GPS, maritime systems and April 6, 2019

Since February, several media outlets—both newspapers and television—have raised concerns about the Week Number Rollover scheduled for April 6, 2019. With alarmist headlines, some even suggest you might need to pull out your paper charts again. So who should we believe? What are the real implications for maritime information systems?

GNSS, GPS… what are we talking about?

On April 10, 2018, the U.S. Department of Homeland Security, and more specifically CERT-ICS, published a memorandum aimed at users of the GNSS (Global Navigation Satellite System), particularly GPS (Global Positioning System).

As a reminder, several GNSS constellations exist. GPS is the most widely known, but there are also others, such as Galileo, expected to reach full operational capability around 2020, and the Russian GLONASS system, among the main systems with global coverage.

A matter of weeks

In its memorandum, CERT-ICS warned users about the possible effects of April 6, 2019 on systems using GPS as a time reference. During the night of April 6–7, 2019, just before midnight UTC, a Week Number (WN) rollover will occur.

The standard GPS signal includes a parameter encoded on 10 bits, representing the week number. Once all bits have reached the value 1, the counter resets to 0, which happens every 1024 weeks. Since the GPS time count started on January 6, 1980, the first WN rollover occurred on August 21, 1999.

Manufacturers enter the picture

In theory, GPS manufacturers are expected to comply with the IS-GPS-200H interface specifications (for those interested, available here), which clearly define the characteristics of the WN parameter and the rollover dates.

The less reassuring news is that implementations differ between manufacturers. If you use an older GPS receiver, there is a possibility that it was not designed to handle a second rollover. Tests performed on certain receivers have shown that some devices handle the rollover poorly, potentially affecting date continuity. In some cases, manufacturers linked the WN parameter to the creation date of their firmware. Others may revert 1024 weeks backward, returning to August 21–22, 1999.

Manufacturers are therefore releasing updates—either automatic or manual—depending on the affected models. Past experiences show that similar issues have occasionally caused operational problems. For example, some Garmin aviation GPS receivers have been affected. A comparable issue also appeared when some devices reached the 1000th week mark, with operational consequences.

In the maritime sector, the main manufacturers include (feel free to suggest others):

• Garmin – no major official announcement on their site; officially everything is fine, although some older maritime receivers such as the 72H do indeed have firmware updates addressing the WN rollover.

• Furuno – probably the manufacturer that has most thoroughly documented the phenomenon and its impact on their receivers. Interestingly, other rollover scenarios exist besides April 2019, including March 17, 2019.

• Raymarine – limited information available; an initial list was published, but it appears rather short.

• Navico, including the brands Lowrance, Simra* (list of affected products), and B&G for leisure and racing vessels.

It is therefore very likely that not all impacts have been publicly documented. Some manufacturers may provide only limited support, particularly for older equipment. Others simply integrate GPS chips from subcontractors such as Trimble (see their notice, which suggests everything is fine—though in reality not entirely) or STMicroelectronics, which has also experienced similar issues in the past.

Potential impacts

Critical infrastructures frequently rely on GPS as a time reference. The maritime sector is no exception, having used GPS for decades, gradually replacing older radio navigation systems such as LORAN (and, for those who remember, SYLEDIS).

Based on the elements discussed above:

1. The complete list of affected equipment is not available.
2. The devices concerned appear to be mostly older models.
3. The rollover will not necessarily occur exactly on April 6 for all systems.
4. The consequences may be minor (potentially “only” the date is affected).
5. In principle, positioning itself (latitude/longitude) should not be impacted.

However, the difficulty in the maritime sector is that GPS is used for far more than just determining position:

1. GPS date and time are transmitted via NMEA messages to many onboard systems. An incorrect date can therefore propagate through the network.
2. If the GPS fails completely (not just the date), the impact could be significant because it may stop transmitting data entirely on the NMEA bus.
3. ECDIS systems may use date/time information to compute estimated arrival times; a sudden change in date could cause abnormal behavior.
4. The GPS time reference may also feed:
   • GMDSS systems, including some INMARSAT receivers (GPS is often used for antenna pointing and ephemeris calculations)
   • SSAS (Ship Security Alert System)
   • VMS (Vessel Monitoring System)
   • VDR (Voyage Data Recorder)
   • AIS (Automatic Identification System)
   • Ephemeris systems, such as tide calculations
5. On larger vessels, date information may also be distributed to other systems for time synchronization (NTP servers) or even used to maintain the clocks of onboard industrial systems. See for example this technical explanation.

For those operating such equipment in the maritime sector, it is therefore important to verify that your GPS devices are not too old and are not affected by the March and April 2019 rollover events, and to apply updates if necessary.

For further information:
https://www.GPS.gov/