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
1DDD DDDD, 1DDD DDDD, 0DDD DDDD
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.
Markus
[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
http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__
Received on Sun Feb 15 2004 - 04:10:36 PST