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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.

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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

In-flight identification of the Galileo spacecraft flexible mode characteristics

The Galileo scan platform is controlled in two degrees of freedom. A clock (Spin Bearing) actuator controls the relative position between the rotor and stator, and a cone actuator controls the position between the stator and the platform. Instruments on the platform are required to point to 140 micro-rad accuracy and 50 micro-rad per second stability. The system identification objectives were to identify dominant structural resonance frequencies, mode shapes, and damping ratio which exist in the transfer function between the clock actuator and the gyro sensor; and position the notch filter to limit undesirable actuator torque output to ensure stability and performance.

Wong, Edward C.↗
NASA NTRS1990

Application specific serial arithmetic arrays

High performance systolic arrays of serial-parallel multiplier elements may be rapidly constructed for specific applications by applying hardware description language techniques to a library of full-custom CMOS building blocks. Single clock pre-charged circuits have been implemented for these arrays at clock rates in excess of 100 Mhz using economical 2-micron (minimum feature size) CMOS processes, which may be quickly configured for a variety of applications. A number of application-specific arrays are presented, including a 2-D convolver for image processing, an integer polynomial solver, and a finite-field polynomial solver.

Winters, K.↗
NASA NTRS1993

MIL-STDS and PTTI: What's available and what needs to be done

The systems developer who needs Precise Time and Time Interval (PTTI) capability has relatively little guidance in the form of military standards, particularly for systems using atomic clocks or other sources of very precise time and frequency. The existing standards, including MIL-STD-188-115, MIL-F-2991(EC), and DOD-STD-1399 are discussed. These documents were written several years ago and do not always reflect current practice or take advantage of more recent technology improvements. User needs have also changed over the years and some of those needs such as more detailed time codes are not being met. We will summarize what's available and what's good and bad about it. The second part will make suggestions about what should be done in the future to promote and facilitate good PTTI design practice. Topics will include clock performance parameters, environmental considerations, time codes, signal isolation, and time dissemination.

Murray, James A.↗
NASA NTRS1993

Analysis of the eastern range multiplexed fiber optic IRIG B120 distribution system

A study was conducted to assess the effects of transmitting a precision clock synchronization signal over a commercial multiplexed fiber optic communication system. This study is an evaluation of the distortion and jitter introduced into the signal by this type of transmission system. An analysis comparing signal quality at the multiplexing and demultiplexing ends of the fiber optic communication system shows that the amplitude and phase distortion added to the clock synchronization signal by the transmission system is minimal.

Duncan, Michael J.↗
NASA NTRS1994

QPPM receiver for free-space laser communications

A prototype receiver developed at NASA Lewis Research Center for direct detection and demodulation of quaternary pulse position modulated (QPPM) optical carriers is described. The receiver enables dual-channel communications at 325-Megabits per second (Mbps) per channel. The optical components of the prototype receiver are briefly described. The electronic components, comprising the analog signal conditioning, slot clock recovery, matched filter and maximum likelihood data recovery circuits are described in more detail. A novel digital symbol clock recovery technique is presented as an alternative to conventional analog methods. Simulated link degradations including noise and pointing-error induced amplitude variations are applied. The bit-error-rate performance of the electronic portion of the prototype receiver under varying optical signal-to-noise power ratios is found to be within 1.5-dB of theory. Implementation of the receiver as a hybrid of analog and digital application specific integrated circuits is planned.

Budinger, J. M.↗
NASA NTRS1992

Detection of feed-through faults in CMOS storage elements

In testing sequential circuits, internal faults in the storage elements (SE's) are sometimes modeled as stuck-at faults in the combinational circuits surrounding the SE. The detection of some transistor-level faults that cannot be modeled as stuck-at are considered. These feed-through faults cause the cell to become either data-feed-through, which makes the cell combinational, or clock-feed-through, which causes the clock signal or its complement to appear at the output. Under such faults, the cell does not function as a memory element. Here it is shown that such faults may or may not be detected depending on delays involved. Conditions under which race-ahead occurs are identified.

