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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

NASA balloon: Aircraft ranging, data and voice experiment

A series of tests to evaluate, at L-band, the ranging, voice, and data communications concepts proposed for the air traffic control experiment of the Applications Technology Satellite-F are described. The ground station facilities, balloon platforms and the aircraft were supplied by the European Space Research Organization. One ground simulation and two aircraft flights at low elevation angles were conducted. Even under high interference conditions good performance was obtained for both voice communications and side tone ranging. High bit errors occurred in the data channels resulting in false commands. As a result of the experience gained in operating the equipment in an aircraft environment several recommendations were made for improving the equipment performance.

Wishna, S.↗
NASA NTRS1972

Hybrid coding systems study

The state of efficiency improvement available with high speed decoders presently in operation or under development is summarized. The required ratio of bit-energy-to-noise-density is given in each case for bit error probabilities.

Odenwalder, J. P.↗
NASA NTRS1971

On the timing problem in optical PPM communications.

Investigation of the effects of imperfect timing in a direct-detection (noncoherent) optical system using pulse-position-modulation bits. Special emphasis is placed on specification of timing accuracy, and an examination of system degradation when this accuracy is not attained. Bit error probabilities are shown as a function of timing errors, from which average error probabilities can be computed for specific synchronization methods. Of significant importance is shown to be the presence of a residual, or irreducible error probability, due entirely to the timing system, that cannot be overcome by the data channel.

Gagliardi, R. M.↗
NASA NTRS1971

Performance results for a hybrid coding system.

Results of computer simulation studies of the hybrid pull-up bootstrap decoding algorithm, using a constraint length 24, nonsystematic, rate 1/2 convolutional code for the symmetric channel with both binary and eight-level quantized outputs. Computational performance was used to measure the effect of several decoder parameters and determine practical operating constraints. Results reveal that the track length may be reduced to 500 information bits with small degradation in performance. The optimum number of tracks per block was found to be in the range from 7 to 11. An effective technique was devised to efficiently allocate computational effort and identify reliably decoded data sections. Long simulations indicate that a practical bootstrap decoding configuration has a computational performance about 1.0 dB better than sequential decoding and an output bit error rate about .0000025 near the R sub comp point.

Hoffman, L. B.↗
NASA NTRS1973

ERTS wideband tape recorder

The ERTS video bandwith tape recorder uses a rotary head to run the tape in transverse mode; the head wheel gives a head-to-tape surface speed of nearly 5080 centimeter per second. The electronics unit handles 15 megabit per second rate with a bit-error rate of 0.00001. An operational unit onboard ERTS A returned images from the 85 to 90 percent of the earth that are not available in real time.

Hayes, J. M.↗
NASA NTRS1973

Balloon atmospheric propagation experiment measurements

High altitude balloon measurements on laser beam fading during propagation through turbulent atmosphere show that a correlation between fading strength and stellar scintillation magnitudes exists. Graphs for stellar scintillation as a function of receiver aperture are used to predict fading bit error rates for neodymium-yag laser communication system.

Minott, P. O.↗
NASA NTRS1973

The CO2 laser frequency stability measurements

Carbon dioxide laser frequency stability data are considered for a receiver design that relates to maximum Doppler frequency and its rate of change. Results show that an adequate margin exists in terms of data acquisition, Doppler tracking, and bit error rate as they relate to laser stability and transmitter power.

Johnson, E. H., Jr.↗
NASA NTRS1972

The ERTS wideband image communication system.

The ERTS payload includes a three camera return beam vidicon (RBV) television system and a four channel multispectral scanner system (MSS). The communications system described includes spacecraft wideband equipment plus unique payload processing equipment located at three NASA tracking stations. The RBV information, transmitted as analog FM with a baseband response of dc to 3.2 MHz, is telemetered, processed and recorded on a wideband video recorder. The MSS data, consisting of 24 analog sensor outputs, is digitized and encoded as 15 megabit per second PCM. The MSS data is then transmitted as analog FM, demultiplexed, and recorded on a 25 channel recorder. It is shown that bit error rate is an excellent indicator of MSS link performance while signal-to-noise ratio and distortion vs frequency adequately define RBV link performance.

Pandelides, J.↗
NASA NTRS1972

High-data-rate, spacecraft tape recorders.

Description of magnetic head designs, tape transport features, encoding techniques, bit error rates, and data rate capabilities of tape recorders used on board satellites for temporary storage of sensor data. Helical- and transverse-scan rotary head mechanisms used for wideband video signals from imaging systems are explained along with associated electronic circuitry. Decoding and decision procedures employed in multiple-track recorders are outlined, and deskewing electronics are discussed.

Thompson, C. R.↗
NASA NTRS1972

Hybrid and concatenated coding applications.

