A leap second is a second that is occasionally inserted (when) into UTC time in order to bring it into line with solar time (because the earth's rotation rate is slightly slowing). Precession of the earth's axis also effects solar time and consequently leap second events are somewhat irregular in nature. Below we can see that sometimes only a year passes between leap second insertion events and other times as much as 7 years have passed:
GPS time - UTC time
(Leap Second Count)
Jul 1982
Jan 2009
Accurate timing is fundamental in all GNSS's, when these leap second events occur it is essential that a receiver's algorithms seamlessly process the event so no loss of the PVT solution is seen. It should be noted that GPS, Galileo and Beidou time systems are continuous, in other words these GNSS's do not insert a leap second into their system time. Therefore these system times diverge from UTC on a leap second event (they get 1 second in front) whereas GLONASS system time follows UTC so there is no divergence in GLONASS from UTC. GPS Navigation data specifies the exact date that the event will happen on and also the current leap second value and future leap second value. Because it is totally unambiguous it could even insert 2 (or more!) leap seconds at a time whenever you wanted (It must be at the end of a day). An event in GPS needs several navigation data "words" to capture the information. GLONASS Navigation data however is ambiguous. This is because in practice leap seconds are only inserted (a maximum of 1 at a time) on the end of the day on either the 30 June or 31 December. An event in GLONASS only requires 2 bits of navigation data: The definition is as follows for the leap second count (LSC): 00 No LSC at ECQ 01 LSC +1 at ECQ 10 Maybe LSC at ECQ 11 LSC -1 at ECQ where ECQ = End of current quarter.
For information on how to simulate a Leap Second Event(LSE) in PosAPP see DAN009 Simulating UTC Leap Second Insertion Events - latest issue. To determine if a leap second is scheduled to happen at the next end of half year period this site can be checked: https://datacenter.iers.org/data/latestVersion/16_BULLETIN_C16.txt It is updated within a couple of weeks of the start of the new half year declaring or not if a leap second event will occur at the end of the current half year period.