[LEAPSECS] aircraft GPS receivers hit by leap second bug

Tom Van Baak tvb at LeapSecond.com
Wed Jun 12 08:49:50 EDT 2019

 > 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.

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.

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

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.


[1] http://www.leapsecond.com/notes/leapsec256.htm

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