GPS Week Rollover Is Coming in April 2019

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
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GPS Week Rollover Is Coming in April 2019

Postby jimh » Thu Dec 06, 2018 10:16 am

In the U.S. Air Force NAVSTAR Global Positioning System (for NAVigation System Timing And Ranging and also known widely as just "GPS"), the underlying basis for position finding is knowledge of the precise time a signal is transmitted from a satellite. In the navigation message sent by a GPS satellite, the exact time a signal is sent is encoded into the navigation message. The GPS system keeps track of the date by counting up in units of weeks. The design of the system allocated a maximum WEEK NUMBER to be 1023, based on the representation of the week number in binary math as a ten-digit number, that is, 2 raised to the 10-power is 1024, thus the week number starts at zero and proceeds to 1023. At that point, the week number rolls over to zero, and begins counting upward again.

A period of 1024-weeks is 19.7-years. The GPS system refers to this an a GPS epoch. GPS timekeeping began on January 6, 1980. The first rollover to a new epoch occurred on August 21,1999. The next GPS epoch rollover will occur on April 6, 2019, or next Spring.

For many users of GPS, the GPS receiver they are using was very likely made after August 21, 1999, and has therefore not experienced the effect of a GPS epoch week rollover. Until the new epoch begins on April 6, 2019, it is difficult to predict how a particular GPS receiver will respond.

A further consideration is the manner in which a particular GPS receiver has implemented its handling of the GPS week notation. Some manufacturers have attempted to extend the useful service life of their GPS receiver by using an implementation that handles the week number counting beginning with the date of their own firmware, thus giving their device an approximate service life of 19.7-years before it encounters a week number rollover.

A number of recreational boaters have already begun to experience problems with their chartplotters and GPS receivers becoming confused about the date and time. The GPS week number in decimal 1000 began on October 21, 2018, and this is when some recreational-grade chart plotters began to have problems with display of data and time. This error may be associated with the method those devices used to handle conversion of the the GPS week number. The onset of these problems seems to have occurred when the actual GPS week number reached 1,000 in decimal. There is an assumption that among the affected devices, their handling of the GPS week was done with only a three digit decimal number, thus 999 would have been the highest GPS week number they could accommodate in their computation of calendar date from GPS epoch week number.

As a random data point, my first-generation Lowrance HDS-8 continues at present to show the proper time and date. And my GlobaSAT BU-353 USB GPS receiver is sending a NMEA-0183 output with the correct date and time at this moment.

HDS_ScreenCaptureSatsInView.png
Fig. 1. Screen capture from HDS-8 first generation showing display of proper time and date, thus immunity to the GPS Week Epoch changing to four digits in decimal, as occurred in October, 2018. Note I have obscured the location of the receiver in this image, but the position finding of this receiver is also working properly.
HDS_ScreenCaptureSatsInView.png (24.45 KiB) Viewed 34740 times


Next Spring, when the current GPS week epoch rolls over to zero, we may see more recreational-grade marine chartplotter and GPS devices affected by this event.

For a very thorough explanation of GPS Week Rollover see
https://www.spirent.com/blogs/positioning/2018/january/gps-rollover-week

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Re: GPS Week Rollover Is Coming in April 2019

Postby Phil T » Thu Dec 06, 2018 8:13 pm

Oh my god, it's another Y2K scenario. That was painful from an IT perspective.
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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Fri Dec 07, 2018 3:18 pm

To give some perspective, the GPS system keeps time and date in its own system. GPS does not use UTC for time, and, from what I understand, that's because UTC often adds leap seconds about every four years to adjust for the earth rotation speed. Similarly with date, GPS does not use a conventional calendar date, again due to leap years adding days to February every four years. Instead, GPS marches along to its own time and day standards, referencing them to a particular starting epoch.

At this moment GPS time is 18-seconds ahead of UTC, an accumulation of all the leap seconds (and other adjustments) that have been added to UTC over the lifetime of GPS. GPS time and UTC were synchronized in 1980 and have been drifting apart since then.

