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At least 127 records · Page 7

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NASA NTRS1983

Optimal trajectories for aeroassisted orbital transfer

Consideration is given to classical and minimax problems involved in aeroassisted transfer from high earth orbit (HEO) to low earth orbit (LEO). The transfer is restricted to coplanar operation, with trajectory control effected by means of lift modulation. The performance of the maneuver is indexed to the energy expenditure or, alternatively, the time integral of the heating rate. Firist-order optimality conditions are defined for the classical approach, as are a sequential gradient-restoration algorithm and a combined gradient-restoration algorithm. Minimization techniques are presented for the aeroassisted transfer energy consumption and time-delay integral of the heating rate, as well as minimization of the pressure. It is shown that the eigenvalues of the Jacobian matrix of the differential system is both stiff and unstable, implying that the sequential gradient restoration algorithm in its present version is unsuitable. A new method, involving a multipoint approach to the two-poing boundary value problem, is recommended.

Miele, A.↗

NASA NTRS1985

In-situ detection of micron-sized dust particles in near-Earth space

In situ detectors for micron sized dust particles based on the measurement of impact ionization have been flown on several space missions (Pioneer 8/9, HEOS-2 and Helios 1/2). Previous measurements of small dust particles in near-Earth space are reviewed. An instrument is proposed for the measurement of micron sized meteoroids and space debris such as solid rocket exhaust particles from on board an Earth orbiting satellite. The instrument will measure the mass, speed, flight direction and electrical charge of individually impacting debris and meteoritic particles. It is a multicoincidence detector of 1000 sq cm sensitive area and measures particle masses in the range from 10 to the -14th power g to 10 to the -8th power g at an impact speed of 10 km/s. The instrument is lightweight (5 kg), consumes little power (4 watts), and requires a data sampling rate of about 100 bits per second.

Gruen, E.↗

NASA NTRS1985

The influence of quiet asymmetric magnetosphere on the cutoff rigidities of the main cone

Some earlier studies show that cutoff rigidities of cosmic-ray particles in the model magnetospheric fields of internal and external sources have daily variations caused by asymmetry of the magnetic field due to the currents induced at the magnetopause and tail currents. Cutoff rigidities of the charged particles coming down at the middle latitudes are examined in this paper. The mathematical model of the magnetospheric field is based on the merged IMP-HEOS experimental data set and includes all known at the present time current systems of the magnetosphere: magnetopause, ring, and magnetotail currents.

Tyasto, M. I.↗

NASA NTRS1986

Space solar cell research: Problems and potential

The value of a passive, maintenance-free, renewable energy source was apparent in the early days of the space program, and the silicon solar cell was pressed into service. Efficiencies of those early space solar arrays were low, and lifetimes shorter than hoped for, but within a decade significant advances had been made in both areas. Better performance was achieved through improvements in silicon single crystal material, better device designs, and a better understanding of the factors that affect the performance of a solar cell in space. Chief among the latter, particularly for the mid-to-high altitude (HEO) and geosynchronous (GEO) orbits, are the effects of the naturally occurring particulate radiation environment. Although not as broadly important to the photovoltaic community at large as increased efficiency, the topic of radiation damage is critically important to use of solar cells in space, and is a major component of the NASA research program in space photovoltaics. A brief overview of some of the opportunities and challenges for space photovoltaic applications is given, and some of the current research directed at achieving high efficiency and controlling radiation damage in space solar cells is discussed.

Flood, D. J.↗

NASA NTRS1986

Space solar cell research - Problems and potential

The value of a passive, maintenance-free, renewable energy source was immediately recognized in the early days of the space program, and the silicon solar cell, despite its infancy, was quickly pressed into service. Efficiencies of those early space solar arrays were low, and lifetimes shorter than hoped for, but within a decade significant advances had been made in both areas. Better performance was achieved because of a variety of factors, ranging from improvements in silicon single crystal material, to better device designs, to a better understanding of the factors that affect the performance of a solar cell in space. Chief among the latter, particularly for the mid-to-high altitude (HEO) and geosynchronous (GEO) orbits, are the effects of the naturally occurring particulate radiation environment. Although not as broadly important to the photovoltaic community at large as increased efficiency, the topic of radiation damage is critically important to use of solar cells in space, and is a major component of the NASA research program in space photovoltaics. This paper will give a brief overview of some of the opportunities and challenges for space photovoltaic applications, and will discuss some of the current reseach directed at achieving high efficiency and controlling the effects of radiation damage in space solar cells.

