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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Adaptive Deadband Synchronization for a Spacecraft Formation

A paper discusses general problems in estimation and control of the states (positions, attitudes, and velocities) of spacecraft flying in formation, then addresses the particular formation-flying-control problem of synchronization of deadbands. The paper presents a deadband synchronization algorithm for the case in which the spacecraft are equipped with pulse-width-modulated thrusters for maintaining their required states. The algorithm synchronizes thruster-firing times across all six degrees of freedom of all the spacecraft. The algorithm is scalable, inherently adapts to disturbances, and does not require knowledge of spacecraft masses and disturbance forces. In this algorithm, one degree of freedom of one spacecraft is designated the leader, and all other degrees of freedom of all spacecraft as followers. The Cassini adaptive optimum deadband drift controller is the subalgorithm for control in each degree of freedom, and the adaptation is run until each spacecraft achieves a specified drift period. The adaptation is critical because a different disturbance affects each different degree of freedom. Then the leader communicates its thruster-firing starting times to the followers. Then, for each follower, a deadband-synchronization subalgorithm determines the shift needed to synchronize its drift period with that of the leader.

Scharf, Daniel↗
NASA NTRS2007

Multispectral Imager With Improved Filter Wheel and Optics

Figure 1 schematically depicts an improved multispectral imaging system of the type that utilizes a filter wheel that contains multiple discrete narrow-band-pass filters and that is rotated at a constant high speed to acquire images in rapid succession in the corresponding spectral bands. The improvement, relative to prior systems of this type, consists of the measures taken to prevent the exposure of a focal-plane array (FPA) of photodetectors to light in more than one spectral band at any given time and to prevent exposure of the array to any light during readout. In prior systems, these measures have included, variously the use of mechanical shutters or the incorporation of wide opaque sectors (equivalent to mechanical shutters) into filter wheels. These measures introduce substantial dead times into each operating cycle intervals during which image information cannot be collected and thus incoming light is wasted. In contrast, the present improved design does not involve shutters or wide opaque sectors, and it reduces dead times substantially. The improved multispectral imaging system is preceded by an afocal telescope and includes a filter wheel positioned so that its rotation brings each filter, in its turn, into the exit pupil of the telescope. The filter wheel contains an even number of narrow-band-pass filters separated by narrow, spoke-like opaque sectors. The geometric width of each filter exceeds the cross-sectional width of the light beam coming out of the telescope. The light transmitted by the sequence of narrow-band filters is incident on a dichroic beam splitter that reflects in a broad shorter-wavelength spectral band that contains half of the narrow bands and transmits in a broad longer-wavelength spectral band that contains the other half of the narrow spectral bands. The filters are arranged on the wheel so that if the pass band of a given filter is in the reflection band of the dichroic beam splitter, then the pass band of the adjacent filter is in the longer-wavelength transmission band of the dichroic beam splitter (see Figure 2). Each of the two optical paths downstream of the dichroic beam splitter contains an additional broad-band-pass filter: The filter in the path of the light transmitted by the dichroic beam splitter transmits and attenuates in the same bands that are transmitted and reflected, respectively, by the beam splitter; the filter in the path of the light reflected by the dichroic beam splitter transmits and attenuates in the same bands that are reflected and transmitted, respectively, by the dichroic beam splitter. In each of these paths, the filtered light is focused onto an FPA. As the filter wheel rotates at a constant angular speed, its shaft angle is monitored, and the shaft-angle signal is used to synchronize the exposure times of the two FPAs. When a single narrowband-pass filter on the wheel occupies the entire cross section of the beam of light coming out of the telescope, the spectrum of light that reaches the dichroic beam splitter lies entirely within the pass band of that filter. Therefore, the beam in its entirety is either transmitted by the dichroic beam splitter and imaged on the longer-wavelength FPA or reflected by the beam splitter and imaged onto the shorter-wavelength FPA.

