SEARCH · Engineering Papers

Results for “SATELLITES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 649 records · Page 36

All sources NASA NTRS DOE OSTI

NASA NTRS1992

Atmospheric gravitational influence on geodetic satellite orbits - Starlette analysis

The atmosphere is constantly in motion. The changing gravitational force due to the air mass movement will slightly perturb the orbit of a satellite. As the instrument accuracy for geodetic satellites improves, failure to model this perturbation can result in significant systematic errors in the orbit determination. The latter, in turn, will degrade the Earth's gravity solutions. A direct modeling technique to analyze the atmospheric gravitational influence on geodetic satellite is developed. We use the global surface pressure data from the ECMWF Initial Analysis Database to compute the gravitational force due to atmospheric perturbation exerted on given satellite as a function of time during selected orbital arcs. Satellite Laser Ranging (SLR) tracking data for selected Starlette (altitude 900 km) orbital arcs are used to test the computed force model. Although only a slight reduction in the rms residuals is observed when the atmospheric gravitational perturbation is included in the force model for data reduction of the SLR data, significant improvement is obtained in the predictability of the satellite orbit. Comprehensive studies involving more definitive test criteria and more refined models are still needed.

Chao, B. F.↗

NASA NTRS1993

Results of multiband (L, S, Ku band) propagation measurements and model for high elevation angle land mobile satellite channel

Signal propagation in the land mobile satellite (LMS) service is an important consideration due to its critical impact on the overall economic and commercial viability of the system. At frequencies allocated for LMS systems, shadowing of the line-of-sight (LOS) signal as well as multipath propagation phenomena can severely impair the link availability. In particular, as most of the studies have shown, the shadowing of LOS signal causes long and deep fades in a variety of mobile environments due to the inherent nature of the channel between the satellite and a mobile. Roadside obstacles, such as buildings, trees, utility poles etc., in the immediate vicinity of a mobile and the surrounding terrain are major sources of signal shadowing in LMS links. Therefore, a proper knowledge of link degradation is essential for cost-effective planning of a satellite based mobile communication system. The results of a propagation campaign undertaken to characterize the fading nature of LMS channel at high elevation angles is presented. It was envisaged that one of the most important physical variables contributing to the amount of LOS signal shadowing is the elevation angle of the satellite. At higher elevation angles to the satellite, less obstructions in the direct satellite-to-mobile path would therefore amount to statistically better link availability. Narrowband channel measurements were carried out at three RF frequencies corresponding to L (1.3 GHz), S (2.32/2.45 GHz), and Ku (10.4 GHz) bands. The campaign itself was divided into two phases to observe the effects of seasonal variation of foliage on the roadside trees. Phase measurements were carried out in September 1991 and in April 1992. Some important aspects from the statistical analysis of the propagation data are presented.

Parks, M. A. N.↗

NASA NTRS1993

Internetworking satellite and local exchange networks for personal communications applications

The demand for personal communications services has shown unprecedented growth, and the next decade and beyond promise an era in which the needs for ubiquitous, transparent and personalized access to information will continue to expand in both scale and scope. The exchange of personalized information is growing from two-way voice to include data communications, electronic messaging and information services, image transfer, video, and interactive multimedia. The emergence of new land-based and satellite-based wireless networks illustrates the expanding scale and trend toward globalization and the need to establish new local exchange and exchange access services to meet the communications needs of people on the move. An important issue is to identify the roles that satellite networking can play in meeting these new communications needs. The unique capabilities of satellites, in providing coverage to large geographic areas, reaching widely dispersed users, for position location determination, and in offering broadcast and multicast services, can complement and extend the capabilities of terrestrial networks. As an initial step in exploring the opportunities afforded by the merger of satellite-based and land-based networks, several experiments utilizing the NASA ACTS satellite and the public switched local exchange network were undertaken to demonstrate the use of satellites in the delivery of personal communications services.

