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Computer program, which predicts effects of optical aberrations on transmitters and receivers, includes effects of Gaussian pupil functions and utilizes algorithm that permits specification of number and location of output points for computed spread function results.
A technique for measuring velocity, referred to as a Delta Doppler technique, was presented. This technique determines scattering source velocities by measuring the difference in Doppler shifts of two different frequencies. By transmitting the two frequencies along the same path, a moving fringe pattern is established such that a nonmoving scatterer at the sensing volume would see an intensity variation exactly equal to the difference in the transmitted frequencies. If the particle has a velocity component along an axis which bisects the angle formed by the transmitter and receiver axes, a Doppler shift in the difference frequency can be measured and the velocity component computed. The frequency measured would correspond to the difference in Doppler frequencies that two laser Doppler velocimeters using separate frequencies (the same frequencies as used previously) would have measured, thus the term Delta Doppler.
A preliminary feasibility study of the microwave multi-applications payload (MMAP) system for the Spacelab has been carried out. The initial objectives of this study have been to determine the minimum equipment requirements of the MMAP and the feasibility of placing the numerous large aperture antennas in the Spacelab. The study was begun by reviewing the experimental objectives and techniques and determining areas of commonality. Emphasis was given to the determination of common RF equipment requirements. These requirementers were considered after agreement among the experiments had been reached on limiting the number of frequencies to be used in the system. This was done so that the number of antennas, transmitters, and receivers could be minimized. The electronics system block diagram and the antenna configurations were considered in some details. It was concluded that the MMAP is feasible and can be an economical method of achieving a large number of experimental goals.
Experiments which are under consideration for possible flight on the Space Shuttle in the 1980's to aid in the development of new communications techniques are outlined. A millimeter wave communications experiment for the study of propagation above 10 GHz and an electromagnetic environment experiment for the characterization of fields in orbit are planned to provide information needed for effective spectrum conservation. An antenna range experiment is being studied to provide an in-orbit far-field RF signal source for measuring ground antenna patterns and gain over a wide frequency range. Additional experiments will be intended to test spacecraft technology developments, including such communications system components as large phased array antennas and solid state transmitters and receivers.
Error rate of RF digital communication link is measured at low signal-to-noise ratios using this technique. In this method, identical pseudo-random sequences of pulses are generated at transmitter and receiver of link. However, comparison of two sequences at receiver involves integration that makes bit-tracking threshold depend on length of pseudo-random sequence, rather than on actual rate of transmission of individual bits.
The application of synthetic aperture radar (SAR) in monitoring and managing earth resources is examined. Synthetic aperture radars form a class of side-looking airborne radar, often referred to as coherent SLAR, which permits fine-resolution radar imagery to be generated at long operating ranges by the use of signal processing techniques. By orienting the antenna beam orthogonal to the motion of the spacecraft carrying the radar, a one-dimensional imagery ray system is converted into a two-dimensional or terrain imaging system. The radar's ability to distinguish - or resolve - closely spaced transverse objects is determined by the length of the pulse. The transmitter components receivers, and the mixer are described in details.
Wrist strap with copper strips is used as coupler of radio-frequency energy. Wires at midpoints of strips lead to radio-frequency device that acts as transmitter or receiver. Varying widths of copper strips produces somewhat different characteristics.
Airspace between high-power microwave transmitters and receivers is dangerous to people, equipment, or birds flying in area. Hazard is reduced or eliminated by subsystem that monitors area and interrupts or alters microwave transmission during accidental intrusions.
Detailed discussions are presented of the measures taken on the Communications Technology Satellite (CTS or Hermes) which provide protection against the effects of spacecraft charging. These measures include: a comprehensive grounding philosophy and implementation; provision of command and data line transmitters and receivers for transient noise immunity; and a fairly restrictive EMI specification. Ground tests were made on materials and the impact of these tests on the CTS spacecraft is described. Hermes, launched on 17 January 1976 on a 2914 Delta vehicle, has successfully completed 10 months of operations. Anomalies observed are being assessed in relation to spacecraft charging, but no definite correlations have yet been established. A list of conclusions with regard to the CTS experience is given and recommendations for future spacecraft are also listed.
S-band communication components include low-pass filter, diplexer, and transmit output filter, which prevent radiation or coupling of unwanted transmitter spurious outputs and to provide isolation while transmitter and receiver operate simultaneously.
