperspectral microwave sensor to augment thermodynamic sounding capability from space, with a focus on the Earth's Planetary Boundary Layer. This research responds to the recommendation expressed in the 2018 National Academies of Sciences decadal survey to accelerate the readiness of high-priority PBL observables not feasible for cost-effective spaceflight in 2017–2027. This paper provides an overview on HyMPI's design, configured as the objective instrument concept needed to fly in the future PBL mission and presents preliminary trade studies aim at demonstrating HyMPI's enhanced thermodynamic sounding skill in the Earth's Planetary Boundary Layer over conventional microwave sounders from the current Program of Record.

We present an overview of the Hyperspectral Microwave Photonic Instrument (HyMPI), a NASA Instrument Incubation Proposal funded research project aimed at developing a hyperspectral microwave instrument intended for enhanced remote sensing of atmospheric temperature and water vapor from space. This paper provides preliminary results on HyMPI's spectral and noise characteristics and a preliminary demonstration of its enhanced water vapor sensitivity and vertical resolution, with a particular focus on the Earth's Planetary Boundary Layer.

Modern multi-band radiometer imagers are designed trading spatial resolution, spectral coverage, and surface sampling characteristics to optimize science return. In this process, trades must be made to usually under-sample the Earth scene. Care is taken to obtain contiguous 3-dB edge-to-edge coverage, but even so, such a design still aliases high spatial frequency content in the image.

A tri-frequency microwave synthetic aperture radar (SAR), designed for the estimation of snow water equivalent (SWE), was recently developed as part of the SWESARR (Snow Water Equivalent Synthetic Aperture Radar and Radiometer) instrument. The SAR operates at 9.65 GHz, 13.6 GHz, and 17.25 GHz, and at two polarizations (VV, VH), with a nominal bandwidth of 140 MHz. The SAR was first flight tested in December 2018 and later, along with SWESARR's radiometer, participated in the SnowEx science flight campaign in the fall 2019. During these flights, the SAR collected comprehensive data sets of a variety of terrains, including calibration sites where several trihedral corner reflectors had been deployed. Analysis of these data sets indicated the radar performed according to the design specifications.

Although considered as stationary and Gaussian in general, radiometer gain is usually a fluctuating signal with non-stationary properties. Analyses of such non-stationary features is challenging as the radiometer signal cannot be observed independently. On the other hand, time series of post-gain voltages constitute an ensemble set for the radiometer gain which can be used to characterize the radiometer gain. This paper presents a novel technique called "Ensemble Detection" which can analytically retrieve the standard deviation of stationary Gaussian radiometer gain or find an equivalent stationary Gaussian process which represents the non-stationary radiometer gain under different calibration schemes. It has been found that the equivalent Gaussian process for non-stationary radiometer gain heavily depends on the calibration structure and the observation times of the measurand and the calibration references.

Multiband passive microwave imagery in X to W Bands has a nearly 40-year history of utilization for measurement of multiple geophysical parameters (e.g., precipitation rate, ocean surface wind speed, sea ice concentration, and land surface temperature). Spatial resolution is limited by aperture size, and although aperture sizes have grown to 1-2 meters, current capability will not meet future spatial resolution needs. As aperture size increases, new antenna feed techniques are needed to maintain contiguous coverage and obtain Nyquist sampling. Here we apply the correlator array-fed radiometer architecture adapted from radio astronomy and show how it can meet emerging needs. Simulation results of a 0.8-m, 36.5-GHz, array-fed reflector (equivalent to 20 meters at 1.41 GHz) show the feasibility of creating multiple over-lapping beams.

A new airborne synthetic aperture radar (SAR) system was recently developed for the estimation of snow water equivalent (SWE). The radar is part of the SWESARR (Snow Water Equivalent Synthetic Aperture Radar and Radiometer) instrument, an active passive microwave system specifically designed for the accurate estimation of SWE. The dual polarization (VV, VH) radar operates at three frequency bands (9.65 GHz, 13.6 GHz, and 17.25 GHz), with bandwidths of up to 200 MHz. The radar flew its first flight campaign in November 2019, along with SWESARR's -already operational - radiometer. The radar collected comprehensive data sets over various terrains that show a successful system performance. The instrument is slated to participate in future SnowEx campaigns.

