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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Coherent summation of spatially distorted laser Doppler signals by using a two-dimensional heterodyne detector array

Phase-sensitive coherent summation of individual heterodyne detector array signals was demonstrated for the enhanced detection of spatially distorted laser Doppler returns. With the use of a 2 x 2 heterodyne detector array, the phase and amplitude of a time-varying speckle pattern was detected, and the signal-to-noise ratio of the Doppler shift estimate was shown to be improved by a factor of 2, depending on the extent of spatial coherence loss. These results are shown to agree with a first-order analysis and indicate the advantage of coherent summation for both short-range laser Doppler velocimetry and long-range atmospheric coherent lidar.

Chan, Kin P.↗
NASA NTRS1991

Coherent Doppler lidar for automated space vehicle, rendezvous, station-keeping and capture

Recent advances in eye-safe, short wavelength solid-state lasers offer real potential for the development of compact, reliable, light-weight, efficient coherent lidar. Laser diode pumping of these devices has been demonstrated, thereby eliminating the need for flash lamp pumping, which has been a major drawback to the use of these lasers in space based applications. Also these lasers now have the frequency stability required to make them useful in coherent lidar, which offers all of the advantages of non-coherent lidar, but with the additional advantage that direct determination of target velocity is possible by measurement of the Doppler shift. By combining the Doppler velocity measurement capability with the inherent high angular resolution and range accuracy of lidar it is possible to construct Doppler images of targets for target motion assessment. A coherent lidar based on a Tm,Ho:YAG 2-micrometer wavelength laser was constructed and successfully field tested on atmospheric targets in 1990. This lidar incorporated an all solid state (laser diode pumped) master oscillator, in conjunction with a flash lamp pumped slave oscillator. Solid-state laser technology is rapidly advancing, and with the advent of high efficiency, high power, semiconductor laser diodes as pump sources, all-solid-state, coherent lidars are a real possibility in the near future. MSFC currently has a feasibility demonstration effort under way which will involve component testing, and preliminary design of an all-solid-state, coherent lidar for automatic rendezvous, and capture. This two year effort, funded by the Director's Discretionary Fund is due for completion in 1992.

Dunkin, James A.↗
NASA NTRS1991

Mars approach navigation using Doppler and range measurements to surface beacons and orbiting spacecraft

Approximate analytical models are developed and used to construct an error covariance analysis for investigating the range of orbit determination accuracies which might be achieved for typical Mars approach trajectories. The sensitivity or orbit determination accuracy to beacon/orbiter position errors and to small spacecraft force modeling errors is also investigated. The results indicate that the orbit determination performance obtained from both Doppler and range data is a strong function of the inclination of the approach trajectory to the Martian equator, for surface beacons, and for orbiters, the inclination relative to the orbital plane. Large variations in performance were also observed for different approach velocity magnitudes; Doppler data in particular were found to perform poorly in determining the downtrack (along the direction of flight) component of spacecraft position. In addition, it was found that small spacecraft acceleration modeling errors can induce large errors in the Doppler-derived downtrack position estimate.

Thurman, Sam W.↗
NASA NTRS1991

A method for computing instantaneous downtrack error from TDRS Doppler residuals

A simple method has been developed which calculates the instantaneous downtrack error in an orbiting vehicle's state vector from TDRS (Tracking and Data Relay Satellite) Doppler observation residuals. The downtrack error is expressed as a delta-time correction to the timetag of the vehicle's state vector. The method requires only a few easily obtained parameters and is easy to implement. This paper details the derivation of computing a delta-time correction from TDRS Doppler observation residuals and evaluates the method's assumptions and limitations. The results presented show that typically the method computes delta-time corrections that are less than five percent off from the true timetag error for Doppler observation residuals within twenty minutes of closest approach to the TDRS.

Haas, Franklin M., Jr.↗
NASA NTRS1993

De-Dopplerization of aircraft acoustic signals

A de-Dopplerization scheme is devised and applied to both a tonal noise source, a XV-15 aircraft, and a broadband noise source, a F-18 aircraft. The procedure is developed from the conservation equations of fluid mechanics and is described in the paper. The corrected time history is constructed by using linear interpolation in the measured time history. This was made possible by knowing the position history of the aircraft provided by radar and/or laser tracking. The XV-15 data established that the scheme can accurately account for Doppler frequency shifts. The F-18 data confirms what has been noted in prediction models and static tests pertaining to broadband shock associated noise. That is, the peak frequency increases and the peaks broaden toward the jet axis. Another issue addressed in the study is the influence of correcting for spherical spreading and Doppler amplitude on the spectral shape and overall sound pressure levels of the source. Results from this investigation confirm that the dominant noise source in high-speed jets is due to turbulent mixing.