Wolff, Richard S.↗

NASA NTRS1993

Optimizing space constellations for mobile satellite systems

Designing a mobile satellite system entails many complex trade-offs between a great number of parameters including: capacity, complexity of the payload, constellation geometry, number of satellites, quality of coverage, etc. This paper aims at defining a methodology which tries to split the variables to give rapidly some first results. The major input considered is the traffic assumption which would be offered by the system. A first key step is the choice of the best Rider or Walker constellation geometries - with different numbers of satellites - to insure a good quality of coverage over a selected service area. Another aspect to be addressed is the possible altitude location of the constellation, since it is limited by many constraints. The altitude ranges that seem appropriate considering the spatial environment, the launch and orbit keeping policy and the feasibility of the antenna allowing sufficient frequency reuse are briefly analyzed. To support these first considerations, some 'reference constellations' with similar coverage quality are chosen. The in-orbit capacity needed to support the assumed traffic is computed versus altitude. Finally, the exact number of satellite is determined. It comes as an optimum between a small number of satellites offering a high (and costly) power margin in bad propagation situation and a great number of less powerful satellites granting the same quality of service.

Roussel, T.↗

NASA NTRS1993

Odyssey personal communications satellite system

The spectacular growth of cellular telephone networks has proved the demand for personal communications. Large regions of the world are too sparsely populated to be economically served by terrestrial cellular communications. Since satellites are well suited to this application, TRW filed with the FCC on May 31, 1993 for the Odyssey construction permit. Odyssey will provide high quality wireless communication services worldwide from satellites. These services will include: voice, data, paging, and messaging. Odyssey will be an economical approach to providing communications. A constellation of 12 satellites will be orbited in three, 55 deg. inclined planes at an altitude of 10,354 km to provide continuous coverage of designated regions. Two satellites will be visible anywhere in the world at all times. This dual visibility leads to high line-of-sight elevation angles, minimizing obstructions by terrain, trees and buildings. Each satellite generates a multibeam antenna pattern that divides its coverage area into a set of contiguous cells. The communications system employs spread spectrum CDMA on both the uplinks and downlinks. This signaling method permits band sharing with other systems and applications. Signal processing is accomplished on the ground at the satellite's 'Gateway' stations. The 'bent pipe' transponders accommodates different regional standards, as well as signaling changes over time. The low power Odyssey handset will be cellular compatible. Multipath fade protection is provided in the handset.

Spitzer, Christopher J.↗

NASA NTRS1993

A study of satellite motion-induced multipath phenomena

Experiments have been undertaken at COMSAT Laboratories to determine some of the propagation effects likely to be encountered by handheld satellite communications devices. L-band pilot tones aboard geosynchronous satellites at 15 and 40 deg. elevations were used to examine diurnal signal variations measured by using a hemispherical antenna. It was found that the receiver with a hemispherical antenna suffered daily peak-to-peak signal level variations of up to 12 dB compared to only 2 to 3 dB for a receiver equipped with a directional antenna. These results were highly repeatable, and extensive tests were conducted to confirm the accuracy of the data. The results suggest that the diurnal variations were due to multipath effects caused by the motion of the satellite with respect to the receiver antenna. Noting that the orbit inclinations of the satellites used in the experiment were only on the order of 2 to 3 deg., the results also suggest a potentially serious signal variation problem for low-gain antenna-based communications systems using low earth orbit satellites, since the satellite elevation angles relative to earth change far more rapidly.

Allnutt, R. M.↗

NASA NTRS1994

Time transfer using geostationary satellites: Implementation of a Kalman filter

Since 1988, various experiments have shown that the TV signals transmitted by direct TV satellites may easily be used to perform time transfers at the level of a few tens of nanoseconds, the main source of error being the uncertainty on the satellite position. We first present the two methods used in our experiment to reduce the effects of the satellite residual motion: the first one consists in estimating the longitude variations of the satellite and then using this information to improve other measurements. This allows reducing the uncertainty to values between 9 and 50 nanoseconds according to the position of the involved stations. In the second method we determine the satellite position by using the data collected by three calibrated stations. Time transfer between each of these stations and a fourth one has been shown to be achievable at the precision level of ten nanoseconds. A new approach based on the use of a Kalman filter is proposed in order to take into account the dynamics of the geostationary satellite. The precisions on orbital elements and clock differences and rates determination given by the first simulated applications of the Kalman filter are presented and compared to those obtained by the other methods.