A millimeter-submillimeter transmitter and receiver system for propagation studies with an optically pumped laser as a source and a quasi-optical superheterodyne receiver is described. Topics are discussed in the areas of extended microwave techniques, quasi-optical devices and methods, radiometry, and measurements of importance to spectroscopy and propagation. Measurements obtained with operational systems are highlighted and their significance to physical questions and future system applications discussed.
Some applications and properties of free-space power transmission are examined. Among the properties discussed are: no mass, either in the form of wire conductors or ferrying vehicles, is needed between the source of energy and the point of consumption; energy can be transferred at the velocity of light; no loss of energy in transferring energy throught the vacuum of space; and the energy transfer is insensitive to a difference in the gravitational potential of the transmitter and receiver locations. Applications discussed include the Solar Power Satellite, high altitude platforms for communications and remote sensing. Also discussed are rectennas and retrodirective arrays. Finally, an expression yielding minimum cost is presented.
Device used in mixers, modulators, and function generators provides stable sine-wave signal compatible with both integrated circuits and discrete-component assemblies. Oscillator's frequency is tunable over narrow band about design value. Frequency range, stability, linearity, and low power drain of device are suited to communications receivers and transmitters and digital microprocessors, computers, and displays. Circuit simplicity allows for easy monolithic construction.
Research and development needs for millimeter-wave space communication systems are presented. Assumed propagation fade statistics are investigated along with high data rate diversity link and storage. The development of reliable ferrite switches, and high performance receivers and transmitters is discussed, in addition to improved tolerance of dish and lens fabrication for the antennas. The typical cost for using a simplex voice channel via a high capacity 40/50 GHz satellite is presented.
An investigation carried out by the Georgia Institute of Technology that quantified the error contributors associated with both far-field and near-field antenna measurement concepts is discussed. Measurement errors are quantified into four sources: antenna range, structural/environmental, transmitter, and receiver. The study is carried out so as to control error sources to yield an overall gain uncertainty of plus or minus 0.04 dB. With regard to antenna positioners, it is found that a small angle positioner provides highly accurate scan capability over a plus or minus 1.5 deg sector for the purpose of beam integration. With regard to near-field measurement techniques, it is found that with adequate probe calibration and precision mechanical scanning, full 30 by 30 m mechanical module antenna measurements may be performed. The performance and relative cost considerations between planar near-field and conventional far-field methods indicate that the overall costs are roughly the same.
A program initiated by NASA was formed to address some key earth resources synthetic aperture radar (SAR) needs. Major steps in the SAR system development are summarized, and the objective of the advanced SAR (ASAR) program is defined. The program rationale and specific objectives are discussed, including system calibration, wide-band technique evaluation, a system resolution of 10 m or better at 45 deg, and an airborne system to cover four bands within 1 to 10 GHz. The ASAR conceptual system design, consisting of a modulator, transmitter, antenna, receiver, calibrator, and data system modules is also presented, and ASAR capability expansions are discussed. The program, scheduled for 1983, will provide a data source which is not available to applications investigations at this time, and will produce design approaches that are flexible and multimission to the maximum extent for application to a Shuttle or a freeflyer system.
The purpose of this paper is to present a model and associated analysis of the deleterious effects which both transmitter and receiver data clock time-base instability have on bit-error probability. The types of time-base instability modeled and analyzed include what is commonly referred to as bit jitter (epoch jitter), bit-jitter rate (time interval jitter), and data asymmetry. The bit-error probability is determined as a function of the statistical characteristics of the time-base instability, the data-transition density, the signal-to-noise ratio in the bit-rate bandwidth and the mechanizations of both the data-bit detector and bit synchronizer. The latter is manifested through the bit synchronizer's natural frequency and loop-damping parameters. Numerical results which indicate the sensitivity of the system bit-error rate to various parameters of the model are provided.
Pseudo noise (PN) spread spectrum systems require a very accurate alignment between the PN code epochs at the transmitter and receiver. This synchronism is typically established through a two-step algorithm, including a coarse synchronization procedure and a fine synchronization procedure. A standard approach for the coarse synchronization is a sequential search over all code phases. The measurement of the power in the filtered signal is used to either accept or reject the code phase under test as the phase of the received PN code. This acquisition strategy, called a single dwell-time system, has been analyzed by Holmes and Chen (1977). A synopsis of the field of sequential analysis as it applies to the PN acquisition problem is provided. From this, the implementation of the variable dwell time algorithm as a sequential probability ratio test is developed. The performance of this algorithm is compared to the optimum detection algorithm and to the fixed dwell-time system.