Flood, Dennis J.↗

NASA NTRS1987

Cusp displacement at the magnetopause for large IMF Y component

The magnetic field orientation just earthward of the dayside magnetopause from 95 ISEE and 11 HEOS crossings with /By/ greater than /Bz/ in the adjacent magnetosheath indicate a statistically significant-1-earth-radius shift of the cusps toward dawn (dusk) in the Northern Hemisphere and dusk (dawn) in the Southern Hemisphere for positive (negative) By. This small shift near the magnetopause and the contrasting large shifts of some reported ionospheric cusp signatures are explained in terms of a finite thickness magnetopause model through which the cusps shift gradually from the inner to the outer edge.

Crooker, N. U.↗

NASA NTRS1989

Fuel-optimal trajectories of aeroassisted orbital transfer with plane change

The problem of minimization of fuel consumption during the atmospheric portion of an aeroassisted, orbital transfer with plane change is addressed. The complete mission has required three characteristic velocities, a deorbit impulse at high earth orbit (HEO), a boost impulse at the atmospheric exit, and a reorbit impulse at low earth orbit (LEO). A performance index has been formulated as the sum of these three impulses. Application of optimal control principles has led to a nonlinear, two-point, boundary value problem which was solved by using a multiple shooting algorithm. The strategy for the atmospheric portion of the minimum-fuel transfer is to start initially with the maximum positive lift in order to recover from the downward plunge, and then to fly with a gradually decreasing lift such that the vehicle skips out of the atmosphere with a flight path angle near zero degrees.

Naidu, Desineni Subbaramaiah↗

NASA NTRS1989

Fuel-optimal trajectories of aeroassisted orbital transfer with plane change

Naidu, D. S.↗

NASA NTRS1990

Mission optimization of the Space Infrared Telescope Facility

The Space Infrared Telescope Facility (SIRTF) mission based on a cryogenically cooled telescope is described. The SIRTF makes it possible to perform background limited measurements with three focal plane instruments in the wavelength range 2-700 microns over the entire celestial sphere for at least five years. A telescope performance limited by the natural astrophysical background between 2 to 300 microns is achieved by using a superfluid helium cryogenic system for maintaining the forebaffle temperature below 8 K. The attitude control system based on Extreme Ultra-Violet Explorer reaction wheels meets the large and small angle slew requirements and provides 0.15 arc sec pointing stability. The present HEO baseline configuration has a mass of 4370 kg. The future Titan IV/Centaur launch capability to 100,000 km for a SIRTF-sized payload is at least 5770 kg.

Brooks, Walter F.↗

NASA NTRS1992

The Germanium GAlactic Plane Patrol mission

The goal of the Germanium GAlactic Plane Patrol mission (GGAPP) is to provide a continuous monitor of the Galactic Plane (GP) for variable sources of gamma-ray lines. Potentially interesting sources include black hole candidates, X-ray binary systems, pulsars, gamma-ray bursts, and solar flares. The GGAPP instrument is an array of Ge detectors cooled by a mechanical refrigerator to achieve a spectral resolution of 2 keV at 1 MeV (1/500). A bismuth germanate (BGO) shield will restrict the field-of-view (FOV) to within 20 deg of the GP, and a modulation collimator system will locate strong sources to less than 0.3 deg in galactic longitude, provide a direct means of subtracting background, and mapping the diffuse emission from the GP. The spacecraft will be rotationally stabilized with the spin axis perpendicular to the GP such that the modulator scans in galactic longitude. A HEO or L1 orbit will keep GGAPP far away from the strong background produced by the Earth. GGAPP will provide a natural bridge between GRO and future missions such as INTEGRAL/NAE.

