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Consideration of the problem of selecting a low-pass sampling bandwidth for the digital mechanization of a matched-filter bit-synchronizer combination. An attempt is made to determine how large a data rate can be provided for a fixed sampling rate (limited by hardware considerations on the sampling device) without paying an excessive penalty in SNR performance.
Preemphasis schedules are given for 11 constant-bandwidth FM subcarriers modulating an S-band transmitter at various receiver SNRs (9 dB, 15 dB and 25 dB). The criterion for establishing these preemphasis curves is the achievement, at various receiver IF SNR, of equal receiver output SNR for all channels. The empirically derived results are compared with a simplified, analytically derived schedule and the primary differences are explained. The S-band preemphasis schedule is also found to differ from the lower frequency VHF case.
A modified complementary filtering technique for estimating aircraft roll rate was developed and flown in a research helicopter to determine whether higher gains could be achieved. Use of this technique did, in fact, permit a substantial increase in system frequency bandwidth because, in comparison with first-order filtering, it reduced both noise amplification and control limit-cycle tendencies.
In a joint USAF-NASA Program, Lewis Research Center is carrying out an efficiency improvement program on traveling wave tubes for use in electronic counter measures by applying multistage depressed collector (MDC) and spent beam refocusing techniques. In the analytic part of the effort, three-dimensional electron trajectories are computed throughout the TWT. On the experimental side, tube performance is evaluated first without the MDC; then, the spent beam is analyzed for symmetry, circularity, and velocity spread. The three-dimensional theory predicts a MDC-efficiency, at mid-band, of 81 per cent for a 2-stage MDC with symmetric, circular, and optimally refocused beams and 85.5 per cent for a 4-stage MDC. Experimental results to date have yielded MDC efficiencies of a minimum of 81 and 83 per cent for a 2- and 4-stage MDC, respectively, across a one-octave bandwidth of a 4.8 to 9.6 GHz 330-to-550-W TWT.
Synchronous switching circuit reduces bandwidth and improves sensitivity of communications receiver. With modified receiver, signals 35 db below level can be detected.
A study was carried out to identify the optimum uplink and downlink frequencies for audio-bandwidth channels for use by a satellite system distributing social services. The study considered functional-user-need models for five types of social services and identified a general baseline system that is appropriate for most of them. Technical aspects and costs of this system and of the frequency bands that it might use were reviewed, leading to the identification of the 620-790 MHz band as a perferred candidate for both uplink and downlink transmissions for nonmobile applications. The study also led to some ideas as to how to configure the satellite system.
New systems have been developed for measuring the average impulse response, the pulse-height spectrum, the transit-time statistics as a function of signal level, and the dark-count spectrum of gigahertz bandwidth photomultipliers. Measurements showed that the 0.53 microns pulse used as an optical test source had a 30 picoseconds and less than 70 ps pulse width. Calibration data showed the system resolution to be less than 20 ps for root mean square transit-time measurements. Test data for a static crossed-field photomultiplier showed 2-photoelectron resolution and less than 30-ps time jitter over the 1- to 100-photoelectron range.
The downlink signal from spacecraft in superior solar conjunction phases suffers a great reduction in signal-to-noise ratio. Responsible in large part for this effect is the line broadening of the signal spectrum. An analytic empirical expression was developed for spectral bandwidth as a function of heliocentric distance from 1 to 20 solar radii. The study is based on spectral broadening data obtained from the superior conjunctions of Helios 1 (1975), Helios 2 (1976), and Pioneer 6 (1968). The empirical fit is based in part on a function describing the electron content in the solar corona.
A million (two to the 20th power) channel, 300 MHz bandwidth, digital spectrum analyzer was considered. The design, fabrication, and maintenance philosophy of the modular, pipelined, fast fourier transform (FFT) hardware are described. The spectrum analyzer will be used to examine the region from 1.4 GHz to 26 GHz for radio frequency interference which may be harmful to present and future tracking missions of the Deep Space Network. The design has application to the search for extraterrestrial intelligence signals and radio science phenomena.
General relations for the energy spectral densities of nonperiodic signals constrained in bandwidth in nonlinear reactances are derived. It is assumed that the charge-voltage characteristics of the reactances are polynomials. In addition certain conditions for the frequency bands of the signals must be met. By contrast with the Manley-Rowe relations integrals with respect to frequency appear instead of the ratios power over frequency. The integrands are energy spectral densities divided by frequency. For parametric devices inequalities can be derived for the ratios of the energy levels in the different circuits and the limiting frequencies of the energy spectral densities. With these inequalities it is possible to determine limits for the energy levels.
