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At least 739 records · Page 41

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NASA NTRS2013

The Accidental Tide Gauge: A GPS Reflection Case Study from Kachemak Bay, Alaska

For the last decade, it has been known that reflected GPS signals observed with specialized instruments could be used to measure sea level. In this letter, data from an existing geodeticquality GPS site near Kachemak Bay, Alaska, are analyzed for a one-year time period. Daily sea-level variations are more than 7 m. Tidal coefficients have been estimated and compared with coefficients estimated from records from a traditional tide gauge at Seldovia Harbor, approximately 30 km away. The GPS and Seldovia estimates of M(sub 2) and S(sub 2) coefficients agree to better than 2%; much of this residual can be attributed to true differences in the tide over 30 km as it propagates up Kachemak Bay. For daily mean sea levels the agreement is 2.3 cm. Because a standard geodetic GPS receiver/antenna is used, this GPS instrument can measure long-term sea-level changes in a stable terrestrial reference frame.

GPS reflections↗

NASA NTRS2014

AG Channel Measurement and Modeling Results for Over-Sea Conditions

This report describes results from flight tests conducted in an over-sea environment, for the purpose of characterizing the air-to-ground (AG) channel, for future unmanned aircraft system (UAS) communication system analysis and design. These results are for the first of a set of several flight tests conducted in different ground site (GS) environments. An ultimate aim of all these tests is the development of models for the AG channel that can be used in communication system evaluation. In this report we provide measured results for propagation path loss, root-mean square delay spread (RMS-DS), and the correlation coefficient of the primary received signal components on the four antennas (two antennas for C-band, two for L-band). For path loss, the curved-earth two-ray model provides a reasonable fit to the measured data, altered by several dB at the shortest link distances by aircraft antenna pattern effects. This two-ray model also accounts for the majority of measured RMS-DS results of a few tens of nanoseconds, except for the occasional intermittent reflections from surface objects. These intermittent reflections yield RMS-DS values up to several hundred nanoseconds. For portions of the flight path that were over a harbor area highly populated with boats, the channel was found to be more "continuously dispersive," with RMS-DS reaching approximately 250 ns. A separate model will be developed for this over-harbor setting. The correlation coefficient results are still undergoing analysis; preliminary observations are that correlation between signals on the same-band antennas is generally large (>0.6) for the C-band straight flight paths, whereas for the L-band signals and for the oval-shaped flight paths the correlation is generally small (below 0.4). Inter-band correlations are typically very small, and are well modeled as zero-mean Gaussian in distribution, with a standard deviation less than 0.2. Hence the over-sea channel effects in the two bands can be considered uncorrelated, which will allow for good diversity gains in dual-band systems. We describe initial modeling approaches for the over-sea channel; complete models for this and the over-harbor setting will appear in a subsequent report.

Aircraft Communications↗

NASA NTRS1985

Contrasts among bidirectional reflectance of leaves, canopies, and soils

Simple models are presented for predicting the bidirectional reflectance distribution functions (BRDFs) for soils and plant canopies viewed from various directions. BRDFs are predicted for bare soil, individual leaves, and plant canopies, and the results are compared with measurements and a three coefficient empirical equation. BRDF measurements for corn and soybean leaves are presented to contrast with canopy and soil distributions. Estimates of the soil, canopy, and leaf BRDFs are combined into a model called Cupid to predict BRDFs for complex natural surfaces.

Norman, J. M.↗

NASA NTRS1985

Simulation of the temperature distribution in crystals grown by Czochralski method

Production of perfect crystals, free of residual strain and dislocations and with prescribed dopant concentration, by the Czochralski method is possible only if the complex, interacting phenomena that affect crystal growth in a Cz-puller are fully understood and quantified. Natural and forced convection in the melt, thermocapillary effect and heat transfer in and around the crystal affect its growth rate, the shape of the crystal-melt interface and the temperature gradients in the crystal. The heat transfer problem in the crystal and between the crystal and all other surfaces present in the crystal pulling apparatus are discussed at length. A simulation and computer algorithm are used, based on the following assumptions: (1) only conduction occurs in the crystal (experimentally determined conductivity as a function of temperature is used), (2) melt temperature and the melt-crystal heat transfer coefficient are available (either as constant values or functions of radial position), (3) pseudo-steady state is achieved with respect to temperature gradients, (4) crystal radius is fixed, and (5) both direct and reflected radiation exchange occurs among all surfaces at various temperatures in the crystal puller enclosure.