Kelly, Jeffrey J.↗
NASA NTRS1992

Doppler experiments with Cassini radio system

The radio system of the Cassini orbiter will include a K-alpha band downlink channel, mainly intended for telemetry. A K-alpha uplink has also been proposed to allow for a highly accurate gravitational wave experiment. The fourfold increase in frequency will reduce the plasma noise by a factor of 12 and will allow a Doppler accuracy better than 10 exp -15 for time scales of 10 exp 3 - 10 exp 4 s. Extensive Doppler measurements of the gravitational field of Saturn and its satellites can be performed, exploiting the induced change in the velocity of the spacecraft. Possible sources of low-frequency gravitational waves and errors in the Doppler link are discussed.

Comoretto, G.↗
NASA NTRS1985

Doppler lidar sampling strategies and accuracies: Regional scale

It has been proposed that a Doppler lidar be placed in a polar orbit and scanned to provide estimates of lower tropospheric winds twice per day and with a spatial resolution of 300 km. Initial feasibility studies conducted primarily by NOAA and NASA presented an optimistic outlook for a space based lidar. The technology appeared within reach and initial computer simulations suggested that acceptable accuracies could be obtained. Those early studies exposed, however, several potential problem areas which included: (1) the algorithms for computing the wind vectors did not perform well when there were coherent gradients in the wind fields; and (2) the lifetime and power requirements of the lidar put severe restrictions on the pulse repetition frequency (PRF). These two basic problems are currently being addressed by a Doppler lidar simulation study focussed upon three primary objectives: (1) to develop optimum scan parameters and shot patterns for a satellite-based Doppler lidar; (2) to develop robust algorithms for computing wind vectors from lidar returns; and (3) to evaluate the impact of coherent mesoscale structures (wind gradients, clouds, aerosols) on up-scale wind estimates. An overview is provided of the simulation efforts with particular emphasis upon rationale and methodology. Since this research is currently underway, any results shown are meant only as evidence of progress.

Emmitt, G. D.↗
NASA NTRS1993

Joint pain and Doppler-detectable bubbles in altitude (Hypobaric) decompression

The observation that altitude decompression sickness (DCS) is associated with pain in the lower extremities is not new, although it is not a consistent finding. DCS in divers is generally in the upper body, an effect often attributed to non-loading of the body while immersed. In caisson workers, DCS is reported more in the lower extremities. Surprisingly, many researchers do not mention the location of DCS joint pain, apparently considering it to be random. This is not the case for the tissue ratios encountered in studying decompression associated with simulated EVA. In NASA/JSC tests, altitude DCS generally presented first in either the ankle, knee, or hip (83 percent = 73/88). There was a definite statistical relation between the maximum Spencer precordial Doppler Grade and the incidence of DCS in the extremity, although this is not meant to imply a casual relation between circulating gas bubbles and joint pain. The risk of DCS with Grade 4 was considerably higher than that of Grades 0 to 3. The DCS risk was independent of the 'tissue ratio.' There was a predominance of lower extremity DCS even when exercise was performed with the upper body. The reason for these locations we hypothesize to be attributed to the formation of tissue gas micronuclei from kinetic and tensile forces (stress-assisted nucleation) and are the result of the individuals ambulating in a 1g environment. Additionally, since these showers of Doppler bubbles can persist for hours, it is difficult to imagine that they are emanating solely from tendons and ligaments, the supposed site of joint pain. This follows from Henry's law linking the volume of joint tissue (the solvent) and the solubility coefficient of inert gas; there is volumetrically insufficient connective tissue to produce the prolonged release of gas bubbles. If gas bubbles are spawned and released from connective tissue, their volume is increased by those from muscle tissue. Therefore, the nexus between Doppler-detectable gas bubbles and joint-pain decompression sickness is essentially a statistical, rather than a direct, one.

