From ladkin@rvs.uni-bielefeld.de Wed May 14 16:00:32 1997 From: "Peter B. Ladkin" Subject: A definitive clarification of time measurement Date: Mon, 28 Apr 1997 09:36:11 +0100 Peter Ladkin, Universitaet Bielefeld, Technische Fakultaet, Postfach 10 01 31, D-33501 Bielefeld, Germany http://www.rvs.uni-bielefeld.de +49 (0)521 106-5326 (From John Laverty via Peter Ladkin) In Britain, the National Physical Laboratory is the canonical site for questions concerning standard time, and the Royal Greenwich Observatory (which is now in Cambridge) refers all questions there. I talked to Dr. John Laverty of Time and Frequency Services, CETM (jrl@taf.npl.co.uk), who kindly supplied me with the following account of time standards. The position of the sun in the sky has been used as a basis for measuring time for many centuries. One simple example is that 12 noon in local solar time occurs when the sun is directly `overhead'. However, local solar time does not provide as uniform a time scale as that based more implicitly on the rotation of the Earth about its axis. The Earth's orbit is elliptical and its axis tilted, so that the actual position of the sun against the background of stars appears a little ahead or behind the expected position. The accumulated error varies from 14 minutes slow in February to 16 minutes fast in November. These effects can be predicted, and a more uniform timescale can be established on the basis of a hypothetical 'mean' sun that moves with uniform speed across the sky. Greenwich Mean Time (GMT) is probably the most well known example of such a time scale: GMT is the local time on the Greenwich meridian based on the position of a hypothetical mean sun. The need to coordinate time measurement and agree on a standard time was driven by improved communications, particularly by the railways, when the differences in the local time at different locations became very noticeable. Greenwich Mean Time was established as a world time standard at the International Meridian Conference in 1884. The time scales in active use today are Universal Time (UT), Coordinated Universal Time (UTC) and International Atomic Time (TAI). They are described below along with some of the reasons for their use. Greenwich Mean Time (GMT) and Universal Time (UT) are very closely related. Before 1925 January 1, the twenty four hour GMT day was taken to commence at noon, while since that date the convention has been for the GMT day to begin at midnight. The term Universal Time (UT) was introduced in 1928 by astronomers to denote GMT measured from Greenwich Mean Midnight, to be clear about the convention for the start of the day. Now there are actually three different definitions: UT_0, UT_1, UT_2 (using underscores to denote subscripting). UT_0 is based on `direct' observation of the earth's rotation on the prime meridian, UT_1 is adjusted to account for the small movements of the Earth relative to the axis of rotation (polar variations), and UT_2 adjusts for seasonal variations. The maximal difference between all three is of the order of a few tens of milliseconds. The term `UT' thus crudely refers to all three for large granularities, and for finer granularity, the term is ambiguous and one needs to specify which of the UT's one is referring to. Starting in the 1930's with the development of quartz crystal oscillators, but particularly in the 1950's with the introduction of atomic clocks, better measurements have been available. As a consequence of studies comparing atomic clocks and astronomical observations, it was realised that atomic clocks offered a more much more stable time standard than one based on the rotation of the Earth. In 1967, the SI second was redefined as "the second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom". The international time scale based on the SI second is International Atomic Time (TAI). TAI was synchronised with UT at the beginning of 1958. It is a more stable time scale than UT, but UT and TAI naturally drift apart because they are based on different principles. Universal Coordinated Time (UTC) is a compromise between TAI and UT and was established in its current form on 1 Jan 1972. It uses the SI definition of the second, but introduces leap seconds by convention in order that the difference between UTC and UT shall never be more than one second. There have been 20 leap seconds introduced since January 1972; the first at 1 July 1972. The 21st leap second is scheduled for 1 July 1997. So UTC and TAI run in lockstep, but with conventional separation, which is now 30 seconds and will become 31 seconds on 1 July 1997 (By the beginning of 1972 TAI and UT had drifted apart by 10 seconds from the `synchronisation' point at the begining of 1958, which accounts for the extra 10 seconds in addition to the leap seconds). UTC is the current world time standard, as indicated by the recommendations of the International Telecommunications Union (ITU) for example. There are