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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A system approach to archival storage

The introduction and viewgraphs of a discussion on a system approach to archival storage presented at the National Space Science Data Center (NSSDC) Mass Storage Workshop is included. The use of D-2 iron particles for archival storage is discussed along with how acceleration factors relating short-term tests to archival life times can be justified. Ampex Recording Systems is transferring D-2 video technology to data storage applications, and encountering concerns about corrosion. To protect the D-2 standard, Battelle tests were done on all four tapes in the Class 2 environment. Error rates were measured before and after the test on both exposed and control groups.

Corcoran, John W.↗
NASA NTRS1992

The Use of Subcarriers in Future DSN Missions

The performance of deep space telemetry signals that employ a residual carrier modulation technique is compared in the presence and absence of a subcarrier. When the subcarrier is present, the performance for the resulting pulse-coded modulation/phase-shift keyed/phase-modulated (PCM/PSK/PM) scheme is evaluated for both sine wave and square wave subcarriers and non-return-to zero (NRZ) data. When the subcarrier is absent, the performance for the resulting PCM/PM technique is evaluated for both the NRZ and the bi-phase data format. The comparison is based on telemetry performance as well as bandwidth efficiency. The first criterion is characterized in terms of the symbol error rate (SER) as a function of symbol SNR, loop bandwidth-to-data rate ratio, and modulation index. The bandwidth efficiency is characterized by the occupancy factor. The results of both the analysis and measurements show that when the interference-to-carrier ratio (ICR) is less than -15 dB, the performance degradation in the absence of a subcarrier is negligible. Various combinations of loop bandwidth-to-data rate ratios and modulation indices that achieve this performance are derived and listed. Bandwidth occupancy comparison indicates that PCM/PM/NRZ is the most efficient in this regard. Therefore, by eliminating the subcarrier and using the PCM/PM/NRZ scheme, many advantages can be realized without any sacrifice in performance.

Shihabi, M. M.↗
NASA NTRS1993

Symbol signal-to-noise ratio loss in square-wave subcarrier downconversion

This article presents the simulated results of the signal-to-noise ratio (SNR) loss in the process of a square-wave subcarrier down conversion. In a previous article, the SNR degradation was evaluated at the output of the down converter based on the signal and noise power change. Unlike in the previous article, the SNR loss is defined here as the difference between the actual and theoretical symbol SNR's for the same symbol-error rate at the output of the symbol matched filter. The results show that an average SNR loss of 0.3 dB can be achieved with tenth-order infinite impulse response (IIR) filters. This loss is a 0.2-dB increase over the SNR degradation in the previous analysis where neither the signal distortion nor the symbol detector was considered.

Feria, Y.↗
NASA NTRS1993

Adaptive laser link reconfiguration using constraint propagation

This paper describes Harris AI research performed on the Adaptive Link Reconfiguration (ALR) study for Rome Lab, and focuses on the application of constraint propagation to the problem of link reconfiguration for the proposed space based Strategic Defense System (SDS) Brilliant Pebbles (BP) communications system. According to the concept of operations at the time of the study, laser communications will exist between BP's and to ground entry points. Long-term links typical of RF transmission will not exist. This study addressed an initial implementation of BP's based on the Global Protection Against Limited Strikes (GPALS) SDI mission. The number of satellites and rings studied was representative of this problem. An orbital dynamics program was used to generate line-of-site data for the modeled architecture. This was input into a discrete event simulation implemented in the Harris developed COnstraint Propagation Expert System (COPES) Shell, developed initially on the Rome Lab BM/C3 study. Using a model of the network and several heuristics, the COPES shell was used to develop the Heuristic Adaptive Link Ordering (HALO) Algorithm to rank and order potential laser links according to probability of communication. A reduced set of links based on this ranking would then be used by a routing algorithm to select the next hop. This paper includes an overview of Constraint Propagation as an Artificial Intelligence technique and its embodiment in the COPES shell. It describes the design and implementation of both the simulation of the GPALS BP network and the HALO algorithm in COPES. This is described using a 59 Data Flow Diagram, State Transition Diagrams, and Structured English PDL. It describes a laser communications model and the heuristics involved in rank-ordering the potential communication links. The generation of simulation data is described along with its interface via COPES to the Harris developed View Net graphical tool for visual analysis of communications networks. Conclusions are presented, including a graphical analysis of results depicting the ordered set of links versus the set of all possible links based on the computed Bit Error Rate (BER). Finally, future research is discussed which includes enhancements to the HALO algorithm, network simulation, and the addition of an intelligent routing algorithm for BP.

