Patrizia Tavella

Demetrios Matsakis

Selected Papers from TimeNav 2007

The US Naval Observatory (USNO) has provided timing for the navy since 1830 and, in cooperation with other institutions, has also provided timing for the United States and the international community. Its Master Clock (MC) is the source of UTC(USNO), the USNO&#39;s realization of Coordinated Universal Time (UTC), which has stayed within 5 nanoseconds RMS of UTC since 1999. The data used to generate UTC(USNO) are based upon 73 cesium and 21 hydrogen maser frequency standards in three buildings at two sites. The USNO disseminates time via voice, telephone modem, LORAN, Network Time Protocol (NTP), GPS, and two-way satellite time transfer (TWSTT). This paper describes some of the changes being made to meet the future needs for precision, accuracy, and robustness.

Time and Frequency Activities at the  U.S. Naval Observatory

Galileo, the European Global Navigation Satellite System, will provide to its users highly accurate global positioning services and their associated integrity information. The element in charge of the computation of integrity messages within the Galileo Ground Mission Segment is the integrity processing facility (IPF), which is developed by GMV Aerospace and Defence. The main objective of this paper is twofold: to present the integrity algorithms implemented in the IPF and to show the achieved performance with the IPF software prototype, including aspects such as: implementation of the Galileo overbounding concept, impact of safety requirements on the algorithm design including the threat models for the so-called feared events, and finally the achieved performance with real GPS and simulated Galileo scenarios.

Carlos Hern&#225;ndez Medel

Carlos Catal&#225;n Catal&#225;n

Miguel Angel Fern&#225;ndez Vidou

Esther Sard&#243;n P&#233;rez

The Galileo Ground Segment Integrity Algorithms: Design and Performance

To compute precise point positioning (PPP) and precise time transfer using GPS code and phase measurements, a new software named <i>Atomium</i> was developed by the Royal Observatory of Belgium. <i>Atomium</i> was also adapted to perform a phase-only analysis with the goal to obtain a continuous clock solution which is independent of the GPS codes. In this paper, the analysis strategy used in <i>Atomium</i> is described and the clock solutions obtained through the phase-only approach are compared to the results from the PPP mode. It is shown that the phase-only solution improves the stability of the time link for averaging times smaller than 7 days and that the phase-only solution is very sensitive to the station coordinates used. The method is, however, shown to perform better than the IGS clock solution in case of changes in the GPS receiver hardware delays which affects the code measurements.

Pascale Defraigne

Nicolas Guyennon

Carine Bruyninx

GPS Time and Frequency Transfer: PPP and Phase-Only Analysis

NICT has completed a set of major upgrades in its systems for the realization of Japan standard time. One of the most significant changes is the introduction of hydrogen masers as signal sources for UTC (NICT) instead of Cs atomic clocks. This greatly improves the short-term stability of UTC (NICT). Another major change is the introduction of a newly developed 24-channel dual-mixer-time-difference system (DMTD) as the main tool for measurements. The reliability of the system is also improved by enhanced redundancy and monitoring systems. The new JST system has been in regular operation since February 2006.

Yuko Hanado

Kuniyasu Imamura

Noboru Kotake

Fumimaru Nakagawa

Yoshiyuki Shimizu

Ryo Tabuchi

Yukio Takahashi

Mizuhiko Hosokawa

Takao Morikawa

The New Generation System of Japan Standard Time at NICT

The stability of the atomic clocks on board the satellites of a navigation system should
remain constant with time. In reality there are numerous physical phenomena that make the
behavior of the clocks a function of time, and for this reason we have recently introduced the
dynamic Allan variance (DAVAR), a measure of the time-varying stability of an atomic clock. In this paper we discuss the dynamic Allan variance for phase and frequency jumps, two common nonstationarities of atomic clocks. The analysis of both numerical simulations and experimental data proves that the dynamic Allan variance is an effective way of characterizing nonstationary behaviors of atomic clocks.

