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At least 289 records · Page 16

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NASA NTRS1991

Pi/4-QPSK modems for satellite sound/data broadcast systems

The use of pi/4-quadrature phase-shift keying (QPSK) modems for satellite sound broadcast systems (SSBS) broadcasting to mobile or portable receivers is proposed. Three different differential detectors (including the FM-discriminator followed by integrate-sample-and-dump filter) and a novel coherent detector are discussed. The degradation caused by the frequency offset between the local oscillator (LO) and the unmodulated carrier (CR) in the baseband differential detector is studied. The performance of both coherently and differentially detected pi/4-QPSK in a Gaussian channel is also studied. It is shown that with a frequency offset of more than 3 percent of the symbol rate, the performance degradation is more than 1 dB at 0.0001. The out-of-band power of the nonlinearly amplified bandlimited pi/4-QPSK signals is reduced from -13 dB to -37 dB if a 2-dB output back-off amplifier is used instead of a hardlimiter. The performance of the pi/4-QPSK is equivalent to that of QPSK, although the pi/4-QPSK has the advantage of less spectrum restoration after nonlinear amplification. The coherent demodulator and differential decoder avoid the three-level detection and achieve the same bit-error-rate performance as DEQPSK with a simple circuit.

Liu, Chia-Liang↗

NASA NTRS2002

Spectral Re-Growth Reduction for CCSDS 8-D 8-PSK TCM

This report presents a study on the CCSDS recommended 8-dimensional 8 PSK Trellis Coded Modulation (TCM) scheme. The important steps of the CCSDS scheme include: conversion of serial data into parallel form, differential encoding, convolutional encoding, constellation mapping, and filtering the 8-PSK symbols using the square root raised cosine (SRRC) pulses. The last step, namely the filtering of the 8 PSK symbols using SRRC pulses, significantly affects the bandwidth of the signal. If a nonlinear power amplifier is used, the SRRC filtered signal creates spectral regrowth. The purpose of this report is to investigate a technique, called the smooth phase interpolated keying (SPIK), that can provide an alternative to SRRC filtering so that good spectral as well as power efficiencies can be obtained with the CCSDS encoder. The results of this study show that the CCSDS encoder does not affect the spectral shape of the SRRC filtered signal or the SPIK signal. When a nonlinear traveling wave tube amplifier (TWTA) is used, the spectral performance of the SRRC signal degrades significantly while the spectral performance of SPIK remains unaffected. The degrading effect of a nonlinear solid state power amplifier (SSPA) on SRRC is found to be less than that due to a nonlinear TWTA. However, in both cases, the spectral performance of the SRRC modulated signal is worse than that of the SPIK signal. The bit error rate (BER) performance of the SRRC signal in a linear amplifier environment is about 2.5 dB better than that of the SPIK signal when both the receivers use algorithms of similar complexity. In a nonlinear TWTA environment, the SRRC signal requires accurate phase tracking since the TWTA introduces additional phase distortion. This problem does not arise with SPIK signal due to its constant envelope property. When a nonlinear amplifier is used, the SRRC method loses nearly 1 dB in the bit error rate performance. The SPIK signal does not lose any performance. Thus the performance gap between SRRC and SPIK reduces. The BER performance of SPIK can be improved even further by using a more optimal receiver. A similar optimal receiver for SRRC is quite complex since the amplifier distorts the pulse shape. However, this requires further investigation and is not covered in this report.

Borah, Deva K.↗

NASA NTRS2018

Assessing C2 Communications for UAS Traffic Management

The National Aeronautics and Space Administration's (NASA) Unmanned Aircraft Systems (UAS) Traffic Management (UTM) project works to develop tools and technologies essential for safely enabling civilian low-altitude UAS operations. Currently there is no established infrastructure to enable and safely manage the widespread use of low-altitude airspace and UAS operations, regardless of the type of UAS. The UTM technical challenge will develop comprehensive and validated airspace operations and integration requirements to safely enable large-scale persistent access to visual line of sight and autonomous beyond visual line of sight small UAS in low-altitude airspace. Within the UTM project, a number of communications technologies to support UTM command and control (C2) are under investigation. In particular, commercial networked cellular systems are being tested and assessed for their ability to meet the reliability, scalability, cybersecurity and redundancy required. NASA Glenn Research Center is studying some of the aspects of employing such networks for UTM C2 communications. This includes the development of a test platform for sensing and characterizing the airborne C2 communications environment at various altitudes and in various terrains and topologies, measuring such aspects as received signal strength and interference. System performance aspects such as latency in the link, handover performance, packet error loss rate, drop outs, coverage gaps and other aspects impacting UTM operation will also be assessed. In this paper we explore some of the C2 approaches being proposed and demonstrated in the UTM project, the reliability, availability and other general C2 performance requirements, and approaches to evaluating and analyzing UTM C2 links based on commercial cellular networks.

