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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

A study of time dissemination via satellite in India

A simultaneous two way clock synchronization experiment between three Earth stations, was performed and improvements over the technique earlier attempted in which transmit/receive roles of the two stations were alternated at regular intervals, were studied. Time signals via two modes high frequency and satellite were critically monitored and analyzed. This time format was modified to include the additional information about time of the day in year, month, day, hour, minute and second as well as DUT1 in BCD code and was disseminated via the satellite. These signals were decoded, displayed and studied. Some preliminary work on time transfer via TV using direct satellite broadcast, was also conducted.

Mathur, B. S.↗
NASA NTRS1980

Coding for spread spectrum packet radios

Packet radios are often expected to operate in a radio communication network environment where there tends to be man made interference signals. To combat such interference, spread spectrum waveforms are being considered for some applications. The use of convolutional coding with Viterbi decoding to further improve the performance of spread spectrum packet radios is examined. At 0.00001 bit error rates, improvements in performance of 4 db to 5 db can easily be achieved with such coding without any change in data rate nor spread spectrum bandwidth. This coding gain is more dramatic in an interference environment.

Omura, J. K.↗
NASA NTRS1980

Alternate symbol inversion for improved symbol synchronization in convolutionally coded systems

Inverting alternate symbols of the encoder output of a convolutionally coded system provides sufficient density of symbol transitions to guarantee adequate symbol synchronizer performance, a guarantee otherwise lacking. Although alternate symbol inversion may increase or decrease the average transition density, depending on the data source model, it produces a maximum number of contiguous symbols without transition for a particular class of convolutional codes, independent of the data source model. Further, this maximum is sufficiently small to guarantee acceptable symbol synchronizer performance for typical applications. Subsequent inversion of alternate detected symbols permits proper decoding.

Simon, M. K.↗
NASA NTRS1980

A large-scale interactive one-dimensional array processing system

The work describes a scientist/user oriented interactive program for processing one-dimensional arrays. It is shown that the program is oriented toward processing spectrophotometric astronomical data and can also be used for general I-D array processing. Further, the program has totally free format input with a sophisticated decoding capability which can cope with typographical plus other possible mistakes. Finally, a description of the program is given to provide information on implementing a large-scale data-reduction facility.

Clark, R. N.↗
NASA NTRS1980

High-altitude resolution stratospheric measurements with the Arecibo 430-MHz radar

Instrumental modification at the Arecibo Observatory are described, which make possible a resolution of 150 m, the maximum allowed by the transmitter bandwidth. Attention is given to digital decoding, the radar controller, and the fact that a complementary coded pulse scheme generating no pulse side lobes has been used for the first time. It is noted that power spectra of the signals corresponding to 256 altitudes are evaluated on line by means of an array processor. A profile of the E-W component of the wind as a function of altitude has been observed, as well as the discrete layered structure of turbulence, especially at higher altitudes; in addition, the maximum range at which an echo has been detected is about 31 km. The results obtained show the importance for stratospheric studies of high resolution when using radar to obtain wind information from the Doppler shift of turbulence echoes.

Woodman, R. F.↗
NASA NTRS1980

Eight microprocessor-based instrument data systems in the Galileo Orbiter spacecraft

Instrument data systems consist of a microprocessor, 3K bytes of Read Only Memory and 3K bytes of Random Access Memory. It interfaces with the spacecraft data bus through an isolated user interface with a direct memory access bus adaptor, and/or parallel data from instrument devices such as registers, buffers, analog to digital converters, multiplexers, and solid state sensors. These data systems support the spacecraft hardware and software communication protocol, decode and process instrument commands, generate continuous instrument operating modes, control the instrument mechanisms, acquire, process, format, and output instrument science data.

Barry, R. C.↗
NASA NTRS1980

Capacity, cutoff rate, and coding for a direct-detection optical channel

It is shown that Pierce's pulse position modulation scheme with 2 to the L pulse positions used on a self-noise-limited direct detection optical communication channel results in a 2 to the L-ary erasure channel that is equivalent to the parallel combination of L completely correlated binary erasure channels. The capacity of the full channel is the sum of the capacities of the component channels, but the cutoff rate of the full channel is shown to be much smaller than the sum of the cutoff rates. An interpretation of the cutoff rate is given that suggests a complexity advantage in coding separately on the component channels. It is shown that if short-constraint-length convolutional codes with Viterbi decoders are used on the component channels, then the performance and complexity compare favorably with the Reed-Solomon coding system proposed by McEliece for the full channel. The reasons for this unexpectedly fine performance by the convolutional code system are explored in detail, as are various facets of the channel structure.

Massey, J. L.↗
NASA NTRS1981

New results on antenna arraying, part 1

Baseband combining with and without combined carrier referencing for antenna arrays are compared under two scenarios for the Voyager 2 Uranus encounter. The combined carrier reference scheme is estimated to outperform the baseband only scheme by less than 0.3 dB E (sub b)/N (sub 0) at a bit error probability of 0.005. These results were attained both with mathematical modeling and software Viterbi decoder simulations.

Deutsch, L. J.↗
NASA NTRS1981

Decommutator patchboard verifier

A system for automatically verifying the connections between terminals of a patchboard is described. The system includes a back plane having a plurality of plugs corresponding to the pins of the patchboard. A number of decoders are connected to the plugs of the back plane so that a signal can be sequentially applied to each plug of the back plane under control of a stepping register and a control circuit. A plurality of data selectors are also connected to the plugs of the back plane and under control of a second external register. This control circuit sequentially makes connections between an output circuit and the plugs of the back plane so as to patch the signal applied to a respective plug through a patch connection to the output circuit. The precise locations of the patches on the patchboard can be identified and compared with previously stored information in a memory unit.

