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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Month-to-month variability of ocean-atmosphere latent heat flux as observed from the Nimbus microwave radiometer

Comparison with in situ measurements shows that the Nimbus/Scanning Multichannel Microwave Radiometer is useful in describing the month-to-month variability of the latent heat flux 'LE' and related parameters during the 1982-1983 El Nino event. The spaceborne measured monthly mean LE was found to be within 30 W/sq m of those derived from ship reports. Absolute accuracy could not be determined, though satellite measurements could extrapolate information on the LE both in space and in time.

Liu, W. Timothy↗
NASA NTRS1990

On-orbit structural dynamic performance of a 15-meter microwave radiometer antenna

The on-orbit structural dynamic performance of a microwave radiometer antenna for Earth science applications is addressed. The radiometer is one of the Earth-observing instruments aboard a proposed geostationary platform as part of the Mission to the Planet Earth. A sequential approach is presented for assessing the ability of an antenna structure to retain its geometric shape subject to a representative onboard disturbance. This approach includes establishing the structural requirements of the antenna, developing the structural and disturbance models, performing modal and forced response analyses, and evaluating the resulting distortions in terms of the antenna's ability to meet stringent structural performance requirements. Two antenna configurations are discussed: free-flying and platform-mounted. These configurations are analyzed for a representative disturbance function which simulates rotation of the subreflector in order to perform a raster-type scan of the Earth disk. Results show that the scanning maneuver modeled would not induce antenna structural errors outside the specified limits.

Wahls, Deborah M.↗
NASA NTRS2004

Progress in Low-Power Digital Microwave Radiometer Technologies

Three component technologies were combined into a digital correlation microwave radiometer. The radiometer comprises a dual-channel X-band superheterodyne receiver, low-power high-speed cross-correlator (HSCC), three-level ADCs, and a correlated noise source (CNS). The HSCC dissipates 10 mW and operates at 500 MHz clock speed. The ADCs are implemented using ECL components and dissipate more power than desired. Thus, a low-power ADC development is underway. The new ADCs arc predicted to dissipated less than 200 mW and operate at 1 GSps with 1.5 GHz of input bandwidth. The CNS provides different input correlation values for calibration of the radiometer. The correlation channel had a null offset of 0.0008. Test results indicate that the correlation channel can be calibrated with 0.09% error in gain.

Piepmeier, Jeffrey R.↗
NASA NTRS1993

TOPEX/Poseidon Microwave Radiometer (TMR): 1. Instrument Description and Antenna Temperature Calibration

The TOPEX/Poseidon Microwave Radiometer (TMR) is a 3-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21 and 37 GHz in a nadir only viewing direction which is co-aligned with the T/P radar altimeters. TMR monitors and corrects for the electrical path delay of the altimeter radar signal due to water vapor and non-precipitating liquid water in the atmosphere. This paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (T_A) from the raw digital data. T_A precision of 0.4 K is predicted on orbit in all expected thermal environments. T_A accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. When uncertainties related to antenna sidelobe corrections are included, this T_A accuracy yields a brightness temperature accuracy of 0.7- 0.8 K...

Ruf, C. S.↗
NASA NTRS2021

Soil Moisture Active/Passive (SMAP) L-band Microwave Radiometer Post-Launch Calibration Revisit: Approach and Performance

The SMAP microwave radiometer is a fully-polarimetric L-band radiometer flown on the SMAP satellite in a 6 AM / 6 PM sun-synchronous orbit at 685-km altitude. After the SMAP L1B_TB data product version4was released in 2018, the radiometer has undergonefurther calibration and validation. The goal isto reducethe difference between antenna temperature (TA) ofascending and descending orbits during the eclipse, andto reduce the dips in the calibration drift over the Cold Sky (CS) during the eclipse seasons in 2017 and 2018. The post-launch calibration algorithmhas been revisitedby retrieving all of the calibration parameters simultaneouslywith two different options for thehot calibration source(theglobal ocean, or the radiometer internal reference load). The performance of the two options are compared here. Theoption with the radiometer internal reference load has been chosen by the SMAP science team for data release version 5. In addition, a correction offset is applied to the input signal to account for offsets during the early-mission stages with theSMAP SAR transmitter operating alongside the radiometer.

