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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Relationships Between the Performance of Time/Frequency Standards and Navigation/Communication Systems

The relationship between system performance and clock or oscillator performance is discussed. Tradeoffs discussed include: short term stability versus bandwidth requirements; frequency accuracy versus signal acquisition time; flicker of frequency and drift versus resynchronization time; frequency precision versus communications traffic volume; spectral purity versus bit error rate, and frequency standard stability versus frequency selection and adjustability. The benefits and tradeoffs of using precise frequency and time signals are various levels of precision and accuracy are emphasized.

Hellwig, H.↗
NASA NTRS2006

Dynamics and Adaptive Control for Stability Recovery of Damaged Aircraft

This paper presents a recent study of a damaged generic transport model as part of a NASA research project to investigate adaptive control methods for stability recovery of damaged aircraft operating in off-nominal flight conditions under damage and or failures. Aerodynamic modeling of damage effects is performed using an aerodynamic code to assess changes in the stability and control derivatives of a generic transport aircraft. Certain types of damage such as damage to one of the wings or horizontal stabilizers can cause the aircraft to become asymmetric, thus resulting in a coupling between the longitudinal and lateral motions. Flight dynamics for a general asymmetric aircraft is derived to account for changes in the center of gravity that can compromise the stability of the damaged aircraft. An iterative trim analysis for the translational motion is developed to refine the trim procedure by accounting for the effects of the control surface deflection. A hybrid direct-indirect neural network, adaptive flight control is proposed as an adaptive law for stabilizing the rotational motion of the damaged aircraft. The indirect adaptation is designed to estimate the plant dynamics of the damaged aircraft in conjunction with the direct adaptation that computes the control augmentation. Two approaches are presented 1) an adaptive law derived from the Lyapunov stability theory to ensure that the signals are bounded, and 2) a recursive least-square method for parameter identification. A hardware-in-the-loop simulation is conducted and demonstrates the effectiveness of the direct neural network adaptive flight control in the stability recovery of the damaged aircraft. A preliminary simulation of the hybrid adaptive flight control has been performed and initial data have shown the effectiveness of the proposed hybrid approach. Future work will include further investigations and high-fidelity simulations of the proposed hybrid adaptive Bight control approach.

Nguyen, Nhan↗
NASA NTRS1996

Precision Lower Stratospheric Temperature Monitoring with the MSU: Technique, Validation, and Results, 1979-1991

Microwave Sounding Unit channel 4 data from the TIROS-N (Television and Infrared Operational Satellite)-N series of NOAA (National Oceanic and Atmospheric Administration) satellites are intercalibrated to provide a continuous global record of deep-layer averaged lower stratospheric temperatures during 1979-1991. A 13-year record of temperature anomalies is time averaged into pentads and months on a 2.5 deg. grid. The monthly gridpoint anomalies are validated with ten years of radiosonde data during 1979-88. The calibration stability of each satellite's measurements is evaluated during satellite overlap periods, the longest of which reveal no measurable instrumental drift at the level of 0.01 C yr (exp -1). Intercomparisons between NOAA-6 and NOAA- 7 anomalies indicate monthly gridpoint precision of 0.05 C in the tropics to around 0.10 C in the extratropics, and signal-to-noise ratios precision and stability statistics are much better than have been previously reported by other investigators for MSU channel 4. Pentad precision is about 0.10 C in the tropics to around 0.25 C at high latitudes and signal-to- noise ratios generally over 250 in the tropics and high latitudes, but 100-200 in the middle latitudes. Radiosonde comparisons to the monthly gridpoint anomalies have correlations ranging from 0.90 in the tropics (where the interannual variability is smallest) to as high as 0.99 at high-latitude stations. The corresponding standard error of estimate is generally around 0.3 C. A significant difference in decadal trends is found between the satellite and radiosonde systems, with a step change of 0.21 C (sondes cooler) compared to the satellite measurements. Investigations of the possible sources of the discrepancy lead us to suspect that the gradual transition from on-site calibration of sondes with thermometers to factory calibration of sondes around 1982 might have caused a change in the calibration, although this conclusion must be viewed as tentative. The largest globally averaged temperature variations during 1979-91 occur after the El Chichon (1982) and Pinatubo (1991) volcanic eruptions. These warm events are superimposed upon a net downward trend in temperatures during the period. This cooling trend has more of a step function than linear character, with the step occurring during the El Chichon warm event. It is strongest in polar regions and the Northern Hemisphere middle latitudes. These characteristics are qualitatively consistent with radiative adjustments expected to occur with observed ozone depletions.