Bremer, James C.↗
NASA NTRS2002

GPS-Based Navigation and Orbit Determination for the AMSAT Phase 3D Satellite

This paper summarizes the results of processing GPS data from the AMSAT Phase 3D (AP3) satellite for real-time navigation and post-processed orbit determination experiments. AP3 was launched into a geostationary transfer orbit (GTO) on November 16, 2000 from Kourou, French Guiana, and then was maneuvered into its HEO over the next several months. It carries two Trimble TANS Vector GPS receivers for signal reception at apogee and at perigee. Its spin stabilization mode currently makes it favorable to track GPS satellites from the backside of the constellation while at perigee, and to track GPS satellites from below while at perigee. To date, the experiment has demonstrated that it is feasible to use GPS for navigation and orbit determination in HEO, which will be of great benefit to planned and proposed missions that will utilize such orbits for science observations. It has also shown that there are many important operational considerations to take into account. For example, GPS signals can be tracked above the constellation at altitudes as high as 58000 km, but sufficient amplification of those weak signals is needed. Moreover, GPS receivers can track up to 4 GPS satellites at perigee while moving as fast as 9.8 km/sec, but unless the receiver can maintain lock on the signals long enough, point solutions will be difficult to generate. The spin stabilization of AP3, for example, appears to cause signal levels to fluctuate as other antennas on the satellite block the signals. As a result, its TANS Vectors have been unable to lock on to the GPS signals long enough to down load the broadcast ephemeris and then generate position and velocity solutions. AP3 is currently in its eclipse season, and thus most of the spacecraft subsystems have been powered off. In Spring 2002, they will again be powered up and AP3 will be placed into a three-axis stabilization mode. This will significantly enhance the likelihood that point solutions can be generated, and perhaps more important, that the receiver clock can be synchronized to GPS time. This is extremely important for real-time and post-processed orbit determination, where removal of receiver clock bias from the data time tags is needed, for time-tagging of science observations. Current analysis suggests that the inability to generate point solutions has allowed the TANS Vector clock bias to drift freely, being perhaps as large as 5-7 seconds by October, 2001, thus causing up to 50 km of along-track orbit error. The data collected in May, 2002 while in three-axis stabilized mode should provide a significant improvement in the orbit determination results.

Davis, George↗
NASA NTRS1980

Critique of the Hughes Aircraft shuttle Ku band leading edge bit synchronizer

A bit synchronizer is analyzed via timing diagrams in a noise-free environment. It is believed that this new bit synchronizer will track the rising edge of the data bits with 25% asymmetry and up to a 90 deg phase shift between the received clock and data bit timing. In addition, the data bits will be demodulated correctly. It is not true that phase shifts larger than 90 deg will necessarily be corrected by this bit synchronizer. However, the specifications currently require the loop to operate over only a + or - 75 deg phase shift between the received data stream leading edges and the bit synchronizer leading edges; consequently, there should be no problem.

Holmes, J. K.↗
NASA NTRS1966

A dual frequency VLF timing system.

Dual frequency VLF timing system for synchronization of remotely located clocks, noting WWVL radio carrier frequency, signal generator, etc

VERY LOW FREQUENCY↗
NASA NTRS1972

Space Satellite Dynamics with Applications to Sunlight Pressure Attitude Control

A research program into three aspects of space satellite dynamics was carried out. First, a four-dimensional space-time formulation of Newtonian mechanics is developed. This theory allows a new physical interpretation of the conservation theorems of mechanics first derived rigorously by Noether. Second, a new concept for estimating the three angles which specify the orientation in space of a rigid body is presented. Two separate methods for implementing this concept are discussed, one based on direction cosines, the other on quaternions. Two examples are discussed: constant orientation in space, and constant rate of change of the three angles with time. Third, two synchronous equatorial orbit communication satellite designs which use sunlight pressure to control their attitude are analyzed. Each design is equipped with large reflecting surfaces, called solar sails, which can be canted in different directions to generate torques to correct pointing errors.

Stuck, B. W.↗
NASA NTRS1975

Double-discharge copper-vapor laser

Power supply for discharge pulses consists of two capacitors that are made to discharge synchronously with adjustable time intervals. First pulse is switched with hydrogen thyratron, and second by spark gap. Lasing action peaks for appropriate combination of these two parameters.

Chen, C. J.↗
NASA NTRS1972

Utilization of FSK communications for time

The method of time control at a radio station for time signal transmission using frequency shift keying, is discussed. Methods for extracting a recovered clock at the receiver are described. Results of a test involving the methods of processing time signals are presented. The requirements for adapting Precise Time and Time Interval control to a communications system are analyzed. A method for reducing the time required for synchronization or identification of messages and the fallout from the communication system to passive timekeeping users are reported.

Stone, R. R., Jr.↗
NASA NTRS1981

Lateral Attenuation of High-By-Pass Ratio Engined Aircraft Noise

A flight experiment was conducted to investigate the lateral attenuation of high by pass ratio engined airplanes. A B-747 was flown at low altitudes over the ends of two microphone arrays. One array covering a lateral distance of 1600 m consisted of 14 microphones positioned over grass. The second array covered a lateral distance of 1200 m and consisted of 6 microphones positioned over a concrete runway. Sixteen runs were flown at altitudes ranging from 30 to 960 m. The acoustic information recorded in the field was reduced to one third octave band spectral time histories and synchronized with tracking and weather information. Lateral attenuation as a function of elevation angle was calculated in overall, A-weighted, tone-corrected perceived noise level, and effective perceived noise level units. The B-747 results are compared with similar results for a turbojet-powered T-38 airplane and the SAE recommended lateral attenuation prediction procedure. Less lateral attenuation was measured for the B-747 than for the T-38. The B-747 lateral attenuation values also fell below the SAE curve.

