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At least 1,099 records · Page 61
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NASA NTRS1992

Stratocumulus cloud properties derived from simultaneous satellite and island-based instrumentation during FIRE

Cloud parameters derived from visible and infrared window data from the Geostationary Operational Environmental Satellite (GOES) are compared to corresponding properties determined from instrumentation on San Nicolas Island off the coast of California during the First ISCCP Regional Experiment marine stratocumulus intensive field observations period in July 1987. Examination of the satellite imagery revealed that the apparent bias can be explained by the persistence of the clouds over the northwest part of the island during periods of clearing around the island. Diurnal variations in the cloud cover were very significant; minimum cloudiness occurred during the late afternoon and maximum cloudiness early in the morning. Relationships were established between the satellite-derived cloud optical depth and two surface-observed quantities: cloud liquid water path and cloud thickness. Simultaneous observations of liquid water path and satellite-derived cloud optical depth were used to infer effective cloud-droplet radius, resulting in good agreement with correlative data. The diurnal variations in cloud amount are accompanied by changes in cloud thickness, cloud-top height, cloud liquid water path, and effective droplet size. These observations provide the most complete picture, to date, of the diurnal cycle of marine stratocumulus clouds, confirming previous satellite-based inferences of the diurnal behavior of marine stratocumulus at larger scales.

Minnis, Patrick↗
NASA NTRS1992

Monitoring global geodynamical parameter variations with GPS and low earth satellites

A low earth satellite enhances global geodynamical parameters determination with GPS in two ways. First, it improves the GPS orbits, which in turn improve the estimates of other parameters. Secondly, a low earth satellite completes an orbit cycle in far shorter time (90 to 120 minutes) than do GPS satellites (12 hours); it observes more GPS satellites than a ground receiver does in shorter time and increases the correlation between GPS orbit errors. This reduces the error in the determination of nonrotational coordinate parameters, i.e., geocentric offset of ground tracking sites. Covariance results with the global geodynamical parameters modeled as constants, and as random-walk parameters to closer reflect the actual variations, are compared. The effects of using different GPS data quality and different ground tracking network are studied. The case of using two earth satellites in orthogonal orbital planes is also investigated.

Wu, S. C.↗
NASA NTRS1992

Energetic particle signatures of satellites and rings in Neptune's magnetosphere

The cosmic ray system on Voyager 2 found a trapped radiation environment in Neptune's inner magnetosphere which is controlled primarily by absorption at the rings and satellite surfaces. The intensity of electrons with kinetic energies approximately greater than 1 MeV shows particularly strong and narrow signatures associated with absorption by the satellite 1989N1 at an orbital radius of 4.75 Neptune radii. Closer to the planet are several signatures of the inner satellites and rings. Absorption limits the intensity of the inner radiation belt sufficiently for the maximum intensity to occur outside the orbit of 1989N1 at a magnetic L shell of about 7. Radial profiles of the electron phase space density show that electrons diffuse inward from a source in the outer magnetosphere. Many of the inward-diffusing electrons are absorbed upon reaching a satellite orbital radius, but the finite absorption efficiency allows some of the electrons to pass by unaffected. The locations of the satellite and ring signatures also provide constraints on the nondipolar components of the planetary magnetic field.

Selesnick, R. S.↗
NASA NTRS1992

On the use of satellites in Molniya orbit for meteorological and oceanographic observations of the high latitudes

Two types of orbits are currently used for meteorological satellites: geostationary orbit and low earth orbit. Neither orbit serves the high latitudes well. The Soviet Molniya satellites fly in a highly elliptic orbit inclined 63.4 deg to the equator. For approximately 8 h of its 12-h orbit, a satellite in Molniya orbit is synchronized with the earth such that it is nearly geostationary in the high latitudes. A meteorological/oceanographic satellite in Molniya orbit would bring nearly the same frequent imaging capability to the high latitudes which the tropics and mid-latitudes now enjoy. The uses of such a satellite include Arctic forecasting, study of polar lows, high latitude precipitation estimation, Antarctic studies, sea ice monitoring, and ozone measurements.

Kidder, Stanley Q.↗
NASA NTRS1992

Direct Broadcast Satellite: Radio Program

NASA is committed to providing technology development that leads to the introduction of new commercial applications for communications satellites. The Direct Broadcast Satellite-Radio (DBS-R) Program is a joint effort between The National Aeronautics and Space Administration (NASA) and The United States Information Agency/Voice of America (USIA/VOA) directed at this objective. The purpose of this program is to define the service and develop the technology for a direct-to-listener satellite sound broadcasting system. The DBS-R Program, as structured by NASA and VOA, is now a three-phase program designed to help the U.S. commercial communications satellite and receiver industry bring about this new communications service. Major efforts are being directed towards frequency planning hardware and service development, service demonstration, and experimentation with new satellite and receiver technology.