An optimal estimation scheme is employed to demonstrate the utility of using multi-band radar observations for estimating supercooled liquid profiles. Qualitative comparisons with microphysical probe images show that the retrievals are capable of producing supercooled liquid consistent with in situ data. Finally, a path forward for quantifying performance and extending the study to a more robust measurement suite is given.

Tracking the radiometer calibration stability is very important for climate monitoring radiometers as long term accuracy of observations is needed to create reliable climate models. This presentation discusses the advantages of 3-point onboard calibration techniques over 2-point methods to track radiometer calibration stability.

Recent technology advances have profoundly changed the landscape of modern radiometry by enabling miniaturized, low-power, and low-noise radio-frequency receivers operating at frequencies near 200 GHz and beyond. These advances enable the practical use of receiver arrays to multiplex multiple broad frequency bands into many spectral channels. We use the term “hyperspectral microwave” to refer generically to microwave sounding systems with approximately 50 spectral channels or more. In this paper, we report on the design and analysis of the receiver subsystem (lensed antenna, RF front-end electronics, and IF processor module) for the Hyperspectral Microwave Atmospheric Sounder (HyMAS) comprising multiple receivers near the oxygen absorption line at 118.75 GHz and the water vapor absorption line at 183.31 GHz. The hyperspectral microwave receiver system will be integrated into a new scanhead compatible with the NASA GSFC Conical Scanning Microwave Imaging Radiometer/Compact Submillimeter-wave Imaging Radiometer (CoSMIR/CoSSIR) airborne instrument system to facilitate demonstration and performance characterization under funding from the NASA ESTO Advanced Component Technology program. Four identical radiometers will be used to cover 108-119 GHz, and two identical receivers will be used to cover 173-183 GHz. Subharmonic mixers will be driven by frequency-multiplied dielectric resonant oscillators, and single-sideband operation will be achieved by waveguide filtering of the lower sideband. A relatively high IF frequency is chosen to facilitate miniaturization of the IF processor module, which will be fabricated using Low Temperature Co-fired Ceramic (LTCC) technology. Corrugated feed antennas with lenses are used to achieve a FWHM beamwidth of approximately 3.5 degrees. Two polarizations are measured by each feed to increase overall channel count, and multiple options will be considered during the design phase for the polarization diplexing approach. Broadband operation over a relatively high intermediate frequency range (18-29 GHz) is a technical challenge of the front-end receiver systems, and a receiver temperature of approximately 2000-3000K is expected over the receiver bandwidth. This performance, together with approximately 100-msec integration times typical of airborne operation, yields channel NEDTs of approximately 0.35K, which is adequate to demonstrate the hyperspectral microwave concept by comparing profile retrievals with high-fidelity ground truth available either by coincident overpasses of hyperspectral infrared sounders and/or in situ radiosonde/dropsonde measurements.

The Digital Beamforming Synthetic Aperture Radar (DBSAR) is a state-of-the-art L-band radar that employs advanced radar technology and a customized data acquisition and real-time processor in order to enable multimode measurement techniques in a single radar platform. DBSAR serves as a test bed for the development, implementation, and testing of digital beamforming radar techniques applicable to Earth science and planetary measurements. DBSAR flew its first field campaign on board the National Aeronautics and Space Administration P3 aircraft in October 2008, demonstrating enabling techniques for scatterometry, synthetic aperture, and altimetry.

Characterization of non stationary and nonlinear processes is a challenge in many engineering and scientific disciplines. Climate change modeling and projection, retrieving information from Doppler measurements of hydrometeors, and modeling calibration architectures and algorithms in microwave radiometers are example applications that can benefit from improvements in the modeling and analysis of non stationary processes. Analyses of measured signals have traditionally been limited to a single measurement series. Ensemble Detection is a technique whereby mixing calibrated noise produces an ensemble measurement set. The collection of ensemble data sets enables new methods for analyzing random signals and offers powerful new approaches to studying and analyzing non stationary processes. Derived information contained in the dynamic stochastic moments of a process will enable many novel applications.