Willshire, W. L., Jr.↗
NASA NTRS1982

Programming real-time executives in higher order language

Methods by which real-time executive programs can be implemented in a higher order language are discussed, using HAL/S and Path Pascal languages as program examples. Techniques are presented by which noncyclic tasks can readily be incorporated into the executive system. Situations are shown where the executive system can fail to meet its task scheduling and yet be able to recover either by rephasing the clock or stacking the information for later processing. The concept of deadline processing is shown to enable more effective mixing of time and information synchronized systems.

Foudriat, E. C.↗
NASA NTRS1983

Phase Calibration for the Block 1 VLBI System

Very Long Baseline Interferometry (VLBI) in the DSN provides support for spacecraft navigation, Earth orientation measurements, and synchronization of network time and frequency standards. An improved method for calibrating instrumental phase shifts has recently been implemented as a computer program in the Block 1 system. The new calibration program, called PRECAL, performs calibrations over intervals as small as 0.4 seconds and greatly reduces the amount of computer processing required to perform phase calibration.

Roth, M. G.↗
NASA NTRS1986

Hard ACTS to follow

The Advanced Communications Technology Satellite (ACTS), the third phase of NASA's 30/20 GHz satellite communications program, is praised for its frugal usage of both the geosynchronous orbital arch and the frequency spectrum resources necessary for communications satellites. Its objective is to verify Ka-band satellite communications concepts and to develop a flight and ground system for validation of the multibeam communications proof-of-concept technologies. The ACTS ground segment (comprised of four types of terminals) is designed to compliment the spacecraft for the SS launch in 1989. Precise coordination between the ground and spacecraft segments is performed by the baseband processor (BBP), which is an in-orbit switchboard, and the tracking error word, which enables the ground terminals to remain synchronized with onboard timing. Fixed spot beams and scan beams, comprising the two types of spot beams used, both operate at the same frequency and hence, conserve frequency resources. In addition, the time division multiple access serves to enhance system efficiency. It is concluded that Ka-band satellites are a practical approach to the better usage of those resources potentially threatened by communications satellites. Comprehensive graphs and block diagrams of the system are included.

Moy, L.↗
NASA NTRS1985

Spread spectrum communications. Volume 1, 2 & 3

The design and operation of spread-spectrum (SS) communication systems are examined in an introductory text intended for graduate engineering students and practicing engineers. Chapters are devoted to an overview of SS systems, the historical origins of SS, basic concepts and system models, antijam communication systems, pseudonoise generators, coherent direct-sequence systems, noncoherent frequency-hopped systems, coherent and differentially coherent modulation techniques, pseudonoise acquisition and tracking in direct-sequence receivers, time and frequency synchronization of frequency-hopped receivers, low-probability-of-intercept communication, and multiple-access communication. Graphs, diagrams, and photographs are provided.

Simon, M. K.↗
NASA NTRS1986

Virtual time and time warp on the JPL hypercube

This paper describes the concept of virtual time and its implementation in the Time Warp Operating System at the Jet Propulsion Laboratory. Virtual time is a distributed synchronization paradigm that is appropriate for distributed simulation, database concurrency control, real time systems, and coordination of replicated processes. The Time Warp Operating System is targeted toward the distributed simulation application and runs on a 32-node JPL Mark II Hypercube.

Jefferson, David↗
NASA NTRS1988

Spacecraft applications of advanced global positioning system technology

The purpose of this study was to evaluate potential uses of Global Positioning System (GPS) in spacecraft applications in the following areas: attitude control and tracking; structural control; traffic control; and time base definition (synchronization). Each of these functions are addressed. Also addressed are the hardware related issues concerning the application of GPS technology and comparisons are provided with alternative instrumentation methods for specific functions required for an advanced low earth orbit spacecraft.

Huth, Gaylord↗
NASA NTRS1988

Performance analysis of FDDI

The Fiber Distributed Data Interface (FDDI) is an imerging ANSI and ISO standard for a 100 megabit per second fiber optic token ring. The performance of the FDDI media access control protocol is analyzed using a simulation developed at NASA Ames. Both analyses using standard measures of performance (including average delay for asynchronous traffic, channel utilization, and transmission queue length) and analyses of characteristics of ring behavior which can be attributed to constraints imposed by the timed token protocol on token holding time (including bounded token rotation time, support for synchronous traffic, and fairness of channel access for nodes transmitting asynchronous traffic) are included.

Johnson, Marjory J.↗