[LEAPSECS] 256-week / leap seconds / in the news

[Tom Van Baak]

Steve,

I'm thinking the problem is not with GPS or with modulo arithmetic. I'm 
in contact with Leo and others for the root cause or the scope of the 
problem as reported on twitter. I'll say more later.

Per the GPS ICD, when dt_LS == dt_LSF there is no leap second to speak 
of; not in recent past; not in near future. The 8-bit WN and DN fields 
are not applicable to a non event.

When dt_LS != dt_LSF you know there is a nearby leap second event. You 
then look at WN and DN to determine when. It could have been in the 
recent past, it could be in the near future, or even in progress. By 
"recent past" and "near future" I mean ±127 weeks (about ±2½ years).

Clearly this design means GPS cannot give indefinite past history of a 
previous leap second(s), nor indefinite future notice of a pending leap 
second(s). This is not a problem given how UTC is currently defined and 
managed. BIPM has a good track record of 6 months (~26 weeks) of notice. 
The official UTC spec is 1 month (~4 weeks) of notice so a 127 week GPS 
limit is more than adequate.

/tvb


On 7/25/2021 8:54 AM, Steve Allen wrote:
> On Sat 2021-07-24T18:50:50-0700 Tom Van Baak hath writ:
>> In the news:
>>
>> "GPS will broadcast a 0 second leap second in 128 days"
>> https://news.ycombinator.com/item?id=27944776
> A tweet from Leo Bodnar electronics about their NTP device
> https://twitter.com/LeoBodnar/status/1419239590532206597
> which famously includes highlighted text from the section of the GPS
> ICD that describes the need to do modulo arithmetic along with the URL
> to the new version of the firmware.
>
> --
> Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
> UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
> 1156 High Street               Voice: +1 831 459 3046         Lng -122.06015
> Santa Cruz, CA 95064           https://www.ucolick.org/~sla/  Hgt +250 m
> _______________________________________________
> LEAPSECS mailing list
> LEAPSECS at leapsecond.com
> https://pairlist6.pair.net/mailman/listinfo/leapsecs
>