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At least 1,477 records · Page 82

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NASA NTRS1973

Study of spacecraft direct readout meteorological systems

Characteristics are defined of the next generation direct readout meteorological satellite system with particular application to Tiros N. Both space and ground systems are included. The recommended space system is composed of four geosynchronous satellites and two low altitude satellites in sun-synchronous orbit. The goesynchronous satellites transmit to direct readout ground stations via a shared S-band link, relayed FOFAX satellite cloud cover pictures (visible and infrared) and weather charts (WEFAX). Basic sensor data is transmitted to regional Data Utilization Stations via the same S-band link. Basic sensor data consists of 0.5 n.m. sub-point resolution data in the 0.55 - 0.7 micron spectral region, and 4.0 n.m. resolution data in the 10.5 - 12.6 micron spectral region. The two low altitude satellites in sun-synchronous orbit provide data to direct readout ground stations via a 137 MHz link, a 400 Mhz link, and an S-band link.

Bartlett, R.↗

NASA NTRS1973

Optimal space communications techniques

The design, development, and analysis are reported of a digital phase-locked loop (DPLL) for FM demodulation and threshold extension. One of the features of the developed DPLL is its synchronous, real time operation. The sampling frequency is constant and all the required arithmetic and logic operations are performed within one sampling period, generating an output sequence which is converted to analog form and filtered. An equation relating the sampling frequency to the carrier frequency must be satisfied to guarantee proper DPLL operation. The synchronous operation enables a time-shared operation of one DPLL to demodulate several FM signals simultaneously. In order to obtain information about the DPLL performance at low input signal-to-noise ratios, a model of an input noise spike was introduced, and the DPLL equation was solved using a digital computer. The spike model was successful in finding a second order DPLL which yielded a five db threshold extension beyond that of a first order DPLL.

Schilling, D. L.↗

NASA NTRS1974

Wide area coverage radar imaging satellite for earth applications

A preliminary study was made of a radar imaging satellite for earth applications. A side-looking synthetic-aperture radar was considered and the feasibility of obtaining a wide area coverage to reduce the time required to image a given area was investigated. Two basic approaches were examined; low altitude sun-synchronous orbits using a multibeam/multifrequency radar system and equatorial orbits up to near-synchronous altitude using a single beam system. Surveillance and mapping of ice on the Great Lakes was used as a typical application to focus the study effort.

Stevens, G. H.↗

NASA NTRS1974

Testing a satellite automatic nutation control system

Testing of a particular nutation control system for the synchronous meteorological satellite (SMS) is described. The test method and principles are applicable to nutation angle control for other satellites with similar requirements. During its ascent to synchronous orbit, a spacecraft like the SMS spins about its minimum-moment-of-inertia axis. An uncontrolled spacecraft in this state is unstable because torques due to fuel motion increase the nutation angle. However, the SMS is equipped with an automatic nutation control (ANC) system which will keep the nutation angle close to zero. Because correct operation of this system is critical to mission success, it was tested on an air-bearing table. The ANC system was mounted on the three-axis air-bearing table which was scaled to the SMS and equipped with appropriate sensors and thrusters. The table was spun up in an altitude chamber and nutation induced so that table motion simulated spacecraft motion. The ANC system was used to reduce the nutation angle. This dynamic test of the ANC system met all its objectives and provided confidence that the ANC system will control the SMS nutation angle.

Hrasiar, J. A.↗

NASA NTRS1973

Analysis of the partially filled viscous ring damper

A ring partially filled with a viscous fluid has been analyzed as a nutation damper for a spinning satellite. The fluid has been modelled as a rigid slug of finite length moving in a tube and resisted by a linear viscous force. It is shown that there are two distinct modes of motion, called the spin synchronous mode and the nutation synchronous mode. Time constants for each mode are obtained for both the symmetric and asymmetric satellite. The effects of a stop in the tube and an offset of the ring from the spin axis are also investigated. An analysis of test results is also given including a determination of the effect of gravity on the time constants in the two modes.

