GPS World

Tag: UTC

  • Directions 2022: Galileo FOC, G2 on the horizon

    Directions 2022: Galileo FOC, G2 on the horizon

    Galileo Second Generation Batch#1A satellites. (Image: ESA).
    Galileo Second Generation Batch#1A satellites. (Image: ESA).

    Successful European Cooperation

    Galileo is Europe’s civil global satellite navigation constellation and a major success, being the world’s most precise satnav system and offering meter-scale accuracy to more than two billion users around the globe.

    The signature of the Financial Framework Partnership Agreement (FFPA) on June 22, 2021, further strengthened effective cooperation between the European Commission (EC), the European Union Agency for the Space Program (EUSPA), and the European Space Agency (ESA) — key to successfully achieving a crucial EU Space Program component like Galileo in the current EU Multi Financial Framework (2021–2028).

    The EC is the program manager, with EUSPA acting as the exploitation manager and ESA as the system development prime.

    Stable Service Performance

    Galileo continues to deliver excellent service performance every month in a safe, secure and seamless manner. Delivery of Galileo services is managed by EUSPA, as the Galileo service provider, with industrial partner SpaceOpal, the Galileo service operator prime contractor. The performance of Galileo services is independently monitored by the Galileo Reference Center (GRC) and regularly published on the GNSS Service Center (GSC) web portal at www.gsc-europa.eu — both agencies were developed by GMV. The security of the Galileo System is monitored by the Galileo Security Monitoring Centers (GSMC), operated by EUSPA.

    With 22 satellites in service, the open service is already delivering more than 99% availability of PDOP <= 6 worldwide. This, together with the excellent ranging accuracy, suggests that most Galileo dual-frequency users are typically experiencing positioning accuracy in the order of only 2 to 3 meters.

    Timing users also continue to receive accurate (in the order of 5 ns) access to Galileo System Time, which they can trace to Universal Coordinated Time (UTC) through the corresponding offset parameters transmitted by the satellites.

    The SAR/Galileo service, contributing to COSPAS/SARSAT, continues to deliver both the Forward Link Service (FLS) and the Return Link Service (RLS) with more than 99% availability, allowing users in distress not only to issue an alert and be located within a few minutes, but also be notified that the alert was successfully processed and rescue is on the way. The SAR/Galileo control center is located in Toulouse (France) and operated by CNES under the authority of EUSPA. The excellent performance of the service has been demonstrated both through several rescue exercises and real-life emergencies.

    Galileo Launch 11

    Soyuz launcher VS-26 lifted off from French Guiana with the first pair of Galileo Batch 3 satellites on Dec. 5, 2021, at 01:19 CET. This marks the 11th Galileo launch of operational satellites in 10 years: a decade of hard work by Europe’s Galileo partners and European industry. With these satellites, the robustness of the constellation has increased, guaranteeing a higher level of service.

    Thanks to an upgrade of the Ground Control Segment, the Launch and Early Orbit Phase has been for the first time conducted directly from the Galileo Control Center, rather than requiring an external mission control site. This version of the ground segment increases overall reliability and cybersecurity and opens the way to significant expansion of the Galileo constellation, allowing command and control of up to 38 satellites. The development has been performed by an industrial consortium led by GMV, harnessing state-of-the-art technology using the latest solutions on the market.

    Galileo launch 11 from Europe’s spaceport in French Guyana. (Photo: ESA)
    Galileo launch 11 from Europe’s spaceport in French Guyana. (Photo: ESA)

    On Route to Full Operational Capability

    This year will pave the way toward Full Operational Capability of Galileo services.

    Industrial prime contractor OHB Systems has nearly completed production of the additional 10 recurrent satellites belonging to Galileo Batch 3. Six of them are undergoing final acceptance testing at the ESA satellite test center, and the other four are under integration at the satellite prime facilities.

    Preparation for Launch 12 has already started, with the satellites’ acceptance for a launch date planned in the first months of 2022, followed by Launch 13 in autumn. This is leading toward completion of the Galileo constellation, providing an increased availability of the Galileo signal in space for both GNSS and search-and-rescue users.

    From 2023 onward, the remaining Batch 3 satellites will be launched with the new Ariane 62 launch vehicle, a variant of Ariane 6 with two strap-on solid boosters. The launcher is undergoing the final stages of development, led by prime contractor ArianeGroup.

    The Galileo Ground Mission Segment will undergo a complete technological refresh, including hardware virtualization and porting of several million lines of code, performed by an industrial consortium led by Thales France. A series of improvements will be introduced to increase system resilience, including an extended mode of operation to improve service continuity and robustness.

    Cybersecurity monitoring of all the ground assets will be introduced as an overlay to the current ground infrastructure. The upgrade will undergo a rigorous level of qualification testing followed by worldwide deployment in a seamless way in both Galileo control centers, in both Galileo security monitoring centers, and at all remote locations without affecting continuity of service.