Crone, M. S.↗
NASA NTRS1993

Analyses of space environment effects on active fiber optic links orbited aboard the LDEF

The results of the 'Preliminary Analysis of WL Experiment no. 701, Space Environment Effects on Operating Fiber Optic Systems,' is correlated with space simulated post retrieval terrestrial studies performed on the M0004 experiment. Temperature cycling measurements were performed on the active optical data links for the purpose of assessing link signal to noise ratio and bit error rate performance some 69 months following the experiment deployment in low Earth orbit. The early results indicate a high correlation between pre-orbit, orbit, and post-orbit functionality of the first known and longest space demonstration of operating fiber optic systems.

Taylor, Edward W.↗
NASA NTRS1991

Performance evaluation of land mobile satellite system under fading and interference using multiple TCM by Monte-Carlo simulation

The performance of the land mobile satellite system (LMSS) was evaluated by using two trellis coded modulation (TCM) schemes under multipath fading and interference. The results were also compared with uncoded QPSK. The trellis coding formats included a trellis code designed for optimum performance on the fading channel that typifies satellite to mobile communications, as well as one designed for optimum performance in the additive white Gaussian noise (AWGN) channel. The results show that, in Rayleigh fading, the TCM code designed for optimum performance in such an environment performs 4 dB better than the TCM code designed for optimum performance in AWGN, and 10 dB better than the uncoded format, at a bit error rate (BER) of 10 exp -4. Additional results on the performance degradation due to the nonlinearities in the satellite transponder and the adjacent and cochannel interference show that the TCM system is more sensitive than the uncoded system to the phase distortion caused by these impairments.

Kwa, S. C.↗
NASA NTRS1991

Bandwidth requirements for direct detection optical communication receivers with PPM signaling

The performance of a direct detection optical communication receiver for pulse position modulation (PPM) signal formats which contains an avalanche photodiode photodetector is calculated as a function of the parameters which characterize a raised cosine filter used in the receiver. It is shown that the performance penalty of this receiver relative to that of an optimal receiver which contains an integrator and assumes a perfectly rectangular PPM pulse shape is less than 1 dB at a receiver bit error rate of 10 exp -6. The advantage of the raised cosine filter is that the electrical bandwidth required is no more than one-half that of the integrator, that is, 1/tau as opposed to 2/tau where tau is the PPM time slot duration.

Davidson, Frederic M.↗
NASA NTRS1991

NASA's flight-technological development program - A 650 Mbps laser communications testbed

A 650 Mbps laser communications testbed under construction for the development of flight qualifiable hardware suitable for near-term operation on geosynchronous-to-geosynchronous crosslink missions is presented. The program's primary purpose is to develop and optimize laser communications unique subsystems. Requirements for the testbed experiments are to optimize the acquisition processes, to fully simulate the long range (up to 21,000 km) and the fine tracking characteristics of two narrow-beam laser communications terminals, and to fully test communications performance which will include average and burst bit error rates, effects of laser diode coalignment, degradation due to internal and external stray light, and the impact of drifts in the optical components.

Hayden, W. L.↗
NASA NTRS1993

Low-cost TDRSS communications for NASA's long duration balloon project

A new transponder and RF ground support equipment for the NASA Tracking and Data Relay Satellite System (TDRSS) intended to support long duration scientific balloon flights in Antarctica are described. The new balloon class transponder features a highly integrated spread spectrum receiver design based on programmable charge coupled device (CCD) correlators and digital signal processing chips. The correlator chip is a Lincoln Labs 4ABC with four CCD channels. The balloon transponder is capable of reporting an estimate of its input bit error rate using digital signal processing. The TDRSS user RF test set is based on a set of RF ground support equipment capable of providing both the RF communications and direct control and monitoring necessary for transponder testing and a two-way RF link for preflight testing.

