SEARCH · Search NASA

Results for “Coherent errors”

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.

SourceResearch connectionFrom yearThrough year

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

At least 307 records · Page 17

All sources NASA NTRS DOE OSTI

NASA NTRS1978

GEOS-3 coherent C-band tracking data reduction and analysis

The availability of a coherent C-Band transponder on an orbiting vehicle has permitted the development of calibration techniques for C-Band radars. Prior coherent tracking was limited to test vehicles in powered flight or skin tracking of orbiting spacecraft. The pulse Doppler range rate data available concurrently and independently with normal range and angle data provides an extremely useful data set for assessing the internal consistency of the radars measurements, particularly when coupled with a trajectory well constrained by orbit dynamics. In order to further assess the precision of the range rate data, a technique was developed to numerically integrate the rate measurements into a pseudo-range data set showing extremely low noise and high sensitivity to systematic radar errors. Evaluation of integrated range data led to improvements in calibration of both the rate and conventional range data. In particular, proper evaluation of the very sensitive integrated ranges required the development of a processing correction for beacon delay variation with signal strength, a significant error in the conventional ranging data.

Krabill, W. B.↗

NASA NTRS2020

Instrument Offset Optimization Technique for Airborne Doppler Wind Lidar Profiling Algorithm

This paper presents the instrument offset optimization technique for the Doppler Aerosol Wind Lidar (DAWN) profiling algorithm at NASA Langley Research Center (LaRC). The 2 um Doppler wind Lidar system at LaRC has a long history of research development and airborne campaigns. The core of the wind Lidar data processing is Airborne Wind Profiling Algorithm for Doppler Wind Lidar (APOLO), which was developed at LaRC. The extraction of wind parameters from the airborne Doppler wind Lidar data is challenging due to many factors such as the speed of aircraft, and the errors in instrument installation and calibration. It is unavoidable to prevent the adverse impact of calibration error and instrument installation offset on the performance of wind parameter estimation in the algorithm. Due to the unsteady environment where the data are collected, even a small offset will result in nonsensical results in the parameter estimation process. A brief introduction of APOLO and the overview of the optimization techniques are presented in this paper.

Wind profile↗

NASA NTRS1990

High-accuracy, high-resolution gravity profiles from 2 years of the Geosat Exact Repeat Mission

Satellite altimeter data from the first 44 repeat cycles (2 years) of the Geosat Exact Repeat Mission (EWRM) were averaged to improve accuracy, resolution and coverage of the marine gravity field. Individual 17-day repeat cycles were first edited and differentiated, resulting in the along-track vertical deflection (i.e., gravity disturbance). To increase the signal-to-noise ratio, 44 of these cycles were then averaged to form a single highly accurate vertical deflection profile. The largest contribution to the vertical deflection error is short-wavelength altimeter noise and longer-wavelength oceanographic variability; the combined noise level is typically 6 microrad. Both types of noise are reduced by averaging many repeat cycles. Over most ocean areas the uncertainty of the average profile is less than 1 microrad which corresponds to 1 mgal of along-track gravity disturbance. However, in areas of seasonal ice coverage, its uncertainty can exceed 5 microrad. To assess the resolution of individual and average Geosat gravity profiles, the cross-spectral analysis technique was applied to repeat profiles. Individual Geosat repeat cycles are coherent (greater than 0.5) for wavelengths greater than about 30 km and become increasingly incoherent at shorter wavelengths.

Sandwell, David T.↗

NASA NTRSDate not supplied

Impact of Near-Angle Scatter on Exo-Earth Coronagraphy

Near-Angle Scatter ( of the host star’s light may limit the ability of a potential Habitable Worlds Observatory (HWO) to detect and characterize an Earth-like planet around a Sun-like star via coronagraphy NAS from each optical surface produces an E-field across the dark hole that is coherent. These E-fields sum and could be as large or larger than the coronagraph mask leakage E-field. NAS E-fields contribute to the dark hole noise floor via both shot noise and heterodyne amplification of the wavefront instability. The amount of NAS is determined entirely by the statistical properties of the optical surface microroughness and the operating wavelength. Surface properties include not only the rms roughness, but also correlation length and the functional form of the distribution itself. We derive an expression that specifies the surface statistics required to achieve a given coronagraph error budget NAS throughput allocation.

