What is the Week Rollover Problem?

Discussion in 'General GPS Discussion' started by Nuvi-Nebie, Feb 21, 2016.

  1. Nuvi-Nebie

    Nuvi-Nebie Moderator

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    This problem only affects early GPS units, symptoms include, a unit's date displaying 10 or more years away from the current date and (or) an Inability to get a satellite 'fix'

    Your GPS unit has a battery backed memory that contains the current time and date as well as an almanac (a list of satellites to look for), waypoints, track, routes, etc. if this 'memory' is lost the the unit will find it much harder to get a 'fix' and sometimes it will be impossible.

    Early units only kept a 20 year record of the date, so if the unit gets reset back to the date of it's manufacture, it gets 'stuck' in the past and is unable to 'see' satellites in their current position, you may be able to fix this on units where you can manually set the current time / date, where this is not possible the date can sometimes be updated with new software

    It is recommended that units not in regular use are stored with main batteries fitted in order to keep the time / date correct especially in early units, batteries will need to be checked to see if they are O.K. periodically

    Technical Explanation


    Satellites transmit the date as a 10 bit 'week counter', this rolled over on 22nd. August 1999, and again on 6th. April 2019 early units only stored this 10 bit week counter, giving them a maximum date span of 20 years, current units store an extra 3 bits to count the rollover 'Blocks'

    Block . . . . . . . . Range
    1 . . . (000) . . . 6th January 1980 to 22nd August 1999
    2 . . . (001) . . . 22nd August 1999 to 6th April 2019
    3 . . . (010) . . . 6th April 2019 to 21 November 2038
    4 . . . (011) . . . 21 November 2038 to 2057

    5 . . . (100) . . . 2057 to 2076

    And so on . . .

    However the 1023 week block is sometimes shifted away from these dates by the manufacturer, so a unit made in say 2005 could count from then to 2024 before seeing a problem, also the problem may only be seen when the unit 'forgets' the current date due to the internal battery failing


    NOTE : This section is for Garmin products only

    There is a computer program produced by Garmin that fixes this problem on some of it's early GPS models, e.g. GPS38 and GPS12, I don't have a full list of the units this works on so some research is required, the program requires that both the computer and the GPS unit has a RS232 COM port although it is possible to use a USB to COM port interface on the computer end.

    For more details on how to connect to the older Garmin Serial port, see this link (Click HERE)

    Note:- It may be necessary to re-allocate the COM port number because GpsEow only allows COM1 to COM4, below are the steps for Windows XP :-


    Device Manager >> COM LPT list >> Right Click on COM >> Properties >> Port Settings >> Advanced >> COM Port Number

    If COMs 1 to 4 are already allocated, you will have to reassign one to a higher number in order to allow it to be used

    com-port.jpg

    A copy of the GpsEow.zip file is attached below

    Garmin GPS Sensors

    Garmin products described as sensors are usually units with no display or control buttons commonly with antenna and GPS receiver electronics in a single weatherproof unit, older sensors that may be affected by the rollover problem included GPS - 10, 10x, 15/15U, 15 H/L, 16A, 16/17, 17N, 18 PC / LVC, 18-5Hz, 25, 35, 36, these sensors used Serial (RS-232) ports to deliver NMEA 0183 ver2 sentences to computers etc., Garmin supplied a Windows sensor configuration program called SNSRCFG_320.exe that can be used to change the date on these sensors which can fix the rollover problem

    A copy of the SNSCFG_320.zip file is attached below

    This website (Click HERE) has details on how to set the date using SNSCFG_320.exe

    Main Points :-
    1) Select Config/Get Configuration from GPS
    2) Select Config/Send Configuration to GPS (This also sends the computer's time/date to the unit)
    3) Check NMEA sentences are displaying current time
    4) If NMEA time is still incorrect Set your computer's clock ahead 9 years and repeat 1 - 3
    5) It may be necessary to repeat this procedure (Including exiting SNSCFG_320 and re-running it) several times
    6) Set computer's time back to current time / date when procedure is completed

    I suspect that the NMEA command sent to sensors (and other Garmin units) to set the time / Date is $PGRMI, here is the syntax of this command :-
    PGRMI.jpg




     

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    Nuvi-Nebie, Feb 21, 2016
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