Today on a calendar is Friday, December 7, 2018, but for GPS today is in GPS Week 2030. (The week 2030 is counting from the first epoch, 1024 weeks, and adding 1006 weeks, the amount we are now into the second epoch.) The day is described by the number of seconds from the start of the week, so today began at 432000 seconds into the week, that is five days of 24-hours of 60-minutes of 60-seconds duration.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Dec 12, 2018 8:11 am

I became more curious about the GPS receiver in my HDS-8 (first generation), and I investigated the NMEA-0183 output data it was sending. At first I looked at the common GPS position sentences like GGA or GLL, but then I noted those sentences do not send the date. A sentence available from the HDS-8 that includes the date is NMEA ZDA - Date and Time.

The format for ZDA is as follows

$GPZDA,hhmmss.ss,dd,mm,yyyy,xx,yy*CC
where:
hhmmss HrMinSec(UTC)
dd,mm,yyy Day,Month,Year
xx local zone hours -13..13
yy local zone minutes 0..59
*CC checksum


Here is the output of my HDS-8 next to the format specification:

$GPZDA,hhmmss.ss,dd,mm,yyyy,xx,yy*CC
$GPZDA,201724,11,12,18,-05,00*68


Decoding this myself, I get the following:
Time is 20-hours, 17-minutes, 24 seconds
Date is day 11, month is 12, year 18
Time offset in hours from GMT is -5
Time offset in minutes from GMT is 00
Checksum is 68.

The information I found for the ZDA sentence structure indicates the year is supposed to be sent as a four-digit value. The HDS is only sending a two-digit value for the year.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Dec 12, 2018 8:54 am

Another NMEA-0183 sentence that sends the date is RMC. The format for RMC is:

$GPRMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,xxxx,x.x,a*hh
  1. Time (UTC)
  2. Status, V = Navigation receiver warning; A=Valid
  3. Latitude
  4. N or S
  5. Longitude
  6. E or W
  7. Speed over ground, knots
  8. Track made good, degrees true
  9. Date, ddmmyy
  10. Magnetic Variation, degrees
  11. E or W
  12. Checksum


Here is a sample output from the HDS-8:
$GPRMC,hhmmss,A,llll.llll,a,yyyyy.yyyy,a,x.x,xx.x,ddmmyy,x.x,a*hh
$GPRMC,200116,A,4231.0000,N,08312.0000,W,0.1,83.0,101218,7.7,W,A*2E


This decodes to
Time = 20-hours 01-minutes 16-seconds
Valid position
Latitude = 42-degrees 31.0000-minutes N
Longitude = 083-degrees 12.0000-minutes W
Speed over ground = 0.1-knots
Track made good = 83.0-degrees true
Date: Day =10 Month = 12 Year = 18
Magnetic variation = 7.7 W

Again the year is being sent as a two digit value.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Sun Dec 30, 2018 11:14 am

Below is a link to a brief memo from Homeland Security that alerts GPS users to a possible problem that may occur in April 2019 related to the GPS Week Number Rollover to zero:

MEMORANDUM FOR U.S. OWNERS AND OPERATORS USING
GPS TO OBTAIN UTC TIME
Upcoming Global Positioning System Week Number Rollover Event

https://ics-cert.us-cert.gov/sites/default/files/documents/Memorandum_on_GPS_2019.pdf

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Re: GPS Week Rollover Is Coming in April 2019

Postby porthole » Wed Jan 23, 2019 6:38 pm

Reading the DHS file leaves me just as confused.
My take is that we'll just go with "business as usual" until April 6, 2019 and see what happens.

If the timing becomes a problem, will the device just not lock on?
Is this similar to what happened the Northstar 9XX series GPS units that could no longer provide tide data due to some clock error?
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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Mon Feb 11, 2019 11:09 am

porthole wrote:...If the timing becomes a problem, will the device just not lock on?


The "timing" problem is not in the GPS receiver, it is in the method the GPS receiver handles the GPS Week Number. There should be no effect on the ability of the GPS receiver to receive signals from GPS satellites.

The error occurs when the navigation message is processed. The GPS Week number is not properly handled by some GPS receiver chips.

In the case of NAVICO (with brands Lowrance, Simrad, and B&G), it appears that they used a GPS chip made by OEM company ST Microelectronics and in the Teseo family. To remedy the problem NAVICO has been providing its customers with a firmware updater patch file. When this file is executed on the affected chart plotter, it communications internally with the GPS chip, identifies the chip model, and then tells the chip to modify its internal firmware to correct the problem. It is a rather complex patch. It took a while for NAVICO to get a patch for every model affected by this problem.