Powell, Michael R.↗
NASA NTRS1994

An atlas of Doppler emission-line tomography of cataclysmic variable stars

Doppler emission-line tomography is a technique similar to medical tomography. In this atlas the emission-line profiles of cataclysmic variable stars, seen at different orbital phases, are transformed into velocity space images. This transformation makes many of the complex line profile changes easier to interpret. The emission contributions of the disk and the s-wave are clearly separated in these images, and any emission from the stream and the secondary star can often be identified. In this atlas, Doppler tomograms of Hbeta, He I lambda 4471, and He II lambda 4686 emission lines of 18 cataclysmic variable stars are presented. The Doppler images provide insights into the individual systems and a better technique for measuring and radial velocity amplitude of the white dwarf.

Kaitchuck, Ronald H.↗
NASA NTRS1994

Discovery of the double Doppler-shifted emission-line systems in the X-ray spectrum of SS 433

We have used the CCD X-ray spectrometers on ASCA and resolved the X-ray emission line from the jet of SS 433 both into Doppler-shifted components with two distinct velocities, and into emission from different ionization states of iron, i.e., Fe XXV and Fe XXVI. This is the first direct detection of the two Doppler shifted beams in the X-ray spectra of SS 433 and allows the radial velocity of the jet along the line of sight to be determined with an accuracy comparable to the optical spectroscopy. We also found pairs of emission lines from other atomic species, such as ionized silicon and sulfur, with the Doppler shifts consistent with each other. This confirms the origin of the X-ray emission in the high temperature plasma in the jets.

Kotani, Taro↗
NASA NTRS1994

Doppler radar signatures of developing thunderstorms and their potential to indicate the onset of cloud-to-ground lightning

The capability of Doppler weather radars to short-term forecast the initiation of thunderstorms and the onset of cloud-to-ground (CG) lightning is examined. Doppler weather radar data from 28 thunderstorms were analyzed from August 1990 in the central Florida environment. These radar echoes were associated with CG lightning strike locations from the National Lightning Detection Network and two lightning detection systems operated by the U.S. Air Force in the vicinity of Kennedy Space Center. From a time history of these radar echoes it was found that a 10-dBZ echo, first detected near the freezing level, may be the first definitive echo of a future thunderstorm. This thunderstorm initiation signature is often accompanied by low-altitude convergence and divergence at the top of the radar echo. The observed lead times between this thunderstorm initiation signature and the first detected CG lightning strike ranged from 5 to 45 min with a median lead time of 15 min. All lightning-producing radar echoes were detected using the thunderstorm initiation signature; however, some echoes exceeded the 10-dBZ threshold and did not produce andy CG lightning. The charecteristics of the WSR-88D and Terminal Doppler Weather Radar systems are evaluated for their capability to detect the thunderstorm initiation signature in central Florida with sufficient temporal and spatial resolution.

Hondl, Kurt D.↗
NASA NTRS1994

Mass flux in the ecliptic plane and near the Sun deduced from Doppler scintillation

During the late declining phase of the solar cycle, the tilt of the solar magnetic dipole with respect to the Sun's rotation axis leads to large-scale organization of the solar wind, such that alternating regions of high- and low-speed solar wind are observed in the ecliptic plane. In this paper, we use Doppler scintillation measurements to investigate mass flux of these two types of solar wind in the ecliptic plane and inside 0.3 AU, where in situ measurements have not been possible. To the extent that Doppler scintillation reflects mass flux, we find that mass flux in high-speed streams: (1) is lower (by a factor of approximately 2.2) than the mass flux of the average solar wind in the heliocentric distance range of 0.3-0.5 AU; (2) is lower still (by as much as a factor of about 4) than the mass flux of the slow solar wind associated with the streamer belt; and (3) appears to grow with heliocentric distance. These Doppler scintillation results are consistent with the equator to pole decrease in mass flux observed in earlier spectral broadening measurements, and with trends and differences between high- and low-speed solar wind observed by in situ measurements in the range of 0.3-0.1 AU. The mass flux results suggest that the solar wind flow in high-speed streams is convergent towards the ecliptic near the Sun, becoming less convergent and approaching radial with increasing heliocentric distance beyond 0.3 AU. The variability of mass flux observed within equatorial and polar high-speed streams close to the Sun is strikingly low. This low variability implies that, as Ulysses currently ascends to higher latitudes and spends more time in the south polar high-speed stream after crossing the heliocentric current sheet, it can expect to observe a marked decrease in variations of both mass flux and solar wind speed, a trend that appears to have started already.

