Dear Respondee, Thank you for having responded to, or somehow been connected with, our questionnaire concerning the effects of a redefinition of UTC. If you are interested in pursuing this issue, we welcome you to join our listserv "chat group", http://clockdev.usno.navy.mil/archives/leapsecs.html. Demetrios Matsakis *************************************************************************** Report of the URSI Commission J Working Group on the Leap Second Date: July 2, 2000 Abstract and Conclusions An e-mail survey to find possible adverse effects of a redefinition of UTC has identified some possibly expensive or unsolvable problems involving software rewriting or checking, which are listed below. Although it was not possible to quantify the financial scale of resolving the software problems, the largest expenses appear to be for satellite systems, of which one estimate of several hundred thousand dollars was supplied. The quantity and quality of the responses opposed to a change indicate that those who favor any change must be prepared to make a very convincing argument to people and groups who initially will disagree with them. To further discuss this issue and inform the community of any developments, an archived electronic listserv has been set up. Anyone wishing to join can do so using http://clockdev.usno.navy.mil/archives/leapsecs.html. Introduction UTC (Coordinated Universal Time), which the public commonly confuses with Greenwich Mean Time, is computed by occasionally adding leap seconds to International Atomic Time (TAI). Since 1972, these leap seconds have been added on December 31 or June 30, at the rate of about one every 18 months, and serve to keep atomic time in step with the Earth's rotation. Although it is recommended that users use only TAI or UTC, as their needs indicate, many major navigation system have used times offset from TAI by fixed amounts. The most important of these is GPS, which is offset by 19 seconds from TAI. A segment of the international timing community has proposed a revision of the definition of UTC to avoid the discontinuities due to intermittent leap seconds. A discussion of the motivations for a change and of possible solutions has been published by McCarthy and Klepczynski in the Innovations Section of the November, 1999 issue of GPS World. The authors consider the most significant reason for a change to be keeping spread-spectrum communication systems and satellite navigation systems compatible with each other and with civil times. Another reason is the emerging need in the financial community to keep all computer time-stamps synchronized. In order to survey the effects of any action, an URSI Commission J Working Group (WG) was formed, whose purpose was to prepare this report and propose further actions. Methodology A questionnaire (Appendix I) was distributed as widely as seemed appropriate (Appendix II). The goal of the questionnaire was to find and categorize those operations that would be adversely affected should a change in UTC's computation be made. The questionnaire focused on the possibility of simply inserting no new leap seconds, although alternative solutions were also solicited. Over 200 responses were received, and no effort was made to separate the responses of URSI members from those of nonmembers. The principal object of the questionnaire was to find what systems would be adversely affected should a change be made in leap second procedures rather than to convince users of the need for a change or to take a vote. However, so many queries on these matters were received that a "standard reply" (Appendix III) was developed and distributed as appropriate. In the spirit of full disclosure, the number of responses in each category is given, but we caution that this was by no means an unbiased sampling of all who would be affected by the change. All responses were counted only once, with preference to the most practical grounds for objection. About half the responses that were received were opposed to any change, while one-fourth were in favor of a change, and one-fourth indifferent. I. Responses Opposed to Changes in the Status Quo A. The expense of rewriting software. Five responses suggested that contractors would have to be hired to scrutinize and adapt large amounts of code for operational satellite systems. Efforts were made to contact these responders for specific dollar amounts, and one off-the-cuff estimate of "several $100,000" was received. The impact on such systems would be lessened if any decision to redefine UTC were announced several years in advance. Twenty-six others indicated that software would be a serious problem - a very few of these were from people who did not seem to understand the proposal. There were 9 responses involving telescope control; one of these, from the Keck Observatory, provided a rough estimate of a few programmer- months. Others pointed out the problems computing eclipses and occultations, for telescope pointing by amateurs, or with code they had themselves written for professional-level projects such as speckle interferometry. One