Re: [LEAPSECS] GPS versus Galileo

From: Markus Kuhn <>
Date: Sun, 15 Feb 2004 12:10:17 +0000

Steve Allen wrote on 2004-02-14 21:53 UTC:
> Or maybe Galileo will do its signal format right, and allow at least
> 16 bits in the field that gives the difference between TAI and UTC.
> That would last for at least 2800 years, which is plenty of foresight.
> 24 bits wouldn't hurt, and would last for at least 44000 years, by which
> date mean solar time would need one leap second per day. Presumably
> by that time humanity will have come up with a better idea.

Modern data formats are a bit more sophisticated than that. Designers
today try to avoid fixed-width fields where possible. For example, even
if you use the old ASN.1 BER syntax [1], which has been widely used in
computer communication protocols since the mid 1980s, an integer is
automatically encoded as a variable-length sequence of bytes, and in
each byte, 7 bits contribute to the number while the most-significant
bit signals whether there is another byte following.

So you have the three byte sequence


to encode the signed 21-bit value D DDDD DDDD DDDD DDDD DDDD
(-2^20..2^20-1). (BTW, what ASN.1 BER actually does is to prefix any
integer value with a length indicator that is encoded in the way above.)

The GPS signal format has been virtually unchanged since prototype
experiments in the early 1970s, when microprocessors became just
available [2]. Galileo will have a higher data rate than GPS and the
protocol format designers can comfortably assume that a 32-bit RISC
microcontroller running at >50 MHz clock frequency is the least that any
Galileo receiver will have on offer; the equivalent of an early 1990s
desktop workstation, which you find today in any lowest-cost mobile
phone. The use of variable-length number formats adds hardly any cost
and leaves it at the discretion of the operator to fine-tune later with
what exact precision and range to broadcast data.


[1] ISO/IEC 8825, Information technology -- ASN.1 encoding rules.

[2] B.W. Parkinson and J.J. Spilker Jr.: Global Positioning System:
Theory and Applications -- Volume I, Progress in Astronautics and
Aeronautics, Volume 163, American Institute of Aeronautics and
Astronautics, Washington DC, 1996.

Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__
Received on Sun Feb 15 2004 - 04:10:36 PST

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