A study is made of the effects of quantization of the radar returns transmitted from aircraft or spacecraft employing a synthetic aperture radar system. The study is based on the output images obtained after one-bit, two-bit, and eight-bit quantizations and comparing the results to ground truth. In this way the degradation resulting from data or bandwidth reduction is determined. Quantization is evaluated in terms of crater scene, number of looks, and transmission error rate. It is found that two-bit quantization of raw radar data from homogeneous scenes processed to 32 looks yields nearly all the details of the original. One-bit quantization of raw radar data from homogeneous scenes processed to 32 looks yields a good visual representation of the scene but some fine detail is lost and the absolute reflectivity level is not reliable. Image quality is observed to improve with more looks and video and intermediate frequency quantization are not distinguishable even for one-bit quantizations. Image quality is not influenced by bit error rates less than about 2 to the -7th power.
An interleaved, solid state expanded memory for a 100 MHz bandwidth waveform recorder is described. The memory development resulted in a significant increase in the storage capacity of a commercially available recorder. The motivation for the memory expansion of the waveform recorder, which is used to support in-flight measurement of the electromagnetic characteristics of lightning discharges, was the need for a significantly longer data window than that provided by the commercially available unit. The expanded recorder provides a data window that is 128 times longer than the commercial unit, while maintaining the same time resolution, by increasing the storage capacity from 1024 to 131 072 data samples. The expanded unit operates at sample periods as small as 10 ns. Sampling once every 10 ns, the commercial unit records for about 10 microseconds before the memory is filled, whereas, the expanded unit records for about 1300 microseconds. A photo of the expanded waveform recorder is shown.
The channel capacity of an optical communications link utilizing direct photon detection can be substantially larger than the heterodyne detection quantum limit. In the limit of a noiseless channel the capacity per received photon can even be infinite. In most communications systems the real constraint is to deliver a certain throughput capacity measured in nats/second. The current investigation has the objective to show that due to physical limitations on the minimum time resolution which can be achieved, and for any given throughput capacity in nats/second, there exists an optimum PPM word size which maximizes the 'power efficiency capacity' expressed in nats/photon. Such an optimization will allow the system-designer to minimize the amount of power needed to achieve the desired throughput without violating his bandwidth constraint.
Bandwidth required to transmit closed circuit television image reduced by transmitting small portion at high resolution and remainder at low resolution. High-resolution portion centered in viewer's gaze so entire image seems to be of high resolution.
The spectral bands of the Multispectral Scanner and Thematic Mapper subsystems of Landsat-4 and Landsat-D' have been analyzed using a bandwidth normalization technique based on analysis of the moments of the spectral responsitivity curves. The results include the effective wavelength, the bandpass, the wavelength limits, and the normalized responsivity for each spectral channel. In addition, temperature coefficients for TM PF Channel 6 have been derived. The moments normalization method employed yields sensor parameters whose derivation is independent of source characteristics (i.e., incident solar spectral irradiance, atmospoheric transmittance, or ground reflectance). The errors expected using these parameters are lower than those expected using other normalization methods.
The objective of this study was to investigate bandwidth concepts for deriving rotorcraft handling-qualities criteria from data obtained in two simulator experiments conducted at the Aeromechanics Laboratory. The first experiment was an investigation of the effects of helicopter vertical-thrust-response characteristics on handling qualities; the second experiment investigated the effects of helicopter yaw-control-response characteristics. In both experiments, emphasis was on low-speed Nap-of-the-Earth (NOE) tasks.
A burst-mode coded trellis modulation system is discussed which is capable of 200-Mbit/s information rate transmission over nonlinear satellite transponders whose center frequencies are separated by 100 MHz. An overall real bandwidth efficiency of 2 bit/s per Hz of the allocated spectrum is projected. Convolutional codec and 8-PSK modulation subsystems together maximize the minimum Euclidean distance between the modulated codeword sequences in the overall coded trellis, resulting in a 4.3-dB asymptotic coding advantage over an uncoded 8-PSK system signalling at the same information rate over the same channel. The system performance has been evaluated by computer simulation.
An eight stage, 32 GHz reflected wave ruby maser was built. The maser operates in a 3 watt closed cycle refrigerator at 4.5 K and is capable of 21 dB of net gain with an instantaneous bandwidth of 400 MHz. The input noise temperature referred to the room temperature flange is approximately 21 K.