Ten underflights of the Special Sensor Microwave Imager/Sounder (SSMIS) with Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) onboard the NASA ER-2 aircraft were conducted over the coastal region of California between March 2004 and March 2005. The measured brightness temperature (Tb) values from both sensors are collocated and compared at frequencies of 50.3, 52.8, 53.6, 91.655, 150, 183.3 plusmn 1,183.3 plusmn 3, and 183.3 plusmn 6.6 GHz. The more transparent channels at 50.3, 91.655, and 150 GHz are strongly affected by the changes in surface emission and low-level liquid clouds. Thus, the average differences in Tbvalues (deltaTb), measured by the two sensors, and their changes from flight to flight are difficult to assess. For the remaining opaque channels, using the CoSMIR measurements as reference, the lowest SSMIS Tbvalues occur when the SSMIS is completely under the Earth's shadow. As the satellite moves out of the Earth's shadow in the ascending passes, the SSMIS Tbvalues are found to gradually increase with more exposure to the sun. The magnitudes of these Tb changes are about 4-5 K for the three 183.3-GHz channels and about 2 K for the 52.8- and 53.6-GHz channels.

An airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) was developed recently for calibration/validation of the new-generation DMSP F-series microwave radiometer, the Special Sensor Microwave/Imager/Sounder. The CoSMIR is a total-power radiometer that measures radiation at nine channels over the frequency range of 50–183 GHz. The instrument employs a two-axis gimbaled mechanism to generate the conical scan with periodic calibration. Its scan geometry is software programmable and can be designed to serve the scientific requirements of an experiment. A series of CoSMIR flights was conducted over the coastal regions of California in March and December of 2004, in which the instrument was programmed to acquire both conical and across-track scan data sets simultaneously. Two of these flights on March 25 and December 2 contained segments over the snow-covered Sierra Mountain Range and were selected to demonstrate the novel features of this new instrument.

Results of concurrent airborne measurements of snowfalls with the Millimeter-wave Imaging Radiometer (MIR) and the Airborne Cloud Radar (ACR) are reported in this paper. The measurements were obtained during January-February 2003 in a field experiment in Wakasa Bay, Japan. The MIR is an imaging radiometer that measures radiation at seven channels between 89 GHz and 340 GHz. The ACR operates at 94 GHz and provides nadir-viewing radar reflectivity profiles with very good accuracy. Three days of snowfall events were observed during this field deployment. It was found that the MIR brightness temperature depressions at the frequencies of 183.3/spl plusmn/1, 183.3/spl plusmn/3, 183.3/spl plusmn/7, and 340 GHz strongly correlate with the ACR reflectivity profiles during these snow events. Radiometric signatures from the remaining less opaque channels of the MIR (89, 150 and 220 GHz) showed ambiguity in snowfall detection due to variation in surface emissivities. An attempt to retrieve the ice water path and the median mass equivalent sphere diameter of the snowfalls would be described and results discussed.

Based on radiometric and radiosonde observations made at the North Slope of Alaska, during March 1991, comparisons of measured and calculated brightness temperatures are made for frequencies ranging from 23.8 to 340 GHz. Three clean air absorption models of Liebe (1987 and 1993) and Rosenkranz (1998) are used.

Results of the first science flight of the airborne Conical Scanning Millimeter-wave Imaging Radiometer (CoSMIR) for high-altitude observations from the NASA ER-2 is discussed. Imagery collected from the flight demonstrates CoSMIR's unique conical/cross-track imaging mode and provides comparison of CoSMIR measurements to those of the SSM/T-2 satellite radiometer.

Radiometers require periodic calibration to correct for instabilities in the receiver response. Various calibration techniques exist that minimize the effect of instabilities in the receivers. The optimal technique depends upon many parameters. Some parameters are constrained by the particular application and others can be chosen in the system design. For example, the measurement uncertainty may be reduced to the limits of the resolution of the measurement (sensitivity) if periodic absolute calibration can be performed with sufficient frequency. However if the period between calibrations is long, a reference-differencing technique, i.e. Dicke-type design, can yield better performance. The measurement uncertainty not only depends upon the detection scheme but also on the number of pixels between calibrations, the integration time per pixel, integration time per calibration reference measurement, calibration reference temperature, and the brightness temperature of what is being measured. The best scheme for reducing the measurement uncertainty also depends, in large part, on the stability of the receiver electronics. A framework for evaluating calibration schemes for a wide range of system architectures is presented. Two methods for treating receiver non-stationarity are compared with radiometer measurements.

