Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “PHASE ERROR”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Distortion Representation of Forecast Errors for Model Skill Assessment and Objective Analysis

We proposed a novel characterization of errors for numerical weather predictions. In its simplest form we decompose the error into a part attributable to phase errors and a remainder. The phase error is represented in the same fashion as a velocity field and is required to vary slowly and smoothly with position. A general distortion representation allows for the displacement and amplification or bias correction of forecast anomalies. Characterizing and decomposing forecast error in this way has two important applications, which we term the assessment application and the objective analysis application. For the assessment application, our approach results in new objective measures of forecast skill which are more in line with subjective measures of forecast skill and which are useful in validating models and diagnosing their shortcomings. With regard to the objective analysis application, meteorological analysis schemes balance forecast error and observational error to obtain an optimal analysis. Presently, representations of the error covariance matrix used to measure the forecast error are severely limited. For the objective analysis application our approach will improve analyses by providing a more realistic measure of the forecast error. We expect, a priori, that our approach should greatly improve the utility of remotely sensed data which have relatively high horizontal resolution, but which are indirectly related to the conventional atmospheric variables. In this project, we are initially focusing on the assessment application, restricted to a realistic but univariate 2-dimensional situation. Specifically, we study the forecast errors of the sea level pressure (SLP) and 500 hPa geopotential height fields for forecasts of the short and medium range. Since the forecasts are generated by the GEOS (Goddard Earth Observing System) data assimilation system with and without ERS 1 scatterometer data, these preliminary studies serve several purposes. They (1) provide a testbed for the use of the distortion representation of forecast errors, (2) act as one means of validating the GEOS data assimilation system and (3) help to describe the impact of the ERS 1 scatterometer data.

Hoffman, Ross N.↗
NASA NTRS1976

Statistics of the radiated field of a space-to-earth microwave power transfer system

Statistics such as average power density pattern, variance of the power density pattern and variance of the beam pointing error are related to hardware parameters such as transmitter rms phase error and rms amplitude error. Also a limitation on spectral width of the phase reference for phase control was established. A 1 km diameter transmitter appears feasible provided the total rms insertion phase errors of the phase control modules does not exceed 10 deg, amplitude errors do not exceed 10% rms, and the phase reference spectral width does not exceed approximately 3 kHz. With these conditions the expected radiation pattern is virtually the same as the error free pattern, and the rms beam pointing error would be insignificant (approximately 10 meters).

Stevens, G. H.↗
NASA NTRS1986

DSN microwave antenna holography. Part 2: Data processing and display of high-resolution effective maps

The results of a recently completed computer graphic package for the process and display of holographically recorded data into effective aperture maps are presented. The term effective maps (labelled provisional on the holograms) signifies that the maps include contributions of surface mechanical errors as well as other electromagnetic factors (phase error due to feed/subreflector misalignment, linear phase error contribution due to pointing errors, subreflector flange diffraction effects, and strut diffraction shadows). While these maps do not show the true mechanical surface errors, they nevertheless show the equivalent errors, which are effective in determining overall antenna performance. Final steps to remove electromagnetic pointing and misalignment factors are now in progress. The processing and display of high-resolution effective maps of a 64m antenna (DSS 63) are presented.

Rochblatt, D. J.↗
NASA NTRS1973

Nonlinear analysis of phased-locked loops with rapidly varying phase

The performance of command and telemetry systems, useful in deep-space communications, is frequently affected by the radio-frequency phase error which is introduced at the point of reception by means of the carrier tracking loop. In low data rate communications, this phase error may vary rapidly over the duration of the signaling interval. In this paper such phase variation is characterized by a sinusoidal input phase, K sin (omega sub o t+, pi/6), which models a typical phase variation in communication over turbulent media. Conditions for synchronization stability and the acquisition behavior are examined by detailed computer study of the phase-plane trajectories for the second and third-order loops with perfect integrator. It is demonstrated that for the phase variation considered the third-order loop has no real advantage over the second-order loop. Finally, it is shown that nonzero initial conditions may result in large steady-state phase error.

Chen, C.↗
NASA NTRS1986

A method to dramatically improve subcarrier tracking

A method is presented for achieving a dramatic improvement in phase tracking of square wave subcarriers or other square waves. The method is to set the amplitude of the phase quadrature reference signal to zero except near the zero crossings of the input signal. Without changing the loop bandwidth, the variance of the phase error can be reduced to approximately W sigma(sub 0)(2), were sigma (sub 0)(2) is the phase error variance without windowing, and W is the fraction of cycle in which the reference signal has a nonzero value. Simulation results confirm the analysis and establish minimum W versus signal-to-noise ratio. Typically, the window can be made so narrow as to achieve a phase error variance of 1.5 sigma(sub 0)(4).