    The service facilities that contribute to the delivery of Galileo services (the European GNSS Service Center, the Galileo Reference Center, and the SAR data service providers) will also evolve to support not only the transition from Initial Services to Full Operational Capability, but also the early roll-out of service evolutions. In this regard, extensive work is ongoing to deliver an exciting set of improvements, some of which are already in development or testing, to reach the users in the year to come:

    • Improvements of the I/NAV signal to increase robustness and time-to-first-fix, while assuring full backward compatibility with legacy receivers.
    • OS Navigation Message Authentication (OS-NMA) to support applications that require trust in the authenticity of the data transmitted by the Galileo satellites (a public observation campaign was launched in November 2021 to engage stakeholders and collect their feedback before moving to the initial service provision).
    • An initial phase of the High Accuracy Service, delivering corrections in the Galileo E6 signal and over terrestrial network to allow users to perform precise point positioning over Europe; test signals were already transmitted with promising results.
    • A Search and Rescue Beacon Command Service complementing the SAR Return Link, providing improved capabilities to timely locate beacons under authorized emergency situations (such as the disappearance of Flight MH370 in the Indian Ocean in 2014).
    • A first implementation of an Emergency Warning Service over Europe, allowing the authorized national emergency-management authorities of the EU Member States to relay alert messages through Galileo signals, which can reach target areas even in case of disrupted terrestrial communications (such as due to floods or earthquakes).
    Galileo worldwide ground segment. (Credit: ESA)
    Galileo worldwide ground segment. (Credit: ESA)

    Second Generation in the Making

    The FFPA will bring Galileo to the next level with the development of the second generation, a further step forward with the use of many innovative technologies to guarantee the system’s unprecedented precision, robustness and flexibility.

    In parallel to the completion of the first generation of Galileo, Europe has conducted in recent years preparation activities for the Second Generation (G2). Elaborating on market, user and exploitation needs collected by EUSPA, ESA identified a number of system evolution scenarios, which were discussed among relevant EU stakeholders to select the second-generation mission and services baseline to build the system infrastructure.

    The evolution of Galileo capabilities will not only provide better services through advanced technical solutions identified by ESA, but will also ensure continuity of service and backward compatibility for
    first-generation legacy users.

    Two parallel contracts to develop and manufacture each of the six Galileo Second Generation Batch#1 satellites were kicked off in the first half of 2021 with Thales Alenia Space (Italy) and Airbus Defence & Space (Germany). The new G2 satellites will be constructed on a short time scale, with their first launch via Ariane-62 expected in less than four years, allowing them to commence operations in space as soon as possible. Both contracts have already undergone preliminary design reviews.

    Development of the G2 satellites is supported by the Galileo Payload Test Bed, which provides an early proof-of-concept of the advanced G2 payload architecture. These satellites will provide, among others, the following key innovations:

    • Reconfigurable fully digital navigation payload.
    • Point-to-point connection between satellites by Inter-Satellite-Link for command and control and ranging functionalities.
    • Electric propulsion for orbit-raising capabilities.
    • Advanced jamming and spoofing protection mechanisms to safeguard Galileo signals.

    System and Ground Segment definition studies, together with the associated technology pre-developments, have been performed, leading to the definition of the preliminary design and technical requirement baseline for the G2 system, a project involving most of Europe’s space industrial partners.

    The G2 In-Orbit Validation Ground Segment and System Test Bed have been defined and relevant procurement procedures are ongoing, with these objectives:

    • G2 Batch#1 satellites launch and early orbit phase, in-orbit testing and enhanced legacy services provision.
    • G2 new capabilities in-orbit validation, including prototyping and validation of all the novel technologies that can exploit the full capabilities of the G2 Batch#1 satellites.
    Galileo Second Generation Batch#1B satellites. (Image: ESA).
    Galileo Second Generation Batch#1B satellites. (Image: ESA).

    Definition activities for the G2 Initial Orbit Capability (IOC) are progressing well and are expected to converge in the first half of 2022, in order to establish the future roadmap for new G2 services provision in the years to come.

    2022 will be a key year for the evolution of Galileo Second Generation activities, through the consolidation of the first batch of G2 satellite design and development activities and the start of development of associated G2G IOV Ground Segment and System Test Beds.

    A bright future awaits Galileo, both through the completion of its Final Operational Capability and the start of evolution towards Galileo Second Generation.

    Guerric Pont is Galileo Exploitation Program manager for the European Union Agency for the Space Program (EUSPA).

    Marco Falcone is Galileo First Generation Project manager for the European Space Agency (ESA).