Lorenzo Galleani

Identifying Nonstationary Clock Noises in Navigation Systems

Diophantine Frequency Synthesis (DFS), a number-theoretic approach to the design of very high resolution frequency synthesizers, was introduced in 2006. Further work concerning the impact of controlling mixing products for high-spectral purity was addressed and reported at the 2007 European Frequency and Time Forum. The focus of this paper is on the implementation of nested DFS architectures targeting microphase-type applications for precision timekeeping systems.  
We have shown that DFS does not impart any extraordinary design constraints on spectral purity in comparison to commonly used high resolution frequency synthesis techniques such as DDS or fractional <svg style="vertical-align:-0.0pt;width:14.8375px;" id="M1" height="10.325" version="1.1" viewBox="0 0 14.8375 10.325" width="14.8375"  xmlns="http://www.w3.org/2000/svg">
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10&#x2009;MHz synthesizers with 1E-13 fractional resolution in consecutive steps ranging <svg style="vertical-align:-1.09097pt;width:26.4625px;" id="M2" height="12.05" version="1.1" viewBox="0 0 26.4625 12.05" width="26.4625"  xmlns="http://www.w3.org/2000/svg">
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</svg>&#x2009;Hz. The synthesizers generate their output  from a 10&#x2009;MHz reference standard. Such synthesizers are essential to accomplishing precision frequency correction in timekeeping systems.

Paul P. Sotiriadis

Gregory L. Weaver

Diophantine Frequency Synthesizer Design for Timekeeping Systems

GPS is widely used for time and frequency transfer. To estimate the clock difference between two ground
stations, a single baseline solution can be performed using external products for the modelling of the geometrical effects (constellation ephemeris and station coordinates). The baseline solution relies on a single-difference formulation, using code and phase ionosphere-free measurements. The phase ambiguities are usually adjusted as floating parameters. Such solutions give very good results when modelling hypotheses are consistent between the external solution (e.g., GPS orbits) and the baseline solution. 
However, the frequency bias in the computed clock is very sensitive to discrepancies in the models, and is only observed thanks to the code measurement, with limitations due to the noise. Here, we propose to solve the integer ambiguities on single-difference phase measurements. The advantage is the complete elimination of the clock drifts observed in floating ambiguities solutions. This formulation allows also a reliable continuous connection between overlapping clock solutions (jumps between such solutions can be completely eliminated). Several time transfer results are analyzed and compared to TWSTFT. The methodology has been extended to a network of stations using integer ambiguities on zero-difference measurements. The corresponding results are given for a few European stations.

J. Delporte

F. Mercier

D. Laurichesse

O. Galy

GPS Carrier-Phase Time Transfer Using Single-Difference Integer Ambiguity Resolution

We discuss the use of some new time transfer techniques for computing TAI time links. Precise point positioning (PPP) uses GPS dual frequency carrier phase and code measurements to compute the link between a local clock and a reference time scale with the precision of the carrier phase and the accuracy of the code. The time link between any two stations can then be computed by a simple difference. We show that this technique is well adapted and has better short-term stability than other techniques used in TAI. We present a method of combining PPP and two-way time transfer that takes advantage of the qualities of each technique, and shows that it would bring significant improvement to TAI links.

G&#xE9;rard Petit

Zhiheng Jiang

Precise Point Positioning for TAI Computation

Two hydrogen masers (HMs) are used in the Precise Timing Facility to provide the physical realization of Galileo System Time, insuring the extremely high short-term stability required for the navigation functions.  In order to allow a smooth switch over between backup and primary HMs, the &#8220;backup HM steering algorithm&#8221; is developed.  This acquires the phase difference measured between two HMs, computes a steering correction, and generates the steering correction to the backup HM via a PicoStepper with a 0.1-picosecond resolution. The algorithm design is based on outlier removal and a proportional-integral filtering controller. To verify the steering operability and the loop performance, the overall backup HM steering system is simulated using real HM-HM measurements, and  with simulated anomalies (phase/frequency spikes, jumps, and drift).

Qinghua  Wang

Pascal  Rochat

Xavier  Stehlin

Backup Hydrogen Maser Steering System for Galileo Precise Timing Facility

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International Journal of Navigation and Observation

Selected Papers from TimeNav 2007

Selected Papers from TimeNav 2007

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