Re: GPS will fail EVEN SOONER

From: Steve Allen <>
Date: Sat, 27 Dec 2003 09:51:44 -0800

On Wed 2003-12-24T13:33:37 -0800, Steve Allen hath writ:
> The current downlink data format for the GPS satellites stores the
> difference between GPS system time and UTC using 8-bits.

oops. I've just re-read the GPS Interface Control Document.
Those 8-bits are a signed quantity. It can count to 127.

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.

Is this the smoking gun for starting the review of leap seconds?

The W1K rollover for GPS was in 1999, and all that year was spent
testing various systems to see how they would fail. It would not be
at all surprising if the impending doom of the leap second counter was
noticed during a review of other deficiencies in the GPS system.

The review process for the new GPS signal formats is most easily
visible at

The most recent document still indicates that the delta between
GPS and UTC is stored in a signed 8-bit quantity.

Is it reasonable that the demise of civil Mean Solar Time should be
caused by lack of foresight by the GPS system designers?

Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064      Voice: +1 831 459 3046
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
Received on Sat Dec 27 2003 - 09:52:03 PST

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