Jinzheng Peng↗
NASA NTRS1980

The Seasat scanning multichannel microwave radiometer /SMMR/ - Radiometric calibration algorithm development and performance

Radiometric calibration algorithms for processing Seasat scanning multichannel microwave radiometer (SMMR) flight data are presented. An equation is derived which accounts for losses and reradiation in the microwave components and nonlinearities in the radiometer. The application of pre-launch calibration data to this derivation is described. A performance evaluation of the post-launch operation of the algorithm is presented, indicating temperature resolution of 0.63 K at 6.6 GHz and 1.39 K at 37 GHz.

Swanson, P. N.↗
NASA NTRS1985

Optimum strategies and performance for the remote sensing of path-delay using ground-based microwave radiometers

Computer simulations were used to study the accuracy of remotely sensed microwave radiometer measurements of excess radio propagation path delay due to atmospheric water vapor. A number of strategies were investigated for remote sensing of path delay in order to define baseline parameters for the design of water vapor radiometers (WVRs) in geodetic applications. Strategies were judged according to their retrieval performance in a variety of climatological regions. An observing approach using the frequency 20.7/22.2/31.4 Gz was found to be close to optimum. A statistical retrieval approach using retrieval coefficients stratified for clear and cloudy weather was identified as a substantial improvement over conventional single-set all-weather retrieval strategies. It is shown that a reasonably well optimized WVR with an estimated calibration uncetainty of 0.5 K can achieve an overall retrieval performance of 0.27 cm in clear weather; and 0.51 cm in cloudy weather. The weather-averaged retrieval performance for individual locations was found to vary by no more than 14 percent from the average for all locations despite a mean path delay of 5 to 26 cm.

Gary, B. L.↗
NASA NTRS2018

NASA's Experiences with Microwave Radiometers from Ground to Space

Passive microwave sensing is sensitive to a wide variety of geophysical parameters in the atmosphere, oceans, cryosphere, and on land. For example, spaceborne passive microwave atmospheric sounders provide the highest-impact observations for state-of-the-art weather forecast models used by meteorological agencies around the world. The ability of passive microwave sensors (radiometers) to observe day or night through cloud cover and other obstacles (wavelength dependent, of course) partially compensates for their sometimes large footprint size compared to other sensor types. Examples will be presented of radiometers deployed on the ground, on aircraft, and on satellites. Each of these environments presents different challenges in obtaining accurate observations. And while many standard radiometer designs and techniques have been developed, every user re-discovers that even the "standard" approach requires careful attention in order to achieve full potential.

calibration↗
NASA NTRS1990

Global atmospheric temperature anomaly monitoring with passive microwave radiometers

The potential of microwave sounding units (MSU) for augmenting the surface-based thermometer record by providing a measurement representing a significant depth of the troposphere is considered. These radiometers measure the thermal emission by molecular oxygen in the atmosphere at different spectral intervals in the oxygen absorption complex near 60 GHz. Brightness temperature variations measured by NOAA-6 and NOAA-7 MSUs during a near-two year period are analyzed and compared with monthly averaged surface air temperature data. It is demonstrated that MSUs, while of limited use for vertical profiling of the atmosphere, provide stable measurements of vertically average atmospheric temperatures, centered at a constant pressure level.

Spencer, Roy W.↗
NASA NTRS1980

A study of radio frequency interference with the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR)

One of the important objectives of the NIMBUS-7 Scanning Multichannel Microwave Radiometer (SMMR) is to demonstrate the feasibility of all weather measurements of various ocean parameters; such as sea surface temperature (SST) and near surface wind speed (WS). These ocean parameters can be determined from multispectral measurements of ocean brightness temperatures in the microwave region of the electromagnetic spectrum. These microwave measurements, however, are distorted if the field of view of the SMMR antenna encounters radio transmissions from terrestrial sources. Sources of terrestrial Radio Frequency Interference (RFI) in the SMMR ocean data were identified. Its extent and characteristics over different ocean areas on the Earth were determined.

