[time-nuts] June 30 2015 leap second

Tom Van Baak tvb at LeapSecond.com
Sun Jan 11 09:02:11 EST 2015

>> I haven't looked at the raw data, but using the windows apps: 
>> a Trimble Resolution SMT is showing the leap pending a Motorola UT+ is not.
>> lady heather is not 
> Interesting.  I don't see any leap-pending from a Z3801A.  (or a KS-24361)

Hi Hal,

It's not that simple. There are many different levels of leap second notification.

1) IERS updates their ftp site (for bots) and sends email (for humans) indicating when the next leap second will occur. Days or weeks (and sometimes months) later,

2) GPS ground control uploads almanac information to the satellites with updated UTC parameters. Four values in page 18 of subframe 4 give the current UTC leap second delta and also an optional future UTC delta, with a GPS week (LS_f) and day number (DN) when that future delta will be current. By definition the leap second will occur at the end of a UTC (not GPS) day.

In this way GPS can provide up to 256 weeks (~4.9 years) of advanced leap second information and support positive or negative leap seconds. Note there is no "leap second warning" *bit* in the GPS spec, per se.

3) Starting up to 12.5 minutes later, GPS receivers will see page 18 of subframe 4 from some or all SV and thus know not only the current UTC offset, bit when/if a different UTC offset will be valid in the distant future. Prior to this (e.g., cold start) there is no certainty of either the UTC offset or leap second state.

4a) Since UTC and leap seconds are not needed for navigation, some GPS receivers do not bother to tell the user about leap seconds at all.

4b) Some GPS receivers only give the user a leap second warning and so they must wait until the month in which the leap second is to be applied before they issue the warning. That means they may sit on the internal leap second information for many months.

4c) Other GPS receivers give the future date of the next leap second (if any). This is not a warning bit, but just the date/time of the next leap second.

4d) Especially dangerous are any GPS receivers that report only a leap second warning bit, but don't tell you which month it will occur.

5) Host software (e.g., GPSDO firmware, operating system, drivers, or apps) take this information and must only operate on it at the end of the appropriate UTC month. Hardcoded rules for June and December are frowned upon.

It would be nice if we pooled together our resources and made a list of which GPS/GNSS receivers are 4a, 4b, 4c, or 4d.


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