摘要:

This work describes a method for phase calibration of spatial interferometry (SI) systems, which have recently seen increasing use in the wind profiling community. The various available techniques are inherently dependent on any phase imbalance in the system. The imbalance is caused by a path length difference between the multiple receivers, which is typically corrected at the time of construction. Aging of the system and other environmental changes can cause a modification in the path lengths, however. An obvious method for calibrating an SI system involves examining the phase differences between the receivers for a signal produced by a source with known location. For physically small systems, this can be accomplished by pointing the antenna beam toward a man‐made source, such as a simple transmitter with the same center frequency as the radar. For large MST (mesosphere/stratosphere/troposphere) radar systems, such as the Middle and Upper (MU) Atmosphere radar, this method is impractical. The present work describes the calibration of the MU radar using the radio star Cygnus A as a source of known location. As the radio star traverses the beam, the MU radar passively tracks it by measuring the phase differences of the cosmic noise signal received on spatially separated receivers. Since the location of Cygnus A is known from astronomical calculations, any phase imbalance can be compensated. Data from May 1992 show the usefulness of the metho

ISSN:0048-6604    

DOI:10.1029/95RS02319

年代:1996

数据来源: WILEY

摘要:

The presented theory ties the properties of a turbulently advected scattering medium to the cross correlation and cross spectrum of signals in a general configuration of receiving and transmitting antennas. The correlation length of Bragg scatterers and antenna diameter are the significant parameters determining the diffraction pattern's correlation length. We examine how vertical anisotropy of the scattering medium affects the diffraction pattern's correlation length. We demonstrate that the cross spectrum can be formulated in terms of a pair of spectral sampling functions (a one‐dimensional Doppler and a three‐dimensional wavenumber function), and closed form solutions are obtained. We give the conditions under which the scattering medium's statistical properties can be represented by a Gaussian correlation or spectral model, and the distance over which the diffraction pattern simply advects without significant change. We show that the diffraction pattern of a pair of scatterers can translate at the speed of the scatterers, not twice their speed as is commonly thou

ISSN:0048-6604    

DOI:10.1029/95RS02318

年代:1996

数据来源: WILEY

摘要:

This paper summarizes the results of recent laboratory investigations into the ultrawideband, polarimetric scattering properties of breaking water waves. Breaking waves were produced in a wave tank through chirped wave packets (0.5–1.0 Hz) generated by a programmable wavemaker. An ultrawideband radar system was then used to measure the polarimetric properties of the breakers over the 6–12‐GHz band at a grazing angle of 10°. An analysis of the data from these experiments shows interesting dynamics in and differences between the dominant VV and HH polarizations. In particular, the RF frequency responses of the orthogonally polarized signals differ substantially, and both the magnitudes as well as the relative phases can change appreciably over relatively short time intervals. A “bench top” model of a breaker is used to support the hypothesis that this behavior is due to the presence of small, nonlinear structures near the breaker crest and a “double‐bounce” scattering mechanism asso

ISSN:0048-6604    

DOI:10.1029/95RS01762

年代:1996

数据来源: WILEY

摘要:

The changes with altitude of the vertical electric field from seven VLF ground‐based transmitters were measured during the ascent of the Extended Life Balloon‐Borne Observatory (ELBBO) balloon test flight from Ft. Sumner, New Mexico. The altitude profiles of the VLF vertical electric field allowed us to determine the propagation modes based on the relative amplitude changes of the VLF signals with height. Three of the VLF transmitters had signals which crossed through the dawn terminator on their paths to the balloon's receiver. The observed height gains from these three stations show that at least two models of sunrise modal interference, those proposed byCrombie[1964] and byKaiser[1968 ], are occurring. For these three cases a time and altitude‐varying height gain signal (the ratioE(,z)/E(0) for altitudezabove the surfacez= 0 ) was measured during the balloon's 70‐min ascent to an altitude of 28.5 km. This method of balloon‐measured VLF height gains allows a high level of discrimination between modes, which is shown to be a useful method for studying the phenomena of sunrise and sunset terminator effects on VLF modal int

ISSN:0048-6604    

DOI:10.1029/95RS02637

年代:1996

数据来源: WILEY

摘要:

The characteristics of three different modes of ELF and VLF excitation using the high‐power auroral simulation (HIPAS) HF heater array are compared under different ionospheric conditions. For each of these three methods, amplitude modulation (AM), phase demodulation (DM), and the double‐frequency excitation (DF), we observed signals on the order of a few picoteslas at two widely separated receiver sites for bothXandOmode heater polarizations. Strong electrojet activity is essential for ELF generation at frequencies close to the Schumann range, whereas at higher frequencies, such as in the VLF range (greater than 1 kHz), signals could be observed under a wider range of ionosopheric conditions. The AM method generally produced the largest signal at both frequency ranges, while the DM mode was approximately half of this signal amplitude. The DF generated signals are comparable in strength with the AM signals at VLF frequencies and are more stable than either method as a function of time. TheXmode polarization also produces a stronger signal than theOmode for these three methods of excitation by a factor of two. The polarization of the received signals follow the same variation at VLF frequencies for all three modes, indicating a common height of ori

ISSN:0048-6604    

DOI:10.1029/95RS01993

年代:1996

数据来源: WILEY

摘要:

For experimental observation of the Jovian decametric emission, it is important to know the low‐frequency limit of the received radiation, which depends on the Jovian elevation (angular position from the local horizon) and the ionospheric electron density distribution. We develop the corresponding analysis and present results for different Jovian positions and different models of the ionospheric electron density profile. This consideration is of special interest for the periods 1993 to 1997 and also 2005 to 2009, years for low Jovian positions observed from northern hemisphere observatories where Jupiter is in the southern hemisphere in the celestial equatorial coordinate system. For several ionospheric models we determine the using range which depends on the Jovian elevations. By the use of empirical ionospheric models fromFeichter and Leitinger,[1990, 1993] of the total electron content (TEC) from which an electron density maximum can be derived, we found that the solar activity highly influences the electron peak density which, in turn, acts on the low‐frequency limit of the detectable Jovian decametric emission from ground‐based observat

ISSN:0048-6604    

DOI:10.1029/95RS02952

年代:1996

数据来源: WILEY

ISSN:0048-6604    

DOI:10.1029/96RS00024

年代:1996

数据来源: WILEY