    Miguel Manteiga Bautista is Galileo Second Generation Project manager for the European Space Agency (ESA).

  • Directions 2022: A new epoch for GLONASS

    Directions 2022: A new epoch for GLONASS

    Figure 1. GLONASS high inclined space complex. (Image: Institute of Navigation Technology JSC)
    Figure 1. GLONASS high inclined space complex. (Image: Institute of Navigation Technology JSC)

    The digital transformation of the global economy requires precise time synchronization and valid object position information. Global navigation satellite systems (GNSS) are the most accurate tool for such tasks.

    This year will be 40th anniversary of the launch of the first GLONASS satellite, and we see that the quality of navigation services is driven by the characteristics of today’s satellite navigation signals.

    The first fourth-generation Glonass-K2 #13L satellite is scheduled for launch in 2022. It will broadcast a full ensemble of navigation signals — both Frequency Division Multiple Access (FDMA) signals in the L1 and L2 bands and Code Division Multiple Access (CDMA) signals in the L1, L2 and L3 bands. This long-awaited launch will cap a 10-year effort and begin to provide a new platform by broadcasting all the CDMA signals through a single antenna array on the satellite’s geometric axis.

    The FDMA antenna array is displaced by 0.9 m from this axis, but this arrangement is done on only two satellites. The next Glonass-K2 satellites, which will be launched beginning in 2024, will have a single antenna array for all navigation signals.

    The final second-generation Glonass-M satellite, scheduled to be placed in orbit next year, will provide services by open FDMA signals in the traditional bands at 1.6 GHz and 1.25 GHz. This satellite will be the seventh Glonass-M vehicle able to broadcast GLONASS L3 CDMA signals. There are only two Glonass-K satellites broadcasting this signal now, but more satellites with such a signal will be activated by the end of testing of the GLONASS modernized ground control facility.

    We expect the number of satellites able to provide this service to increase by two per year as we replace Glonass-M satellites with Glonass-K and Glonass-K2 satellites.

    As of this writing, 15 satellites (62% of the constellation) are working past their guaranteed life times, limiting our ability to increase the system’s accuracy. For the last decade, the signal-in-space range error (SISRE) was 1.4 m, despite the fact that newly launched satellites provide a SISRE of about 0.8 m.

    Glonass-K satellites will be launched to maintain the orbit constellation within the next three years, and the accuracy of their signals will be the same or even better. These satellites have a single antenna array for all three bands and could broadcast either FDMA or CDMA signals.

    In 2022, the constellation orbits will increase to six satellites in three planes, as we aim to increase the navigation service accuracy and availability (FIGURE 1). See TABLE 1 for satellite orbit parameters. This constellation will make it possible to increase navigation accuracy in the Eastern Hemisphere by about 25% through decreasing the value of the geometric factor.

    Table 1. Augmented orbit constellation parameters.
    Table 1. Augmented orbit constellation parameters.

    Additionally, this will greatly improve the availability of the GLONASS navigation service in difficult conditions, such as locations where current users can only receive navigation signals from satellites at least 25° above the horizon. New constellation satellites will be based on the Glonass-K platform, which has passed in-orbit qualification and proved it can provide SISRE at 0.3 m. The preliminary design proved that satellites on this platform could provide an in-orbit SISRE below 0.4 m with standard cesium on-board clocks. This signal-in-space accuracy level with valid ionospheric and tropospheric model data from the navigation signal will allow users to receive a position determination error below 2 m in the plane. Navigation services from these satellites will be provided by the CDMA signal in three frequency bands.

    The new satellite will weigh about 1,000 kg and be launched into orbit from both Russian spaceports (northern Plesetsk and eastern Vostochny) by the highly reliable Soyuz-2 rocket. The first launch is scheduled for 2026.

    One of GLONASS’ important tasks is to transmit the UTC (SU) national time scale to consumers. Over the past few years, significant results have been achieved in this area.

    • A complex of high-precision measuring instruments to compare the national coordinate timescale UTC (SU) with the GLONASS timescale was put into operation. These instruments include a transported quantum clock that provides timescale storage with an uncertainty of no more than 1 ns at an observation interval of one day, and with a transportation time of no more than 12 hours. It also provides duplex comparisons of timescales, comparing objects with the permissible uncertainty of ±1.5 ns.
    • Timescale storage complexes of secondary and working standards of time and frequency VET1-5 (Irkutsk), VET 1–19 (Novosibirsk), VET 1–7 (Khabarovsk) and RET1-1 (Petropavlovsk-Kamchatsky) were modernized and put into operation, providing an overall uncertainty of 3 · 10-15 and with a maximum permissible shift of the timescale of the complex relative to the national timescale UTC (SU) of ± 10 ns.
    • An optical ground-based frequency reference on cold strontium atoms was developed with an uncertainty of reproduction of the frequency unit and time of no more than 1 · 10-17 .
    • A keeper of time and frequency units was developed on the basis of a “fountain” of rubidium atoms having a frequency instability of no more than 2 · 10-16 for equipping the standards of time and frequency units and subsequent transmission of more accurate time-frequency information to precision ground and onboard equipment and GLONASS systems.
    • A developmental prototype of the national timescale storage complex of the Russian Federation was developed on the basis of a new generation of hydrogen keepers.