Concurrent measurements of atmospheric water vapor profiles were conducted over the Atlantic Ocean on September 25, 1995 with both the Millimeter-wave Imaging Radiometer (MIR) and Lidar Atmospheric Sounding Experiment (LASE) on board the NASA ER-2 aircraft. LASE provides high precision measurements of both aerosol backscatter and water vapor profiles; aerosol backscatter has a vertical resolution of 60 m while the water vapor profiles have a resolution of 330 m in the low-to-mid troposphere and 550 m in the upper troposphere. Therefore, LASE measurements provide an excellent resource for assessing the capabilities and limitations of MIR as a water vapor profiler. Previously, the water vapor profiles retrieved from the MIR measurements have been compared with those of rawinsonde and Raman lidar observations at point locations. The frequency and extent of the comparisons made in that fashion were largely constrained by the requirement of near coincidence in time and space. The data acquired concurrently by MIR and LASE from this ER-2 aircraft flight enable the comparison of MIR-retrieved and LASE-measured moisture profiles over a long stretch of time and space. In addition, the LASE-measured profiles of aerosol backscatter provide a resource to assess the impact of clouds on the retrieval of water vapor profiles from the MIR measurements. It is shown that profiles of water vapor mixing ratio retrieved from the MIR data generally conform to those measured by the LASE; however, differences in the values of mixing ratio at individual altitude levels are quite often not small. The standard deviations of these differences are found to be about. /spl plusmn/0.98, /spl plusmn/0.84, /spl plusmn/0.95, /spl plusmn/0.42, and /spl plusmn/0.06 g/kg at altitudes of 1.25, 2.75, 4.75, 7.25, and 10.25 km. It is demonstrated that a substantial portion of these differences are due to the poor vertical resolution inherent in the profile retrieval using the MIR radiometric measurements. Additionally, MIR water vapor profiling under cloudy conditions is demonstrated, and it is shown that location and height of the low-altitude clouds estimated from the retrieval process were generally consistent with those observed by the LASE. For study cases where cirrus clouds are present, retrievals from the MIR data over-estimate the mixing ratio; this over-estimate is provoked by brightness temperature decreases that occur at 183-220 GHz within these regions. Undoubtedly, the retrieval method needs an additional procedure to account for the mm-wave scattering by cloud ice particles so that water vapor profiling can be improved within regions where cirrus clouds are present.

The airborne Millimeter-wave Imaging Radiometer (MIR) measurements conducted over the Midwest region of the continental United States during January/February 1997 and over the Alaska-Arctic region during May 1998 are used to estimate column water vapor W<0.8 g/cm/sup 2/ under a clear sky. On board the same aircraft are two other instruments, the Cloud Lidar System (CLS) and MODerate-resolution Imaging Spectrometer (MODIS) Airborne Simulator (MAS), which provide cloud cover information and independent measurements of W, respectively. The MIR-estimated W values are compared and found to be in very good agreement with those measured by rawinsondes at near concurrence. A close correlation is found between the MIR-estimated W and that estimated from the MAS near-IR reflectance ratios. Water surface emissivities at several MIR frequencies are obtained in the process of the W retrieval from several flights over the Midwest lakes. These estimated emissivities compared favorably with values calculated for a calm water surface, which are based on a di-electric permittivity model and MAS-measured surface temperatures. The results from all comparisons strongly demonstrate the soundness of the technique for estimating W.

