Re: [LEAPSECS] GPS will fail EVEN SOONER, not

From: Tom Van Baak <>
Date: Fri, 2 Jan 2004 01:37:57 -0800

> The current GPS data format will fail in approximately 2057, 2079, or
> 2095 for decelerations of 42, 31, or 25.6 s/cy2, respectively.
> In terms of deployed systems, that's Real Soon Now.

Not to worry. It won't fail. The "solution" is simply
to let delta t sub LS in page 18 subframe 4 roll over
every 128 leap seconds.

This is the same approach as was used to "solve"
the GPS week number rollover problem.

Some people would argue WNRO was neither a
solution nor a problem. I think most GPS receiver
manufacturers got the week number roll over right
in the first place; those that got caught just issued
an upgrade in most cases. True, the GPS ICD was
updated, but only to explicitly remind "users" (i.e.,
GPS receiver firmware writers) to take WN roll over
into account. The signal format did not change.

So if that was good news, here's the better news:

Recall with GPS WNRO there were several years of
public notice and the event occurs every 19.6 years.

At a leap second rate of one per 500 days we have
some 150 years of notice and a LSRO (leap second
roll over) event every 180 years. Even with a leap
second rate of two per year, we still have 57 years
of advanced notice and a LSRO every 64 years.
And (just for fun) at today's weird rate of one leap
second every 5 years we have 500 years of notice
and a LSRO every 640 years.

One suggestion, though. Sometime in the next few
decades ICD-GPS-200 should be updated with a
comment such as "delta t sub LS is modulo 128
and may roll over and users must account for this".
The signal format does not need to change.

Received on Fri Jan 02 2004 - 01:38:30 PST

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