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At least 1,531 records · Page 85
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Maximum frequency of the decametric radiation from Jupiter

The upper frequency limits of Jupiter's decametric radio emission are found to be essentially the same when observed from the earth or, with considerably higher sensitivity, from the Voyager spacecraft close to Jupiter. This suggests that the maximum frequency is a real cut-off corresponding to a maximum gyrofrequency of about 38-40 MHz at Jupiter. It no longer appears to be necessary to specify different cut-off frequencies for the Io and non-Io emission as the maximum frequencies are roughly the same in each case.

Barrow, C. H.↗
NASA NTRS1980

A high frequency silicon pressure sensor

Theoretical and design considerations as well as fabrication and experimental work involved in the development of high-frequency silicon pressure sensors with an ultra-small diaphragm are discussed. A sensor is presented with a rectangular diaphragm of 0.0127 cm x 0.0254 cm x 1.06 micron; the sensor has a natural frequency of 625 kHz and a sensitivity of 0.82 mv/v-psi. High-frequency results from shock tube testing and low-frequency (less than 50 kHz) comparison with microphones are given.

Kahng, S. K.↗
NASA NTRS1982

Recent developments and proposed schemes for trapped ion frequency standards

Ion traps are exciting candidates as future precision frequency sources. Recent developments demonstrate that mercury ion frequency standards are capable of a stability performance comparable to commercial cesium standards. There is, however, considerable room for improvement with regard to the signal to noise problem. The 40 GHz microwave frequency implies that a careful design should be implemented to ensure the elimination of the unwanted side bands in the microwave pump signal. A long life, high performance light source to be used in a trapped mercury ion microwave standard must be developed and the long term performance of a trapped mercury ion microwave standard must be investigated. While newly proposed two photon pumping schemes in conjuction with mercury ions promise exciting developments for both microwave and optical frequency standards, other ions that may be potential candidates should be evaluated for their usefulness.

Maleki, L.↗
NASA NTRS1982

Frequency stability of maser oscillators operated with cavity Q

The short term frequency stability of masers equipped with an external feedback loop to increase the cavity quality factor was studied. The frequency stability of a hydrogen and a rubidium maser were measured and compared with theoretical evaluation. It is shown that the frequency stability passes through an optimum when the cavity Q is varied. Long term fluctuations are discussed and the optimum mid term frequency stability achievably by small size active and passive H-masers is considered.

Tetu, M.↗
NASA NTRS1981

Radiometric measurements over bare and vegetated fields at 1.4 GHz and 5 GHz frequencies

Microwave emission from bare and vegetated fields was measured with dual polarized radiometers at 1.4 GHz and 5 GHz frequencies. The measured brightness temperatures over bare fields are shown to compare favorably with those calculated from radiative transfer theory with two constant parameters characterizing surface roughness effect. The presence of vegetation cover is found to reduce the sensitivity to soil moisture variation. This sensitivity reduction is generally pronounced the denser, the vegetation cover and the higher the frequency of observation. The effect of vegetation cover is also examined with respect to the measured polarization factor at both frequencies. With the exception of dry corn fields, the measured polarization factor over vegetated fields is found appreciably reduced compared to that over bare fields. A much larger reduction in this factor is found at 5GHz than at 1.4GHz frequency.

Wang, J. R.↗
NASA NTRS1982

Development of low-frequency kernel-function aerodynamics for comparison with time-dependent finite-difference methods

Finite difference methods for unsteady transonic flow frequency use simplified equations in which certain of the time dependent terms are omitted from the governing equations. Kernel functions are derived for two dimensional subsonic flow, and provide accurate solutions of the linearized potential equation with the same time dependent terms omitted. These solutions make possible a direct evaluation of the finite difference codes for the linear problem. Calculations with two of these low frequency kernel functions verify the accuracy of the LTRAN2 and HYTRAN2 finite difference codes. Comparisons of the low frequency kernel function results with the Possio kernel function solution of the complete linear equations indicate the adequacy of the HYTRAN approximation for frequencies in the range of interest for flutter calculations.