H Philip Stahl↗

NASA NTRSDate not supplied

Impact of Near-Angle Scatter on Exo-Earth Coronagraphy

Near-Angle Scatter (NAS) of the host star’s light may limit the ability of a potential Habitable Worlds Observatory (HWO) to detect and characterize an Earth-like planet around a Sun-like star via coronagraphy. NAS from each optical surface produces an E-field across the dark hole that is coherent. These E-fields sum and could be as large or larger than the coronagraph mask leakage E-field. NAS E-fields contribute to the dark hole noise floor via both shot noise and heterodyne amplification of the wavefront instability. The amount of NAS is determined entirely by the statistical properties of the optical surface microroughness and the operating wavelength. Surface properties include not only the rms roughness, but also correlation length and the functional form of the distribution itself. This paper derives an expression that specifies the surface statistics required to achieve a given coronagraph error budget NAS throughput allocation. Analysis does not include scatter from coating columnar structure, edges, contamination, micrometeoroid impacts, or polarization.

H Philip Stahl↗

NASA NTRS1988

Digital carrier demodulation for the DSN Advanced Receiver

The digital in-phase and quadrature (I and Q) carrier demodulation for the Deep Space Network's (DSN) Advanced Receiver is described and analyzed. The system provides coherent demodulation for a variety of modulation formats including Binary Phase Shift Keying (BPSK), BPSK with a carrier residual, Quadrature Phase Shift Keying (QPSK), Offset-QPSK (OQPSK), and Minimum Shift Keying (MSK). The focus is on the theory and hardware design of the halfband filters which are the integral part of the demodulator. The underlying theory of the filters is summarized, a breadboard hardware design is described, and a VLSI implementation is proposed which significantly decreases the hardware. A second important problem analyzed is DC-offset in the demodulator. This is a serious problem which translates into bias error in the residual carrier phase detector. The dynamic range of the complex mixer is analyzed using a probabilistic approach. It is deduced that the resulting static phase error is less than 0.2 deg when the ratio of carrier power to noise power in the demodulator input bandwidth is -72 dB or higher. Thus, the static phase error is negligible at a carrier power to noise spectral density of 0 dB-Hz for a 15 MHz bandwidth demodulator.

Sadr, R.↗

NASA NTRS1997

Geosat Data Assimilation with Application to the Eastern North Atlantic

An attempt is made to determine the three-dimensional ocean circulation from satellite altimeter measurements by assimilating Geosat sea surface height data into an eddy-resolving QuasiGeostrophic (QG) model of the eastern North Atlantic Ocean. Results are tested against independent information from hydrographic field observations and moored current meter data collected during the Geosat ERM. The comparison supports the concept of inferring aspects of the three-dimensional flow field from sea surface height observations by combining altimetric measurements with the dynamics of ocean circulation models. A Holland-type QG model with open boundaries is set up on a 2000 km X 2000 km domain of the eastern North Atlantic between 25 deg. and 45 deg. N, 32 deg. and 8 deg. W. By using a simple nudging technique, about two years of Geosat altimeter data are assimilated into the model every five days as space-time objective analyses on the model grid. The error information resulting from the analysis is used during the assimilation procedure to account for data uncertainties. Results show an intense eddy field, which in the surface layer interacts with a meandering Azores Front. Compared to Geosat, the model leads to smoothed fields that follow the observations. Model simulations are significantly correlated with hydrographic data from March 1988 and June 1989, both close to the surface and in the subsurface. Good agreement is also found between the model velocity fields and moored current meter data in the top two model layers. The agreement is visually weak in the bottom layer, although a coherence analysis reveals an agreement between the model simulation and current meter data over the full water column at periods exceeding 80 days.

