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NASA NTRS1978

An analytic study of impact ejecta trajectories in the atmospheres of Venus, Mars, and earth

Calculations have been made to determine the effects of atmospheric drag and gravity on impact ejecta trajectories on Venus, Mars, and earth. The equations of motion were numerically integrated for a broad range of body sizes, initial velocities, and initial elevation angles. A dimensionless parameter was found from approximate analytic solutions which correlated the ejecta range, final impact angle, and final impact velocity for all three planets.

Tauber, M. E.↗

NASA NTRS1977

Effects of detector threshold, location of the sun, and flight altitude upon spectral variations in remote sensing over water

Photographic flights with Hasselblad cameras were flown on August 28, 1975, at altitudes from 2.66 and 5.3 kilometers over an ocean acid waste dump site while acid dumping was in progress. Repeated flights resulted in broadband spectral radiance data between the wavelengths of 500 to 900 nanometers for sun elevation angles that varied from 26 to 48 degrees, and at all sun azimuth angles over the range of off-nadir angles from 0 to 35 degrees. From film densitometer data, it is shown that before spectral variations in remotely sensed data can be used to quantify substances in water, the longer wavelength data must be above the detection level of the detector, radiance data between + or - 45 degrees in the direction of the sun must be avoided, and off-nadir camera correction factors must be applied to the observed radiance data.

Bressette, W. E.↗

NASA NTRS1978

Ray tracing evaluation of a technique for correcting the refraction errors in satellite tracking data

Errors may be introduced in satellite laser ranging data by atmospheric refractivity. Ray tracing data have indicated that horizontal refractivity gradients may introduce nearly 3-cm rms error when satellites are near 10-degree elevation. A correction formula to compensate for the horizontal gradients has been developed. Its accuracy is evaluated by comparing it to refractivity profiles. It is found that if both spherical and gradient correction formulas are employed in conjunction with meteorological measurements, a range resolution of one cm or less is feasible for satellite elevation angles above 10 degrees.

Gardner, C. S.↗

NASA NTRS1978

Application of global positioning system to determination of tectonic plate movements and crustal deformations

It is shown that pseudo-range measurements to four GPS satellites based on correlation of the pseudo random code transmissions from the satellites can be used to determine the relative position of ground stations which are separated by several hundred kilometers to a precision at the centimeter level. Carrier signal measurements during the course of passage of satellites over a pair of stations also yield centimeter precision in the relative position, but oscillator instabilities limit the accuracy. The accuracy of solutions based on either type of data is limited by unmodeled tropospheric refraction effects which would reach 5 centimeters at low elevation angles for widely separated stations.

Anderle, R. J.↗

NASA NTRS1977

Antenna bias rigging for performance objective

An optimum bias rigging for a paraboloidal antenna can be used to minimize the expected average mean-square half-pathlength surface deviations. Statistics of deep space planetary missions are employed to develop weighting factors for antenna elevation angles during these missions. A procedure is shown to compute the optimum rigging angle from these weights.

Levy, R.↗

NASA NTRS1979

Optimum frequencies of passive microwave radiometer for tropospheric path-length correction

Radio-astronomical observations require accurate calibration of tropospheric path length. Such calibration can be achieved by microwave radiometers operating near the 22-GHz water vapor line. However, the performances of current passive microwave radiometers are meteorology-profile dependent. This is due mainly to incorrect frequency combinations and to saturation of brightness temperatures. By properly selecting an optimum frequency pair and removing the saturation effect, the dependency is alleviated and can be further adjusted by surface measurements alone. Hence, a universal calibration equation is applicable to all environmental conditions. Optimum frequency pairs are systematically searched. Simulation analysis indicates that calibration for the tropospheric water-vapor path-length error is better than 0.3 cm at zenith and better than 2 cm for an elevation angle as low as 10 deg.

Wu, S.-C.↗

NASA NTRS1979

Application of non-attenuating frequency radars for prediction of rain attenuation and space diversity performance

In order to establish transmitter power and receiver sensitivity levels at frequencies above 10 GHz, the designers of earth-satellite telecommunication systems are interested in cumulative rain fade statistics at variable path orientations, elevation angles, climatological regions, and frequencies. They are also interested in establishing optimum space diversity performance parameters. In this work are examined the many elements involved in the employment of single non-attenuating frequency radars for arriving at the desired information. The elements examined include radar techniques and requirements, phenomenological assumptions, path attenation formulations and procedures, as well as error budgeting and calibration analysis. Included are the pertinent results of previous investigators who have used radar for rain attenuation modeling. Suggestions are made for improving present methods.

