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Ahead of the impending GPS number rollover, Homeland Security issues memo for users

Image Courtesy: Septentrio

On April 6, 2019, GPS will be rolled over to zero. Mark the date if you extensively use GPS! This could lead to glitches and system malfunction if users and operators are not well prepared in advance and program their receivers efficiently.

In order to forewarn users and enable them to take required measures, National Cybersecurity & Communications Integration Center and National Coordinating Center for Communications of the Department for Homeland Security have released a memorandum for obtaining UTC time on GPS once the Week Number rollover happens on April 6.

Need for GPS rollover

Individual users, operators and organizations and anyone who gets Coordinated Universal Time (UTC) from GPS will have to be aware of the rollover and plan accordingly.

GPS transmits the precise date and time to a receiver by feeding the receiver with the current week and the current number of seconds into the week. Using this, the receiver is able to transmit date and time into the usual day, month, year, time format.

However, the field with the week number is a 10-bit binary, which limits the range of week number from 0 to 1024.

GPS Zero week started on January 6, 1980 when the GPS was first launched. The counter ran out and had to be reset from zero on August 21, 1999.

The WN (week number) parameter in the GPS navigation message “rolls over” to zero every 1024 weeks starting from 0000Z January 6, 1980. The next WN rollover will occur April 6, 2019.

The WN parameter is provided via a ten (10) bit parameter – or “counter.” The valid range of values for the WN parameter is 0 to 1023 (or 1024 total values). The WN parameter is incremented by one each week. At the end of the 1024th week, the counter experiences a rollover (resets) to 0. Each WN rollover event defines a new GPS Time Epoch.

What users and operators should do?

The memorandum has squarely asked the users to understand their dependence on GPS for UTC and to contact the manufacturers of the GPS device for UTC. The users have also been advised to understand the preparedness of the manufacturers for the April 6, 2019 rollout and to ensure that the firmware is upgraded so that after the rollout the smooth functioning is not hampered.

GPS devices confirming to IS-GPS-200 should not be affected by the rollover. Though, a number of tests have revealed that there are other implementations by manufacturers that won’t handle the impending rollover in the right manner. Also, there are certain manufacturer implementations that interpret WN to a date that is different than January 5, 1980. It is expected that rollover will have no effect on these devices but there would be a similar rollover in the future.

For instance, a specific GPS may interpret WN parameter according to the date of firmware creation and hence the rollouts occur 1024 weeks post the firmware creation date.

The GPS Internal Navigation Time Scale “GPS Time” is based on the weighted average of GPS satellites and ground station clocks, as per the memorandum.

Even a minuscule error in GPS leads to large scale consequences. For example, the error of one nanosecond in GPS time can lead to one foot in position error.