Alfriend, K. T.↗

NASA NTRS1973

Analysis of the dynamics and frequency spectrum synthesis of an optical-mechanical scanning device

A two-coordinate optical-mechanical scanning device (OMSD), the operating unit of which is a scanning disk, with directional and focusing optics and a board, on which the data carrier is placed, is examined. The disk and board are kinematically connected by a transmission mechanism, consisting of a worm and complex gear drive and a tightening screw-nut with correcting device, and it is run by a synchronous type motor. The dynamic errors in the system depend, first, on irregularities in rotation of the disk, fluctuations in its axis and vibrations of the table in the plane parallel to the plane of the disk. The basic sources of the fluctuations referred to above are residual disbalance of the rotor and other rotating masses, the periodic component of the driving torque of the synchronous motor, variability in the resistance, kinematic errors in the drive and other things. The fluctuations can be transmitted to the operating units through the kinematic link as a flexural-torsional system, as well as through vibrations of the housing of the device.

Andryushkevichyus, A. I.↗

NASA NTRS1972

The effect of support flexibility and damping on the dynamic response of a single mass flexible rotor in elastic bearings

The dynamic unabalance response and transient motion of the single mass Jeffcott rotor in elastic bearings mounted on damped, flexible supports are discussed. A steady state analysis of the shaft and the bearing housing motion was made by assuming synchronous precession of the system. The conditions under which the support system would act as a dynamic vibration absorber at the rotor critical speed were studied. Plots of the rotor and support amplitudes, phase angles, and forces transmitted were evaluated by the computer and the performance curves were plotted by an automatic plotter unit. Curves are presented on the optimization of the support housing characteristics of attenuate the rotor synchronous unbalance response.

Kirk, R. G.↗

NASA NTRS1974

Digital communication system

A digital communication system is reported for parallel operation of 16 or more transceiver units with the use of only four interconnecting wires. A remote synchronization circuit produces unit address control words sequentially in data frames of 16 words. Means are provided in each transceiver unit to decode calling signals and to transmit calling and data signals. The transceivers communicate with each other over one data line. The synchronization unit communicates the address control information to the transceiver units over an address line and further provides the timing information over a clock line. A reference voltage level or ground line completes the interconnecting four wire hookup.

Monford, L. G., Jr.↗

NASA NTRS1973

Centaur D-1T propulsion and propellant systems

Modifications are described which have been introduced to the Centaur high-energy upper stage (which is being integrated with the Titan IIIC booster) to improve mission flexibility and reliability, and to provide three-burn synchronous orbit capability. The addition of a sidewall radiation shield significantly reduces heat transfer to the liquid hydrogen tank. This radiation field, in conjunction with a newly developed computer controlled propellant tank venting and pressurization system, permits the use of improved techniques for propellant management during orbital coast. Some minor changes in the propulsion and propellant systems which were necessary to adapt these systems for synchronous orbit trajectories are described, together with the redundancy incorporated to reduce the number of single failure points.-

Goette, W. E.↗

NASA NTRS1974

Stability of a dual-spin satellite with two dampers

The rotational stability of a dual-spin satellite consisting of a main body and a symmetric rotor, both spinning about a common axis, is investigated. The main body is equipped with a spring-mass damper, while a partially filled viscous ring damper is mounted on the rapidly spinning rotor. The effect of fluid motion on the rotational stability of the satellite is calculated, considering the fluid as a single particle moving in a tube with viscous damping. Time constants are obtained by solving approximate equations of motion for the nutation-synchronous and the spin-synchronous modes, and the results are found to agree well with the numerical integrations of the exact equations. A limit cycle may exist for some configurations; the nutation angle tends to increase in such cases.

Alfriend, K. T.↗

NASA NTRS1974

Further characteristics of the evening energetic electron decreases during substorms

Evaluation of the results of studying the evening energetic electron decreases observed at synchronous altitudes with the use of high-resolution data (25.6-sec averages). The high-resolution data show that the evening particle events sometimes undergo rapid time variations, faster than the 6-min time-averaged data previously used by Lezniak and Winckler (1970). Periodic structures of 2- to 12-min period often accompany the evening particle decreases. Interpretation of these observations in terms of enhanced plasma energy density at synchronous altitudes during substorms, as evidenced by DeForest and McIlwain (1971) and Sharp and Johnson (1972), suggests that such plasma regions are highly nonuniform and turbulent.

Lin, C. S.↗

NASA NTRS1973

Instrumentation for one-way satellite PTTI applications

A review of general principles and operational procedures illustrates how the typical passive user and omni receiving antenna can recover Precise Time and Time Interval (PTTI) information from a low altitude navigation satellite system for clock calibration and synchronization. Detailed discussions of concepts and theory of the receiver design are presented. The importance of RF correlation of the received and local PN encoded sequences is emphasized as a means of reducing delay uncertainties of the instrumentation to values compatible with nanosecond to submicrosecond PTTI objectives. Two receiver configurations were fabricated for use in satellite-to-laboratory experiments. In one receiver the delay-locked loop for PN signals synchronization used a dithered amplitude detection process while the second receiver used a complex sums phase detection method for measurement of delay error. The necessity for compensation of Doppler shift is discussed. Differences in theoretical signal acquisition and tracking performance of the design concepts are noted.

