GPS units impacted by rollover
A U.S. government agency is warning boaters about a “rollover”or resetting of a date in the GPS navigatation system in early April that may affect older types of GPS units.
A joint U.S. military and civilian agency called the National Space-Based Positioning, Navigation and Timing (PNT) Advisory Board announced that one of the parameters used in the GPS location system — the week number — will reset to zero just before midnight on April 6, which may play havoc with some older GPS units used by boaters.
Satellites used in the Global Positioning System (GPS), which was initially developed for the U.S. military and is now used by boaters in chartplotters and hand-held devices, regularly sends out information to assist in position location.
One of the parameters currently used by the GPS navigation system is the so-called “week number”, which has been successively counting since Jan. 5 1980.
It’s scheduled to reach 1024 at 18 seconds before midnight on April 6 2019 and reset, which may throw off older GPS receivers or those without software upgrades.
The “week number” rollover has happened once before, in August 1999, and the time period from then until this next date rollover has been considered the “second epoch” in “GPS” time. The GPS system is moving from a 10-bit to a 13-bit parameter for the week number, which should solve any future rollover problems, say government agencies.
“Older GPS receivers, or receivers that have not been provided manufacturer updates, may be impacted by the rollover. The impact might occur in April, or could affect such equipment at a later date,” says the U.S. PNT advisory board.
“On these receivers, the date might revert back to August 1999 or may revert to another date. Since this issue does not affect the other parts of the GPS navigation message, (it only affects the date), the receiver’s ability to calculate the position and to display the exact time of day should not be impacted.”
“If you are operating a relatively recently-made piece of GPS equipment, it has likely been designed to handle this rollover event. If you regularly update your equipment’s software/firmware with manufacturer updates, it has likely been prepared to handle it.”
Those GPS units built to Global Positioning Systems Directorate Systems Engineering and Integration Interface Specification, IS-GPS-200, are not expected to be affected.
The U.S. Department of Homeland Security has indicated that the rollover will affect GPS units that use Coordinated Universal Time (UTC) and which have not been updated. Tests have indicated that not all GPS receivers will properly handle the date rollover.
The GPS navigation time scale, or “GPS Time”, “is based on the weighted average of GPS satellites and ground station clocks.
“A nanosecond error in GPS Time can equate to one foot of position (ranging) error,” says a memorandum from Homeland Security.
The U.S. agencies advise boaters to check their GPS units, and to contact the manufacturer if unsure on whether the software has been updated.
Some GPS units were affected by a date rollover early in 2000, and would not function properly. For further information, visit www.gps.gov.