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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Is the signal from Cyg X-3, as recorded in some underground experiments, real?

Most of the excitement concerning the underground detection of signals from Cyg X-3 comes not from astrophysical grounds, but from the contradiction with surface experimental data. Believing in the Cyg X-3 signal underground and also that the main processes of muon production are well knownm the conclusion was drawn that the signal in EAS Cherenkov or counter experiments could be remarkably high, which is not the case. Possible solutions to this discrepancy are discussed.

Chudakov, A. E.↗
NASA NTRS1986

Measurements of Siple transmitter signals on the DE 1 satellite - Wave normal direction and antenna effective length

A new experimental technique is developed to simultaneously measure the wave propagation direction and the effective length of a small (L value much smaller than wavelength) electric dipole antenna on a spin-stabilized satellite in the magnetosphere. The technique relies on the near simultaneous measurement of single components of the electric and magnetic fields of a coherent VLF signal injected into the medium from a ground-based source. The spin fading characteristics of the signal received by the electric dipole and the magnetic loop antenna permit the measurement of the wave normal direction assuming whistler-mode propagation. In-situ and remote measurements of the local cold plasma density are used to determine the refractive index. The wave electric field is then inferred from the wave magnetic field as measured on the loop antenna, the refractive index and the direction of propagation. Comparing this electric field with the measured voltage across the dipole antenna leads to the determination of the effective length of the receiving electric dipole. The technique is applied to data from the Dynamics Explorer 1 satellite observations of whistler mode signals injected into the magnetosphere from the Siple, Antarctica, VLF transmitter. In one case, with the measured background cold plasma density being 15 el/cu cm, the effective length of the 200 m-long electric dipole antenna is found to be 222 + or - 56 m, i.e., about twice the conventional value.

Sonwalker, V. S.↗
NASA NTRS1985

A new type of VLF emission triggered at low altitude in the subauroral region by Siple Station VLF transmitter signals

The present paper has the objective to report the existence of a new phenomenon in which coherent VLF signals from the Siple Station VLF transmitter (Antarctica, 76 deg S, 84 deg W) are observed to trigger a new type of VLF emission, taking into account the propagation of these signals upward through the ionosphere and low-altitude magnetosphere to the ISIS 2 satellite at 1400 km altitude. The existence of the impulsive emission (IE) effect lends credence to the idea that coherent VLF signals can act as catalysts to trigger natural plasma instabilities in the subauroral ionosphere and lower magnetosphere. Controlled studies of these instabilities may, therefore, be possible.

Bell, T. F.↗
NASA NTRS1985

Generation, modulation and reception of orthogonally polarized signals

The radiating electromagnetic field of a dipole antenna is considered. By allowing the dipole to rotate about its midpoint, one can construct an entire set of signal polarizations wherein distinct members of the set are mutually orthogonal. It is shown how these signals can be modulated and demodulated to convey information. These ideas are then generalized, and both balanced, as well as unbalanced quadrapole modulations are defined. Methods of receiver synchronization to such signals are described, as well as their potential application to multiple access and anti-jam communications.

Lesh, J. R.↗
NASA NTRS1986

Coherent lidar signal fluctuation reduction by means of frequency diversity technique

The atmospheric return measured by a coherent lidar is typically characterized by rapid and deep fluctuations in signal strength. These fluctuations result from the interference of the fields backscattered to the lidar from randomly located aerosol particles which move relative to the lidar pulse. In many applications, it is necessary to determine the average value of the lidar signal intensity at some range. A new method utilizes frequency diversity initially suggested by Goldstein and subsequently studied in the microwave radar domain by others. It is expected that the application of the frequency diversity method in the coherent lidar domain will eventually provide greater efficiency and speed in the return signal averaging needed to obtain accurate intensity estimates. The frequency diversity method recognizes that the transmitted lidar pulse is very long compared to a wavelength and consequently a given phase, theta sub i, is repeated many times within the pulse. In order to test this concept, a fairly simple laboratory experiment was designed which simulates scattering of a lidar pulse from atmospheric aerosol. The testing of the frequency diversity method is discussed.

Schotland, R. M.↗
NASA NTRS1986

Antenna arraying of Voyager telemetry signals by symbol stream combining

Telemetry signals received from the Voyager 2 spacecraft at Deep Space Stations at Parkes and Canberra, Australia, on February 6, 1986, were combined by the method of symbol stream combining. This second demonstration of symbol stream combining followed the International Cometary Explorer (ICE) demonstration at Giacobini-Zinner encounter in September 1985. The Voyager demonstration was at a symbol rate of 43.2 ksymb/s, compared to 2 ksymb/s for ICE. Recording, playback, and combining at this higher rate were demonstrated. The average symbol signal-to-noise ratio (SNR) of the combined data was 2.84 dB, or 0.23 dB less than the sum of the SNRs of the two imput symbol streams. This 0.23 loss from ideal combining was due to use of 4-bit quantization of the input symbol stream and imperfect scaling. A practical implementation with 8-bit quantization could achieve combining losses of under 0.05 dB over a wide dynamic range of input signal levels.

Hurd, W. J.↗
NASA NTRS1986

SMI adaptive antenna arrays for weak interfering signals

The performance of adaptive antenna arrays in the presence of weak interfering signals (below thermal noise) is studied. It is shown that a conventional adaptive antenna array sample matrix inversion (SMI) algorithm is unable to suppress such interfering signals. To overcome this problem, the SMI algorithm is modified. In the modified algorithm, the covariance matrix is redefined such that the effect of thermal noise on the weights of adaptive arrays is reduced. Thus, the weights are dictated by relatively weak signals. It is shown that the modified algorithm provides the desired interference protection.