Al-Assadi, Waleed K.↗
NASA NTRS1993

A simulation of GPS and differential GPS sensors

The Global Positioning System (GPS) is a revolutionary advance in navigation. Users can determine latitude, longitude, and altitude by receiving range information from at least four satellites. The statistical accuracy of the user's position is directly proportional to the statistical accuracy of the range measurement. Range errors are caused by clock errors, ephemeris errors, atmospheric delays, multipath errors, and receiver noise. Selective Availability, which the military uses to intentionally degrade accuracy for non-authorized users, is a major error source. The proportionality constant relating position errors to range errors is the Dilution of Precision (DOP) which is a function of the satellite geometry. Receivers separated by relatively short distances have the same satellite and atmospheric errors. Differential GPS (DGPS) removes these errors by transmitting pseudorange corrections from a fixed receiver to a mobile receiver. The corrected pseudorange at the moving receiver is now corrupted only by errors from the receiver clock, multipath, and measurement noise. This paper describes a software package that models position errors for various GPS and DGPS systems. The error model is used in the Real-Time Simulator and Cockpit Technology workstation simulations at NASA-LaRC. The GPS/DGPS sensor can simulate enroute navigation, instrument approaches, or on-airport navigation.

Rankin, James M.↗
NASA NTRS1994

Loran-C time management

As of 1 Oct. 1993, the US Coast Guard (USCG) supports and operates fifteen Loran-C chains. With the introduction of the Global Positioning Systems (GPS) and the termination of the Department of Defense (DOD) overseas need for Loran-C, the USCG will cease operating the three remaining overseas chains by 31 Dec. 1994. Following this date, the USCG Loran-C system will consist of twelve chains. Since 1971, management of time synchronization of the Loran-C system has been conducted under a Memorandum of Agreement between the US Naval Observatory (USNO) and the USCG. The requirement to maintain synchronization with Coordinated Universal Time (UTC) was initially specified as +/- 25 microseconds. This tolerance was rapidly lowered to +/- 2.5 microseconds in 1974. To manage this synchronization requirement, the USCG incorporated administrative practices which kept the USNO appraised of all aspects of the master timing path. This included procedures for responding to timing path failures, timing adjustments, and time steps. Conducting these aspects of time synchronization depended on message traffic between the various master stations and the USNO. To determine clock adjustment the USCG relied upon the USNO's Series 4 and 100 updates so that the characteristics of the master clock could be plotted and controls appropriately applied. In 1987, Public Law 100-223, under the Airport and Airway Improvement Act Amendment, reduced the synchronization tolerance to approximately 100 nanoseconds for chains serving the National Airspace System (NAS). This action caused changes in the previous administrative procedures and techniques. The actions taken by the USCG to meet the requirements of this law are presented.

Justice, Charles↗
NASA NTRS1994

Time and position accuracy using codeless GPS

The Global Positioning System has allowed scientists and engineers to make measurements having accuracy far beyond the original 15 meter goal of the system. Using global networks of P-Code capable receivers and extensive post-processing, geodesists have achieved baseline precision of a few parts per billion, and clock offsets have been measured at the nanosecond level over intercontinental distances. A cloud hangs over this picture, however. The Department of Defense plans to encrypt the P-Code (called Anti-Spoofing, or AS) in the fall of 1993. After this event, geodetic and time measurements will have to be made using codeless GPS receivers. However, there appears to be a silver lining to the cloud. In response to the anticipated encryption of the P-Code, the geodetic and GPS receiver community has developed some remarkably effective means of coping with AS without classified information. We will discuss various codeless techniques currently available and the data noise resulting from each. We will review some geodetic results obtained using only codeless data, and discuss the implications for time measurements. Finally, we will present the status of GPS research at JPL in relation to codeless clock measurements.

Dunn, C. E.↗
NASA NTRS1994

Cesium and rubidium frequency standards status and performance on the GPS program

This paper is an update of the on-orbit operational performance of the frequency standards on the Block I GPS 8 to 11 Navstar satellites, the Block II GPS 13 to 21 Navstar satellites and the Block IIA GPS 22 to 40 Navstar satellites. A brief history of the frequency standards depicting improvements incorporated in the GPS Program with corresponding results will be presented. The frequency standards configurations from the start to the present, including the various Block IIA clock configurations will be discussed. Topics such as clock observations and long term trending analysis which could enhance on-orbit performance analysis will be covered.