For Lowrance plotters you can find the updaters at

https://downloads.lowrance.com

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Mon Feb 11, 2019 11:11 am

porthole wrote:...Is this similar to what happened the Northstar 9XX series GPS units that could no longer provide tide data due to some clock error?


I have no information about what caused legacy devices from NORTHSTAR to malfunction. If the onset of the problem was at the same date as the GPS Week rolled over to 1000 from 999, then the problem is likely the same problem: poor handling of the GPS Week number.

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Re: GPS Week Rollover Is Coming in April 2019

Postby Tom Hemphill » Sun Mar 31, 2019 11:40 am

Recently my National Weather Service local forecasts added a link to the following information:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE BOSTON/NORTON MA
1151 AM EDT THU MAR 28 2019

...UPCOMING LOSS OF MARINE OBSERVATIONS...

ON OR AFTER 8 PM EDT ON APRIL 5, 2019, IT IS EXPECTED THAT A
NEWLY IDENTIFIED GPS SOFTWARE concern WILL CAUSE TWENTY COASTAL AND
MARINE AUTOMATED NETWORK (C-MAN) STATIONS TO STOP TRANSMITTING ALL
DATA.

FOR NEW ENGLAND, THIS INCLUDES
BUZZARDS BAY, MA (BUZM3)
MATINICUS ROCK, ME (MISM1)
MT. DESERT ROCK, ME (MDRM1)

LOCAL NATIONAL WEATHER SERVICE FORECAST OFFICES HAVE NO CONTROL
OVER FIXING OR MAINTAINING THESE SITES. THEY ARE MAINTAINED BY THE
NATIONAL DATA BUOY CENTER (NDBC). NDBC IS WORKING TO UPDATE THE
GPS SOFTWARE AS SOON AS POSSIBLE, BUT THIS REQUIRES ONSITE VISITS.
THESE VISITS WILL MOST LIKELY BE DONE BETWEEN AUGUST AND NOVEMBER
OF 2019.

YOU CAN READ THE NATIONAL WEATHER SERVICE HEADQUARTERS SERVICE
CHANGE NOTICE ON THIS AT

HTTPS://WWW.WEATHER.GOV/MEDIA/NOTIFICAT ... N_BUOY.PDF
[Note: the above hyperlink is a dead-link, 404-NOT FOUND result--jimh]


THE NDBC ONLINE SCHEDULE OF THEIR PROJECTED SITE MAINTENANCE IS AT:

HTTPS://WWW.NDBC.NOAA.GOV/OPS.SHTML
[Note: the above hyperlink is a dead-link, 404-NOT FOUND result--jimh]


YOU CAN READ A FURTHER EXPLANATION OF THE GPS SOFTWARE concern AT:

HTTPS://WWW.NWS.NOAA.GOV/OM/MARINE/GPSR ... FFECTS.PDF
[Fixed bad URL--jimh]

NOAA APOLOGIZES FOR THE INCONVENIENCE THAT THIS WILL CAUSE OUR
CUSTOMERS.

Apparently the GPS "Week Rollover" event will impact a number of aids to navigation.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Apr 03, 2019 4:48 pm

It sounds like the notice is talking about automated weather information sites.

If aids to navigation were affected, the USCG would post information in NOTICE TO MARINERS.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Fri Apr 05, 2019 7:50 pm

Tomorrow is April 6, and we will discover if our GPS receivers are going to be affected. I just checked my Lowrance HDS-8 this afternoon: it is still showing the proper date and time. Tomorrow I will re-check and see if the GPS receiver has survived its first GPS-WEEK Rollover event.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Mon Apr 08, 2019 3:54 pm

This morning, April 8, 2019, is well past the GSP Week Rollover event that occurred sometime on April 6, 2019, and I checked my Lowrance HDS-8 chart plotter to see if the internal GPS receiver was still working properly. Curiously, it took unusually long for the GPS receiver to produce a position fix and show me the date and time, perhaps due to an unusual arrangement of GPS satellites in the overhead sky and the indoor location of the receiver. After about ten minutes the GPS receiver had a position fix, a time, and a data, and all was normal.

