However, care must be taken if tests are run and the receiver date is changed across several epochs. The simulator has to provide the GPS receiver with a simulated GPS satellite signal where the week number rolls over, so you can watch the output to make sure the outgoing date, time and hardware signals continue normally when the week number rollover occurs. Since the rollover occurs in the week number included in the data stream received from the satellites, you'd need a GPS simulator to run a real test. Meinberg Devices With 3rd Party GNSS Receiver Modules However, once the correct date has been set via the user interface the firmware computes the correct extended week number and continues working normally with the correct date, which is kept in the RTC as long as the battery is OK. Only if the device's backup battery has failed and thus the RTC has no valid time and date the epoch number is unknown, so the receiver may start with the wrong epoch, resulting in date in the the range 1980.1999, or 1999.2019, depending on the receiver's firmware version. The initial extended week number is computed after power-up from the date kept in the battery-buffered real time clock chip (RTC). Tests have been run to verify that this still works as expected. The extended week number is used for all kinds of computation, and even the earliest receivers already handled the first GPS week number rollover in August 1999 and the next one in April 2019 properly.Ĭurrent models as well as older models of the GPS receivers manufactured by Meinberg use the 16 bit extended week number internally, and thus can handle the week number rollovers properly, regardless of the firmware version. The firmware uses a 16 bit week number internally, and the week number is simply incremented at the end of each week, so when the week number sent by the satellites first rolled over from 1023 to 0 in August 1999 the internal week number just rolled over from 1023 to 1024, at the next rollover in April 2019 it was incremented from 2047 to 2048, and so on. When Meinberg started to develop their first own GPS receivers in the 1990s, care was taken right from the beginning that those receivers could handle GPS week number rollovers properly. Meinberg's Approach To Handle GPS Week Number Rollovers The exact time at which these kind of problems will occur depends on the GPS receiver model and firmware version, and is not related to the GPS system-specific rollover dates mentioned above. This has happened with some 3rd party GPS receivers in February 2016, and some other 3rd party GPS receivers are expected to have problems after July, 2016. Instead, it can occur at an arbitrary weekend when the number reaches the threshold determined by the firmware developers. The worst thing with this approach is that this switchback doesn't occur at the same time as the scheduled week number rollover in the satellite data. 1884 for the extended week number, but after real week number 1884 the calculation yields a number in the range 0.860, thus a wrong date 1024 weeks in the past. If you take 90 as a limit the resulting range would be 1990.2089.įor the GPS week number example this results in a valid range like 860. This approach is similar to extending a 2 digit year number to a full 4 digit year number. If the week number is above 860 we assume a rollover has not yet occurred.” “If the transmitted week number is below 860 we assume a rollover has already occurred, and thus add 1024 to get a full week number. There are 3rd party GPS receivers out there which take a very simply approach with a fixed week number limit, e.g.: The first GPS week number rollover occured 1024 weeks later, on Saturday, 21st August 1999, and the next rollover occured another 1024 weeks later, on Saturday, 6th April 2019.įor pure navigational receivers this doesn't matter much since they work based on time differences only, but GPS receivers used for time synchronization have to properly account for these rollovers to determine the correct calendar date.Īll Meinberg devices with integrated GPS or GNSS receivers handle the week number rollovers without problems.Ī more detailed explanation follows below. The “start date” of the GPS system time is Sunday, 6th January 1980. This behavior is intentional and has been clearly specified in the GPS Interface Control Document Unfortunately most GPS satellites send the GPS week number encoded in 10 bits only, covering a range of 1024 weeks (week numbers 0 through 1023), and after week 1023 the week number transmitted by the satellites rolls over to 0. The GPS system time is based on week numbers since an epoch, and seconds of the current week.
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