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At least 577 records · Page 32

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NASA NTRS1977

Experiments in data collection technology using satellites

A variety of techniques potentially useful to data collection have been tested. An automatic data collection platform with a minicomputer collects and preprocesses data, then sends desired information when interrogated through a communication satellite. Position surveillance by tone-code ranging through communication satellites is automatic, real time and accurate. Emergency medical data transmissions from ambulances to hospitals can be extended to rural and remote areas by direct satellite links. A small platform can send emergency-related data through a satellite while the satellite is routinely relaying powerful communication signals. A low orbit satellite provides means to locate existing emergency locator beacons.

Anderson, R. E.↗

NASA NTRS1978

Satellite formation. II

A satellite formation model is extended to include evolution of planetary ring material and elliptic orbital motion. In this model the formation of the moon begins at a later time in the growth of the earth, and a significant fraction of the lunar material is processed through a circumterrestrial debris cloud where volatiles might have been lost. Thus, the chemical differences between the earth and moon are more plausibly accounted for. Satellites of the outer planets probably formed in large numbers throughout the growth of those planets. Because of rapid inward evolution of the orbits of small satellites, the present satellite systems represent only satellites formed in the last few percent of the growths of their primaries. The rings of Saturn and Uranus are most plausibly explained as the debris of satellites disrupted within the Roche limit. Because such a ring would collapse onto the planet in the course of any significant further accretion by the planet, the rings must have formed very near or even after the conclusion of accretion.

Harris, A. W.↗

NASA NTRS1978

System implementation for Earth Radiation Budget Satellite System

A description is presented of the instrument system which is needed for the Earth Radiation Budget Satellite System (ERBSS). The system is to be composed of instruments on two of NOAA's near-polar sun-synchronous Tiros-N/NOAA A through G series of operational satellites and on a NASA midinclination satellite of the Applications Explorer Mission (AEM) type referred to as ERBS-A/AEM. The Tiros-N/NOAA satellites will be in nominal 833 km altitude circular orbits with orbital inclinations of 98 deg. The AEM satellite will be in a circular orbit with an inclination of approximately 56 deg and a nominal altitude of 600 km. Each satellite will carry wide field-of-view (WFOV) and medium field-of-view (MFOV) sensors, a sensor for measuring the solar constant, and a narrow field-of-view (NFOV) cross-track scanner. The conceptual design of the W/MFOV instrument is discussed along with the conceptual design of the scanner.

Cooper, J. E.↗

NASA NTRS1978

Solar power satellites: The Engineering Challenges

Certain elements of solar power satellite design and system engineering studies are reviewed analyzing solar power satellites as a potential baseload electric power source. The complete system concept concept includes not only the satellites and their ground stations, but also the space transportation for delivery of the satellites, piece by piece, into space, and the factories for their construction in space. Issues related to carrying the solar power satellite concept from the present design study phase through implementation of actual hardware are considered. The first issue category is environmental aspects of the SPS systems. The second category of issues is the technology risks associated with achieving the necessary component and subsystem performances. The third category includes the engineering issues associated with carrying out such a large scale project. The fourth issue category is financial: the funding required to bring such a project into being and the costs of the satellites and resulting cost of the power produced as compared to potential alternative energy sources.

Woodcock, G. R.↗

NASA NTRS1979

Effects of tether attachments on the Shuttle/Tethered Satellite System dynamics

The dynamics of the Shuttle Tethered Satellite System are influenced by attaching the tether at some point other than the center-of-masses of the Shuttle and the subsatellite. At the Shuttle, the tether attachment is made at the end of a boom deployed out of the payload bay. This attachment noticeably affects retrieval dynamics of the satellite pendulous motion. At the satellite, the tether attachment is assumed to be made on the circumference of the satellite. This attachment greatly affects the attitude motion of the satellite about its own center-of-mass. Computer simulation results are presented showing the effects of the Shuttle boom in a three-dimensional model and the effects of satellite attachment in a planar model.

Gresham, L. L.↗

NASA NTRS1980

Modelling of environmentally induced discharges in geosynchronous satellites

The NASCAP computer code was used to compute the charging and discharging characteristics of a typical communications satellite in geosynchronous orbit. For the case of a severe substorm satellite surface differential charging in sunlight was found to be substantially less than that required to produce discharges in ground simulation studies. A discharge process was postulated involving discharges triggered at edges (or imperfection) followed by discharges to space. The characteristics of such discharges was parametrically varied to evaluate the possible effects on the satellite. Results indicated that discharge characteristics inferred from satellite monitors could be caused by predicted space discharges, that single cell discharges to space can reduce surface potentially over entire satellite, and that low density electron trajectory computations indicate that discharge generated electrons do not return to the satellite by long trajectories. Current transients predicted do not agree with available ground simulation results indicating that additional work must be done both analytically and experimentally to understand and fully explain these discrepancies.