Spencer, Roy W.↗
NASA NTRS1989

Stabilized fiber-optic frequency distribution system

A technique for stabilizing reference frequencies transmitted over fiber-optic cable in a frequency distribution system is discussed. The distribution system utilizes fiber-optic cable as the transmission medium to distribute precise reference signals from a frequency standard to remote users. The stability goal of the distribution system is to transmit a 100-MHz signal over a 22-km fiber-optic cable and maintain a stability of 1 part in 10(17) for 1000-second averaging times. Active stabilization of the link is required to reduce phase variations produced by environmental effects, and is achieved by transmitting the reference signal from the frequency standard to the remote unit and then reflecting back to the reference unit over the same optical fiber. By comparing the phase of the transmitted and reflected signals at the reference unit, phase variations of the remote signal can be measured. An error voltage derived from the phase difference between the two signals is used to add correction phase.

Primas, L. E.↗
NASA NTRS1999

A Frequency Transfer and Cleanup System for Ultra-High Stability at Both Long and Short Times for the Cassini Ka-Band Experiment

New radio science experiments, including a gravitational wave search and several atmospheric occultation studies, are planned for the Cassini Ka-band experiment. These experiments are made possible by reduced solar-induced phase fluctuations at the high-frequency (32 GHZ) of the radio link between the earth and the spacecraft. In order to match the improved link performance, a significant upgrade is under way to improve the frequency stability capabilities of NASA's Deep Space Network (DSN). Significant improvements are being undertaken in many areas, including antenna vibration and (wet) tropospheric calibration, in addition to frequency generation and distribution. We describe here the design and development of a system to provide a reference signal with the highest possible frequency stability for both long-term, short-term, and phase noise, at an antenna (DSS 25) that is remote from the frequency standards room at SPC-10 at the Goldstone site. The new technologies were developed in order to meet the very tight requirements. They are: 1) a Stabilized Fiber-Optic Distribution Assembly (SFODA) that includes active compensation of thermal variations to transfer long-term stability over 16 km of ordinary fiber-optic cable, and 2) a Compensated Sapphire Oscillator (CSO) that provides short-term performance in a cryocooled sapphire oscillator with ultra-high short-term stability and low phase noise.

Calhoun, M. D.↗
NASA NTRS2013

Spring-Summer Temperatures Since AD 1780 Reconstructed from Stable Oxygen Isotope Ratios in White Spruce Tree-Rings from the Mackenzie Delta, Northwestern Canada

High-latitude delta(exp 18)O archives deriving from meteoric water (e.g., tree-rings and ice-cores) can provide valuable information on past temperature variability, but stationarity of temperature signals in these archives depends on the stability of moisture source/trajectory and precipitation seasonality, both of which can be affected by atmospheric circulation changes. A tree-ring delta(exp 18)O record (AD 1780-2003) from the Mackenzie Delta is evaluated as a temperature proxy based on linear regression diagnostics. The primary source of moisture for this region is the North Pacific and, thus, North Pacific atmospheric circulation variability could potentially affect the tree-ring delta(exp 18)O-temperature signal. Over the instrumental period (AD 1892-2003), tree-ring delta(exp 18)O explained 29% of interannual variability in April-July minimum temperatures, and the explained variability increases substantially at lower-frequencies. A split-period calibration/verification analysis found the delta(exp 18)O-temperature relation was time-stable, which supported a temperature reconstruction back to AD 1780. The stability of the delta(exp 18)O-temperature signal indirectly implies the study region is insensitive to North Pacific circulation effects, since North Pacific circulation was not constant over the calibration period. Simulations from the NASA-GISS ModelE isotope-enabled general circulation model confirm that meteoric delta(exp 18)O and precipitation seasonality in the study region are likely insensitive to North Pacific circulation effects, highlighting the paleoclimatic value of tree-ring and possibly other delta(exp 18)O records from this region. Our delta(exp 18)O-based temperature reconstruction is the first of its kind in northwestern North America, and one of few worldwide, and provides a long-term context for evaluating recent climate warming in the Mackenzie Delta region.