Vanmelle, M. J.↗
NASA NTRS1994

The NIST Internet time service

We will describe the NIST Network Time Service which provides time and frequency information over the Internet. Our first time server is located in Boulder, Colorado, a second backup server is under construction there, and we plan to install a third server on the East Coast later this year. The servers are synchronized to UTC(NIST) with an uncertainty of about 0.8 ms RMS and they will respond to time requests from any client on the Internet in several different formats including the DAYTIME, TIME and NTP protocols. The DAYTIME and TIME protocols are the easiest to use and are suitable for providing time to PC's and other small computers. In addition to UTC(NIST), the DAYTIME message provides advance notice of leap seconds and of the transitions to and from Daylight Saving Time. The Daylight Saving Time notice is based on the US transition dates of the first Sunday in April and the last one in October. The NTP is a more complex protocol that is suitable for larger machines; it is normally run as a 'daemon' process in the background and can keep the time of the client to within a few milliseconds of UTC(NIST). We will describe the operating principles of various kinds of client software ranging from a simple program that queries the server once and sets the local clock to more complex 'daemon' processes (such as NTP) that continuously correct the time of the local clock based on periodic calibrations.

Levine, Judah↗
NASA NTRS1994

The Mars Observer differential one-way range demonstration

Current methods of angular spacecraft positioning using station differenced range data require an additional observation of an extragalactic radio source (quasar) to estimate the timing offset between the reference clocks at the two Deep Space Stations. The quasar observation is also used to reduce the effects of instrumental and media delays on the radio metric observable by forming a difference with the spacecraft observation (delta differential one-way range, delta DOR). An experiment has been completed using data from the Global Positioning System satellites to estimate the station clock offset, eliminating the need for the quasar observation. The requirements for direct measurement of the instrumental delays that must be made in the absence of a quasar observation are assessed. Finally, the results of the 'quasar-free' differential one-way range, or DOR, measurements of the Mars Observer spacecraft are compared with those of simultaneous conventional delta DOR measurements.

Kroger, P. M.↗
NASA NTRS1994

Testing and Performance Analysis of a 650 Mbps QPPM Modem for Free-space Laser Communications

The testing and performance of a prototype modem developed at NASA Lewis Research Center for high-speed free-space direct detection optical communications is described. The testing was performed under laboratory conditions using computer control with specially developed test equipment that simulates free-space link conditions. The modem employs quaternary pulse position modulation (QPPM) at 325 Megabits per second (Mbps) on two optical channels, which are multiplexed to transmit a single 650 Mbps data stream. The measured results indicate that the receiver's automatic gain control (AGC), phased-locked-loop slot clock recovery, digital symbol clock recovery, matched filtering, and maximum likelihood data recovery circuits were found to have only 1.5 dB combined implementation loss during bit-error-rate (BER) performance measurements. Pseudo random bit sequences and real-time high quality video sources were used to supply 650 Mbps and 325 Mbps data streams to the modem. Additional testing revealed that Doppler frequency shifting can be easily tracked by the receiver, that simulated pointing errors are readily compensated for by the AGC circuits, and that channel timing skew affects the BER performance in an expected manner. Overall, the needed technologies for a high-speed laser communications modem were demonstrated.

Mortensen, Dale J.↗
NASA NTRS1994

Properties of nearly perfect crystals at very low temperatures

Data shows that the frequency stability of 5 MHz AT-cut quartz crystal oscillators is improved by lowering the temperature to 4.3 K. The resultant level of stability is apparently not, at this point, sufficient for a clock accurate to 1 part in 10(exp 17). However, many improvements are possible in the scheme presented here. These would involve better temperature regulation, better crystal mounting and vibration isolation, and lower temperatures. Below the lambda-transition at 2.17 K the residual bubbling of the bath would be eliminated. Thus, the prospect of a successful clock built on this scheme is not ruled out.

Weber, J.↗
NASA NTRS1994

Highly parallel reconfigurable computer architecture for robotic computation having plural processor cells each having right and left ensembles of plural processors

In a computer having a large number of single-instruction multiple data (SIMD) processors, each of the SIMD processors has two sets of three individual processor elements controlled by a master control unit and interconnected among a plurality of register file units where data is stored. The register files input and output data in synchronism with a minor cycle clock under control of two slave control units controlling the register file units connected to respective ones of the two sets of processor elements. Depending upon which ones of the register file units are enabled to store or transmit data during a particular minor clock cycle, the processor elements within an SIMD processor are connected in rings or in pipeline arrays, and may exchange data with the internal bus or with neighboring SIMD processors through interface units controlled by respective ones of the two slave control units.