observatory indicated its station clock can not accommodate a large UT1-UTC correction. Fourteen more indicated that software issues would be a problem, but that they are probably solvable. Some of these actually indicated support for the change. B. Inherent inability to rewrite software to allow for |UT1-UTC| exceeding .9 seconds. Ten responses involved navigational software. Taking the example of a software product of the US Naval Observatory (USNO), pilots and sailors are given the option to input UT1-UTC. However, it is expected that many users would not understand this and enter 0, leading to noticeable errors within a decade. These are similar to the telescope-control problems covered above, except that one could not and should not expect the general public to ever understand these issues. Problems amateur astronomers might have are also included here, and were brought up by many responders in other contexts. One of the problems anticipated is that UT1-UTC could be applied with the wrong sign, just as the leap second is occasionally applied with the wrong sign. An example of "buffer overflow" problem would happen in NIST's WWV, WWVH and WWWB transmissions, which do not allow enough space for |UT1-UTC| to exceed .9 sec. Any users of these broadcasts who might need this information and who are unaware of the problem, decoding in hardware, or relying on old software would be adversely affected. Under the best of future circumstances only the sub-second (tenths) digit would be available, requiring the user to keep track of the digits to the left of the decimal point. Not tabulated is an informal comment seriously made to the Working Group's Chairman, by a respected and competent scientist from a non-western nation, that astrologers would be adversely affected. C. Philosophical Objections Eight thought this would confuse or antagonize the public, religious authorities, or even scientists - in essence because solar time is "true time". (In contrast, one responder in favor of a change thought this would help educate the public to the fact that Earth rotation is not "true" time). Three pointed out that legal complications might occur in countries where laws are specified in terms of solar time, or GMT (which has not existed for thirty years). Although one of these responders feared governments would not follow the scientist's lead, we find it difficult to believe that governments would, on their own, choose to add leap seconds. Others thought that any legal system flexible enough to handle daylight savings time and the past abandonment of GMT in favor of UTC would easily accommodate a seamless change in UTC's computation, especially if no other time standard were available. Some of the history of legal issues concerning past changes in time definition can be found in the book "Greenwich Time and Longitude" by Derek Howse. Three thought we should not adopt a system which will fail in the long run, even if that is a very distant time in the future. (It could be pointed out that all current time systems will eventually fail. Well before 2050 we could be routinely adding more than one leap second per year, and when we reach the point where a day is 48 hours long we would have to add a leap second every second. Even the Gregorian calendar will eventually need revision because in a few million years the Earth will rotate less than 365 times per year, and leap days will not be necessary.) Two thought this would deprive the timing community of free publicity when leap seconds are inserted. Thirty-eight expressed opposition, but gave no specific reason. Eleven of those also indicated that a problem would exist with their system, but did not specify it. Some of these pointed out that TAI was readily available, or indicated that they had seen no justification for a change. (We had intentionally provided no justification in the initial questionnaire, but two responders replied that they still believed there was not enough reason to change even after having read "standard response".) Four were against it because they thought problems would happen if some systems did not use the new system, and because they thought one would have to separate analyses based upon whether data were recorded with the current system or the old system. (We believe these to be based upon a misunderstanding of the "no new leap seconds" proposal, but entirely possible if the more drastic measure of re-defining SI, the International Second, were adopted.) II. Alternative Suggestions Five suggested that it would be better to redefine the second to be longer, add 1 second every 18 months forever, change on leap years or century-ends, or change when the number exceeded a fixed amount. These possibilities are also discussed in the GPS World Article. A suggestion was received to add enough "negative leap seconds" to bring UTC in line with GPS. However, a different responder pointed out that many of NASA's programs assume UT1-UTC always grows and that problems would happen with any change that would sometimes lead to a "negative leap second". (According to Dennis McCarthy of