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At least 109 records · Page 6

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NASA NTRS1977

Estimation of regional heat flux from scanning radiometer measurements

The paper presents a statistical method for extracting regional information on the radiative heat flux at the top of the atmosphere from a satellite-borne scanning radiometer. The radiative heat flux distribution is denoted by a double Fourier series with coefficients to be determined from measurements. The method determines, in the sense of minimum variance, the best linear estimator of the total flux over a region. The heat flux distribution is represented by a truncated series which is unbiased.

Bess, T. D.↗

NASA NTRS1978

Kalman filter divergence and aircraft motion estimators

Kalman filters designed for many aerospace systems turn out to be unsatisfactory. The estimate errors become large compared to the errors predicted by the theory ('divergence'). One of the principal causes of this failure is that the system model contains states or modes that are undisturbed by the modeled process noise, and are neutrally stable (NS). One cure for such problems is periodic restarting of a time-varying Kalman filter. Other cures include minimum variance observers with eigenvalue constraints, added noise, pole-shifting, and destabilization. Several examples are given, including effective time-invariant estimators for the longitudinal and lateral motions of an airplane where several NS modes are undisturbed by wind gusts. An interpretation of these estimators as a 'strapdown IMU' without accelerometers, gimbaled gyros, or servos is given.

Bryson, A. E., Jr.↗

NASA NTRS1978

Large-scale Gulf Stream frontal study using GEOS 3 radar altimeter data

From data obtained by the GEOS 3 radar altimeter, sea surface heights are found by both editing and filtering the raw sea surface height measurements and then referencing these processed data to a 5 foot by 5 foot geoid. Any trend between the processed data and the geoid is removed by subtracting out a linear fit to the residuals in the open ocean. Data from individual passes are further processed by applying a minimum variance technique at the subsatellite crossing points to produce surface topography maps for the 6 months and an overall mean map which reveal important details about the Gulf Stream system. The differences between the monthly mean and the overall mean are calculated for each of the 6 months to show the temporal and spatial changes of the Gulf Stream front and spawned eddies. The standard deviation map is even more informative and shows preferred locations of Gulf Stream meanders.

Huang, N. E.↗

NASA NTRS1978

Magnetic dips in the solar wind

Using magnetic data from the HELIOS 1 fluxgate magnetometer, with a 0.2 sec resolution, the structures of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector were investigated. A minimum variance analysis illustrates the behavior of the magnetic field through the transition in the plane of its maximum variation. Using this analysis, quite different structures have been individuated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 1,000 km), results show the observed structures could be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.

Dobrowolny, M.↗

NASA NTRS1979

Magnetopause surface fluctuations observed by Voyager 1

Moving out of the dawnside of the earth's magnetosphere, Voyager 1 crossed the magnetopause apparently seven times, despite the high spacecraft speed of 11 km/sec. Normals to the magnetopause and their associated error cones were estimated for each of the crossings using a minimum variance analysis of the internal magnetic field. The oscillating nature of the ecliptic plane component of these normals indicates that most of the multiple crossings were due to a wave-like surface disturbance moving tailward along the magnetopause. The wave, which was aperiodic, was modeled as a sequence of sine waves. The amplitude, wavelength, and speed were determined for two pairs of intervals from the measured slopes, occurrence times, and relative positions of six magnetopause crossings. The magnetopause thickness was estimated to lie in the range 300 to 700 km with higher values possible. The estimated amplitude of these waves was obviously small compared to their wavelengths.

Lepping, R. P.↗

NASA NTRS1979

Interplanetary sector boundaries, 1971 - 1973

Eighteen interplanetary sector boundary crossings observed at 1 AU by the magnetometer on the IMP-6 spacecraft are discussed. The events were examined on many different time scales ranging from days on either side of the boundary to high resolution measurements of 12.5 vectors per second. Two categories of boundaries were found, one group being relatively thin and the other being thick. In many cases the field vector rotated in a plane from one polarity to the other. Only two of the transitions were null sheets. Using the minimum variance analysis to determine the normals to the plane of rotation, and assuming that this is the same as the normal to the sector boundary surface, it was found that the normals were close to the ecliptic plane. An analysis of tangential discontinuities contained in 4-day periods about the events showed that their orientations were generally not related to the orientations of the sector boundary surface, but rather their characteristics were about the same as those for discontinuities outside the sector boundaries.

