Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “VHF”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1971

VHF Ranging and Position Fixing Experiment Using ATS Satellites: Final Report on Phases 1 and 2, 25 November 1968 - 1 May 1971

The testing program with the ATS-1 and ATS-3 spacecraft showed that geostationary satellites can provide superior communications and position surveillance for mobile craft. Inexpensive modifications to conventional mobile communications equipment aboard the craft can provide reliable, high quality voice and digital communications with distant ground stations and other vehicles, and automatic surveillance of the positions of all the craft by a ground facility. The tests also demonstrated the location and automatic readout of remote data collection platforms. Frequency modulation signals with the narrow audio and radio frequency bandwidths of terrestrial mobile radio communications were relayed through the VHF transponders of the geostationary satellites. The voice and digital communications were far superior in reliability and quality to long-distance mobile communications by other means. It was shown that one satellite can provide nearly uniform high quality performance over approximately one-third of the earth's surface. Position fixes by range measurement from the two satellites were accurate to approximately one nautical mile, except near the equator and the poles.

Source record↗
NASA NTRS1972

Amplitude fading of simultaneous transionospheric L-band and VHF signals received at the geomagnetic equator

At Ancon, Peru, simultaneous observations of 1550-MHz and 136-MHz signals from the ATS 5 and Intelsat-1 spacecraft showed ionospheric fading as great as 27 db at 136 MHz and 6 db at 1550 MHZ. The observations were made on 48 days during the 1970 autumnal and 1971 vernal equinox periods. Comparison of the two frequencies, in respect to rates and depths of fades, is made. Statistical distributions of the received signal levels during ionospheric scintillation occurrences are presented which may be of use to communications system engineers with operational requirements in the equatorial regions. The distributions show that during expected periods of scintillation, the L band signal typically falls 1.1 db below the median for 1.0 percent of the time, and the VHF signal falls 11.5 db below the median for 1.0 percent of the time.

Sessions, W. B.↗
NASA NTRS1972

Very High-Frequency (VHF) ionospheric scintillation fading measurements at Lima, Peru

During the spring equinox of 1970, scintillating signals at VHF (136.4 MHz) were observed at Lima, Peru. The transmission originated from ATS 3 and was observed through a pair of antennas spaced 1200 feet apart on an east-west baseline. The empirical data were digitized, reduced, and analyzed. The results include amplitude probability density and distribution functions, time autocorrelation functions, cross correlation functions for the spaced antennas, and appropriate spectral density functions. Results show estimates of the statistics of the ground diffraction pattern to gain insight into gross ionospheric irregularity size, and irregularity velocity in the antenna planes.

Blank, H. A.↗
NASA NTRS1972

L-band orthogonal-mode crossed-slot antenna and VHF crossed-loop antenna

A low gain, circularly polarized, L-band antenna; a low gain, linealy polarized, L-band antenna; and a low gain, circularly polarized, upper hemisphere, VHF satellite communications antenna intended for airborne applications are described. The text includes impedance and antenna radiation pattern data, along with physical description of the construction of the antennas.

Olsson, T.↗
NASA NTRS1973

Preflight SL-1/SL-3 Skylab VHF ranging coverage (nominal TPI). Antenna and propagation studies for spacecraft systems, task E-531

A preflight assessment of the Skylab VHF ranging coverage for the rendezvous portion of the nominal SL-1/SL-3 mission is reported, assuming a 27 July 1973 SL-3 launch. Data are based on a nominal attitude trajectory, which has the Saturn workshop in a solar inertial attitude throughout the rendezvous; the CSM terminal phase initiation maneuver is nominal. An addendum to this report is being prepared, which considers the effects of early and late TPI maneuvers. Curves are presented which show the variation in received power levels on both spacecraft-to-spacecraft links from about 600 n.mi. range to CSM and SWS station keeping. Appropriate threshold levels are shown on these received power curves to indicate zero circuit margins for the ranging function.

Eisenhauer, D. R.↗
NASA NTRS1975

Quasi-isotropic VHF antenna array design study for the International Ultraviolet Explorer satellite

Results of a study to design a quasi-isotropic VHF antenna array for the IUE satellite are presented. A free space configuration was obtained that has no nulls deeper than -6.4 dbi in each of two orthogonal polarizations. A computer program named SOAP that analyzes the electromagnetic interaction between antennas and complicated conducting bodies, such as satellites was developed.

