[LEAPSECS] aircraft GPS receivers hit by leap second bug

[Martin Burnicki]

Paul Hirose wrote:
> Thinking I had missed a pending leap second, I checked the IERS site,
> but Bulletin C says the offset is still 37 seconds and nothing is
> scheduled. ???

The GPS satellites transmit the week number of the nearest leap second
(WNlsf) as 8 bit value only, giving a valid range of ~+/- 128 weeks from
"now".

The last leap second was on 2016-12-31, and counting the number of weeks
in 2017, 2018, and 2019 until now, this sounds like close to 128. I
haven't computed the exact number, but I'm assuming that the range was
just recently exceeded. I'm currently out of the office, but I've heard
from my colleagues that the currently sent WNlsf number is 0x89.

However, the current GPS/UTC offset numbers before and after the nearest
leap seconds are still 18/18, so there is no current leap second
announcement, and WNlsf may be ambiguous. When the next announcement
starts the offsets will change to 18/19, and WNlsf will also be updated
to be in range.

The "mbgstatus -vvv" program for the Meinberg GPS PCI cards now shows a
wrong leap second date for 2021, too:

UTC correction parameters:
  t0t: 2057|405504.0000000, A0: -9.31323e-10 A1: -2.66454e-15
  WNlsf: 2185, DN: 7, offs: 18/18
Last leap second eventually at UTC midnight at the end of Sat, 2021-11-27

while just recently the displayed leap second date was (correctly)
2016-12-31.

However, Meinberg GPS receivers evaluate WNlsf *only* if the 2 offset
values differ, i.e. there is really an announcement of a leap second,
and WNlsf is in a valid range, so these receivers don't have a problem
with a wrong leap second.

If no leap second is announced then extended week number and thus the
date *can* be wrong, but it's just informational, and the word
"eventually" is used to point out the ambiguity.

Martin