Nº 7 2013 > The future of time –
To abolish or not to abolish the leap second?

Geosciences and international time-scales
Status of the discussions at the International Union of Geodesy and Geophysics

Claude Boucher, International Union of Geodesy and Geophysics representative to the Consultative Committee for Time and Frequency

The Earth's time zones in an abstract image of a map set in a clock face Artist’'s impression of the orbits of the Navstar satellites used in the Global Positioning System (GPS). The system uses a constellation of 24 satell
The Earth's time zones in an abstract image of a map set in a clock face
Artist’'s impression of the orbits of the Navstar satellites used in the Global Positioning System (GPS). The system uses a constellation of 24 satellites. Small ground-based receivers calculate the user's position by measuring the time it takes for signals to arrive from the satellites

Discussions thus far

The International Union of Geodesy and Geophysics (IUGG) is concerned with international time-scales, and specifically with Coordinated Universal Time (UTC), both as a user and a provider. On one hand, IUGG uses these time-scales as the time tag for measurements and as the time variable in models. On the other hand, because space geodesy plays a key role in the realization of time-scales, IUGG is a provider of essential data.

The possibility of redefining UTC with the removal of the leap second has been extensively discussed over the past few years. The topic was one of the major items on the agenda of the 19th Consultative Committee for Time and Frequency meeting held in Sèvres, France, in 2012, and IUGG, like the International Astronomical Union and ITU, gave its views. IUGG has also received a formal request from ITU to express its opinion on the subject, and the outcome of discussions in an internal group set up to gather opinions from members of the IUGG community is given below.

UTC as a recommended international time-scale for geoscience

Geoscientists and others are faced with a multiplicity of time-scales — Barycentric Dynamical Time TDB), Terrestrial Dynamical Time (TDT), Geocentric Coordinate Time (TCG), Terrestrial Time (TT), International Atomic Time (TAI), Coordinated Universal Time (UTC), Global Positioning System (GPS) Time and others. The definitions and physical realizations of these time-scales differ, and some effort is required to achieve a clear view of their interrelationships.

IUGG considers that the adoption of a unique preferred international time-scale as a fundamental reference is very important. Like the generally accepted International Terrestrial Reference System, it would be highly desirable for the communities involved to agree on the choice of an international time-scale. At this stage, IUGG considers UTC to be the best choice, in particular because of the decisions already approved by countries through ITU, such as the link between UTC and legal time.

Redefining UTC

The discussion within IUGG concerning the possible redefinition of UTC presupposes the suppression of the leap second. This implies that UTC would be a uniform continuous time-scale strictly derived from International Atomic Time (TAI), and that the constraint on the value of UT1-UTC would be abandoned.

If redefinition is adopted as the way forward, it has several positive aspects that should be underlined. One is that UTC is continuous and uniform. In addition, the difference between UTC and GPS Time is more or less constant.

But there are also counter arguments. One is that if UTC is redefined, it would not provide a good estimate of UT1, although that difficulty could be overcome by making better direct estimates of UT1. Also, some people see the occurrence of a leap second as an opportunity to communicate and coordinate action with related organizations. IUGG will have to adopt a formal position, bearing in mind these points.

Estimating UT1

It is now technically possible to estimate UT1 to an accuracy of 0.001 second, which is a hundred times better than the estimates made using UTC. The International Earth Rotation and Reference Systems Service (IERS) provides the UT1 estimation service at present. IUGG recommends that this IERS service should be supported and provided with the necessary resources to guarantee its quality and continuance.

Role of global navigation satellite systems in disseminating Coordinated Universal Time

Global navigation satellite systems — such as GPS, GLONASS, and the upcoming BeiDou and Galileo systems — play a tremendous role in disseminating time-scales. In the opinion of IUGG, it is important to support the services provided by global navigation satellite systems in regard to time-scales. In particular, global navigation satellite systems provide the measurements and information required to achieve instantaneous synchronization at the nanosecond level. Furthermore, these systems broadcast the necessary information to estimate time according to the UTC scale.

The use of the global navigation satellite systems is therefore recommended as an important point to ensure the universal adoption of UTC.

The geoscience world

IUGG is the nongovernmental international organization dedicated to scientific research in all fields of geosciences (geodesy, seismology, volcanology, geomagnetism, the atmospheric sciences, oceanography, hydrology and so on).

IUGG includes various committees, as well as eight associations dealing with specific areas of geoscience: the International Association of Geodesy for geodesy; the International Association of Seismology and Physics of the Earth’s Interior for seismology and internal geophysics; the International Association of Volcanology and Chemistry of the Earth’s Interior for volcanology and geochemistry; the International Association of Geomagnetism and Aeronomy; the International Association of Meteorology and Atmospheric Sciences; the International Association of Hydrological Sciences; the International Association for the Physical Sciences of the Oceans; and the International Association of Cryospheric Sciences for ice studies.

IUGG and the International Astronomical Union jointly govern the International Earth Rotation and Reference Systems Service (IERS), while IUGG also governs several other international services, such as the International Global Navigation Satellite Systems (GNSS) Service.

IUGG is formally represented in the Consultative Committee for Time and Frequency, one of the consultative committees of the International Committee for Weights and Measurements (CIPM).



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