air traffic control↗

NASA NTRS2018

Assessing C2 Communications

aircraft communications↗

NASA NTRSDate not supplied

Curating AI-Ready Datasets for Equity and Environmental Justice: A Data-Centric AI Case Study

An equitable and environmentally just community is essentialin order to avoid disproportionate burden borne by vulnerablecommunities. This need becomes pressing in the aftermathof an extreme event such as disaster or hazard when it is diffi-cult for the governing bodies to implement resource allocationas per the need. Artificial Intelligence (AI) algorithms canhelp surface Equity and Environmental Justice (EEJ) issueswhen trained on EEJ datasets. However, curating AI-readyEEJ training datasets is challenging due to differences in fac-tors such as heterogeneity, resolution, modality, and level ofexpertise in labeling. Additionally, EEJ issues involve sensi-tive information where uncertainties and errors could degradethe performance of AI algorithms. For eg. Error in seasonalcrop yield information can highly affect the prediction of an-nual crop yield. To address these challenges, Data-centricAI (DCAI) methods are employed, which enhance AI algo-rithm performance even with limited training samples. DCAIprioritizes data quality, thereby reducing the adverse effectsof uncertainties and errors during the model training process.This research proposes a novel dataset and benchmark for an-alyzing the effect of the Maui Wildfire of 2023 for Equityand Environmental Justice (EEJ) issues. The proposed datasetaligns with the concepts of DCAI such as annotation quality,data preprocessing, privacy, feature engineering, governanceand provenance. We firmly believe that the proposed datasetwould lay a foundation to implement robust and reliable mod-ern AI algorithms for addressing EEJ issues.

Paridhi Parajuli↗

NASA NTRS1981

Coherent detection of frequency-hopped quadrature modulations in the presence of jamming. I - QPSK and QASK modulations

This paper examines the performance of coherent QPSK and QASK systems combined with FH or FH/PN spread spectrum techniques in the presence of partial-band multitone or noise jamming. The worst-case jammer and worst-case performance are determined as functions of the signal-to-background noise ratio (SNR) and signal-to-jammer power ratio (SJR). Asymptotic results for high SNR are shown to have a linear dependence between the jammer's optimal power allocation and the system error probability performance.

Simon, M. K.↗

NASA NTRS1982

The performance of M-ary FH-DPSK in the presence of partial-band multitone jamming

Using a geometric approach, the performance of M-ary FH-DPSK in the presence of partial-band multitone jamming is evaluated. The optimal jamming strategy is determined as a function of the number of signaling levels M and the ensuing results are used to determine worst case bit error probability performance as a function of this same parameter. It is demonstrated that, for M = 2 to the m power (where m is an integer), the best performance is obtained for M = 4.

Simon, M. K.↗

NASA NTRS1986

DSA's subcarrier demodulation losses

The degradation in bit error rate performance due to imperfect subcarrier tracking by the Demodulation Synchronization Assembly (DSA) is investigated. Results apply to any type of digital loop and received signal dynamics. A type four loop causes the least amount of loss, because it tracks phase jerk with zero steady-state error. However, when frequency rate and frequency acceleration are as large as in the extended Magellan mission, it will be necessary to decrease the loop update time in order to minimize the losses.

Simon, M.↗

NASA NTRS2019

A comprehensive line spread function error for the Off-plane Grating Rocket Experiment

The Off-plane Grating Rocket Experiment (OGRE) is a soft X-ray grating spectrometer to be flown on a suborbital rocket. The payload is designed to obtain the highest-resolution soft X-ray spectrum of Capella to date with a resolution goal of R(lambda/delta lambda) > 2000 at select wavelengths in its 10 - 55 A bandpass of interest. The optical design of the spectrometer realizes a maximum resolution of R 5000 with all components performing optimally, in perfect alignment, and not considering in-flight pointing errors; however, performance errors, misalignments, and in-flight pointing errors work to degrade this performance. A comprehensive line spread function (LSF) error budget has been constructed for the spectrometer to identify errors contributing to the LSF, to determine how each affects the LSF, and to inform performance requirements and alignment tolerances for the spectrometer. In this document, the comprehensive LSF error budget for the OGRE spectrometer is presented and its implications are discussed.

Benjamin D Donovan↗

NASA NTRS1988

Dynamic analysis of multimesh-gear helicopter transmissions

A dynamic analysis of multimesh-gear helicopter transmission systems was performed by correlating analytical simulations with experimental investigations. The two computer programs used in this study, GRDYNMLT and PGT, were developed under NASA/Army sponsorship. Parametric studies of the numerical model with variations on mesh damping ratios, operating speeds, tip-relief tooth modifications, and tooth-spacing errors were performed to investigate the accuracy, application, and limitations of the two computer programs. Although similar levels of dynamic loading were predicted by both programs, the computer code GRDYNMLT was found to be superior and broader in scope. Results from analytical work were also compared with experimental data obtained from the U.S. Army's UH-60A Black Hawk 2240-kW (3000-hp) class, twin-engine helicopter transmission tested at the NASA Lewis Research Center. Good correlation in gear stresses was obtained between the analytical model simulated by GRDYNMLT and the experimental measurements. More realistic mesh damping can be predicted through experimental data correlation.

