How Operational Oceanography Can Benefit from Dynamic Topography Estimates as Derived from Altimetry and Improved Geoid

With a precise geoid, GOCE will allow an estimation of absolute dynamic topography from altimetry. The projected benefits to operational oceanography and its applications are analyzed herein. After a brief overview of operational oceanography, we explain how the new geoids will be used in the future to improve real time altimeter products and to better constrain modelling and data assimilation systems. A significant impact is expected both for mesoscale (e.g. better estimations and forecasts of currents for pollution monitoring, marine safety, offshore industry) and climate (better initialization of coupled ocean/atmosphere models) applications. This revised version was published online in August 2006 with corrections to the Cover Date.     

Blind adaptive decision fusion for distributed detection

We consider the problem of decision fusion in a distributed detection system. In this system, each detector makes a binary decision based on its own observation, and then communicates its binary decision to a fusion center. The objective of the fusion center is to optimally fuse the local decisions in order to minimize the final error probability. To implement such an optimal fusion center, the performance parameters of each detector (i.e., its probabilities of false alarm and missed detection) as well as the a priori probabilities of the hypotheses must be known. However, in practical applications these statistics may be unknown or may vary with time. We develop a recursive algorithm that approximates these unknown values on-line. We then use these approximations to adapt the fusion center. Our algorithm is based on an explicit analytic relation between the unknown probabilities and the joint probabilities of the local decisions. Under the assumption that the local observations are conditionally independent, the estimates given by our algorithm are shown to be asymptotically unbiased and converge to their true values at the rate of O(1/k/sup 1/2/) in the rms error sense, where k is the number of iterations. Simulation results indicate that our algorithm is substantially more reliable than two existing (asymptotically biased) algorithms, and performs at least as well as those algorithms when they work.     

Pitch-angle diffusion and the origin of temporal and spatial structures in morningside aurorae

Morningside aurorae at latitudes below about 70° display complex spatial and temporal structures unlike anything seen in the evening or midnight sectors. The morningside structures are believed to be formed by the precipitation of trapped electrons injected in auroral substorms; no significant role has yet been identified in the morningside auroral regions for the large-scale parallel electric fields that dominate the evening side. How those spatial and temporal structures originate has been the subject of much speculation; most theoretical mechanisms focus on the wave-particle interactions that drive pitch-angle diffusion. The principal evidence pertaining to the role of pitch-angle diffusion in the auroral regions is reviewed here. The observational evidence concerns mainly auroral emissions in the atmosphere, energetic particles observed from rockets and satellites, VLF waves at high altitudes, magnetospheric cold plasma, and magnetic pulsations detected on the ground. With the aid of such evidence, plus observations and theories related to the outer permanently trapped radiation belts, several theoretical models for the modulation of VLF wave growth in the equatorial regions have been pieced together. Those models, and the observational data supporting them, are examined to see how well they fit the observational picture and to see where they might lead in future research. The models fall into two categories: those in which the modulations are externally imposed and those in which the modulations are self-excited. For the temporal variations the self-excited mechanisms are now favored. The leading candidate involves a nonlinear relaxation oscillator; the nonlinearity may have important consequences. There are several contenders in both categories for the origin of the spatial structures, none of which agrees fully with inferences from the observations. All the theories involve critical parameters that have not yet been precisely fixed. The critical research needs are listed and discussed.     

Optimization of observer trajectories for bearings-only targetlocalization

The problem of bearings-only target localization is to estimate the location of a fixed target from a sequence of noisy bearing measurements. Although, in theory, this process is observable even without an observer maneuver, estimation performance (i.e., accuracy, stability and convergence rate) can be greatly enhanced by properly exploiting observer motion to increase observability. This work addresses the optimization of observer trajectories for bearings-only fixed-target localization. The approach presented herein is based on maximizing the determinant of the Fisher information matrix (FIM), subject to state constraints imposed on the observer trajectory (e.g., by the target defense system). Direct optimal control numerical schemes, including the recently introduced differential inclusion (DI) method, are used to solve the resulting optimal control problem. Computer simulations, utilizing the familiar Stansfield and maximum likelihood (ML) estimators, demonstrate the enhancement to target position estimability using the optimal observer trajectories     

A chirp scaling approach for processing squint mode SAR data

Image formation from squint mode synthetic aperture radar (SAR) is limited by image degradations caused by neglecting the range-variant filtering required by secondary range compression (SRC). Introduced here is a nonlinear FM chirp scaling, an extension of the chirp scaling algorithm, as an efficient and accurate approach to range variant SRC. Two methods of implementing the approach are described. The nonlinear FM filtering method is more accurate but adds a filtering step to the chirp scaling algorithm, although the extra computation is less than that of a time domain residual compression filter. The nonlinear FM pulse method consists of changing the phase modulation of the transmitted pulse, thus avoiding an increase in computation. Simulations show both methods significantly improve resolution width and sidelobe level, compared with existing SAR processors for squint angles above 10 deg for L-band and 20 deg for C-band     

SOFIA: Stratospheric Observatory For Infrared Astronomy

SOFIA is a planned 2.5 meter telescope to be installed in a Boeing 747 aircraft and operated at altitudes from 41,000 to 46,000 feet. It will permit routine measurement of infrared radiation inaccessible from the ground-based sites, and observation of astronomical objects and transient events from anywhere in the world. The concept is based on 18 years of experience with NASA's Kuiper Airborne Observatory (KAO), which SOFIA would replace.     

Receiver characteristics of laser altimeters with avalanchephotodiodes

The receiver characteristics of a laser altimeter system containing an avalanche photodiode photodetector are analyzed using the Gaussian approximation, the saddlepoint approximation, and a nearly exact analysis. The last two methods are shown to yield very similar results except when the background noise is extremely low and the probability of false alarm is high. However, the Gaussian approximation method is shown to cause significant errors even under relatively high levels of background noise and received signal energy