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At least 793 records · Page 44

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NASA NTRS1982

Low gain and steerable vehicle antennas for communications with land mobile satellite

Current development activities at JPL for ground mobile vehicle antennas to be used with the Land Mobile Satellite Service (LMSS) system are described. Both low gain and electronically steerable high gain type antennas are discussed in terms of their design concept and RF performance. For the low gain type, three classes of antennas are under various stages of development. These are the crossed-drooping dipole, quadrifilar helix, and microstrip patch designs. The antennas are intended to provide circularly-polarized radiation with a minimum of 3-dB gain in the angular region from 19 degrees to 60 deg from the horizon in elevation plane and with an omnidirectional pattern in azimuthal plane. For the electronically steerable high gain type, circularly-polarized microstrip patch phased arrays formed on a planar surface and on the surface of a truncated cone are under study. The arrays are intended to provide a minimum of 12 dB gain in the same angular region in elevation plane at all azimuthal angles. This coverage is accomplished by scanning the high gain pencil beam in both elevation and azimuthal directions. Both types of antennas are to transmit at 821-831 MHz band and to receive at 866-876 MHz band. They must be of low cost design and reasonably conformal to the vehicle.

Woo, K.↗

NASA NTRS1983

The input mobility of an infinite circular cylindrical elastic shell filled with fluid

The force input mobility of an infinite elastic circular cylindrical shell filled with fluid is derived by using the spectral equations of motion. Mobilities are evaluated and their physical interpretations are discussed for a steel shell of thickness h/a = 0.05 filled with water and vibrating in the n = 0, 1 and 2 circumferential modes. The results are subsequently used to analyze the related situations of wave transmission through a radial ring constraint and the far field vibrational energy distributions between the contained fluid and the shell wall for line and point driving forces.

Fuller, C. R.↗

NASA NTRS1984

Effective mobility limit in small-bubble, low-damping bubble materials

Examination of the implications of the basic equations that govern the motion of bubbles with nonzero winding number (S) yields an unexpected result. Bubble velocity is limited to the speed implied by the effective wall mobility (gamma)r/2S, no matter how large the actual wall mobility. For large bubbles, this limit will not usually be significant. However, for small bubbles (r less than 1 micron) in low-damping materials, this limit will profoundly affect the bubble motion. A derivation of the effect and experimental confirmation of it are presented.

Slusky, S. E. G.↗

NASA NTRS1984

Chemical studies of L chondrites. III - Mobile trace elements and Ar-40/Ar-39 ages

Radiochemical neutron activation analysis has been used to determine the content of the mobile trace elements Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Se, Te, Tl, and Zn, in L4-6 chondrites with undisturbed Ar-40 release patterns or patterns showing some 4.4-4.6 Gyr plateau disturbance that indicates some shock-induced loss. The mean concentrations are significantly lower in 16 chondrites with disturbed patterns than in 4 with undisturbed ones, consistent with shock-induced mobilization. Significantly lower mean concentrations of nearly all trace elements in 26 strongly shocked chondrites than in 14 mildly shocked ones indicate loss in shock-formed FeS-Fe eutectic and/or by means of vaporization during cooling of shock-heated collisional debris.

Huston, T. J.↗

NASA NTRS1984

ARQ schemes for data transmission in mobile radio systems

The problem of providing data communications over a land mobile radio channel is considered, taking into account error detection combined with retransmission on request as a means of obtaining reliability in digital data transmission. The considered systems are referred to as automatic RQ (repeat-request) or ARQ. It is pointed out that the distinguishing features of a land mobile radio (LMR) system, as far as the ARQ protocol is concerned, are a frequency or pair of frequencies shared by a large population of half-duplex users which cannot transmit and receive simultaneously. A description is presented of those variations of ARQ which are adaptable to an LMR system. A new channel model for a fading LMR channel is developed, and several ARQ protocols and some new protocol combinations are described. The relationships are developed for a description of the various protocols in terms of channel failure probabilities. Undetected error probabilities are developed and compared.

Comroe, R. A.↗

NASA NTRS1984

RF characteristics of the hoop column antenna for the land mobile satellite system mission

A communication system using a satellite with a 118 meter diameter quad aperture antenna to provide telephone service to mobile users remotely located from the large metropolitan areas where the telephone companies are presently implementing their cellular system is described. In this system, which is compatible with the cellular system, the mobile user communicates with the satellite at UHF frequencies. The satellite connects him at S-Band, to the existing telephone network via a base station. The results of the RF definition work for the quad aperture antenna are presented. The elements of the study requirements for the LMSS are summarized, followed by a beam topology plan which satisfies the mission requirements with a practical and realiable configuration. The geometry of the UHF antenna and its radiation characteristics are defined. The various feed alternatives, and the S-band aperture are described.