Raines, J. K.↗
NASA NTRS1977

Emergency-vehicle VHF antenna

Helical VHF antenna mounts on roof of moving vehicle to communicate with distant stations via earth satellites. Antenna requires no pointing and can provide two-way communication while vehicle moves at high speed. Device has proved extremely successful in electrocardiogram transmission tests between medical services vehicle and hospital emergency room.

Anderson, R. E.↗
NASA NTRS1977

The temporal structure of HF and VHF radiations during Florida lightning return strokes

Measurements have been made of the HF and VHF radiation at 3, 30, 139, and 295 MHz produced during lightning return strokes, in correlation with wideband electric field measurements covering from about 300 Hz to 2 MHz. Intense pulses of RF are observed during the first return stroke in a discharge to ground, but with a 10-30 µsec time delay between the beginning of the stroke and the occurrence of significant RF radiation. The delay occurs at all frequencies and is short enough to suggest that the main channel and/or branches are the main initial sources of RF during the first stroke. During a subsequent return stroke, the RF starts an average of 265 µsec prior to the onset of the stroke. At 139 and 295 MHz, the subsequent stroke RF lasts only about 115 µsec; however, at 3 MHz, the radiation frequently persists up to and during the stroke.

David M Le Vine↗
NASA NTRS1979

VHF downline communication system for SLAR data

A real time VHF downlink communication system is described for transmitting side-looking airborne radar (SLAR) data directly from an aircraft to a portable ground/shipboard receiving station. Use of this receiving station aboard the U.S. Coast Guard icebreaker Mackinaw for generating real-time photographic quality radar images is discussed. The system was developed and demonstrated in conjunction with the U.S Coast Guard and NOAA National Weather Service as part of the Project Icewarn all weather ice information system for the Great Lakes Winter Navigation Program.

Schertler, R. J.↗
NASA NTRS1978

First measurements of mesospheric vertical velocities by VHF radar at temperate latitudes

A program of ground-based VHF measurements of turbulence and of wave and tidal motions in the mesosphere has been initiated by the University of Illinois using a high-power radar facility located at Urbana, Illinois. Observations show scattering occurring intermittently during the daytime throughout the mesosphere. Vertical velocities exhibit the presence of gravity waves with dominant periods near 10 min and amplitudes ranging from less than 1 m/s to approximately 5 m/s.

Miller, K. L.↗
NASA NTRS1979

VHF downlink communication system for SLAR data

This paper describes a real-time VHF downlink communication system for transmitting side-looking airborne-radar (SLAR) data directly from an aircraft to a portable ground/shipboard receiving station. Use of this receiving station aboard the U.S. Coast Guard icebreaker Mackinaw for generating real-time photographic quality radar images will be discussed. The system was developed and demonstrated in conjunction with the U.S. Coast Guard and NOAA National Weather Service as part of the Project Icewarn all-weather ice information system for the Great Lakes Winter Navigation Program.

Schertler, R. J.↗
NASA NTRS1982

VHF power scattered from the mesosphere at mid-latitudes

Scattered power profiles from the Urbana VHF radar have been analyzed. Coherent power returns from the mesosphere (60-90 km) show that a large portion of these returns occur in well-defined stable layers lasting for more than 6 hours in some cases. It is concluded that some of these layers may be caused by standing diurnal tides. Short time variation in scattered power is attributed to internal gravity waves modifying the amplitude and altitude of maximum vertical shear in the horizontal wind. Correlation time of the scattered signal varies with altitude as well as with scattered power. Seasonal averages of the scattered power show a broad peak around 76 km. The decrease in scattered power below 76 km appears because of a decrease in the electron density gradient, while the decrease above 76 km is due to a decrease in the intensity of turbulence. The amplitude of short-period gravity waves shows a decrease with altitude below 66 km, attributed to the Brunt-Vaisala barrier, and a slight increase with altitude above 70 km. The dominant period of the vertical oscillations shows an increase above 63 km, giving further evidence that the high-frequency oscillations present below 63 km cannot propagate to higher altitudes.