Meyer, F.↗

NASA NTRS1994

Looking at Earth from space: Direct readout from environmental satellites

Direct readout is the capability to acquire information directly from meteorological satellites. Data can be acquired from NASA-developed, National Oceanic and Atmospheric Administration (NOAA)-operated satellites, as well as from other nations' meteorological satellites. By setting up a personal computer-based ground (Earth) station to receive satellite signals, direct readout may be obtained. The electronic satellite signals are displayed as images on the computer screen. The images can display gradients of the Earth's topography and temperature, cloud formations, the flow and direction of winds and water currents, the formation of hurricanes, the occurrence of an eclipse, and a view of Earth's geography. Both visible and infrared images can be obtained. This booklet introduces the satellite systems, ground station configuration, and computer requirements involved in direct readout. Also included are lists of associated resources and vendors.

Source record↗

NASA NTRS1994

Improved small satellite access of the space network

This report contains the results of a study performed under the sponsorship of the National Aeronautics and Space Administration (NASA) made as a grant to the Center for Space Telemetering and Telecommunication Systems at New Mexico State University. The purpose of this phase of the grant is to increase user access to the Space Network (SN) run by NASA for supplying space-to-ground communications for satellites and associated control centers. The identified need is to bring more users into the community of those accessing the SN, especially those in the small satellite class of users. The initial phase of the study concerned the potential for modifications to the standard transponder used in the SN. The results of that investigation are summarized in Section 4. As the hardware modifications were being investigated, a second option was developed, namely to consider changes to the operational mode for the small satellites. This operational concept was to use a single, fixed-pointing antenna in a spin-stabilized satellite and let the antenna pattern sweep past the Tracking and Data Relay Satellites (TDRS) in the SN. The question to be answered by this phase of the study was twofold: could enough contact time per day be made available using this simple operating mode and could the data rate be high enough to allow for sufficient data throughput to satisfy the user community using existing components. Section 2 outlines the methodology and simulation results to answer these questions. Section 3 contains a summary of an operational simulation of a simple satellite payload using these contact scenarios. The simulation is not all inclusive but shows how a payload simulation could be configured to utilize variable contact times. The answer to both of the questions desired to be answered is affirmative. By carefully choosing the correct system transmission power and antenna pattern, the system will allow support to the 50th percentile of expected systems. It is recommended that based on this initial study, further work be done to quantify the exact parameters for transmission through the space network and to optimize usage of the contact time to maximize throughput.

Horan, Stephen↗

NASA NTRS1994

Gravity model improvement using the DORIS tracking system on the SPOT 2 satellite

A high-precision radiometric satellite tracking system, Doppler Orbitography and Radio-positioning Integrated by Satellite system (DORIS), has recently been developed by the French space agency, Centre National d'Etudes Spatiales (CNES). DORIS was designed to provide tracking support for missions such as the joint United States/French TOPEX/Poseidon. As part of the flight testing process, a DORIS package was flown on the French SPOT 2 satellite. A substantial quantity of geodetic quality tracking data was obtained on SPOT 2 from an extensive international DORIS tracking network. These data were analyzed to assess their accuracy and to evaluate the gravitational modeling enhancements provided by these data in combination with the Goddard Earth Model-T3 (GEM-T3) gravitational model. These observations have noise levels of 0.4 to 0.5 mm/s, with few residual systematic effects. Although the SPOT 2 satellite experiences high atmospheric drag forces, the precision and global coverage of the DORIS tracking data have enabled more extensive orbit parameterization to mitigate these effects. As a result, the SPOT 2 orbital errors have been reduced to an estimated radial accuracy in the 10-20 cm RMS range. The addition of these data, which encompass many regions heretofore lacking in precision satellite tracking, has significantly improved GEM-T3 and allowed greatly improved orbit accuracies for Sun-synchronous satellites like SPOT 2 (such as ERS 1 and EOS). Comparison of the ensuing gravity model with other contemporary fields (GRIM-4C2, TEG2B, and OSU91A) provides a means to assess the current state of knowledge of the Earth's gravity field. Thus, the DORIS experiment on SPOT 2 has provided a strong basis for evaluating this new orbit tracking technology and has demonstrated the important contribution of the DORIS network to the success of the TOPEX/Poseidon mission.