    The application of the newly developed technical equipment made it possible to significantly reduce the maximum displacement of the national timescale relative to the International Coordinated Time Scale (UTC), which in 2020 was less than ± 3 ns (FIGURE 2). The UTC (SU) timescale ranks among the best national implementations of UTC, according to the International Bureau of Weights and Measures (BIMP).

    Figure 2. Displacement of national timescales relative to Universal Coordinated Time (UTC). (Image: VNIIFTRI FSUE)
    Figure 2. Displacement of national timescales relative to Universal Coordinated Time (UTC). (Image: VNIIFTRI FSUE)

    Many important events are coming for GLONASS users in 2022. They will improve the user characteristics and lay the foundation for further development of the system.

    Sergey Karutin is general designer of the Russian GNSS program GLONASS.

    Nicolay Testoedov is CEO of JSC Information Satellite Systems Reshetnev (ISS JSC), a Russian satellite manufacturing company.

    Sergey Donchenko is general director of the Federal State Unitary Enterprise, Russian metrological institute of technical physics and radio engineering, VNIIFTRI FSUE.

  • NIST confirms STL as reliable timing source, even indoors

    NIST confirms STL as reliable timing source, even indoors

    Study by U.S. government agency responsible for maintaining national time scale shows that Satelles provides a signal that is independent of GNSS and delivers exceptional timing stability

    Following a detailed performance study in 2020, the U.S. National Institute of Standards and Technology (NIST) determined that Satellite Time and Location (STL) is a reliable source of timing highly consistent with Coordinated Universal Time (UTC). The secure STL services are provided by Satelles Inc.

    STL is based on a signal independent from GPS and other GNSS. The STL service was able to deliver this consistent performance in a deep indoor environment where GNSS signals did not reach.

    The results of the study were shared by Elizabeth Donley, chief of the Time and Frequency Division at NIST, in a keynote speech at the Workshop on Synchronization and Timing Systems (WSTS) conference on April 1.

    Donley articulated the details of the NIST study, in which a GPS-disciplined clock and a Satelles EVK-2 evaluation unit with a quartz oscillator were compared to UTC for 50 days. In this evaluation, the GPS device received its signal from an outdoor antenna, whereas the Satelles device was connected to an indoor antenna in a deep indoor environment where GNSS signals were not able to reach.

    Time deviation calculations estimated the stability of the two signals with respect to the UTC time scale. Based on one day of averaging, the GPS instability was less than two nanoseconds, and the STL instability was only slightly higher at under three nanoseconds (see chart). These measurements demonstrated that STL delivers stability comparable to GNSS and does so in an indoor location where GPS signals usually cannot penetrate.

    Image: NIST
    Image: NIST

    STL delivers a positioning, navigation and timing (PNT) service from satellites in low Earth orbit (LEO) to back up or augment GPS and other GNSS. The evaluation by NIST confirms that users of PNT-reliant applications can obtain accurate and reliable timing without using GNSS.

    “We are thrilled that NIST has performed these independent tests that confirm what we have long known, which is that STL delivers an independent timing source that is reliable and highly consistent with UTC,” said Gregory Gutt, president and CTO of Satelles. “This report complements and reinforces the findings of the U.S. Department of Transportation, which identified STL as a top-ranked PNT system in its technology demonstration report released earlier this year, and showed STL to be the only solution that demonstrated a wide-area timing capability that works indoors and out.”

  • USDOT seeks university to help secure automated navigation

    USDOT seeks university to help secure automated navigation

    The U.S. Department of Transportation (DOT) announced competitive grant solicitation for a new University Transportation Center (UTC) specifically on Highly Automated Transportation Research.

    Photo: Resilient PNT Foundation
    Photo: Resilient PNT Foundation

    “People will not be comfortable getting into an automated vehicle or with platooning driverless trucks heading down the highway if they think… their GPS might be spoofed,” according to Diana Furchtgott-Roth, Deputy Assistant Secretary at the U.S. Department of Transportation at a recent presentation. She believes getting robust and resilient navigation right is a prerequisite for success with automated and autonomous systems.

    As part of this effort, her office is looking for a U.S. non-profit institution of higher learning to become a University Transportation Center (UTC) focusing on “Highly Automated Transportation Research.” A critical component of this new center’s efforts will be work on resilient positioning, navigation and timing (PNT) services.

