GNSS Clocks: History and Present	Ron Beard, U.S. Naval Research Laboratory
The Practice of Frequency Stability Analysis and Time Domain Statistics to Clock and Oscillator Measurements	Gregory Weaver, John Hopkins University Applied Physics Laboratory
Timescales and GNSS: A GPS Clock Ensemble	Huascar Ascarrunz, Harris Corporation
Anatomy of GNSS Signals and Receivers for Time/Frequency Determination	Douglas Boehm, U.S. Naval Research Laboratory and O. Jay Oaks, U.S. Naval Research Laboratory
Precise GNSS Clock Corrections from Geodetic Analysis - Procedure, Examples, Applications	Prof. Urs Hugentobler, Universität Technische München, Germany
Distributing Time and Frequency Data: Requirements and Methods	Judah Levine, US National Institute of Standards and Technology Time and Frequency Division

GNSS Clocks: History and Present

Time: Monday, January 29, 9:00 a.m. - 10:00 a.m.

The first Global Navigation Satellite System (GNSS) was the U.S. Navy Navigation Satellite System, also known as Transit, developed in the 1960’s as part of the U.S. Navy’s Fleet Ballistic Missile Program. That low altitude system was based on the Doppler shift of a signal transmitted from a low altitude orbiting satellite. As such it was only visible to a user on the surface of the Earth for about 20 minutes. The satellite’s oscillator needed to be stable over that time in order to provide an accurate measurement. The Global Positioning System (GPS) followed in the 1970’s developed out of investigations into passive ranging techniques and techniques for establishing constellations of satellites to provide global uniform coverage. Passive techniques offered the promise of rapid and three dimensional positioning measurements. However, passive techniques rely upon the capability of a ranging signal transmitted from a satellite to precisely measure the time of propagation and hence the range to a user’s receiver. Precise synchronization between the transmitter and receiver is required. The means of establishing and maintaining synchronization of the satellites over the period of time that each satellite is visible to a point on or near the surface of the Earth has been implemented using especially developed space qualified atomic clocks. Since the initial development by GPS there have been several development efforts into perfecting space qualified atomic clocks. This tutorial will describe those developments of the GPS program and of GNSS in general. The resulting capabilities provided will be introduced as well as performance measures. Statistical performance techniques will be introduced as a means of comparing the clocks involved and their contribution to GNSS performance.

Ron Beard was the Head of the Advanced Space PNT Branch (formerly the Space Applications Branch) at the Naval Research Laboratory (NRL) until he retired the end of September 2015, and is now re-employed with NRL as an annuitant. He has made significant contributions to the generation and dissemination of precise time from space over the course of his 40-year career.

Mr. Beard joined the Navy and later NRL as the DoD was beginning to explore new concepts for a second-generation satellite-based navigation system. At NRL he worked for Roger Easton, who proposed "passive ranging" from satellites with synchronized clocks in what became the TIMATION project. There was very limited experience with precise clocks in the space environment during those early days.

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The Practice of Frequency Stability Analysis and Time Domain Statistics to Clock and Oscillator Measurements

Time: Monday, January 29, 10:20 a.m. - 11:20 a.m.

Most users of precise timekeeping devices and frequency sources are familiar with phase noise and Allan deviation as performance specifications defining the quality of the device, where lower values are better (and typically more costly). Also, the concepts of short term stability, long term stability, time deviation and time interval error are often brought forward by device providers and system requirements in a self-referenced manner. More specifically, the presentations and proceedings of the PTTI generally require an attendee to have a basic working knowledge of these performance metrics to discern the extent of contribution to the improvement of the community’s practice.

