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At least 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Shuttle-tethered satellite system definition study. Volume 1: Executive study

The Tethered Satellite System has great prospects for extending orbital operations capability of the Space Transportation System to science, applications, and technology projects not otherwise attainable. The system will installed in the Shuttle Orbiter and will have the capability to deploy a captive satellite up to 100 km away from the Orbiter. Control and retrieval of the satellite are accomplished by means of a tether line connecting the satellite and the cargo bay mounted equipment in the Orbiter. At low satellite altitudes, the system will permit investigations of a duration that could not be pursued with sounding rockets of free-flying spacecraft. The propose of the Shuttle/Tethered Satellite System Definition Study was to produce the preliminary design, preliminary specifications, gross program plans, and program cost estimate for a 1982 operational verification flight. This was accomplished during a fifteen month effort under by the NASA George C. Marshall Space Flight Center (MSFC). The MSFC Phase 1 and related studies demonstrated the feasibility of the system and served as a starting point for the Phase 2 definition study.

Source record↗
NASA NTRS1984

NASA's mobile satellite communications program; ground and space segment technologies

This paper describes the Mobile Satellite Communications Program of the United States National Aeronautics and Space Administration (NASA). The program's objectives are to facilitate the deployment of the first generation commercial mobile satellite by the private sector, and to technologically enable future generations by developing advanced and high risk ground and space segment technologies. These technologies are aimed at mitigating severe shortages of spectrum, orbital slot, and spacecraft EIRP which are expected to plague the high capacity mobile satellite systems of the future. After a brief introduction of the concept of mobile satellite systems and their expected evolution, this paper outlines the critical ground and space segment technologies. Next, the Mobile Satellite Experiment (MSAT-X) is described. MSAT-X is the framework through which NASA will develop advanced ground segment technologies. An approach is outlined for the development of conformal vehicle antennas, spectrum and power-efficient speech codecs, and modulation techniques for use in the non-linear faded channels and efficient multiple access schemes. Finally, the paper concludes with a description of the current and planned NASA activities aimed at developing complex large multibeam spacecraft antennas needed for future generation mobile satellite systems.

Naderi, F.↗
NASA NTRS1985

The enigma of the Uranian satellites' orbital eccentricities

The eccentricity decay times for the Uranian satellites are calculated using recent observations (Brown et al., 1982) of the diameters and orbital elements of the satellites and assuming reasonable dissipation functions and rigidities for icy satellites. For the outer two satellites, Titania and Oberon, the decay times are found to be very long, whereas the inner three satellites, Miranda, Ariel, and Umbriel, have decay times on the order of 10 to the 7th to 10 to the 8th years and have a near-commensurability in their mean motions that cannot force their eccentricities. There are several possible solutions for the lack of resonant forcing: (1) the reported eccentricities are incorrect, and are very nearly zero, (2) the reported mean motions are incorrect, and an exact commensurability exists, (3) the physical properties assumed for the satellites are grossly in error, and (4) the system is evolving rapidly, perhaps from a previous state of higher eccentricity. A new lower bound of about 17,000 on the dissipation function of Uranus is calculated from the mass of Ariel and its proximity to Uranus.

Squyres, S. W.↗
NASA NTRS1985

Analysis of Satellite Drag and Spin Decay Data

Work was begun as an effort to obtain satellite drag and upper atmospheric wind data from a spinning spherical satellite to be launched from the space shuttle scheduled for the STS-11 flight. The satellite was an inflated sphere having high area-to-mass ratio which would result in measurable orbital decay due to aerodynamic drag forces experienced by the satellite in the Earth's upper atmosphere. The satellite would also experience a spin decay due to the aerodynamic torque acting to slow the satellite spinning. The original proposal was to develop data analysis procedures to be employed to extract the desired information. Work was performed concerning the data analysis procedure prior to the STS-11 launch; however, the major effort was to begin after the data had been obtained. Unfortunately, the launch of the inflatable sphere on STS-11 was a failure in that the sphere did not inflate properly and no data was obtained. The emphasis of the work planned under this effort was then changed to develop a proposal for a series of experiments similar to the one originally planned for STS-11.