Osborne, A. E.↗

NASA NTRS1974

The temporal organization of processes of cell reproduction and its connection with rhythms of radiosensitivity of the body

Radiosensitivity of individual phases of the mitotic cycle was studied in synchronous cell cultures and in several biological objects. It was found that radiosensitivity changed essentially according to phases of the mitotic cycle, depending on the kind of cells, evaluation criteria and the radiation dosage. Tests on partially synchronized HeLa cell populations, according to the criterion of survival, showed them most sensitive during mitosis, as well as in later G sub 1- or early DNA-synthesizing stages. With radiation in doses of 300 rad, the proportion of surviving cells showed a sensitivity directly before DNA synthesis of approximately 4 times higher than the later S-phase and during the major portion of G sub 1- and G sub 2-periods. Sensitivity of cells in mitosis was approximately 3 times higher than in late G sub 1- and early S-phases.

Druzhinin, Y. P.↗

NASA NTRS1974

Prototype color field sequential television lens assembly

The design, development, and evaluation of a prototype modular lens assembly with a self-contained field sequential color wheel is presented. The design of a color wheel of maximum efficiency, the selection of spectral filters, and the design of a quiet, efficient wheel drive system are included. Design tradeoffs considered for each aspect of the modular assembly are discussed. Emphasis is placed on achieving a design which can be attached directly to an unmodified camera, thus permitting use of the assembly in evaluating various candidate camera and sensor designs. A technique is described which permits maintaining high optical efficiency with an unmodified camera. A motor synchronization system is developed which requires only the vertical synchronization signal as a reference frequency input. Equations and tradeoff curves are developed to permit optimizing the filter wheel aperture shapes for a variety of different design conditions.

Source record↗

NASA NTRS1975

Computerized processing of data from Interkosmos-7 and Interkosmos-8 satellite experiments

Computerized data processing problems are analyzed using telemetry data received from Intercosmos-7 and Intercosmos-8 satellites. Problems related to the restoration of measurement data in the form of piecewise continuous and smooth calibrated functions of time and problems encountered in the scientific interpretation of the experimental results are considered. Curve smoothing is discussed in the case of distinct assignments of calibration levels, synchronization, and synchronization checks over time. A proposal is presented for checking a piecewise linear function providing a simple method for the elimination of isolated values due to channel failures and the separation of individual intervals.

Pfau, H.↗

NASA NTRS1974

Use of precision time and time interval (PTTI)

A review of range time synchronization methods are discussed as an important aspect of range operations. The overall capabilities of various missile ranges to determine precise time of day by synchronizing to available references and applying this time point to instrumentation for time interval measurements are described.

Taylor, J. D.↗

NASA NTRS1974

Frame sync acquisition for biorthogonally coded data

The paper analyzes the optimum frame-synchronization problem for biorthogonally coded data received over the additive white Gaussian noise (AWGN) channel and decoded prior to sync acquisition. In deriving the optimum frame-sync algorithm, the discussion is restricted to the case in which each frame contains F complete code words, including a frame-sync prefix of P complete code words. The k-bit source words and k-bit decoded words are regarded as super symbols from an alphabet of binary k-tuples. The biorthogonal encoder, the AWGN channel, and the decoder are lumped into a super symbol channel whose characteristics determine the form of the optimum frame-sync decision rule. A union-bound approach is used to provide a quantitative measure of the probability of false synchronization for the algorithm derived.

Levitt, B. K.↗

NASA NTRS1975

Reformulation of the relativistic conversion between coordinate time and atomic time

The relativistic conversion between coordinate time and atomic time is reformulated to allow simpler time calculations relating analysis in solar system barycentric coordinates (using coordinate time) with earth-fixed observations (measuring 'earth-bound' proper time or atomic time). After an interpretation in terms of relatively well-known concepts, this simplified formulation, which has a rate accuracy of about 10 to the minus 15th, is used to explain the conventions required in the synchronization of a worldwide clock network and to analyze two synchronization techniques - portable clocks and radio interferometry. Finally, pertinent experimental tests of relativity are briefly discussed in terms of the reformulated time conversion.

Thomas, J. B.↗