This tutorial will provide the information to unwrap the interpretation of clock and frequency source measurements, clock statistical characterization, and frequency stability analysis to bring about a workable understanding to the PTTI attendee. The tutorial will use NIST Special Publication 1065, Handbook of Frequency Stability Analysis by William J. Riley as a reference, so that subsequently, the user may be familiar with its application and techniques. The tutorial will also inject the work of Victor S. Reinhardt, David A. Howe, and Patrizia Tavella to supplement the material of NIST 1065. The tutorial will demonstrate analysis of measurement data from devices such as the Chip Scale Atomic Clock, GPS disciplined composite clock, and the ultra-stable oscillators on-board the New Horizons spacecraft. The use of these devices is intended to provide illustrative working examples for the identification, characterization, and assessment of both deterministic (systematic) processes and stochastic (noise) properties.

Gregory L. Weaver is a member of the Principle Professional Staff of JHU/APL and works within the RF Engineering Group of the Space Department. He is a technologist with extensive background in both the technical and business aspects of the frequency control industry and has held positions as a senior design engineer, technical manager and marketing strategist over a 30 year career history. He is a frequent contributor to the IEEE International Frequency Control Symposium, Precise Time and Time Interval Systems and Application Meeting and the European Frequency and Time Forum.

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Timescales and GNSS: A GPS Clock Ensemble

Time: Monday, January 29, 11:30 a.m. - 12:30 p.m.

In an earth centered, earth-fixed spatial and temporal reference frame, using timing signals from four known reference locations a navigator can solve for their position and time. Likewise, a timing user can solve for their time using a single ranging signal from a known reference location. In practice, generating, transmitting, receiving and utilizing timing signals for navigation and time transfer in a real-world system is challenging and requires tradeoffs in the overall enterprise system design. Currently, GNSS systems such as GPS trade greater complexity in the Space and Control Segments for simplicity, accuracy, reliability and lower power in the User Segment.

Using a network of surveyed monitor station receivers, the GPS Control Segment (CS) estimates position and time for every (satellite) transmitter using the same satellite ranging signals as the User Segment. Position and time are delivered to the navigation and timing users in the form of predictions relative to a fixed time reference frame, GPS Time, and a fixed spatial reference frame, WGS-84.

This tutorial will discuss the role of the broadcast clock reference time in user range error, and time transfer error. It outlines the formulation of a basic timescale from range measurements and briefly discusses the role of environmental clock effects on time transfer performance. For this tutorial, the estimation of transmitter position and the estimation of transmitter time are assumed to be largely uncorrelated.

Huascar Ascarrunz has been a Systems Engineer at Harris Corporation since 2008. At Harris, he worked on the GPS III Space Vehicle Timekeeping System and has been doing algorithm analysis work for the GPS Next Generation Control Segment Navigation Software since 2010. Huascar holds a Bachelor’s degree in Electrical Engineering from the University of Colorado at Boulder.

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Anatomy of GNSS Signals and Receivers for Time/Frequency Determination

Time: Monday, January 29, 1:30 p.m. - 2:30 p.m.

The use of the United States Global Positioning System (GPS) for affordable nanosecond-level time recovery and synchronization has become ubiquitous globally, permeating countless industries and institutions. The success of GPS has led other countries to implement or begin to implement their own Global Navigation Satellite Systems (GNSS) to provide timing and navigation either cooperatively with or independently from GPS. GNSS receivers are now available that can utilize individual constellations or a mixture of them to determine time and position. Time recovery from the GNSS signals is performed through a combination of satellite signal tracking, decoding of navigation data messages containing satellite ephemeris and other parameters and corrections, and implementation of the positioning solution. GNSS receivers track the transmitted signals code and integrated Doppler or carrier using correlators to detect the delay and Doppler of the satellite generated pseudo random code (PRN) against the receivers generated PRN and processed through code and carrier tracking loops. The GNSS constellations each have their own characteristics and structure and their civil broadcast service timing performances will be compared in this tutorial using multi-GNSS receivers that can track multiple constellations.