Karr, G. R.↗
NASA NTRS1985

The northern Utah satellite (NUSAT) communications link

During the planning Stages of the NUSAT satellite, an obvious issue to be discussed was the method of communications to be used. The frequencies would have to be high enough to pass through the atmosphere relatively unattenuated but low enough that antennas and transmission lines would not be so critical in length and properties that unexperienced students would have difficulty with handling them. In conversations with the Federal Communications Commission (FCC) in Washington D.C., the frequencies of 450.000 MHz and 137.900 MHz were decided upon and applied for licensing. The 450.000 MHt up-link is used on a "non-interference" basis to control the satellite while the 137.900 MHz frequency down-links data from the satellite. A reasonable amount of time was spent discussing the mods of transmission to be used in communication with the satellite. Representatives of the amateur radio satellite organization, AMSAT, were contacted for ides. This organization seems to favor AM types of emissions such as CW to control their OSCAR series of satellites and also the current Phase 111 unit. NUSAT personnel felt, however, that there would be merit in the improved signal to noise ratio usually obtained in an FM mode. Doppler shift of the transmitted information on the NUSAT also had to be considered. The final decision WBS to use Audio Frequency Shift Keying (AFSK) modulated on an FM carrier. In this mode the audio tones used would not shift frequency with Doppler - only the carrier would shift in frequency. If necessary, the receivers could be fitted with a form of AFC control or simply have enough bandwidth to handle the Doppler shift with an acceptable increase in noise.

Lee Barrett↗
NASA NTRS1986

Network design consideration of a satellite-based mobile communications system

Technical considerations for the Mobile Satellite Experiment (MSAT-X), the ground segment testbed for the low-cost spectral efficient satellite-based mobile communications technologies being developed for the 1990's, are discussed. The Network Management Center contains a flexible resource sharing algorithm, the Demand Assigned Multiple Access scheme, which partitions the satellite transponder bandwidth among voice, data, and request channels. Satellite use of multiple UHF beams permits frequency reuse. The backhaul communications and the Telemetry, Tracking and Control traffic are provided through a single full-coverage SHF beam. Mobile Terminals communicate with the satellite using UHF. All communications including SHF-SHF between Base Stations and/or Gateways, are routed through the satellite. Because MSAT-X is an experimental network, higher level network protocols (which are service-specific) will be developed only to test the operation of the lowest three levels, the physical, data link, and network layers.

Yan, T.-Y.↗
NASA NTRS1986

Differential GPS approaches to orbit determination of high-altitude earth satellites

Orbit determination of earth satellites using conventional (up-looking) differential GPS is limited to lower altitudes due to the finite antenna beamwidth and altitude of GPS satellites. Higher user satellites suffer from having short GPS view period; at sufficiently high altitudes, up-looking GPS visibility becomes so poor that no differential measurements are possible. For such higher user satellites, two alternative differential GPS approaches are suggested. A covariance analysis is performed investigating the capabilities of these approaches at various user satellite altitudes. Best differential GPS data types are identified for satellites within different altitude ranges. Error sources and other factors affecting the orbit determination accuracy are examined. Relative merits of these approaches are compared. When applied to QUASAT, the proposed earth orbiting radio telescope with an elliptical orbit, a sub-meter orbit determination accuracy is expected.

Wu, S.-C.↗
NASA NTRS1986

Radiation physics and modelling for off-nadir satellite-sensing of non-Lambertian surfaces

The primary objective of this paper is to provide a deeper understanding of the physics of satellite remote-sensing when off-nadir observations are considered. Emphasis is placed on the analysis and modeling of atmospheric effects and the radiative transfer of non-Lambertian surface reflectance characteristics from ground-level to satellite locations. The relative importance of spectral, spatial, angular, and temporal reflectance characteristics for satellite-sensed identification of vegetation types in the visible and near-infrared wavelength regions is evaluated. The highest identification value is attributed to angular reflectance signatures. Using radiative transfer calculations to evaluate the atmospheric effects on angular reflectance distributions of vegetation surfaces, atmosphere-invariant angular reflectance features such as the 'hot spot' and the 'persistent valley' are identified. A new atmospheric correction formalism for complete angular reflectance distributions is described. A sample calculation demonstrates that a highly non-Lambertian measured surface reflectance distribution can be retrieved from simulated satellite data in the visible and near infrared to within about 20 percent accuracy for almost all view directions up to 60 deg off-nadir. Thus the high value of angular surface reflectance characteristics (the 'angular signature') for satellite-sensed feature identification is confirmed, which provides a scientific basis for future off-nadir satellite observations.