Nerem, R. S.↗

NASA NTRS1994

Simultaneous observations of VLF ground transmitter signals on the DE 1 and COSMOC 1809 satellites: Detection of a magnetospheric caustic and a duct

Khabarovsk transmitter signals (15.0 kHz, 48 deg N, 135 deg E) were observed on the high-altitude (approximately 15000 km) Dynamic Explorer 1 (DE 1) and the low-altitude (approximately 960) km COSMOS 1809 satellites during a 9-day period in August 1989. On 7 out of 9 days the linear wave receiver (LWR) on the DE 1 satellite also detected signals from the Alpha transmitter (11.9-15.6 kHz) in Russia and an Omega transmitter (10.2-13.6 kHz) in Australia, as well as natural VLF emissions such as hiss, chorus, whistlers, and wideband impulsive signals. On two days, August 23 and 27, 1989, observations of the Khabarovsk transmitter signals were simultaneously carried out at high altitude on thre DE 1 satellite and at low altitude on the COSMOS 1809 satellite. Analysis of data from these 2 days has led to several new results on the propagation of whistler mode signals in the Earth's magnetosphere. New evidence was found of previously reported propagation phenomena, such as (1) confinement of transmitter signals in the conjugate hemisphere at ionospheric heights (approximately 1000 km), (2) observation of direct multipath propagation on both DE 1 and COSMOS 1809, (3) detection of ionospheric irregularities of is less than or equal to 100 km scale size with a few percent enhancement in electron density. We also report the first direct detection of a magnetospheric dust at L = 2.94 which was believed to be responsible for the ducted propagation of Khabarovsk signals observed in the COSMOS 1809 satellite. Our study also indicates that duct end points can extend down to at least approximately 1000 km. The peak electric and magnetic fields of ducted Khabarovsk transmitter signals at approximately 1000 km were 520 micro V/m and 36 pT respectively. Estimated field strengths of these signals inside the dust at the geomagnetic equator were 57 micro V/m and 12 pT for electric and magnetic field respectively. The results of two-dimensional ray tracing simulations were consistent with the observations of the nonducted whistler-mode propagation of Khabarovsk (15 kHz) and Alpha (11.9 kHz) signals from the transmitter location to the DE 1 and COSMOS 1809 satellites. Our results have direct implications for the question of accessibility of waves injected from the ground to various regions of the ionosphere and the magnetosphere.

Chmyrev, V. M.↗

NASA NTRS1996

Investigation of Adaptive-threshold Approaches for Determining Area-Time Integrals from Satellite Infrared Data to Estimate Convective Rain Volumes

The principal goal of this project is to establish relationships that would allow application of area-time integral (ATI) calculations based upon satellite data to estimate rainfall volumes. The research is being carried out as a collaborative effort between the two participating organizations, with the satellite data analysis to determine values for the ATIs being done primarily by the STC-METSAT scientists and the associated radar data analysis to determine the 'ground-truth' rainfall estimates being done primarily at the South Dakota School of Mines and Technology (SDSM&T). Synthesis of the two separate kinds of data and investigation of the resulting rainfall-versus-ATI relationships is then carried out jointly. The research has been pursued using two different approaches, which for convenience can be designated as the 'fixed-threshold approach' and the 'adaptive-threshold approach'. In the former, an attempt is made to determine a single temperature threshold in the satellite infrared data that would yield ATI values for identifiable cloud clusters which are closely related to the corresponding rainfall amounts as determined by radar. Work on the second, or 'adaptive-threshold', approach for determining the satellite ATI values has explored two avenues: (1) attempt involved choosing IR thresholds to match the satellite ATI values with ones separately calculated from the radar data on a case basis; and (2) an attempt involved a striaghtforward screening analysis to determine the (fixed) offset that would lead to the strongest correlation and lowest standard error of estimate in the relationship between the satellite ATI values and the corresponding rainfall volumes.

