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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Noise analysis of spaceborne IR spatial interferometer

Analysis is performed to determine the relationship between the signal-to-noise ratio and the integration time for a spaceborne IR heterodyne spatial interferometer. The analysis includes consideration of the transformation of the interferogram to obtain the angular intensity distribution. The results presented show that for an IR source such as IRC + 10216, an integration time of 50 seconds per interferogram point will yield a signal-to-noise ratio greater than 10 dB in the angular intensity profile obtained from a 30-point transformation.

Wolfe, R. H., Jr.↗
NASA NTRS1976

The effect of instrument performance upon the calculated spectrum from a spaceborne IR interferometer

The data processing equations used to transform an interferometer output signal into a spectrum have been applied to the instrument noise to find the relationship between the signal to noise ratio (S/N) in the interferogram and that in the computed spectrum. Both photon and thermal noises have been taken into account. The results show that the voltage S/N in the spectrum is inversely proportional to the square root of the number of samples so that taking a larger number of samples requires a larger S/N in the interferogram to achieve the same S/N in the spectrum. Numerical examples are given relating the exposure time to design and performance parameters for stellar and atmospheric applications.

James, D. A.↗
NASA NTRS1978

Improved double-pass michelson interferometer

Interferometer design separates beams by offsetting centerlines of cat's-eye retroreflectors vertically rather than horizontally. Since beam splitter is insensitive to minimum-thickness condition in this geometry, relatively-low-cost, optically flat plate can be used.

Schindler, R. A.↗
NASA NTRS1977

Over-under double-pass interferometer

An over-under double pass interferometer in which the beamsplitter area and thickness can be reduced to conform only with optical flatness considerations was achieved by offsetting the optical center line of one cat's-eye retroreflector relative to the optical center line of the other in order that one split beam be folded into a plane distinct from the other folded split beam. The beamsplitter is made transparent in one area for a first folded beam to be passed to a mirror for doubling back and is made totally reflective in another area for the second folded beam to be reflected to a mirror for doubling back. The two beams thus doubled back are combined in the central, beamsplitting area of the beamsplitting and passed to a detector. This makes the beamsplitter insensitive to minimum thickness requirements and selection of material.

Schindler, R. A.↗
NASA NTRS1978

Zodiacal light dynamics experiment: A wideband imaging Fabry-Perot interferometer

The Solar Probe will provide an ideal platform from which to study dynamics of dust particles near the sun by measuring the detailed character of the Fraunhofer structure of the zodiacal light. The suggested instrument is a wideband imaging Fabry-Perot interferometer with state of the art technology in both the optics and the detector. The instrument would function as a high-resolution imaging device providing wavelength resolution of 0.03 A over about a 20 A range. The wideband imaging capability would provide sky maps of the zodiacal light on a despun spacecraft without mechanical scanning. The Solar Probe mission would allow the velocity distribution of the dust to be mapped along most of the trajectory of the spacecraft.

Torr, D. G.↗
NASA NTRS1977

Fresnel-Fizeau effect in a rotating optical fiber ring interferometer

Measurements of the Fresnel-Fizeau effect in a fiber ring interferometer gyroscope are reported. The presence of the dispersion term in the Fresnel drag coefficient was discernable. The temperature dependence of the coefficient bears on the temperature sensitivity of the optical fiber gyroscope. An alternate approach to measuring fiber dispersion is an added benefit, leading to determination of maximum pulse rate transmitted through single-mode fibers.

Vali, V.↗
NASA NTRS1977

The Tidbinbilla interferometer

This paper describes a technique to detect and identify weak, small-diameter radio sources with flux densities in the millijansky range. An interferometer system is proposed which will use existing 64 m and 24 m antennas of the Tidbinbilla Deep Space Network near Canberra. Design parameters of the system are described, and a schematic is presented with attention to low-noise traveling wave masers, and phase oscillation devices.

Batty, M. J.↗
NASA NTRS1978

Passive imaging through the turbulent atmosphere - Fundamental limits on the spatial frequency resolution of a rotational shearing interferometer

The signal-to-noise (S/N) ratio to be expected when a 180 deg rotationally shearing interferometer is used for image recovery at the diffraction limit of a large telescope is computed. The variance and covariance of the irradiance fluctuations at the detector array are shown to yield measures of the high-frequency spatial spectrum of the source. Four fundamental sources of noise are considered: temporal fluctuations of the source, space-time fluctuations of the atmosphere, shot noise in the detected photocurrents, and the effects of finite sampling. S/N is found to be directly proportional to the angular resolution of the telescope, the single-frame integration time, the square root of the number of frames, the cube of the operating wavelength, the quantum efficiency of the detector, and the average spectral irradiance from the source on the pupil. It is inversely proportional to the cube of the field angle subtended by the source (or part thereof) under study.

Burke, J. J.↗
NASA NTRS1979

Apparatus for providing a servo drive signal in a high-speed stepping interferometer

An analog voltage approximately linearly proportional to a desired offset from the present null position of a moving mirror in an interferometer is applied to the mirror moving means. As the mirror moves to the next null position, as determined by the analog voltage, the fringes of a laser reference interference pattern are detected. At the occurrence of each fringe the analog voltage is reduced proportionally so that when the next null position is reached, this driving analog is effectively zero. A binary up/down counter, by its internal count, causes a digital/analog converter to supply the analog voltage to the mirror moving means. Fringe detection and direction of movement logic cause the binary up/down counter to be decremented from its offset count as the mirror is moved to the new null position. Undesirable movement of the mirror due to vibration or other sources causes a correcting drive signal to be applied to the mirror moving means that is proportional to the distance of movement.

