【材料】GPS周期翻转问题！JRC，Furuno用户看过来！

2019-03-26 14:58来源： 信德海事网

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Notice for upcoming GPS week roll-over

All users of GlobalPositioning System (GPS) receivers should be aware that on 6 April 2019, theGPS Week Number will rollover into the third epoch. This will be only thesecond time the GPS week number has rolled over; as such, this may affectCoordinated Universal Time (UTC) derived from GPS. Tests have shown that someolder GPS receivers that have not been kept up to date may not handle therollover correctly, giving incorrect UTC following the rollover. Navigationsolution should not be affected, but associated time tags could be incorrectthus still corrupting navigation data.

AA: BACKGROUND

The GPS Internal Navigation Time Scale “GPS Time” is based onthe weighted average of GPS satellites and ground station clocks. GPS Time isused for user navigation solutions. A nanosecond error in GPS Time can equateto one foot of position (ranging) error. The WN parameter is provided via a ten(10) bit parameter – or “counter.” The valid range of values for the WN parameteris 0 to 1023 (or 1024 total values). The WN parameter is incremented by oneeach 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. The WNvalue is referenced to the start of the current GPS Time Epoch. The last WNrollover was August 21, 1999. GPS Time is currently in the second Epoch. Thenext WN rollover is April 6, 2019.

GPS Time is adjusted bythe U.S. Air Force GPS Directorate to maintain alignment with UTC as providedby the U.S. Naval Observatory. A GPS device that provides UTC time does so byconverting GPS Time to UTC using multiple parameters – including WN – conveyedin page 18 of GPS sub-frame 4. GPS devices with a poorly implemented GPSTime-to-UTC conversion algorithm may provide incorrect UTC following a WNrollover. Additionally, some GPS devices that calculate the WN value from adevice-specific date rather than the start of the current GPS Time Epoch mayprovide incorrect UTC at some other device-specific date.

BB: SUMMERY OF ISSUE

Critical Infrastructure(CI) owners and operators and other users who obtain Coordinated Universal Time(UTC) from Global Positioning System (GPS) devices should be aware of the GPSWeek Number (WN) rollover events and the possible effect a GPS WN rolloverevent may have on the reliability of the reported UTC. The IS-GPS-200Hinterface specification identifies both the ten bit WN parameter and the WNrollover events. A GPS device that conforms to the latest IS-GPS-200 andprovides UTC should not be adversely affected. However, tests of some GPSdevices revealed that not all manufacturer implementations correctly handle theApril 6, 2019 WN rollover. Additionally, some manufacturer implementationsinterpret the WN parameter relative to a date other than January 5, 1980. Thesedevices should not be affected by the WN rollover on April 6, 2019 but mayexperience a similar rollover event at a future date. For example, a particularGPS device may interpret the WN parameter relative to a firmware creation dateand would experience a similar rollover event 1024 weeks after that firmwarecreation date.

Thereare products at a higher risk of going wrong if they are;

- units older than 10 years, and

-units which have had no firmware updates.

Productsare at a lower risk if;

- they are relatively new units, and

-units that are regularly updated with firmware.

CC: ACTIONS TO TAKE

It is recommended that all GPS users should:

1.Ensure all recent firmware updates are completed;

2.Check with your GPS manufacturer for further information on GPS-dependentequipment and identify what action, if any is, required by the operator;

3. Identify what effect this could have onyour current operations, PNT (position, navigation and timing) solutions andpotential issues if failure occurred; and

a. to understandthe manufacturers’ preparedness for the April 6, 2019 WN rollover,

b. to understand actions required by CI andother owners and operators to ensureproper operation through the April 6,2019 WN rollover

4. Operators of suchequipment should monitor and, wherever possible, verify their equipment for anyerrors.

Forexample:

Øthe following guidance notefrom FURUNO manufacturer

Please comply with the procedure in item 3.

1.List of equipment that may be affected by the GPS weeknumber rollover

Rollover Occurrence Date: 2019/03/17

NOTE: The datespecified above is the date in which week number rollover occurs in ourproducts listed above, and the date is independent from the one announced bythe US Department of Homeland Security, namely, the 6th of April 2019, which isthe date when GPS week number rollover occurs in the GPS as a system per se.

The differencestems from different initial dates of counting week between GPS system and theGPS chipset used in our products.

2.Natures ofthe error and its cause

The GPS systemis synchronized to a different time scale than Coordinated Universal Time (UTC)and calculates time by counting the number of weeks up to 1,023rd week.Afterweek 1,023, the week number count is reset to zero. This is called "GPSweek number rollover". Subsequently, the erroneous date information on theGPS receiver clock may result in instability in positioning, causing positionerrors, including failure of GPS position acquisition, etc.

3.Procedure forchecking the integrity of the equipment and counter measures to be taken

Please carryout the following counter measures:

1. Since the nature of the problem of "GPS week number roll-over"is the erroneous date information on the receiver clock, the equipment thatdoes not require date information would be unaffected by the problem inquestion. If, by any chance, the positioning performance became instable, youwould be able to resolve the problem of positioning integrity by cold-startingthe equipment through the operating menu. Please note, however, that eventhough you regain the GPS position acquisition, the date information cannot berestored merely by cold-starting.

2. Ifthe positioning integrity is not restored after cold start, the problem maystem from the failure of the equipment, not from GPS week number roll-over. Ifsuch is the case, please consult with our sales representative closest to yourlocation.

3. Ifyou wish to restore the date information, you will need to replace the GPSreceiver within the equipment for value. Also, please note that the above listof equipment includes some models whose maintenance service provision periodhas been expired.

NOTE

If the GPSproduct is interfaced with any of the following equipment and systems, thecountermeasures would have to be taken, for the date information is utilized bythe interfaced equipment and systems.

* GMDSS radio installation, incl. internal GNSS receiver of Inmarsat-Cterminal

* Ship Security Alert System (SSAS)

* Vessel Monitoring System (VMS)

* Equipment that displays route information such as ECDIS

* Voyage Data Recorder (VDR)

* Automatic Identification System (AIS)

* Other equipment and systems that utilizes the date information from GPSreceiver clock

Øthe following guidance notefrom JRC manufacturer