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NASA NTRS1981

Analysis of coherent satellite communication systems in the presence of interference and noise

The general analysis presented can be used to evaluate the performance of M-ary phase shift keying (MPSK) used over a nonlinear satellite channel in the presence of interference (CW tone and ISI) and channel noise. The numerical evaluation is based on the classical moment technique for approximating the probability distribution from moments of a random variable. The mathematical formulation of error probabilities of MPSK signals transmitted over nonlinear satellite repeaters is presented to include the effect of CW tone interference as well as the intersymbol interference. The approach is general and can be extended to include the effect of the synchronization.

Huang, T.-C.↗

NASA NTRS1981

Practical codes for photon communication

It is shown that even assuming a noiseless model for photon communication for which capacity (measured in nats/photon) is infinite, it is unlikely that a signaling efficiency of even 10 nats/photon could be achieved in practice. Pulse-position modulation plus Reed-Solomon coding is shown to yield practical results in the range of 2 to 3 nats/photon. In addition, a channel model appropriate for the study of noiseless photon communication, referred to as a photon channel, is presented. The use of q-ary pulse position modulation (q-PPM) on the photon channel is investigated. It is shown that q-PPM channel capacity is log q nats/photon, and performance curves (error probability versus signaling efficiency) are given for coded and uncoded q-PPM.

Mceliece, R. J.↗

NASA NTRS1982

Description of the Deep Space Network

The facilities, functions, and operations capabilities of the Deep Space Network are summarized. Network activities in support of planned future as well as ongoing deep space missions are described as are activities in support of radio astronomy experiments and the Astronomical Radio Interferometric Earth Surveying (ARIES) Network.

Renzetti, N. A.↗

NASA NTRS1982

Error probabilities in optical PPM receivers with Gaussian mixture densities

A Gaussian mixture density arises when a discrete variable (e.g., a photodetector count variable) is added to a continuous Gaussian variable (e.g., thermal noise). Making use of some properties of photomultiplier Gaussian mixture distributions, some approximate error probability formulas can be derived. These appear as averages of M-ary orthogonal Gaussian error probabilities. The use of a pure Gaussian assumption is considered, and when properly defined, appears as an accurate upper bound to performance.

Gagliardi, R. M.↗

NASA NTRS1981

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

It is shown that when Pierce's pulse-position modulation scheme with 2 to the L power positions (where L is some positive integer) is used on a self-noise-limited direct-detection optical communication channel, there results a (2 to the L power)-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 (1979) 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 NTRS1982

Differentially coherent detection of QASK for frequency-hopping systems. II - Performance in the presence of jamming

The performance of differentially coherent detection of frequency-hopped QASK in the presence of partial-band noise and partial-band multitone jamming is presented. In each case, the worst case jamming strategy is determined which consists of specifying the worst case partial-band fraction and the corresponding maximum average error probability. The results obtained are compared with those of M-ary FH-DPSK operating in the same jamming environment.

Simon, M. K.↗

NASA NTRS1982

EUV emission from Titan's upper atmosphere - Voyager 1 encounter

Most of the observed emission short of Lyman-alpha is shown to be accounted for by electron impact on N2 above 3600 km, in an analysis of Titan's EUV emission spectra obtained at the Voyager 1 encounter. It is determined that N2 is the major component of Titan's upper atmosphere, with 3900-km upper limit mixing ratios of NeI, ArI, CO, H2, and HI of 0.01, 0.06, 0.05, 0.06 and 0.1, respectively. Magnetospheric electron precipitation produces an average dayside electron density of about 3000/cu cm between 3600 and 4000 km, which is the region of bright limb emission, and magnetospheric electron impact dissociation of N2 generates an N atom escape rate of 3 x 10 to the 26th/sec from Titan's exosphere when Titan is within Saturn's magnetosphere.

Strobel, D. F.↗

NASA NTRS1982

Determination of semi-transparent cirrus cloud temperature from infrared radiances - Application to Meteosat

A bispectral technique was employed to determine the cirrus cloud-top temperature. Data were gathered from aircraft equipped with the ARIES radiometer, which on sixteen flights provided IR readings at 6.5 and 11.5 microns of cirrus tops. The flights were made on a NASA Convair to a maximum ceiling of 12,500 m. The brightness temperatures above the cirrus were found to be consistently colder, by up to 40 K, than the 11.5 microns window channel, and good spatial correlations were displayed over whole clouds. Effective emissivities are calculated for both channels and applied to Meteosat digital imagery. The subsequent derived temperatures were significantly colder than black-body temperatures, which is explained by taking into account the absorption of emitted radiation by water vapor.