Foldes, P.↗

NASA NTRS1985

Challenges in the development of the shuttle extravehicular mobility unit

The development of the Shuttle extravehicular mobility unit (EMU) has required significant technology advances in the design of the astronaut life support system and space-suit assembly. The life support system and space-suit assemblies are integrated into a single system and optimized for the primary function of supporting astronaut extravehicular operations. Rather than accommodating a limited, male-only astronaut population, the EMU must satisfy size requirements for both males and females with a minimum of sized parts. In addition, the Shuttle EMU has been designed to implement Space Shuttle Program philosophy of long operating life and mission reuse capability to minimize program lifetime cost. The advancement in life support system and space-suit technology achieved by the development of the Shuttle extravehicular mobility unit (EMU) is illustrated by comparison with the requirements for and the design features of the Apollo EMU.

Mcmann, H. J.↗

NASA NTRS1985

Determination of the mobility profile in GaAs-MESFETs

A process for measuring charge carrier mobility for gallium-arsenide metal semiconductor field effect transistors is described in an attempt to optimize the relationship between this factor and production. The measuring procedure allows an actual determination of local mobility in the channel. The physical basis for the process and features of the measuring room are outlined. The measuring technique is described and recommendations are made for setting measuring parameters.

Prost, W.↗

NASA NTRS1985

NASA mobile satellite program

A three phase development program for ground and space segment technologies which will enhance and enable the second and third generation mobile satellite systems (MSS) is outlined. Phase 1, called the Mobile Satellite Experiment (MSAT-X), is directed toward the development of ground segment technology needed for future MSS generations. Technology validation and preoperational experiments with other government agencies will be carried out during the two year period following launch. The satellite channel capacity needed to carry out these experiments will be obtained from industry under a barter type agreement in exchange for NASA provided launch services. Phase 2 will develop and flight test the multibeam spacecraft antenna technology needed to obtain substantial frequency reuse for second generation commercial systems. Industry will provide the antenna, and NASA will fly it on the Shuttle and test it in orbit. Phase 3 is similar to Phase 2 but will develop an even larger multibeam antenna and test it on the space station.

Knouse, G.↗

NASA NTRS1985

A Concept for a Mobile Remote Manipulator System

A conceptual design for a Mobile Remote Manipulator System (MRMS) is presented. This concept does not require continuous rails for mobility (only guide pins at truss hardpoints) and is very compact, being only one bay square. The MRMS proposed is highly maneuverable and is able to move in any direction along the orthogonal guide pin array under complete control at all times. The proposed concept would greatly enhance the safety and operational capabilities of astronauts performing EVA functions such as structural assembly, payload transport and attachment, space station maintenance, repair or modification, and future spacecraft construction or servicing. The MRMS drive system conceptual design presented is a reasonably simple mechanical device which can be designed to exhibit high reliability. Developmentally, all components of the proposed MRMS either exist or are considered to be completely state of the art designs requiring minimal development, features which should enhance reliability and minimize costs.

Mikulus, M. M., Jr.↗

NASA NTRS1985

Propagation considerations in land mobile satellite transmission

It appears likely that the Land Mobile Satellite Services (LMSS) will be authorized by the FCC for operation in the 800 to 900 MHz (UHF) and possibly near 1500 MHz (L-band). Propagation problems are clearly an important factor in the effectiveness of this service, but useful measurements are few, and produced contradictory interpretations. A first order overview of existing measurements is presented with particular attention to the first two NASA balloon to mobile vehicle propagation experiments. Some physical insight into the interpretation of propagation effects in LMSS transmissions is provided.

Vogel, W. J.↗

NASA NTRS1985

Electrophoretic mobility of cells in a vertical Ficoll gradiant

The upward migration of living cells and test particles under the influence of a constant electric field in a low conductivity Ficoll gradient occurs at nearly constant velocity. Viscosity and neutral polymer concentration affect migration rate. Decreasing viscosity speeds up the particle migration, decreasing neutral polymer (Ficoll) concentration, slows particle migration, since electrophoretic mobility increases approximately linearly with neutral polymer concentration. Neutral polymers interact with the cell surface to effectively raise its zeta potential. An analytic function was developed from the known dependence of these physical variables on migration distance; the analysis expresses migration velocity as an explicit function of position in the density gradient. It predicts an almost linear increase in velocity of about 12 to 16% over the working region of the gradient. It was numerically integrated and correctly predicts cell migration distance vs time curves without the use of any fitted parameters. The resulting migration curves follow the expected slowly varying exponential form that closely resembles a straight line. The ability to determine standard electrophoretic mobilities from such curves depends on knowledge of the effect of Ficoll on the zeta potential of the cell type that is separated.