Hollansworth, James E.↗
NASA NTRS1992

Applications of satellite technology to broadband ISDN networks

Two satellite architectures for delivering broadband integrated services digital network (B-ISDN) service are evaluated. The first is assumed integral to an existing terrestrial network, and provides complementary services such as interconnects to remote nodes as well as high-rate multicast and broadcast service. The interconnects are at a 155 Mbs rate and are shown as being met with a nonregenerative multibeam satellite having 10-1.5 degree spots. The second satellite architecture focuses on providing private B-ISDN networks as well as acting as a gateway to the public network. This is conceived as being provided by a regenerative multibeam satellite with on-board ATM (asynchronous transfer mode) processing payload. With up to 800 Mbs offered, higher satellite EIRP is required. This is accomplished with 12-0.4 degree hopping beams, covering a total of 110 dwell positions. It is estimated the space segment capital cost for architecture one would be about $190M whereas the second architecture would be about $250M. The net user cost is given for a variety of scenarios, but the cost for 155 Mbs services is shown to be about $15-22/minute for 25 percent system utilization.

Price, Kent M.↗
NASA NTRS1992

Vibration and acoustic testing of TOPEX/Poseidon satellite

The satellite was subjected to a 1.5G swept sine vibration test and a 146 dB overall level acoustic test, in accordance with Ariane launch vehicle requirements, at the NASA Goddard Space Flight Center. Extensive pretest analysis of the sine test was conducted to plan the input notching and to justify vibration testing the satellite only in the longitudinal axis. A unique measurement system was utilized to determine the six components of interface force between the shaker and the satellite in the sine vibration test. The satellite was heavily instrumented in both the sine vibration and acoustic test in order to insure that the launch loads were enveloped with appropriate margin and that satellite responses did not exceed the compatibilities of the structure and equipment. The test specification, objectives, instrumentation, and test results are described herein.

Boatman, Dave↗
NASA NTRS1992

Tethered Satellite System Contingency Investigation Board

The Tethered Satellite System (TSS-1) was launched aboard the Space Shuttle Atlantis (STS-46) on July 31, 1992. During the attempted on-orbit operations, the Tethered Satellite System failed to deploy successfully beyond 256 meters. The satellite was retrieved successfully and was returned on August 6, 1992. The National Aeronautics and Space Administration (NASA) Associate Administrator for Space Flight formed the Tethered Satellite System (TSS-1) Contingency Investigation Board on August 12, 1992. The TSS-1 Contingency Investigation Board was asked to review the anomalies which occurred, to determine the probable cause, and to recommend corrective measures to prevent recurrence. The board was supported by the TSS Systems Working group as identified in MSFC-TSS-11-90, 'Tethered Satellite System (TSS) Contingency Plan'. The board identified five anomalies for investigation: initial failure to retract the U2 umbilical; initial failure to flyaway; unplanned tether deployment stop at 179 meters; unplanned tether deployment stop at 256 meters; and failure to move tether in either direction at 224 meters. Initial observations of the returned flight hardware revealed evidence of mechanical interference by a bolt with the level wind mechanism travel as well as a helical shaped wrap of tether which indicated that the tether had been unwound from the reel beyond the travel by the level wind mechanism. Examination of the detailed mission events from flight data and mission logs related to the initial failure to flyaway and the failure to move in either direction at 224 meters, together with known preflight concerns regarding slack tether, focused the assessment of these anomalies on the upper tether control mechanism. After the second meeting, the board requested the working group to complete and validate a detailed integrated mission sequence to focus the fault tree analysis on a stuck U2 umbilical, level wind mechanical interference, and slack tether in upper tether control mechanism and to prepare a detailed plan for hardware inspection, test, and analysis including any appropriate hardware disassembly.

Source record↗
NASA NTRS1992

Downward-deployed tethered satellite systems, measurement techniques, and instrumentation - A review

This paper describes a number of scheduled and proposed Shuttle-based downward-deployed tethered satellite systems (TSSs) the purpose of which is to determine the structure of the lower thermosphere and to measure the atmospheric and aerodynamic effects in the vicinity of the satellite, the aerothermodynamic effects on the satellite's surface, and the dynamics of the tether and its endmass, the satellite. The instruments for the downward-deployed tethered missions will include mass spectrometers and other density sensors, plasma instrumentation, optical spectrophotometers, magnetometers, and instrumentation to measure the effects on satellite surface (such as the surface temperature, heat transfer, and pressure; gas adsorption on surfaces, chemistry with other gas molecules and surface material, and desorption from the surface; and surface charging).