    From the abstract:

    “Progress on the path toward integrating automated systems into the transportation domain for all modes of transportation (surface, aviation, and maritime) will be aided by additional research addressing challenges associated with ensuring resilient Positioning, Navigation, and Timing (PNT) services to support automated systems.

    “This University Transportation Center will complement the new Center of Excellence for Highly Automated Transportation Systems being implemented within the USDOT. Among the critical needs is research to support securing cyber resilient PNT receivers for use in automated systems. This UTC will carry out research to support the development of standards and or prototypes and incorporate existing U.S. Government guidance.“

    The department is also looking for UTCs for Communications Technology and E-Commerce Effects on Travel Demand; Implications of Accessible Automated Vehicles and Mobility Services for People with Disabilities; and Strategic Implications of Changing Public Transportation Travel Trends.

    According to a press release issued March 30, one institution will be selected in each of the four areas. The total initial funding available to is just shy of $5 million.

    Interested parties can find the opportunity here, and can call Nancy Wilochka at (202) 366-5128.

  • USDOT seeks to create 4 new university transportation centers

    USDOT seeks to create 4 new university transportation centers

    Photo: adamkaz/E+/Getty Images
    Photo: adamkaz/E+/Getty Images

    The U.S. Department of Transportation (DOT) has announced competitive grant solicitation for nearly $5 million for four new University Transportation Centers (UTCs).

    Through the Notice of Funding Opportunity, DOT intends to fund one UTC in each of the following specific topic areas:

    • Highly automated transportation systems research
    • Communications technology and e-commerce effects on travel demand
    • Implications of accessible automated vehicles and mobility services for people with disabilities
    • Strategic implications of changing public transportation travel trends.

    Only U.S. non-profit institutions of higher education are eligible to apply or to be members of a proposed UTC consortium. Non-profit institutions of higher education may include qualifying two-year institutions.

    “Our University Transportation Centers are not only the seeds of our future transportation system, they serve as living labs, bringing research to reality. Four new UTCs will address a variety of important 21st century transportation topics,” said Deputy Assistant Secretary Diana Furchtgott-Roth.

    Thee Tier 1 UTCs will support research needs that DOT has identified under two of the FAST Act research priorities (49 U.S.C. § 6503(c)(1)): “Promoting Safety” and “Improving Mobility of People and Goods.”

    The Notice of Funding Opportunity (NOFO) is available now. For more information on the UTC program, contact Nancy Wilochka, (202) 366-5128.

  • United Technologies Corp. acquires Rockwell Collins

    United Technologies Corp. acquires Rockwell Collins

    Logo: Collins AerospaceUnited Technologies Corp. (UTC) has completed its acquisition of Rockwell Collins. The two companies will now combine to form Collins Aerospace Systems and be based in Palm Beach City, Florida.

    Collins Aerospace Systems will have about 70,000 employees. Rockwell Collins CEO and President Kelly Ortberg will be the CEO of the newly formed company. UTC Aerospace President Dave Gitlin will serve as president.

    There are no immediate changes planned as to how many employees will be based in Cedar Rapids, Iowa, or Collins Aerospace’s other locations in Eastern Iowa, said Collins Aerospace spokeswoman Pam Tvrdy-Cleary.

    However, the company’s avionics and mission systems will be based in Cedar Rapids. Kent Statler, former Rockwell chief operating officer for commercial systems, will head the avionics unit, and Phil Jasper, former Rockwell chief operating officer for government systems, will be in charge of the mission systems.

  • Expert Advice: A Leap Second — One More Time!

    Expert Advice: A Leap Second — One More Time!

    From left: Dennis McCarthy, Wayne Hanson, Ronald Beard and William Klepczynski
    From left: Dennis McCarthy, Wayne Hanson, Ronald Beard and William Klepczynski

    By Dennis McCarthy, Wayne Hanson, Ronald Beard and William Klepczynski

    Once again we are going to adjust the world’s clocks by one second. This time it will happen on June 30, when we insert another leap second in Coordinated Universal Time (UTC), the standard international time scale. In theory, all UTC clocks should insert a second labeled 23h 59m 60s (the leap second) following one labeled 23h 59m 59s UTC. This is equivalent to having all of the clocks in the world stop for one second at that time.

    Are you ready for it?

    The last leap second occurred two years ago on June 30, 2012, and the continuation of the process of making these one-second adjustments has stirred a growing controversy over the last few years.

    How did the leap second come about — and why do we continue making these sporadic adjustments?