Tueller, Jack↗

NASA NTRS1993

ESA personal communications and digital audio broadcasting systems based on non-geostationary satellites

Personal Communications and Digital Audio Broadcasting are two new services that the European Space Agency (ESA) is investigating for future European and Global Mobile Satellite systems. ESA is active in promoting these services in their various mission options including non-geostationary and geostationary satellite systems. A Medium Altitude Global Satellite System (MAGSS) for global personal communications at L and S-band, and a Multiregional Highly inclined Elliptical Orbit (M-HEO) system for multiregional digital audio broadcasting at L-band are described. Both systems are being investigated by ESA in the context of future programs, such as Archimedes, which are intended to demonstrate the new services and to develop the technology for future non-geostationary mobile communication and broadcasting satellites.

Logalbo, P.↗

NASA NTRS1993

Global coverage Mobile Satellite Systems: System availability versus channel propagation impairments

Mobile Satellite Systems (MSS) in Highly Elliptical (HEO) and circular Earth orbits at Medium (MEO) and Low (LEO) altitudes have been intensively studied in the last few years as an effective means of providing global communication services. Such global coverage MSS networks are also expected to mitigate typical channel impairments usually encountered in geostationary Land Mobile Satellite (LMS) systems. In the design stages of these satellite networks, information regarding the mobile propagation channel is needed to assess the overall link availability versus elevation angle and environmental scenarios. For multisatellite LMS configurations, the mobile user on the Earth surface sees, at any given time, more than one satellite of the constellation. In our paper, it is shown that, under certain working assumptions regarding the statistics of the propagation channel, an improvement of the link availability may be achieved through the use of a multisatellite constellation. The analyses have been carried out using the European Space Agency (ESA) LMS propagation data base which presently covers a wide range of elevation angles and environmental scenarios.

Sforza, M.↗

NASA NTRS1994

Multisatellite constellation configuration selection for multiregional highly elliptical orbit constellations

The Archimedes Project is a joint effort of the European Space Agency (ESA) and the National Space Development Agency of Japan (NASDA). The primary goal of the Archimedes project is to perform a technical feasibility analysis and preliminary design of a highly inclined multisatellite constellation for direct broadcast and mobile communications services for Europe, Japan and much of North America. This report addresses one aspect of this project, specifically an analysis of continuous satellite coverage using multiregional highly elliptical orbits (M-HEO's). The analysis methodology and ensuing software tool, named SPIFF, were developed specifically for this project by the author during the summer of 1992 under the STA/NSF Summer Institute in Japan Program at Tsukuba Space Center.

Matossian, Mark G.↗

NASA NTRS1993

The possibilities for mobile and fixed services up to the 20/30 GHz frequency bands

Satellite Communications and broadcasting is presently in a period of considerable change. In the fixed service there is strong competition from terrestrial fiber optic systems which have virtually arrested the growth of the traditional satellite market for long distance high capacity communications. The satellite has however made considerable progress in areas where it has unique advantages; for example, in point to multipoint (broadcasting), multipoint to point (data collection) and generally in small terminal system applications where flexibility of deployment coupled with ease of installation are of importance. In the mobile service, in addition to the already established geostationary systems, there are numerous proposals for HEO, MEO and LEO systems. There are also several new frequency allocations as a result of the WARC 92 to be taken into account. At one extreme there are researchers working on Ka band 20/30 GHz mobile systems and there are other groups who foresee no future above the L-band frequency allocations. Amongst all these inputs it is difficult to see the direction in which development activities both for satellites and for earth segment should be focused. However, as an aid to understanding, this paper seeks to find some underlying relationships and to clarify some of the variables.

