Engineering topics

Weber, W. J., III

Publications and source records attributed to Weber, W. J., III.

11 records

NASA NTRS1987

Antenna Technology Shuttle Experiment (ATSE)

Numerous space applications of the future will require mesh deployable antennas of 15 m in diameter or greater for frequencies up to 20 GHz. These applications include mobile communications satellites, orbiting very long baseline interferometry (VLBI) astrophysics missions, and Earth remote sensing missions. A Lockheed wrap rip antennas was used as the test article. The experiments covered a broad range of structural, control, and RF discipline objectives, which is fulfilled in total, would greatly reduce the risk of employing these antenna systems in future space applications. It was concluded that a flight experiment of a relatively large mesh deployable reflector is achievable with no major technological or cost drivers. The test articles and the instrumentation are all within the state of the art and in most cases rely on proven flight hardware. Every effort was made to design the experiments for low cost.

Freeland, R. E.↗

NASA NTRS1982

Land Mobile Satellite Systems (LMSS): Single aperture system design

The large size of the UHF antenna on the satellite provides two advantages with respect to the Land Mobile Satellite System design. It provides tremendous gain for both transmitting and receiving at the satellite and enables very simple, low power equipment in the mobile vehicle. The UHF antenna for this study was selected to have an aperture of 55 meters. This implies a beamwidth of roughly 0.45 degree and requires 87 beams to cover the continental United States.

Weber, W. J., III↗

NASA NTRS1979

Decision-directed automatic gain control

Logic circuitry determines whether gain fluctuation are result of signal-strength changes or of atypical strings of like data symbols. Automatic Gain Control (AGC) system provides tight control that is independent of short-term, average, received signal energy and has negligible degrading effect on probability of error for signal.

Weber, W. J., III↗

NASA NTRS1979

Eliminating ambiguity in digital signals

Multiamplitude minimum shift keying (mamsk) transmission system, method of differential encoding overcomes problem of ambiguity associated with advanced digital-transmission techniques with little or no penalty in transmission rate, error rate, or system complexity. Principle of method states, if signal points are properly encoded and decoded, bits are detected correctly, regardless of phase ambiguities.

Weber, W. J., III↗

NASA NTRS1979

A decision-directed network for dual-polarization crosstalk cancellation

Frequency reuse in the specific form of dual-polarized microwave communication systems has grown in importance in recent years as a practical means of radio spectrum conservation. Ideally the capacity of a given frequency allocation can be doubled through dual-polarization. However, hardware imperfections and propagation effects, particularly rain depolarization, prevent the achievement of this doubling without severe system performance degradation. A decision-directed cross-polarization correction network is presented whose operation depends on only simple base-band signal processing. No pilot tones or frequency offsets are required. The loop can work with any two-dimensional signal set for digital data transmission. The loop has been experimentally verified and provides a means of doubling the data capacity with little performance degradation.

Weber, W. J., III↗

NASA NTRS1978

A bandwidth compressive modulation system using multi-amplitude minimum shift keying /MAMSK/

A bandwidth compressive modem making use of multi-amplitude minimum shift keying (MAMSK) has been designed and implemented in a laboratory environment at microwave frequencies. This system achieves a substantial bandwidth reduction over binary PSK and operates within 0.5 dB of theoretical performance. A number of easily implemented microwave transmitters have been designed to generate the required set of 16 signals. The receiver has been designed to work at 1 Mbit/s and contains the necessary phase tracking, AGC, and symbol synchronization loops as well as a lock detector, SNR estimator and provisions for differential decoding. This paper describes this entire system and presents the experimental results.

Weber, W. J., III↗

NASA NTRS1978

Differential encoding for multiple amplitude and phase shift keying systems

Because of the symmetry in most two-dimensional signal constellations, ambiguities exist at the receiver as to the exact phase orientation of the received signal set. In PSK systems, this ambiguity is resolved by the use of differential encoding. This paper presents differential encoding techniques which can be used with a variety of symmetric signal sets to remove their phase ambiguity. While not proven to be optimum, the techniques do have low performance penalties relative to the uncoded performance. The key to reducing the performance penalty is to use the minimum amount of differential encoding necessary to resolve the ambiguity. Examples of encoding techniques for several common signal constellations are given, including their performance penalties.

Weber, W. J., III↗

NASA NTRS1976

Viking X-band telemetry experiment

In order to uncover operational and design problems in the use of X-band by the 1977 Mariner Jupiter-Saturn mission and future spacecraft using the Deep Space Network, the Viking X-band telemetry experiment was conducted. The experiment was conducted during the months of December 1975 and January 1976. During each of the five successful passes, a periodic sequence (in lieu of ranging) was transmitted to the spacecraft and returned by the spacecraft transponder on both S- and X-bands. These telemetry-like signals were received, demodulated, and detected. From a variety of measurements at the station, four independent measurements were made of the received signal-to-noise ratio (SNR). These four SNRs were later compared with each other and the predicted SNR. The principal result of the experiment is that X-band telemetry works as expected. That is, the measured SNRs were consistent relative to each other and to the predicted values within the accuracy of the experiment.

Weber, W. J., III↗

NASA NTRS1976

Cascaded coherent tracking systems with time-varying channels

A performance analysis is presented of a two-way coherent tracking system (of the type used for spacecraft tracking and navigation) in which the transmitted signals have pass through linear time-varying channels. The performance of the system is characterized by the steady state probability density function of the reduced phase error process in the second tracking loop, the system considered consisting of two first-order phase locked loops in cascade. While the log normal channel, which arises in communication through planetary atmospheres, was used as the channel model, the results can be extended to other channels, such as the Rice and Rayleigh channels.

Weber, W. J., III↗

NASA NTRS1975

The use of TWT amplifiers in M-ary amplitude and phase shift keying systems

A great deal of research is presently being conducted to study modulation schemes in order to reduce the bandwidth of digital communications. These M-ary amplitude and phase shift keying (MAPSK) modulation systems place new requirements on TWT amplifiers in space applications in that the various signal sets usually include two or more discrete output power levels as well as discrete phases at each power level. This paper presents a variety of predistortion schemes to circumvent the TWT problems of AM distortion and AM-to-PM conversion in order to produce MAPSK signal sets. A feedback scheme is presented to correct for long-term phase and amplitude drifts in the transmitter. A complete transmitter is proposed using predistortion to produce the desired MAPSK signal set at very high data rates including the possible use of a feedback stabilization scheme.

Weber, W. J., III↗

NASA NTRS1974

Decision-directed automatic gain control for MAPSK systems

An automatic gain control (AGC) loop is presented for use with M-ary amplitude and phase shift keying (MAPSK) systems. The gain control amplifier is regulated by an error signal formed by the difference between the estimated amplitude level and the received amplitude level. The AGC performance is thus independent of the short-term average received signal energy. AGC loop analysis and simulation is presented for M-ary amplitude shift keying and quadrature amplitude shift keying. The AGC is shown to have a negligible degradation on the symbol probability of error for most practical cases. A generalized AGC for an arbitrary MAPSK system is presented.

Weber, W. J., III↗