    From Sun to Caesium

    Historically, it has been easy to make use of the apparently uniform repetition of various astronomical phenomena to measure the passage of time. We’re familiar with the Sun rising and setting, and this regularity provides us a convenient measure of time: the solar day. In recent times until 1960, the average solar day was used as the basis for timekeeping, and if we divide the day into 24 hours, each containing 60 minutes made up of 60 seconds, we can define the second as 1/86,400 of the mean solar day. This meant that the length of the second depended on the Earth’s rate of rotation because it is the rotating Earth that causes the Sun to appear to move across the sky.

    In the mid-1930s, astronomers concluded that the Earth did not rotate uniformly as measured by the most precise clocks then available. This causes the duration of a second to vary as the Earth’s rotation rate varies. We now know that a variety of physical phenomena affect the Earth’s rotational speed, and consequently this definition of a second became impractical for applications that require a truly uniform time scale. So, in 1960, the second was redefined in terms of the Earth’s yearly orbital motion around the Sun. The time scale provided by this astronomical phenomenon was called Ephemeris Time (ET), to call attention to the fact that its realization depended on the conventionally adopted positions and motions (that is, the ephemeris) of the Sun (or Moon) that was used in the analyses of the required astronomical observations. The second defined in this manner was called the Ephemeris second.

    Although Ephemeris Time does provide a more uniform measure of the duration of a second, it is inconvenient to make the necessary astronomical observations that would be required to maintain a practical time scale for applications that demand high precision. So, in 1967, the second was redefined again, this time in terms of the frequency of an energy level transition in the Caesium atom, which had already been calibrated with respect to Ephemeris Time by using astronomical observations of the Moon’s motion. Caesium frequency standards, by the early ’60s, had become known as reliable, uniform, accurate and precise clocks. The second defined in this way provided, and continues to provide, a uniform standard of time that can easily be measured in a laboratory with greater precision and accuracy than any astronomical phenomena.

    Lab Clocks Rule

    Although the second defined using the frequency of an atomic energy level transition does provide a unit of time duration that is precise and uniform, it does mean that the passage of time measured in this way is no longer connected to astronomical phenomena. Indeed, with the advent of more accurate observational techniques, astronomers could measure variations in the Earth’s rotation rate by measuring its changing orientation in space and comparing the rate of change with laboratory clocks. They established that among the various variations in the Earth’s rotation rate is the gradual slowing down with respect to a uniform atomic time scale. This deceleration is consistent with theoretical tidal effects and observed terrestrial deglaciation.It is also apparently consistent with ancient observations of solar eclipses, indicating that that this slowing has been going on for thousands of years

    As a result, if we were to observe a recurring astronomical event, we would see it happening earlier from day to day. To bring our clock back into agreement with the astronomical event, we would have to add some time to the face of our atomic clock. While astronomers can cope with this situation by applying the appropriate corrections derived from astronomical observations that measure the Earth’s rotation rate, navigators that relied on astronomical observations to determine their positions considered this situation problematic.

    When the definition of the second based on the Caesium atom was introduced, it was known that there would be a time varying discrepancy between a clock running at a uniform rate and a theoretical one using a second defined by the Earth’s rotation rate. Starting from 1961, the observed discrepancy was modeled by making small adjustments on the order of a few milliseconds (thousandths of a second) to our clocks at first, and later by making small adjustments to the frequency of the atomic clocks from time to time, usually on an annual basis. This meant that the duration of a second could vary depending on when it was measured.

    No More Changes

    In 1970 the International Radio Consultative Committee (CCIR and now known as the International Telecommunications Union Radiocommunications Sector, or ITU-R) in collaboration with other international agencies adopted a definition of UTC that did away with any periodic changes to the duration of the second. Instead it was decided that the discrepancy between UTC and the observed rotation angle of the Earth would be accounted for by making one-second adjustments when needed, so that the absolute difference between UTC and the Earth’s rotation angle measured in time units would always be less than 0.9 seconds. A finer correction would also be provided frequently so that the Earth’s rotation angle in time units designed as Universal Time 1 (UT1) could be derived to 0.1 second precision.

    It was specified that the one-second adjustments, either positive or negative, were to be made preferably at 23h 59m 59s on the last day of the months of December or June, but could also be made, if necessary, at 23h 59m 59s on the last day of the months of March and September, and further if required at 23h 59m 59s on the last day of any month. The implementation of this definition actually began in 1972, a year in which two leap seconds were introduced.

    These one-second adjustments came to be known as “leap” seconds by analogy with the “leap” day inserted in calendars. This definition then fixed the second in UTC to be uniformly established as the international standard atomic second defined by the resonance frequency of Caesium and known as the SI (Système International) second.

    Compromise Overcome by GNSS

    The introduction of the concept of the leap second was historically a compromise with practitioners of celestial navigation who needed to base their observations on astronomical time to determine their longitude. If UTC doesn’t differ from the observed rotation angle of the Earth by more than a second, navigators could use UTC directly as a substitute without introducing a systematic error greater than a quarter of a mile. However, the routine practice of using celestial navigation has been overcome by the success of Global Navigation Satellite Systems (GNSS), inertial navigation systems, and radar navigation.