Schindler, R. A.↗
NASA NTRS1978

Miniature interferometer terminals for earth surveying

A system of miniature radio interferometer terminals was proposed for the measurement of vector baselines with uncertainties ranging from the millimeter to the centimeter level for baseline lengths ranging, respectively, from a few to a few hundred kilometers. Each terminal would have no moving parts, could be packaged in a volume of less than 0.1 cu m, and would operate unattended. These units would receive radio signals from low-power (10 w) transmitters on earth-orbiting satellites. The baselines between units could be determined virtually instantaneously and monitored continuously as long as at least four satellites were visible simultaneously.

Counselman, C. C., III↗
NASA NTRS1978

CIMATS - A correlation interferometer for the measurement of atmospheric trace species

A multigas correlation interferometer, based on the previously developed and tested COPE instrument, has been designed and fabricated and is presently undergoing laboratory testing. The CIMATS instrument has the capability to remotely measure densities of selected gaseous atmospheric species in the spectral range 2 to 2.5 and 4 to 10 microns. The sensor has been designed for both nadir viewing and earth limb viewing. As presently configured the sensor is capable of measuring ammonia, nitrous oxide, and carbon monoxide in the nonthermal infrared spectral region and carbon monoxide and sulfur dioxide in the thermal infrared. Other gaseous species can be measured by inclusion of additional narrow band filters.

Goldstein, H. W.↗
NASA NTRS1979

Satellite interferometer as an advanced navigation/communication system

An orthogonal system of interferometry baselines aboard a geostationary satellite is proposed to be used for navigation, surveillance, and traffic control. Position determination is based on the relationship between the measured phase differences, the known and unknown transmitter locations, and other systematic error model parameters using a generalized least-squares estimation procedure. It is shown that position accuracy is critically dependent on the baseline length and on the magnitude of the random component of the measuring errors. Several trade-offs with respect to the system accuracies, the number of users, data rates, hardware costs, etc., are examined. Simultaneous use of the same channels for navigation and data transfer is found feasible and five candidate or 'strawman' interferometer system proposals are presented. It is concluded that the system's performance is excellent in comparison with present or planned systems, and that it is viable in meeting potential user's requirements.

Gopalapillai, G. S.↗
NASA NTRS1979

Description of the ISEE satellite-to-satellite kilometric wavelength interferometer system

The International Sun-Earth Explorer (ISEE) spacecraft 1 and 2 carry receivers for detecting electromagnetic waves with kilometric wavelengths. For selected receiver frequencies from 30 kHz to 2 MHz, a 10-kHz bandwidth channel is single-sideband mixed down to baseband. These analog data and a reference frequency, which is a submultiple of the local oscillator frequency, are transmitted to ground stations and tape recorded along with the precise time and frequency information. Cross correlation of these tape recorded signals constitutes a satellite-to-satellite interferometer with a fringe spacing of 0.4 to 41 arcmin (at a range of 10,000 to 100 km spacing between spacecraft for 250 kHz in frequency) and with a time-delay resolution of 32 microsec for a 10-kHz bandwidth, which gives an angular resolution of 3 min to 6 deg of arc. For kilometric radiation at earth, sources in the size range of 25 to 2500 km can be identified (from 20 earth radii) and located in relative position ranging from 0.02 to 2 earth radii, depending on spacecraft spacing. Reception and analysis of solar and Jovian bursts may also be possible.

Shawhan, S. D.↗
NASA NTRS1980

The USNO/NRL Green Bank interferometer program

Application of the connected-element radio interferometer of the National Radio Astronomy Observation in Green Bank, West Virginia to the determination of improved source coordinates, astronomical constants, and variations in Earth rotation parameters is discussed. It is concluded that because of the brevity and discontinuity of the data so far no reliable conclusions regarding the accuracy of the data can be drawn.

Klepczynski, W. J.↗
NASA NTRS1980

Phase and group delay calibration of a very long baseline interferometer by East Coast VLBI Group

Precisely timed pulses injected into the input of each receiver are used to calibrate the phase and group delay through each interferometer terminal. The short duration pulses are generated at a 1 MHz rate directly from the output of the frequency standard. The pulses are injected into the receiver at a level low enough to produce less than one percent increase in system temperature, yet can be extracted during processing with a high enough signal to noise ratio to determine the phase of the calibration rails within 1 degree in 1 second of integration. The calibration system also includes precise cable measurement electronics and a pulse echo for clock synchronization.

Rogers, A. E. E.↗
NASA NTRS1980

The sensitivity of a very long baseline interferometer

The theoretical sensitivity of various methods of acquiring and processing interferometer data are compared. It is shown that for a fixed digital recording capacity one bit quantization of single sideband data filtered with a rectangular bandpass and sampled at the Nyquist rate yields the optimum signal to noise ratio. The losses which result from imperfect bandpass, poor image rejection, approximate methods of fringe rotation, fractional bit correction, and loss of quadrature are discussed. Also discussed is the use of the complex delay function as a maximum likelihood fringe estimator.

Rogers, A. E. E.↗
NASA NTRS1980

Accurate radio positions with the Tidbinbilla interferometer

The Tidbinbilla interferometer, designed specifically to provide accurate radio position measurements of compact radio sources in the Southern Hemisphere with high sensitivity using the 26 m and 64 m antennas of the Deep Space Network at Tidbinbilla, near Canberra, is discussed. The instrument also provides high accuracy flux density measurements for compact radio sources.

Batty, M. J.↗