Gerstl, S. A.↗
NASA NTRS1986

Satellite attitude motion models for capture and retrieval investigations

The primary purpose of this research is to provide mathematical models which may be used in the investigation of various aspects of the remote capture and retrieval of uncontrolled satellites. Emphasis has been placed on analytical models; however, to verify analytical solutions, numerical integration must be used. Also, for satellites of certain types, numerical integration may be the only practical or perhaps the only possible method of solution. First, to provide a basis for analytical and numerical work, uncontrolled satellites were categorized using criteria based on: (1) orbital motions, (2) external angular momenta, (3) internal angular momenta, (4) physical characteristics, and (5) the stability of their equilibrium states. Several analytical solutions for the attitude motions of satellite models were compiled, checked, corrected in some minor respects and their short-term prediction capabilities were investigated. Single-rigid-body, dual-spin and multi-rotor configurations are treated. To verify the analytical models and to see how the true motion of a satellite which is acted upon by environmental torques differs from its corresponding torque-free motion, a numerical simulation code was developed. This code contains a relatively general satellite model and models for gravity-gradient and aerodynamic torques. The spacecraft physical model for the code and the equations of motion are given. The two environmental torque models are described.

Cochran, John E., Jr.↗
NASA NTRS1987

A multipurpose satellite ejection system

A design is presented for a pneumatic ejection system capable of ejecting a spin stabilized satellite from the cargo bay of space vehicles. This system was orginally designed for use on the Spacelab 6 shuttle mission, but is now being considered for use with expendable rockets for launching satellites. The ejection system was designed to launch a 150 lb satellite at an initial ejection velocity of 32 ft/sec with a spin rate of 30 rev/min. The ejection system consists of a pneumatic cylinder, satellite retaining mechanism, and bearing assembly to allow the satellite to rotate during the spin up phase. As the cylinder is pressurized rapidly causing movement of the actuation piston, the mechanism automatically releases the spinning satellite by retracting a pneumatic locking pin and three spring loaded holddown pins. When the piston reaches the end of its stroke, it encounters a crushable aluminum honeycomb shock absorber which decelerates the piston and retaining mechanism. The assembly is designed for multiple uses except for the crushable shock absorber and pyrotechnic valves. The advantage of the design is discussed and patent no. and date given.

Moore, Michael B.↗
NASA NTRS1986

Communications satellites in the fiber optics era

Three new satellite technologies, i.e., electronically hopped antenna beams, on-board switching, and laser intersatellite links, will soon be tested on experimental satellites. These technologies will enable future satellites to capture significant amounts of data and voice traffic. Fiber cables on the continents will dominate trunking routes, but satellites functioning as switches will be more flexible and reliable, and will cost less than terrestrial-based switches. When satellite switches and laser intersatellite links are combined, the resulting satellite networks will provide cost-effective, independent voice/data networks in the coming global village. They will augment existing terrestrial networks for thin route and low data rate links.

Lovell, R. R.↗
NASA NTRS1986

Satellites of Uranus and Neptune, and the Pluto-Charon system

The orbital properties, surface compositions, opposition surges, masses, radii, and densities of the satellites of Uranus are presented. It is noted that the Uranian satellites are comparable in size to the largest of Saturn's icy satellites while density measurements suggest that the bulk compositions of Ariel and Umbriel might be different from those of Titania and Oberon. Consideration is given to the two satellites of Neptune and the question of a third satellite is addressed. The elements of Charon, determined from astrometric observations by photographic and speckle interferometric techniques, and then from eclipse observations, are given. The diurnal period of Pluto and its photometric lightcurve are discussed. The similarities and differences existing between the satellites of Uranus and Neptune and the Pluto-Charon pair are mentioned briefly.

Cruikshank, Dale P.↗
NASA NTRS1986

Precise orbit determination of a geosynchronous satellite with differential very-long-baseline interferometry

The use of differential VLBI (DVLBI) to determine the orbit of a U.S. geosynchronous satellite with a position accuracy of a few meters is examined. DVLBI was employed to track the geosynchronous satellite DSCS-II; two observation channels with 2 MHz bandwidth each in a 2 GHz band were used to receive the telemetry signal of the satellite and quasar radio signals. The random observation, delay, station location, and quasar position errors are discussed. The correlation processing of the quasar and satellite signals is described. The accuracy of the estimated satellite position is evaluated using a covariance analysis. It is determined that the satellite position was estimated with an accuracy of about 4 m at the center of the observation arc. It is noted that the modeling error of the solar radiation pressure factor, ionospheric correction error, and observation error need to be corrected in order to provide more accurate orbit determinations.