Bland, S. R.↗
NASA NTRS1982

Selection of frequencies for deep-space telecommunications

The procedures used by the Jet Propulsion Laboratory to select and recommend frequencies to be used for deep space telecommunications are explained. The frequency selection process described deals only with the potential for radio frequency interference between deep space telecommunication links. Channel plans based on bandwidth, hardware implementation, and frequency ratio considered are used. The channel selection process is based on calculations and analysis of interference-to-signal power ratios as a function of time for each possible pair of missions. The modes of interference and the interference protection ratio are explained, and the interference analysis procedure described. Equations used in calculating the downlink and uplink interference analyses are presented.

Degroot, N. F.↗
NASA NTRS1981

The AKR emission cone at low frequencies

It is noted that certain of the ISEE-1 observations between the plasmasphere and the auroral zone have revealed the emission cone of auroral kilometric radiation (AKR) unaffected by plasmaspheric refraction. At some distance from the source, the cone produced a sharp low-frequency boundary in the AKR signals, which was displaced above the cyclotron frequency. The variation of this boundary, together with other aspects of the AKR signals, suggested that the AKR emission cone closed toward a hollow, roughly 45 deg limit cone with decreasing frequency, duplicating the behavior previously found with ISIS-1 at the opposite end of the AKR spectrum. It is pointed out that the hollow limit cone at low frequencies is a new feature, not previously reported.

Calvert, W.↗
NASA NTRS1981

An analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites

A four frequency moment characterization of backscatter from the near-vertical is applied to an analysis of the short pulse and dual frequency microwave techniques. The range reflectivity modulation spectrum closely approximates the directional wave slope spectrum, while harmonic distortion is small and is a minimum near 10 deg incidence. The short pulse measurement signal-to-noise ratio (SNR) is typically greater than the narrowband dual frequency SNR, with the difference being the ratio of the range beam extent to pulse length, minus the ratio of beam-limited to pulse-limited Doppler spreads. It is concluded that dual frequency measurements are basically impractical, although short pulse measurements are useful and can employ existing space-qualified microwave hardware.

Jackson, F. C.↗
NASA NTRS1981

Absorption and emission of extraordinary-mode electromagnetic waves near cyclotron frequency in nonequilibrium plasmas

An investigation is presented of two cases: (1) weakly relativistic electrons with a loss-cone type distribution, and (2) electrons with a drift velocity parallel to the ambient magnetic field. Numerical computations are given for physical parameters close to those in the polar region of the earth magnetosphere and laboratory experiments, with attention to the fast extraordinary-mode radiation whose frequency is near that of the electron cyclotron frequency. The fast extraordinary mode can escape from a strong field region to the weaker field region and may therefore be measured outside the plasma. It is found that the X mode radiation can be amplified by means of a cyclotron maser effect when the electrons have a loss-cone distribution, and it is concluded that, when the electron energy is sufficiently high, the X mode cutoff frequency may be lower than the cyclotron frequency.

Wu, C. S.↗
NASA NTRS1982

Computer-aided prediction of high-frequency performance limits in silicon bipolar integrated circuits

A circuit model for an existing silicon integrated bipolar junction transistor (IBJT) is used to evaluate presently achievable high frequency circuit performance. The relationship between circuit model and processing parameters are semi-quantitatively explored to make predictions on the frequency response, which can be achieved through realistic device fabrication modifications. A new figure of merit is introduced, which is defined as the signal frequency at which an integrated bipolar junction transistor can deliver a power gain of G. The most sensitive parameter influencing attainable high frequency IBJT performance is base resistance.

Burns, J. L.↗
NASA NTRS1982

Refined Monte Carlo method for simulating angle-dependent partial frequency redistributions

A refined algorithm for generating emission frequencies from angle-dependent partial frequency redistribution functions R sub II and R sub III is described. The improved algorithm has as its basis a 'rejection' technique that, for absorption frequencies x less than 5, involves no approximations. The resulting procedure is found to be essential for effective studies of radiative transfer in optically thick or temperature varying media involving angle-dependent partial frequency redistributions.