Woo, Richard↗
NASA NTRS1995

Optimal Estimation Of Range And Doppler Centroid In ScanSAR

Algorithm processes synthetic-aperture-radar (SAR) returns into optimal estimates of ranges, pointing angles, and Doppler centroids. Derives radar-beam-pointing angles partly from SAR data. Exploits information available in overlapping regions between adjacent image bursts or, equivalently, between adjacent radar beams corresponding to adjacent image bursts. Range or Doppler centroid obtained from estimate of range or Doppler value at which intensity in one image burst equals that in adjacent burst.

Jin, Michael Y.↗
NASA NTRS1998

Airborne/Space-Based Doppler Lidar Wind Sounders Sampling the PBL and Other Regions of Significant Beta and U Inhomogeneities

This final report covers the period from April 1994 through March 1998. The proposed research was organized under four main tasks. Those tasks were: (1) Investigate the vertical and horizontal velocity structures within and adjacent to thin and subvisual cirrus; (2) Investigate the lowest 1 km of the PBL and develop algorithms for processing pulsed Doppler lidar data obtained from single shots into regions of significant inhomogeneities in Beta and U; (3) Participate in OSSEs including those designed to establish shot density requirements for meso-gamma scale phenomena with quasi-persistent locations (e.g., jets, leewaves, tropical storms); and (4) Participate in the planning and execution of an airborne mission to measure winds with a pulsed CO2 Doppler lidar. Over the four year period of this research contract, work on all four tasks has yielded significant results which have led to 38 professional presentations (conferences and publications) and have been folded into the science justification for an approved NASA space mission, SPARCLE (SPAce Readiness Coherent Lidar Experiment), in 2001. Also this research has, through Task 4, led to a funded proposal to work directly on a NASA field campaign, CAMEX III, in which an airborne Doppler wind lidar will be used to investigate the cloud-free circulations near tropical storms. Monthly progress reports required under this contract are on file. This final report will highlight major accomplishments, including some that were not foreseen in the original proposal. The presentation of this final report includes this written document as well as material that is better presented via the internet (web pages). There is heavy reference to appended papers and documents. Thus, the main body of the report will serve to summarize the key efforts and findings.

Emmitt, Dave↗
NASA NTRS1999

Modeling the Performance of Direct-Detection Doppler Lidar Systems in Real Atmospheres

Previous modeling of the performance of spaceborne direct-detection Doppler lidar systems has assumed extremely idealized atmospheric models. Here we develop a technique for modeling the performance of these systems in a more realistic atmosphere, based on actual airborne lidar observations. The resulting atmospheric model contains cloud and aerosol variability that is absent in other simulations of spaceborne Doppler lidar instruments. To produce a realistic simulation of daytime performance, we include solar radiance values that are based on actual measurements and are allowed to vary as the viewing scene changes. Simulations are performed for two types of direct-detection Doppler lidar systems: the double-edge and the multi-channel techniques. Both systems were optimized to measure winds from Rayleigh backscatter at 355 nm. Simulations show that the measurement uncertainty during daytime is degraded by only about 10-20% compared to nighttime performance, provided a proper solar filter is included in the instrument design.

McGill, Matthew J.↗
NASA NTRS1998

High Speed A/D DSP Interface for Carrier Doppler Tracking

As on-board satellite systems continue to increase in ability to perform self diagnostic checks, it will become more important for satellites to initiate ground communications contact. Currently, the NASA Space Network requires users to pre-arranged times for satellite communications links through the Tracking and Data Relay Satellite (TDRS). One of the challenges in implementing an on-demand access protocol into the Space Network, is the fact that a low Earth orbiting (LEO) satellite's communications will be subject to a doppler shift which is outside the capability of the NASA ground station to lock onto. In a prearranged system, the satellite's doppler is known a priori, and the ground station is able to lock onto the satellite's signal. This paper describes the development of a high speed analog to digital interface into a Digital Signal Processor (DSP). This system will be used for identifying the doppler shift of a LEO satellite through the Space Network, and aiding the ground station equipment in locking onto the signal. Although this interface is specific to one application, it can be used as a basis for interfacing other devices with a DSP.

