To: C.K. Houston who wrote (20 ) 4/14/1998 10:55:00 AM From: C.K. Houston Read Replies (3) | Respond to
"It's a known, and publicly acknowledged FACT that GPS will fail August 21,1999."C.K. HOUSTON BILL WEXLER THE GLOBAL POSITIONING SYSTEM Before January 1, 2000 comes August 21, 1999. The US Naval Observatory warns that some Global Position System (GPS) "receivers may display inaccurate date information [and] some may also calculate incorrect navigation solutions" after midnight August 21, 1999. The world's aircraft control software must be modified to expect and compensate for this Year 1999 Problem. Shipping and trucking software may also need to be fixed.tycho.usno.navy.mil According to Mitre Corporation, "Global Position System is a satellite positioning system developed by the Department of Defense that provides precise position, velocity, and time information to users. There are 24 satellites in six orbital planes with four satellites per plane. They orbit the earth every 12 hours at a height of approximately 20,000 km, and transmit ranging signals modulated with satellite identification and location information. A user's receiver determines its position by determining the pseudorange to four or more satellites. A pseudorange is the range to a satellite plus the user's clock offset from GPS time. The pseudoranges are used to determine the four unknowns of the user's 3-D position and clock offset [Center for Advanced Aviation System Development]. So what's the problem? The GPS Week Number count began at midnight on January 5, 1980. Since that time, the count has been incremented by one each week, and broadcast as part of the GPS message. The GPS Week Number field is modulo 1,024. This means that at the completion of week 1,023, the GPS week number will rollover to 0 on midnight of the evening of August 21, 1999. The US Navy warns, "Once the rollover has occurred, it is the responsibility of the user (i.e., user of equipment or software) to account for the previous 1,024 weeks. >Depending upon the manufacturer of your GPS receiver, you may or may not be affected by the GPS Week Number Rollover on August 22, 1999. Contact the manufacturer of your GPS receiver to determine if you will be affected by the GPS week number rollover." yardeni.com GPS ... EXCERPT FROM NAVAL DISCUSSION GROUP. The firmware in all older units will have to be replaced. This would involve replacement of PROMs; some are socketed, some are soldered. New units presumably will know better than to claim dates from before they were manufactured, and/or will allow the user to directly or indirectly tell the firmware which 1024-week cycle to assume, without requiring replacement of that firmware at the second rollover, in 1980+(2*1024/52)= 2019 AD. Some of this equipment will still be in use then, long after the manufacturer has forgotten the product.However, in spite of everything, not everybody will get the message , so system software will forever have to have an independent idea of what year it is, to know when to disbelieve a receiver or receivers (they could all be wrong), and to handle arguments between various GPS receivers (if only some are wrong).Without a GPS Simulator, there is no way for users to test a GPS receiver for this problem. All most users can do is to ask their manufacturer for a solution, and also to imbue the system software with a suitable degree of skepticism about GPS receivers' sense of time. As a technical matter, the solution is quite simple. It's the logistics that will take some years. stl.nps.navy.mil