Szejwach, G.↗

NASA NTRS1981

Trellis phase codes for power-bandwith efficient satellite communications

Support work on improved power and spectrum utilization on digital satellite channels was performed. Specific attention is given to the class of signalling schemes known as continuous phase modulation (CPM). The specific work described in this report addresses: analytical bounds on error probability for multi-h phase codes, power and bandwidth characterization of 4-ary multi-h codes, and initial results of channel simulation to assess the impact of band limiting filters and nonlinear amplifiers on CPM performance.

Wilson, S. G.↗

NASA NTRS1981

Coding tradeoffs for improved performance of FH/MFSK systems in partial band noise

Partial band noise jamming can severely degrade the performance of frequency-hopped, M-ary frequency-shift keyed communciation systems. This paper illustrates the tradeoffs between channel coding, diversity, and block orthogonal (MFSK) modulation as a means of overcoming the advantage of worst case, non-adaptive (as opposed to repeat-back) partial band jamming. For ease of computation, the analysis relies on exponentially tight error bounds, and is based on a noncoherent detection metric that requires jamming state information for each hop. A more robust, less complex receiver structure which eliminates the jamming knowledge requirement is shown to degrade performance less than 2-1/2 dB. The coding tradeoffs discussed in this report are exemplified in the design of a hypothetical 32 kb/s military frequency-hopped communication link.

Levitt, B. K.↗

NASA NTRS1982

Development of a 1m-normal-incidence-EUV-Telescope

A brief description is given of the 1m-EUV-Telescope and its focal plane instrumentation, namely an EUV spectrometer and six EUV/FUV photometers. The telescope is scheduled for launch on an Aries rocket on June 17, 1982. The principal goals are the white dwarf HZ43 and a photometric scan across the sky in an area of the sky where 21 cm line observations reveal a steep density gradient. The optical bench of the telescope is a cylinder made of a graphite epoxy compound. Despite its low specific weight, the bench shows an excellent mechanical performance, with an elasticity modulus of approximately 70,000 N/cu mm. It is pointed out that by carefully combining layers with different winding angles of the carbon fiber, the thermal expansion along the cylinder axis is almost negligible, even under severe thermal loads

Grewing, M.↗

NASA NTRS1983

Integrating computer programs for engineering analysis and design

The design of a third-generation system for integrating computer programs for engineering and design has been developed for the Aerospace Vehicle Interactive Design (AVID) system. This system consists of an engineering data management system, program interface software, a user interface, and a geometry system. A relational information system (ARIS) was developed specifically for the computer-aided engineering system. It is used for a repository of design data that are communicated between analysis programs, for a dictionary that describes these design data, for a directory that describes the analysis programs, and for other system functions. A method is described for interfacing independent analysis programs into a loosely-coupled design system. This method emphasizes an interactive extension of analysis techniques and manipulation of design data. Also, integrity mechanisms exist to maintain database correctness for multidisciplinary design tasks by an individual or a team of specialists. Finally, a prototype user interface program has been developed to aid in system utilization.

Wilhite, A. W.↗

NASA NTRS1983

The correlated X-ray and optical time variability of TT Arietis

The results of Einstein X-ray observation and simultaneous optical observations of the cataclysmic variable TT Arietis extending over several binary orbits are reported. Evidence is found for correlations between the X-ray and optical variability of TT Ari on three distinct time scales: the 3.3 hour variability that has been associated with the orbital period of the star, the about 1000 s time scale of the irregular flickering activity, and the time scale of quasi-coherent oscillations which have periods of order 10 s. There is a modulation of the X-ray flux with a period consistent with the orbital period of approximately 200 minutes, but there is no apparent modulation of the X-ray spectrum. The optical flux is modulated with a similar period and may lag the X-ray modulation by about 0.1 in phase. The X-ray spectrum is well fitted by a thermal bremsstrahlung plus Gaunt factor model. The results suggest that the hard X-ray emission may be produced in a corona above and below the inner accretion disk.