Results of a study to evaluate the performance and implementation complexity of a concatenated and a hybrid coding system for moderate-speed deep-space applications. It is shown that with a total complexity of less than three times that of the basic Viterbi decoder, concatenated coding improves a constraint length 8 rate 1/3 Viterbi decoding system by 1.1 and 2.6 dB at bit error probabilities of 0.0001 and one hundred millionth, respectively. With a somewhat greater total complexity, the hybrid coding system is shown to obtain a 0.9-dB computational performance improvement over the basic rate 1/3 sequential decoding system. Although substantial, these complexities are much less than those required to achieve the same performances with more complex Viterbi or sequential decoder systems.

Hofman, L. B.↗
NASA NTRS1973

Improved detection technique of a band-limited PCM/NRZ signal.

A simple tapped-delay-line filter in tandem with the receiver that optimally detects a single pulse over a band-limited channel in the presence of Gaussian noise is proposed for reducing the effect of the intersymbol interference in an ideal band-limited PCM/NRZ system. The improvement is described in terms of the gain in signal-to-noise ratio for a specific bit-error probability.

Shehadeh, N. M.↗
NASA NTRS1974

Development of a wideband pulse quaternary modulation system

Laboratory data verifying the pulse quaternary modulation (PQM) theoretical predictions is presented. The first laboratory PQM laser communication system was successfully fabricated, integrated, tested and demonstrated. System bit error rate tests were performed and, in general, indicated approximately a 2 db degradation from the theoretically predicted results. These tests indicated that no gross errors were made in the initial theoretical analysis of PQM. The relative ease with which the entire PQM laboratory system was integrated and tested indicates that PQM is a viable candidate modulation scheme for an operational 400 Mbps baseband laser communication system.

Federhofer, J. A.↗
NASA NTRS1974

Performance of convolutional codes on fading channels typical of planetary entry missions

The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail. The signal fading is due primarily to turbulent atmospheric scattering of the RF signal transmitted from an entry probe through a planetary atmosphere. Short constraint length convolutional codes are considered in conjunction with binary phase-shift keyed modulation and Viterbi maximum likelihood decoding, and for longer constraint length codes sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms are considered. Careful consideration is given to the modeling of the channel in terms of a few meaningful parameters which can be correlated closely with theoretical propagation studies. For short constraint length codes the bit error probability performance was investigated as a function of E sub b/N sub o parameterized by the fading channel parameters. For longer constraint length codes the effect was examined of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms. The effects of simple block interleaving in combatting the memory of the channel is explored, using the analytic approach or digital computer simulation.

Modestino, J. W.↗
NASA NTRS1975

Space vehicle Viterbi decoder

The design and fabrication of an extremely low-power, constraint-length 7, rate 1/3 Viterbi decoder brassboard capable of operating at information rates of up to 100 kb/s is presented. The brassboard is partitioned to facilitate a later transition to an LSI version requiring even less power. The effect of soft-decision thresholds, path memory lengths, and output selection algorithms on the bit error rate is evaluated. A branch synchronization algorithm is compared with a more conventional approach. The implementation of the decoder and its test set (including all-digital noise source) are described along with the results of various system tests and evaluations. Results and recommendations are presented.

Source record↗
NASA NTRS1974

Performance analysis of a frame sync algorithm for uncoded PSK telemetry

The optimum procedure for locating a frame sync word periodically inserted in uncoded binary data received over a binary symmetric channel is based on the Hamming distance metric. In this paper, a practical frame sync acquisition and maintenance algorithm is described, and its performance is analyzed. Specifically, with respect to this algorithm, an upper bound on the probability of false sync acquisition, the mean time to sync acquisition, and the subsequent mean time to loss of sync are computed for arbitrary bit error rates, frame lengths, sync word lengths, and algorithm parameters.

Levitt, B. K.↗
NASA NTRS1974

Convolutional code performance in fading channels

The performance of convolutional codes in fading channels typical of the planetary entry channel is examined in detail. Short constraint length codes are considered in conjunction with binary phase-shift-keyed (BPSK) modulation and Viterbi maximum likelihood decoding while for longer constraint length codes we consider sequential decoding utilizing both the Fano and Zigangirov-Jelinek (ZJ) algorithms. For short constraint length codes we are primarily interested in the bit error probability performance parameterized by the fading channel parameters. For longer constraint length codes interest will center on the effect of the fading channel parameters on the computational requirements of both the Fano and ZJ algorithms. In either case the effects of simple block interleaving in combatting the memory of the channel is thoroughly explored. The approach is analytic where possible otherwise resort is made to digital computer simulation.

Modestino, J. W.↗
NASA NTRS1974

Performance of noncoherent MFSK channels with coding

Computer simulation of data transmission over a noncoherent channel with predetection signal-to-noise ratio of 1 shows that convolutional coding can reduce the energy requirement by 4.5 dB at a bit error rate of 0.001. The effects of receiver quantization and choice of number of tones are analyzed; nearly optimum performance is attained with eight quantization levels and sixteen tones at predetection S/N ratio of 1. The effects of changing predetection S/N ratio are also analyzed; for lower predetection S/N ratio, accurate extrapolations can be made from the data, but for higher values, the results are more complicated. These analyses will be useful in designing telemetry systems when coherence is limited by turbulence in the signal propagation medium or oscillator instability.

Butman, S. A.↗