Baggett, Timothy↗
NASA NTRS1999

Direct Detection Doppler Lidar for Spaceborne Wind Measurement

Aerosol and molecular based versions of the double-edge technique can be used for direct detection Doppler lidar spaceborne wind measurement. The edge technique utilizes the edge of a high spectral resolution filter for high accuracy wind measurement using direct detection lidar. The signal is split between an edge filter channel and a broadband energy monitor channel. The energy monitor channel is used for signal normalization. The edge measurement is made as a differential frequency measurement between the outgoing laser signal and the atmospheric backscattered return for each pulse. As a result the measurement is insensitive to laser and edge filter frequency jitter and drift at a level less than a few parts in 10(exp 10). We have developed double edge versions of the edge technique for aerosol and molecular-based lidar measurement of the wind. Aerosol-based wind measurements have been made at Goddard Space Flight Center and molecular-based wind measurements at the University of Geneva. We have demonstrated atmospheric measurements using these techniques for altitudes from 1 to more than 10 km. Measurement accuracies of better than 1.25 m/s have been obtained with integration times from 5 to 30 seconds. The measurements can be scaled to space and agree, within a factor of two, with satellite-based simulations of performance based on Poisson statistics. The theory of the double edge aerosol technique is described by a generalized formulation which substantially extends the capabilities of the edge technique. It uses two edges with opposite slopes located about the laser frequency at approximately the half-width of each edge filter. This doubles the signal change for a given Doppler shift and yields a factor of 1.6 improvement in the measurement accuracy compared to the single edge technique. The use of two high resolution edge filters substantially reduces the effects of Rayleigh scattering on the measurement, as much as order of magnitude, and allows the signal to noise ratio to be substantially improved in areas of low aerosol backscatter. We describe a method that allows the Rayleigh and aerosol components of the signal to be independently determined using the two edge channels and an energy monitor channel. The effects of Rayleigh scattering may then subtracted from the measurement and we show that the correction process does not significantly increase the measurement noise for Rayleigh to aerosol ratios up to 10. We show that for small Doppler shifts a measurement accuracy of 0.4 m/s can be obtained for 5000 detected photon, 1.2 m/s for 1000 detected photons, and 3.7 m/s for 50 detected photons for a Rayleigh to aerosol ratio of 5. Methods for increasing the dynamic range of the aerosol-based system to more than +/- 100 m/s are given.

Korb, C. Laurence↗
NASA NTRS1999

Measuring Tropospheric Winds from Space Using a Coherent Doppler Lidar Technique

The global measurement of tropospheric wind profiles has been cited by the operational meteorological community as the most important missing element in the present and planned observing system. The most practical and economical method for obtaining this measurement is from low earth orbit, utilizing a Doppler lidar (laser radar) technique. Specifically, this paper will describe the coherent Doppler wind lidar (CDWL) technique, the design and progress of a current space flight project to fly such a system on the Space Shuttle, and plans for future flights of similar instruments. The SPARCLE (SPAce Readiness Coherent Lidar Experiment) is a Shuttle-based instrument whose flight is targeted for March, 2001. The objectives of SPARCLE are three-fold: Confirm that the coherent Doppler lidar technique can measure line-of-sight winds to within 1-2 m/s accuracy; Collect data to permit validation and improvement of instrument performance models to enable better design of future missions; and Collect wind and backscatter data for future mission optimization and for atmospheric studies. These objectives reflect the nature of the experiment and its program sponsor, NASA's New Millennium Program. The experiment is a technology validation mission whose primary purpose is to provide a space flight validation of this particular technology. (It should be noted that the CDWL technique has successfully been implemented from ground-based and aircraft-based platforms for a number of years.) Since the conduct of the SPARCLE mission is tied to future decisions on the choice of technology for free-flying, operational missions, the collection of data is intrinsically tied to the validation and improvement of instrument performance models that predict the sensitivity and accuracy of any particular present or future instrument system. The challenges unique to space flight for an instrument such as SPARCLE and follow-ons include: Obtaining the required lidar sensitivity from the long distance of orbit height to the lower atmosphere; Maintaining optical alignments after launch to orbit, and during operations in "microgravity"; Obtaining pointing knowledge of sufficient accuracy to remove the speed of the spacecraft (and the rotating Earth) from the measurements; Providing sufficient power (not a problem on the Shuttle) and cooling to the instrument. The paper will describe the status and challenges of the SPARCLE project, the value of obtaining wind data from orbit, and will present a roadmap to future instruments for scientific research and operational meteorology.

Miller, Timothy L.↗