Klein, L.↗

NASA NTRS1979

Magnetic dips in the solar wind

Using magnetic data from the Helios-1 fluxgate magnetometer, with a 0.2 s resolution, we have investigated the structure of several interplanetary discontinuities involving magnetic dips and rotations of the magnetic field vector. A minimum variance analysis illustrates the behaviour of the magnetic field through the transition. Using this analysis, quite different structures have been isolated and, in particular, narrow transitions resembling almost one dimensional reconnected neutral sheets. For the thinner cases (scale lengths of the magnetic rotation of the order or smaller than 1000 km), we find that the observed structures can be the nonlinear effect of a resistive tearing mode instability having developed on an originally one dimensional neutral sheet at the solar corona.

Dobrowolny, M.↗

NASA NTRS1979

Geomagnetopause surface fluctuations observed by Voyager 1

The paper discusses the geomagnetopause surface fluctuations observed by Voyager 1. Normals to the magnetopause were determined for the crossings of Voyager 1 by minimum variance analysis of the internal magnetic field; the oscillating nature of the ecliptic plane component of these normals indicates that the multiple crossings were due to a wavelike surface disturbance moving tailward along the magnetopause. The estimated amplitude of these waves was small compared to their wavelengths; this conclusion is independent of any bulk normal motion of the magnetopause.

Lepping, R. P.↗

NASA NTRS1980

Interplanetary sector boundaries 1971-1973

The large-scale morphology and internal structure of sector boundaries (SB) are investigated, using observations at 1 AU of interplanetary SB crossings. It was found that the durations of the SBs were either relatively short (less than about 10 min) or relatively long (longer than about 3 hours). Using the minimum variance technique, it was found that the SB surfaces were inclined appreciably with respect to the ecliptic at this epoch of the solar cycle (1971-1973). Magnetic holes were found in thick SBs at a rate about three times that elsewhere. In addition, an analysis of tangential discontinuities showed that their orientations were generally not related to the orientations of the SB surface, but their characteristics were very similar to those for discontinuities outside the SBs.

Klein, L.↗

NASA NTRS1981

An analytic technique for statistically modeling random atomic clock errors in estimation

Minimum variance estimation requires that the statistics of random observation errors be modeled properly. If measurements are derived through the use of atomic frequency standards, then one source of error affecting the observable is random fluctuation in frequency. This is the case, for example, with range and integrated Doppler measurements from satellites of the Global Positioning and baseline determination for geodynamic applications. An analytic method is presented which approximates the statistics of this random process. The procedure starts with a model of the Allan variance for a particular oscillator and develops the statistics of range and integrated Doppler measurements. A series of five first order Markov processes is used to approximate the power spectral density obtained from the Allan variance.

Fell, P. J.↗

NASA NTRS1981

Configuration of Jupiter's magnetic tail and equatorial current sheet

Recent research reports by Behannon et al. (1981) and Connerney et al. (1981) are summarized. It is noted that the analysis made of the detailed neutral sheet crossings by the minimum variance method shows a consistent result with regard to the orientation of the neutral sheet in the magnetic tail as a two-dimensional surface rocking back and forth about the Jupiter sun-line as the rotation of the planet leads to a precession of the tilted dipole magnetic axis. The occurrence of neutral sheet crossings is found not to be consistent with any of the axially symmetric theoretical models proposed earlier on the basis of the 1974 Pioneer 10 observations. It is noted that a simple nonaxially symmetric model has been developed on the basis of the Voyager results which indicates the strong control upon orientation by the interaction of the solar wind with the Jovian magnetosphere. The model is described as simple because it improves the fit of theory to observation but uses fewer parameters. A quantitative model of the magnetodisc equatorial current sheet has been developed for the inner magnetosphere region which matches well the in-situ magnetic field observations.

Ness, N. F.↗

NASA NTRS1981

Structure of Jupiter's magnetopause - Pioneer 10 and 11 observations

The magnetic structure of Jupiter's magnetopause, as observed by the space probes Pioneer 10 and 11, is compared with terrestrial magnetopause structures from the OGO 5 mission. The Jovian magnetopause thickness, deduced from a rapid triple crossing, is found to lie in the range 3500-5200 km, while the adjoining plasma boundary layer was 5600-8400 km thick. Comparison with the terrestrial situation suggests that the dayside magnetopause thickness in both cases is a few times the ion gyroradius and that the dayside boundary layer is also of about the same width. The magnetopause normal vector and normal magnetic field component are determined for each crossing by use of minimum variance analysis. The results indicate a blunt and floppy magnetopause surface, for the most part with an insignificant normal magnetic field component. Only for three of the 14 crossings did this component exceed 1.5 times its error estimate. In two of these cases, the magnetic field tangential to the magnetopause displayed the characteristic features of a rotational discontinuity.