Choy, Fred K.↗

NASA NTRS1993

Interferometric detection of freeze-thaw displacements of Alaskan permafrost using ERS-1 data

The possibility of making large scale (50 km) measurements of motions of the earth's surface with high resolution (10 m) and very high accuracy (1 cm) from multipass SAR interferometry was established in 1989. Other experiments have confirmed the viability and usefulness of the method. Work is underway in various groups to measure displacements from volcanic activity, seismic events, glacier motion, and in the present study, freeze-thaw cycles in Alaskan permafrost. The ground is known to move significantly in these cycles, and provided that freezing does not cause image decorrelation, it should be possible to measure both ground swelling and subsidence. The authors have obtained data from multiple passes of ERS-1 over the Toolik Lake region of northern Alaska of suitable quality for interferometry. The data are processed into images, and single interferograms are formed in the usual manner. Phase unwrapping is performed, and the multipass baselines are estimated from the images using both orbit ephemerides and scene tie points. The phases are scaled by the baseline ratio, and a double-difference interferogram (DDI) is formed. It is found that there is a residual 'saddle-shape' phase error across the image, which is postulated to be caused by a small divergence (10(exp -2) deg.) in the orbits. A simulation of a DDI from divergent orbits confirms the shape and magnitude of the error. A two-dimensional least squares fit to the error is performed, which is used to correct the DDI. The final, corrected DDI shows significant phase (altitude) changes over the period of the observation.

Werner, Charles L.↗

NASA NTRS1995

Computer simulation results for PCM/PM/NRZ receivers in nonideal channels

This article studies, by computer simulations, the performance of deep-space telemetry signals that employ the pulse code modulation/phase modulation (PCM/PM) technique, using nonreturn-to-zero data, under the separate and combined effects of unbalanced data, data asymmetry, and a band-limited channel. The study is based on measuring the symbol error rate performance and comparing the results to the theoretical results presented in previous articles. Only the effects of imperfect carrier tracking due to an imperfect data stream are considered. The presence of an imperfect data stream (unbalanced and/or asymmetric) produces undesirable spectral components at the carrier frequency, creating an imperfect carrier reference that will degrade the performance of the telemetry system. Further disturbance to the carrier reference is caused by the intersymbol interference created by the band-limited channel.

Anabtawi, A.↗

NASA NTRS2008

Analyzing and Detecting Problems in Systems of Systems

Many software systems are evolving complex system of systems (SoS) for which inter-system communication is mission-critical. Evidence indicates that transmission failures and performance issues are not uncommon occurrences. In a NASA-supported Software Assurance Research Program (SARP) project, we are researching a new approach addressing such problems. In this paper, we are presenting an approach for analyzing inter-system communications with the goal to uncover both transmission errors and performance problems. Our approach consists of a visualization and an evaluation component. While the visualization of the observed communication aims to facilitate understanding, the evaluation component automatically checks the conformance of an observed communication (actual) to a desired one (planned). The actual and the planned are represented as sequence diagrams. The evaluation algorithm checks the conformance of the actual to the planned diagram. We have applied our approach to the communication of aerospace systems and were successful in detecting and resolving even subtle and long existing transmission problems.

Lindvall, Mikael↗

NASA NTRS2023

The CHROMA Cloud-Top Pressure Retrieval Algorithm for the Plankton, Aerosol, Cloud, Ocean Ecosystem (PACE) Satellite Mission

This paper provides the theoretical basis and simulated retrievals for the Cloud Height Retrieval from O2 Molecular Absorption (CHROMA) algorithm. Simulations are performed for the Ocean Color Instrument (OCI), which is the primary payload on the forthcoming NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, and the Ocean Land Colour Instrument (OLCI) currently flying on the Sentinel 3 satellites. CHROMA is a Bayesian approach which simultaneously retrieves cloud optical thickness (COT), cloud-top pressure and height (CTP and CTH respectively), and (with a significant prior constraint) surface albedo. Simulated retrievals suggest that the sensor and algorithm should be able to meet the PACE mission goal for CTP error, which is ±60 mb for 65 % of opaque (COT ≥3) single-layer clouds on global average. CHROMA will provide pixel-level uncertainty estimates, which are demonstrated to have skill at telling low-error situations from high-error ones. CTP uncertainty estimates are well-calibrated in magnitude, although COT uncertainty is overestimated relative to observed errors. OLCI performance is found to be slightly better than OCI overall, demonstrating that it is a suitable proxy for the latter in advance of PACE's launch. CTP error is only weakly sensitive to correct cloud phase identification or assumed ice crystal habit/roughness. As with other similar algorithms, for simulated retrievals of multi-layer systems consisting of optically thin cirrus clouds above liquid clouds, retrieved height tends to be underestimated because the satellite signal is dominated by the optically thicker lower layer. Total (liquid plus ice) COT also becomes underestimated in these situations. However, retrieved CTP becomes closer to that of the upper ice layer for ice COT ≈3 or higher.