Fijany, Amir↗
NASA NTRS1993

CFD Parametric Study of Consortium Impeller

Current design of high performance turbopumps for rocket engines requires effective and robust analytical tools to provide design impact in a productive manner. The main goal of this study is to develop a robust and effective computational fluid dynamics (CFD) pump model for general turbopump design and analysis applications. A Finite Difference Navier-Stokes flow solver, FDNS, which includes the extended k-epsilon turbulence model and appropriate moving interface boundary conditions, was developed to analyze turbulent flows in turbomachinery devices. A second-order central difference scheme plus adaptive dissipation terms was employed in the FDNS code, along with a predictor plus multi-corrector pressure-based solution procedure. The multi-zone, multi-block capability allows the FDNS code to efficiently solve flow fields with complicated geometry. The FDNS code has been benchmarked by analyzing the pump consortium inducer, and it provided satisfactory results. In the present study, a CFD parametric study of the pump consortium impeller was conducted using the FDNS code. The pump consortium impeller, with partial blades, is a new design concept of the advanced rocket engines. The parametric study was to analyze the baseline design of the consortium impeller and its modification which utilizes TANDEM blades. In the present study, the TANDEM blade configuration of the consortium impeller considers cut full blades for about one quarter chord length from the leading edge and clocks the leading edge portion with an angle of 7.5 or 22.5 degrees. The purpose of the present study is to investigate the effect and trend of the TANDEM blade modification and provide the result as a design guideline. A 3-D flow analysis, with a 103 x 23 x 30 mesh grid system and with the inlet flow conditions measured by Rocketdyne, was performed for the baseline consortium impeller. The numerical result shows that the mass flow rate splits through various blade passages are relatively uniform. Due to the complexity of blade geometries, the TANDEM blade configurations were analyzed with the multi-zone grid structure. Both the 7.5 deg- and the 22.5 deg-clocking TANDEM blade cases utilized a 80K mesh system. The numerical result of two TANDEM blade modifications indicates the efficiency and the head are worse than those of the baseline case due to larger flow distortion. The gap between the TANDEM blade and the full blade allows the flow passes through and heavily loads the pressure side of the partial blade such that flow reversal occurs near the suction side of the splitter. The flow split at the exit of impeller blades is very non-uniform for TANDEM blade cases, and this will greatly induce the side load on the diffuser. Therefore, the TANDEM blade modification in the present CFD analysis does not improve the performance of the consortium impeller.

Cheng, Gary C.↗
NASA NTRS1993

The 1993 Gordon Research Conference on Chronobiology

The study of biological timekeeping is now at a particularly fertile stage, encompassing multiple levels of biological organization, recruiting a wide array of disciplines and methodologies and uniting a host of investigators. This report summarizes a research conference on Chronobiology. Some of the topics focused on transcriptional and translational mechanisms of circadian rhythmicity, with discussions of putative 'clock genes' in cyanobacteria, algae, fungi, fruitflies, and hamsters. Cellular analysis, with emphasis on photoreceptors in frogs, neurons in molluscs, and testis in moths was addressed. New methods for investigating the circadian clock in the suprachiasmatic nucleus were introduced.

Schwartz, William J.↗
NASA NTRS1995

High accuracy time transfer synchronization

In July 1994, the U.S. Naval Observatory (USNO) Time Service System Engineering Division conducted a field test to establish a baseline accuracy for two-way satellite time transfer synchronization. Three Hewlett-Packard model 5071 high performance cesium frequency standards were transported from the USNO in Washington, DC to Los Angeles, California in the USNO's mobile earth station. Two-Way Satellite Time Transfer links between the mobile earth station and the USNO were conducted each day of the trip, using the Naval Research Laboratory(NRL) designed spread spectrum modem, built by Allen Osborne Associates(AOA). A Motorola six channel GPS receiver was used to track the location and altitude of the mobile earth station and to provide coordinates for calculating Sagnac corrections for the two-way measurements, and relativistic corrections for the cesium clocks. This paper will discuss the trip, the measurement systems used and the results from the data collected. We will show the accuracy of using two-way satellite time transfer for synchronization and the performance of the three HP 5071 cesium clocks in an operational environment.

Wheeler, Paul J.↗