Shiomi, Tadashi↗
NASA NTRS1987

An upper limit on the sizes of shepherding satellites at Saturn's ring G

An accurate analytical theory of the absorption of energetic magnetospheric particles by an inert satellite is developed for the case in which the radius b of the satellite is much less than the equatorial gyroradius r(g) of the particles and in which r(g) is in turn much less than the radius r of the satellite's orbit. In previous interpretations of Pioneer 11 observations, an estimate of the lifetime against absorption of energetic protons at Saturn's ring G has been made. This lifetime is used in the framework of the absorption theory to establish an upper limit on the sizes of shepherding satellites associated with the ring. The resulting upper limit, ignoring the absorption of the optically observed particulate matter, is given for an assemblage of N satellites of various radii. It is noted that ring G lies outside the Roche limit. No shepherding satellites at ring G were detected optically by Voyagers 1 and 2, and the searches were not comprehensive in longitudinal coverage.

Van Allen, James A.↗
NASA NTRS1986

Concept definition study for recovery of tumbling satellites. Volume 2: Supporting research and technology report

A number of areas of research and laboratory experiments were identified which could lead to development of a cost efficient remote, disable satellite recovery system. Estimates were planned of disabled satellite motion. A concept is defined as a Tumbling Satellite Recovery kit which includes a modular system, composed of a number of subsystem mechanisms that can be readily integrated into varying combinations. This would enable the user to quickly configure a tailored remote, disabled satellite recovery kit to meet a broad spectrum of potential scenarios. The capability was determined of U.S. Earth based satellite tracking facilities to adequately determine the orientation and motion rates of disabled satellites.

Cable, D. A.↗
NASA NTRS1988

A k-permutation algorithm for Fixed Satellite Service orbital allotments

A satellite system synthesis problem, the satellite location problem (SLP), is addressed in this paper. In SLP, orbital locations (longitudes) are allotted to geostationary satellites in the Fixed Satellite Service. A linear mixed-integer programming model is presented that views SLP as a combination of two problems: (1) the problem of ordering the satellites and (2) the problem of locating the satellites given some ordering. A special-purpose heuristic procedure, a k-permutation algorithm, that has been developed to find solutions to SLPs formulated in the manner suggested is described. Solutions to small example problems are presented and analyzed.

Reilly, Charles H.↗
NASA NTRS1988

Estimating noise levels of remotely sensed measurements from satellites using spatial structure analysis

A technique is presented whereby the noise level of satellite measurements of the atmosphere and earth can be estimated. The technique analyzes a spatial array of data measured by a satellite instrument. A minimum of about 200 satellite measurements is required, preferably in a regular pattern. Statistical structure analysis is used to describe a combination of the mean gradient and noise in the data. The noise level is then estimated by separating out the gradient information and leaving only the noise. Results are presented for four satellite sounding instruments, and effective blackbody or brightness temperature noise levels were compared to prelaunch specifications or inflight calibrations for each instrument. Comparisons showed that in the absence of cloud-contaminated measurements (in the case of infrared data) and away from the highly variable ground surface, the noise level of various satellite instruments can be obtained without the need for calibration data. The noise levels imply how much spatial averaging is possible, without smearing the detected geophysical gradient, and how much is necessary, to meet the absolute signal accuracy requirements for the intended use of the satellite measurements.

Hillger, Donald W.↗
NASA NTRS1988

Accretional heating of the satellites of Saturn and Uranus

A model developed specifically for accretional heating of small icy satellites of massive planets was used to investigate the effects of various parameters on the accretional heating of the Saturn and Uranus satellites. It was shown that accretional heating is completely dominated by cisplanetary impactors, with substantial deposition of heat occurring well below a satellite's surface. Three parameters that have large uncertainties were found to affect accretional temperature profile: the Safronov parameter for cisplanetary impactors, the fraction of impactor energy deposited below the surface, and the exponent in the impactor size-distribution equation. For most satellites, the depth of maximum heating and the maximum temperature reached have smooth positive dependences on satellite size. Accretional heating of these satellites is insufficient to melt H2O ice, but, if NH3 or CH4 are present, accretional heating may produce a warm buoyant mobile zone tens of kilometers below the surface.

Squyres, Steven W.↗