If you have not checked your GPS receiver lately, now would be a good time. The GPS Week number has rolled over to zero and is counting upward toward 1023, which will take about 19.7-years. If your GPS receiver is working today, it should be good for about two decades.

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Re: GPS Week Rollover Is Coming in April 2019

Postby ConB » Mon Apr 08, 2019 7:31 pm

Both of my Garmin hand helds are working. One is newer and fired right up. My older Garmin is always slow if it has sat for a while. The bad batteries may have had something to do with that too.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Tue Apr 09, 2019 9:08 am

There is nothing in the GPS Week Rollover event that would stop a GPS receiving from "booting up" or being "fired up." The GPS Week Rollover event only affects the date function. To know if a GPS receiver has properly handled the GPS Week Rollover Event, the year, month, and day have to be checked.

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Re: GPS Week Rollover Is Coming in April 2019

Postby Acseatsri » Tue Apr 09, 2019 4:13 pm

[Gave a a second-hand report that a] Lowrance Gen 2 multi-function display with internal GPS receiver did not get a position fix on April 8, 2019.

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Re: GPS Week Rollover Is Coming in April 2019

Postby f8f1 » Wed Apr 10, 2019 9:18 am

After booting up normally, my Garmin 547xs displayed the correct date and its position fix matched its location in my driveway.
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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Apr 10, 2019 1:57 pm

If a GPS receiver does not have a proper date and time available when it is started from a cold start, the receiver cannot predict the orbits of the GPS satellites that will be in view. Without a hint about what satellites are in view, the receiver will begin to search for satellites in a systematic way and many of them might not be in view. This can result in a very long time to obtain an initial position fix, possibly an hour or more, particularly on older GPS receivers with limited numbers of correlators (referred to as "channels" by boaters), if the actual satellites that are in view don't happen to be searched for in the early stages of the methodical search pattern.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Apr 10, 2019 2:30 pm

Acseatsri wrote:[Gave a a second-hand report that a] Lowrance Gen 2 multi-function display with internal GPS receiver did not get a position fix on April 8, 2019.


The person with the Lowrance device should visit the Lowrance website. Lowrance has a comprehensive list of devices that may require updating to function properly. Only a few models are affected among dozens of legacy devices.

See

https://www.lowrance.com/news-videos/gps-week-rollover/

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Wed Apr 10, 2019 2:32 pm

Several manufacturers have information regarding their products and how they are affected by the April 6, 2019 GPS Week Rollover Event.

NAVICO brand current models are mostly not affected by the GPS Week Rollover, and for older devices that have been affected there are or will be patches that can be applied in the field to modify the internal GPS receivers to provide a remedy:

https://www.simrad-yachting.com/news/gps-week-rollover/

https://www.lowrance.com/news-videos/gps-week-rollover/

https://www.bandg.com/news/gps-week-rollover/

GARMIN seems to think none of its products are affected:

https://support.garmin.com/ms-MY/?faq=zWQY6Z2kFiAuY9kDnDBgZ6

RAYMARINE seems to think that many of its older products are affected. They offer no remedy other than to purchase a replacement GPS receiver:

http://forum.raymarine.com/showthread.php?tid=7453

http://forum.raymarine.com/attachment.php?aid=2852

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Fri Apr 12, 2019 1:11 pm

I was able to check the operation of my GlobalSAT BU-353 GPS receiver on April 12, 2019 around 12:59 EDST (GMT -4). It was able to obtain an accurate position fix, and it sent the date information via NMEA-0183:

$GPRMC,hhmmss.sss,A,llll.llll,a,yyyyy.yyyy,a,x.xx,xxx.xx,ddmmyy,x.x,a*hh
$GPRMC,165914.000,A,4231.0000,N,08312.0000,W,0.04,323.65,120419,,


This is decoded at follows:
UTC Time = 16-hours 59-minutes 14.000-seconds
Position = VALID
Latitude = 42-degrees 31.0000-minutes N
Longitude = 083-degrees 12.0000-minutes W
Speed over ground = 0.04-knots
Track made good = 323.65-degrees
Day: Day=12 Month=04 Year=19
Magnetic variation = no data
Checksum = no data

Based on this outcome, the GlobalSAT BU-353 GPS receiver appears to have no effects from the GPS Week Rollover event.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Fri Apr 12, 2019 1:57 pm

Most manufacturers of recreational grade marine chart plotters that have internal GPS receivers did not manufacture the GPS receiver themselves; they typically bought a entire GPS receiver on a chip from a GPS receiver chip manufacturer. The only exception to that might be GARMIN. I don't know what chips they use; maybe GARMIN is big enough in GPS development to make their own.

