GPS will fail

From: Steve Allen <>
Date: Wed, 24 Dec 2003 13:33:37 -0800

The current downlink data format for the GPS satellites stores the
difference between GPS system time and UTC using 8-bits. This means
that the maximum difference ( GPS - UTC ) that can be stored is 255
seconds. If leap seconds continue to be inserted into UTC, then the
current fleet of GPS satellites and all receivers of GPS signals will
eventually become obsolete.

When will the current GPS data format fail? It depends on the
deceleration of earth rotation. The approximate year of failure is
2114, 2149, or 2174 for decelerations of 42, 31, or 25.6 s/cy2,
respectively. This means that a new series of GPS satellites and a
new set of receivers must be designed and deployed sometime within the
next century.

Furthermore, navigational satellite systems such as GPS and Galileo
place constraints on what can and cannot be used for civil time.
Suppose that human society deems that atomic time should be used for
system operations, but that some form of mean solar time other than
UTC with leap seconds is desirable for civil purposes. This new form
of mean solar time might have leap-milliseconds and be something like
UTC was before 1972. It is not feasible to consider use of a time
scale like this unless the satellite systems can broadcast it.
Therefore it is imperative as soon as possible to decide what form of
time is needed for civil purposes.

I have not kept up on the progress of the Galileo signal format
of the next generation of GPS satellites. I don't know if they
are considering any such issues.

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 Wed Dec 24 2003 - 13:33:58 PST

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