Brunson, J. W.↗
NASA NTRS1981

The effect of noisy carrier reference on telemetry with baseband arraying

Imperfect carrier reference causes a degradation in detection performance in coherent communication systems. A measure of this degradation is the radio loss, which is the amount of increase in data signal to noise ratio (SNR) per bit required to achieve the same bit error rate when carrier reference is perfect. Performance analysis and numerical results are obtained for the Voyager high rate telemetry link with maximum likelihood convolutional decoding. The arraying of antennas provides not only improved performance due to an increase in effective antenna aperture, but also a decrease in radio loss with respect to a single antenna. This telemetry link performance improvement is a function of the carrier loop SNR and data bit error rate. When the carrier loop SNR's are low, it provides a significant improvement in the telemetry link performance since the decrease in radio loss with respect to a single station is substantial.

Divsalar, D.↗
NASA NTRS1981

The effects of receiver tracking phase error on the performance of the concatenated Reed-Solomon/Viterbi channel coding system

Analytical and experimental results are presented of the effects of receiver tracking phase error, caused by weak signal conditions on either the uplink or the downlink or both, on the performance of the concatenated Reed-Solomon (RS) Viterbi channel coding system. The test results were obtained under an emulated S band uplink and X band downlink, two way space communication channel in the telecommunication development laboratory of JPL with data rates ranging from 4 kHz to 20 kHz. It is shown that, with ideal interleaving, the concatenated RS/Viterbi coding system is capable of yielding large coding gains at very low bit error probabilities over the Viterbi decoded convolutional only coding system. Results on the effects of receiver tracking phase errors on the performance of the concatenated coding system with antenna array combining are included.

Liu, K. Y.↗
NASA NTRS1980

Joint map data/phase sequence estimation for trellis phase codes

A unified estimation procedure is formulated for the coherent detection of trellis phase codes, including partial response FM codes and multi-h codes. The unknown carrier phase is imbedded in the system state vector, and by simple augmentation of the decoder's state space, joint data and carrier estimation may be accomplished. The procedure is general to all trellis phase codes analyzed thus far. The effect of a static phase error on detection performance is also treated.

Wilson, S. G.↗
NASA NTRS1980

Universal delta modulation receivers using the bit slice approach

The architecture of programmable processors is described, with particular reference to the design of a programmable delta-modulation decoder capable of receiving DM encoders employing different schemes. A switching receiver implementation, allowing communication between different DM algorithms, is designed and shown.

Apelewicz, T.↗
NASA NTRS1980

The impact of pulsed RFI on the coded BER performance of the nonlinear satellite communication channel

Communication links from low orbiting spacecraft to a central ground station, via satellite repeater, may be susceptible to significant performance degradation if high-level, pulsed radio frequency interference (RFI) must be contended with. The present paper examines the coded bit error rate (BER) performance of a communication system in which BPSK modulation is employed, pulsed CW or pulsed noise RFI is present, and the transponder contains a nonlinearity characterized by arbitrary AM/AM and AM/PM characteristics. A major result of this paper is an analytic method for determining soft decision statistics of the receiver demodulator output, in which pulsed RFI effects are accounted for. It is demonstrated that this result can be analytically applied to the approximate determination of the BER at the output of a Viterbi decoder, when convolutional coding is employed.

Weinberg, A.↗
NASA NTRS1981

A bit transition density encoder for the Space Shuttle 2 MHz data channel

As a result of several unique factors, the 2 MHz data channel which carries science data from the various experiments aboard the Space Shuttle through the Tracking and Data Relay Satellite System (TDRSS) lacks sufficient bit transition density to satisfy the bit synchronizer requirements at the ground station. It is noted that six encoding techniques were examined, and all but one, a PN cover sequence (Reset Bit Scrambler) were dismissed because of incompatibility with system constraints. The rationale behind the particular PN sequence chosen, the way it was truncated, and the new sequence properties are presented. The properties of the raw data stream are analyzed, as are the encoded sequence properties, including transition density. Diagrams of the encoder and decoder are included.

Schoggen, W. O.↗
NASA NTRS1981

Asynchronous multiple-access channel capacity

The capacity region for the discrete memoryless multiple-access channel without time synchronization at the transmitters and receivers is shown to be the same as the known capacity region for the ordinary multiple-access channel. The proof utilizes time sharing of two optimal codes for the ordinary multiple-access channel and uses maximum likelihood decoding over shifts of the hypothesized transmitter words.

Cover, T. M.↗
NASA NTRS1981

Radiographic applications of spatial frequency multiplexing

The application of spacial frequency encoding techniques which allow different regions of the X-ray spectrum to be encoded on conventional radiographs was studied. Clinical considerations were reviewed, as were experimental studies involving the encoding and decoding of X-ray images at different energies and the subsequent processing of the data to produce images of specific materials in the body.

Macovski, A.↗
NASA NTRS1981

Memory-based frame synchronizer

A frame synchronizer for use in digital communications systems wherein data formats can be easily and dynamically changed is described. The use of memory array elements provide increased flexibility in format selection and sync word selection in addition to real time reconfiguration ability. The frame synchronizer comprises a serial-to-parallel converter which converts a serial input data stream to a constantly changing parallel data output. This parallel data output is supplied to programmable sync word recognizers each consisting of a multiplexer and a random access memory (RAM). The multiplexer is connected to both the parallel data output and an address bus which may be connected to a microprocessor or computer for purposes of programming the sync word recognizer. The RAM is used as an associative memory or decorder and is programmed to identify a specific sync word. Additional programmable RAMs are used as counter decoders to define word bit length, frame word length, and paragraph frame length.

Stattel, R. J.↗