The Marine Department has urged shipmasters, coxswains and person-in-charge of vessels fitted with Global Positioning System (GPS) receivers to pay attention to the impact of the End-of-Week (EOW) rollover problem that will occur on Sunday week (August 22).

"They are advised to seek confirmation before that date from the suppliers of their GPS receivers that the equipment has been tested and will handle the rollover correctly," a spokesman for the department said today (Thursday).

Shipboard GPS receiver has a built-in number of weeks counter to match with the week number count encrypted in the GPS satellite broadcast data message.

Based on the week number count, a GPS receiver is able to determine which satellites are above the horizon and hence available for tracking.

The week number count will rollover from 1,023 weeks to zero week again at 8 am local time (or 0000 hour Coordinated Universal Time) on August 22 (Sunday).

"The GPS EOW rollover problem could manifest itself in the operation of GPS receivers in many unpredictable ways," the spokesman said.

"Some receivers may not be able to correctly recognise the date and may generate incorrect time, position or velocity information," he said, adding that the accuracy of navigation might be severely affected.

But he stressed that the rollover problem would be dependent upon how individual GPS receiver manufacturer has designed the receiver firmware.

The spokesman advised navigators that notwithstanding assurance from the manufacturers, they should use the information from the GPS receivers on that date with caution.

"The information should be regularly cross-checked by other position fixing methods," he added.

Thursday 12 August, 1999