Canada↗
NASA NTRS1995

The Deep Space Network stability analyzer

A stability analyzer for testing NASA Deep Space Network installations during flight radio science experiments is described. The stability analyzer provides realtime measurements of signal properties of general experimental interest: power, phase, and amplitude spectra; Allan deviation; and time series of amplitude, phase shift, and differential phase shift. Input ports are provided for up to four 100 MHz frequency standards and eight baseband analog (greater than 100 kHz bandwidth) signals. Test results indicate the following upper bounds to noise floors when operating on 100 MHz signals: -145 dBc/Hz for phase noise spectrum further than 200 Hz from carrier, 2.5 x 10(exp -15) (tau =1 second) and 1.5 x 10(exp -17) (tau =1000 seconds) for Allan deviation, and 1 x 10(exp -4) degrees for 1-second averages of phase deviation. Four copies of the stability analyzer have been produced, plus one transportable unit for use at non-NASA observatories.

Breidenthal, Julian C.↗
DOE OSTI2025

The Relative Importance of Forced and Unforced Temperature Patterns in Driving the Time Variation of Low-Cloud Feedback

Abstract Atmospheric models forced with observed sea surface temperatures (SSTs) suggest a trend toward a more-stabilizing cloud feedback in recent decades, partly due to the surface cooling trend in the eastern Pacific (EP) and the warming trend in the western Pacific (WP). Here, we show model evidence that the low-cloud feedback has contributions from both forced and unforced feedback components and that its time variation arises in large part through changes in the relative importance of the two over time, rather than through variations in forced or unforced feedbacks themselves. Initial-condition large ensembles (LEs) suggest that the SST patterns are dominated by unforced variations for 30-yr windows ending prior to the 1980s. In general, unforced SSTs are representative of an ENSO-like pattern, which corresponds to weak low-level stability in the tropics and less-stabilizing low-cloud feedback. Since the 1980s, the forced signals have become stronger, outweighing the unforced signals for the 30-yr windows ending after the 2010s. Forced SSTs are characterized by relatively uniform warming with an enhancement in the WP, corresponding to a more-stabilizing low-cloud feedback in most cases. The time-evolving SST pattern due to this increasing importance of forced signals is the dominant contributor to the recent stabilizing shift of low-cloud feedback in the LEs. Using single-forcing LEs, we further find that if only greenhouse gases evolve with time, the transition to the domination of forced signals occurs 10–20 years earlier compared to the LEs with full forcings, which can be understood through the compensating effect between aerosols and greenhouse gases.

58 GEOSCIENCES↗
NASA NTRS2002

Motor scaling by viewing distance of early visual motion signals during smooth pursuit