Hurd, W. J.↗
NASA NTRS1988

A method to dramatically improve subcarrier tracking

A method is presented for achieving a dramatic improvement in phase tracking of square wave subcarriers or other square waves. The method is to set the amplitude of the phase quadrature reference signal to zero except near the zero crossings of the input signal. Without changing the loop bandwidth, the variance of the phase error can be reduced to approximately W sigma(sub 0)(2), where sigma (sub 0)(2) is the phase error variance without windowing, and W is the fraction of cycle in which the reference signal has a nonzero value. Simulation results confirm the analysis and establish minimum W versus signal-to-noise ratio. Typically, the window can be made so narrow as to achieve a phase error variance of 1.5 sigma(sub 0)(4).

Hurd, William J.↗
NASA NTRS2010

Atmospheric Compensation for Uplink Arrays via Radiometry

Uplink arrays for communications applications are gaining increased visibility within the NASA and military community due to the enhanced flexibility and reliability they provide. When compared with the conventional large, single aperture antennas currently comprising the Deep Space Network (DSN), for example, smaller aperture antenna arrays have the benefits of providing fault tolerance (reduced single-point failure), reduced maintenance cost, and enhanced capabilities such as electronic beam-steering and multi-beam operation. However, signal combining of antenna array elements spaced many wavelengths apart becomes problematic due to the inherent instability of earth's turbulent atmosphere, particularly at the frequencies of interest to the DSN (i.e., Ka-band). Degradation in the power combining of the individual elements comprising the array arises due to uncorrelated phase errors introduced as the signals propagate through the troposphere. It is well known that the fundamental source of this error is due to the inhomogeneous distribution of water vapor in the atmosphere [1]. Several techniques have been proposed to circumvent this issue, including the use of phase calibration towers and a moon bounce to generate a feedback loop which would provide a means of intermittent calibration of the system phase errors (thermal drifts, atmosphere) [2,3]. However, these techniques require repositioning of the antenna elements to perform this operation which ultimately results in reduced system availability. And, though they are sufficient for compensating for slow varying phase drifts, they are insufficient to compensate for faster varying phase errors, such as those introduced by the atmosphere. In this paper, preliminary radiometry and interferometry measurements collected by the NASA Glenn Research Center are analyzed and indicate that the use of optimized water vapor radiometers as a feedback system in a communications platform could provide the necessary atmospheric compensation technique to enhance the beamforming of uplink arrays.

Nessel, James A.↗
NASA NTRS1994

An analysis of carrier phase jitter in an MPSK receiver utilizing map estimation

The use of 8 and 16 PSK TCM to support satellite communications in an effort to achieve more bandwidth efficiency in a power-limited channel has been proposed. This project addresses the problem of carrier phase jitter in an M-PSK receiver utilizing the high SNR approximation to the maximum aposteriori estimation of carrier phase. In particular, numerical solutions to the 8 and 16 PSK self-noise and phase detector gain in the carrier tracking loop are presented. The effect of changing SNR on the loop noise bandwidth is also discussed. These data are then used to compute variance of phase error as a function of SNR. Simulation and hardware data are used to verify these calculations. The results show that there is a threshold in the variance of phase error versus SNR curves that is a strong function of SNR and a weak function of loop bandwidth. The M-PSK variance thresholds occur at SNR's in the range of practical interest for the use of 8 and 16-PSK TCM. This suggests that phase error variance is an important consideration in the design of these systems.

Osborne, William P.↗
NASA NTRS1989

Slot clock recovery in optical PPM communication systems with avalanche photodiode photodetectors

Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The rms phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (wavelength = 834 nm) and a silicon avalanche photodiode photodetector. The system used Q = 4 PPM signaling and operated at a source data rate of 25 Mbits/s. The mathematical model developed to compute the rms phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10 to the -6th at a received optical signal energy of 55 detected photons per information bit.

Davidson, Frederic M.↗
NASA NTRS1989

Real-Time Optimization Of Receiver Bandwidth

Estimates of signal and noise spectra enhance reception of weak signals. Carrier-tracking phase-locked loop represented by linear mathematical model at small rms phase errors. Loop continuously generates estimates of received phase. Bandwidth (in effect, scale of complex-frequency variable p) optimized to minimize rms phase error. Minimum signals tracked 5 to 15 dB below those tracked by current receivers. Improvement accomplished by use of bandwidths of 0.1 to 1.0 Hz, in contrast with 3-Hz bandwidth in current use. Principle of real-time optimization of bandwidth adapted to other situations to enhance reception of weak signals otherwise "buried" in noise.