Brown, Kenneth G.↗
NASA NTRS1992

MEDSAT - A remote sensing satellite for malaria early warning and control

A remote sensing, medical satellite (MEDSAT) aids in the control of carrier (vector) borne disease. The prototype design is a light satellite to test for control of malaria. The design features a 340-kg satellite with visual/IR and SAR sensors in a low inclination orbit observing a number of worldwide test sites. The approach is to use four-band visual/IR and dual-polarized L-band SAR images obtained from MEDSAT in concert with in-situ data to estimate the temporal and spatial variations of malaria risk. This allows public health resources to focus on the most vulnerable areas at the appropriate time. It is concluded that a light-satellite design for a MEDSAT satellite with a Pegasus launch is feasible.

Vesecky, John↗
NASA NTRS1992

Radiometric gains of satellite sensors of reflected solar radiation - Results from NASA ER-2 aircraft measurements

A method for using congruent aircraft-satellite observations to calibrate a satellite sensor is presented. A calibrated spectroradiometer at an altitude of 19 km above White Sands, NM, is oriented to view White Sands at the satellite overpass time along the same view vector as the satellite sensor. Collected data are transformed into corresponding estimates of sensor band radiance at the satellite (derived from the aircraft measurements), and average count (from the sensor measurements). These are both averaged across the footprint of the spectroradiometer. Results are presented for the evolution of NOAA-11 Advanced Very High Resolution Radiometer (AVHRR) (Bands 1 and 2) gain between November 1988 and October 1990, and for GOES-6 and GOES-7 VISSR/VAS visible bands during the same period. Estimates of uncertainty in the results are presented, as well as ideas for their reduction in future flights.

Abel, Peter↗
NASA NTRS1989

Petite Amateur Navy Satellite (PANSAT)

The Naval Postgraduate School's (NPS) Space Systems Academic Group (SSAG) is designing and developing a small communications satellite for launch aboard the shuttle as a complex autonomous payload (CAP). The objectives of PANSAT are three-fold. First, PANSAT will provide an ideal educational tool for the officer students at NPS supporting Space Systems Engineering and Space Systems Operations with hands-on hardware development. Second, the satellite will provide digital store-and-forward communications, or packet radio, for the amateur radio community. The third objective is to provide a low-cost, space-based platform for small experiments. PANSAT will be launched from the shuttle at a nominal altitude of 200 n.m. and an inclination of at least 37 degrees. The satellite weight is 150 lbs. Since there is no attitude control, eight dipole whip antennas will be used to provide isotropic ground coverage for communications. FM digital communications will be used with up-link and down-link on a single frequency in the amateur band of 437.25 MHz. A maximum 50 kHz of bandwidth is envisioned for the satellite. The expected lifetime of the satellite is 1 1/2 to 2 years before atmospheric reentry. The PANSAT design consists of the following: communications subsystem (COMM); computer, or data processor and sequencer (DP&S); power subsystem; structure subsystem; and experiment payload.

Sakoda, D.↗
NASA NTRS1993

Potential markets for advanced satellite communications

This report identifies trends in the volume and type of traffic offered to the U.S. domestic communications infrastructure and extrapolates these trends through the year 2011. To describe how telecommunications service providers are adapting to the identified trends, this report assesses the status, plans, and capacity of the domestic communications infrastructure. Cable, satellite, and radio components of the infrastructure are examined separately. The report also assesses the following major applications making use of the infrastructure: (1) Broadband services, including Broadband Integrated Services Digital Network (BISDN), Switched Multimegabit Data Service (SMDS), and frame relay; (2) mobile services, including voice, location, and paging; (3) Very Small Aperture Terminals (VSAT), including mesh VSAT; and (4) Direct Broadcast Satellite (DBS) for audio and video. The report associates satellite implementation of specific applications with market segments appropriate to their features and capabilities. The volume and dollar value of these market segments are estimated. For the satellite applications able to address the needs of significant market segments, the report also examines the potential of each satellite-based application to capture business from alternative technologies.