the USNO, this is possible even with the current system, but unlikely). It was also pointed out that rubidium atomic fountains or optical standards could lead to a redefinition of the second using an element other than cesium. If so, it could be redefined using a scale factor of sufficient magnitude to avoid the short-term need for leap seconds (As noted in the GPS World article, such a change would alter the values of those physical quantities that depend upon time.) One suggested that a redefined UTC should have a new name. III. Responses in favor of changing the current leap second procedures Forty-eight responses in favor were received, several from people who experienced minor problems now in handling leap seconds, such as confusion due to GPS time being currently 13 seconds offset from UTC and computer errors at the time of the leap seconds. Along with responses based upon reasons already covered in the GPS World Article, there were also three from the highly undersampled group of computer programmers, which pointed out the growing need to synchronize diverse computers to one second accuracy and the difficulties of doing so in an when dates of future leap seconds were unpredictable. Many of the responses indicated that periodic addition of leap seconds on a scheduled basis would also be acceptable. One of the responders, who was opposed to a change, suggested that leap seconds were not a problem because computer users could program an automatic extraction of the needed information from some publicly available source; however it must be pointed out that no one can guarantee that a given file, whether it be for leap seconds or for daily values of UT1-UTC (see part IV, below), will remain forever available in a given format or given IP address. Four responses were from people who indicated that the convenience in not having to update files every 18 months outweighed the expense and problems rewriting software. One of these suggested the use of sufficient negative leap seconds to bring UTC in line with TAI, and several noted that UT1-users easily incorporate the offsets due to time zones and daylight time. IV. Other Responses Forty-seven responders checked the "indifferent" option. Most indicated that there would be no problems with their system, or that the problems were small enough that they were indifferent to a change. Seven other responders indicated that the change would be okay if they could somehow reliably obtain UT1-UTC. (The USNO, as a sub-bureau of the IERS, freely provides this information on its web site, http://www. maia.usno.navy.mil, and via weekly emailings. Other organizations also provide this information.) Some responses tabulated here were phrased in the negative because the responders were apparently unaware that the information was available. One responder was concerned that the GPS system itself would be degraded for this reason, however this is not the case because GPS currently uses UT1-UTC values derived from USNO products. ________________________________________________________________________ Appendix I. Questionnaire It is being proposed to change the definition of Coordinated Universal Time (UTC) regarding the insertion of leap seconds, possibly even eliminating their use. Leap seconds are introduced so as to keep UTC synchronized (within 0.9 s) to the time scale determined from the Earth's rotation. Should no new leap seconds be inserted, solar time will diverge from atomic time at the rate of about 2 seconds every 3 years, and after about a century |UT1-UTC| would exceed 1 minute. Although no fundamental problems are anticipated, it is very likely that Y2K-like problems may result in software that assumes UT1=UTC, or |UT1-UTC|< some value, or whose input/output records use a field size that can only accommodate |UT1-UTC| values up to one second. To gather information, an URSI Commission J Working Group was formed, consisting of Don Backer, Wim. N. Brouw, Barry Clark, Irwin Shapiro, Ir. E. Van Lil, with Demetrios Matsakis being the chair. We would like to ask you to consult with the members of your institute who currently deal with UT1-UTC, and give us a considered response to the following two questions: A. If the appropriate international bodies decide to eliminate the insertion of new leap seconds, would you foresee any practical problems for your institution/instrument/observations? no ____ * yes ___ * possibly ____ (* please explain any known or possible problems) B. Would you be in favor of such a proposal? yes ____ no ____ indifferent ___ have better idea ___ (feel free to comment) C. Is there anyone else you would recommend we contact? (feel free to forward this email directly) I would appreciate your assistance, and a response by January 15 to dnm@orion.usno.navy.mil. I am attaching a list of institutions and persons contacted, except for 931 institutions whose emails were obtained from the AAS. I would like to apologize to anyone contacted twice, but also appreciate it if you would forward this email to anyone we have missed. Also, if you are an URSI Commission J national chair, we