Royrvik, O.↗
NASA NTRS1983

Lightning activity observed in upper and lower portions of storms and its relationship to storm structure from VHF mapping and Doppler radar

Space time mapping of very high frequencies (VHF) sources reveals lightning processes for cloud to ground (CG) and for large intracloud (IC) flashes are confined to an altitude below about 10 km and closely associated with the central high reflectivity region of a storm. Another class of IC flashes was identified that produces a splattering of small sources within the main electrically active volume of a storm and also within a large divergent wind canopy at the top of a storm. There is no apparent temporal association between the small high altitude IC flashes occurring almost continuously and the large IC and CG flashes sporadically occurring in the lower portions of storms.

Taylor, W. L.↗
NASA NTRS1983

Is VHF Fresnel reflectivity due to low frequency buoyancy waves?

VHF radar echoes are greatly enhanced near the zenith relative to other directions. This enhancement must be due to reflection from horizontally stratified laminate of refractive index. The refractivity laminate are due to the displacements of low frequency buoyancy (internal gravity) waves acting on the background vertical gradient of refractivity. VANZANDT (1982) has shown that the observed spectra of mesoscale wind fluctuations in the troposphere and lower stratosphere are modeled by a universal spectrum of buoyancy (internal gravity) waves. Since the observed frequency spectrum is red, the buoyancy wave model of the vertical displacement spectrum is strongly enhanced near the zenith. In other terms, the resulting refractivity irregularities are strongly stratified.

Vanzandt, T. E.↗
NASA NTRS1983

Complementary code and digital filtering for detection of weak VHF radar signals from the mesoscale

The SOUSY-VHF-Radar operates at a frequency of 53.5 MHz in a valley in the Harz mountains, Germany, 90 km from Hanover. The radar controller, which is programmed by a 16-bit computer holds 1024 program steps in core and controls, via 8 channels, the whole radar system: in particular the master oscillator, the transmitter, the transmit-receive-switch, the receiver, the analog to digital converter, and the hardware adder. The high-sensitivity receiver has a dynamic range of 70 dB and a video bandwidth of 1 MHz. Phase coding schemes are applied, in particular for investigations at mesospheric heights, in order to carry out measurements with the maximum duty cycle and the maximum height resolution. The computer takes the data from the adder to store it in magnetic tape or disc. The radar controller is programmed by the computer using simple FORTRAN IV statements. After the program has been loaded and the computer has started the radar controller, it runs automatically, stopping at the program end. In case of errors or failures occurring during the radar operation, the radar controller is shut off caused either by a safety circuit or by a power failure circuit or by a parity check system.

Schmidt, G.↗
NASA NTRS1983

Design considerations of a proposed UK VHF radar

The major features of a proposed UK VHF radar for meso-, strato-, and tropospheric studies are discussed. The research interests of potential users which were considered in the design analysis are listed.

Hall, A. J.↗
NASA NTRS1983

Design considerations for high-power VHF radar transceivers (keynote paper)

A review and analysis of conference papers addressing the design of high power VHF radar tranceivers is presented. Specific design topics include: (1) distributed versus single transmitters; (2) coded pulses and decoder design; (3) large transmitter design considerations; and (4) T/R switch design. The topics in the discussion on future directions include the following: (1) enhanced receiver performance; (2) transmitter systems for unattended operation; (3) limitations of the current T/R switch designs; (4) data processing (is eight bits enough); and (5) new frequencies (200 MHz and/or 400 MHz).

Brosnahan, J. W.↗
NASA NTRS1984

VHF Radar Observations in the Stratosphere and Mesosphere During a Stratospheric Warming

The SOUSY-VHF-radar was used to carry out measurements during minor and a major stratospheric warming in February and March 1980, respectively. Echoes have been received from the stratosphere up to an altitude of about 30 km continuously during day and night, whereas echoes from the mesosphere were restricted to the daytime and occurred sporadically at different heights within the altitude range from 60 to 90 km. The three dimensional velocity vector was derived from Doppler measurements made in three different antenna beam directions with a height resolution of 1.5 km. In particular, the results obtained during disturbed conditions show the change of the zonal winds at mesospheric heights from westerly to easterly. A spectral analysis reveals a diurnal and a weaker semidiurnal tide of the zonal wind component.

Ruster, R.↗