Israel, David J.↗
NASA NTRS1992

Comparison of direct detection 50 Mbps Q-PPM receiver performance for AlGaAs and Nd doped laser transmitters

We present a theoretical and experimental performance comparison of a free space direct detection 50 Mbps quaternary pulse position modulation (Q-PPM) communications receiver for Nd:YAG and AlGaAs laser transmitters. Using silicon avalanche photodiodes (APD), we achieved a receiver sensitivity of 2.6 nW (264 incident photons/bit) at 1060 nm and 0.77 nW (59 incident photons/bit) at 810 nm (bit error rate (BER) of 10 exp -6). This is the best sensitivity ever reported for a direct detection receiver at 1060 nm. The experimental results were in good agreement with theoretical predictions.

Unger, G. L.↗
NASA NTRS1992

Numerical simulation of a 325 Mbit/s QPPM optical communication system

Optical digital receivers are being considered for intersatellite laser communication links. A demonstration system is being designed to operate at 325 MBit/s, using quaternary pulse position modulation (QPPM). Laboratory experiments have been conducted using a 50 MBit/s prototype system. A numerical model has been developed to simulate a QPPM optical receiver. The 50 MBit/s system was simulated to verify the validity of the model. The model was then used to simulate the projected behavior of the 325 MBit/s system. The model predicts a bit error rate of 10.6 at 38 incident photons per bit for 820 nm light.

Martino, Anthony J.↗
NASA NTRS1993

Single event test methodology for integrated optoelectronics

A single event upset (SEU), defined as a transient or glitch on the output of a device, and its applicability to integrated optoelectronics are discussed in the context of spacecraft design and the need for more than a bit error rate viewpoint for testing and analysis. A methodology for testing integrated optoelectronic receivers and transmitters for SEUs is presented, focusing on the actual test requirements and system schemes needed for integrated optoelectronic devices. Two main causes of single event effects in the space environment, including protons and galactic cosmic rays, are considered along with ground test facilities for simulating the space environment.

Label, Kenneth A.↗
NASA NTRS1993

Enhanced decoding for the Galileo S-band mission

A coding system under consideration for the Galileo S-band low-gain antenna mission is a concatenated system using a variable redundancy Reed-Solomon outer code and a (14,1/4) convolutional inner code. The 8-bit Reed-Solomon symbols are interleaved to depth 8, and the eight 255-symbol codewords in each interleaved block have redundancies 64, 20, 20, 20, 64, 20, 20, and 20, respectively (or equivalently, the codewords have 191, 235, 235, 235, 191, 235, 235, and 235 8-bit information symbols, respectively). This concatenated code is to be decoded by an enhanced decoder that utilizes a maximum likelihood (Viterbi) convolutional decoder; a Reed Solomon decoder capable of processing erasures; an algorithm for declaring erasures in undecoded codewords based on known erroneous symbols in neighboring decodable words; a second Viterbi decoding operation (redecoding) constrained to follow only paths consistent with the known symbols from previously decodable Reed-Solomon codewords; and a second Reed-Solomon decoding operation using the output from the Viterbi redecoder and additional erasure declarations to the extent possible. It is estimated that this code and decoder can achieve a decoded bit error rate of 1 x 10(exp 7) at a concatenated code signal-to-noise ratio of 0.76 dB. By comparison, a threshold of 1.17 dB is required for a baseline coding system consisting of the same (14,1/4) convolutional code, a (255,223) Reed-Solomon code with constant redundancy 32 also interleaved to depth 8, a one-pass Viterbi decoder, and a Reed Solomon decoder incapable of declaring or utilizing erasures. The relative gain of the enhanced system is thus 0.41 dB. It is predicted from analysis based on an assumption of infinite interleaving that the coding gain could be further improved by approximately 0.2 dB if four stages of Viterbi decoding and four levels of Reed-Solomon redundancy are permitted. Confirmation of this effect and specification of the optimum four-level redundancy profile for depth-8 interleaving is currently being done.