Stevens, N. J.↗

NASA NTRS1980

Satellite Power Systems (SPS) concept definition study. Volume 5: Special emphasis studies

Satellite configurations based on the Satellite Power System baseline requirements were analyzed and a preferred concept selected. A satellite construction base was defined, precursor operations incident to establishment of orbital support facilities identified, and the satellite construction sequence and procedures developed. Rectenna construction requirement were also addressed. Mass flow to orbit requirements were revised and traffic models established based on construction of 60 instead of 120 satellites. Analyses were conducted to determine satellite control, resources, manufacturing, and propellant requirements. The impact of the laser beam used for space-to-Earth power transmission upon the intervening atmosphere was examined as well as the inverse effect. The significant space environments and their effects on spacecraft components were investigated to define the design and operational limits imposed by the environments on an orbit transfer vehicle. The results show that LEO altitude 300 nmi and transfer orbit duration 6 months are preferrable.

Hanley, G. M.↗

NASA NTRS1980

Tidal dissipation, orbital evolution, and the nature of Saturn's inner satellites

Estimates of tidal damping times of the orbital eccentricities of Saturn's inner satellites place constraints on some satellite rigidities and dissipation functions Q. These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturn's inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite.

Peale, S. J.↗

NASA NTRS1980

Detection of tides on the Patagonian shelf by the SEASAT satellite radar altimeter - An initial comparison

The detection of tides on the Patagonian shelf off Argentina by the SEASAT satellite radar altimeter is reported. The satellite data was obtained during two passes of the satellite over the shelf on August 1 and August 4, 1978, and compared with empirical estimates of the tidal elevation along the satellite ground tracks based on coastal tide gage data, an estimate of tidal variation over the shelf, and the tidal prediction equations of Shureman (1958). Good agreement was observed between the satellite data of sea surface height with respect to a standard ellipsoid and the empirical estimate for the near-shore pass, with agreement degraded slightly for the pass further off shore. It is concluded that the generally good agreement found demonstrates the applicability of satellite radar altimeter data to the determination of tides and that small length scale fluctuations in the geoid are minor along the two ground tracks.

Parke, M. E.↗

NASA NTRS1980

Modelling of environmentally induced discharges in geosynchronous satellites

The NASCAP computer code is used to compute the charging and discharging characteristics of a typical communications satellite in geosynchronous orbit. For the case of a severe substorm, satellite surface differential charging in sunlight is found to be substantially less than that required to produce discharges in ground simulation studies. A discharge process is postulated involving discharges triggered at edges (or imperfection) followed by discharges to space. The characteristics of such discharges are parametrically varied to evaluate the possible effects on the satellite. It has been found that discharge characteristics inferred from satellite monitors could be caused by predicted space discharges, that single cell discharges to space can reduce surface potential over entire satellite, and that low-density electron trajectory computations indicate that discharge generated electrons may not return to the satellite by long trajectories. Current transients predicted do not agree with the available ground simulation results indicating that additional work must be done both analytically and experimentally to understand and fully explain these discrepancies.

Stevens, N. J.↗

NASA NTRS1982

The satellites of Uranus

Observations and the probable natures of the five known satellites of Uranus are reviewed. Photographic, photoelectric and CCD photometry of the satellites since 1961, although in agreement within experimental error, is not as mutually consistent as may be expected, and broadband JHK photometry falls in a portion of the J-H, H-K color diagram difficult to interpret. Spectrophotometry in the range 0.3-1.1 microns taken on two separate occasions is inconsistent, with only the relatively neutral reflectances of Titania and Oberon regarded as well established. Near-infrared spectrophotometry has revealed the presence of water ice or frost on the satellite surfaces, possibly in a very pure state, with spectra most similar to Ganymede. Estimations of the properties of the satellites from their surface geometric albedos, assumed mean densities and dynamics yield radii in the range 160-520 km, albedos on the order of 0.5 and densities of about 1.3 g/cu cm, similar to the icy Saturn satellites. The satellites are also believed to have formed after the event that caused the planet to tilt to its present obliquity.

Cruikshank, D. P.↗

NASA NTRS1981

Results and analyses of ground-based and satellite ozone observations A review

The ground-based network that has been in existence for several decades is described. Attention is given to two basic questions, namely the way in which monthly satellite analyses compare with those from the ground-based network and the way in which the trends from the satellite and ground-based data compare. Synoptic analyses of total ozone for December 1970 are compared with a subjective analysis of the ground-based network and an objective backscatter ultraviolet (BUV) analysis. It is found that the BUV analysis is the more consistent of the two. Considering monthly analyses for September 1970, satellite and ground-based data are found to agree quite well. In considering the question of trend determination by the ground-based versus the satellite systems, attention is given to the sources of error in the satellite system. The principal sources of error in BUV analysis are thought to be calibration drift with time and the fact that, over the lifetime of the satellite spacecraft, power limitations result in a decrease in the number of data points per month, possibly with significant gaps in the coverage.

