Russell Eberst asked: > Whatever happened to 'leap seconds'? They were designed to be > inserted at the end of December or June (today) to allow the > Earth to catch up with a regular time standard. However, > there hasn't been a leap second announced for many months. I had not noticed this, but now that Russell has mentioned it, a long time does seem to have passed since the last leap second was inserted. I know little about leap seconds, but it turns out that there is quite a bit of information on the web. I have learned that by international agreement, the decision whether or not to insert a leap second is made by the organization International Earth Rotation Service (IERS). Its web site provides lots of information: http://www.iers.org The relevant information is found among the four standard bulletins issued by the IERS, which can be accessed here: http://www.iers.org/iers/publications/bulletins/ Bulletin C "contains announcements of the leap seconds in UTC". Following the URLs leads me to: http://hpiers.obspm.fr/eop-pc/products/bulletins.html The last Bulletin C, issued on 2002 Jan 14, announces that no leap second will be introduced at the end of June 2002: http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.dat The history of UTC offsets, provided at the following URL, reports that the last leap second insertion was on 1998 Dec 31: http://hpiers.obspm.fr/eop-pc/earthor/utc/UTC-offsets_tab.html Now, to answer Russell's question, I went back to the bulletins page, and found that Bulletin A "contains rapid determinations for earth orientation parameters". This sounded promising, so I followed the URLs to: http://maia.usno.navy.mil/ The URL labelled, "IERS Bulletin A -- Rapid Service/Prediction of Earth Orientation" leads to: ftp://maia.usno.navy.mil/ser7/ser7.dat The table of COMBINED EARTH ORIENTATION PARAMETERS informs that on 2002 Jun 27, UT1-UTC = -.228762 s. Since the purpose of the leap second is to maintain the difference between UT1 and UTC to within 0.9 s, it is now clear that a leap second will not be issued because it is not necessary. Judging by the table of PREDICTIONS on the same page, it appears that UT1-UTC will be -0.37118 s one year from now, so no leap second is likely to be required through at least Jun 2003. How is this relevant to visual satellite observation? By convention, the time of positional observations is reported as UTC. Experienced observers regularly achieve timing accuracy of 0.1 s or better. The analysts who produce updated orbits from these observations find that errors of 1 s tend to stick out like a sore thumb. Depending upon the circumstances, errors that large may result in the observation being assigned a rather low statistical weight or being rejected outright. To an analyst attempting to fit an orbit to a set of observations spanning 30 Jun or 31 Dec, the insertion of a leap second on those dates results in an apparent 1 second error in the UTC positional timings, when comparing observations made before and after the insertion. A practical solution is to subtract 1 s from the observations made prior to the insertion of the leap second. This eliminates the 1 s error, and results in an epoch that is accurate going forward from the date of the leap second insertion. Here is an exercise. The first elset was issued about 19 h before the leap second insertion at the end of 1997 Jun 30; the second one about three after the insertion: 1 00271U 62010A 97181.18205596 +.00000006 +00000-0 +10000-3 0 09014 2 00271 086.6536 054.4864 0329540 060.7273 302.6159 09.41297471210592 1 00271U 62010A 97182.13850598 +.00000006 +00000-0 +10000-3 0 09063 2 00271 086.6543 054.3470 0329535 059.5568 303.7463 09.41297490210680 Compare predictions made using both elsets on 1997 Jun 30 and 1997 Jul 01. Ted Molczan ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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