Combined Use of Altimetry and In Situ Gravity Data for Coastal Dynamics Studies

Accurate local geoids derived from in situ gravity data will be valuable in the validation of GOCE results. In addition it will be a challenge to use GOCE data in an optimal way, in combination with in situ gravity, to produce better local geoid solutions. This paper discusses the derivation of a new geoid over the NW European shelf, and its comparison with both tide gauge and altimetric sea level data, and with data from ocean models. It is hoped that over the next few years local geoid methods such as these can be extended to cover larger areas and to incorporate both in situ and satellite measured gravity data. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Genesis Solar-Wind Collector Materials

Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection. Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector arrays and elsewhere targeted for the analyses of specific solar-wind components. Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability. A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community throughout the 21st Century. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spectral-domain covariance estimation with a priori knowledge

A knowledge-aided spectral-domain approach to estimating the interference covariance matrix used in space-time adaptive processing (STAP) is proposed. Prior knowledge of the range-Doppler clutter scene is used to identify geographic regions with homogeneous scattering statistics. Then, minimum-variance spectral estimation is used to arrive at a spectral-domain clutter estimate. Finally, space-time steering vectors are used to transform the spectral-domain estimate into a data-domain estimate of the clutter covariance matrix. The proposed technique is compared with ideal performance and to the fast maximum likelihood technique using simulated results. An investigation of the performance degradation that can occur due to various inaccurate knowledge assumptions is also presented     

Current Injected Equivalent Circuit Approach to Modeling Switching DC-DC Converters

A new current injected equivalent circuit approach (CIECA) to modeling switching dc-dc converter power stages is developed, which starts with the current injected approach and results in either a set of equations which completely describe input and out-put properties or an equivalent linear circuit model valid at small signal, low frequency levels. This approach to modeling switching dc-dc converter power stages has the merits but not the demerits of both the electronic equivalent circuit state space average approach and the current injected control type approach, namely, 1) the modeling is very clear and is simple whether the converter operates in continuous or discontinuous inductor conduction modes, 2) the modeling results in an equivalent circuit which is very close to the actual converter, and 3) the equivalent circuit can be used directly in the computer for theoretical predictions like SPICE, etc.     

ATS-6 Experiment Summary

The ATS-6 is the most advanced experimental satellite that has evolved from the Application Technology Satellite Program conducted and implemented by NASA Goddard Space Flight Center (NASA/GSFC). This project utilizes a state-of-the-art spacecraft and ground terminal network to perform advance studies and to conduct technological demonstrations in a large number of scientific areas. The design and implementation of this unique spacecraft permitted multiple experimentation simultaneously. The control of the spacecraft is performed at ATS Operational Control Center (ATSOCC) located at NASA/GSFC. Experimentation which was performed covered a wide spectrum of communications, technological, meterorological, and scientific subjects. Three principal ground terminals are utilized to assist the experimenters to acquire data. Data reduction and analysis are performed by the many facilities at NASA/GSFC in support of the experimenters.     

Geostationary tether satellite system and its application tocommunication systems

The geostationary tether satellite system expands the geostationary orbit resource from a one-dimensional arc into a two-dimensional disk. The tethered satellites, each several thousand kilometers apart and aligned along the local vertical, are stabilized at the altitude of the geosynchronous orbital speed. When this system is applied to communications systems, it is estimated that the number of satellites can be increased as much as thirteenfold and the communication capacity can be increased more than seventeenfold, compared with a conventional geostationary satellite orbit system     

Transient Modeling of Synchronous Machine-Transformer-SCR Bridge

A rigorous mathematical model of a synchronouschine-machine-transformer-thyristor bridge unit is presented in state-space form with coefficients that are explicit functions of standard parameters. The developed model has a minimum order for each of the various operation modes of the bridge, and its form is such that it can be readily interfaced with any type of dc network connected to the bridge. In addition, the model has the capability to consider different types of transformer winding connections. In a digital simulation, the explicit form of the state model eliminates the need of matrix inversions at each integration step. This and the use of the ?, ? reference frame result in a high computing efficiency and accuracy. As an illustration, simulation results are shown of a case study where the unit is part of a high-power pulse generating system.     

Photoelectron Counting Statistics and Error Performance of m-Distributed Optical Signal

By the term "m-distributed optical signal" we mean a noise-like optical signal whose envelope (or intensity) fluctuation probability is modeled by Nakagami's "m-distribution." Using the m-distribution which has been widely used as an analytical model of the fading envelope in radio communications, it is shown that one can generally analyze the statistical properties such as the photoelectron count probabilities and error probabilities for the wider class of noise-like optical signals; some numerical results are also given.     

AC/DC converter topologies for the Space Station

A new class of AC/DC converter topologies (Type-1 converters) is described, suitable for use in an advanced single-phase sine-wave voltage, high-frequency power distribution system, of the type that was proposed for a 20 kHz Space Station primary electrical power distribution system. The converter comprises a transformer, a resonant network, a current controller, a diode rectifier, and an output filter. The input AC voltage source is converted into a sinusoidal current source using the resonant network. The output of this current source is rectified by the diode rectifier and is controlled by the current controller. The controlled rectified current is then filtered by the output filter to obtain a constant voltage across the load. Three distinct converter topologies, Type-1A, Type-1B, and Type 1-C, are described, and their performance characteristics are presented. All three types have a close-to-unity rated power factor (greater than 0.98), low total harmonic distortion in input current (less than 5%), and high conversion efficiency (greater than 96%)