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A comparison is made of three device configurations which can be used as tunable filters: a Fabry-Perot etalon used in transmission (FPT), a Fabry-Perot etalon used in reflection (FPR), and a variable birefringence filter used in transmission (VBF). Initial conditions are defined for the achievement of a contrast ratio of 100:1, and the implications of these results on material choice discussed. The effect of nonuniform device thickness on contrast ratio is analyzed for all three cases. Analytic expressions are derived which predict the behavior of the contrast ratio under differing device conditions and which give rule-of-thumb values for the maximum acceptable thickness variation for a given contrast ratio. Using these expressions it is shown that the configuration least sensitive to variations in thickness is the VBF, with the FPT coming second and the FPR last. Experimental studies were carried out on three devices-a piezoelectric tunable air-spaced Fabry-Perot etalon, an electrooptically tunable liquid-crystal Fabry-Perot etalon, and an electrooptically tunable liquid-crystal variable birefringence filter. >

A means of approximating light-traffic performance of RAD (random-arrivals-with-deadlines) systems for four basic preemptive scheduling policies is presented. The design goal is to keep congestion low enough to make the probability of rejection acceptably small. These designs must have low processor utilization. The study analyzes rejection probabilities at utilizations up to 20% and rejection probabilities up to about 10% for various well-known preemptive scheduling disciplines (shortest job first, earliest due date, and least laxity first), as well as first-come, first-served. Good approximations for the rejection probability and for a number of other properties, such as the distribution of time-to-go at rejection, are found. >

The Global Hawk system is a high altitude endurance unmanned aerial vehicle developed under the United States Air Force Advanced Concept Demonstrator program primarily as a reconnaissance system for use against fixed and mobile targets. The Global Hawk system deployed to Australia in April 2001 for six weeks and in this period conducted 11 missions with the focus on maritime surveillance. The Australian deployment was the culmination of two years collaboration between the United States and Australia that included modifications to the radar sensor, system control and exploitation to support a surveillance focus. This paper presents aspects of the Australian contribution to the Global Hawk deployment including the rationale behind the sensor modifications and employment that achieved a surveillance capability with a system primarily designed for land reconnaissance.

Consideration is given to a homogeneous distributed network of nodes with a quasi-dynamic random task-sharing discipline and identical Poisson hard real-time traffic at each node. The task-sharing discipline under consideration permits tasks to be shared randomly around the network as long as remaining task laxity permits. Analysis of task loss in such a network requires the assumption that the transferred task stream from all nodes form a Poisson stream. The authors study the requirements for such an assumption to be justified by using computer simulation to examine transferred traffic for both independence and its fit to an exponential distribution as a function of network size. A rough rule of thumb for estimating independence is to assume the transfers are independent after the system size exceeds the average number of transfers between nodes. >

Previous experimental measurements of beam noise have shown that spectrum levels measured with towed line arrays have considerable temporal and spatial variability. In the ambient noise region dominated by shipping noise (a few Hz to a few hundred Hz), variations in spectral levels of over 30 dB can occur. Attempts to model the predicted beam noise cumulative distribution function (R.M. Heitmeyer, L.T. Davis and N. Yen, NRL Report 8863, February 1985) required approximations for both the beam pattern and transmission loss to achieve an analytic solution. The computed detection gain in regions of reduced noise resulting from resolution of individual noise-interferers, termed "ship resolution gain" (SRG), is dependent on source distribution, acoustic transmission loss and beam pattern approximation. The paper uses numerical computation of SRG to treat arbitrary hydrophone shading and realistic ocean environments. Results are compared to earlier analytic predictions to show dependence on system and environmental parameters. >

The Global Hawk high altitude endurance unmanned aerial vehicle was deployed to Australia in April 2001 for six weeks. The system conducted eleven missions with the focus on maritime surveillance. This was the culmination of two years collaboration between the United States and Australia which included development of three new sensor modes, sensor control and exploitation techniques to efficiently perform surveillance with a system primarily developed for the reconnaissance role. The paper describes the Australian contribution to the Global Hawk deployment. It describes the sensor, system integration, system control, mission management and ground element challenges presented by shifting the operational focus from reconnaissance to surveillance.

Artificial birefringence in a silicon slit structure has been demonstrated using 10·6 µm radiation. The 4 µm period grating of slits, 2 µm wide and up to 50 µm deep, was formed by an anistropic etching technique. The birefringence was investigated as a function of temperature and found to agree with theory. The possibility of electronic control of devices based on the birefringent properties, such as waveplates and shutters, is discussed.