What are GPS Leap Seconds?
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The global time standard is called Coordinated Universal Time (UTC), all national time standards around the world are based on this UTC time with the appropriate offset for their time zone. To keep UTC time synchronised to the Earth’s rotation, additional seconds can be added or removed to compensate for slight variations in the rotation rate of the Earth, as well as other factors. When required, leap seconds are applied either on December 31st or June 30th. The Global Positioning System (GPS) system uses GPS Time, which does not include these leap seconds. GPS time started on the 1st January, 1980, and was accurate to UTC at this point. Since then, there have been 18 leap seconds introduced, such that there is now an 18 second difference between GPS time and UTC time. Instead of reprogramming each satellite’s internal clock with every leap second added, it was decided that an easier method would be to include the current number of leap seconds in the downloaded navigational data. GPS engines would then be able to apply this offset to allow an accurate UTC time to be determined. |
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Wikipedia: ‘Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service. When needed to ensure that the difference between the UTC and UT1 (GMT) readings will never exceed 0.9 second.’
On December 31st 2016, the offset was increased to 18 seconds:
Before the 2016 leap second: GPS-UTC was 17 (GPS ahead of UTC by 17 seconds)
After the 2016 leap second: GPS-UTC became 18 (GPS ahead of UTC by 18 seconds)
The sequence of dates of the UTC second markers for the last offset increase are as follows:
2016 December 31, 23h 59m 59s
2016 December 31, 23h 59m 60s
2017 January 1, 00h 00m 00s
GPS Leap Second Date Table
| GPS Leap Second Dates | Leap Seconds |
|---|---|
| 01/01/1980 - 01/07/1981 | 0 |
| 01/07/1981 - 01/07/1982 | 1 |
| 01/07/1982 - 01/07/1983 | 2 |
| 01/07/1983 - 01/07/1985 | 3 |
| 01/07/1985 - 01/01/1988 | 4 |
| 01/01/1988 - 01/01/1990 | 5 |
| 01/01/1990 - 01/01/1991 | 6 |
| 01/01/1991 - 01/07/1992 | 7 |
| 01/07/1992 - 01/07/1993 | 8 |
| 01/07/1993 - 01/07/1994 | 9 |
| 01/07/1994 - 01/01/1996 | 10 |
| 01/01/1996 - 01/07/1997 | 11 |
| 01/07/1997 - 01/01/1999 | 12 |
| 01/01/1999 - 01/01/2006 | 13 |
| 01/01/2006 - 01/01/2009 | 14 |
| 01/01/2009 - 01/07/2012 | 15 |
| 01/07/2012 - 01/07/2015 | 16 |
| 01/07/2015 - 31/12/2016 | 17 |
| 31/12/2016 - xx/xx/xxxx | 18 |