PACE↗

NASA NTRS1993

The Communication Link and Error ANalysis (CLEAN) simulator

During the period July 1, 1993 through December 30, 1993, significant developments to the Communication Link and Error ANalysis (CLEAN) simulator were completed and include: (1) Soft decision Viterbi decoding; (2) node synchronization for the Soft decision Viterbi decoder; (3) insertion/deletion error programs; (4) convolutional encoder; (5) programs to investigate new convolutional codes; (6) pseudo-noise sequence generator; (7) soft decision data generator; (8) RICE compression/decompression (integration of RICE code generated by Pen-Shu Yeh at Goddard Space Flight Center); (9) Markov Chain channel modeling; (10) percent complete indicator when a program is executed; (11) header documentation; and (12) help utility. The CLEAN simulation tool is now capable of simulating a very wide variety of satellite communication links including the TDRSS downlink with RFI. The RICE compression/decompression schemes allow studies to be performed on error effects on RICE decompressed data. The Markov Chain modeling programs allow channels with memory to be simulated. Memory results from filtering, forward error correction encoding/decoding, differential encoding/decoding, channel RFI, nonlinear transponders and from many other satellite system processes. Besides the development of the simulation, a study was performed to determine whether the PCI provides a performance improvement for the TDRSS downlink. There exist RFI with several duty cycles for the TDRSS downlink. We conclude that the PCI does not improve performance for any of these interferers except possibly one which occurs for the TDRS East. Therefore, the usefulness of the PCI is a function of the time spent transmitting data to the WSGT through the TDRS East transponder.

Ebel, William J.↗

NASA NTRS1979

Effect of mid- and high-spatial frequencies on optical performance

In many of today's telescopes the effects of surface errors on image quality and scattered light are very important. The influence of optical fabrication surface errors on the performance of an optical system is discussed. The methods developed by Hopkins (1957) for aberration tolerancing and Barakat (1972) for random wavefront errors are extended to the examination of mid- and high-spatial frequency surface errors. The discussion covers a review of the basic concepts of image quality, an examination of manufacturing errors as a function of image quality performance, a demonstration of mirror scattering effects in relation to surface errors, and some comments on the nature of the correlation functions. Illustrative examples are included.

Noll, R. J.↗

NASA NTRS1975

Performance of coded MFSK in a Rician fading channel

The performance of convolutional codes in conjunction with noncoherent multiple frequency shift-keyed (MFSK) modulation and Viterbi maximum likelihood decoding on a Rician fading channel is examined in detail. While the primary motivation underlying this work has been concerned with system performance on the planetary entry channel, it is expected that the results are of considerably wider interest. Particular attention is given to modeling the channel in terms of a few meaningful parameters which can be correlated closely with the results of theoretical propagation studies. Fairly general upper bounds on bit error probability performance in the presence of fading are derived and compared with simulation results using both unquantized and quantized receiver outputs. The effects of receiver quantization and channel memory are investigated and it is concluded that the coded noncoherent MFSK system offers an attractive alternative to coherent BPSK in providing reliable low data rate communications in fading channels typical of planetary entry missions.

Modestino, J. W.↗

NASA NTRS1971

Investigation of techniques for improving Saturn 5 RF tracking and ranging systems

Results of theoretical analyses of 12 problems are presented and comparisons of theoretical and experimental results are given. The investigations covered were: (1) techniques for improving the Saturn radar altimeter, (2) performance of the phase lock loop of the offset Doppler transponder, (3) signal processing equipment design for an orbital altitude radar return experiment, (4) spectral studies of signals present in the command and communication system (CCS) up-link transmitter, (5) intermodulation in the CCS down-link data demodulators, (6) error rate performance of the CCS transponder command demodulator, (7) flame attenuation effects on telemetry transmissions during Saturn launches, (8) design of a digital television system which converts a standard monochrome picture to a slow-scan picture, (9) methods of obtaining an additional CCS 72-kilobit/sec telemetry channel, (10) computation of the CCS S-band down-link spectra, (11) modeling of portions of the communications systems, and (12) modeling of a telemetry transmitter.

Walsh, J. R., Jr.↗