Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
GLONASS and Coordinated Universal Time
Igor V. Zheltonogov, D. Aronov and S. Sorokin, Geyser-Telecom, Russian Federation
The purpose of the Russian Federation’s Global Navigation Satellite System (GLONASS) is to provide an unlimited number of air, marine and other users with all-weather three-dimensional positioning, velocity measurement and timing anywhere in the world or in near-Earth space.
GLONASS has three components: a constellation of satellites (space segment); ground-based control facilities (control segment); and user equipment (user segment). The current GLONASS constellation is composed of 29 satellites, of which four are in reserve and one is in testing mode. The most recent launch was on 26 April 2013.
Currently, the GLONASS system implements a time-scale with the leap second, in accordance with international standards (Recommendation ITU–R TF.460-6).
The GLONASS time-scale is periodically corrected simultaneously with the Coordinated Universal Time (UTC) leap-second insertions, which are made following announcement by the International Earth Rotation and Reference Systems Service (IERS).
GLONASS users are notified in advance (at least three months before) of these planned leap-second corrections (including their value and sign) through relevant bulletins, notifications and so on. The GLONASS satellite navigation messages themselves do not include any data concerning the UTC leap-second correction.
Typically, these corrections (±1 second) are performed once a year (or 1.5 years) at midnight 00 hours 00 minutes 00 seconds UTC from 31 December to 1 January (or from 31 March to 1 April or from 30 June to 1 July or from 30 September to 1 October) simultaneously by all UTC users.
As a result of the periodic leap-second corrections, there is no integer-second difference between GLONASS Time and the UTC (SU) time-scale maintained by the Russian Federation. However, there is a constant three-hour difference between these two time-scales.
The GLONASS view of changing the time-scale
The World Radiocommunication Conference in 2012 (WRC‑12) decided that WRC‑15 should, under its Agenda item 1.14, consider redefining Coordinated Universal Time (UTC).
Under this agenda item, and in accordance with Resolution 653 (WRC‑12), ITU–R is conducting the necessary studies on the feasibility of achieving a continuous reference time-scale for dissemination by radiocommunication systems. ITU–R is also studying issues related to the possible implementation of a continuous reference time-scale (including technical and operational factors). Based on the studies, WRC‑15 is invited “to consider the feasibility of achieving a continuous reference time-scale, whether by the modification of UTC or some other method, and take appropriate action, taking into account ITU–R studies” (emphasis added).
In 30 years of operating the GLONASS system and implementing the time-scale with leap seconds in accordance with international standards, a large amount of existing hardware and software has been adapted for the insertion of leap seconds.
In many cases, for example in that of space-borne receivers, this equipment cannot be updated during its operational life. It should be noted that the guaranteed operational life of spacecraft is more than 10 years, and the GLONASS system will have to maintain its existing time-scale with leap seconds to ensure the continued operation of this hardware.
Navigation receivers are widely used for the safeguarding and rescue of human life, for example the COSPAS-SARSAT International Satellite System for Search and Rescue. The Radio Regulations pay special attention to such applications, and their provision No. 4.10 states: “Member States recognize that the safety aspects of radionavigation and other safety services require special measures to ensure their freedom from harmful interference; it is necessary therefore to take this factor into account in the assignment and use of frequencies.” This makes it clear that WRC decisions should not create adverse consequences for systems used for the safeguarding and rescue of human life.
If a decision were to be taken to move to a continuous time-scale in the near future and this decision were to be implemented in the GLONASS system without retaining the existing time-scale (with the leap second), then a large amount of existing equipment and the corresponding systems would provide incorrect navigation information or even be totally unable to operate. In some cases (for example in aviation, maritime and satellite systems) this could lead to disaster. To avoid this disastrous outcome, the existing navigation receivers that use the reference time-scale with the leap second would have to be updated or replaced in order to be able to operate with the continuous time-scale. In many cases this would also involve updating all the approved technical documents, and carrying out a complete cycle of retesting and recertification of the systems and equipment (for example, spacecraft and launch vehicles).
Given the scale of the use of GLONASS system navigation applications in aviation, space and maritime services, there would be significant difficulties for the GLONASS system if the existing time-scale with a leap second is not kept.
The opinion with regard to GLONASS is that WRC‑15 should consider solutions that ensure backward compatibility, enabling existing equipment to operate fully without updates and replacements. One possibility would be to keep the current UTC time-scale (with the leap second) without change and bring into use a continuous time-scale on an equal basis. Such a decision would allow systems using the current UTC time-scale to continue to operate without any changes and the associated costs. It would also avoid problems arising from applying corrections to conform to a continuous time-scale.
By maintaining the current UTC time-scale on an equal basis with a new continuous time-scale, it would be possible in each case to apply the more suitable time-scale for any particular system.