ALBERT

Beschreibung: An α -Chapman function with a continuously varying scale height H ( h ) can be used to describe the topside ionosphere electron density profile that seamlessly connects the ionosphere with the plasmasphere. Huang and Reinisch [2001] have derived the analytic expression of the scale height from the Vary-Chap function, which can be applied to the topside F2 region. However, the H ( h ) is still difficult to obtain because H ( h ) is a function of the unknown value H ( h ) at hmF2. In this paper, the maximum entropy method is introduced to solve this ill-posed problem, and the Vary-Chap scale height for each electron density profile is derived. Then the average percentage errors that defined as the differences between the measured electron density profiles and the Vary-Chap profiles obtained from the Vary-Chap scale heights are calculated with the Alouette/ISIS satellites topside sounder database. The results reveal that the average percentage errors are generally less than 5% and the Vary-Chap profiles can well represent the topside electron density profiles. What is more, the local time, seasonal, solar cycle, and latitudinal variations of the Vary-Chap scale height are preliminarily analyzed.

Beschreibung: Understanding and modeling the rainfall drop size distribution is important in a number of applications, in particular predicting and mitigating attenuation of satellite signals in the millimetre band. Various standard statistical distributions have been proposed as suitable models, the first widely accepted being the exponential distribution. Subsequently, gamma and lognormal distributions have been shown to provide better rainfall rate computations. Some empirical studies have revealed bimodal distributions under some circumstances. A natural question to ask therefore is how often gamma and lognormal distributions fit the empirical data. In this paper we fit lognormal and gamma distributions to one minute slices of rainfall drop size distributions (DSD) taken from seven years’ data from the Chilbolton Observatory in southern England. The chi-square goodness-of-fit of the models against the data is calculated and it is found that failure to fit is greater than would normally be expected. This failure to fit is broken down and examined against seasonal variations, different rain rates, atmospheric temperature and wind speed. Possible reasons for the lack of fit are explored and alternative fits using models based on Gaussian Mixture Models are developed and found to be an improvement.

Beschreibung: For decades analog beacon satellite receivers have generated multi-frequency narrow-band complex data streams that could be processed directly to extract total electron content (TEC) and scintillation diagnostics. With the advent of software-defined radio, modern digital receivers generate baseband complex data streams that require intermediate processing to extract the narrow-band modulation imparted to the signal by ionospheric structure. This paper develops and demonstrates a processing algorithm for digital beacon satellite data that will extract TEC and scintillation components. For algorithm evaluation a simulator was developed to generate noise-limited multi-frequency complex digital signal realizations with representative orbital dynamics and propagation disturbances. A frequency tracking procedure is used to capture the slowly changing frequency component. Dynamic demodulation against the low-frequency estimate captures the scintillation. The low-frequency reference can be used directly for dual-frequency TEC estimation.

Beschreibung: No abstract is available for this article.

Beschreibung: A novel technique for the full-wave analysis of 3D complex waveguide devices is presented. This new formulation, based on the Boundary Integral-Resonant Mode Expansion (BI-RME) method, allows the rigorous full-wave electromagnetic characterization of 3D arbitrarily-shaped metallic structures making use of extremely low CPU resources (both time and memory). The unknown electric current density on the surface of the metallic elements is represented by means of Rao-Wilton-Glisson basis functions, and an algebraic procedure based on a singular value decomposition is applied to transform such functions into the classical solenoidal and non-solenoidal basis functions needed by the original BI-RME technique. The developed tool also provides an accurate computation of the electromagnetic fields at an arbitrary observation point of the considered device, so it can be used for predicting high-power breakdown phenomena. In order to validate the accuracy and efficiency of this novel approach, several new designs of band-pass waveguides filters are presented. The obtained results (S-parameters and electromagnetic fields) are successfully compared both to experimental data, and to numerical simulations provided by a commercial software based on the finite-element technique. The results obtained show that the new technique is specially suitable for the efficient full-wave analysis of complex waveguide devices considering an integrated coaxial excitation, where the coaxial probes may be in contact with the metallic insets of the component.