some 50 or so centers around the world which measure TAI/UTC using commercial atomic clocks, with just a few laboratory based 'primary' caesium standards which are are able to measure the time with greatest accuracy. The PTB in Germany has the distinction of having the longest running and most reliable primary caesium standards. The NPL, having developed the first caesium atomic clock in the 1950's, is currently working on the `next generation' standard based on the caesium fountain method demonstrated at the LPTF in Paris. There are other primary caesium standards at NIST in the US, NRC in Canada, CRL in Japan and in Moscow. The institute responsible for maintaining TAI and UTC is the BIPM in Paris, and the decision as to when to introduce leap seconds is made by the IERS, also in Paris, who measure UT also. The Royal Greenwich Observatory (RGO) no longer maintain their own time standard. It is recognised that GMT and UT are equivalent, so that now the IERS provide the information necessary to determine GMT. However, the appropriate definition of UT should be used instead of GMT if the distinction between UT_0, UT_1 and UT_2 is important for a given application. The time standards that are so carefully measured by astronomers and metrologists need to be made available if they are to be of use, and radio time signals are one of the most common ways of making UTC available. In Western Europe, NPL broadcasts the UK time on 60 kHz from the BT Radio Station at Rugby (call sign MSF), and similarly, the PTB broadcasts Central European Time from Frankfurt (call sign DCF77) on 77.5 kHz. There are similar transmitters operated by other countries around the globe. The other common means of accessing standard time is through the Global Positioning System (GPS) navigation system, where accurate position and time information allow a receiver to calculate its position from the times of flight (at the speed of light) of signals from a number of GPS satellites. The GPS system was developed, as its name implies, for positioning, but a welcome spin-off is accurate time. The GPS time signals offer high-accuracy UTC (one microsecond time accuracies are readily achievable) and global coverage, but the LF radio time signals, although limited to a range of typically 1500 km, offer the advantage of broadcasting the local time including summertime changes (to millisecond time accuracies). ------------------------------ Date: Tuesday, May 13, 1997, 7:35PM EDT From: Phillip G. Felker <70672.2100@compuserve.com Subject: Y2K fixed? But what about the month? Last evening I discovered a probable side-effect of the Y2K problem. I have used a credit card to pay the monthly charges of my on-line service provider. It happens to be the service noted in my return address. A week or so ago, when I signed onto the service, I was prompted that my credit card information must be updated. As I had no other choice, I dutifully followed the instructions and submitted the information. My access was granted and I thought no more about it even though I had been using the same card for several years. Yesterday, I was again prompted for an update of my credit card information. I entered the information but noticed a message stating that they would resubmit the charge to the credit card company. This peaked my interest to the point I contacted the company and inquired as to whether there was any problem with my account. Fortunately, I was able to contact a real person, (score one) who was very helpful and seemed knowledgeable about the system, (score two) and even offered some information as to the problem (score three)! Seems that the on-line service provider was submitting the charge using only one digit for the month in the expiration date and the credit card company required two (zero fill to the left for all months less than 10). I advised her that I was apparently being victimized by a program bug but also indicated that the company's program was perhaps an accomplice (i.e. accept a single digit month as valid, except for zero, which would reduce the possibility of a false negative from 75% (9 out of 12 months) to a fraction since the only positive errors would be confined to those bum transmissions of only 2 months). I then signed back onto the on-line service and checked my billing information only to discover that there was no way to force the lead zero in the expiration date! I did leave a message for customer service. My suspicion is that in "fixing" one or both computer programs for the Y2K problem, the program "broke" the month. The risks are quite obvious; thorough testing, customer requirements, et al. I am reminded of a time many years ago when a particular technical support person began cataloging the application programmers excuses. This one falls under the heading of, "But I didn't change anything in that part of the program!" I can only hope that I will be able to sign onto the service to send this. Phillip