The geometry of gaze stabilization during head translation requires eye movements to scale proportionally to the inverse of target distance. Such a scaling has indeed been demonstrated to exist for the translational vestibuloocular reflex (TVOR), as well as optic flow-selective translational visuomotor reflexes (e.g., ocular following, OFR). The similarities in this scaling by a neural estimate of target distance for both the TVOR and the OFR have been interpreted to suggest that the two reflexes share common premotor processing. Because the neural substrates of OFR are partly shared by those for the generation of pursuit eye movements, we wanted to know if the site of gain modulation for TVOR and OFR is also part of a major pathway for pursuit. Thus, in the present studies, we investigated in rhesus monkeys whether initial eye velocity and acceleration during the open-loop portion of step ramp pursuit scales with target distance. Specifically, with visual motion identical on the retina during tracking at different distances (12, 24, and 60 cm), we compared the first 80 ms of horizontal pursuit. We report that initial eye velocity and acceleration exhibits either no or a very small dependence on vergence angle that is at least an order of magnitude less than the corresponding dependence of the TVOR and OFR. The results suggest that the neural substrates for motor scaling by target distance remain largely distinct from the main pathway for pursuit.

Non-NASA Center↗
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 NTRS2004

Stochastic Stability of Nonlinear Sampled Data Systems with a Jump Linear Controller

This paper analyzes the stability of a sampled- data system consisting of a deterministic, nonlinear, time- invariant, continuous-time plant and a stochastic, discrete- time, jump linear controller. The jump linear controller mod- els, for example, computer systems and communication net- works that are subject to stochastic upsets or disruptions. This sampled-data model has been used in the analysis and design of fault-tolerant systems and computer-control systems with random communication delays without taking into account the inter-sample response. To analyze stability, appropriate topologies are introduced for the signal spaces of the sampled- data system. With these topologies, the ideal sampling and zero-order-hold operators are shown to be measurable maps. This paper shows that the known equivalence between the stability of a deterministic, linear sampled-data system and its associated discrete-time representation as well as between a nonlinear sampled-data system and a linearized representation holds even in a stochastic framework.

Gonzalez, Oscar R.↗
NASA NTRS2004

Laser Sounder for Global Measurement of CO2 Concentrations in the Troposphere from Space: Progress

We describe progress toward developing a laser-based technique for the remote measurement of the tropospheric CO2 concentrations from orbit. Our goal is to demonstrate a lidar technique and instrument technology that will permit measurements of the CO2 column abundance in the lower troposphere from aircraft at the few ppm level, with a capability of scaling to permit global CO2 measurements from orbit. Accurate measurements of the tropospheric CO2 mixing ratio from space are challenging due to the many potential error sources. These include possible interference from other trace gas species, the effects of temperature, clouds, aerosols & turbulence in the path, changes in surface reflectivity, and variability in dry air density caused by changes in atmospheric pressure, water vapor and topographic height. Some potential instrumental errors include frequency drifts in the transmitter, small transmission and sensitivity drifts in the instrument. High signal-to-noise ratios and measurement stability are needed for mixing ratio estimates at the few ppm level. We have been developing a laser sounder approach as a candidate for a future space mission. It utilizes multiple different laser transmitters to permit simultaneous measurement of CO2 and O2 extinction, and aerosol backscatter in the same measurement path. It directs the narrow co-aligned laser beams from the instrument's fiber lasers toward nadir, and measures the energy of the strong laser echoes reflected from the Earth's land and water surfaces. During the measurement its narrow linewidth lasers are rapidly tuned on- and off- selected CO2 line near 1572 nm and an O2 absorption line near 770 nm. The receiver measures the energies of the laser echoes from the surface and any clouds and aerosols in the path with photon counting detectors. Ratioing the on- to off-line echo pulse energies for each gas permits the column extinction and column densities of CO2 and O2 to be estimated simultaneously via the differential absorption lidar technique. For the on-line wavelengths, the side of the selected absorption lines are used, which due to pressure broadening, weights the measurements to the lower troposphere, where CO2 variations caused by surface sources and sinks are largest. Simultaneous measurements of O2 column abundance are made using an identical approach using an O2 line. The laser backscatter profiles from clouds and aerosols are measured with other lidar channels, which permits identifying measurements influenced by clouds and/or aerosol scattering in the path. For space use, our lidar would continuously measure at nadir in near polar circular orbit. Using dawn and dusk measurements made over the same region will make it possible to sample the diurnal variations in CO2 mixing ratios. A 1-m diameter telescope is used for the receiver for all wavelengths. When averaging over 50 seconds, our calculations show a SNR of approximately 1500 is achievable for each gas at each on- and off-line measurement. Measurements from such a mission can be used to generate monthly global maps of the lower tropospheric CO2 column abundance. Our calculations show global coverage with an accuracy of a few ppm with a spatial resolution of approximately 50,000 sq. km are achievable each month. We have demonstrated some key elements of the laser, detector and receiver approaches in the laboratory and with measurements over a 206 m horizontal path. These include stable measurements of CO2 line shapes in an absorption cell using a fiber laser amplifier seeded by a tunable diode laser, measurement of small amplitude changes at low optical signal levels with the PMT receiver, and comparison of the horizontal path measurements of CO2 against those from an in-situ instrument.