For recreational marine chart plotter makers who bought someone else's GPS receiver on a chip, I don't think it is likely they subjected those chips to rigorous testing with regard to GPS Week Rollover Events. Clearly many of the chip manufacturers did not do that testing themselves. To hold a recreational chart plotter maker to rigorous standard with regard to GPS Week Rollover Events seems quite a strange demand coming from recreational boaters who buy these recreational grade chart plotters. I doubt if one in a million boaters was even aware of GPS Week Rollover Events when they bought their recreational grade chart plotter with a GPS receiver made by someone else, and most likely neither were the recreational grade chart plotter makers.

In order to detect how a particular GPS chip is going to behave when a GPS Week Rollover Event occurs, the most realistic test would be to have a sophisticated GPS simulator test generator that could be set ahead to create signals that would occur just before and then just after a GPS Week Rollover Event. Test gear like that is not cheap, and to expect that every recreational chart plotter maker would have an engineering and test center with that sort of gear is not reasonable. Nor is it reasonable to demand, even if they had the test gear to do it, that they would have thoroughly tested for the behavior of the GPS receiver they bought for inclusion in their recreational grade chart plotter products with regard to a GPS Week Rollover.

For recreational chart plotter makers who did end up with products having an internal GPS receiver with bad behavior following a GPS Week Rollover event, there is no simple fix available for them. They did not make the GPS receiver chip. To alter the firmware in a GPS receiver chip will require the manufacturer of the chip to come up with a fix. The chart plotter maker will have to wait for the chip manufacturer to figure out how to remedy the bad behavior of the GPS chip. There is no way the chart plotter maker can know anything about the embedded operating system inside someone else's GPS chip. Assuming the GPS maker comes up with a fix, then the chart plotter maker has to figure out some method of loading the new firmware into the chip using the existing hardware and software capabilities of their product. I am surprised that some companies like NAVICO have been able to figure out a method that an internal chip containing firmware created by someone else can be updated using the NAVICO operating system. Accomplishing that is really quite a feat for NAVICO to pull off.

When the chip manufacturer provides an updater, and the recreational chart plotter maker figures out some crafty method to update that chip inside their recreational chart plotter, it seems a bit harsh to blame the recreational chart plotter maker when the chip maker's new firmware does not solve all the problems.

There is also a possibility that there may be no remedy developed by the GPS chip manufacturer. There could be limitation in the hardware in the chip that prevent a remedy from being possible. For example, there might not be enough memory available to implement new firmware.

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Re: GPS Week Rollover Is Coming in April 2019

Postby Don McIntyre - MI » Fri Apr 12, 2019 2:49 pm

Whew. Want to talk about a cluster-bomb that's brewing?

How about a very major thermal imaging company that's supplied mobile imaging systems for years, for world wide customers. It's hit them. Fleets are being affected. And it looks like some systems will be required to be shipped back to the factory for updating.

Big case of not good.

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Re: GPS Week Rollover Is Coming in April 2019

Postby jimh » Fri Apr 12, 2019 3:07 pm

I can understand how users of commercial grade gear that has a GPS receiver may have bigger problems than a recreational boater. I only own one recreational grade chart plotter, and it is working fine. If I were a commercial operator with a fleet of a thousand vehicles whose GPS stopped working last weekend, I would not be as complacent.

The event has demonstrated the reliance on GPS for position, navigation, and timing (PNT) that has become pervasive in modern society.

There won't likely be another GPS Week Rollover Event like this: the newer GPS III phase satellites will use a much longer week epoch of 8192 weeks (or 2 to the 13th power). That means the newer GPS WEEK system week rollover will occur only once every 157-years. None of us will be around when that date comes up.

I don't know if GPS will maintain its legacy format for week number forever so that current GPS receivers will still be useful. Maybe in 19.7-years we will find out. And maybe by then all legacy GPS receivers that could be fixed will have been fixed. And all the ones that couldn't be fixed will have been replaced.