Miller, A. J.↗

NASA NTRS1982

Recommended satellite imagery capabilities for disaster management

This study explores the role that satellite imaging systems might play in obtaining information needed in the management of natural and manmade disasters. Information requirements which might conceivably be met by satellite were identified for over twenty disasters. These requirements covered pre-disaster mitigation and preparedness activities, disaster response activities, and post-disaster recovery activities. The essential imaging satellite characteristics needed to meet most of the information requirements are 30 meter (or finer) spatial resolution, frequency of observations of one week or less, data delivery times of one day or less, and stereo, synoptic all-weather coverage of large areas in the visible, near infrared, thermal infrared and microwave bands. Of the current and planned satellite systems investigated for possible application to disaster management, Landsat-D and SPOT appear to have the greatest potential during disaster mitigation and preparedness activities, but all satellites studied have serious deficiencies during response and recovery activities. Several strawman concepts are presented for a satellite system optimized to support all disaster management activities.

Richards, P. B.↗

NASA NTRS1982

Fokker-Planck response of stochastic satellites

The present investigation is concerned with the effects of stochastic geometry and random environmental torques on the pointing accuracy of spinning and three-axis stabilized satellites. The study of pointing accuracies requires a knowledge of the rates of error growth over and above any criteria for the asymptotic stability of the satellites. For this reason the investigation is oriented toward the determination of the statistical properties of the responses of the satellites. The geometries of the satellites are considered stochastic so as to have a phenomenological model of the motions of the flexible structural elements of the satellites. A widely used method of solving stochastic equations is the Fokker-Planck approach where the equations are assumed to define a Markoff process and the transition probability densities of the responses are computed directly as a function of time. The Fokker-Planck formulation is used to analyze the response vector of a rigid satellite.

Huang, T. C.↗

NASA NTRS1982

The satellites of Jupiter and Saturn

The nature of the Jovian and Saturnian satellites as places, and their probable history, are discussed. Orbital data on the satellites are presented, including the radius, period, eccentricity, inclination, mass, density, albedo, and surface composition. The relationship between cratering and geologic time scales is reviewed. The major and intermediate satellites are categorized as giant, rocky, or icy, and their surface topography, internal heat sources, geologic activity, and impact crater density are discussed. The characteristics of Titan's atmosphere are covered. What is known of the small satellites of Saturn, the inner satellites of Jupiter, and the irregular outer satellites of Jupiter is assessed.

Morrison, D.↗

NASA NTRS1982

An advanced generation land mobile satellite system and its critical technologies

A conceptual design for a Land Mobile Satellite System (LMSS) for the 1990s is presented. LMSS involves small tranceivers accessing satellites directly, with ground reception through small car-top antennas. The satellite would have a large antenna and blanket coverage areas in the UHF. The call may originate from a home, be carried by wire to a gateway, transmitted to satellite on the S-band, converted to UHF on the satellite, and transmitted to the vehicle. The system design is constrained by the number of users in an area during the busiest hours, Shuttle storage, controllability factors, and the total area served. A 55-m antenna has been selected, with 87 spot beams and two 10 MHz UHF bands in the 806-890 MHz band. A 17 dB interbeam isolation level is required, implying that sufficient sub-bands can be generated to assure 8265 total channels. The mobile satellite (MSAT) would have an 83 m mast lower segment, a 34 m upper segment, and a second, 10 m antenna made of a deployable mesh. Various antenna function modes are considered.

Naderi, F.↗

NASA NTRS1982

Signalling characteristics in satellite-aided land mobile communications

The feasibility of land mobile radio communications has been demonstrated by a large number of experiments with NASA's ATS satellites. Significant differences in the propagation characteristics of satellite and terrestrial mobile signal paths were observed in the experiments. Terrestrial paths are best in cities where they can provide frequency reuse and assure communication by bouncing signals around obstructions. Satellites may be best in thinly populated areas because they eliminate the need for many tower mounted relays. The satellite paths do not have the severe Rayleigh fading that limits the range and signal quality of terrestrial paths if the satellite is above approximately ten degrees elevation, a value easily achieved for the United States. The experiments verified that high quality voice communications and other functions, such as data transmission and vehicle position surveillance, are easily accomplished through geostationary satellites with vehicle transmitter power and antenna gain no different than those of terrestrial mobile communications.

Anderson, R. E.↗

NASA NTRS1982

Cloud cover sampling capability of various satellite combinations for ISCCP

One of the most important objectives of the International Satellite Cloud Climatology Project (ISCCP) is the quantification of cloud cover and associated radiation parameters for climate research. The time-varying nature of cloud cover requires a measurement system that obtains cloud radiance at sufficient time intervals for determining accurate values of cloudiness for the appropriate averaging period. Current plans for the ISCCP call for such day to be acquired every 3 hours during both data and night to ensure that the diurnal cycle of cloud cover is adequately sampled. The satellite system proposed is an array of geostationary and polar-orbiting satellites. The error in mean cloud cover estimates for observation systems which do not include geostationary satellites are quantified. Candidates for such a system include two NOAA Sun-synchronous satellites with daylight equatorial-crossing times of 0730 and 1430, and a USSR satellite in an 81 deg. inclination, 900-km altitude orbit.

Harrison, E. F.↗