Stammer, Detlef↗

NASA NTRS1999

Phase-Shifted Laser Feedback Interferometry

Phase-shifted, laser feedback interferometry is a new diagnostic tool developed at the NASA Lewis Research Center under the Advanced Technology Development (ATD) Program directed by NASA Headquarters Microgravity Research Division. It combines the principles of phase-shifting interferometry (PSI) and laser-feedback interferometry (LFI) to produce an instrument that can quantify both optical path length changes and sample reflectivity variations. In a homogenous medium, the optical path length between two points is the product of the index of refraction and the geometric distance between the two points. LFI differs from other forms of interferometry by using the laser as both the source and the phase detector. In LFI, coherent feedback of the incident light either reflected directly from a surface or reflected after transmission through a region of interest will modulate the output intensity of the laser. The combination of PSI and LFI has produced a robust instrument, based on a low-power helium-neon (HeNe) gas laser, with a high dynamic range that can be used to measure either static or oscillatory changes of the optical path length. Small changes in optical path length are limited by the fraction of a fringe that can be measured; we can measure nonoscillatory changes with a root mean square (rms) error of the wavelength/1000 without averaging.

Ovryn, Benjie↗

NASA NTRSDate not supplied

Using the GEOS 5 Nature Run to Simulate 2053 nm Coherent Doppler Wind Lidar Observations

Wind observations are a critical part of the current global observation system used for Numerical Weather Prediction (NWP). Wind lidars have been cited as precise instruments that can provide 3-dimensional wind measurements. Several studies have conducted observing system experiments (OSEs) with existing lidar observations or observing system simulation experiments (OSSEs) with simulated lidar observations highlighting the benefits of wind lidar measurements to NWP. Previous studies using simulated lidar observations have typically tied aerosol optical properties to functions of relative humidity instead of to aerosol properties. A methodology is presented for simulating wind measurements from a novel 2053 nm lidar using aerosol properties derived using the GEOS-5 Nature Run, along with estimating winds derived from cloud information. Some assumptions regarding aerosol scattering and the distribution of clouds are explored, along with the role of observation weighting, and implications for representativeness error. Results from a preliminary OSSE are presented highlighting the importance of assumptions used to derive data from cloud returns and aerosol scattering. While a longer duration study is required, results show a general reduction in analysis error when lidar measurements are ingested.

Bryan M. Karpowicz↗

NASA NTRSDate not supplied

Using the GEOS 5 Nature Run to Simulate 2053 nm Coherent Doppler Wind Lidar Observations

Bryan M. Karpowicz↗

NASA NTRS1982

Investigation of continuously traversing microphone system for mode measurement

The continuously Traversing Microphone System consists of a data acquisition and processing method for obtaining the modal coefficients of the discrete, coherent acoustic field in a fan inlet duct. The system would be used in fan rigs or full scale engine installations where present measurement methods, because of the excessive number of microphones and long test times required, are not feasible. The purpose of the investigation reported here was to develop a method for defining modal structure by means of a continuously traversing microphone system and to perform an evaluation of the method, based upon analytical studies and computer simulated tests. A variety of system parameters were examined, and the effects of deviations from ideal were explored. Effects of traverse speed, digitizing rate, run time, roundoff error, calibration errors, and random noise background level were determined. For constant fan operating speed, the sensitivity of the method to normal errors and deviations was determined to be acceptable. Good recovery of mode coefficients was attainable. Fluctuating fan speed conditions received special attention, and it was concluded that by employing suitable time delay procedures, satisfactory information on mode coefficients can be obtained under realistic conditions. A plan for further development involving fan rig tests was prepared.