Kogut, J. A.↗
NASA NTRS1972

Salinity surveys using an airborne microwave radiometer

The Barnes PRT-5 infrared radiometer and L-band channel of the multifrequency microwave radiometer are used to survey the distribution of surface water temperature and salinity. These remote sensors were flown repetitively in November 1971 over the outflow of the Mississippi River into the Gulf of Mexico. Data reduction parameters were determined through the use of flight data obtained over a known water area. With these parameters, the measured infrared and microwave radiances were analyzed in terms of the surface temperature and salinity.

Paris, J. F.↗
NASA NTRS2012

Single-Pole Double-Throw MMIC Switches for a Microwave Radiometer

In order to reduce the effect of gain and noise instabilities in the RF chain of a microwave radiometer, a Dicke radiometer topology is often used, as in the case of the proposed surface water and ocean topography (SWOT) radiometer instrument. For this topology, a single-pole double-throw (SPDT) microwave switch is needed, which must have low insertion loss at the radiometer channel frequencies to minimize the overall receiver noise figure. Total power radiometers are limited in accuracy due to the continuous variation in gain of the receiver. High-frequency SPDT switches were developed in the form of monolithic microwave integrated circuits (MMICs) using 75 micron indium phosphide (InP) PIN-diode technology. These switches can be easily integrated into Dicke switched radiometers that utilize microstrip technology.

Montes, Oliver↗
NASA NTRS1980

Airborne microwave radiometer remote sensing of lake ice

The NASA Langley C-Band Stepped Frequency Microwave Radiometer was used to conduct airborne remote sensing measurements of the Great Lakes ice cover during the winters of 1978 and 1979. In order to evaluate the use of microwave radiometry for remote sensing of fresh water ice, an initial experiment was conducted in February 1978. After the results were analyzed an algorithm was developed, and a more comprehensive mission was completed in March 1979, which related the thermal emission from lake ice to meaningful geophysical quantities. A clear discrimination of pressure ridges and rubble from base ice was observed, and evidence was found which showed gradual changes in the thickness of the base itself. Results indicate that passive microwave sensors can provide a measure of lake ice within uncertainties presented by the attenuation coefficient, and the changes in surface reflectivity imposed by surface roughness.

Swift, C. T.↗
NASA NTRS1972

Soil moisture measurements with microwave, radiometers

One technique of measuring moisture content that appears promising is that of microwave radiometry. In the microwave region of the spectrum, the emissivity of water is approximately 0.4, whereas that of dry soil is approximately 0.9. Therefore, the emissivity of the soil can range from about 0.6 to 0.9 as the soil changes from a wet to a dry condition. Recent ground base measurements have demonstrated emissivity changes of this magnitude. To test the use of this approach for remote sensing of soil moisture, flights were made over agricultural test sites in the vicinity of Phoenix, Ariz., during late February 1971. On the same day, soil moisture measurements were made on the ground for 200 fields. On board the aircraft were six microwave radiometers, ranging in wavelength from 21 cm to 8 mm. The results of one of these radiometers are presented.

Schmugge, T. J.↗
NASA NTRS1982

Observations of oceanic surface-wind fields from the Nimbus-7 microwave radiometer

Brightness temperatures from the five-frequency dual-polarized scanning multichannel microwave radiometer (SMMR) on Nimbus 7 have been used to obtain surface wind fields over the ocean. The satellite-derived wind field for 1200Z, Feb. 19, 1979, in the eastern North Pacific has been compared with an operationally generated surface-wind analysis field. Previous point comparisons at selected locations have indicated that satellite winds are accurate to 3 m/sec. The results, although of a preliminary nature, indicate that SMMR-derived winds may be used to determine large-scale wind fields over the ocean, particularly in areas of strong wind gradients such as found in cyclonic systems.

Miller, J. R.↗