[LEAPSECS] aircraft GPS receivers hit by leap second bug

[Martin Burnicki]

Tom Van Baak wrote:
>> 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.
> 
> Perhaps call it "immaterial" rather than "ambiguous"? The fact that it's
> 18/18 means no positive or negative leap second is pending. Period.

Yes, the WNlsf number is immaterial for timing computations as long as
the 2 offset values don't differ.

What I meant is that if you try to derive the date of the last recent
leap second from WNlsf if the 2 offsets *are* the same, the result is
ambiguous since you don't know if you are in a +/- 128 weeks interval,
or if another 256 weeks interval has passed. That's exactly what we are
observing right now.

> In other words, the value of WNlsf doesn't matter in this case. It's an
> 8-bit value so obviously it must always be something between 0x00 and
> 0xFF, or -128 to +127. Maybe it's a recent old value, maybe it's zero,
> maybe a future new value, or maybe random. It doesn't matter. What
> matters first are the tLS and tFLS values, which are currently 18/18 --
> which means no leap second. Period.

Yes, and as I wrote in my previous email the Meinberg GPS firmware
doesn't evaluate WNlsf for timing in this case.

Only some of our utility programs try to find out the date of the last
recent leap second, and display it just for informational reasons.

As mentioned before we used the word "eventually" ("probably" in the
next software release) to say that due to the ambiguity this is not
necessarily a distinct date.

> A similar issue arose in some GPS receivers during the 7 year "leap
> second drought" of 1998 to 2005. [1]

Yes.

> Here's the math:
> 
>   * GPS start date: 1980-01-06 (MJD 44244)
>   * Most recent leap second: end of the day 2016-12-31 (MJD 57753)
>   * Most recent leap second: prior to the day 2017-01-01 (MJD 57754)
>   * Date of first Collins / GPS / ADS-B anomaly: 2019-06-09 (MJD 58643)
>   * Date that Collins says their systems will start working again:
>     2019-06-16 (MJD 58650)
> 
> Note that (58650 - 57754) / 7 = 128.000
> 
> So it's a big Rockwell-Collins oops. Both the GPS receiver firmware
> engineer who wrote the embedded code and the tester using a fancy GPS
> simulator dropped the ball.

Exactly.

Martin