Products that rely on Global Navigation Satellite Systems (GNSS) have become an essential part of daily life for millions of people around the world. In addition to enabling navigation, these constellations of satellites and the signals they transmit provide a global, precise timing source, used in everything from electrical power grid phasing to synchronization of financial networks. This lecture introduces the concept of radio navigation, describes the features of GNSS signals that make navigation possible, and explains how these signals are processed by GNSS receivers. The resulting measurements and error sources, such as atmospheric effects and multipath, are discussed. Special consideration is given to the challenges of using GNSS in space, and the innovations that make it possible. A survey of space applications and recent flight experiences is provided. Active areas of research are discussed, including the use of GNSS for missions to the Moon.
Free space optical communication channels can transmit high-speed data between sites over the air. We show here that a FSO digital optical communication channel can be re-purposed and used directly for two-way time transfer. We demonstrate real-time synchronization between two sites over a turbulent air path of 4 km using binary phase modulated CW laser light. Under synchronization, the two sites have a sub-3-ps time deviation below the synchronization bandwidth and a fractional frequency deviation below 10- 15 at one hour averaging time. Over an 8-hour period the peak-to-peak wander is 16 ps. Work of the US government not subject to copyright.
We measured the absolute frequency of the 1S 0 –3P 0 transition of 171Yb atom relative to the SI second with an uncertainty reduced by a factor of 14 compared with our previous measurement. The determined frequency was 518 295 836 590 863.38(57) Hz. The experimental setup is being improved for the independent uncertainty evaluation by measuring the frequency shift coefficients of our own system.
This work deals with an acquiring system dedicated to the measurement of the resonance frequency and the quality factor of organic piezoelectric resonators. These resonators consist of a free-standing cantilever and its non-released counterpart used to compensate electrical signals that are not due to mechanical behavior. The system uses a capacitive half-bridge with two AC voltages in opposite phases, associated with an IQ demodulator. The magnitudes of the two AC voltages can be set independently from one another, which enables a powerful compensation of parasitic capacitance. Thanks to this dedicated system, measurements of the resonance frequency of organic piezoelectric resonators both in air and liquid media have been performed. Several methods to determine accurately the resonance frequency have been tested and compared in terms of measurement noise.
In this work, we present an unprecedented level of integration of piezoelectric actuation means on arrays of functional nanoresonators at the wafer scale. We use 150-nm thin lead titanate zirconate (PZT) as piezoelectric material mainly because of its excellent actuation properties even when geometrically constrained at extreme scale. This work paves promising ways for NEMS to be used in configurations where transduction capabilities are integrated at the nanodevice level providing effective fabrication process flow at the wafer-scale.
Cross-spectral analysis is a mathematical tool for extracting the power spectral density of a correlated signal from two time series in the presence of uncorrelated interfering signals. We demonstrate and explain a set of amplitude and phase conditions where the detection of the desired signal using cross-spectral analysis fails partially or entirely in the presence of a second uncorrelated signal [1-2]. Not understanding when and how this effect occurs can lead to dramatic under-reporting of the desired signal.
The design, modeling, fabrication, and characterization of a vibrationally trapped thickness-shear MEMS resonator is presented. This device is intended to avoid various limitations of flexural MEMS resonators, including nonlinearity, clamping losses, thermoelastic damping, and high damping in liquid. It includes a silicon bridge and a reference line on an SOI wafer, a coupled Au/Cr coplanar waveguide, Lorentz-force coupling, variations in waveguide thickness for vibrational trapping, and circuitry for nulling the components of the signal that are unrelated to the acoustic resonance. Finite-element vibrational modeling shows the lowest thickness-shear mode with a bridge thickness of 4.9 μm to be dominated by shear displacements, with the magnitude of out-of-plane displacements decreasing with increasing bridge width. Two-dimensional modeling of vibrational trapping, with central regions of the waveguides having 43 nm greater thickness, indicates that amplitudes are reduced by several orders of magnitude at the ends of the bridges for the fundamental ~400 MHz thickness-shear resonance. Swept-frequency network-analyzer measurements of fabricated devices reveal no evidence for an acoustic resonance, despite a calculated prediction of levels of acoustic power absorption that are well above the measured noise level. A possible explanation for this result is stiction of the bridges to the substrate.