Goldhirsh, J.↗

NASA NTRS1979

Transionospheric propagation predictions

The current status and future prospects of the capability to make transionospheric propagation predictions are addressed, highlighting the effects of the ionized media, which dominate for frequencies below 1 to 3 GHz, depending upon the state of the ionosphere and the elevation angle through the Earth-space path. The primary concerns are the predictions of time delay of signal modulation (group path delay) and of radio wave scintillation. Progress in these areas is strongly tied to knowledge of variable structures in the ionosphere ranging from the large scale (thousands of kilometers in horizontal extent) to the fine scale (kilometer size). Ionospheric variability and the relative importance of various mechanisms responsible for the time histories observed in total electron content (TEC), proportional to signal group delay, and in irregularity formation are discussed in terms of capability to make both short and long term predictions. The data base upon which predictions are made is examined for its adequacy, and the prospects for prediction improvements by more theoretical studies as well as by increasing the available statistical data base are examined.

Klobucher, J. A.↗

NASA NTRS1980

The estimation of the propagation delay through the troposphere from microwave radiometer data

The uncertainity in propagation delay estimates is due primarily to tropospheric water, the total amount and vertical distribution of which is variable. Because water vapor both delays and attenuates microwave signals, the propagation delay, or wet path length, can be estimated from the microwave brightness temperature near the 22.235 GHz transition of water vapor. The data from a total of 240 radiosonde launches taken simultaneously were analyzed. Estimates of brightness temperature at 19 and 22 GHz and wet path length were made from these data. The wet path length in the zenith direction could be estimated from the surface water vapor density to an accuracy of 5 cm for the summer data and 2 cm for winter data. Using the brightness temperatures, the wet path could be estimated to an accuracy of 0.3 cm. Two dual frequency radiometers were refurbished in order to test these techniques. These radiometers were capable of measuring the difference in the brightness temperature at 30 deg elevation angle and at the zenith to an accuracy of about 1 K. In August 1975, 45 radiosondes were launched over an 11 day period. Brightness temperature measurements were made simultaneously at 19 and 22 GHz with the radiometers. The rms error for the estimation of wet path length from surface meteorological parameters was 3.2 cm, and from the radiometer brightness temperatures, 1.5 cm.

Moran, J. M.↗

NASA NTRS1980

Microwave radiometry as a tool to calibrate tropospheric water-vapor delay

Microwave radiometers were used to measure the emission line due to the water vapor molecules of atmospheric emission. Four separate field tests were completed which compared radiometers to other techniques which measure water vapor. It is shown that water vapor induced delay can be estimated with an accuracy of plus or minus 2 cm for elevation angles above 17 degrees.

Resch, G. M.↗

NASA NTRS1980

Millimeter wave attenuation prediction using a piecewise uniform rain rate model

A piecewise uniform rain rate distribution model is introduced as a quasi-physical model of real rain along earth-space millimeter wave propagation paths. It permits calculation of the total attenuation from specific attenuation in a simple fashion. The model predications are verified by comparison with direct attenuation measurements for several frequencies, elevation angles, and locations. Also, coupled with the Rice-Holmberg rain rate model, attenuation statistics are predicated from rainfall accumulation data.

Persinger, R. R.↗

NASA NTRS1980

Dependence of the spectral surface irradiance on aerosol properties and surface reflectivity

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.

Otterman, J.↗

NASA NTRS1980

Hectometer and kilometer solar observations

Three dimensional 'snapshots' of the large scale solar magnetic field topology as well as the solar wind electron density distribution from about 0.1 to 1 AU are obtained by tracking traveling solar radio bursts at the hectometer and kilometer wavelengths with instruments aboard the ISEE-3 satellite and the HELIOS-2 solar probe. Both instruments observe in the frequency range from 30 kHz to a 1 MHz and both are equipped with dipole antennas located in the vehicle spin plane. ISEE-3 also has a dipole along the spin axis and the signals from the two ISEE-3 antennas are combined to give the aximuth and elevation angles of the radio source. Triangulation between HELIOS-2 and ISEE-3 provides the additional observation necessary to determine uniquely the position of the radio source in space at each observing frequency.

Stone, R. G.↗

NASA NTRS1980

Radio frequency performance of DSS 14 64-meter antenna at X-band using an improved subreflector

The performance of the Deep Space Station (DSS) 14 64-meter antenna X-band gain determined using the X- and K-band Radar feedcone is discussed. Tests prior to, and following the installation of an improved subreflector, proved the unit largely (if not totally) restored initial performance levels at that station. The X-band peak gain with the subreflector is +17.8dBi(area efficiency of 47.3 percent); an increase of +0.47 dB due to the unit installation. The test further showed a significant shift in the axial focus required (function of elevation angle) which, if not implemented operationally, will cause a serious degradation. An historical summary of all documented X-band gain measurements at DSS 14 is included and reviewed. The initial performance is traced through various major configuration changes such as the installation of the X-band dual hybrid mode horn within the operational XRO feedcone. Finally, based on the summary and recent data, a peak X-band antenna gain of +72.1 dBi (area efficiency of 50.8 percent) is projected for the operational feedcone.