Dudokovic, M. P.↗

NASA NTRS1993

AVHRR Surface Temperature and Narrow-Band Albedo Comparison with Ground Measurements for the Greenland Ice Sheet

An ice-surface temperature retrieval algorithm for the Greenland ice sheet was developed using NOAA 11 thermal radiances from channels 4 and 5. Temperature, pressure and humidity profiles, cloud observations and skin temperatures from the Swiss Federal Institute of Technology (ETH) camp, located at the equilibrium line altitude at 49 deg17 min W, 69 deg 34 min N, were used in the LOWTRAN 7 model. Through a statistical analysis of daily clear sky profiles, the coefficients that correct for the atmospheric effects were determined for the ETH-Camp field season (May to August). Surface temperatures retrieved by this method were then compared against the in situ observations with a maximum difference of 0.6 K. The NOAA 11 narrow-band planetary albedo values for channels 1 and 2 were calculated using pre-launch calibration coefficients. Scattering and absorption by the atmosphere were modelled with LOWTRAN 7. Then, narrow-band albedo values for the AVHRR visible and near infrared channels were compared with in situ high resolution spectral reflectance measurements. In the visible band (580-680 nm), AVHRR-derived narrow-band albedo and the in situ measurements corrected with radiative transfer model LOWTRAN 7 showed a difference of less than 2%. For the near infrared channel (725-1100 nm) the difference between the measured and modelled narrow-band albedo was 14%. These discrepancies could be either the result of inaccurate aerosol scattering modelling (lack of the in situ observation), or the result of sensor drift due to degradation.

Haefliger, M.↗

NASA NTRS2021

Assessing within-Field Corn and Soybean Yield Variability from WorldView-3, Planet, Sentinel-2, and Landsat 8 Satellite Imagery

Crop yield monitoring is an important component in agricultural assessment. Multispectral remote sensing instruments onboard space-borne platforms such as Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Visible Infrared Imaging Radiometer Suite (VIIRS) have shown to be useful for efficiently generating timely and synoptic information on the yield status of crops across regional levels. However, the coarse spatial resolution data inherent to these sensors provides little utility at the management level. Recent satellite imagery collection advances toward finer spatial resolution (down to 1 m) alongside increased observational cadence (near daily) implies information on crops obtainable at field and within-field scales to support farming needs is now possible. To test this premise, we focus on assessing the efficiency of multiple satellite sensors, namely WorldView-3, Planet/Dove-Classic, Sentinel-2, and Landsat 8 (through Harmonized Landsat Sentinel-2 (HLS)), and investigate their spatial, spectral (surface reflectance (SR) and vegetation indices (VIs)), and temporal characteristics to estimate corn and soybean yields at sub-field scales within study sites in the US state of Iowa. Precision yield data as referenced to combine harvesters’ GPS systems were used for validation. We show that imagery spatial resolution of 3 m is critical to explaining 100% of the within-field yield variability for corn and soybean. Our simulation results show that moving to coarser resolution data of 10 m, 20 m, and 30 m reduced the explained variability to 86%, 72%, and 59%, respectively. We show that the most important spectral bands explaining yield variability were green (0.560 µm), red-edge (0.726 µm), and near-infrared (NIR - 0.865 µm). Furthermore, the high temporal frequency of Planet and a combination of Sentinel-2/Landsat 8 (HLS) data allowed for optimal date selection for yield map generation. Overall, we observed mixed performance of satellite-derived models with the coefficient of determination (R^2) varying from 0.21 to 0.88 (averaging 0.56) for the 30 m HLS and from 0.09 to 0.77 (averaging 0.30) for 3 m Planet. R^2 was lower for fields with higher yields, suggesting saturation of the satellite-collected reflectance features in those cases. Therefore, other biophysical variables, such as soil moisture and evapotranspiration, at similar fine spatial resolutions are likely needed alongside the optical imagery to fully explain the yields.

agriculture; yield; within-field; corn; soybean; r↗

NASA NTRS1974

Six-color photometry of Iapetus, Titan, Rhea, Dione and Tethys

The extensive photometric observations of Titan, Iapetus, Rhea, Dione and Tethys have made it possible to separate the solar phase and orbital phase contributions to the observed light variations of these satellites. For Titan, the wavelength dependence of its solar phase coefficient has been obtained. This dependence should prove useful in constructing future model atmospheres. The other four satellites show a surprising array of different photometric behaviors. Despite these differences, all four satellites have similar spectral reflectivities. Clearly Iapetus, Rhea, Dione and Tethys are complex objects, varying substantially from one another in surface structure and/or composition.

