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A lamellar grating has been developed to be used for very far infrared observations on the airborne observatory. The design characteristics and performance during laboratory testing and initial observations of Jupiter at wavelengths between 50 and 500 microns are presented.
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An introductory overview of very long baseline interferometry (VLBI) as it exists and is used today is given and the scientific advances that have been achieved with this technique in the past decade are described. The report briefly reviews developments now in progress that will improve ground station VLBI in the next few years, and the limitations that still will exist. The advantages and the scientific return on investment that may be expected from a VLBI terminal in space are described. Practical problems that have to be faced range from system design through hardware implementation, to data recovery and analysis.
The accuracy of direction measurements is substantially improved by Very Long Baseline Interferometry (VLBI). The technique is geometric in that it operates independently of the earth's gravity field, and dynamic in that observations of polar motion, nutation, length of day variations, and precession are also possible. Attention is given to a VLBI study of earth rotation within a framework of network geometry parameters.
System immune from interference locates signals in space without ambiguity. Signals from each antenna element are processed through three different mixing stages.
An error analysis is performed for a global positioning and navigation system which relies on an orthogonal array of interferometry baselines on board a geostationary satellite. The error analysis involves linearization of a mathematical model which is based on the relationship between the measured phase differences, the known transmitter positions, and other systematic error parameters. According to the error analysis, position accuracy is critically dependent on the baseline length and the magnitude of the random component of the measuring errors. It is shown that a satellite interferometry system with baselines of about 50 m can yield position accuracies on the order of 20 m.
The theoretical feasibility inherent in the correlation interferometry technique is demonstrated. High signal to noise and increased information content in a given partial interferogram are attractive features of the technique. The separation of interfering gas species is overcome by a precise measurement program for spectral line parameters of methane and other species. There are optical, mechanical, and electronic difficulties with the interferometry hardware design. Flight data from Cessna 402 over southeastern Virginia during April 1978 yielded amplitude fluctuations that are explainable only as internal nonlinearities. Misalignment of the AGC/interferometer optical axis also contributed to a large error source in the flight data. It is concluded that no useful data on gas burdens was obtained from these flights.
A cryogenic system to be utilized with a mounted pointing system is designed. The analysis indicates that the resulting design is significantly heavier than previous design due to the increased diameter of the system, resulting in greater tank and vacuum enclosure weights. Additional weight is also required in the area where the payload is attached to the pointing system in order to provide additional stiffness to the payload. The vacuum shell required to enclose the instrument is shorter and therefore an attendant reduction in instrument package weight would be expected. The primary resonance of the cooler instrument package occurs at 21 Hz, a mode in which the flexibility of the vacuum shell allows motion of the cryogen tank and instrument, and which can be increased by additional stiffening in the region of the pointing system vacuum shell connection at the cost of additional weight. The second resonance at 59 Hz is due to vibration of the helium tank on the fiberglass support tubes. The parasitic heat load can be reduced to 0.28 W by the utilization of a sandwich support tube structure in place of the monocoque structure assumed in the design.
Interference fringes from superimposed beams move across a photodetector, creating electrical pulses. Frequency of pulses is direct measure of angular velocity of rotating body. If separation of the sources changes, it affects separation of all fringes equally and does not affect angular velocity measurement.
A system for focusing and pointing the SPS power beam is discussed. The system is ground based and closed loop. One receiving antenna is required on earth. A conventional uplinked data channel transmits an 8-bit phase error correction back to the SPS for sequential calibration of each power module. Beam pointing resolution is better than 140 meters at the rectenna.
Ultrasonic waves in composite materials using optical fiber waveguide as the detecting device were studied. The possibility of using embedded probes in composites for nondestructive evaluation is discussed. It is found that interferometric system is sensitive but requires accurate optical positioning at both fiber input and output.
A system for focusing and pointing the SPS power beam is discussed. The system is ground based and closed loop. One receiving antenna is required on Earth. A conventional uplink data channel transmits an 8-bit phase error correlation back to the SPS for sequential calibration of each power module. Beam pointing resolution is better than 140 meters at the Rectenna.
A method is described for monitoring the changing thickness of a thin oil film subject to an aerodynamic shear stress using two focused laser beams. The measurement is then simply analyzed in terms of the surface skin friction of the flow. The analysis includes the effects of arbitrarily large pressure and skin friction gradients, gravity, and time varying oil temperature. It may also be applied to three dimensional flows with unknown direction. Applications are presented for a variety of flows including two dimensional flows, three dimensional swirling flows, separated flow, supersonic high Reynolds number flows, and delta wing vortical flows.
Both ISEE 1 and 2 are equipped with identical wide-band analog plasma wave receivers which makes long baseline interferometry possible. A cross-correlation of the received signals from each spacecraft during periods of intense Auroral Kilometric Radiation (AKR) provides information on the source size of AKR. From the magnitude of the cross-correlation coefficient, measured over baselines from 40 Km to 8000 Km, an upper limit to the statistical source size can be determined. Thirty-five AKR events at 125 kHz are being analyzed. Preliminary results indicate that the apparent source sizes are smaller than .1 Re.
The etalon is shown to remain stable to less than 5-m/sec equivalent wind over one orbit and less than 100 m/sec over several months in orbit. The thermal stability considerations for the highly stable etalon are discussed, including the passive thermal control used in flight. The various physical processes which give rise to drifts in the peak transmission wavelength are quantified, and the kinematic mount used to isolate the etalon from large incremental forces caused by changes in the thermal environment is presented. The performance of the etalon in its flight configuration is evaluated.
Previously announced in STAR as N82-28605
A method is described for monitoring the changing thickness of a thin oil film subject to an aerodynamic shear stress using two focused laser beams. The measurement is then simply analyzed in terms of the surface skin friction of the flow. The analysis includes the effects of arbitrarily large pressure and skin friction gradients, gravity, and time varying oil temperature. It may also be applied to three dimensional flows with unknown direction. Applications are presented for a variety of flows, including two dimensional flows, three dimensional swirling flows, separated flow, supersonic high Reynolds number flows, and delta wing vortical flows. Previously announced in STAR as N83-12393
Embedded single-mode optical fibers sample internal-stress fields directly. Statically loaded composite matrix emits pulsed ultrasonic waves which mechanically modulate embedded fiber and phase-modulate transmitted optical field. Modulation detected by optical interferometry and Fourier optical processing converted to electronic signal proportional to acoustic field amplitude integated along length of fiber embedded in specimen. Technique used for measurements of both high- and low-frequency CW acoustic fields as well as high-frequency transients.