Neil Ashby

First accuracy evaluation of NIST-F2

We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (NIST) with a cryogenic (liquid nitrogen) microwave cavity and flight region. The 80?K atom interrogation environment reduces the uncertainty due to the blackbody radiation shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>50?000) at this low temperature, resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine in-house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems and the accuracy evaluation methods. The type B fractional uncertainty of NIST-F2 is shown to be 0.11???10?15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44???10?15.

Power dependence of distributed cavity phase-induced frequency biases in atomic fountain frequency standards

We discuss the implications of using high-power microwave tests in a fountain frequency standard to measure the frequency bias resulting from distributed cavity-phase shifts. We develop a theory which shows that the frequency bias from distributed cavity phase depends on the amplitude of the microwave field within the cavity. The dependence leads to the conclusion that the frequency bias associated with the distributed cavity phase is typically both misestimated and counted twice within the error budget of fountain frequency standards.

An assessment of relativistic effects for low Earth orbiters: the GRACE satellites

The GRACE mission consists of two identical satellites orbiting the Earth at an altitude of ∼500 km. Dual-frequency carrier-phase Global Positioning System (GPS) receivers are flying on both satellites. They are used for precise orbit determination and to time-tag the K-band ranging system used to measure changes in the distances between the two satellites. The satellites are also flying ultra-stable oscillators (USOs) to achieve the mission’s need for short-term (<1 s) oscillator stability. Because of the high quality of both the GPS receivers and the oscillators, relativistic effects in the GRACE GPS data can be examined. An expression is developed for relativistic effects that explicitly includes the effects of the Earth’s oblateness ( J2). Use of this expression significantly reduces the twice per orbital period energy in the GRACE clock solutions, indicating that the effect of J2 can be significant and should be modeled for satellite clocks in low Earth orbit. After relativistic effects have been removed, both GRACE USOs show large (2 ns to 3 ns) once per orbital period signatures that correlate with voltage variations on the spacecraft.

PARCS: a Primary Atomic Reference Clock in Space

NIST, in collaboration with the Jet Propulsion Laboratories (JPL), the University of Colorado, Politecnico di Torino and Harvard Smithsonian Center for Astrophysics (SAO) is building a laser-cooled cesium-beam atomic clock for flight on the International Space Station (ISS). The clock, named PARCS (Primary Atomic Reference Clock in Space) is designed to perform certain tests of relativity and fundamental physics and to serve as a primary frequency standard.

Vibration Sensitivity of Microwave Components

Vibration sensitivity is an important specification for oscillators on mobile systems, unmanned aerial vehicles (UAVs) etc. These systems must provide superior performance when subject to severe environmental conditions. Electronic oscillators often can provide sufficiently low intrinsic phase modulation (PM) noise to satisfy particular system requirements when in a quiet environment. However, mechanical vibration and acceleration can introduce mechanical deformations that degrade the oscillators otherwise low PM noise. This degrades the performance of an electronic system that depends on this oscillators low phase noise. Not only an oscillator, but most microwave components, such as microwave cables, circulators, and amplifiers are sensitive to vibration to some extent. Therefore, it is very important to select vibration-tolerant components in order to build a system with less vibration sensitivity. We study the performance of different microwave cables (flexible, semi-rigid as well as rigid) under vibration for different vibration profiles. Some good cables provide a vibration-sensitivity noise floor that provides sensitivity of 10-11-10-12 per g for an oscillator under test. We also verify the reproducibility of each measurement after disassembly and reassembly. We study the vibration sensitivity of a SiGe amplifier-based surface transverse wave (STW) oscillator and an air-dielectric cavity resonator oscillator (ACRO) and compare their performances with a commercially available dielectric resonator oscillator (DRO). We also describe passive and active vibration cancellation schemes to reduce vibration induced noise in oscillators.

On the power dependence of extraneous microwave fields in atomic frequency standards

We show that the frequency bias caused by distributed cavity phase has a strong dependence on microwave power. We also show that frequency biases associated with microwave leakage have distinct signatures in their dependence on microwave power and the physical location of the leakage interaction with the atom.

Probability distributions and confidence intervals for simulated power law noise

A method for simulating power law noise in clocks and oscillators is presented based on modification of the spectrum of white phase noise, then Fourier transforming to the time domain. Symmetric real matrices are introduced whose traces-the sums of their eigenvalues-are equal to the Allan variances, in overlapping or non-overlapping forms, as well as for the corresponding forms of the modified Allan variance. We show that the standard expressions for spectral densities, and their relations to Allan variance, are obtained with this method. The matrix eigenvalues determine probability distributions for observing a variance at an arbitrary value of the sampling interval τ, and hence for estimating confidence in the measurements. Examples are presented for the common power-law noises. Extension to other variances such as the Hadamard variance, and variances with dead time, are discussed.

Design studies for a laser-cooled space clock

We present a theoretical comparison between a TE/sub 01n/ cavity and a traditional Ramsey cavity when used with a laser-cooled atom source in a microgravity clock.

The global positioning system, relativity, and extraterrestrial navigation

Relativistic effects play an important role in the performance of the Global Positioning System (GPS) and in world-wide time comparisons. The GPS has provided a model for algorithms that take relativistic effects into account. In the future exploration of space, analogous considerations will be necessary for the dissemination of time and for navigation. We discuss relativistic effects that are important for a navigation system such as at Mars. We describe relativistic principles and effects that are essential for navigation systems, and apply them to navigation satellites carrying atomic clocks in orbit about Mars, and time transfer between Mars and Earth. It is shown that, as in the GPS, relativistic effects are not negligible.

A laser-cooled Atomic Clock in Space

This paper describes work aimed at producing a “Primary Atomic Reference Clock in Space (PARCS),” scheduled for flight on the International Space Station in 2004. The microgravity environment of space will allow this clock to achieve an uncertainty ten times lower than can be achieved on earth. The experimental objectives are to measure several relativistic effects on clocks and to provide both time and frequency references available as an international standards accessible to anyone on earth.