Noland, M.↗

NASA NTRS1979

Dynamic mechanical analysis of fiber reinforced composites

Dynamic mechanical and thermal properties were determined for unidirectional epoxy/glass composites at various fiber orientation angles. Resonant frequency and relative logarithmic decrement were measured as functions of temperature. In low angle and longitudinal specimens a transition was observed above the resin glass transition temperature which was manifested mechanically as an additional damping peak and thermally as a change in the coefficient of thermal expansion. The new transition was attributed to a heterogeneous resin matrix induced by the fiber. The temperature span of the glass-rubber relaxation was found to broaden with decreasing orientation angle, reflecting the growth of fiber contribution and exhibiting behavior similar to that of Young's modulus. The change in resonant frequency through the glass transition was greatest for samples of intermediate fiber angle, demonstrating behavior similar to that of the longitudinal shear modulus.

Reed, K. E.↗

NASA NTRS1979

Dynamic mechanical analysis of fiber reinforced composites

Reed, K. E.↗

NASA NTRS2003

Multi-Sensor Approach for Assessing the Taiga-Tundra Boundary

Monitoring the dynamics of the tundra-taiga boundary is critical for our understanding of the causes and consequences of the changes in this area. Because of its inaccessibility, remote sensing data will play an important role. In this study we examined the use of several remote sensing techniques for identifying the existing tundra-taiga ecotone. These include Landsat, MISR and RADARSAT data. High-resolution IKONOS images were used for local ground truth. It was found that on Landsat ETM+ summer images, reflectance from tundra and taiga at band 4 (NIR) is similar, but different at other bands such as red, and MIR bands. When the incidence angle is small, C-band HH-pol backscattering coefficients from both tundra and taiga are relatively high. The backscattering from tundra targets decreases faster than taiga targets when the incidence angle increases, because the tundra targets look smoother than taiga. Because of the shading effect of the vegetation, the MISR data, both multi-spectral data at nadir looking and multi-angle data at red and NIR bands, clearly show the transition zone.

Ranson, K. J.↗

NASA NTRSDate not supplied

Blade Displacement Measurements of a Rotor in Forward Flight in the Langley 14- by 22-Ft Wind Tunnel

Stereo photogrammetry was used to measure the elastic bending and twist of a three-bladed rotor in forward flight in the NASA Langley 14-by 22-Ft Subsonic Wind Tunnel. The rotor was imaged from below by two cameras mounted at windows of a large turntable in the floor of the test section. At each test condition, the turntable and cameras were rotated and the cameras were triggered to allow measurements over a range of blade azimuth angles. Retro-reflective targets were applied along the leading and trailing edges of the lower surface of each blade. Image blur due to blade motion was minimized by illuminating the targets with high-intensity LED flash lamps mounted next to each camera. The cameras were re-calibrated at each angular position of the turntable using the positions in each image of retro-reflective targets on the ceiling of the test section whose spatial coordinates had been very accurately measured. The blade displacement measurements yielded elastic bending and twist along each blade as functions of blade azimuth, thrust coefficient, and advance ratio, all at a constant rotor angle-of-attack. In addition, the measurements produced estimates of the rigid-body blade pitch, flap, and lag angles.

rotor blade displacement↗

NASA NTRSDate not supplied

Blade Displacement Measurements of a Rotor in Forward Flight in the Langley 14-by 22-Foot Wind Tunnel

Stereo photogrammetry was used to measure the elastic bending and twist of a three-bladed rotor in forward flight in the NASA Langley 14-by 22-Ft Subsonic Wind Tunnel. The rotor was imaged from below by two cameras mounted at windows of a large turntable in the floor of the test section. At each test condition, the turntable and cameras were rotated and the cameras were triggered to allow measurements over a range of blade azimuth angles. Retro-reflective targets were applied along the leading and trailing edges of the lower surface of each blade. Image blur due to blade motion was minimized by illuminating the targets with high-intensity LED flash lamps mounted next to each camera. The cameras were re-calibrated at each angular position of the turntable using the positions in each image of retro-reflective targets on the ceiling of the test section whose spatial coordinates had been very accurately measured. The blade displacement measurements yielded elastic bending and twist along each blade as functions of blade azimuth, thrust coefficient, and advance ratio, all at a constant rotor shaft angle. In addition, the measurements produced estimates of the rigid-body blade pitch, flap, and lag angles.

rotor↗

NASA NTRS1973

Experimental study of shock-wave reflection from a thermally accommodating wall.

