A review of global satellite-derived snow products

Snow cover over the Northern Hemisphere plays a crucial role in the Earth’s hydrology and surface energy balance, and modulates feedbacks that control variations of global climate. While many of these variations are associated with exchanges of energy and mass between the land surface and the atmosphere, other expected changes are likely to propagate downstream and affect oceanic processes in coastal zones. For example, a large component of the freshwater flux into the Arctic Ocean comes from snow melt. The timing and magnitude of this flux affects biological and thermodynamic processes in the Arctic Ocean, and potentially across the globe through their impact on North Atlantic Deep Water formation.     

Reducing geometric dilution of precision using ridge regression

A signal processing technique is proposed for improving position-fix navigation system accuracy performance when the geometry of the navigation landmarks (e.g. sensors) are nearly collinear. In the navigation literature, the accuracy degradation associated with a nearly collinear measure geometry is termed the geometric dilution of precision (GDOP). Its presence causes not only the variance of the position estimates to be highly inflated but also any bias terms which may be present in the model. Since a nearly collinear predictor matrix is mathematically equivalent to GDOP, it is proposed to use the ridge regression technique in a navigation signal processor. A position-fix algorithm based on ridge regression reduces the bias and variance inflation caused by GDOP and the overall mean-squared position error as well. Ridge regression contains the GDOP-sensitive least-mean-square (LMS) estimator as a special case. Even with a matched model, GDOP can inflate the mean-square error (MSE) of the ordinary least-squares estimator, whereas the ridge regression technique chooses a suitable biased estimator that will reduce the MSE, which is the main goal. The ridge concept is extended to include GDOP-amplified bias errors. A simple range/range navigation system is analyzed to illustrate the underlying principles of ridge regression     

An Adaptive Detection Algorithm

A general problem of signal detection in a background of unknown Gaussian noise is addressed, using the techniques of statistical hypothesis testing. Signal presence is sought in one data vector, and another independent set of signal-free data vectors is available which share the unknown covariance matrix of the noise in the former vector. A likelihood ratio decision rule is derived and its performance evaluated in both the noise-only and signal-plus-noise cases.     

System Considerations for the New DME/P International Standard

The International Civil Aviation Organization (ICAO) has completed technical standards for the precision distance measuring equipment (DME/P). The DME/P is an integral element of the microwave landing system (MLS) and will provide the precision ranging function to complement the already standardized azimuth and elevation guidance functions of the system. In 1978 an ICAO meeting concluded that the DME/P should be integrated into the existing standards for the conventional DME (DME/N) as a compatible service. The motivation for this was economic. It was reasoned that a single L-band airborne unit could satisfy the need for both existing enroute and the new precision approach and landing services, thereby avoiding unnecessary duplication of on-board ranging equipment. Furthermore, this approach would permit existing conventional airborne equipment to obtain service from the new DME/P ground facilities at least during the initial stages of MLS implementation. These compatibility requirements were later incorporated into the DME/P statement of operational requirements which was accepted at an ICAO meeting in April 1981. Further, they have been a primary focus in the DME/P standardization effort which has been actively pursued since late 1980. The conceptual design of the DME/P that evolved from the ICAO process is discussed here. The comptability issues are highlighted, and it is shown how they are accommodated in the system synthesis which also treats the obvious need for a guidance function of high integrity and robust performance in the severe multipath environment encountered in approach and landing operations.     

Lithium-ion cell performance under environmental extremes

High energy density, lithium secondary cells are very attractive for use in many future military applications. However, a number of technical challenges remain. Specifically, the development and qualification of a system capable of withstanding the harsh environmental conditions encountered during normal and abnormal zones of operation. This paper focuses on the environmental extremes that the Eagle-Picher lithium-ion system has tested to date. Emphasis is placed on low temperature performance, high temperature performance, power capability, and cycle life at these extremes. Other areas including safety and environmental issues have also been investigated     

Continuous Measurement of Shock and Contact Discontinuities Velocity

An experimental method applying microwave techniques to obtain continuous measurement of both the shock and contact discontinuities bounding an air plasma generated in a cylindrical hypersonic shock tube is developed. X-band microwave signals excited in the TE11 mode reflect from the moving shock and contact surfaces. The resulting FM interference pattern is demodulated, yielding continuous velocity versus time data. Results depicting subtle detail of velocity behavior, particularly of the shock front, were obtained over a shock Mach number range of 9 to 13.     

Impact disruption and recovery of the deep subsurface biosphere

Although a large fraction of the world's biomass resides in the subsurface, there has been no study of the effects of catastrophic disturbance on the deep biosphere and the rate of its subsequent recovery. We carried out an investigation of the microbiology of a 1.76 km drill core obtained from the ～35 million-year-old Chesapeake Bay impact structure, USA, with robust contamination control. Microbial enumerations displayed a logarithmic downward decline, but the different gradient, when compared to previously studied sites, and the scatter of the data are consistent with a microbiota influenced by the geological disturbances caused by the impact. Microbial abundance is low in buried crater-fill, ocean-resurge, and avalanche deposits despite the presence of redox couples for growth. Coupled with the low hydraulic conductivity, the data suggest the microbial community has not yet recovered from the impact ～35 million years ago. Microbial enumerations, molecular analysis of microbial enrichment cultures, and geochemical analysis showed recolonization of a deep region of impact-fractured rock that was heated to above the upper temperature limit for life at the time of impact. These results show how, by fracturing subsurface rocks, impacts can extend the depth of the biosphere. This phenomenon would have provided deep refugia for life on the more heavily bombarded early Earth, and it shows that the deeply fractured regions of impact craters are promising targets to study the past and present habitability of Mars.     

Ideal Limiting of Periodic Signals in Random Noise

A method based on Hermite polynomials is developed for analysis of the output resulting from passing Fourier expandable signals and noise through general zero-memory devices. New results are developed for the output time and autocorrelation functions. Special results are generated for an ideal clipper. Techniques for obtaining results for practical cases are discussed. The method itself can be modified and applied to non-Gaussian noise and general nonlinear devices with nondeterministic signals.     

Engineering challenges to the long term operation of the International Space Station

The U.S. Congress has maintained an intense interest in the ISS program since its inception. In the Appropriations Act of 1997, the Senate of the United States included language directing National Aeronautics and Space Administration (NASA) to have the National Research Council (NRC) under take a study that evaluates the engineering challenges posed by extravehicular activity (EVA) requirements, United States and non-United States space launch requirements, the potential need to upgrade or replace equipment and components after Assembly Complete, and the requirement to decommission and disassemble the facility. NASA and the NRC decided the focus should be on the anticipated challenges in the continuous operation and maintenance of the ISS after assembly of the on-orbit facility has been completed. This would encompass the operational years, from late 2004 (if the current schedule holds) to 2020-2025. This executive summary overviews the results of this NRC study. It focuses on the U.S. operation of the ISS after Assembly Complete, including cooperative efforts by the United States and Russia. The paper summarizes the primary findings and recommendations in each of the areas considered during this two-year NRC study.