would appreciate your forwarding this email to your complete membership and in particular to the directors of observatories. Sincerely, Demetrios Matsakis ________________________________________________________________________ Dr. Demetrios N. Matsakis Director, Time Service Department (202) 762-1587 DSN 762-1587 U. S. Naval Observatory FAX (202) 762-1511 3450 Massachusetts Avenue NW dnm@orion.usno.navy.mil Washington DC, USA 20392-5420 ________________________________________________________________________ Appendix II. Distribution List Efforts were made to conduct an international survey, and responses were received from Europe, Japan, China, Australia, Canada, Russia, Saudi Arabia, Brazil, and other countries. The majority of the responses, however, were from the U.S. Commercial interests, such as satellite companies, were highly undersampled. The questionnaire asked people to forward it to others, and we know the following were contacted: 1. 128 prominent scientists, who were mostly URSI Commission J members and official representatives of institutions associated with URSI. 2. 27 URSI Commission J national chairs. 3. 931 Institutions extracted from a database kept by the American Astronomical Society, including many from outside the United States. Several hundred of those email addresses proved invalid. 4. All on the USNO Series 7 mailing list. 5. All on the "Gazette" mailing list of the International Earth Rotation Service (IERS). This would be expected to include most scientific groups that are directly involved with UT1. 6. One responder who worked in SETI indicated he had forwarded it to SETI circles. 7. The Internet Engineering Task Force's mail exploder. 8. One responder, who was against any change, indicated he had forwarded the questionnaire extensively around Jet Propulsion Labs (JPL). ________________________________________________________________________ Appendix III. "Standard Reply" to queries concerning the questionnaire Thank you for your response, which will be tabulated. Unix-technology willing, we also plan to send a copy of our report all responders. Many people have asked me why there is a move to rethink the leap seconds, with solutions such as (but not limited to) adding no new leap seconds. I unfortunately edited out some of the reasons from early drafts of the questionnaire, because I was afraid that people would not read a long email. Since then, I have developed the following "standard reply": 1.) Many high-tech navigation systems, particularly those using spread-spectrum techniques, can't handle leap seconds very well. GLONASS, the Russian equivalent to GPS, goes off-line for leap second adjustments. Also, problems can occur in interfacing between systems that handle leap seconds differently. There is also the practical problem of inserting a second every year and half - people often do it the wrong way. The response one person sent me is below, and it concerns Network Time Protocol (NTP), which uses the internet to transfer time.
If leap seconds went away, the NTP community would worship the ground you walk on. Leap secs introduce a manual discontinuity in the NTP time scale. It takes a while to propagate leap secs through the hierarchy. Leap seconds are a tremendous headache in the NTP world because they cannot be predicted. One must set a flag to indicate that one is coming. I think it is a very true statement that all GPS users would vote against continuing leap seconds, not just NTP users. Many telecommunications circuits use GPS or atomic clocks to keep cellular phones in operation, and leap seconds are a nuisance to them as well.
2.) I also received many comments about the effects on society when UT1 diverges. Note that we are talking about a minute in the next century. Society routinely handles a one-hour switch with every daily savings time, and a half-hour offset if they live at the edge of a time zone. By the time leap seconds add up to an hour, the world will be very different. If we have settled the solar system, a whole new scheme will probably have evolved. Even if we have not changed our system, society has enough slop in its timekeeping that people will slowly shift without even knowing it. More people will start showing up to work at 9:00 AM, and less at 8:30 AM, etc. 3) The "Innovations" section in November's GPS World is on this subject, and discusses other possibilities aside from "no new leap seconds". These are inserting larger discontinuities less frequently, inserting integer leap seconds at predefined times, simply using TAI, and redefining the length of the second. 4) It should be pointed out that UT1-UTC is readily available on the web. The USNO, as a subbureau of the International Rotation Service, makes this information available via a weekly mailing and from a web page at http://www.maia.usno.navy.mil, and other organizations also provide this service. 5) My final comment is not to worry about any "surprise" decisions - before/if the international bodies all decide to do this, it will be fully debated and publicized. My role, here, is simply to gather information.