Douglas Boehm is an Electrical Engineer and a Senior Lead Engineer of the Advanced Space Precision Navigation and Timing Branch at the Naval Research Laboratory. Mr. Boehm received his Bachelors and Masters in Electrical Engineering at Rensselaer Polytechnic Institute spending summers and winters working at NRL. Mr. Boehm has been a full time Navy civilian employee at NRL for 10 years. While at NRL, Mr. Boehm has performed research, design, development, test and evaluation of satellite communications and precision network time transfer systems as well as for technologies that augment GPS receive systems. Mr. Boehm is currently the lead engineer testing the quality and precision of multi GNSS receivers.

O. Jay Oaks is an Electronics Engineer and Deputy Head of the Advanced Space Precision Navigation and Timing Branch at the Naval Research Laboratory. Mr. Oaks has been a Navy civilian employee for 45 years and at the NRL for 39 years. While at NRL, he was responsible for the design and development of several precision time transfer satellite receivers used in special scientific and military applications. Mr. Oaks led efforts that investigated GPS vulnerabilities in PNT applications and developed augmentation solutions. He currently leads a group of scientists and engineers on projects that require precise positioning, navigation, and time synchronization.

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Precise GNSS Clock Corrections from Geodetic Analysis - Procedure, Examples, Applications

Time: Monday, January 29, 2:40 p.m. - 3:40 p.m.

Precise positioning with GNSS is based on precise measurement of signal travel times. That GNSS is a one-way measurement system has important advantages. The user e.g. does not require a heavy communication unit. He can use the signals without registering for the service. A one-way system however has the fundamental drawback that two clocks are required to measure the signal travel time. As a consequence a precise synchronization - for precise applications at the few ps level - is crucial. The ingenious concept of GNSS - developed in the 1970s - is the capability of the system to synchronize the involved clocks based on the GNSS ranging signals themselves. As a consequence precise ranges can be measured even without involving ultra stable clocks. As a matter of fact every GNSS user carries in his iPhone a virtual atomic clock. Likewise the GNSS system is by concept a precise time and frequency transfer tool, e.g. used in the generation of International Atomic Time. With precise modeling of propagation effects - despite high correlation between parameter groups such as clock parameters, station coordinates, troposphere delay parameters - precise satellite and receiver clock corrections can be derived. This tutorial highlights the GNSS concept with focus on determination of clock corrections, gives examples for the performance of current GNSS clocks, and addresses the potential of using the stability of upcoming clocks for applications such as precise orbit determination or reconstruction precise kinematic trajectories and of troposphere parameters.

Prof. Urs Hugentobler is a full professor at the Institute for Astronomical and Physical Geodesy of Technische Universität München, Germany, and head of the Research Establishment Satellite Geodesy. His research activities focus on precise applications of GNSS such as positioning, precise orbit determination, reference frame realization, and time transfer.

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Distributing Time and Frequency Data: Requirements and Methods

Time: Monday, January 29, 4:00 p.m. - 5:00 p.m.

This course will describe the methods that are used to distribute time and frequency information, with special emphasis on methods that are independent of global navigation satellite systems. The course will illustrate these methods with the requirements of commercial and financial institutions and distributors of electrical power. In addition to the purely technical requirements, additional requirements that result from the need to demonstrate traceability to national standards will also be discussed. The level of accuracy that is required to support these applications is relatively modest from the perspective of the internal time scales of most National Metrology Institutes and timing laboratories, but satisfying the requirements becomes much more challenging when the need for extreme reliability and the limitations of many of the common distribution channels are included. None of the alternative solutions that have been proposed is completely adequate now and all of them will have increasing difficulty satisfying the increasing accuracy requirements in the future.

Dr. Judah Levine is a Fellow of the National Institute of Standards and Technology and is the leader of the Network Synchronization Project in the Time and Frequency Division, which is located in the NIST laboratories in Boulder, Colorado. He received his Ph.D. in Physics from New York University in 1966. Dr. Levine is a member of the IEEE and a Fellow of the American Physical Society.

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