Jensen, K. A.↗

NASA NTRS1983

Experimental limits of the extreme ultraviolet background

Photometric observations of the diffuse extreme ultraviolet background with two photometers having bandpasses of 750-940 A and 1040-1080 A are reported. The payload, which was flown aboard an ARIES sounding rocket in June 1982, is described, including the electron detectors, filters, and calibration. The operation of the probe during the experiment, including its motions, are described. The primary experiment involved spectroscopic observation of the hot white dwarf HZ43. The photometer count rate is shown and the measurements of the diffuse background are compared with theoretical predictions. Despite the lower limits obtained using a narrowband detector, the measurements are not sensitive enough to draw any relevant astrophysical conclusions.

Wulf-Mathies, C.↗

NASA NTRS1983

User manual of the CATSS system (version 1.0) communication analysis tool for space station

The Communication Analysis Tool for the Space Station (CATSS) is a FORTRAN language software package capable of predicting the communications links performance for the Space Station (SS) communication and tracking (C & T) system. An interactive software package was currently developed to run on the DEC/VAX computers. The CATSS models and evaluates the various C & T links of the SS, which includes the modulation schemes such as Binary-Phase-Shift-Keying (BPSK), BPSK with Direct Sequence Spread Spectrum (PN/BPSK), and M-ary Frequency-Shift-Keying with Frequency Hopping (FH/MFSK). Optical Space Communication link is also included. CATSS is a C & T system engineering tool used to predict and analyze the system performance for different link environment. Identification of system weaknesses is achieved through evaluation of performance with varying system parameters. System tradeoff for different values of system parameters are made based on the performance prediction.

Tsang, C. S.↗

NASA NTRS1984

Timing performance of phased-locked loops in optical pulse position modulation communication systems

An optical digital communication system requires that an accurate clock signal be available at the receiver for proper synchronization with the transmitted signal. Phase synchronization is especially critical in M-ary pulse position modulation (PPM) systems where the optimum decision scheme is an energy detector which compares the energy in each of M time slots to decide which of M possible words was sent. Timing errors cause energy spillover into adjacent time slots (a form of intersymbol interference) so that only a portion of the signal energy may be attributed to the correct time slot. This effect decreases the effective signal, increases the effective noise, and increases the probability of error. A timing subsystem for a satellite-to-satellite optical PPM communication link is simulated. The receiver employs direct photodetection, preprocessing of the detected signal, and a phase-locked loop for timing synchronization. The variance of the relative phase error is examined under varying signal strength conditions as an indication of loop performance, and simulation results are compared to theoretical calculations.

Lafaw, D. A.↗

NASA NTRS1983

Infrared observations of RS CVn stars

The paper presents infrared photometry of the RS CVn binary stars AR Lac (1.2-10 microns) and MM Her (1.2-3.5 microns) as they egressed from their primary and secondary eclipses; of the eclipsing systems RS CVn and Z Her at maximum light (1.2-10 microns) and of the non-eclipsing systems UX Ari and HR 1099 (1.2-10 microns). An analysis of these and published V data based on flux ratio diagrams (linear analogues of color-color diagrams) shows that G and K stars supply the infrared light of these systems. In AR Lac, the combined light of a G5-K0 subgiant and either a late F dwarf or an early F subgiant can account for the observed visual and infrared light curves. None of these systems shows infrared emission from circumstellar matter. This result is simply understood: dust grains would not be expected to form in the physical conditions surrounding the subgiant, and the corona and chromosphere (whose properties have been deduced from spectroscopic X-ray observations) should not produce appreciable infrared emission.

Berriman, G.↗

NASA NTRS1982

Demodulation of PPM for Reed-Solomon coded optical space channel

The paper proposes a delta-max demodulator for Reed-Solomon coded M-ary PPM modulation over optical (laser) channels including satellite-to-satellite and deep-space-vehicle-to-relay-satellite communications. This demodulator is compared with the threshold demodulator currently in use, and the two demodulators are shown to have identical performance in the absence of background noise. The delta-max demodulator is found to outperform the threshold demodulator as the intensity of background noise increases.

Divsalar, D.↗