From time to time, we hear that the GPS GROUND CONTROL SEGMENT or the GPS SATELLITES are going to malfunction either on the rollover of the WEEK counter or due to the calendar rollover December 31, 1999. What is the scoop? One of our GPS experts has provided the following information to us relative to the question. Here are his remarks: ==================================================================== Joe - Since it appears that the humor of the last missile (or is it missive?) went way over the heads of several of the newsgroup readers, let's try some plainer English -- The Space Segment (= space vehicles = SVs = satellites = birds) is, always has been, always will be Y2k compliant. In fact, Y2k is irrelevant to the Space Segment. Y2k is also irrelevant to the navigation message transmitted from the SVs. The week count (10 bits, value 0 through 1023) will roll over and restart in the navigation message in August 1999. Week count is just a counter generated by the on-board clock on each vehicle, with synchronization done several times a day by the Ground Segment. There is no week count cycle flag, indicator, or counter in the navigation message, never has been, never will be unless there is a major revision of the System Specification, which means a whole new system. Tracking week count cycles is and always has been assigned to the User Segment (your GPS receiver), not the Space Segment. The Ground Segment (also called the Operational Control Segment) does the clock synchronization and generates the other information for the navigation message. The upgrade is running ahead of schedule and looks like it will be complete by the end of this year (1998), so the OCS will also be completely compliant. The sky will not fall in August 1999 nor at midnight Dec 31, 1999, nor will the satellites. Your pocket (or wrist) GPS receiver will not go up in flames at either of those times (well, at least mine won't, I don't know about yours, but just to be safe I would stay well away from MB's units, judging by his posts). Further expansion -- A. The SVs (= space vehicle = satellite = bird) are, always have been, and always will be Y2k compliant. Hey, folks, that's like saying the Sun or Moon are Y2k compliant. The Sun, Moon, Earth, other planets, stars, and yes, folks, even man-made satellites take no notice of human-imposed calendars. They just keep rotating on their axes and revolving in their orbits. B. The navigation message is Y2k compliant as transmitted from the satellites, always has been, always will be. The GPS time system has no connection to human year counts in any system, so actually the navigation message has nothing whatsoever to do with Y2k, Gregorian calendars, Julian calendars, Jewish calendars, Moslem calendars, Chinese calendars. Your consumer pocket GPS receiver (part of the User Segment) was assigned the responsibility of translating the display of time and location into terms you the human want (and presumably understand) via its User Interface (that means the display screen, in case you were wondering). C. The week count number will continue stepping along with no hitches and never was a problem. That, of course, is the same as saying that the minute hand on an analog watch keeps going around and around. There are 1024 weeks in one cycle (0 through 1023, 10 bits), just as there are 60 minutes in an hour, 60 seconds in a minute, 24 hours in a day (12 each AM and PM), 7 days in a week. It is, and has been since the original System Specification was written, the User Segment's responsibility to keep track of which week count cycle you are in. That's like saying, "Look at Mickey's other hand." Okay, so most analog watches have an hour hand as well as a minute hand, but that means you, the wearer of the watch are responsible for keeping track of AM and PM, which day of the week and month, which month, which year. All present consumer GPS receivers, according to statements by their manufacturers, keep track of the week count cycle. It is trivial to do so by several different methods. D. The orbit calculations done in your receiver (not on the satellite, since this again was assigned to the User Segment from the original System Spec) depend only on taking the differences (that's simple subtraction, folks) between time count number pairs contained in the navigation message. Y2k and other artificial human-imposed calendar systems are completely irrelevant to those orbit calculations. Some older GPS receivers which do not keep track of the week count cycle would have a problem for a short time (less than a day) at the week count rollover, but all current consumer GPS receivers, according to their manufacturers, track the week count cycle and will have no problem. Now, I recognize that there are several regular posters to the newsgroup (notably MB) who believe and frequently post that the manufacturers are lying about their compliance. However, most of us have every reason to believe that the manufacturers are acting in good faith on this point. E. The Ground Segment originally used mainframe computers and, like much of the business world, had to compress coding to fit the limitations of the machine. The typical under-$1000 desktop computer of today has more compute power and more memory than those 1980-era mainframes. There were a number of places where 2-digit years were used in the user interface sections, and potentially introduce Y2k problems. These were recognized years ago and workaround patches were introduced where needed. Several years ago 2SOPS (that's Second Space Operations unit of the Air Force, the folks running the GPS program) started a program to upgrade and modernize the Ground Segment, also called GPSOCS (for GPS Operational Control Segment). Both hardware and software were to be replaced, the software being re-written by current software engineering methods. Published progress reports state that the upgrade is ahead of schedule. Again, I recognize that certain posters on the newsgroup believe and frequently post that the Air Force, along with GPS manufacturers, is lying to the public. These posters also believe, based on their posts, that no software or hardware devised by humans could possibly work and inevitably will fail catastrophically at midnight Dec 31, 1999 (excuse me, which time zone, or is it a rolling collapse of the sky, time zone by time zone, with Newfoundland being on the half-hour?) 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