Gupta, Inder J.↗
NASA NTRS1986

Frequency domain laser velocimeter signal processor

A new laser velocimeter signal processor is described which uses advanced digital circuitry to configure itself to maximize measurement accuracy based on the characteristics of the input signals. This processor maintains measurement independence from burst to burst and yields measurement accuracies better than 0.2 percent for signal frequencies up to 100 MHz.

Meyers, James F.↗
NASA NTRS1988

Synchronous Photodiode-Signal Sampler

Synchronous sampling circuit increases signal-to-noise ratio of measurements of chopped signal of known phase and frequency in presence of low-frequency or dc background noise. Used with linear array of photoelectric sensors for locating edge of metal plate. Multiplexing circuit cycles through 16 light-emitting-diode/photodiode pairs, under computer control. Synchronized with multiplexer so edge detector makes one background-subtracted signal measurement per emitter/detector pair in turn.

Primus, Howard K.↗
NASA NTRS1988

Video Analog Signal Divider

Video analog signal divider produces black-and-white composite video signal based on color ratio. Device inexpensive, uses signal from standard red/green/blue camera as input. Used to produce quantitative thermal images of two-color phosphor coatings.

Buck, Gregory M.↗
NASA NTRS1987

Intelligent signal analysis and recognition

Progress in the research and development of self-organizing database system that can support the identification and characterization of signals in an RF environment is described. As the radio frequency spectrum becomes more crowded, there are a number of situations that require a characterization of the RF environment. This database system is designed to be practical in applications where communications and other instruments encounter a time varying and complex RF environment. The primary application of this system is the guidance and control of NASA's SETI Microwave Observing Project. Other possible applications include selection of telemety bands for communication with spacecraft, and the scheduling of antenna for radio astronomy are two examples where characterization of the RF environment is required. In these applications, the RF environment is constantly changing, and even experienced operators cannot quickly identify the multitude of signals that can be encountered. Some of these signals are repetitive, others appear to occur sporadically.

Levinson, Robert↗
NASA NTRS1988

Planetographic clustering of low-altitude impulsive electric signals in the night ionosphere of Venus

This paper presents maps of the locations of electromagnetic signals, presumably caused by lightning, which were observed at 730 Hz during the first three nighttime observing seasons of Pioneer Venus Orbiter when the spacecraft reached altitudes low enough to detect the signals. The maps show that there are specific 'active regions' of enhanced signal occurrence, and that at least some of these active regions appear to remain fixed in planetographic coordinates from year to year.

Russel, C. T.↗
NASA NTRS1989

Signal Preprocessor For Laser-Fringe Anemometers

Signal-preprocessing unit contains digital and analog circuitry added to existing equipment in laser-fringe anemometer system to filter raw anemometer signal and relieve researcher of some tedious and difficult control functions. Preprocessor operates under control of same digital computer that acquires measurement data. Both automatically and in response to control settings applied manually by researcher, preprocessor helps system to establish conditions that produce signals of highest quality.

Oberle, Lawrence G.↗
NASA NTRS1989

Fast Correction For Doppler In MDPSK Signals

Detector of multiple differential phase-shift-keyed (MDPSK) signals designed to effect combination of open-loop estimation of frequency and differential detection of digital modulation. Improved design expected to decrease probability of bit errors in situations in which carrier frequency uncertain and signals transmitted in bursts too short to allow closed-loop tracking of carrier frequency - for example, in transmission of digitally-coded voice signals in land-mobile/satellite communications at ultra-high frequencies.

Simon, M. K.↗
NASA NTRS1989

Digital Signal Processor For GPS Receivers

Three innovative components combined to produce all-digital signal processor with superior characteristics: outstanding accuracy, high-dynamics tracking, versatile integration times, lower loss-of-lock signal strengths, and infrequent cycle slips. Three components are digital chip advancer, digital carrier downconverter and code correlator, and digital tracking processor. All-digital signal processor intended for use in receivers of Global Positioning System (GPS) for geodesy, geodynamics, high-dynamics tracking, and ionospheric calibration.

Thomas, J. B.↗
NASA NTRS1988

Estimating the signal-to-noise ratio of AVIRIS data

To make the best use of narrowband airborne visible/infrared imaging spectrometer (AVIRIS) data, an investigator needs to know the ratio of signal to random variability or noise (signal-to-noise ratio or SNR). The signal is land cover dependent and varies with both wavelength and atmospheric absorption; random noise comprises sensor noise and intrapixel variability (i.e., variability within a pixel). The three existing methods for estimating the SNR are inadequate, since typical laboratory methods inflate while dark current and image methods deflate the SNR. A new procedure is proposed called the geostatistical method. It is based on the removal of periodic noise by notch filtering in the frequency domain and the isolation of sensor noise and intrapixel variability using the semi-variogram. This procedure was applied easily and successfully to five sets of AVIRIS data from the 1987 flying season and could be applied to remotely sensed data from broadband sensors.

Curran, Paul J.↗
NASA NTRS1989

On sampling band-pass signals

Four techniques for uniform sampling of band-bass signals are examined. The in-phase and quadrature components of the band-pass signal are computed in terms of the samples of the original band-pass signal. The relative implementation merits of these techniques are discussed with reference to the Deep Space Network (DSN).

Sadr, R.↗
NASA NTRS1988

An experimental adaptive array to suppress weak interfering signals

An experimental adaptive antenna system to suppress weak interfering signals is described. It is a sidelobe canceller with two auxiliary elements. Modified feedback loops are used to control the array weights. The received signals are simulated in hardware for parameter control. Digital processing is used for algorithm implementation and performance evaluation. The experimental results are presented. They show that interfering signals as much as 10 dB below the thermal noise level in the main channel are suppressed by 20-30 dB. Such a system has potential application in suppressing the interference encountered in direct broadcast satellite communication systems.

Walton, Eric K.↗