Abshire, J. B.↗
NASA NTRS1996

The JPL Hg(sup +) Extended Linear Ion Trap Frequency Standard: Status, Stability, and Accuracy Prospects

Microwave frequency standards based on room temperature (sup 199)Hg(sup +) ions in a Linear Ion Trap (LITS) presently achieve a Signal to Noise and line Q inferred short frequency stability. Long term stability has been measured for averaging intervals up to 5 months with apparent sensitivity to variations in ion number/temperature limiting the flicker floor.

Hg+ linear ion trap frequency LITE LITS microwave ↗
NASA NTRS1977

The trapped mercury ion frequency standard

Singly ionized mercury atoms have a structure similar to neutral alkali atoms. They can be maintained as ions for very long times in an RF quadrupole ion trap. Thus, their ground state hyperfine structure can be used to make a frequency standard using optical pumping techniques similar to the well-known rubidium standard. The mass 199 isotope of mercury has an ionic hyperfine structure of 40.5 GHz. In a trap system a linewidth of 10 Hz has been measured. An expression is presented for the short-term stability of a proposed mercury standard as set by the achieved signal to noise ratio. There is prospect of further improvement. Long-term stability is affected by second order doppler effect, and by pressure, light, and Stark shifts. However, these appear either sufficiently small or sufficiently controlable that the proposed mercury ion standard would be competitive with existing standards.

Mcguire, M. D.↗
NASA NTRS1976

A computer-aided telescope pointing system utilizing a video star tracker

The Video Inertial Pointing (VIP) System is being developed to satisfy the acquisition and pointing requirements of astronomical telescopes. VIP employs a single video sensor to generate three-axis pointing error signals and to provide inputs for a cathode ray tube (CRT) display of the star field. The pointing error signals update the telescope's gyro stabilization system. The CRT display facilitates target acquisition and positioning of the telescope by a remote operator. The present paper describes the analysis, simulation, and hardware development of a prototype, advanced VIP system. An early model of the system utilizing a silicon-intensified target vidicon camera has flown on a balloon-borne telescope and is briefly described. The advanced system, which can employ either a vidicon camera or a charge-coupled device video sensor, has been tested using an analog/digital hybrid simulation. The advanced VIP hardware is described, and the simulation results presented.

Lorell, K. R.↗
NASA NTRS1984

Nimbus-earth radiation budget sensor characterization for improved data reduction fidelity

Detailed characterizations of flight spare earth flux sensors from the Nimbus Earth Radiation Budget (ERB) program have been performed which, when coupled with a more careful accounting of the orbital instrument environment, provide the potential for improved accuracy in the final data products. The characterizations included detailed field-of-view mappings, responses to transient long and short wavelength radiation, and response to sensor temperature changes. These sensor and environment characterizations, along with the outstanding low noise and stability properties of the ERB instrument signal processing system, promise improvement of the data accuracy to levels sufficient for long-term budget and climatological purposes. The combined data sets from Nimbus 6 and 7 are expected to span a period in excess of 10 years. The improvements in data accuracy are particularly significant over zonal latitude bands because the corrections are strongly latitude dependent.

Maschhoff, R.↗