    In fact, the U.S. Naval Academy stopped including celestial navigation in its curriculum in 1998. In the time span since the introduction of the idea of a leap second, computer networks, wireless telecommunication systems, satellite communications, telephone networks, air traffic control systems and even industrial processes have developed to the point where precise time is an essential component of their successful operation. Users and suppliers of these systems are concerned with the impact of sporadic, essentially unpredictable, one-second adjustments.

    Most of these modern systems derive their time using GPS timing receivers. Although the navigational solutions make use of GPS System Time, these receivers provide UTC by means of a broadcast correction that provides the time-varying difference between GPS System Time and UTC. This correction normally provides the varying difference between the two times to less than a microsecond but must also keep track of when a leap second is introduced. As the leap second changes occur sporadically, there may be worries that problems could arise because hardware or software may never have been tested thoroughly for a leap second occurrence. As a result of these concerns, as well as the cost of stopping all of the clocks in the world for one second, the ITU-R has been discussing a possible revision of the definition of UTC by dropping the future use of leap seconds.

    Leap or Not Leap?

    The question of the future of UTC was raised in 2000 with the suggestion of modifying it to be a continuous timescale without leap seconds. Consideration of this question is still ongoing. The 2012 World Radiocommunication Conference (WRC-12) identified this issue as urgent, requiring further examination by the 2015 World Radiocommunication Conference (WRC-15) “to consider the feasibility of achieving a continuous reference time-scale, whether by the modification of Coordinated Universal Time (UTC) or some other method, and take appropriate action…”.

    With the aim of providing adequate technical background for WRC-15 to make an informed decision on this issue, the International Bureau of Weights and Measures (BIPM) and the ITU agreed to organize jointly a workshop on the future of the international time scale. This workshop was held in Geneva, Switzerland, in September 2013. It provided a unique opportunity to present available information on current and possible future precise frequency and time standards, sources and their characteristics, time scales and dissemination systems and different views on the future of UTC.

    Contributions to the workshop were specifically invited to ensure that the breadth of the issue would be covered. Included were the relevant international organizations (the International Astronomical Union, the International Earth Rotation and Reference Systems Service, the International Union of Geodesy and Geophysics, the International Organization for Standardization, the International Maritime Organization, the International Civil Aviation Organization, the Union Radio-scientifique Internationale), the providers of GNSS services (GPS, GLONASS, Galileo and BeiDou), the national metrology institutes that realize and maintain local representations of UTC, the ITU member administrations, and the ITU-T and authorities responsible for electronic time services. Information on the workshop, agenda and presentations is available.

    Final Decision in November

    A special issue of ITU News magazine dedicated to the workshop has also been published; an online version is available. It did not provide a decision on the issues, but rather a forum for issues to be discussed, since there is some controversy over modifying the global reference time scale. The final decision is to be made at the WRC-15 in November when the method for satisfying the feasibility of achieving a continuous time scale will be determined as well as how it would be implemented.

    As preparations begin for the June leap second, hardware and software will undergo testing. This process is likely to be repeated for some time to come, even if the decision to eliminate the use of leap seconds in UTC is made. Legacy systems reliant on the use of leap seconds will require an adequate period of time to adapt to any change in the definition of UTC. If the suppression of leap seconds would be decided, it is recommended that a period of time no less than five years be allowed  before the Final Acts of the WRC-15 go into effect. So, leap seconds could be with us for some time yet.

    Editor’s Note: For an earlier discussion on the leap second by McCarthy and Klepczynski, download the Innovation article “GPS and Leap Seconds: Time to Change?” from the November 1999 issue of GPS World.

    Dennis McCarthy is retired, and serves as a contractor with the U. S. Naval Observatory, where he was science advisor, director of the Directorate of Time, and head of the Earth Orientation Department. Internationally, he has served as president of the Commissions on Time, Commission on Earth Orientation, and Division 1  (Fundamental Astronomy) of the International Astronomical Union (IAU). He was also secretary of Commission 5 of the International Association of Geodesy.

    Wayne Hanson has been a consultant and president of Time Signal Engineering since his retirement in 2001 as chief of the Time and Frequency Services Group in the Time and Frequency Division of the National Institute of Standards and Technology. He is the U.S. chairman of the International Telecommunication Union – Radiocommunication Sector, Working Party 7A concerned with Time Signal and Frequency Standard Emissions.

    Ron Beard is the head of the Advanced Space PNT Branch at the Naval Research Laboratory and International Chairman of ITU-R Working Party 7A, Precise Time and Frequency Broadcast Services. During the early development of GPS in the 1970s, he was the project scientist in the NRL GPS Program Office that developed Navigation Technology Satellites One and Two that operated the first atomic clocks in space.