Vilnrotter, V. A.↗
NASA NTRS1983

The effects of digitizing rate and phase distortion errors on the shock response spectrum

Some of the methods used for acquisition and digitization of high-frequency transients in the analysis of pyrotechnic events, such as explosive bolts for spacecraft separation, are discussed with respect to the reduction of errors in the computed shock response spectrum. Equations are given for maximum error as a function of the sampling rate, phase distortion, and slew rate, and the effects of the characteristics of the filter used are analyzed. A filter is noted to exhibit good passband amplitude, phase response, and response to a step function is a compromise between the flat passband of the elliptic filter and the phase response of the Bessel filter; it is suggested that it be used with a sampling rate of 10f (5 percent).

Wise, J. H.↗
NASA NTRS1989

Unsymmetrical QPSK demodulator

The present invention is a demodulator for demodulating an unsymmetrical QPSK signal (1), i.e., one in which the amplitudes of the I and Q channels are different, and the I and Q channels have been modulated by signals having differnt bit rates. Analog versions of the I and Q modulating baseband signals are first extracted from the carrier. Each baseband signal is then subjected to a bit-rate-matched low pass filter (23, 13). The channel having the higher bit rate is subjected to a delay (32) to time-align the two baseband signals. The time-aligned filtered signals are then amplified by amplifiers (22, 12) having unequal gains that are preselected to substantially equalize the amplifier (22, 12) outputs. The amplified time-aligned filtered signals are then subjected to a phase error (E) generating means (15, 25, 16, 26, 34). The phase error (E) is passed through a loop integrator (38) and a VCO (30) and back to the baseband signal extracting means to complete a phase lock loop. Frequency searching means (40), frequency lock detect means (42, 44), and AGC control ( 46, 8) are optionally present.

Guyer, Edward↗
NASA NTRS1993

On the Effects of a Spacecraft Subcarrier Unbalanced Modulator

This paper presents mathematical models with associated analysis of the deleterious effects which a spacecraft's subcarrier unbalanced modulator has on the performance of a phase-modulated residual carrier communications link. The undesired spectral components produced by the phase and amplitude imbalances in the subcarrier modulator can cause (1) potential interference to the carrier tracking and (2) degradation in the telemetry bit signal-to-noise ratio (SNR). A suitable model for the unbalanced modulator is developed and the threshold levels of undesired components that fall into the carrier tracking loop are determined. The distribution of the carrier phase error caused by the additive White Gaussian noise (AWGN) and undesired component at the residual RF carrier is derived for the limiting cases. Further, this paper analyses the telemetry bit signal-to-noise ratio degradations due to undesirable spectral components as well as the carrier tracking phase error induced by phase and amplitude imbalances. Numerical results which indicate the sensitivity of the carrier tracking loop and the telemetry symbol-error rate (SER) to various parameters of the models are also provided as a tool in the design of the subcarrier balanced modulator.

Nguyen, Tien Manh↗
NASA NTRS1981

Experimental results on tracking performance of the MTDD Costas loop with UQPSK signal

The carrier tracking performance of the breadboard Costas Loop of the Multimegabit Telemetry Demodulator Detector System was tested with unbalanced quadriphase shift keying (UQPSK) signals. An S-band UQPSK modulator was built for the tests. The experimental results are very close to the theoretical rms phase error calculations. The test and analysis show that the rms phase error for UQPSK signals is less than 5 deg at the design point if the I-to-Q channel power ratio is larger than 6 dB.

Park, Y. H.↗
NASA NTRS1986

Phase noise from aircraft motion: Compensation and effect on synthetic aperture radar images

Image degradation of airborne SAR imagery caused by phase errors introduced in the received signal by aircraft motion is discussed. Mechanical motion has a small bandwidth and does not affect the range signal, where the total echo time is typically 60 microsec. However, since the aperture length can be several seconds, the synthesized azimuth signal can have significant errors of which phase noise is the most important. An inertial navigation system can be used to compensate for these errors when processing the images. Calculations to evaluate how much improvement results from compensation are outlined.

Gabriel, Andrew K.↗
NASA NTRS1974

Analysis of multiple pulse NMR in solids. II

A systematic method, an extension of the average Hamiltonian formalism, is presented for calculating the effects of pulse errors and imperfections in the multiple pulse nuclear magnetic resonance experiments. Application of this method to account for effects of pulse nonidealities such as phase errors, phase transient effects, pulse size errors, and rf inhomogeneity is found to agree with experimental observation, and the results furnish a basis for understanding the complex couplings between the pulse errors and other interactions such as the dipolar and the chemical shift Hamiltonians.

Rhim, W.-K.↗
NASA NTRS1975

MLS airborne antenna research

The geometrical theory of diffraction was used to analyze the elevation plane pattern of on-aircraft antennas. The radiation patterns for basic elements (infinitesimal dipole, circumferential and axial slot) mounted on fuselage of various aircrafts with or without radome included were calculated and compared well with experimental results. Error phase plots were also presented. The effects of radiation patterns and error phase plots on the polarization selection for the MLS airborne antenna are discussed.

Yu, C. L.↗