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ROLLOVERS IN TIME
In August of 1999, the GPS week number will rollover from 1023 to 0 due to the
limited length of the GPS week field in the navigation data stream. Motorola
Oncore receivers (GT, UT, VP, XT, and Basic) have been designed and tested to
properly distinguish the correct 20 year window (1024 weeks is just shy of 20
years). They will not need reprogramming or replacement come 1999. In fact,
the transition will be completely transparent to users of Motorola Oncore GPS
receivers. Motorola Oncore GPS receivers are also year 2000 compliant.
Multichannel GPS satellite simulators have been used to test each rollover
condition in GPS. For example, each week there is a rollover in the GPS seconds
on midnight Saturday. The August 1999, year 2000, leap day, and leap second
rollovers have all been successfully tested using simulators.
In order to handle the 1999 rollover, Motorola GT and UT Oncore GPS receivers
use a date stored in flash memory. For example, if the firmware date is 1996, a
defaulted receiver that does not have back-up power (or battery) will start 
with an
internal time of 12:00 on 1/1/96 and begin acquiring satellites. Once the first
satellite is acquired, the time and week number will be downloaded from the
navigation data message. The receiver determines the current date by starting
from the week number of 1/1/96 (week 834) and searching for the first
occurrence of the current week number (week 860 for 7/1/96).

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