A major challenge in using GPS guidance for aircraft final approach and landing is to reject interference that can jam reception
of the GPS signals. Antenna arrays, which use space–time adaptive processing (STAP), significantly improve the signal to interference
plus noise ratio, but at the possible expense of distorting the received signals, leading to timing biases that may degrade
navigation performance. Rather than a sophisticated calibration approach to remove biases introduced by STAP, this paper demonstrates
that a relatively compact calibration strategy can substantially reduce navigation biases, even under elevated interference
conditions. Consequently, this paper develops an antenna bias calibration strategy for two classes of adaptive array algorithm
and validates this method using both simulated and experimental data with operational hardware in the loop. A proof-of-concept
system and an operational prototype are described, which implement the adaptive antenna algorithms and deterministic corrections.
This investigation demonstrates that systems with adaptive antenna arrays can approach the accuracy and integrity requirements
for automatic aircraft landing, and in particular for sea-based landing on board aircraft carriers, while simultaneously providing
significant attenuation of interference. Evidence suggests that achieving these goals is possible with minimal restrictions
on system hardware configuration—specifically, limitations on the permissible level of antenna anisotropy and the use of sufficient
analog-to-digital converter resolution. 相似文献
Increased interest in paleoenvironmental studies is a result of climatic changes occurring at present and predicted for the future. Such studies could be done using the stable isotope compositions (δ2H and δ18O) of kaolins, which provide knowledge on the paleoenvironmental conditions prevailing during the time of kaolinisation. In this study, the stable isotopic compositions of clay-size fraction of kaolins occurring in Cretaceous and Tertiary Formations of the Douala Sub-Basin in Cameroon are presented, with the aim of reconstructing the paleoenvironmental conditions of the Sub-Basin. To achieve this, the clay-size fraction (< 2 μm fraction) of 8 kaolinite-rich samples were analysed for their δ2H and δ18O compositions, and results were reported as part per mil (‰) relative to the SMOW standard. The δ18O values of kaolins found in the Cretaceous–Tertiary Formations of the Douala Sub-Basin varied between +18.2 and +21.0‰, whereas the δ2H values varied between –69 and –53‰. Nine of the eleven samples plotted on the right of the supergene–hypogene line. Five of these nine samples plotted very close to the kaolinite line, which represents the composition of kaolinite in equilibrium with meteoric water at 20 °C; suggesting a supergene weathering origin of these kaolins. The determination of the temperature of kaolinisation yielded mean formation temperatures of 22 ± 2 °C and 27 ± 6 °C for Cretaceous and Tertiary kaolins, respectively. Excluding the two samples falling in the hypogene field, averages of kaolinisation temperatures were 20 and 25 °C during the Cretaceous and Tertiary periods, respectively. These temperatures are slightly below the present mean annual temperature in Douala (27 °C), thereby suggesting that the climate was becoming warmer from the Cretaceous to the Present. Therefore, Douala had a cooler and rainy climate during the Cretaceous, and the climate is gradually becoming hotter and more humid, favouring the refinement of existing kaolins and the kaolinisation of kaolin-forming minerals in the Sub-Basin. 相似文献
A sediment core spanning 7000 cal yr BP recovered from Stella Lake, a small sub-alpine lake located in Great Basin National Park, Nevada, was analyzed for subfossil chironomids (non-biting midges), diatoms, and organic content (estimated by loss-on-ignition (LOI)). Subfossil chironomid analysis indicates that Stella Lake was characterized by a warm, middle Holocene, followed by a cool “Neoglacial” period, with the last two millennia characterized by a return to warmer conditions. Throughout the majority of the core the Stella Lake diatom-community composition is dominated by small, periphytic taxa which are suggestive of shallow, cool, alkaline, oligotrophic waters with extensive seasonal ice cover. A reconstruction of mean July air temperature (MJAT) was developed by applying a midge-based inference model for MJAT (two-component WA-PLS) consisting of 79 lakes and 54 midge taxa (rjack2 = 0.55, RMSEP = 0.9°C). Comparison of the chironomid-inferred temperature record to existing regional paleoclimate reconstructions suggests that the midge-inferred temperatures correspond well to regional patterns. This multi-proxy record provides valuable insight into regional Holocene climate and environmental conditions by providing a quantitative reconstruction of peak Holocene warmth and aquatic ecosystem response to these changes in the Great Basin, a region projected to experience increased aridity and higher temperatures. 相似文献
