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At least 649 records · Page 36

Estimating Surface Radiation Fluxes in the Arctic from TOVS Brightness Temperatures

A new method for estimating downwelling shortwave and longwave radiation fluxes in the Arctic from TOVS brightness temperatures has been developed. The method employs a neural network to bypass computationally intensive inverse and for-ward radiative transfer calculations, Results from two drifting ice camps (CEAREX, LeadEx) and from one coastal station show that downwelling fluxes can be estimated with r.m.s. errors of 20 W/sq m for longwave radiation and 35 W/sq m for shortwave radiation. Mean errors are less than 4 W/sq m and are well within the bounds required for many climate process studies.

Schweiger, A. J.↗

Satellite estimates of downwelling longwave irradiance at the surface during FIFE

Two methods of estimating the downward longwave flux at the earth's surface from satellite measurements are evaluated by comparing estimate calculations to in situ measurements. Attention is given to the low degree of accuracy inherent in longwave irradiance measurements and the subsequent difficulty in validating radiative transfer models. The fluxes are determined by using the models by Morcrette et al. (1986) and by Gupta (1989), and the models are found to be highly correlated for the same atmospheric profiles although an inherent bias is noted. The differences between the model computations and in situ measurements are found to be of the same order of magnitude as the differences among the measurements themselves. The difficulty of quantifying the accuracy of radiative transfer models is related to the paucity of reliable surface measurements.

Breon, Francois-Marie↗

Comparisons of Crosswind Velocity Profile Estimates Used in Fast-Time Wake Vortex Prediction Models

Five methods for estimating crosswind profiles used in fast-time wake vortex prediction models are compared in this study. Previous investigations have shown that temporal and spatial variations in the crosswind vertical profile have a large impact on the transport and time evolution of the trailing vortex pair. The most important crosswind parameters are the magnitude of the crosswind and the gradient in the crosswind shear. It is known that pulsed and continuous wave lidar measurements can provide good estimates of the wind profile in the vicinity of airports. In this study comparisons are made between estimates of the crosswind profiles from a priori information on the trajectory of the vortex pair as well as crosswind profiles derived from different sensors and a regional numerical weather prediction model.

Pruis, Mathew J.↗

Estimation of frequency offset in mobile satellite modems

In mobilesat applications, frequency offset on the received signal must be estimated and removed prior to further modem processing. A straightforward method of estimating the carrier frequency offset is to raise the received MPSK signal to the M-th power, and then estimate the location of the peak spectral component. An analysis of the lower signal to noise threshold of this method is carried out for BPSK signals. Predicted thresholds are compared to simulation results. It is shown how the method can be extended to pi/M MPSK signals. A real-time implementation of frequency offset estimation for the Australian mobile satellite system is described.

Cowley, W. G.↗

On the Relation between Stratospheric Chlorine/Bromine Loading and Short-Lived Tropospheric Source Gases

Current methods for estimating the concentrations of inorganic chlorine/bromine species Cl(y)/Br(y) in the stratosphere due to decomposition of tropospheric source gases assume that the Cl(y)/Br(y) concentration in the stratosphere is determined mainly by the balance between production from in situ oxidation of the source gases in the stratosphere and removal by transport of Cl(y)/Br(y) out of the stratosphere. The rationale being that for source gases whose lifetimes are of the order of several months or longer the concentration of Cl(y)/Br(y) in the troposphere is small because they are produced at a relatively slow rate and also removed efficiently by washout processes. As a result of the small concentration, the rate at which Cl(y)/Br(y) is transported to the stratosphere is expected to be small compared to the in situ stratospheric production. Thus the transport of Cl(y)/Br(y) from the troposphere contributes little to the stratospheric concentration. In contrast, the origin of stratospheric Cl(y)/Br(y) from reactive source gases with tropospheric lifetimes comparable to the washout lifetime of Cl(y)/Br(y) (of the order of 10-30 days) in the troposphere is distinctly different. The in situ source in the stratosphere is expected to be significantly smaller because only a small portion of the source gas is expected to survive the troposphere to be transported into this region. At the same time these short-lived source gases produce appreciable amounts of Cl(y)/Br(y) in the troposphere such that transport to the stratosphere offers a larger source for stratospheric Cl(y)/Br(y) than in situ production. Thus, for reactive source species, simple methods of estimating the concentration of stratospheric Cl(y)/Br(y) that ignore the tropospheric contribution will seriously underestimate the loading. Therefore estimation of the stratospheric Cl(y)/Br(y) loading requires not only measurements of tropospheric source gases but also measurements of Cl(y)/Br(y) at the tropopause. This paper illustrates the mechanism by using results from a two-dimensional chemistry-transport model. However, in view of the importance of tropospheric transport on stratospheric loading the detailed values should be further evaluated using a three-dimensional model with appropriate treatment of convective transport.

Ko, Malcolm K. W.↗

Deviation angle estimation for axial-flow compressors using inviscid flow solutions

Development of a more satisfactory method of estimating deviation angles was attempted using analytical procedures. Solutions for inviscid, irrotational flow in the blade-to-blade plane were obtained with a finite-difference calculation method. Deviation angles for three cascades with rounded trailing edges were estimated using the inviscid flow solutions and four existing trailing edge hypotheses. The estimated deviation angles were compared with experimental values for a range of incidence angle and inlet flow angle. None of the hypotheses were satisfactory at all incidence angles for all the cascades considered. An alternative hypothesis is presented which is suitable for one of the cascade profiles over a range of incidence and blade setting angle.