Lee, J.-S.↗
NASA NTRS1982

Interplanetary plasma concentration measurements by use of dual frequencies from separated sites

In the interplanetary plasma studies considered, signals at two frequencies are sent simultaneously from a single site on the earth to a spacecraft, or vice versa. Changes in group and phase velocity are proportional to the number density of the plasma and inversely proportional to the square of the signal frequency. Plasma-induced radio timing changes can be measured through comparisons of the changes at widely different frequencies. Such a system has, however, certain weaknesses, which can be overcome by making use of an approach which uses separated sites for sources of the low and high frequencies. The practical advantages gained by site separation are balanced somewhat by several technical problems, but each problem can be solved. The main barrier appears to be a very exacting timekeeping requirement. These requirements can be satisfied with the aid of the cesium clock.

Croft, T. A.↗
NASA NTRS1981

Time synchronization of a frequency-hopped MFSK communication system

In a frequency-hopped (FH) multiple-frequency-shift-keyed (MFSK) communication system, frequency hopping causes the necessary frequency transitions for time synchronization estimation rather than the data sequence as in the conventional (nonfrequency-hopped) system. Making use of this observation, this paper presents a fine synchronization (i.e., time errors of less than a hop duration) technique for estimation of FH timing. The performance degradation due to imperfect FH time synchronization is found in terms of the effect on bit error probability as a function of full-band or partial-band noise jamming levels and of the number of hops used in the FH timing estimate.

Simon, M. K.↗
NASA NTRS1982

Flute mode waves near the lower hybrid frequency excited by ion rings in velocity space

Discrete emissions at the lower hybrid frequency are often seen on the S3-3 satellite. Simultaneous observation of perpendicularly heated ions suggests that these ions may provide the free energy necessary to drive the instability. Studies of the dispersion relation for flute modes excited by warm ion rings in velocity space show that waves are excited with real frequencies near the lower hybrid frequency and with growth rates ranging from about 0.01 to 1 times the ion cyclotron frequency. Numerical results are therefore consistent with the possibility that the observed ions are the free energy source for the observed waves.

Cattell, C.↗
NASA NTRS1981

High Frequency Plasma Generators for Ion Thrusters

The results of a one year program to experimentally adapt two new types of high frequency plasma generators to Argon ion thrusters and to analytically study a third high frequency source concept are presented. Conventional 30 cm two grid ion extraction was utilized or proposed for all three sources. The two plasma generating methods selected for experimental study were a radio frequency induction (RFI) source, operating at about 1 MHz, and an electron cyclotron heated (ECH) plasma source operating at about 5 GHz. Both sources utilize multi-linecusp permanent magnet configurations for plasma confinement. The plasma characteristics, plasma loading of the rf antenna, and the rf frequency dependence of source efficiency and antenna circuit efficiency are described for the RFI Multi-cusp source. In a series of tests of this source at Lewis Research Center, minimum discharge losses of 220+/-10 eV/ion were obtained with propellant utilization of .45 at a beam current of 3 amperes. Possible improvement modifications are discussed.

Divergilio, W. F.↗
NASA NTRS1983

Low frequency acoustic and electromagnetic scattering

This paper deals with two classes of problems arising from acoustics and electromagnetics scattering in the low frequency stations. The first class of problem is solving Helmholtz equation with Dirichlet boundary conditions on an arbitrary two dimensional body while the second one is an interior-exterior interface problem with Helmholtz equation in the exterior. Low frequency analysis show that there are two intermediate problems which solve the above problems accurate to 0(k(2) log k) where k is the frequency. These solutions greatly differ from the zero frequency approximations. For the Dirichlet problem numerical examples are shown to verify the theoretical estimates.

Hariharan, S. I.↗
NASA NTRS1983

Radiation hardness of Efratom M-100 rubidium frequency standard

The effects of nuclear radiation on rubidium gas cell frequency standards and components are presented, including the results of recent tests where a continuously operating rubidium frequency standard (Effratom, Model M-100) was subjected to simultaneous neutron/gamma radiation. At the highest neutron fluence 7.5 10 to the 12th power n/sq cm and total dose 11 krad(Si) tested, the unit operated satisfactorily; the total frequency change over the 2 1/2 hour test period due to all causes, including repeated retraction from and insertion into the reactor, was less than 1 x 10 to the -10th power. The effects of combined neutron/gamma radiation on rubidium frequency standard physics package components were also studied, and the results are presented.

English, T. C.↗