Dolinar, S.↗
NASA NTRS1991

Technology, design, simulation, and evaluation for SEP-hardened circuits

This paper describes the technology, design, simulation, and evaluation for improvement of the Single Event Phenomena (SEP) hardness of gate-array and SRAM cells. Through the use of design and processing techniques, it is possible to achieve an SEP error rate less than 1.0 x 10(exp -10) errors/bit-day for a 9O percent worst-case geosynchronous orbit environment.

Adams, J. R.↗
NASA NTRS1993

The Composite Analytic and Simulation Package or RFI (CASPR) on a coded channel

CASPR is an analysis package which determines the performance of a coded signal in the presence of Radio Frequency Interference (RFI) and Additive White Gaussian Noise (AWGN). It can analyze a system with convolutional coding, Reed-Solomon (RS) coding, or a concatenation of the two. The signals can either be interleaved or non-interleaved. The model measures the system performance in terms of either the E(sub b)/N(sub 0) required to achieve a given Bit Error Rate (BER) or the BER needed for a constant E(sub b)/N(sub 0).

Freedman, Jeff↗
NASA NTRS1993

A laboratory system for the investigation of rain fade compensation techniques for Ka-band satellites

The design and performance of a rain fade simulation/counteraction system on a laboratory simulated 30/20 GHz, time division multiple access (TDMA) satellite communications testbed is evaluated. Severe rain attenuation of electromagnetic radiation at 30/20 GHz occurs due to the carrier wavelength approaching the water droplet size. Rain in the downlink path lowers the signal power present at the receiver, resulting in a higher number of bit errors induced in the digital ground terminal. The laboratory simulation performed at NASA Lewis Research Center uses a programmable PIN diode attenuator to simulate 20 GHz satellite downlink geographic rain fade profiles. A computer based network control system monitors the downlink power and informs the network of any power threshold violations, which then prompts the network to issue commands that temporarily increase the gain of the satellite based traveling wave tube (TWT) amplifier. After the rain subsides, the network returns the TWT to the normal energy conserving power mode. Bit error rate (BER) data taken at the receiving ground terminal serves as a measure of the severity of rain degradation, and also evaluates the extent to which the network can improve the faded channel.

Svoboda, James S.↗
NASA NTRS1994

QPPM receiver for free-space laser communications

A prototype receiver developed at NASA Lewis Research Center for direct detection and demodulation of quaternary pulse position modulated (QPPM) optical carriers is described. The receiver enables dual-channel communications at 325-Megabits per second (Mbps) per channel. The optical components of the prototype receiver are briefly described. The electronic components, comprising the analog signal conditioning, slot clock recovery, matched filter and maximum likelihood data recovery circuits are described in more detail. A novel digital symbol clock recovery technique is presented as an alternative to conventional analog methods. Simulated link degradations including noise and pointing-error induced amplitude variations are applied. The bit-error-rate performance of the electronic portion of the prototype receiver under varying optical signal-to-noise power ratios is found to be within 1.5-dB of theory. Implementation of the receiver as a hybrid of analog and digital application specific integrated circuits is planned.

Budinger, J. M.↗
NASA NTRS1993

Aeronautical audio broadcasting via satellite

A system design for aeronautical audio broadcasting, with C-band uplink and L-band downlink, via Inmarsat space segments is presented. Near-transparent-quality compression of 5-kHz bandwidth audio at 20.5 kbit/s is achieved based on a hybrid technique employing linear predictive modeling and transform-domain residual quantization. Concatenated Reed-Solomon/convolutional codes with quadrature phase shift keying are selected for bandwidth and power efficiency. RF bandwidth at 25 kHz per channel, and a decoded bit error rate at 10(exp -6) with E(sub b)/N(sub o) at 3.75 dB are obtained. An interleaver, scrambler, modem synchronization, and frame format were designed, and frequency-division multiple access was selected over code-division multiple access. A link budget computation based on a worst-case scenario indicates sufficient system power margins. Transponder occupancy analysis for 72 audio channels demonstrates ample remaining capacity to accommodate emerging aeronautical services.

Tzeng, Forrest F.↗