Shock-tube experiments have been conducted to study the nonequilibrium gas-surface interaction which occurs when a thick shock wave in argon reflects from a coplanar, heat-conducting wall. Fast-response instrumentation was used to monitor variations in temperature and normal stress on the surface of the shock-tube end wall during and immediately following reflection of the incident shock wave. The laboratory observations are compared with computer predictions obtained by Deiwert using the direct-simulation Monte Carlo method, and excellent agreement is obtained when a suitable average thermal accommodation coefficient is chosen for the wall surface.

Hanson, R. K.↗

NASA NTRS1989

Bidirectional canopy reflectance and its relationship to vegetation characteristics

A method for determining the relationship between bidirectional canopy reflectance (CR) of a vegetated area and its characteristics (type and growth stage) is presented. In the method, the three-dimensional CR surface is expressed in terms of spherical harmonics which are weighted by appropriate coefficients, and the coefficients are analyzed by a cluster technique. The method is applied to four vegetation types (soybeans, corn, shinnery oak, and orchard grass) at various stages of growth.

Goel, Narendra S.↗

NASA NTRS1989

Dissociative recombination in aeronomy

The importance of dissociative recombination in planetary aeronomy is summarized, and two examples are discussed. The first is the role of dissociative recombination of N2(+) in the escape of nitrogen from Mars. A previous model is updated to reflect new experimental data on the electronic states of N produced in this process. Second, the intensity of the atomic oxygen green line on the nightside of Venus is modeled. Use is made of theoretical rate coefficients for production of O (1S) in dissociative recombination from different vibrational levels of O2(+).

Fox, J. L.↗

NASA NTRS1992

Experimental study of performance degradation of a rotating system in the NASA Lewis RC icing tunnel

The Helicopter Icing Consortium (HIC) conducted one of the first U.S. tests of a heavily instrumented model in the controlled environment of a refrigerated tunnel. In the Icing Research Tunnel (IRT) at NASA LeRC, ice was accreted on the main rotor blade of the BMTR-1 Sikorsky model helicopter under a variety of environmental conditions, such that liquid water content (LWC) and volume mean droplet diameter (VMD) ranges reflected the Federal Aviation Agency and Department of Defence icing condition envelopes. This report gives the correlated results of the data provided by NASA LeRC. The method of statistical analysis is discussed. Lift, thrust, and torque coefficients are presented as a function of icing time, as correlated with changes in ambient temperature, LWC, and VMD. The physical significance of these forces is discussed.

Korkan, Kenneth↗

NASA NTRS1991

Radiometric rectification - Toward a common radiometric response among multidate, multisensor images

A method is developed for relating scene digital counts among several images of the same scene by identifying radiometric control sets with mean reflectances that are basically constant. The average digital-count values of the control sets are utilized to compute linear transforms that relate digital count values between images. Two Landsat TM images are studied by means of the technique using simulations of a wide range of atmospheric conditions. In the visible and near-IR bands the algorithm effectively adjusts the surface reflectance for the effects of relative atmospheric differences to within 1 percent. The proposed method is found to be an effective relative correction procedure that can be used when atmospheric optical-depth data and calibration coefficients are not available.

Hall, F. G.↗

NASA NTRS2002

Modelling the effect of diffuse light on canopy photosynthesis in controlled environments

A layered canopy model was used to analyze the effects of diffuse light on canopy gross photosynthesis in controlled environment plant growth chambers, where, in contrast to the field, highly diffuse light can occur at high irradiance. The model suggests that high diffuse light fractions (approximately 0.7) and irradiance (1400 micromoles m-2 s-1) may enhance crop life-cycle canopy gross photosynthesis for hydroponic wheat by about 20% compared to direct light at the same irradiance. Our simulations suggest that high accuracy is not needed in specifying diffuse light fractions in chambers between approximately 0.7 and 1, because simulated photosynthesis for closed canopies plateau in this range. We also examined the effect of leaf angle distribution on canopy photosynthesis under growth chamber conditions, as these distributions determine canopy extinction coefficients for direct and diffuse light. We show that the spherical leaf angle distribution is not suitable for modeling photosynthesis of planophile canopies (e.g., soybean and peanut) in growth chambers. Also, the absorption of the light reflected from the surface below the canopy should generally be included in model simulations, as the corresponding albedo values in the photosynthetically active range may be quite high in growth chambers (e.g., approximately 0.5). In addition to the modeling implications, our results suggest that diffuse light conditions should be considered when drawing conclusions from experiments in controlled environments.

Non-NASA Center↗