Cicon, D. E.↗

NASA NTRSDate not supplied

Performance of Doppler Lidar Velocity and Range Sensor Operating in Highly Dynamic Environments

The operation of a coherent Doppler lidar, developed by NASA for missions to planetary bodies, is analyzed and its projected performance is described. The lidar transmits three laser beams at different but fixed directions and measures line-of-sight range and velocity along each beam. The three line-of-sight measurements are then combined in order to determine the three components of the vehicle velocity vector and its altitude relative to the ground. Operating from over five kilometers altitude, the NDL provides velocity and range data with a few cm/sec and a few meters precision, respectively, depending on the vehicle dynamics. This paper explains the sources of measurements error and analyzes the impacts of vehicle dynamics on the lidar performance.

Doppler Lidar↗

NASA NTRSDate not supplied

Analysis of Navigation Doppler Lidar performance for Moon and Mars Landing

The operation of a coherent Doppler lidar, developed by NASA for missions to planetary bodies, is analyzed and its projected performance is described. The lidar transmits three laser beams at different but fixed directions and measures line-of-sight range and velocity along each beam. The three line-of-sight measurements are then combined in order to determine the three components of the vehicle velocity vector and its altitude relative to the ground. Operating from over five kilometers altitude, the NDL provides velocity and range data with a few cm/sec and a few meters precision, respectively, depending on the vehicle dynamics. This paper explains the sources of measurement error and analyzes the impact of vehicle dynamics on the lidar performance.

Farzin Amzajerdian↗

NASA NTRS2019

Terrestrial Single-Station Analog for Constraining the Martian Core and Deep Interior: Implications for InSight

We used a terrestrial single-station seismometer to quantify the uncertainty of InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) data for determining Martian core size. To mimic Martian seismicity, we formed a catalog using 917 terrestrial earthquakes, from which we randomly selected events. We stacked ScS amplitudes on modeled arrival times and searched for where ScS produced coherent seismic amplitudes. A core detection was defined by a coherent peak with small offset between predicted and user-selected arrival times. Iterating the detection algorithm with varying signal-to-noise (SNR) ranges and quantity of events determined the selection frequency of each model and quantified core depth uncertainty. Increasing the quantity of events reduced core depth uncertainty while increasing the recovery rate, while increasing event SNR had little effect. Including ScS2 multiples increased the recovery rate and reduced core depth uncertainty when we used low quantities of events. The most-frequent core depths varied by back azimuth, suggesting our method is sensitive to the presence of mantle heterogeneities. When we added 1° in source distance errors, core depth uncertainty increased by up to 11 km and recovery rates decreased by <5%. Altering epicentral distances by 25% added ~35 km of uncertainty and reduced recovery rates to <50% in some cases. From these experiments, we estimate that if InSight can detect five events with high location precision (<10% epicentral distance errors), that there is at least an 88% chance of core depth recovery using ScS alone with uncertainty in core depth approaching 18 km and decreasing as more events are located.

Marusiak, Angela G.↗

NASA NTRS2019

Terrestrial Single-Station Analog for Constraining the Martian Core and Deep Interior: Implications for InSight

We used a terrestrial single-station seismometer to quantify the uncertainty of InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) data for determining Martian core size. To mimic Martian seismicity, we formed a catalog using 917 terrestrial earthquakes, from which we randomly selected events. We stacked ScS amplitudes on modeled arrival times and searched for where ScS produced coherent seismic amplitudes. A core detection was defined by a coherent peak with small offset between predicted and user-selected arrival times. Iterating the detection algorithm with varying signal-to-noise (SNR) ranges and quantity of events determined the selection frequency of each model and quantified core depth uncertainty. Increasing the quantity of events reduced core depth uncertainty while increasing the recovery rate, while increasing event SNR had little effect. Including ScS2 multiples increased the recovery rate and reduced core depth uncertainty when we used low quantities of events. The most-frequent core depths varied by back azimuth, suggesting our method is sensitive to the presence of mantle heterogeneities. When we added 1° in source distance errors, core depth uncertainty increased by up to 11 km and recovery rates decreased by <5%. Altering epicentral distances by 25% added ~35 km of uncertainty and reduced recovery rates to <50% in some cases. From these experiments, we estimate that if InSight can detect five events with high location precision (< 10% epicentral distance errors), that there is at least an 88% chance of core depth recovery using ScS alone with uncertainty in core depth approaching 18 km and decreasing as more events are located.

