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__