Hughes, Clifford D.↗

NASA NTRS1995

Observations of the magnetopause current layer: Cases with no boundary layer and tests of recent models

Evidence for the probable existence of magnetospheric boundary layers was first presented by Hones, et al. (1972), based on VELA satellite plasma observations (no magnetic field measurements were obtained). This magnetotail boundary layer is now known to be the tailward extension of the high-latitude boundary layer or plasma mantle (first uniquely identified using HEOS 2 plasma and field observations by Rosenbauer et al., 1975) and the low-latitude boundary layer (first uniquely identified using IMP 6 plasma and field observations by Eastman et al., 1976). The magnetospheric boundary layer is the region of magnetosheath-like plasma located Earthward of, but generally contiguous with the magnetopause. This boundary layer is typically identified by comparing low-energy (less than 10 keV) ion spectra across the magnetopause. Low-energy electron measurements are also useful for identifying the boundary layer because the shocked solar wind or magnetosheath has a characteristic spectral signature for electrons as well. However, there are magnetopause crossings where low-energy electrons might suggest a depletion layer outside the magnetopause even though the traditional field-rotation signature indicates that this same region is a boundary layer Earthward of the current layer. Our analyses avoided crossings which exhibit such ambiguities. Pristine magnetopause crossings are magnetopause crossings for which the current layer is well defined and for which there is no adjoining magnetospheric boundary layer as defined above. Although most magnetopause models to date apply to such crossings, few comparisons between such theory and observations of pristine magnetopause crossings have been made because most crossings have an associated magnetospheric boundary layer which significantly affects the applicable boundary conditions for the magnetopause current layer. Furthermore, almost no observational studies of magnetopause microstructure have been done even though key theoretical issues have been discussed for over two decades. This is because plasma instruments deployed prior to the ISEE and AMPTE missions did not have the required time resolution and most ISEE investigations to-date have focused on tests of MHD plasma models, especially reconnection. More recently, many phenomenological and theoretical models have been developed to explain the existence and characteristics of the magnetospheric boundary layers with only limited success to date. The cases with no boundary layer treated in this study provide a contrary set of conditions to those observed with a boundary layer. For the measured parameters of such cases, a successful boundary layer model should predict no plasma penetration across the magnetopause. Thus, this research project provides the first direct observational tests of magnetopause models using pristine magnetopause crossings and provides important new results on magnetopause microstructure and associated kinetic processes.

Eastman, Timothy E.↗

NASA NTRS1994

Spectral polarization analysis of the interplanetary magnetic field fluctuations

A new computational method and algorithm, based on complex Fourier analysis, is used to derive the spectral density of plane and circularly polarized fluctuation components of the interplanetary magnetic field. Applications of the method have been made using HEOS 2 (1 AU), Pioneer 10 (5 AU), Pioneer 11 (20 AU), and International Cometary Explorer (ICE) (Giocabini-Zinner's comet) data sets. The results show the existence of circularly polarized magnetohydrodynamic (MHD) waves in all cases.

Polygiannakis, J. M.↗

NASA NTRS1995

Narrow- and wide-band channel characterization for land mobile satellite systems: Experimental results at L-band

The results of an airborne measurement campaign aimed at the characterization of the mobile satellite link are presented in this paper. The experimental tests were carried out at 1.8 GHz. The objective of the campaign was to obtain results applicable to all proposed satellite constellations: LEO, HEO, and GEO. Therefore, the measurements were performed for elevation angles from 10 deg...80 deg using a light aircraft. A set of different environments and operational scenarios have been investigated, typically for hand-held and car-mounted applications. We present a survey of wide- and narrowband results for a wide range of elevation angles and environments. For the wideband characterization, the power delay profiles of the channel impulse response are presented and discussed. Figures for the delay spread versus elevation and for the carrier-to-multipath ratio versus time are also given. The narrowband behaviour of the channel is described by power series.

Jahn, Axel↗

NASA NTRS1995

Hand-held terminal antennas for personal satellite communications

This paper deals with two hand-held antenna types operating with geostationary and medium earth orbit (GEO and MEO) satellite systems. They could be applied to the low earth orbit (LEO) and highly elliptical orbit (HEO) systems respectively doing the appropriate frequency scale designs. The first one is a lambda/2 (1/2 turn) quadrifilar helix (quasi-hemispherical coverage), and the second one is a self-diplexed antenna made of a circular patch and a short-circuited ring patch in stacked configuration (zenithal coverage).

Caballero, J. E.↗