Millimeter-wave radiometric measurements obtained from the NASA ER-2 aircraft over the arctic region on May 20, 1998, were used to estimate precipitable water (PW) in the range/spl les/0.60 g/cm/sup 2/. The approach is a modified version of the recent work by J. Miao (1998), which utilized the radiometric measurements at 150, 183.3/spl plusmn/3, and 183.3/spl plusmn/7 GHz of the SSM/T-2 sensor to retrieve PW over the antarctic region. However, Miao has implicitly assumed a surface emissivity that is frequency independent over the 150-183 GHz range. This assumption turns out not to be a good one based on the airborne measurements described below and the errors introduced in the PW estimation were substantial in many cases. It is shown below that four-frequency radiometric measurements in the frequency range of 150-220 GHz provided a robust retrieval of PW, while allowing for a surface emissivity that varied linearly with frequency. The retrieved PW compared favorably with that calculated from rawinsonde data at two widely separated locations. The differences between the retrieved and calculated values are not more than /spl plusmn/0.02 g/cm/sup 2/, which is smaller than errors associated with measurement uncertainty. It is found necessary to account for the double side-band nature of the 183.3 GHz measurements in the radiative transfer calculations for development of the retrieval algorithm. The PW values estimated from the algorithm developed from single side band, 183.3 GHz radiative transfer calculations could be in error by as much as /spl plusmn/0.10 g/cm/sup 2/. Finally, the effect of surface temperature variations is shown to introduce only a small error in the estimation of PW.

Three subjects related to atmospheric water vapor profiling using the 183.3 GHz absorption line are discussed. First, data acquired by an airborne millimeter-wave imaging radiometer (MIR) over ocean surface in the western Pacific are used to estimate three-dimensional (3D) distribution of atmospheric water vapor. The instrument's radiometric measurements with mixed vertical and horizontal polarizations require modifications to the retrieval algorithm used in the past. It is demonstrated that, after the modifications, the new algorithm can provide adequate retrieval of water vapor profiles, even though the measured data are of mixed polarizations. Next, the retrieved profiles, in terms of water vapor mixing ratio /spl rho/ (g/kg), are compared with those measured in near concurrence by dropsondes from a research aircraft in the western Pacific and by a ground-based Raman lidar at Wallops Island, Virginia. The ratio of the standard deviation to the mean /spl rho/ is found to be 0.12 at 0.25 km altitude and gradually degraded to 0.67 at the highest altitude of the retrieval of 10.25 km. Finally, the effect of the "initial guess" relative humidity profile on the final retrieved product is analyzed with respect to the condition for the convergent retrieval. It is found that the effect is minimal if the initial profile is not unrealistically different from the true one. If the initial profile is very different from the true one, the final retrieved product could be subject to a substantial error. Tightening of the convergent condition in the retrieval helped reduce magnitude of the error, but not remove it totally. It is concluded that an initial profile based on climatology is likely to provide most reliable retrieval results.

Airborne microwave measurements of precipitation associated with Super Typhoon Flo in the western North Pacific were conducted during September 16-18, 1990. The sensor package aboard the NASA DC-8 aircraft included a dual-frequency precipitation radar at 10 GHz and 34 GHz and a host of radiometers operating at 10 GHz, 18 GHz, 19 GHz, 34 GHz, and 92 GHz, as well as three frequencies near the strong water vapor absorption line of 183.3 GHz. The measurements were made during a few passes over the storm center, and active and passive microwave signatures of the rainbands were detected with a fine spatial resolution. The relationship between the measured brightness temperature and radar-estimated rain rate is examined at the frequencies between 10-92 GHz. At both 34 and 92 GHz this relationship is analyzed with the 10 GHz radar reflectivity factor measured at altitudes above the freezing layer as a further constraint. The results show that frozen hydrometeors strongly scatter radiation at these frequencies, especially at 92 GHz. It was shown from a close examination of both active and passive microwave signatures that a significant scattering of radiation at frequencies 118 GHz occurred in the inner eyewall at altitudes of 3-8 km. This scattering of microwave radiation by hydrometeors in both liquid and frozen forms is discussed under the authors' current understanding of the scattering mechanism.>

The Luneburg lens, which is commonly used to calibrate radar scatterometers, may not be appropriate for calibration of dual-antenna systems. Theory and results from measurements of the Luneburg lens reradiation pattern indicate that gross errors may be incurred when using the lens to calibrate a dual-antenna system. Measurements of the reradiation pattern indicate that special attention must be paid to the angular separation of the antennas when using the lens to calibrate a dual-antenna radar system. The angular separation of the antennas must be less than the half-power beamwidth of the lens to ensure accurate repeatable measurements. A formula for estimating the half-power beamwidth is given.>