Sonnerup, B. U. O.↗

NASA NTRS1983

Trajectory-Estimation Error Analysis

Orbital and Geodetic Parameter Estimation Error Analysis Program (ORAN) is Bayesian least-squares simulation program for orbital trajectories. ORAN does not process data; rather, computes accuracy of results of data reduction if measurements are processed by minimum-variance data-reduction program. Actual data used to provide time when given measurement was available and estimated noise on that measurement.

Putney, B.↗

NASA NTRS1982

Self-tuning regulators for multicyclic control of helicopter vibration

A class of algorithms for the multicyclic control of helicopter vibration and loads is derived and discussed. This class is characterized by a linear, quasi-static, frequency-domain model of the helicopter response to control; identification of the helicopter model by least-squared-error or Kalman filter methods; and a minimum variance or quadratic performance function controller. Previous research on such controllers is reviewed. The derivations and discussions cover the helicopter model; the identification problem, including both off-line and on-line (recursive) algorithms; the control problem, including both open-loop and closed-loop feedback; and the various regulator configurations possible within this class. Conclusions from analysis and numerical simulations of the regulators provide guidance in the design and selection of algorithms for further development, including wind tunnel and flight tests.

Johnson, W.↗

NASA NTRS1982

Software for the grouped optimal aggregation technique

The grouped optimal aggregation technique produces minimum variance, unbiased estimates of acreage and production for countries, zones (states), or any designated collection of acreage strata. It uses yield predictions, historical acreage information, and direct acreage estimate from satellite data. The acreage strata are grouped in such a way that the ratio model over historical acreage provides a smaller variance than if the model were applied to each individual stratum. An optimal weighting matrix based on historical acreages, provides the link between incomplete direct acreage estimates and the total, current acreage estimate.

Brown, P. M.↗

NASA NTRS1981

The Jovian magnetotail and its current sheet

A summary is given of the results of a further analysis of the magnetic field measurements in Jupiter's magnetic tail and in the region of the tail current sheet by both V1 and V2. Four studies are discussed. The first is a determination of the variation of the average magnetic field magnitude in the lobes of the magnetotail, that is, outside the plasma sheet, as a function of distance from the planet. The second is an examination of the variation with distance from Jupiter of the ratio of the azimuthal component of the magnetic field to the product of the radial distance and the radial component of the magnetic field. The third is a study of the detailed structure and orientation of the tail current sheet system, including both detailed illustrations of typical sheet crossings and the results of a minimum variance analysis of all V2 tail sheet traversals. The fourth is an investigation of various models which predict the position of the current sheet in the outer Jovian magnetosphere as a function of both time and location.

Behannon, K. W.↗

NASA NTRS1981

An optimal numerical filter for wide-field-of-view measurements of earth-emitted radiation

A technique is described in which all data points along an arc of the orbit may be used in an optimal numerical filter for wide-field-of-view measurements of earth emitted radiation. The statistical filter design is derived whereby the filter is required to give a minimum variance estimate of the radiative exitance at discrete points along the ground track of the satellite. An equation for the optimal numerical filter is given by minimizing the estimate error variance equation with respect to the filter weights, resulting in a discrete form of the Wiener-Hopf equation. Finally, variances of the errors in the radiant exitance can be computed along the ground track and in the cross track directions.

Smith, G. L.↗

NASA NTRS1983

Polarized-interferometer feasibility study

The feasibility of using a polarized-interferometer system as a rendezvous and docking sensor for two cooperating spacecraft was studied. The polarized interferometer is a radio frequency system for long range, real time determination of relative position and attitude. Range is determined by round trip signal timing. Direction is determined by radio interferometry. Relative roll is determined from signal polarization. Each spacecraft is equipped with a transponder and an antenna array. The antenna arrays consist of four crossed dipoles that can transmit or receive either circularly or linearly polarized signals. The active spacecraft is equipped with a sophisticated transponder and makes all measurements. The transponder on the passive spacecraft is a relatively simple repeater. An initialization algorithm is developed to estimate position and attitude without any a priori information. A tracking algorithm based upon minimum variance linear estimators is also developed. Techniques to simplify the transponder on the passive spacecraft are investigated and a suitable configuration is determined. A multiple carrier CW signal format is selected. The dependence of range accuracy and ambiguity resolution error probability are derived and used to design a candidate system. The validity of the design and the feasibility of the polarized interferometer concept are verified by simulation.

Raab, F. H.↗