Smith, Paul L.↗

NASA NTRS1995

Concept and implementation of the Globalstar mobile satellite system

Globalstar is a satellite-based mobile communications system which provides quality wireless communications (voice and/or data) anywhere in the world except the polar regions. The Globalstar system concept is based upon technological advancements in Low Earth Orbit (LEO) satellite technology and in cellular telephone technology, including the commercial application of Code Division Multiple Access (CDMA) technologies. The Globalstar system uses elements of CDMA and Frequency Division Multiple Access (FDMA), combined with satellite Multiple Beam Antenna (MBA) technology and advanced variable-rate vocoder technology to arrive at one of the most efficient modulation and multiple access systems ever proposed for a satellite communications system. The technology used in Globalstar includes the following techniques in obtaining high spectral efficiency and affordable cost per channel: (1) CDMA modulation with efficient power control; (2) high efficiency vocoder with voice activity factor; (3) spot beam antenna for increased gain and frequency reuse; (4) weighted satellite antenna gain for broad geographic coverage; (5) multisatellite user links (diversity) to enhance communications reliability; and (6) soft hand-off between beams and satellites. Initial launch is scheduled in 1997 and the system is scheduled to be operational in 1998. The Globalstar system utilizes frequencies in L-, S- and C-bands which have the potential to offer worldwide availability with authorization by the appropriate regulatory agencies.

Schindall, Joel↗

NASA NTRS1995

The Ellipso (TM) mobile satellite system

The Ellipso(trademark) Mobile Satellite System is conceived to extend telecommunication services throughout the world to users that are not well, or not at all served by existing mobile or fixed telephone systems. Unlike cellular telephones, Ellipso(trademark) offers fully nationwide service to every served country, thereby providing service to users located anywhere within the national boundaries, no matter how isolated or remote. With Ellipso(trademark), a user in the middle of a wilderness area will have the same mobile telecommunications service as a user in a major metropolitan area. Ellipso(trademark) uses medium earth orbiting (MEO) satellites and an efficient system design to reach its subscribers directly and at a price that is competitive with terrestrial telephone services. The subscriber only requires a clear view of a serving satellite to achieve a connection and to connect to anyone else served by the national telecommunications system. Subscribers within view of two or more satellites will benefit from Ellipso's(trademark) unique satellite diversity processing, using all available satellites simultaneously to optimize circuit quality.

Castiel, David↗

NASA NTRS1996

A technique for determining cloud free vs cloud contaminated pixels in satellite imagery

Since the first earth orbiting satellite sent pictures of the earth back to them, atmospheric scientists have been focused on the possibilities of using that information as both a forecasting tool and as a meteorological research tool. With the latest generation of Geostationary Operational Environmental Satellites (GOES) now entering service, that view of the earth yields views at a frequency and resolution never before available. These satellites have imagers with a five band multi-spectral capability with high spatial resolution. In addition, the sounder has eighteen thermal infrared (IR) channels plus one low-resolution visible band. With a resolution as small as one kilometer, GOES provides scientists with a powerful eye on the atmosphere. Menzel and Purdom (1994) detail both the imager and sounder capability as well as other systems on the GOES satellites. Immediately apparent in the visible channel are the patterns of clouds swirling over both oceans and continents. These clouds range in size from huge planetary systems covering thousands of kilometers to puffy fair weather cumulus clouds on the order of half a kilometer in size. With the IR sensors temperature patterns are observed. High clouds appear very cold, while low stratus field show temperatures near that of the surface. The surface, in turn, generally appears warmer than the clouds. It would seem then a simple manner to determine cloud and surface temperature from the imagery, but such is not the case. While most of the atmospheric constituents are well mixed and homogeneous, water vapor is not. The water molecule, because of its unique structure and vibration modes, affects the transmittance of the atmosphere most notably in the infrared regions. There are regions of the IR spectrum where water vapor acts as a strong absorber, and at others it is nearly transparent. The transparent wavelengths are called windows, and one such window occurs at 11.2 microns. Adjacent to this window at 12.7 microns which is strongly absorbed by water vapor. These two wavelengths form what is known as a split window, the utility of which was used. Using the linearized form of the radiative transfer equation, they were able to use the split window to determine the amount of water vapor present in the atmosphere. Jedlovec developed the physical split-window (PSW) technique which determines the integrated water content (IWC). The PSW method using Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) found on the older versions of the GOES satellites was used. Recently, Jedlovec and colleagues have been attempting to apply the PSW method using full disk IR imagery obtained by the new generation of GOES satellites. IWC is essential for improved analysis and prediction of convective storms which have been observed to develop in regions of both strong and rapidly evolving moisture gradients. It has also been used in the prediction of clouds and precipitation.