    William Klepczynski is now retired. During his career, he was a consultant to the Institute for Defense Analyses and the head of the Time Service Department of the U.S. Naval Observatory, where he managed the USNO Master Clock, timing operations for GPS and time distribution systems that utilize communications and navigation systems.

  • Galileo Now Tells UTC Time

    Europe’s four Galileo satellites are now working as clocks accurate to a few billionths of a second, disseminating the exact time through their signals expressed as the UTC Universal Coordinated Time global standard, reports the European Space Agency.

    “A billionth of a second equals a nanosecond, a time interval far beyond our own human capacity of appreciation,” explains Marco Falcone, ESA’s Galileo System Manager.

    The prediction error for the offset between Galileo System Time and UTC, expressed in nanoseconds. The UTC value available to the user via Galileo is expected to be accurate within 26 nanoseconds, but in spring 2013 it has been even better, with a prediction error in the last two months of less than five nanoseconds.
    The prediction error for the offset between Galileo System Time and UTC, expressed in nanoseconds. The UTC value available to the user via Galileo is expected to be accurate within 26 nanoseconds, but in spring 2013 it has been even better, with a prediction error in the last two months of less than five nanoseconds.

    “A single lightning flash across the sky during a thunderstorm lasts about ten milliseconds, which is already 10 000 000 nanoseconds. But for high-tech applications, as well as navigation services, nanosecond accuracy is essential.”

    The replacement for Greenwich Mean Time, UTC is part of all our daily lives: it is the timing used for Internet, banking and aviation standards as well as precise scientific experiments, maintained by the Paris-based Bureau International de Poids et Mesures (BIPM).

    The BIPM computes UTC based on inputs from collections of atomic clocks maintained by institutions around the world, including ESA’s ESTEC technical centre in Noordwijk, the Netherlands.

    ‘Galileo time’ is derived independently of UTC but is being kept close to it, with a precise ‘offset’ between the two values being calculated continuously and then disseminated through Galileo’s navigation message.

    Galileo, like all other satellite navigation systems, is based on the highly precise measurement of time. A receiver on the ground pinpoints its position by calculating how long signals from satellites in orbit take to reach it.

    Matching the receiver and satellite clocks then multiplying the time taken by the speed of light gives the range between user and satellite, allowing the receiver to fix its own location relative to four or more satellites.

    “Each navigation system has its internal reference system time used to synchronise all system clocks and maintain overall coherence,” adds Marco.

    Galileo's navigation message embedded in its signals include precise timings based on Galileo System Time, kept close to global time standard UTC with a precise offset given, accurate to at least 26 nanoseconds.
    Galileo’s navigation message embedded in its signals include precise timings based on Galileo System Time, kept close to global time standard UTC with a precise offset given, accurate to at least 26 nanoseconds.

    “Galileo runs on Galileo System Time, GST, which is fixed on the ground at the Galileo Control Centre in Fucino, Italy, by the Precise Timing Facility, based on the average of different atomic clocks.

    “Strictly speaking, for navigation purposes alone this internal reference system time does not need to be in agreement with UTC at the highest level of accuracy but with this agreement being the case, it is therefore possible to immediately disseminate UTC to the users to the best  accuracy and this is the aim of Galileo.”

    The offset between GST and UTC is currently estimated in Turin, Italy, by the Istituto Nazionale di Ricerca Metrologica (INRIM), where time measurements are performed every day with the most precise techniques available to check GST status.

    INRIM has been supporting ESA’s Galileo development since the early phases of the project. More recently INRIM has overseen the creation of a ‘Time Validation Facility’ for Galileo in collaboration with five other European time-measurement institutions: the Physikalisch Technische Bundesanstalt in Germany, the National Physics Laboratory in the UK, the Systeme de References Temps Espace/Observatoire de Paris in France, the Real Instituto y Observatorio de la Armada in Spain and Observatoire Royale de Belgique.

    Galileo's Ground Control Segment (GCS) in the Fucino Control Centre in Italy oversees Galileo navigation services and satellite payload operations.
    Galileo’s Ground Control Segment (GCS) in the Fucino Control Centre in Italy oversees Galileo navigation services and satellite payload operations.

    Each day, the most precise European clocks and national time scales are compared to GST and the offset compared to UTC is estimated and provided to the Galileo Control Centre. This offset is then uploaded to the Galileo satellites for transmission in the navigation message available to users.

    As explained by Patrizia Tavella from INRIM, “The UTC value available to the user via Galileo is expected to be accurate within 26 nanoseconds, but in the last two months it was even better, with a prediction error in the last two months of less than five nanoseconds.”