Motorola Inc. is advising users of several models of its Oncore Global Positioning Satellite (GPS) receiver units that a software glitch will cause the devices to display the incorrect date for a one-second period towards the end of November.
The problem will occur when, according to Universal Time (UTC), Nov. 27 becomes Nov. 28. While the time output from the receivers will remain correct, the date reported by the units will be incorrect for the first second of the new day. Instead of advancing from Nov. 27 to Nov. 28, the time out put will jump to Nov. 29 before correcting to Nov. 28 from one second past midnight. Positioning functions will not be affected.
Motorola has issued a technical application note to its users and is advising them to check any systems that rely on the GPS date signal, although the company says it thinks the glitch will cause few problems.”We are in the process of analyzing that now,” said Tim McCarthy, director of Motorola’s GPS Product Group. “For the vast majority, 99 per cent of users, it won’t cause any problem at all but we’re digging into the details.”
The reason for the problem is an usually long gap between leap seconds, or extra seconds inserted into UTC to ensure it does not stray too far from the Earth’s rotational time. The software in some models of Motorola GPS units keeps count of the number of elapsed weeks since the last leap second and stores this in an 8-bit long field, which allows for a count of up to 256 weeks. This number will cycle over on Nov. 28 when the 257th week since the last leap second begins.
“It will be the first time 256 weeks have elapsed without a leap second needing to be inserted in Universal Time,” said McCarthy. “No one imagined this would happen and no one thinks it will happen again.”
The problem affects the VP-, UT-, GT- and M12 Oncore units. Motorola wouldn’t disclose the number of affected units shipped. However, in September 2002 the company announced total shipments of all of the company’s GPS units had reached five million units.
The modules are typically used in two broad classes of application, McCarthy said.
The first is in systems that use the GPS system for navigation and location finding purposes and the second is in systems that use the GPS clock as an accurate source of time. The former includes applications such as car navigation systems and Qualcomm Inc.’s OmniTracs system, while the latter includes such apparatus as cellular base stations where customers include Telefonaktiebolaget LM Ericsson and Lucent Technologies Inc.
“I am absolutely sure there will be no problem,” said