Wohlman, Richard A.↗

NASA NTRS1995

A Micromechanical INS/GPS System for Small Satellites

The cost and complexity of large satellite space missions continue to escalate. To reduce costs, more attention is being directed toward small lightweight satellites where future demand is expected to grow dramatically. Specifically, micromechanical inertial systems and microstrip global positioning system (GPS) antennas incorporating flip-chip bonding, application specific integrated circuits (ASIC) and MCM technologies will be required. Traditional microsatellite pointing systems do not employ active control. Many systems allow the satellite to point coarsely using gravity gradient, then attempt to maintain the image on the focal plane with fast-steering mirrors. Draper's approach is to actively control the line of sight pointing by utilizing on-board attitude determination with micromechanical inertial sensors and reaction wheel control actuators. Draper has developed commercial and tactical-grade micromechanical inertial sensors, The small size, low weight, and low cost of these gyroscopes and accelerometers enable systems previously impractical because of size and cost. Evolving micromechanical inertial sensors can be applied to closed-loop, active control of small satellites for micro-radian precision-pointing missions. An inertial reference feedback control loop can be used to determine attitude and line of sight jitter to provide error information to the controller for correction. At low frequencies, the error signal is provided by GPS. At higher frequencies, feedback is provided by the micromechanical gyros. This blending of sensors provides wide-band sensing from dc to operational frequencies. First order simulation has shown that the performance of existing micromechanical gyros, with integrated GPS, is feasible for a pointing mission of 10 micro-radians of jitter stability and approximately 1 milli-radian absolute error, for a satellite with 1 meter antenna separation. Improved performance micromechanical sensors currently under development will be suitable for a range of micro-nano-satellite applications.

Barbour, N.↗

NASA NTRS1995

Comparison of Satellite-Derived Wind Measurements with Other Wind Measurement Sensors

The purpose of this paper is to compare the good data from the Jimsphere launches with the data from the satellite system. By comparing the wind speeds from the Fixed Pedestal System 16 (FPS-16) Radar/Jimsphere Wind System and NASA's 50-MHz Radar Wind Profiler, the validation of winds from Geostationary Operational Environmental Satellite 7 (GOES-7) is performed. This study provides an in situ data quality check for the GOES-7 satellite winds. Comparison was made of the flowfields in the troposphere and the lower stratosphere of case studies of pairs of Jimsphere balloon releases and Radar Wind Profiler winds during Space Shuttle launches. The mean and standard deviation of the zonal component statistics, the meridional component statistics, and the power spectral density curves show good agreement between the two wind sensors. The standard deviation of the u and v components for the STS-37 launch (consisting of five Jimsphere/Radar Wind Profiler data sets) was 1.92 and 1.67 m/s, respectively; for the STS-43 launch (there were six Jimsphere/Wind Profiler data sets) it was 1.39 and 1.44 m/s, respectively. The overall standard deviation was 1.66 m/s for the u component and 1.55 m/s tor the v component, and a standard deviation of 2.27 m/s tor the vector wind difference. The global comparison of satellite with Jimsphere balloon vector winds shows a standard deviation of 3.15 m/s for STS-43 and 4.37 m/s for STS-37. The overall standard deviation of the vector wind was 3.76 m/s, with a root-mean-square vector difference of 4.43 m/s. These data have demonstrated that this unique comparison of the Jimsphere and satellite winds provides excellent ground truth and a frame of reference during testing and validation of satellite data

Susko, Michael↗

NASA NTRS1996

NASA SSTI CLARK 3-meter imaging satellite

Within the framework of NASA's Small Satellite Technology Initiative (SSTI) CLARK program, the development of a high technology small satellite is reported on. The satellite will provide 3 m resolution panchromatic and 15 m resolution multispectral image capabilities. The satellite, programmed for launch in 1996, will be in a sun-synchronous orbit and includes the following systems: three-axis zero momentum attitude control; hydrazine propulsion for stationkeeping; Global Positioning System satellite position and star tracker attitude determination; and onboard image storage capability. The objectives of the SSTI CLARK program, the technical characteristics of the satellite, the techniques employed to reduce costs, and the image processing, archiving and distribution are described.

Sebestyen, George↗