Freiley, A. J.↗

NASA NTRS1981

Radio-wave propagation for space communications systems

The most recent information on the effects of Earth's atmosphere on space communications systems is reviewed. The design and reliable operation of satellite systems that provide the many applications in space which rely on the transmission of radio waves for communications and scientific purposes are dependent on the propagation characteristics of the transmission path. The presence of atmospheric gases, clouds, fog, precipitation, and turbulence causes uncontrolled variations in the signal characteristics. These variations can result in a reduction of the quality and reliability of the transmitted information. Models and other techniques are used in the prediction of atmospheric effects as influenced by frequency, geography, elevation angle, and type of transmission. Recent data on performance characteristics obtained from direct measurements on satellite links operating to above 30 GHz have been reviewed. Particular emphasis has been placed on the effects of precipitation on the Earth/space path, including rain attenuation, and ice particle depolarization. Other factors are sky noise, antenna gain degradation, scintillations, and bandwidth coherence. Each of the various propagation factors has an effect on design criteria for communications systems. These criteria include link reliability, power margins, noise contribution, modulation and polarization factors, channel cross talk, error rate, and bandwidth limitations.

Ippolito, L. J.↗

NASA NTRS1980

Satellite-aided mobile communications limited operational test in the trucking industry

An experiment with NASA's ATS-6 satellite, that demonstrates the practicality of satellite-aided land mobile communications is described. Satellite communications equipment for the experiment was designed so that it would be no more expensive, when mass produced, than conventional two-way mobile radio equipment. It embodied the operational features and convenience of present day mobile radios. Vehicle antennas 75 cm tall and 2 cm in diameter provided good commercial quality signals to and from trucks and jeeps. Operational applicability and usage data were gathered by installing the radio equipment in five long-haul tractor-trailer trucks and two Air Force search and rescue jeeps. Channel occupancy rates are reported. Air Force personnel found the satellite radio system extremely valuable in their search and rescue mission during maneuvers and actual rescue operations. Propagation data is subjectively analyzed and over 4 hours of random data is categorized and graded as to signal quality on a second by second basis. Trends in different topographic regions are reported. An overall communications reliability of 93% was observed despite low satellite elevation angles ranging from 9 to 24 degrees.

Anderson, R. E.↗

NASA NTRS1981

Lateral Attenuation of High-By-Pass Ratio Engined Aircraft Noise

A flight experiment was conducted to investigate the lateral attenuation of high by pass ratio engined airplanes. A B-747 was flown at low altitudes over the ends of two microphone arrays. One array covering a lateral distance of 1600 m consisted of 14 microphones positioned over grass. The second array covered a lateral distance of 1200 m and consisted of 6 microphones positioned over a concrete runway. Sixteen runs were flown at altitudes ranging from 30 to 960 m. The acoustic information recorded in the field was reduced to one third octave band spectral time histories and synchronized with tracking and weather information. Lateral attenuation as a function of elevation angle was calculated in overall, A-weighted, tone-corrected perceived noise level, and effective perceived noise level units. The B-747 results are compared with similar results for a turbojet-powered T-38 airplane and the SAE recommended lateral attenuation prediction procedure. Less lateral attenuation was measured for the B-747 than for the T-38. The B-747 lateral attenuation values also fell below the SAE curve.

Willshire, W. L., Jr.↗

NASA NTRS1980

Development of a universal water signature for the LANDSAT-3 Multispectral Scanner, part 1

A generalized four channel hyperplane to discriminate water from nonwater was developed using LANDSAT-3 multispectral scaner (MSS) scenes and matching same/next day color infrared aerial photography. The MSS scenes varied in sun elevation angle from 40 to 58 deg. The 28 matching air photo frames contained over 1400 water bodies larger than one surface acre. A preliminary water discriminant, was used to screen the data and eliminate from further consideration all pixels distant from water in MSS spectral space. A linear discriminant was iteratively fitted to the labelled pixels. This discriminant correctly classified 98.7% of the water pixels and 98.6% of the nonwater pixels. The discriminant detected 91.3% of the 414 water bodies over 10 acres in surface area, and misclassified as water 36 groups of contiguous nonwater pixels.

Schlosser, E. H.↗