Beschreibung: We present a computationally efficient method to predict joint rain fade on arbitrary networks of microwave links. Methods based on synthetic rain fields composed of a superposition of rain cells have been shown to produce useful predictions of joint fade, with low computational overhead. Other methods using rain fields derived from radar systems, have much higher computational overhead but provide better predictions. The proposed method combines the best features of both methods by using a small number of measured rain fields to produce annual fade predictions. Rain fields are grouped into Heavy Rain and Light Rain groups by maximum rain rate. A small selection of rain fields from each group are downscaled and fade predictions generated by pseudo-integration of specific attenuation. This paper presents a method to optimize the weights used to combine the heavy rain and light rain fade predictions to yield an estimate of the average annual distribution. The algorithm presented yields estimates of average annual fade distributions with an error small compared to year-to-year variation, using only 0.2% of the annual dataset of rain fields.

Beschreibung: In this work we will describe the technique of using an RF Sweeping Impedance Probe (SIP) to measure the AC impedance of an electrically short monopole immersed in a plasma. We analyze the SIP measurements which are taken from the payload of the Storms sounding rocket, launched from Wallops Island, Virginia, in 2007. The scientific objective of the Storms mission was to concentrate on whether density irregularities observed in midlatitude spread-F could arise from ionospheric coupling to terrestrial weather. As such, independent measurements of the electron density profile are crucial. Since the inherent nature of the SIP technique makes it relatively insensitive to errors introduced through spacecraft charging, probe contamination, and other DC effects, it is an ideal instrument to employ under disturbed plasma conditions. The instrument measures both the magnitude and phase of the AC impedance from 100 kHz to 20 MHz in 128 frequency steps, performing 45776 sweeps over the entire flight. From these measurements we infer both the absolute electron density n e and the electron neutral collision frequencies v en throughout the flight trajectory. The SIP data can be approximately analyzed using a fluid formulation and thin sheath approximation particularly at altitudes below 200 km, which allows us to match the measurements to quasistatic analytical formulas. At about 265 km on the upleg, the magnitude data transitioned to a highly damped response with increasing altitude. The phase data on the other hand continued to indicate increased plasma density and reduced collisionality as expected. For a large portion of the flight, the payload of the Storms mission exhibited an uncontrolled coning motion, making the local magnetic field orientation with respect to the dipole difficult to decipher. Despite these difficulties, we were able to obtain robust estimates of the electron density profile, using the phase information from each sweep. In addition, the electron neutral collision frequency obtained from matching to phase data alone was on the correct order of magnitude with respect to NRL-MSISE model values in the ionosphere between 100 km to 150 km.

Beschreibung: Inadequate representation of the environment is a limitation for prediction of radar system performance as well as for validation of propagation codes. To improve understanding of how different environmental effects/parameters compete and compare, this study examines the sensitivity of radar wave propagation to a suite of environmental parameters for low grazing angle near-surface radar systems at 3-15 GHz at horizontal and vertical polarizations. A global sensitivity analysis method is used, which accounts for parameter interactions, and propagation is modeled using the parabolic equation method. Environmental parameters examined include 8 sea state parameters and 8 parameters characterizing the vertical structure and character of range-independent refractivity profiles. The relative importance of parameters varies more with frequency than polarization and parameter interactions are found to be significant. Atmospheric mixed layer parameters are found to be the most sensitive, particularly the thickness of the mixed layer. The most significant ocean surface parameter is swell period; although, sea directionality is important at 3 GHz, and sea surface roughness and salinity are important at 9 and 15 GHz. Because of the spatial variability of sensitivity throughout the domain, regional analysis is performed to determine the most important parameters in different regions of the domain (1000 m in altitude and 60 km in range). These regional sensitivity results, along with those for the whole domain, provide guidance on prioritization of environmental characterization in numerical weather prediction and inversion studies (e.g., refractivity from clutter studies), which are two common methods currently used to address environmental effects on propagation.

Beschreibung: No abstract is available for this article.

Beschreibung: Key Points Professor Joshua Le-Wei Li