Plank, L. D.↗

NASA NTRS1985

Electrophoretic mobilities of cultured human embryonic kidney cells in various buffers

Data on the electrophoretic mobility distributions of cells in the new D-1 buffer and the interlaboratory standardization of urokinase assay methods are presented. A table of cell strains and recent data on cell dispersal methods are also included. It was decided that glycerol in A-1 electrophoretic mobility data on cultured human embryonic kidney cells subjected to electrophoresis in this buffer. The buffer composition is presented.

Source record↗

NASA NTRS1985

Effect of passage number on electrophoretic mobility distributions of cultured human embryonic kidney cells

A systematic investigation was undertaken to characterize population shifts that occur in cultured human embryonic kidney cells as a function of passage number in vitro after original explantation. This approach to cell population shift analysis follows the suggestion of Mehreshi, Klein and Revesz that perturbed cell populations can be characterized by electrophoretic mobility distributions if they contain subpopulations with different electrophoretic mobilities. It was shown that this is the case with early passage cultured human embryo cells.

Kunze, M. E.↗

NASA NTRS1985

Second-generation mobile satellite system. A conceptual design and trade-off study

In recent years, interest has grown in the mobile satellite (MSAT) system, a satellite-based communications system capable of providing integrated voice and data services to a large number of users. To explore the potential of a commercial mobile satellite system (MSS) beyond the horizon of the first generation, using technologies of the 1990's and to assist MSAT-X in directing its efforts, a conceptual design has been performed for a second-generation system to be launched around the mid-1990's. The design goal is to maximize the number of satellite channels and/or minimize the overall life-cycle cost, subject to the constraint of utilizing a commercial satellite bus with minimum modifications. To provide an optimal design, a series of trade-offs are performed, including antenna sizing, feed configurations, and interference analysis. Interference is a serious problem for MSAT and often an overlapping feed design is required to reduce interbeam interference. The trade-off studies will show that a simple non-overlapping feed is sufficient for the second-generation system, thus avoiding the need for the complicated beam-forming network that is associated with the overlapping feed designs. In addition, a system that operates at L-band, an alternative frequency band that is being considered by some for possible MSAT applications, is also presented.

Sue, M. K.↗

NASA NTRS1984

An integrated voice and data multiple-access scheme for a land-mobile satellite system

An analytical study is performed of the satellite requirements for a land mobile satellite system (LMSS). The spacecraft (MSAT-X) would be in GEO and would be compatible with multiple access by mobile radios and antennas and fixed stations. The FCC has received a petition from NASA to reserve the 821-825 and 866-870 MHz frequencies for the LMSS, while communications with fixed earth stations would be in the Ku band. MSAT-X transponders would alter the frequencies of signal and do no processing in the original configuration considered. Channel use would be governed by an integrated demand-assigned, multiple access protocol, which would divide channels into reservation and information channels, governed by a network management center. Further analyses will cover tradeoffs between data and voice users, probability of blocking, and the performance impacts of on-board switching and variable bandwidth assignment. Initial calculations indicate that a large traffic volume can be handled with acceptable delays and voice blocking probabilities.

Li, V. O. K.↗

NASA NTRS1985

Mobile trace element contents in Jilin chondrite

A determination is conducted of ppm-ppt levels of Co, Se, Ga, Rb, Cs, Te, Bi, Ag, In, Tl, Zn and Cd (arranged empirically in order of increasing mobility at 1000 C) by radiochemical neutron activation analysis in eight Jilin samples of known Ar-40 content and a large vein from one of these. The trace element contents of the vein do not differ markedly from those of Jilin whole-rock samples. Only Rb correlates with Ar-40, possibly due to chance. All elements but Cd are present at levels similar to thdse in H4-6 chondrites with long K/Ar age (i.e., presumably mildly shocked). These levels are low relative to those in analogous L4-6 chondrites suggesting that H chondrites formed and/or evolved under higher pre-shock temperatures than did mildly shocked L chondrites. Time-temperature conditions during shock-loading of Jilin parent material were mild relative to those in strongly shocked L chondrites, being sufficient at most only to mobilize Cd.

Sakuragi, Y.↗

NASA NTRS1984

Differential detection of Gaussian MSK in a mobile radio environment

Minimum shift keying with Gaussian shaped transmit pulses is a strong candidate for a modulation technique that satisfies the stringent out-of-band radiated power requirements of the mobil radio application. Numerous studies and field experiments have been conducted by the Japanese on urban and suburban mobile radio channels with systems employing Gaussian minimum-shift keying (GMSK) transmission and differentially coherent reception. A comprehensive analytical treatment is presented of the performance of such systems emphasizing the important trade-offs among the various system design parameters such as transmit and receiver filter bandwidths and detection threshold level. It is shown that two-bit differential detection of GMSK is capable of offering far superior performance to the more conventional one-bit detection method both in the presence of an additive Gaussian noise background and Rician fading.

Simon, M. K.↗