Adamson, Steven↗
NASA NTRS1993

Simulated retrieval of land surface parameters using multichannel satellite microwave radiometry

The current global change research emphasis on understanding water and energy fluxes at the land-atmosphere interface provided renewed interest in using passive microwave satellite data for land studies. Radiative transfer models of microwave emission and scattering in the soil-vegetation-atmosphere column, though still in the process of development, can be used to estimate the accuracies with which land surface parameters can be derived from satellite data. These parameters include surface soil moisture, surface temperature, vegetation water content, and atmospheric water content. A microwave radiative transfer model is used to develop linear and nonlinear versions of a multichannel retrieval algorithm to simulate retrievals of the surface and atmospheric parameters. These simulations include the effect of additive noise, and examine the effects of nonlinearities in the radiative transfer models. However, effects of spatial heterogeneity are not considered. Satellite data to which the retrieval algorithms may be applied include the SSMR and SSM/I, and in future will include the Multifrequency Imaging Microwave Radiometer (MIMR) to be launched as part of the Earth Observing System (EOS). The Scanning Multichannel Microwave Radiometer (SMMR) on the Nimbus-7 satellite provided data from 1978 to 1987 at five microwave frequencies between 6.6 and 37 GHz. The series of Special Sensor Microwave/Imager (SSM/I) instruments launched on the DMSP satellites provide data from 1987 to the present at four frequencies between 19.35 and 8.5 GHz. Spatial resolutions of the data range from approximately 12 to 120 km depending on the frequency. Examples using data from these sensors are shown to indicate the results of applying retrieval algorithms based on model simulations to real data.

Njoku, Eni G.↗
NASA NTRS1993

Architectures and protocols for an integrated satellite-terrestrial mobile system

This paper aims to depict some basic concepts related to the definition of an integrated system for mobile communications, consisting of a satellite network and a terrestrial cellular network. In particular three aspects are discussed: (1) architecture definition for the satellite network; (2) assignment strategy of the satellite channels; and (3) definition of 'internetworking procedures' between cellular and satellite network, according to the selected architecture and the satellite channel assignment strategy.

Delre, E.↗
NASA NTRS1993

The Globalstar mobile satellite system for worldwide personal communications

Loral Aerospace Corporation along with Qualcomm Inc. have developed a satellite system which offers global mobile voice and data services to and from handheld and mobile user terminals with omni-directional antennas. By combining the use of low-earth orbit (LEO) satellites with existing terrestrial communications systems and innovative, highly efficient spread spectrum techniques, the Globalstar system provides users with low-cost, reliable communications throughout the world. The Globalstar space segment consists of a constellation of 48 LEO satellites in circular orbits with 750 NM (1389 km) altitude. Each satellite communicates with the mobile users via the satellite-user links and with gateway stations. The gateway stations handle the interface between the Globalstar network and the OSTN/PLMN systems. Globalstar transceivers are similar to currently proposed digital cellular telephones in size and have a serial number that will allow the end user to make and receive calls from or to that device anywhere in the world. The Globalstar system is designed to operate as a complement to existing local, long-distance, public, private and specialized telecommunications networks. Service is primarily designed to serve the rural and thin route communications needs of consumers, government users, and private networks.

Wiedeman, Robert A.↗
NASA NTRS1993

K- and Ka-band mobile-vehicular satellite-tracking reflector antenna system for the NASA ACTS mobile terminal

This paper describes the development of the K- and Ka-band mobile-vehicular satellite-tracking reflector antenna system for NASA's ACTS Mobile Terminal (AMT) project. ACTS is NASA's Advanced Communications Technology Satellites. The AMT project will make the first experimental use of ACTS soon after the satellite is operational, to demonstrate mobile communications via the satellite from a van on the road. The AMT antenna system consists of a mechanically steered small reflector antenna, using a shared aperture for both frequency bands and fitting under a radome of 23 cm diameter and 10 cm height, and a microprocessor controlled antenna controller that tracks the satellite as the vehicle moves about. The RF and mechanical characteristics of the antenna and the antenna tracking control system are discussed. Measurements of the antenna performance are presented.

Densmore, Art↗
NASA NTRS1993

Aeronautical satellite antenna steering using magnetic field sensors

Designers of aeronautical satellite terminals are often faced with the problem of steering a directive antenna from an airplane or helicopter. This problem is usually solved by using aircraft orientation information derived from inertial sensors on-board the aircraft in combination with satellite ephemeris information calculated from geographic coordinates. This procedure works well but relies heavily on avionics that are external to the terminal. For the majority of small aircraft and helicopters which will form the bulk of future aeronautical satcom users, such avionics either do not exist or are difficult for the satellite terminal to interface with. At the Communications Research Center (CRC), work has been undertaken to develop techniques that use the geomagnetic field and satellite antenna pointing vectors (both of which are stationary in a local geographical area) to track the position of a satellite relative to a moving platform such as an aircraft. The performance of this technique is examined and a mathematical steering transformation is developed within this paper. Details are given regarding the experimental program that will be undertaken to test the concepts proposed herein.

Sydor, John↗