Upgrading, maintenance and rehabilitation of road infrastructure is expensive, especially in view of the growing scarcity and cost of suitable road building materials. In areas with high mica content and secondary minerals such as smectite in the natural materials, stabilisation with cement is not viable. The Council for Scientific and Industrial Research of South Africa has embarked on a research programme to evaluate the performance of substandard materials improved with anionic nanosilane modified bitumen emulsions for use in base and subbase layers. This work comprises laboratory testing as well as Accelerated Pavement Testing using the Heavy Vehicle Simulator (HVS). The results of a full-scale HVS test on a light pavement as well as initial analysis on a medium traffic road are discussed. It has been shown that stabilisation of available substandard materials using an anionic nano-silane modified bitumen emulsion compared with the standard approach of importing high quality crushed aggregate can lead to savings as high as 40%-50% for equivalent performance. In addition, there was also a significant reduction in construction effort and time.
We introduce concepts from optimal estimation to the stabilization of precision frequency standards limited by noisy local oscillators. We develop a theoretical framework casting various measures for frequency standard variance in terms of frequency-domain transfer functions, capturing the effects of feedback stabilization via a time-series of Ramsey measurements. Using this framework we introduce a novel optimized hybrid predictive feedforward measurement protocol which employs results from multiple past measurements and transfer-function-based calculations of measurement covariance to improve the accuracy of corrections within the feedback loop. In the presence of common non-Markovian noise processes these measurements will be correlated in a calculable manner, providing a means to capture the stochastic evolution of the LO frequency during the measurement cycle. We present analytic calculations and numerical simulations of oscillator performance under competing feedback schemes and demonstrate benefits in both correction accuracy and long-term oscillator stability using hybrid feedforward. Simulations verify that in the presence of uncompensated dead time and noise with significant spectral weight near the inverse cycle time predictive feedforward outperforms traditional feedback, providing a path towards developing a new class of stabilization "software" routines for frequency standards limited by noisy local oscillators.High-performance passive frequency standards play a major role in technological applications such as network synchronization and GPS [1] as well as many fields of physical inquiry, including radioastronomy (very-longbaseline interferometry) [2], tests of general relativity [3], and particle physics [4]. Atomic clocks exploiting the stability of Cs [5][6][7][8] or other atomic references [9-13] to stabilize an oscillator are known as the most precise timekeeping devices available, but constant performance gains are sought for technical and scientific applications.In many settings, such as miniaturized deployable frequency standards or in GPS-denied environments, a major performance limitation aries from the quality of the local oscillator (LO) that probes and is locked to the atomic transition. The LO frequency may evolve randomly in time due to intrinsic noise processes in the underlying hardware [10,11], leading to time-varying deviations of the LO frequency from that of the stable atomic reference. These instabilities are partially compensated through use of a feedback protocol designed to transfer the stability of the reference to the LO, but their effects cannot be mitigated completely.Early work characterizing the so-called Dick effect [14] demonstrated that no matter how good the reference becomes, LO noise will still produce residual instabilities in the locked LO (LLO) through the feedback protocol itself. The dominant mechanism for this is evolution of the LO's frequency on timescales rapid compared with the shortest measurement and feedback cycle. Major contributors to this phenome...
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