摘要:

The ultimate precision with which very long baseline interferometry (VLBI) can determine the angular position of a spacecraft is determined by the quantum mechanical limitations on the performance of the interferometer receivers and the quantum mechanical uncertainty relation ΔNΔΦ ≥ 1. It is shown that for the navigation of a typical deep space mission using present‐day techniques, fundamental physics imposes the following limits on the precision of spacecraft navigation: (1) Minimum noise on determination of phase of spacecraft navigation tone, ΔΦmin≈ 1.9 × 10−5radians per AU, (2) minimum noise on determination of phase of VLBI navigation fringes, ΔΘmin≈ 2.6 × 10−5radians per AU, (3) minimum noise on determination of VLBI navigation fringe frequency, Δfmin≈ 2.9 × 10−9Hz per AU, (4) minimum noise on determination of VLBI group delay, Δτgmin≈ 0.5 ps per AU, and (5) minimum noise on determination of spacecraft angular position, ΔΨmin≈ 2.9 × 10−11radians per AU. The above limitations which are a consequence of quantum mechanical uncertainty on the determinations of the phase of a spacecraft tracking signal may be circumvented in principle by the a

ISSN:0048-6604    

DOI:10.1029/RS025i002p00097

年代:1990

数据来源: WILEY

摘要:

The scattering characteristics of a radar target can be specified by its impulse response. The impulse response can be computed using Fourier spectral estimation techniques on samples of the radar scattering obtained at a number of equally spaced frequencies. Usually, there is a clear relationship between the location of specific scattering mechanisms and the time such mechanisms appear in the impulse response. One of the difficulties in this type of analysis, however, is that complex targets often have multiple scattering interactions. Many of the terms in the impulse response are due to these interactions rather than specular subcomponent scattering. The bispectrum displays the specifics of the interactions of both single‐ and multiple‐reflection mechanisms. In addition, the suppression of Gaussian noise makes the bispectrum a source of robust features for classification of radar targets. Identification of multiple interaction terms as well as noise suppression are demonstrated in this paper. Finally, classification of scale models of commercial aircraft based on their bispectral signatures is perfor

ISSN:0048-6604    

DOI:10.1029/RS025i002p00101

年代:1990

数据来源: WILEY

摘要:

Three years' observations of the HILAT satellite from stations Sondre (Sondre Stromfjord), Churchill, and Tromso have been used to study the distributions of scintillations over the northern polar region. Two regions showed enhancement. One was an enhancement of phase scintillations when the line of sight to the satellite lay along anLshell and the observing station was under the auroral oval. The second enhancement is revealed most clearly by amplitude scintillations and maximizes in an annular region several degrees poleward of the auroral oval. TheLshell enhancement is most likely associated with large‐scale “blobs” of ionization in the auroral zone, whereas the second enhancement appears to be due to kilometer scale irregularities generated in the pola

ISSN:0048-6604    

DOI:10.1029/RS025i002p00115

年代:1990

数据来源: WILEY

摘要:

Systematic time differences observed in the onset of postsunset VHF scintillations, recorded simultaneously at a meridian chain of stations in the Indian sector, have been analyzed in order to determine the plasma bubble rise velocities over the magnetic equator. The method is based on the assumption that the postsunset scintillations up to several degrees on either side of the magnetic equator are generated by the equatorial plasma bubble and associated irregularities which are strongly field aligned. As the plasma bubble rises in the equatorial ionosphere, after its generation in the bottomside of the F layer, the low‐latitude extremities of the bubble propagate away from the equator in such a way that the upper height limit of the irregularities defines the latitudinal limit of the scintillation occurrence associated with the given event. From the relative time delays observed in the onset of postsunset scintillations at a chain of different latitude stations situated nearly along a common meridian plane and the altitudinal differences over the magnetic equator between their respective field lines corresponding to the F region heights, we have determined the average bubble rise velocities between three altitudinal slabs. The velocities so determined are found to vary from about 128 to 416 m/s between the altitudes of 450 and 550 km, 38 to 327 m/s between 550 and 1140 km, and 15 to 200 m/s between 1140 and 1270 km, which shows that the bubble rise velocities decrease with altitude. The results are found to be comparable to the values measured by VHF radar and satellite techniques. The calculated values of bubble rise velocity during the early postsunset hours are found to have good correlation with the equatorial F layer vertical drift velocities. Also, as compared to the results derived by using spaced ionosondes, present results should yield the plasma bubble rise velocities with better precision because of better spatial and temporal resolution. Thus the present technique provides an effective and rather simple way of investigating the equatorial ionospheric dynamic

ISSN:0048-6604    

DOI:10.1029/RS025i002p00125

年代:1990

数据来源: WILEY

摘要:

A method for the analysis of infinite planar circular cylindrical phased arrays of waveguide fed apertures embedded in a lossy surface is presented. Use is made of the surface impedance concept and an appropriate equivalence principle to reduce considerably the number of simultaneous equations arising in the Galerkin procedure. Numerical results for the matched element gain pattern are presented for circular feed waveguides and compared with the lossless case.

ISSN:0048-6604    

DOI:10.1029/RS025i002p00133

年代:1990

数据来源: WILEY

摘要:

The electromagnetic field generated by a vertical electric dipole on or near the surface of the Earth is studied for continuous‐wave excitation. The analytical foundation established by Sommerfeld and developed by many others is reviewed, and simple explicit new formulas are derived for the three components of the electromagnetic field and represented graphically. Their range of validity is defined. They show that the surface waves constitute the entire far field along the air‐earth bound

ISSN:0048-6604    

DOI:10.1029/RS025i002p00149

年代:1990

数据来源: WILEY

摘要:

A radial propagation integral representation of the microstrip electric dyadic surface Green's function is developed. This representation is very efficient for a numerical evaluation of the field when the source and observation points are laterally rather than vertically separated with respect to the plane of the substrate. Furthermore, when the integration contour is deformed to the steepest descent path, the Green's function exhibits an even faster convergence. In contrast, the conventional Sommerfeld integral representation of the microstrip Green's function converges very poorly for this case. Numerical examples are presented which indicate that the representations obtained here are surprisingly efficient even for relatively small lateral separation of the source and field points. This work is especially useful in the moment method analysis of microstrip antenna arrays where the mutual coupling effects are important.

ISSN:0048-6604    

DOI:10.1029/RS025i002p00161

年代:1990

数据来源: WILEY

摘要:

The high frequency diffraction (transverse electric case) by a partially dielectric‐coated, perfect electric conducting (PEC) half plane is considered. The coating is of finite length and is the same on both sides of the half plane. The length of the two dielectric strips can be different; however, to simplify the analysis and without loss of generality, it is assumed to be the same. This problem requires the solution of two canonical problems which are solved via the Wiener‐Hopf method. The coating is assumed to be thin, and it is modeled by a generalized impedance boundary condition ofO(t), wheretis the thickness of the coating on each face of the PEC half plane. The interactions between the edges of the coating, i.e., surface wave diffraction, doubly and triply diffracted fields, are also considered. These multiply diffracted fields, including two new terms, are obtained via an extended spectral ray method which is related to the spectral theory of diffraction. The backscattered and bistatic echo widths are computed with the solutions developed here and compared with an independent moment method solution. The agreement between the two solutions is very g

ISSN:0048-6604    

DOI:10.1029/RS025i002p00175

年代:1990

数据来源: WILEY