Re: GPS versus Galileo
More news of the recent GPS/Galileo summit: http://europa.eu.int/rapid/start/cgi/guestfr.ksh?p_action.gettxt=gt&doc=IP/04/264|0|RAPID&lg=EN&display= "The main issues covered by this round of negotiations are: * Adoption of a common baseline signal structure for their respective open services (the future GPS intends to use a BOC 1,1 signal whereas the Galileo open service intends to use a fully compatible optimized version of the same signal which guarantees a high-level of performance). * Confirmation of a suitable baseline signal structure for the Galileo Public Regulated Service (PRS) * A process allowing optimization, either jointly or individually, of the baseline signal structures in order to further improve performances * Confirmation of interoperable time and geodesy standards to facilitate the joint use of GPS and Galileo * Non-discrimination in trade in satellite navigation goods and services * Commitment to preserve national security capabilities * Agreement not to restrict use of or access to respective open services by end-users * Agreement to jointly finalize associated documents after which the agreement will be presented for signature" Does anyone here know where the actual technical details of that agreement may be available to the interested taxpayer, to help one untangle this diplomatic language? Markus -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__
Re: GPS versus Galileo
On Sun 2004-02-15T12:10:17 +, Markus Kuhn hath writ: > The GPS signal format has been virtually unchanged since prototype > experiments in the early 1970s I'm just now reviewing the very recent GPS L2 ICD https://gps.losangeles.af.mil/Gpsarchives/1000-public/1200-cm/documents/review/icdgps200/pirn200c008/PIRN-200C-008,%2027Jan2004.pdf An insignificant difference is the blue-pencilling of all occurrences of "Universal Coordinated Time" and replacement with "Coordinated Universal Time. A significant difference is that there is now a 16-bit field intended to give the calendar year. This will certainly alleviate any need to worry about violating the week rollover patent that Tom Van Baak mentioned a while back. But I still find no indication that the leap second counters have a different width. That would leave the GPS receiver implementor with a need to guess the value of Delta-T to within 256 seconds. Fortunately that will be a relatively robust guess for many, many centuries. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E49 89 0E FE 26 B4 14 93
Re: GPS versus Galileo
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 , 1DDD , 0DDD to encode the signed 21-bit value D (-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__
Re: GPS versus Galileo
On Thu 2004-02-05T12:56:03 +, Markus Kuhn hath writ: > Robustness against U.S. "navigation warfare" was one of the main funding > rationales for Galileo. And the fact that GPS will fail to be able to report UTC in about 70 years seems intimately entangled with the desire to discontinue leap seconds. The earliest evident suggestion that leap seconds should be discontinued was in a 1999-03 talk by Klepczynski at CGSIC: http://www.navcen.uscg.gov/cgsic/meetings/summaryrpts/33rdmeeting/Presentations/klepczyn.ppt The September meetings of CGSIC have, of late, been coincident with the ION meetings. ION is intimately connected with the GPS establishment, and Klepcynski is notable in that organization: http://www.ion.org/awards/fellowship_programs.cfm This mentions that Klepczynski is currently placed with the US Department of State. I had not realized the relevance of this until Markus Kuhn posted about GPS vis a vis Galileo, but now it seems likely to be related to issues evident here: http://www.state.gov/g/oes/sat/ http://www.state.gov/r/pa/prs/ps/2004/28005.htm http://www.state.gov/r/pa/prs/ps/2004/28006.htm The EU view of the results of last month's meeting in Washington are delicately discussed in http://europa.eu.int/rapid/start/cgi/guestfr.ksh?p_action.gettxt=gt&doc=IP/04/173|0|RAPID&lg=EN&display= with the note that more meetings will happen within the next 2 weeks. > Is seems the Temporal Cold War has begun ... And mean solar time may be its first casualty. 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. -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 [EMAIL PROTECTED] Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E49 89 0E FE 26 B4 14 93
Re: GPS versus Galileo
Steve Allen wrote on 2003-12-23 19:46 UTC: > Of course no agreement can stop some entity from flying a jamming > rig for both systems over particular theatres of interest. Robustness against U.S. "navigation warfare" was one of the main funding rationales for Galileo. The U.S. are unlikely to jam easily their own military (M-code) navigation signal. As I understood it, the original plan for Galileo was to put its own Public Regulated Signal (PRS) into the spectrum in a way such that the U.S. cannot jam it without jamming their own M-code as well. This improves robustness against adverse US DoD capabilities and also simplifies tremendously the design of receivers that can listen to both GPS and Galileo (which I expect will be all new receivers as soon as Galileo is up and running). Status of Galileo Frequency and Signal Design: http://europa.eu.int/comm/dgs/energy_transport/galileo/doc/galileo_stf_ion2002.pdf http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=61244 Status of new GPS M-code design: http://www.mitre.org/work/tech_papers/tech_papers_00/ betz_overview/betz_overview.pdf DoD versus EU battle: http://www.globalsecurity.org/org/news/2002/020514-gps.htm >From a recent local press review: - EU and US fail to agree on interoperability of satellite navigation systems Discussions between the European Union and the US in Washington concerning the interoperability of the EU's proposed Galileo satellite navigation system and America's existing GPS service have ended without agreement, according to reports in the New Scientist. The sticking point is said to be the standard signal that the EU would like to use for Galileo. Europe's preferred option, known as binary offset carrier (BOC) 1.5, 1.5, would give users of Galileo the most accurate information possible, but the US argues that this would interfere with the GPS system's proposed new encrypted military signal. The US intends to introduce the new signal, known as the M-code, in 2012. During a military conflict, the US would attempt to jam all civilian satellite systems so as not to allow enemies to use satellite navigation. But jamming Galileo's BOC 1.5, 1.5 signal, argue US officials, would also disrupt its own M-code. The US proposes that Galileo uses an alternative signal, such as BOC 1.1, which does not overlap the M-code signal, but the EU is concerned that this will result in a less accurate system for commercial users of Galileo. Officials from the EU and the US will meet later in February to try to resolve the issue. For further information on Galileo, please consult the following web address: http://europa.eu.int/comm/dgs/energy_transport/galileo/index_en.htm - The use of the word "interoperability" for the feature that the operator of one system can jam the other one without affecting its own has a neat Orwellian ring to it. >From what I hear behind the scenes, plans for Galileo are now to make the transmitter and receiver designs highly flexible, such that code rates, spreading sequences, BOC offsets, and perhaps even carrier center frequencies can be reprogrammed smoothly on-the-fly while the system is in operation, to be able to adapt to adverse actions and the current political climate. Apart from moving the center frequency around significantly (which clearly affects the design of the RF hardware very much on each end), most of the remaining DSP and PLL parameters can today quite easily be made reconfigurable in software at little extra cost. We may consider our deliberations on leap second rather abstract and academic here, but outside the ivory tower, the reliable distribution of nanosecond-accuracy timing signals has meanwhile become not only a military concern, but also the topic of a serious turf fight between the Pentagon and the EU Commission. Is seems the Temporal Cold War has begun ... Markus -- Markus Kuhn, Computer Lab, Univ of Cambridge, GB http://www.cl.cam.ac.uk/~mgk25/ | __oo_O..O_oo__