Angela G Marusiak↗

NASA NTRS1995

High-frequency variations in Earth rotation and the planetary momentum budget

The major focus of the subject contract was on helping to resolve one of the more notable discrepancies still existing in the axial momentum budget of the solid Earth-atmosphere system, namely the disappearance of coherence between length-of-day (l.o.d.) and atmospheric angular momentum (AAM) at periods shorter than about a fortnight. Recognizing the importance of identifying the source of the high-frequency momentum budget anomaly, the scientific community organized two special measurement campaigns (SEARCH '92 and CONT '94) to obtain the best possible determinations of l.o.d. and AAM. An additional goal was to analyze newly developed estimates of the torques that transfer momentum between the atmosphere and its underlying surface to determine whether the ocean might be a reservoir of momentum on short time scales. Discrepancies between AAM and l.o.d. at sub-fortnightly periods have been attributed to either measurement errors in these quantities or the need to incorporate oceanic angular momentum into the planetary budget. Results from the SEARCH '92 and CONT '94 campaigns suggest that when special attention is paid to the quality of the measurements, better agreement between l.o.d. and AAM at high frequencies can be obtained. The mechanism most responsible for the high-frequency changes observed in AAM during these campaigns involves a direct coupling to the solid Earth, i.e, the mountain torque, thereby obviating a significant oceanic role.

Rosen, Richard D.↗

NASA NTRS1996

Diffractive Optical Elements for Lidar Beam Scanning

Wind measurement from space can provide critical data for understanding weather patterns and large-scale storm phenomena. An instrument for providing such measurements is currently under development at NASA's Marshall Space Flight Center. The instrument utilizes a pulsed coherent lidar system operating at a wavelength of 2.06 micrometers in order to achieve decreased weight, size, and cost compared to systems operating at longer wavelengths, and it is being developed to be compatible with the capabilities of small satellites. A key aspect of such an orbital lidar system is that the beam must be conically scanned after it exits the final beam expansion telescope. Previous work indicates that the aperture of the beam expansion telescope should be 50 cm with a scanner half-angle of 300 and a rotation rate of 10 RPM. The critical requirements for the beam scanning element include a 50 cm aperture, an induced wavefront error of less than lambda/10, and high efficiency deflection (i.e., 95+ % of the incident light is deflected). This report is intended to provide a brief overview and discussion of potential technologies for space-borne laser radar (lidar) beam scanning.

Nordin, Gregory P.↗

NASA NTRS1995

Carrier arraying: Revisited

Depending on the basis used for comparison, it is possible to predict different gains when applying carrier arraying to a conventional coherent receiver. In the past, the specific performance comparisons were made between the arrayed and nonarrayed cases assuming a fixed carrier-tracking closed-loop bandwidth. While the notion of loop bandwidth for the nonarrayed loop with a single input is well defined and meaningful, the comparable notion for an arrayed loop with multiple inputs is not uniquely defined since it depends on the knowledge of the statistical relation, e.g., degree of correlation, among the various antenna inputs. To circumvent the need for such knowledge, which is often not completely available, we suggest in this article an alternate criterion for comparing arrayed and nonarrayed loops, namely, fixed steady-state phase error, which, in the arrayed case, is independent of the statistical relation among the antenna inputs. We show that, in this case, the gain derived from carrier arraying is quite different from that obtained under the assumption of fixed-loop bandwidth, which suggests that one should exercise care when comparing arrayed and unarrayed loops in that the criterion used for comparison should be directly applicable to the situation at hand.

Simon, M. K.↗