Antenna Beam Direction Research Articles

Cupri observations of PMSE during Salvo B of NLC‐91: Evidence of both partial reflection and turbulent scatter

During the first rocket sequence (called Salvo B) of the NLC‐91 campaign, the Cornell University Portable Radar Interferometer (CUPRI) observed two simultaneously occurring layers of Polar Mesosphere Summer Echoes (PMSE). During the time of the Turbo B flight, the high time‐resolution CUPRI Doppler spectra exhibited sawtooth‐like discontinuities in the lower layer which we interpret to be a distorted partial reflection layer which was advected across the radar beam. The upper layer, on the other hand, appeared to be caused by turbulent scatter and we estimate the turbulence energy dissipation rate in the upper layer at the time of the Turbo B flight to have been approximately 0.04 W/kg. Furthermore, a shift in the antenna beam direction from vertical to 8° off zenith revealed an aspect sensitivity of approximately 5 dB in the lower layer but none in the upper layer. We conclude that, at this particular time, turbulent scatter was responsible for the upper layer while some form of partial reflection was dominant in the lower layer.

Calculation method of separation angles and interference between satellite and radio-relay links in atmospheric refraction condition

When the frequency sharing between the fixed satellite communication system and the radio relay system is realized, it is indispensable to evaluate the interference noise in the design of each channel. Especially, to treat the interference between the satellite station and the terrestrial radio-relay station, it is necessary to find the separation angle between the radio-relay antenna beam and the geostationary satellite orbit. This paper presents a method for evaluation of the separation angle with the atmospheric beam refraction. The beam refraction angle is given as an integration along the beam path of the infinitesimal rate of change of the elevation angle for the altitude. A simple approximate expression is derived for facilitating the derivation of the separation angle. By comparison with the rigorous theory, the approximation errors are shown to be small enough for all practical purposes. A computation method for the separation angle is described for a terrestrial radio-relay station position and an antenna beam direction from the geostationary satellite orbit. As an application, an actual radio-relay link is chosen for which the separation angle distribution and the interference noise distribution are derived. By means of the computational method presented in this paper, the interference noise from the satellite station to the terrestrial radio-relay station is now evaluated rather easily.

On the role of parametric instability of internal gravity waves in atmospheric radar observations

In the presence of a monochromatic finite‐amplitude internal gravity wave, many properties of tropospheric and mesospheric radar observations can be explained consistently in terms of parametric instability. At small primary wave amplitudes, the fastest growing instability modes can be divided into two classes. In one class, the instability modes have scales comparable to that of the primary wave leading to a broadening of the wave number spectrum. The second class consists of small‐scale instability modes with a continuous wave number spectrum so that the primary wave can be “seen” by radars even in the absence of conventional instability mechanisms such as shear or static instability. The small‐scale instability modes generated by long‐period primary waves propagate almost vertically and form extended layers moving with the phase velocity of the primary wave. Thus, in accordance with observational results, the signal power of backscattered radar signals is strongest at near vertical antenna beam directions and shows a layered structure. The small‐scale instability modes further satisfy Taylor's frozen turbulence field hypothesis so that the Doppler shift of scattered radar signals yields the space and time dependent fluid velocity of the primary wave. At sufficiently large primary wave amplitudes, there is in particular an isolated fast growing instability mode which has a frequency close to the mean Väisälä‐Brunt frequency and is probably due to shear instability. It may give rise to a cascade of successive instability modes that can explain organized substructures in layers of enhanced tropospheric UHF radar returns and the frequent occurrence of short‐period evanescent waves in mesospheric VHF radar observations which cannot be attributed to Kelvin‐Helmholtz instability of the mean shear flow.

VHF radar observations in the stratosphere and mesosphere during a stratospheric warming

For the first time the SOUSY-VHF-Radar (lat. 52°N, long. 10°E, Germany) was used to carry out measurements during a minor and 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 has been derived from Doppler measurements made in three different antenna beam directions with a height resolution of 1.5 km. The resulting variation in height and time of the observed stratospheric and mesospheric wind speed and direction is discussed with respect to the stratospheric warming and compared to results obtained during undisturbed conditions.

Radio wave scattering from the tropical mesosphere observed with the Jicamarca radar

Using the VHF radar (49.92 MHz) at Jicamarca (12.0°S, 76.9°W), radio wave scattering from the tropical mesosphere was observed for more than 60 hours on November 14–16, 1977. The 60‐ to 90‐km region was probed at 2.5‐km intervals in two antenna beam directions: the vertical and 3.45° from the zenith to the west. Strong aspect sensitive scattering that is accompanied by a marked positive correlation between the temporal variation of the echo power and the signal correlation time is observed below about 75 km as in previous measurements. Above that altitude, the correlation becomes negative while the scattering is virtually isotropic. This difference appears to be directly related to the stability of the atmosphere, and enhanced reflectivity regions seem to be narrow structures that are horizontally stratified below 75 km. The rms turbulent velocity fluctuations generally increase above 75 km. Signal correlation time depends upon the period of data used to calculate power spectra in the period of 10–60 s, suggesting that there exist substantial velocity fluctuations on short time scales.

Electronically scanned antenna systems

Electronic scanning allows the direction, shape, or rate of change of direction of an antenna beam to be varied faster than is possible by mechanical movement of the aperture. The principal application is in advanced radar systems. Scanning may be in one or two dimensions, using linear, planar, circular or orthogonal apertures. Control may be by mechanical, ferrite, diode or circuit-type phase shifters, or by frequency variation. This control may be applied directly to the aperture, or indirectly by means of some collimation system. The most cost-effective solution to any given scanning problem is usually the configuration that provides no more facilities than the application is able to exploit. During the last two decades, antenna-scanning capabilities have exceeded requirements and few advanced systems have gone into production. However, appropriate applications are now emerging, and a steady but unspectacular growth in electronic scanning may be expected.