摘要:

The potential of imaging large ionospheric structures by applying tomographic techniques has only been proposed recently and has already received interest and increasing attention by workers in ionospheric research. In the combined case of an idealized plane geometry and complete and continuous data extending to ±∞ the inversion is exact and unique. This is no longer the case if any of the idealized conditions are removed. Specifically, we investigate the limitations arising from limited angles, sampled data of finite receiving aperture and circular geometry. Under these various nonidealized but more realistic conditions, fidelity of the reconstructed image is investigated by computing the response to an impulse source function. The deterioration of the reconstructed image, the sensitivity to the position in the image plane, and the possible existence of ghost images are all numerically investigated and illustrated. Possible methods of improvement are suggest

ISSN:0048-6604    

DOI:10.1029/91RS01873

年代:1991

数据来源: WILEY

摘要:

The internal consistency of the baseline length measurements derived from analysis of several independent very long baseline interferometry (VLBI) experiments is an estimate of the measurement precision. In this paper we investigate whether the inclusion of water vapor radiometer (WVR) data as an absolute calibration of the propagation delay due to water vapor improves the precision of VLBI baseline length measurements. We analyzed 28 International Radio Interferometric Surveying runs between June 1988 and January 1989; WVR measurements were made at the G. R. Agassiz Station of the Harvard College Observatory in Ft. Davis, Texas, during each session. We found that the addition of WVR data decreased the scatter of the length measurements of Ft. Davis baselines by 5–10%. The observed reduction in the scatter of the baseline lengths is less than what we would expect from the behavior of the formal errors, which suggest that the baseline length measurement precision should improve 10–20% if WVR data at Ft. Davis are included in the analysis. The discrepancy between the formal errors and the baseline length results could be explained as the consequence of systematic errors in the dry‐mapping function parameters, instrumental biases in the WVR and Ft. Davis barometer, or

ISSN:0048-6604    

DOI:10.1029/91RS02020

年代:1991

数据来源: WILEY

摘要:

We have proposed more stable and accurate recursing equations and a computing procedure to calculate scattering coefficients for a multilayered sphere. The procedure involves three logarithmic derivatives of Ricatti‐Bessel functions ψn′(z)/ ψn(z), χn′(z)/ χn(z), and ξn′(z)/ξn(z), as well as the ratio ψn(z)/χn(z). The asymptotic behavior, stability, and accuracy of the procedure and scattering coefficientsanandbnare discussed for any complex refractive indices. This procedure can be used for large and small inhomogeneous sphe

ISSN:0048-6604    

DOI:10.1029/91RS01192

年代:1991

数据来源: WILEY

摘要:

Measurements of phase path P of lower F region reflections at normal incidence at Kodaikanal (10°14′N, 77° 28′E, dip 3.0°N) revealed the ubiquitous presence of 30–300 s quasi‐sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift) during day time. A study is made of the influence of the irregularities in the equatorial electroject on the P fluctuations using simultaneous observations of F region phase path at Kodaikanal and of equatorial electrojet with the VHF backscatter radar at Thumba (08° 29′N, 76° 56′E, dip 0.9°S). It is shown that the spectral content of the Doppler fluctuations (quantified in terms of variance, σ2computed from P time series synthesized through FFT−1(fast Fourier transfrom) in the chosen period bands, 30–300 s/30–120 s of the FFT of original P time series) bears a significant positive linear relationship to the horizontal phase velocity (Vp) of electrojet irregularities (3‐m scale size)on a hourly basis. This result is in consonance with our earlier findings (Sastri et al., 1990) of a significant linear relationship of σ2to the electrojet strength (estimated from H field data) and a practical cessation of the P fluctuations at times of disappearance of Esq on ionograms (partial/complete counterelectrojet). The present work substantiates the interpretation that the short‐period Doppler frequency fluctuations are due to phase path changes imposed on lower F region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (type I and II) in the

ISSN:0048-6604    

DOI:10.1029/91RS02021

年代:1991

数据来源: WILEY

摘要:

It is shown that the calculation of the transmission and reflection coefficients and the fields of electromagnetic waves in a magnetized plasma layer can be solved as a two‐point boundary value problem. Test results are presented to demonstrate the applicability of this metho

ISSN:0048-6604    

DOI:10.1029/90RS02330

年代:1991

数据来源: WILEY

摘要:

An error analysis is presented for the image reconstruction algorithm currently employed by the electronically steered thinned array radiometer aircraft instrument. This instrument samples components of the Fourier transform of the brightness temperature distribution at 1.4 GHz. Sources of error are identified, including systematic offsets and stochastic noise in the measurements, errors in the interference pattern antenna calibration, multipath scattering from the antenna support structure, mutual coupling between nearby array elements, and variations in the antenna patterns of the array elements. These errors are shown to produce artifacts in the image which appear as “streaks” running along the instrument flight line. Possible hardware corrections are discussed. These include a reduction in the profile of the antenna support structure and a decoupling of adjacent and other nearby array elements using corrugated “skirts.” Also discussed are several image enhancement techniques which detect and correct for some of the streaking. These techniques include a renormalization of the interference patterns based on the measured element patterns, a perturbation based interpretation of image contrast, and a covariance analysis of noise present in th

ISSN:0048-6604    

DOI:10.1029/91RS02355

年代:1991

数据来源: WILEY

ISSN:0048-6604    

DOI:10.1029/91RS02815

年代:1991

数据来源: WILEY

ISSN:0048-6604    

DOI:10.1029/91RS02825

年代:1991

数据来源: WILEY