Miller, M. J.↗

Eclipse design limits for circular orbits.

By means of simple geometric construction compact expressions for the maximum and minimum eclipse durations of a circular orbit, whose semimajor axis and inclination are specified, can be obtained. The method outlined in this note provides the designer with the necessary envelope of eclipse durations that a spacecraft will experience throughout its lifetime, and, therefore, provides a quick method of estimating necessary thermal constraints. Moreover, the closed form method presented eliminates the need for extensive machine time expenditure previously required for this type of analysis. Some numerical data to aid in the design of the spacecraft is provided.

Escobal, P. R.↗

Signature extension methods in crop area estimation

The Procedure B multispectral processing system is both multisegment and multistratum. It uses data from several LACIE-sized segments together and makes a proportion estimate for the entire group of segments as well as for the individual segments. In the clustering of data features, Procedure B produces multiple classes or strata rather than just two strata (as in Procedure 1), and performs stratified sampling on each of these mutliple strata in order to make a proportion estimate. Tests results for the components and for the overall performance of Procedure B are presented, and conclusions that can be drawn from these tests are discussed. The rationale for signature extension for crop area estimation is summarized.

Kauth, R. J.↗

Estimating proportions of materials using mixture models

An approach to proportion estimation based on the notion of a mixture model, appropriate parametric forms for a mixture model that appears to fit observed remotely sensed data, methods for estimating the parameters in these models, methods for labelling proportion determination from the mixture model, and methods which use the mixture model estimates as auxiliary variable values in some proportion estimation schemes are addressed.

Heydorn, R. P.↗

Application of an aerodynamic analysis method including attainable thrust estimates to low speed leading-edge flap design for supersonic cruise vehicles

A study of low speed leading-edge flap design for supersonic cruise vehicle was conducted. Wings with flaps were analyzed with the aid of a newly developed subsonic wing program which provides estimates of attainable leading-edge thrust. Results indicate that the thrust actually attainable can have a significant influence on the design and that the resultant flaps can be smaller and simpler than those resulting from more conventional approaches.

Carlson, H. W.↗

Reliability analysis of structural ceramic components using a three-parameter Weibull distribution

Described here are nonlinear regression estimators for the three-Weibull distribution. Issues relating to the bias and invariance associated with these estimators are examined numerically using Monte Carlo simulation methods. The estimators were used to extract parameters from sintered silicon nitride failure data. A reliability analysis was performed on a turbopump blade utilizing the three-parameter Weibull distribution and the estimates from the sintered silicon nitride data.

Duffy, Stephen F.↗

Reliability analysis of structural ceramic components using a three-parameter Weibull distribution

Described here are nonlinear regression estimators for the three-parameter Weibull distribution. Issues relating to the bias and invariance associated with these estimators are examined numerically using Monte Carlo simulation methods. The estimators were used to extract parameters from sintered silicon nitride failure data. A reliability analysis was performed on a turbopump blade utilizing the three-parameter Weibull distribution and the estimates from the sintered silicon nitride data.

Duffy, Stephen F.↗

System and method for correcting attitude estimation

A system includes an angular rate sensor disposed in a vehicle for providing angular rates of the vehicle, and an instrument disposed in the vehicle for providing line-of-sight control with respect to a line-of-sight reference. The instrument includes an integrator which is configured to integrate the angular rates of the vehicle to form non-compensated attitudes. Also included is a compensator coupled across the integrator, in a feed-forward loop, for receiving the angular rates of the vehicle and outputting compensated angular rates of the vehicle. A summer combines the non-compensated attitudes and the compensated angular rates of the to vehicle to form estimated vehicle attitudes for controlling the instrument with respect to the line-of-sight reference. The compensator is configured to provide error compensation to the instrument free-of any feedback loop that uses an error signal. The compensator may include a transfer function providing a fixed gain to the received angular rates of the vehicle. The compensator may, alternatively, include a is transfer function providing a variable gain as a function of frequency to operate on the received angular rates of the vehicle.

Josselson, Robert H.↗

Modeling Meteor Flares for Spacecraft Safety

NASA's Meteoroid Environment Office (MEO) is tasked with assisting spacecraft operators and engineers in quantifying the threat the meteoroid environment poses to their individual missions. A more complete understanding of the meteoroid environment for this application requires extensive observations. One manner by which the MEO observes meteors is with dedicated video camera systems that operate nightly. Connecting the observational data from these video cameras to the relevant physical properties of the ablating meteoroids, however, is subject to sizable observational and theoretical uncertainties. Arguably the most troublesome theoretical uncertainty in ablation is a model for the structure of meteoroids, as observations clearly show behaviors wholly inconsistent with meteoroids being homogeneous spheres. Further complicating the interpretation of the observations in the context of spacecraft risk is the ubiquitous process of fragmentation and the flares it can produce, which greatly muddles any attempts to estimating initial meteoroid masses. In this talk a method of estimating the mass distribution of fragments in flaring meteors using high resolution video observations will be dis- cussed. Such measurements provide an important step in better understanding of the structure and fragmentation process of the parent meteoroids producing these flares, which in turn may lead to better constraints on meteoroid masses and reduced uncertainties in spacecraft risk.

Ehlert, Steven↗