Palynological studies document forest disappearance during the late Holocene in the tropical Maya lowlands of northern Guatemala. The question remains as to whether this vegetation change was driven exclusively by anthropogenic deforestation, as previously suggested, or whether it was partly attributable to climate changes. We report multiple palaeoclimate and palaeoenvironment proxies (pollen, geochemical, sedimentological) from sediment cores collected in Lake Petén Itzá, northern Guatemala. Our data indicate that the earliest phase of late Holocene tropical forest reduction in this area started at ∼ 4500 cal yr BP, simultaneous with the onset of a circum-Caribbean drying trend that lasted for ∼ 1500 yr. This forest decline preceded the appearance of anthropogenically associated Zea mays pollen. We conclude that vegetation changes in Petén during the period from ∼ 4500 to ∼ 3000 cal yr BP were largely a consequence of dry climate conditions. Furthermore, palaeoclimate data from low latitudes in North Africa point to teleconnective linkages of this drying trend on both sides of the Atlantic Ocean. 相似文献
Transport infrastructure is at significant risk of direct damage from extreme climate events such as flooding, where the cost implications of delayed recovery are generally significant. Previous research in this regard has focused on the technical and engineering aspects of infrastructure construction. The risk management of resilient transport infrastructure is poorly considered, and little has been done to quantify the capacity of transport infrastructure to recover from the impact of natural disasters under varying conditions. This paper applies Cox’s proportional hazards regression model to determine the rate of recovery and cumulative probability that recovery occurs for transport infrastructure across regional areas in New South Wales, Australia. Data for post-disaster reconstruction projects over the period 1992–2012 are used to analyze recovery rate against geographic region, natural disaster type and post-disaster transport infrastructure reconstruction cost. Results demonstrate that transport infrastructure recovered slowest when the failure is the result of a flood rather than bushfire or storm, and in regions with a riverine geography. To validate the accuracy of the model, a bootstrap resampling technique is used. The bootstrap result confirms that the model is robust and reasonable.
Simulation of biodegradation of chlorinated solvents in dense non-aqueous phase liquid (DNAPL) source zones requires a model that accounts for the complexity of processes involved and that is consistent with available laboratory studies. This paper describes such a comprehensive modeling framework that includes microbially mediated degradation processes, microbial population growth and decay, geochemical reactions, as well as interphase mass transfer processes such as DNAPL dissolution, gas formation and mineral precipitation/dissolution. All these processes can be in equilibrium or kinetically controlled. A batch modeling example was presented where the degradation of trichloroethene (TCE) and its byproducts and concomitant reactions (e.g., electron donor fermentation, sulfate reduction, pH buffering by calcite dissolution) were simulated. Local and global sensitivity analysis techniques were applied to delineate the dominant model parameters and processes. Sensitivity analysis indicated that accurate values for parameters related to dichloroethene (DCE) and vinyl chloride (VC) degradation (i.e., DCE and VC maximum utilization rates, yield due to DCE utilization, decay rate for DCE/VC dechlorinators) are important for prediction of the overall dechlorination time. These parameters influence the maximum growth rate of the DCE and VC dechlorinating microorganisms and, thus, the time required for a small initial population to reach a sufficient concentration to significantly affect the overall rate of dechlorination. Self-inhibition of chlorinated ethenes at high concentrations and natural buffering provided by the sediment were also shown to significantly influence the dechlorination time. Furthermore, the analysis indicated that the rates of the competing, nonchlorinated electron-accepting processes relative to the dechlorination kinetics also affect the overall dechlorination time. Results demonstrated that the model developed is a flexible research tool that is able to provide valuable insight into the fundamental processes and their complex interactions during bioremediation of chlorinated ethenes in DNAPL source zones. 相似文献
Persistent organic pollutants (POPs) and heavy metals have been reported in a number of green turtle (Chelonia mydas) populations worldwide. However, due to ethical considerations, these studies have generally been on tissues from deceased and stranded animals. The purpose of this study was to investigate the use of blood samples to estimate the tissue contamination of live C. mydas populations. This study analysed 125 POP compounds and eight heavy metals in the blood, liver, kidney and muscle of 16 C. mydas from the Sea World Sea Turtle Rehabilitation Program, Gold Coast, Australia. Strong correlations were observed between blood and tissue concentrations for a number of POPs and metals. Furthermore, these correlations were observed over large ranges of turtle size, sex and condition. These results indicate that blood samples are a reliable non-lethal method for predicting chemical contamination in C. mydas. 相似文献